Dayside Magnetosphere Interactions (Geophysical Monograph Series) [1 ed.] 1119509637, 9781119509639

Citation preview

Geophysical Monograph Series

Geophysical Monograph Series 197 Auroral Phenomenology and Magnetospheric Processes: Earth and Other Planets Andreas Keiling, Eric Donovan, Fran Bagenal, and Tomas Karlsson (Eds.) 198 Climates, Landscapes, and Civilizations Liviu Giosan, Dorian Q. Fuller, Kathleen Nicoll, Rowan K. Flad, and Peter D. Clift (Eds.) 199 Dynamics of the Earth’s Radiation Belts and Inner Magnetosphere Danny Summers, Ian R. Mann, Daniel N. Baker, and Michael Schulz (Eds.) 200 Lagrangian Modeling of the Atmosphere John Lin (Ed.) 201 Modeling the Ionosphere‐Thermosphere Jospeh D. Huba, Robert W. Schunk, and George V. Khazanov (Eds.) 202 The Mediterranean Sea: Temporal Variability and Spatial Patterns Gian Luca Eusebi Borzelli, Miroslav Gacic, Piero Lionello, and Paola Malanotte‐Rizzoli (Eds.) 203 Future Earth – Advancing Civic Understanding of the Anthropocene Diana Dalbotten, Gillian Roehrig, and Patrick Hamilton (Eds.) 204 The Galápagos: A Natural Laboratory for the Earth Sciences Karen S. Harpp, Eric Mittelstaedt, Noemi d’Ozouville, and David W. Graham (Eds.) 205 Modeling Atmospheric and Oceanic Flows: Insightsfrom Laboratory Experiments and Numerical Simulations Thomas von Larcher and Paul D. Williams (Eds.) 206 Remote Sensing of the Terrestrial Water Cycle Venkat Lakshmi (Ed.) 207 Magnetotails in the Solar System Andreas Keiling, Caitriona Jackman, and Peter Delamere (Eds.) 208 Hawaiian Volcanoes: From Source to Surface Rebecca Carey, Valerie Cayol, Michael Poland, and Dominique Weis (Eds.) 209 Sea Ice: Physics, Mechanics, and Remote Sensing Mohammed Shokr and Nirmal Sinha (Eds.) 210 Fluid Dynamics in Complex Fractured‐Porous Systems Boris Faybishenko, Sally M. Benson, and John E. Gale (Eds.) 211 Subduction Dynamics: From Mantle Flow to Mega Disasters Gabriele Morra, David A. Yuen, Scott King, Sang Mook Lee, and Seth Stein (Eds.) 212 The Early Earth: Accretion and Differentiation James Badro and Michael Walter (Eds.) 213 Global Vegetation Dynamics: Concepts and Applications in the MC1 Model Dominique Bachelet and David Turner (Eds.) 214 Extreme Events: Observations, Modeling and Economics Mario Chavez, Michael Ghil, and Jaime Urrutia‐Fucugauchi (Eds.) 215 Auroral Dynamics and Space Weather Yongliang Zhang and Larry Paxton (Eds.) 216 Low‐Frequency Waves in Space Plasmas Andreas Keiling, Dong‐Hun Lee, and Valery Nakariakov (Eds.) 217 Deep Earth: Physics and Chemistry of the Lower Mantle and Core Hidenori Terasaki and Rebecca A. Fischer (Eds.) 218 Integrated Imaging of the Earth: Theory and Applications Max Moorkamp, Peter G. Lelievre, Niklas Linde, and Amir Khan (Eds.) 219 Plate Boundaries and Natural Hazards Joao Duarte and Wouter Schellart (Eds.) 220 Ionospheric Space Weather: Longitude and Hemispheric Dependences and Lower Atmosphere Forcing Timothy Fuller‐Rowell, Endawoke Yizengaw, Patricia H. Doherty, and Sunanda Basu (Eds.) 221 Terrestrial Water Cycle and Climate Change Natural and Human‐Induced Impacts Qiuhong Tang and Taikan Oki (Eds.) 222 Magnetosphere‐Ionosphere Coupling in the Solar System Charles R. Chappell, Robert W. Schunk, Peter M. Banks, James L. Burch, and Richard M. Thorne (Eds.)

223 Natural Hazard Uncertainty Assessment: Modeling and Decision Support Karin Riley, Peter Webley, and Matthew Thompson (Eds.) 224 Hydrodynamics of Time‐Periodic Groundwater Flow: Diffusion Waves in Porous Media Joe S. Depner and Todd C. Rasmussen (Auth.) 225 Active Global Seismology Ibrahim Cemen and Yucel Yilmaz (Eds.) 226 Climate Extremes Simon Wang (Ed.) 227 Fault Zone Dynamic Processes Marion Thomas (Ed.) 228 Flood Damage Survey and Assessment: New Insights from Research and Practice Daniela Molinari, Scira Menoni, and Francesco Ballio (Eds.) 229 Water‐Energy‐Food Nexus – Principles and Practices P. Abdul Salam, Sangam Shrestha, Vishnu Prasad Pandey, and Anil K Anal (Eds.) 230 Dawn–Dusk Asymmetries in Planetary Plasma Environments Stein Haaland, Andrei Rounov, and Colin Forsyth (Eds.) 231 Bioenergy and Land Use Change Zhangcai Qin, Umakant Mishra, and Astley Hastings (Eds.) 232 Microstructural Geochronology: Planetary Records Down to Atom Scale Desmond Moser, Fernando Corfu, James Darling, Steven Reddy, and Kimberly Tait (Eds.) 233 Global Flood Hazard: Applications in Modeling, Mapping and Forecasting Guy Schumann, Paul D. Bates, Giuseppe T. Aronica, and Heiko Apel (Eds.) 234 Pre‐Earthquake Processes: A Multidisciplinary Approach to Earthquake Prediction Studies Dimitar Ouzounov, Sergey Pulinets, Katsumi Hattori, and Patrick Taylor (Eds.) 235 Electric Currents in Geospace and Beyond Andreas Keiling, Octav Marghitu, and Michael Wheatland (Eds.) 236 Quantifying Uncertainty in Subsurface Systems Celine Scheidt, Lewis Li, and Jef Caers (Eds.) 237 Petroleum Engineering Moshood Sanni (Ed.) 238 Geological Carbon Storage: Subsurface Seals and Caprock Integrity Stephanie Vialle, Jonathan Ajo‐Franklin, and J. William Carey (Eds.) 239 Lithospheric Discontinuities Huaiyu Yuan and Barbara Romanowicz (Eds.) 240 Chemostratigraphy Across Major Chronological Eras Alcides N.Sial, Claudio Gaucher, Muthuvairavasamy Ramkumar, and Valderez Pinto Ferreira (Eds.) 241 Mathematical Geoenergy:Discovery, Depletion, and Renewal Paul Pukite, Dennis Coyne, and Daniel Challou (Eds.) 242 Ore Deposits: Origin, Exploration, and Exploitation Sophie Decree and Laurence Robb (Eds.) 243 Kuroshio Current: Physical, Biogeochemical and Ecosystem Dynamics Takeyoshi Nagai, Hiroaki Saito, Koji Suzuki, and Motomitsu Takahashi (Eds.) 244 Geomagnetically Induced Currents from the Sun to the Power Grid Jennifer L. Gannon, Andrei Swidinsky, and Zhonghua Xu (Eds.) 245 Shale: Subsurface Science and Engineering Thomas Dewers, Jason Heath, and Marcelo Sánchez (Eds.) 246 Submarine Landslides: Subaqueous Mass Transport Deposits From Outcrops to Seismic Profiles Kei Ogata, Andrea Festa, and Gian Andrea Pini (Eds.) 247 Iceland:Tectonics, Volcanics, and Glacial Features Tamie J. Jovanelly

Geophysical Monograph 248

Dayside Magnetosphere Interactions Qiugang Zong Philippe Escoubet David Sibeck Guan Le Hui Zhang Editors

This Work is a co‐publication of the American Geophysical Union and John Wiley and Sons, Inc.

This Work is a co‐publication between the American Geophysical Union and John Wiley & Sons, Inc. This edition first published 2020 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009 © 2020 the American Geophysical Union All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions

Published under the aegis of the AGU Publications Committee Brooks Hanson, Executive Vice President, Science Lisa Tauxe, Chair, Publications Committee For details about the American Geophysical Union visit us at www.agu.org. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging‐in‐Publication Data is available. Hardback: 9781119509639 Cover image: © Naeblys/Shutterstock Cover design: Wiley Set in 10/12pt Times New Roman by SPi Global, Pondicherry, India 10 9 8 7 6 5 4 3 2 1

CONTENTS Contributors..........................................................................................................................................................vii Preface...................................................................................................................................................................xi

1. A Brief History of Dayside Magnetospheric Physics A. Otto............................................................................................................................................................1

Part I: Physics of Dayside Magnetospheric Response to Solar Wind Discontinuities 2. Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures: Summary of the Geospace Environment Modeling (GEM) Focus Group Findings Between 2012 and 2016 Hui Zhang and Qiugang Zong.......................................................................................................................13 3. Transient Solar Wind–Magnetosphere–Ionosphere Interaction Associated with Foreshock and Magnetosheath Transients and Localized Magnetopause Reconnection Y. Nishimura, B. Wang, Y. Zou, E. F. Donovan, V. Angelopoulos, J. I. Moen, L. B. Clausen, and T. Nagatsuma..........................................................................................................................................39 4. Dayside Magnetospheric Interactions Inferred from Dayside Diffuse Aurora and Throat Aurora De‐Sheng Han...............................................................................................................................................55 5. Magnetosphere Response to Solar Wind Dynamic Pressure Change: Vortices, ULF Waves, and Aurorae Q. Q. Shi, X.‐C. Shen, A. M. Tian, A. W. Degeling, Qiugang Zong, S. Y. Fu, Z. Y. Pu, H. Y. Zhao, Hui Zhang, and S. T. Yao................................................................................................................................77

Part II: Structure and Dynamics of Dayside Boundaries 6. Cluster Mission’s Recent Highlights at Dayside Boundaries Philippe Escoubet, A. Masson, H. Laakso, and M. L. Goldstein......................................................................101 7. Structure and Dynamics of the Magnetosheath Katariina Nykyri...........................................................................................................................................117 8. An Examination of the Magnetopause Position and Shape Based Upon New Observations Z. Němeček, J. Šafránková, and J. Šimůnek...................................................................................................135 9. Methods for Finding Magnetic Nulls and Reconstructing Field Topology: A Review H. S. Fu, Z. Wang, Qiugang Zong, X. H. Chen, J. S. He, A. Vaivads, and V. Olshevsky....................................153

v

vi Contents

Part III: The Roles of Solar Wind Sources on Wave Generations and Dynamic Processes in the Inner Magnetosphere 10. Theoretical Studies of Standing Toroidal Alfvén Waves in Dipole‐Like Magnetosphere A. S. Leonovich and D. A. Kozlov.................................................................................................................175 11. Ultra-Low-Frequency Wave–Particle Interactions in Earth’s Outer Radiation Belt R. Rankin, C. R. Wang, Y. F. Wang, Qiugang Zong, X. Z. Zhou, A. W. Degeling, D. Sydorenko, and G. Whittall-Scherfee.......................................................................................................189 12. Recent Advances in Understanding Radiation Belt Electron Dynamics Due to Wave–Particle Interactions W. Li, Q. Ma, J. Bortnik, and R. M. Thorne....................................................................................................207 13. Current Status of Inner Magnetosphere and Radiation Belt Modeling Mei‐Ching Fok.............................................................................................................................................231

Part IV: Cold Plasmas of Ionospheric Origin and Their Role in Coupling Different Regions in Geospace 14. Multi‐Point Observations of the Geospace Plume J. C. Foster, P. J. Erickson, B. M. Walsh, J. R. Wygant, A. J. Coster, and Qing‐He Zhang...................................245 15. Interactions Between ULF Waves and Cold Plasmaspheric Particles Qiugang Zong, Jie Ren, and X. Z. Zhou........................................................................................................265 16. Formation and Evolution of Polar Cap Ionospheric Patches and Their Associated Upflows and Scintillations: A Review Qing‐He Zhang, Zan‐Yang Xing, Yong Wang, and Yu‐Zhang Ma...................................................................285 17. Dayside Magnetosphere Interactions: Progress in Our Understanding and Outstanding Questions Qiugang Zong, Philippe Escoubet, David Sibeck, Guan Le, and Hui Zhang..................................................303 Index...................................................................................................................................................................307

CONTRIBUTORS V. Angelopoulos Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA, USA

H. S. Fu School of Space and Environment, Beihang University, Beijing, China

J. Bortnik Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA

S. Y. Fu Institute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

X. H. Chen School of Space and Environment, Beihang University, Beijing, China

M. L. Goldstein Space Science Institute and Goddard Space Flight Center, Greenbelt, MA, USA

L. B. Clausen Department of Physics, University of Oslo, Oslo, Norway

De‐Sheng Han State Key Laboratory of Marine Geology, School of Ocean and Earth Science, Tongji University, Shanghai, China

A. J. Coster Massachusetts Institute of Technology Haystack Observatory, Westford, MA, USA

J. S. He Institute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

A. W. Degeling Shandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

D. A. Kozlov Institute of Solar‐Terrestrial Physics SB RAS, Irkutsk, Russia H. Laakso ESA European Space Astronomy Centre, Madrid, Spain

E. F. Donovan Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada

Guan Le NASA Goddard Space Flight Center, Greenbelt, MD, USA

P. J. Erickson Massachusetts Institute of Technology Haystack Observatory, Westford, MA, USA

A. S. Leonovich Institute of Solar‐Terrestrial Physics SB RAS, Irkutsk, Russia

Philippe Escoubet ESA European Space Research and Technology Centre, Noordwijk, The Netherlands

W. Li Center for Space Physics, Boston University, Boston, MA, USA

Mei-Ching Fok Geospace Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA

Q. Ma Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA; and Center for Space Physics, Boston University, Boston, MA, USA

J. C. Foster Massachusetts Institute of Technology Haystack Observatory, Westford, MA, USA

vii

viii Contributors

Yu‐Zhang Ma Shandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China A. Masson ESA European Space Astronomy Centre, Madrid, Spain J. I. Moen Department of Physics, University of Oslo, Oslo, Norway T. Nagatsuma National Institute of Information and Communications Technology, Tokyo, Japan Z. Němeček Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic Y. Nishimura Department of Electrical and Computer Engineering and Center for Space Physics, Boston University, Boston, MA, USA; and Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA Katariina Nykyri Centre of Space and Atmospheric Research, Department of Physical Sciences, Embry‐Riddle Aeronautical University, Daytona Beach, FL, USA V. Olshevsky Center for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Leuven, Belgium

J. Šafránková Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic X.‐C. Shen Shandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China; and Center for Space Physics, Boston University, Boston, MA, USA Q. Q. Shi Shandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China David Sibeck NASA Goddard Space Flight Center, Greenbelt, MD, USA J. Šimůnek Institute of Atmospheric Physics, Czech Academy of Science, Prague, Czech Republic D. Sydorenko Department of Physics, University of Alberta, Edmonton, Alberta, Canada R. M. Thorne Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA

A. Otto Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA

A. M. Tian Shandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

Z. Y. Pu Institute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

A. Vaivads Swedish Institute of Space Physics, Uppsala, Sweden

R. Rankin Department of Physics, University of Alberta, Edmonton, Alberta, Canada Jie Ren Institute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

B. M. Walsh Department of Electrical and Computer Engineering, Boston University, Boston, MA, USA B. Wang Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA; and Department of Astronomy and Center for Space Physics, Boston University, Boston, MA, USA

Contributors  ix

C. R. Wang Department of Physics, University of Alberta, Edmonton, Alberta, Canada

Hui Zhang Geophysical Institute and Physics Department, University of Alaska Fairbanks, Fairbanks, AK, USA

Yong Wang Shandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

Qing‐He Zhang Shandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

Y. F. Wang Institute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

H. Y. Zhao Institute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

Z. Wang School of Space and Environment, Beihang University, Beijing, China

X. Z. Zhou Institute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

G. Whittall-Scherfee Department of Physics, University of Alberta, Edmonton, Alberta, Canada J. R. Wygant Department of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA Zan‐Yang Xing Shandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China S. T. Yao Shandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

Qiugang Zong Institute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China Y. Zou Department of Astronomy and Center for Space Physics, Boston University, Boston, MA, USA; and Cooperative Programs for the Advancement of Earth System Science, University Corporation for Atmospheric Research, Boulder, CO, USA

PREFACE Magnetospheric physics addresses a vast array of topics, including the interaction of the solar wind with the magnetosphere, how the magnetosphere interacts with the ionosphere, and a host of processes that occur within the dayside magnetosphere. The AGU Chapman Conference on Dayside Magnetosphere Interactions held in July 2017 in Chengdu, China, addressed the processes by which solar wind mass, momentum, and energy enter the magnetosphere. Topics discussed included the foreshock, bow shock, magnetosheath, magnetopause, and cusps; the dayside magnetosphere; and both the dayside polar and equatorial ionosphere. The meeting was particularly timely due to the results expected from NASA’s magnetospheric multiscale (MMS) mission that was launched in March 2015, arrays of new ground‐based instrumentation being installed, as well as the ongoing operations of NASA’s Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Van Allen Probes missions, European Space Agency (ESA)’s Cluster mission, and Japan Aerospace Exploration Agency (JAXA)’s Geotail mission. Parallel processes occur at other planets, and recent results from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission to Mars, as well as ESA’s Mars and Venus Express missions were also discussed. The 2017 Chapman Conference built upon two previous Chapman Conferences on the dayside boundary of the magnetosphere and their related publications: Earth’s Low‐Latitude Boundary Layer (Geophysical Monograph 133, 2003) and Physics of the Magnetopause (Geophysical Monograph 90, 1995). These two Chapman Conferences on dayside dynamics were held more than one or two solar cycles ago. Thus, a Chapman Conference on dayside interactions was very much overdue given the new data sets brought by the constellation missions launched since then. This monograph includes papers presented at the 2017 Chapman Conference as well as invited papers from experts who did not attend. It starts with a brief history of dayside magnetospheric physics and transients (Otto, Chapter 1). Part I considers the physics of dayside magnetospheric response to solar wind discontinuities. This section presents a summary by the Geospace Environment Modeling (GEM) Focus Group of findings on transient phenomena at the magnetopause and bow shock, and their geoeffects (Zhang and Zong, Chapter 2), solar

ionosphere interactions driven by wind–magnetosphere–­ foreshock transients, magnetosheath high‐speed jets, and localized magnetopause reconnection (Nishimura et al., Chapter 3), and solar wind dynamic pressure changes (Shi et al., Chapter 5). Throat aurora that might be driven by magnetosheath high‐speed jets is also discussed (Han, Chapter 4). Part II is devoted to the structure and dynamics of dayside boundaries. This section includes Cluster mission’s recent highlights at dayside boundaries (Escoubet et al., Chapter 6), the structure and dynamics of the magnetopause and the magnetosheath (Nykyri, Chapter 7; Němeček et al., Chapter 8), and a review of different methods to find magnetic nulls and reconstruct magnetic field topology (Fu et  al., Chapter 9). Part III examines the roles of solar wind sources on wave generations and dynamic processes in the inner magnetosphere. This includes a theoretic study on the spatial structure of toroidal standing Alfvén waves in the magnetosphere (Leonovich and Kozlov, Chapter 10), wave–­particle interactions in Earth’s outer radiation belt (Rankin et  al., Chapter 11; Li et  al., Chapter 12), and a review of the current status of radiation belt and ring current modeling (Fok, Chapter 13). Part IV addresses cold plasmas of the ionospheric origin including the geospace plume (Foster, Chapter 14), ionospheric patches (Zhang et al., Chapter 16), and their interaction with ULF waves in the magnetosphere (Zong et al., Chapters 15 and 17). Over 128 scientists from more than 20 countries participated in the conference. We acknowledge help from AGU staff for the success of the conference as well as the completion of this monograph. Also we acknowledge financial support from National Science Foundation and Peking University. Qiugang Zong Peking University, China Philippe Escoubet ESA European Space Research and Technology Centre, The Netherlands David Sibeck, Guan Le NASA Goddard Space Flight Center, USA Hui Zhang University of Alaska Fairbanks, USA

xi

1 A Brief History of Dayside Magnetospheric Physics A. Otto

ABSTRACT Dayside magnetospheric physics has an early history that is closely related to our understanding of the ­magnetosphere as a whole. The early years of magnetospheric physics are somewhat reminiscent of the gold rush era or the exploration of the American west. Moving into the satellite era, our field had, for the first time, the opportunity to examine in‐situ dayside plasma processes to confirm or reject theories, something that neither solar nor astrophysics can do. Since the late 1970s, with better and faster instrumentation, we have been able to develop a detailed understanding of magnetopause and bow shock plasma physics, where transient phenomena play a critical role. This article provides a brief history of these periods of time and how these led into a modern understanding of dayside physics and transient events. 1.1. SETTING THE STAGE: THE PRE‐SATELLITE ERA

studying the paths of electrons in a dipole representing Earth, Birkeland was convinced that the aurora and associated magnetic perturbations were caused by precipitating electrons from the sun (Birkeland, 1908). He also provided a reasonable estimate of the electric currents and the power associated with the auroral activity. Some years later, Sydney Chapman, a brilliant mathematician, published his first model for geomagnetic storms (Chapman, 1918a). Although most of this work involved horizontal currents in the upper atmosphere, the batteries for these currents were “vertical motions.” These he assumed to be provided by a mostly single charged particle precipitation of solar origin although he noted that this idea was not well appreciated in the science community (Chapman, 1918b). It was only a year later that Frederick Lindemann pointed out that the supposed solar corpuscular stream cannot be single‐charged and must contain ions and electrons to be charge neutral (Lindemann, 1919). Based on a charge neutral, ideally conducting solar stream Chapman and Ferraro presented a new theory of magnetic storms where the geomagnetic field is compressed facing the stream and extended in its wake (Chapman & Ferraro, 1931) somewhat similar to our picture of the magnetosphere (Figure 1.1). They called this a magnetic

At the turn of the nineteenth century, it was known that the Earth’s magnetic field could at times undergo strong perturbations that seemed to correlate with auroral activity. It was also hypothesized that these magnetic ­perturbations were caused by processes on the sun. The most prominent example of this relation was the great flare observed by Richard Carrington on 1 September 1859 (Carrington, 1859) and the geomagnetic response. However, such a connection between solar processes and geomagnetism was met by strong criticism at the time. In the years around the turn of the nineteenth century, Kristian Birkeland undertook a number of expeditions to the auroral zone. He was the first to identify what he called the polar elementary storm which is now known as the auroral substorm. Birkeland provided a highly detailed description and analysis of his observations and implied the existence of vertical currents in the upper atmosphere as closure for the horizontal currents he inferred from magnetic observations. Based on the ­observations and his gas discharge “Terella” experiments Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA

Dayside Magnetosphere Interactions, Geophysical Monograph 248, First Edition. Edited by Qiugang Zong, Philippe Escoubet, David Sibeck, Guan Le, and Hui Zhang. © 2020 American Geophysical Union. Published 2020 by John Wiley & Sons, Inc. 1

2  DAYSIDE MAGNETOSPHERE INTERACTIONS

Stream

To the sun Earth +++

++

+

Stream

Figure 1.1 Illustration of the “magnetic hollow” (magnetic cavity) exposed to the ideally conducting solar plasma. Source: From Chapman and Ferraro (1931).

hollow where solar wind particles could access the upper atmosphere only through “two horns” at the location of the cusps of the magnetic field. This model presented for the first time the concept of a magnetopause as the boundary between the solar plasma and the Earth’s closed magnetosphere, and this model dominated the view in the science community for decades. The model agreed qualitatively with most magnetic storm properties particularly for the initial increase of the magnetic field (sudden commencement), however, it was not convincing for the main phase magnetic depression. Chapman and Bartels (1940, p. 810) remarked that a more efficient particle entry and energization were needed than provided in the closed magnetic field model. A different model for magnetic storms and plasma entry in the form of clouds was suggested by Hannes Alfvén (1940) that generated an ongoing controversy for two decades (e.g., Alfvén, 1958). It should be noted that, at the time, the stream of solar plasma was generally assumed to be transient and localized although Biermann (1951) demonstrated through cometary tail observations that the stream of solar material must, in fact, be continuous. However, Chapman shared the view with some in the community of an invisible solar corona that extended beyond Earth’s orbit and expanded at a low velocity of a few 10 km s−1 (Parker, 1997). Eugene Parker realized that not both views on the stream of solar plasma could be true, and, almost coincident with the launch of the first satellites, and Parker (1958) published his famous theory of the solar wind and

coined the name. Somewhat typical of this time is an ­episode around this publication (Parker, 1997). Parker had submitted his paper to ApJ where Chandrasekhar was editor. So, Chandrasekhar came to Parker’s office one day and told him that all (highly qualified) reviewers regarded the paper as wrong and whether he really wanted to publish it. Parker said “yes,” since the reviewers had no explicit objection to the physical arguments, and after a moment, Chandrasekhar responded “Alright, I will publish it.” Still, 2 years later on an international conference, Chamberlain argued that the supposed supersonic solar wind was the result of a wrong integration constant and the limited heat supply allowed only for a slow expansion of about 20 km s−1 at 1 AU (Chamberlain, 1960, 1961). Fortunately, Parker’s work and reputation were saved by the first satellite observations of the solar wind (Bridge et al., 1962; Gringauz et al., 1962; Snyder & Neugebauer, 1963). It should be mentioned, however, that for very rare  conditions the solar wind can indeed be almost absent such that Chamberlain’s view on the topic was not entirely wrong. 1.2. INTO THE SATELLITE ERA Similar to the importance of a new understanding of electrodynamics and electricity for progress in the first half of the nineteenth century, plasma physics and particularly the formulation of the magnetohydrodynamic (MHD) equations by Alfvén, Schlüter, and others enabled the theoretical understanding of the newly discovered magnetosphere. Even though there had been and still is criticism for the MHD approach, the rapid progress in the late 1950s and early 1960s is inconceivable without the framework of a magnetofluid description, the work by Parker on the solar wind being an excellent example. This theoretical framework and the new in‐situ satellite measurements that became available since 1958 advanced our knowledge of the dayside bow shock and magnetopause physics rapidly. Gold (1955) realized that a shock likely propagated in the stream of solar plasma to cause the sudden rapid compression associated with the sudden commencement of magnetic storms. Based on the short duration (few minutes), he also implied that the solar plasma must be magnetized because otherwise the shock width, based on the very large mean free path, would be too large to explain the fast compression. Several years later, the existence of a bow shock in front of the newly discovered magnetosphere had been suggested (Axford, 1962; Gold, 1962; Kellogg, 1962; Zhigulev, 1959). For instance, Ian Axford produced the teardrop shape of the magnetosphere with a bow shock and discussed the stability of the magnetospheric boundary. He also provided the familiar estimate of the magnetopause standoff distance and

A Brief History of Dayside Magnetospheric Physics  3

Earth

Line of force Direction of flow

Figure 1.2 The “open magnetosphere” as suggested with x lines on the day and night side. Source: From Dungey (1961).

argued correctly that the magnetic boundary encountered by Pioneers 1 and 5 (Sonett, 1960; Sonett et  al., 1960) was the bow shock rather than the magnetopause as had been originally assumed. In the following years, properties of the bow shock such as shape, motion, and upstream particle acceleration were examined based on the newly available observations. Burlaga and Ogilvie (1968) carried out a detailed comparison of Explorer 34 observations with theoretical shock predictions and found good agreement. Some diffuse shock encounters were interpreted as the shock moving. Models using hydrodynamic flow around a model obstacle were employed to make predictions on the shape of the bow shock (Spreiter & Jones, 1963; Spreiter et al., 1966), and satellite observations of bow shock locations provided empirical models of the bow shock shape and distance that agreed well with hydrodynamic predictions (Fairfield, 1967; Fairfield & Ness, 1967; Gosling et  al., 1967). Also, at the time, satellites provided the first evidence of upstream moving electrons (Fan et  al., 1966) and ions (Asbridge et  al., 1968) upstream of the bow shock. Satellite observations also made it apparent that these were regions of increased wave turbulence (Fairfield, 1969). The foreshock regions were a surprise because they had not been predicted by theory demonstrating the kinetic character of the shock structure and showing limitations of the MHD framework. Sonnerup (1969) offered a simple explanation of the upstream acceleration in terms of a displacement of particles along the interplanetary electric field during the reflection process, and Greenstadt (1976) discussed the geometry and energy distribution of the reflected particles, both of which seemed to agree reasonably well

with observations (Paschmann et al., 1980). Early indications of transient structure or events at the bow shock were identified in Vela 3 observations by Greenstadt et al. (1968). This understanding of the bow shock was important for the shape of the magnetosphere but did not offer directly an explanation for the entry of solar wind particles into the magnetosphere and the causes of the aurora and magnetic perturbations during geomagnetic activity. Motivated by solar magnetic field eruptions, Sweet (1958) and Parker (1957) derived the first model of magnetic reconnection based on magnetic neutral lines, at the time called magnetic field annihilation or magnetic field merging. Jim Dungey, who was familiar with this work and with auroral observations, became convinced that some auroral boundaries were so thin that they should be topological boundaries. Therefore, he postulated two neutral (x) lines for the magnetosphere (Figure  1.2), which meant that the magnetosphere should, in fact, be open (Dungey, 1961). Dungey was well aware that the associated electric field in his model would cycle closed geomagnetic flux into open flux on the dayside and vice versa in the tail. However, Sweet and Parker reconnection was far too slow in a highly collisionless plasma because it depended on slow magnetic diffusion in stretched thin current sheet. Arthur Kantrowitz suggested to Harry Petschek that waves might be important for this problem, and Petschek realized that the dissipation does not have to occur in a thin sheet along the x line but can happen all along the boundaries of the reconnection outflow region in the form of shocks. Therefore, Petschek’s (1964) reconnection model depended only weakly on the actual dissipation at the x line and reconnection was much faster

4  DAYSIDE MAGNETOSPHERE INTERACTIONS

with a normalized rate of Order 0.1 which agrees well with most current observations of magnetic reconnection. Petschek presented his theory at a solar flare conference and noteworthy is the comment by Sweet: “Dr. Parker and I have been living with this problem for several years… Your solution struck me at once as the solution for which we have been seeking.” Dungey’s and Petschek’s work, however, also opened a new controversy as to whether the magnetosphere was open or closed based on Chapman’s work. Note that Petschek’s basic consideration is still valid for fast reconnection, although the physics of the outflow region is more complicated and depends on geometry and kinetic processes. Related to reconnection, it should be mentioned that Furth et al. (1963) developed the theory of the resistive tearing mode as the linear instability leading to reconnection, and Coppi et  al. (1966) applied the collisionless ­tearing mode for the first time to explain the onset of reconnection in the magnetotail. In order to explain high-latitude phenomena like aurora, magnetic perturbations, and polar cap convection, Axford and Hines (1961) suggested viscous interaction at the magnetospheric flank boundaries (Figure 1.3). They were unspecific other than mentioning some instability or eddy viscosity that would provide the viscous coupling, and, in fact, Dungey had considered viscosity in a different context. Both Axford and Parker had speculated Solar wind

Figure 1.3 Sketch illustrating viscous momentum transfer from the solar wind and the resulting magnetospheric convection in the equatorial plane. Source: From Axford and Hines (1961).

about Kelvin–Helmholtz (KH) waves at the magnetospheric boundary. Although various studies looked at implications of reconnection and viscous interaction, such as global flux transport, and the response of the convection and tail dynamics based on the interplanetary magnetic field (IMF) orientation, an explicit confirmation from satellite observations would not become available for almost another 20 years. In fact, until the late 1970s, in‐situ observations had no clear identification of low‐latitude dayside reconnection (Fairfield, 1979; Haerendel et  al., 1978) or at best indicated that reconnection might occur (Sonnerup & Ledley, 1979). There was still important work on reconnection such as the suggestion of multiple dayside reconnection patches (Nishida & Maezawa, 1971), models of steady reconnection (Vasyliunas, 1975), and indirect evidence such as cusp latitude control of reconnection (Burch, 1973). However, the lack of actual in‐situ signatures also generated increasing skepticism (Heikkila, 1975) and alternative models for the plasma entry termed “impulsive penetration” (Lemaire, 1977). 1.3. INTO A MATURE FIELD: DAYSIDE TRANSIENT PROCESSES In October 1977, International Sun-Earth Explorer (ISEE) 1 and 2 were launched to study the solar wind‐ magnetosphere interaction, and in August 1984, the Active Magnetospheric Particle Tracer Explorers (AMPTE) satellites were launched to study access of solar wind ions to the magnetosphere and to provide two‐ point observations. Compared to prior missions, the spacecraft had superior instrumentation providing better and much higher time resolution data. Immediately after the ISEE launch, two different signatures of reconnection have been identified. Russell and Elphic (1978) saw the frequent occurrence of strong dipolar perturbations of the magnetic field close to the magnetopause and interpreted this as the signature of magnetic flux ropes that swept past the satellite, connecting the magnetosphere with the magnetosheath (Figure  1.4). They proposed that these flux ropes were generated by a patch of magnetopause reconnection and termed these events magnetic flux transfer events (FTEs). Quite different from this signature, Paschmann et  al. (1979) reported strong plasma acceleration tangential to the magnetopause that satisfied the conditions of steady reconnection as formulated by Petschek and other reconnection models for more general current sheet geometries. It was almost as if a levee had broken. After the initial publications there was a flurry of excellent work that identified in‐situ signatures and properties of both, steady reconnection (Eastman & Frank, 1982; Gosling et  al.,

A Brief History of Dayside Magnetospheric Physics  5

Magnetosphere

Magnetosheath

Figure 1.4 Sketch of the proposed magnetic flux rope connecting the magnetosheath and magnetosphere which causes the dipolar magnetic field signature for flux transfer events (FTEs). Source: From Russell and Elphic (1978).

1982; Scholer et  al., 1981; Sonnerup et  al., 1981) and FTEs (Daly et al., 1984; Daly & Keppler, 1982; Paschmann et  al., 1982; Russell & Elphic, 1979) in much detail. In  1984, this evolution culminated in a Geophysical Monograph on Magnetic Reconnection (Hones, 1984) but the flood of exciting studies on the topic has continued unbroken until today. Following the observations of these quite different reconnection signatures, there has been a debate on how FTEs were formed, and whether FTEs and signatures of fast flows represented different modes of magnetopause reconnection. This discussion was also stimulated by another technological advance, that is the development of sophisticated two‐ and three‐dimensional computer simulations. In part based on these, several models for FTE formation were proposed, that is, impulsive single x‐line reconnection (Scholer, 1988), multiple x‐line reconnection (Lee & Fu, 1985), reconnection in localized single patches (Otto, 1990; Russell & Elphic, 1978), or multiple patches (Nishida, 1989; Otto, 1995) distributed over the magnetopause. However, it is likely fair to say that the type of reconnection signature depends on where or how the reconnected flux geometry is encountered. For some signatures attributed to FTEs, it can also not be excluded that they may be caused by transient solar wind pressure variations (Otto, 1995; Sibeck, 1990).

Computer simulations also contributed much to another transient process that is the viscous transport of momentum through KH waves (Miura, 1984) and the transport of mass through reconnection within KH waves for north‐ and southward IMF conditions (Ma et al., 2014; Nykyri & Otto, 2001; Otto, 2006; Otto & Fairfield, 2000). Better instrumentation and higher temporal resolution also enabled the discovery and investigation of transient bow shock events. Steven Schwartz reported the observation of highly unexpected events termed “active current sheets” upstream of the Earth’s bow shock (Schwartz et al., 1985), which are now termed hot flow anomalies (HFAs) (example in Figure  1.5). Soon afterwards, Michelle Thomson reported similar observations and used the term “hot diamagnetic cavities” (Thomsen et al., 1986) for the events characterized by a large increase in temperature, depletion of the magnetic field, and strong deceleration and deflection of the solar wind velocity. Although the foreshock regions were reasonably explored and understood at the time, these events were a mystery  because they were large scale, and common ­ understanding was that no information could travel upstream of a fast shock except for kinetic processes. The initial reports were again followed by quite a number of further observations that showed that the events were often at the transition between the quasiparallel and quasi‐perpendicular shocks (Thomsen et al., 1988), did not necessarily have a strong magnetic field depletion at their core (Paschmann et al., 1988) which was often flanked by fast shocks (Fuselier et al., 1987), and had magnetosheath manifestations (Schwartz et al., 1988) indicating that they may have been the result of a disruption and reformation of the bow shock. In the late 1980s, I frequently visited the Max–Planck Institute in Garching for a collaboration and remember quite well some intriguing discussions with Götz Paschmann on these extraordinary events. We speculated whether reconnection could provide the observed enormous change of momentum because the events seemed to be associated with tangential discontinuities. An answer was provided by hybrid simulations demonstrating that the interaction of a discontinuity with a fast shock can indeed generate structures like the observed HFAs, provided the motional electric field has the correct sign. Later these results were confirmed by global hybrid simulations (Lin, 1997; Omidi & Sibeck, 2007). The decades after the original HFA discovery produced much more information about these fascinating events such as the distribution, dependence on solar wind, and their impact on the magnetosheath, magnetopause, and global magnetosphere. The large interest in this field also revealed that HFAs are not the only transient structures upstream of the bow shock. Observations and simulation identified transients termed short large amplitude magnetic

6  DAYSIDE MAGNETOSPHERE INTERACTIONS

Ne (cm–3)

79 SEPT 1 20 10 0 107 Temperature (K)

ISEE–2

Tl

106 Te

Ve (km/s)

105 600 400 200 0 180

ϕe (deg)

120 60 0 –60 –120 –180

BTot (nT)

30 20 10 0 1747

1749

1751 Time (UT)

1753

Figure 1.5  Example of the (from top to bottom) density, temperature, magnitude and direction of velocity, and magnetic field for typical hot flow anomalies (HFAs). Source: From Thomsen et al. (1988).

structures (SLAMS) (Schwartz et  al., 1992), foreshock density cavities (Sibeck et  al., 2002), foreshock density holes (Parks et al., 2007), foreshock bubbles (Omidi et al., 2010), and distinguished spontaneous HFAs where no tangential discontinuity is present (Zhang et al., 2013). 1.4. FINAL REMARKS In particular, the earlier history might cause the impression that major progress was made by a few brilliant minds. While brilliance certainly did not hurt, there has

always been a background in technological and methodological development that benefitted our field. Birkeland’s and Chapman’s research would not have been possible without the major new understanding of electrodynamics and the exploration of electricity in many laboratories at the time. Similarly, the framework of MHD equations provided a much more appropriate plasma description on large scales than the formulation of an Ohm’s law with perpendicular and parallel conductivities. In fact, with the latter, one would be entirely at a loss to describe shocks or reconnection. Without rocket experiments and in‐situ space observations that confirm theory, our understanding of magnetospheric processes would have been severely limited. Our field is a wonderful example, how the combination of technology, observation, theory, and simulation provides a synergy that is extremely ­powerful for our understanding, and we have always had the bright people to use this for progress. The unique ­possibility to examine theory by in‐situ observation also provides insight that is not possible in solar or astrophysical plasma. During the decades, our field has seen many con­ troversies, some of which lasted for decades. Examples of  these are the open versus closed magnetosphere (Chapman & Ferraro, 1931; Dungey, 1961), stagnant versus supersonic solar wind (Chamberlain, 1960; Parker, 1958), reconnection versus viscous interaction (Axford & Hines, 1961; Petschek, 1964), B, v (magnetic field, velocity) versus E, j (electric field, current density) plasma description paradigm (Parker, 1996), time‐dependent ­ versus stationary reconnection, and several others. There is sometimes the view (e.g., Dessler, 1984) that these are obstacles to progress. Remembering some substorm conferences in the 1990s, where the same old arguments were repeated over and over again, I can understand this view but don’t share it. A good portion of skepticism and critical examination of existing paradigms must be part of this field and is part of the scientific method. The example of Parker’s solar wind model is one of many examples which illustrate how progress has been and will be achieved. On a final note, it seems there are some (mostly older) colleagues in our field who appear to believe that all the great problems have been solved and there is nothing important left to solve. This view ignores entirely the huge progress that has been achieved in the last few decades. For instance, today, it is undisputed that transients at the magnetopause play a major role for the plasma and magnetic flux transport at the magnetopause. We have quantitative models for FTE formation and KH waves that are in good agreement with observations and the required global transport. Observations confirm the occurrence of reconnection in KH waves. Similarly, it is well established that large HFAs play a

A Brief History of Dayside Magnetospheric Physics  7

major role not only for the shock structure but also for the magnetosheath, magnetopause, and the magnetosphere proper. We have quantitative models for some of these transients that seem consistent with observations. However, there are still many fundamentally important and fascinating unresolved problems such as the following: Is there a relation between the different bow shock transients? What exactly is the role of kinetic physics versus fluid mechanisms? What is the three‐dimensional structure of transients? What are the specific conditions for the formation of different types of transients? And, what is the impact and importance for different bow shock transients on the physics of the magnetopause? Similar questions exist for magnetopause transients. This exciting research provides the motivation for the vigorous activity with which the current generation of researchers is working on many fascinating problems. ACKNOWLEDGMENTS This work was partially supported by ISSI Beijing and benefitted from many discussions with colleagues and friends in this field of research with special thanks to Hui Zhang, Peter Delamere, Kathariina Nykyri, and Xuanye Ma. REFERENCES Alfvén, H. (1940). A theory of magnetic storms and of the aurorae, II, the aurorae; III, the magnetic disturbances. Kungliga Svenska Vetenskapsakademiens Handlingar, Series 3, 18(9), 104–116. Alfvén, H. (1958). On the theory of magnetic storms and aurorae. Tellus Series A, 10, 104–116. https://doi. org/10.1111/j.2153‐3490.1958.tb01991.x Asbridge, J. R., Bame, S. J., & Strong, I. B. (1968). Outward flow of protons from the Earth’s bow shock. Journal of Geophysical Research, 73, 5777–5782. https://doi.org/10.1029/ JA073i017p05777 Axford, W. I. (1962). The interaction between the solar wind and the Earth’s magnetosphere. Journal of Geophysical Research, 67, 3791–3796. https://doi.org/10.1029/JZ067i010p03791 Axford, W. I., & Hines, C. O. (1961). A unifying theory of high‐ latitude geophysical phenomena and geomagnetic storms. Canadian Journal of Physics, 39, 1433. https://doi.org/10.1139/ p61–172 Biermann, L. (1951). Kometenschweife und solare korpuskularstrahlung. Zeitschrift für Astrophysik, 29, 274. Birkeland, K. (1908). The Norwegian aura polaris expedition, 1902–1903. On the Cause of Magnetic Storms and the Origin of Terrestrial Magnetism. (Vol. 1). H. Aschehpug and Co., Christiana. Bridge, H. S., Dilworth, C., Lazarus, A. J., Lyon, E. F., Rossi, B., & Scherb, F. (1962). Direct observations of the interplanetary plasma. Journal of the Physical Society of Japan Supplement, 17, 553.

Burch, J. L. (1973). Rate of erosion of dayside magnetic flux based on a quantitative study of the dependence of polar cusp latitude on the interplanetary magnetic field. Radio Science, 8, 955–961. https://doi.org/10.1029/RS008i011p00955 Burlaga, L. F., & Ogilvie, K. W. (1968). Observations of the magnetosheath‐solar wind boundary. Journal of Geophysical Research, 73, 6167. https://doi.org/10.1029/JA073i019p06167 Carrington, R. C. (1859). Description of a singular appearance seen in the Sun on September 1, 1859. Monthly Notices of the Royal Astronomical Society, 20, 13–15. https://doi.org/10.1093/ mnras/20.1.13 Chamberlain, J. W. (1960). Interplanetary gas. II. Expansion of a model solar corona. The Astrophysical Journal, 131, 47. https://doi.org/10.1086/146805 Chamberlain, J. W. (1961). Interplanetary gas. III. A hydrodynamic model of the corona. The Astrophysical Journal, 133, 675. https://doi.org/10.1086/147070 Chapman, S. (1918a). An outline of a theory of magnetic storms. Proceedings of the Royal Society of London Series A, 95, 61–83. https://doi.org/10.1098/rspa.1918.0049 Chapman, S. (1918b). The energy of magnetic storms. Monthly Notices of the Royal Astronomical Society, 79, 70–83. https:// doi.org/10.1093/mnras/79.1.70 Chapman, S., & Bartels, J. (1940). Geomagnetism, vol. II: Analysis of the data, and physical theories. London: Oxford University Press. Chapman, S., & Ferraro, V. C. A. (1931). A new theory of magnetic storms  –  Part 2. Terrestrial Magnetism and Atmospheric Electricity (Journal of Geophysical Research), 36, 171. https://doi.org/10.1029/TE036i003p00171 Coppi, B., Laval, G., & Pellat, R. (1966). Dynamics of the geomagnetic tail. Physical Review Letters, 16, 1207–1210. https://doi.org/10.1103/PhysRevLett.16.1207 Daly, P. W., & Keppler, E. (1982). Observation of a flux transfer event on the earthward side of the magnetopause. Planetary and Space Science, 30, 331–337. https://doi.org/10.1016/0032 ‐0633(82)90038‐1 Daly, P. W., Saunders, M. A., Rijnbeek, R. P., Sckopke, N., & Russell, C. T. (1984). The distribution of reconnection geometry in flux transfer events using energetic ion, plasma and magnetic data. Journal of Geophysical Research, 89, 3843–3854. https://doi.org/10.1029/JA089iA06p03843 Dessler, A. J. (1984). The evolution of arguments regarding the existence of field‐aligned currents. In T. A. Potemra (Ed.), Magnetospheric Currents, Geophysical Monograph Series (Vol. 28, pp. 22). Washington, DC: American Geophysical Union. Dungey, J. W. (1961). Interplanetary magnetic field and the auroral zones. Physical Review Letters, 6, 47–48. https://doi. org/10.1103/PhysRevLett.6.47 Eastman, T. E., & Frank, L. A. (1982). Observations of high‐ speed plasma flow near the earth’s magnetopause – Evidence for reconnection. Journal of Geophysical Research, 87, 2187–2201. https://doi.org/10.1029/JA087iA04p02187 Fairfield, D. H. (1967). The ordered magnetic field of the magnetosheath. Journal of Geophysical Research, 72, 5865–5877. https://doi.org/10.1029/JZ072i023p05865 Fairfield, D. H. (1969). Bow shock associated waves observed in the far upstream interplanetary medium. Journal of Geophysical Research, 74, 3541. https://doi.org/10.1029/JA074i014p03541

8  DAYSIDE MAGNETOSPHERE INTERACTIONS Fairfield, D. H. (1979). Structure of the magnetopause  – Observations and implications for reconnection. Space Science Reviews, 23, 427–448. https://doi.org/10.1007/ BF00172249 Fairfield, D. H., & Ness, N. F. (1967). Magnetic field measurements with the IMP 2 satellite. Journal of Geophysical Research, 72, 2379–2402. https://doi.org/10.1029/JZ072i009p02379 Fan, C. Y., Gloeckler, G., & Simpson, J. A. (1966). Acceleration of electrons near the Earth’s bow shock and beyond. Journal of Geophysical Research, 71, 1837–1856. https://doi. org/10.1029/JZ071i007p01837 Furth, H. P., Killeen, J., & Rosenbluth, M. N. (1963). Finite‐ resistivity instabilities of a sheet pinch. Physics of Fluids, 6, 459–484. https://doi.org/10.1063/1.1706761 Fuselier, S. A., Thomsen, M. F., Gosling, J. T., Bame, S. J., Russell, C. T., & Mellott, M. M. (1987). Fast shocks at the edges of hot diamagnetic cavities upstream from the earth’s bow shock. Journal of Geophysical Research, 92, 3187–3194. https://doi.org/10.1029/JA092iA04p03187 Gold, T. (1955). Discussion on shock waves and rarified gas dynamics. In Gas Dynamics of Cosmic Clouds, IAU Symposium (Vol. 2, pp. 103). Amsterdam: North Holland Pub. Co. Gold, T. (1962). Magnetic storms. Space Science Reviews, 1, 100–114. https://doi.org/10.1007/BF00174637 Gosling, J. T., Asbridge, J. R., Bame, S. J., Feldman, W. C., Paschmann, G., Sckopke, N., & Russell, C. T. (1982). Evidence for quasi‐stationary reconnection at the dayside magnetopause. Journal of Geophysical Research, 87, 2147–2158. https://doi.org/10.1029/JA087iA04p02147 Gosling, J. T., Asbridge, J. R., Bame, S. J., & Strong, I. B. (1967). Vela 2 measurements of the magnetopause and bow shock positions. Journal of Geophysical Research, 72, 101–112. https://doi.org/10.1029/JZ072i001p00101 Greenstadt, E. W. (1976). Energies of backstreaming protons in the foreshock. Geophysical Research Letters, 3, 553–556. https://doi.org/10.1029/GL003i009p00553 Greenstadt, E. W., Green, I. M., Inouye, G. T., Hundhausen, A. J., Bame, S. J., & Strong, I. B. (1968). Correlated magnetic field and plasma observations of the Earth’s bow shock. Journal of Geophysical Research, 73, 51. https://doi. org/10.1029/JA073i001p00051 Gringauz, K. I., Bezrukikh, V. V., Ozerov, V. D., & Rybchinskii, R. E. (1962). The study of interplanetary ionized gas, high‐ energy electrons and corpuscular radiation of the sun, employing three‐electrode charged particle traps on the second Soviet space rocket. Planetary and Space Science, 9, 103– 107. https://doi.org/10.1016/0032‐ 0633(62)90180‐0 Haerendel, G., Paschmann, G., Sckopke, N., Rosenbauer, H., & Hedgecock, P. C. (1978). The frontside boundary layer of the magnetosphere and the problem of reconnection. Journal of Geophysical Research, 83, 3195–3216. https://doi.org/10.1029/ JA083iA07p03195 Heikkila, W. J. (1975). Is there an electrostatic field tangential to the dayside magnetopause and neutral line. Geophysical Research Letters, 2, 154–157. https://doi.org/10.1029/GL002i004p00154 Hones, E. W., Jr. (1984). Magnetic reconnection in space and laboratory plasmas. Geophysical monograph series. (Vol. 30). Washington DC: American Geophysical Union. https://doi. org/10.1029/GM030

Kellogg, P. J. (1962). Flow of plasma around the earth. Journal of Geophysical Research, 67, 3805–3811. https://doi. org/10.1029/JZ067i010p03805 Lee, L. C., & Fu, Z. F. (1985). A theory of magnetic flux transfer at the earth’s magnetopause. Geophysical Research Letters, 12, 105–108. https://doi.org/10.1029/GL012i002p00105 Lemaire, J. (1977). Impulsive penetration of filamentary plasma elements into the magnetospheres of the earth and Jupiter. Planetary and Space Science, 25, 887–890. https://doi.org/10. 1016/0032‐0633(77)90042‐3 Lin, Y. (1997). Generation of anomalous flows near the bow shock by its interaction with interplanetary discontinuities. Journal of Geophysical Research, 102, 24,265–24,282. https:// doi.org/10.1029/97JA01989 Lindemann, F. (1919). Note on the theory of magnetic storms. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 38(228), 669–684. https://doi. org/10.1080/14786441208636000 Ma, X., Otto, A., & Delamere, P. A. (2014). Interaction of magnetic reconnection and Kelvin‐Helmholtz modes for large magnetic shear: 1. Kelvin‐Helmholtz trigger. Journal of Geophysical Research, 119, 781–797. https://doi.org/ 10.1002/2013JA019224 Miura, A. (1984). Anomalous transport by magnetohydrodynamic Kelvin‐Helmholtz instabilities in the solar wind magnetosphere interaction. Journal of Geophysical Research, 89, 801. Nishida, A. (1989). Can random reconnection on the magnetopause produce the low latitude boundary layer? Geophysical Research Letters, 16, 227–230. https://doi.org/10.1029/ GL016i003p00227 Nishida, A., & Maezawa, K. (1971). Two basic modes of interaction between the solar wind and the magnetosphere. Journal of Geophysical Research, 76, 2254. https://doi. org/10.1029/JA076i010p02254 Nykyri, K., & Otto, A. (2001). Plasma transport at the magnetospheric boundary due to reconnection in Kelvin‐ Helmholtz vortices. Geophysical Research Letters, 28, 3565–3568. https:// doi.org/10.1029/2001GL013239 Omidi, N., Eastwood, J. P., & Sibeck, D. G. (2010). Foreshock bubbles and their global magnetospheric impacts. Journal of Geophysical Research, 115, A06204. https://doi.org/10.1029/ 2009JA014828 Omidi, N., & Sibeck, D. G. (2007). Formation of hot flow anomalies and solitary shocks. Journal of Geophysical Research, 112, A01203. https://doi.org/10.1029/2006JA011663 Otto, A. (1990). 3D resistive MHD computations of magnetospheric physics. Computer Physics Communications, 59, 185–195. https://doi.org/10.1016/0010‐4655(90)90168‐Z Otto, A. (1995). Forced three‐dimensional magnetic reconnection due to linkage of magnetic flux tubes. Journal of Geophysical Research, 100, 11. https://doi.org/10.1029/94JA03341 Otto, A. (2006). Mass Transport at the Magnetospheric Flanks Associated with Three‐Dimensional Kelvin‐Helmholtz Modes. AGU Fall Meeting Abstracts, pp. B365+. Otto, A., & Fairfield, D. H. (2000). Kelvin‐Helmholtz instability at the magnetotail boundary: MHD simulation and comparison with Geotail observations. Journal of Geophysical Research, 105, 21. https://doi.org/10.1029/1999JA000312

A Brief History of Dayside Magnetospheric Physics  9 Parker, E. N. (1957). Sweet’s mechanism for merging magnetic fields in conducting fluids. Journal of Geophysical Research, 62, 509–520. https://doi.org/10.1029/JZ062i004p00509 Parker, E. N. (1958). Dynamics of the interplanetary gas and magnetic fields. The Astrophysical Journal, 128, 664. https:// doi.org/10.1086/146579 Parker, E. N. (1996). The alternative paradigm for magnetospheric physics. Journal of Geophysical Research, 101, 10,587– 10,626. https://doi.org/10.1029/95JA02866 Parker, E. N. (1997). In C. S. Gillmor & J. R. Spreiter (Eds.), Adventures with the geomagnetic field, discovery of the magnetosphere, Series: History of Geophysics, ISBN: 0‐87590‐288‐X (Vol. 7, pp. 143–156). Washington, DC: American Geophysical Union. https://doi.org/10.1029/HG007p0143 Parks, G. K., E. Lee, N. Lin, F. Mozer M. Wilber, E. Lucek et  al., Density holes in the upstream solar wind, in Turbulence and nonlinear processes in astrophysical plasmas, American Institute of Physics Conference Series, vol. 932, edited by D. Shaikh and G. P. Zank, pp. 9–15, American Institute of Physics (Online Publication). doi:https://doi. org/10.1063/1.2778939, 2007. Paschmann, G., Haerendel, G., Papamastorakis, I., Sckopke, N., Bame, S. J., Gosling, J. T., & Russell, C. T. (1982). Plasma and magnetic field characteristics of magnetic flux transfer events. Journal of Geophysical Research, 87, 2159–2168. https://doi.org/10.1029/JA087iA04p02159 Paschmann, G., Haerendel, G., Sckopke, N., Möbius, E., Lühr, H., & Carlson, C. W. (1988). Three‐dimensional plasma structures with anomalous flow directions near  the earth’s bow shock. Journal of Geophysical Research, 93, 11,279– 11,294. https://doi.org/10.1029/JA093iA10p11279 Paschmann, G., Sckopke, N., Asbridge, J. R., Bame, S. J., & Gosling, J. T. (1980). Energization of solar wind ions by  reflection from the earth’s bow shock. Journal of Geophysical Research, 85, 4689–4693. https://doi.org/10.1029/ JA085iA09p04689 Paschmann, G., et al. (1979). Plasma acceleration at the earth’s magnetopause  –  Evidence for reconnection. Nature, 282, 243–246. https://doi.org/10.1038/282243a0 Petschek, H. E. (1964). Magnetic Field Annihilation. NASA Special Publication. AAS‐NASA Symposium on the Physics of Solar Flares, 50, p. 425. Russell, C. T., & Elphic, R. C. (1978). Initial ISEE magnetometer results  –  Magnetopause observations. Space Science Reviews, 22, 681–715. https://doi.org/10.1007/BF00212619 Russell, C. T., & Elphic, R. C. (1979). ISEE observations of flux transfer events at the dayside magnetopause. Geophysical Research Letters, 6, 33–36. https://doi.org/10.1029/GL006i001 p00033 Scholer, M. (1988). Magnetic flux transfer at the magnetopause based on single X line bursty reconnection. Geophysical Research Letters, 15, 291–294. https://doi.org/10.1029/ GL015i004p00291 Scholer, M., Ipavich, F. M., Gloeckler, G., HOvestadt, D., & Klecker, B. (1981). Leakage of magnetospheric ions into the magnetosheath along reconnected field lines at the dayside magnetopause. Journal of Geophysical Research, 86, 1299–1304. https://doi.org/10.1029/JA086i A03p01299

Schwartz, S. J., Burgess, D., Wilkinson, W. P., Kessel, R. L., Dunlop, M., & Luehr, H. (1992). Observations of short large‐ amplitude magnetic structures at a quasi‐parallel shock. Journal of Geophysical Research, 97, 4209–4227. https://doi. org/10.1029/91JA02581 Schwartz, S. J., Kessel, R. L., Brown, C. C., Woolliscroft, L. J. C., Dunlop, M. W., Farrugia, C. J., & Hall, D. S. (1988). Active current sheets near the earth’s bow shock. Journal of Geophysical Research, 93, 11,295–11,310. https://doi. org/10.1029/JA093iA10p11295 Schwartz, S. J., Chaloner, C. P., Christiansen, P. J., Coates, A. J., Hall, D. S., Johnstone, A. D., et al. (1985). An active current sheet in the solar wind. Nature, 318, 269–271. https://doi. org/10.1038/318269a0 Sibeck, D. G. (1990). A model for the transient magnetospheric response to sudden solar wind dynamic pressure variations. Journal of Geophysical Research, 95, 3755–3771. https://doi. org/10.1029/JA095iA04p03755 Sibeck, D. G., Phan, T.‐D., Lin, R., Lepping, R. P., & Szabo, A. (2002). Wind observations of foreshock cavities: A case study. Journal of Geophysical Research, 107, 1271. https://doi. org/10.1029/2001JA007539 Snyder, C. W., & Neugebauer, M. (1963). Direct observations of the solar wind by the Mariner II spacecraft. International Cosmic Ray Conference, 1, 210. Sonett, C. P. (1960). Coupling of the solar wind and the exosphere. Physical Review Letters, 5, 46–48. https://doi. org/10.1103/PhysRevLett.5.46 Sonett, C. P., Judge, D. L., Sims, A. R., & Kelso, J. M. (1960). A  radial rocket survey of the distant geomagnetic field. Journal of Geophysical Research, 65, 55. https://doi.org/ 10.1029/JZ065i001p00055 Sonnerup, B. U. Ö. (1969). Acceleration of particles reflected at a shock front. Journal of Geophysical Research, 74, 1301. https://doi.org/10.1029/JA074i005p01301 Sonnerup, B. U. O., & Ledley, B. G. (1979). Ogo 5 magnetopause structure and classical reconnection. Journal of Geophysical Research, 84, 399–405. https://doi.org/10.1029/JA084iA02p00399 Sonnerup, B. U. O., Paschmann, G., Papamastorakis, I., Sckopke, N., Haerendel, G., Bame, S. J., et al. (1981). Evidence for magnetic field reconnection at the earth’s magnetopause. Journal of Geophysical Research, 86, 10,049–10,067. https:// doi.org/10.1029/JA086iA12p10049 Spreiter, J. R., & Jones, P. W. (1963). On the effect of a weak interplanetary magnetic field on the interaction between the  solar wind and the geomagnetic field. Journal of Geophysical Research, 68, 3555–3564. https://doi.org/10.1029/JZ068i012p03555 Spreiter, J. R., Summers, A. L., & Alksne, A. Y. (1966). Hydromagnetic flow around the magnetosphere. Planetary and Space Science, 14, 223. https://doi.org/10.1016/0032‐0633 (66)90124‐3 Sweet, P. A. (1958). The neutral point theory of solar flares. In B. Lehnert (Ed.), Electromagnetic Phenomena in Cosmical Physics (pp. 123–134). London: Cambridge University Press. Thomsen, M. F., Gosling, J. T., Bame, S. J., Quest, K. B., Russell, C. T., & Fuselier, S. A. (1988). On the origin of hot diamagnetic cavities near the earth’s bow shock. Journal of Geophysical Research, 93, 11,311–11,325. https://doi.org/10.1029/JA093iA10 p11311

10  DAYSIDE MAGNETOSPHERE INTERACTIONS Thomsen, M. F., Gosling, J. T., Fuselier, S. A., Bame, S. J., & Russell, C. T. (1986). Hot, diamagnetic cavities upstream from the earth’s bow shock. Journal of Geophysical Research, 91, 2961–2973. https://doi.org/10.1029/JA091iA03p02961 Vasyliunas, V. M. (1975). Theoretical models of magnetic field line merging. I. Reviews of Geophysics and Space Physics, 13, 303–336. https://doi.org/10.1029/RG013i001p00303

Zhang, H., Sibeck, D. G., Zong, Q.‐G., Omidi, N., Turner, D., & Clausen, L. B. N. (2013). Spontaneous hot flow anomalies at quasi‐parallel shocks: 1. Observations. Journal of Geophysical Research, 118, 3357–3363. https://doi.org/10.1002/jgra.50376 Zhigulev, V. N. (1959). On the phenomenon of magnetic “detachment” of the flow of a conducting medium. Soviet Physics Doklady, 4(514).

Part I Physics of Dayside Magnetospheric Response to Solar Wind Discontinuities

2 Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures: Summary of the Geospace Environment Modeling (GEM) Focus Group Findings Between 2012 and 2016 Hui Zhang1 and Qiugang Zong2

ABSTRACT Dayside transients are frequently observed upstream from the bow shock (such as hot flow anomalies, foreshock cavities, and foreshock bubbles) and at the magnetopause (such as flux transfer events and surface waves). They play a significant role in the mass, energy, and momentum transport from the solar wind into the magnetosphere and impact the whole magnetosphere–ionosphere system. The Geospace Environment Modeling (GEM) Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures focus group has employed both observations and simulations to investigate transient phenomena at the magnetopause and bow shock and their impacts on the magnetosphere–ionosphere. There are over 80 peer‐reviewed papers (only those presented in this focus group at GEM were included) and 8 PhD and MS theses of the students who were actively working on the focus group topics. 2.1. INTRODUCTION

nature such as solar flares and magnetospheric substorms. It is the primary mechanism for transferring momentum and energy from the solar wind to the magnetosphere. FTEs and their ionospheric signatures have often been taken as evidence for intermittent reconnection. Surface waves on the magnetopause can be excited by solar wind pressure variations, transient phenomena ­generated near the bow shock, or the Kelvin–Helmholtz (KH) instability on the magnetopause, and have been observed at the Earth (e.g., Hasegawa et al., 2004), Mercury (Sundberg et al., 2012), and Saturn (Masters et al., 2012). The dynamic pressure inside HFAs is lower than the ambient solar wind due to the density depletion and flow deflection. The passage of HFAs will therefore result in local negative pressure impulses. The depletion of the total pressure in HFAs leads to a local sunward expan­ sion of the magnetopause that has been observed (e.g., Sibeck et al., 1999). HFAs can also transmit compressional waves into the magnetosphere that can excite resonant ultra‐low‐frequency (ULF) waves (Eastwood et al., 2011)

Dayside transients are frequently observed upstream from the bow shock (such as hot flow anomalies [HFAs], foreshock cavities, and foreshock bubbles [FBs]) and at the magnetopause (such as flux transfer events [FTEs] and surface waves). They play a significant role in the mass, energy, and momentum transport from the solar wind into the magnetosphere and impact the whole mag­ netosphere–ionosphere system. A zoo of transient phe­ nomena at the bow shock has been identified and an overview is provided in Section 2.2. Magnetic reconnection efficiently converts magnetic to kinetic energy and affects the changes in magnetic topology that underlie many important phenomena in 1   Geophysical Institute and Physics Department, University of Alaska Fairbanks, Fairbanks, AK, USA 2   Institute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

Dayside Magnetosphere Interactions, Geophysical Monograph 248, First Edition. Edited by Qiugang Zong, Philippe Escoubet, David Sibeck, Guan Le, and Hui Zhang. © 2020 American Geophysical Union. Published 2020 by John Wiley & Sons, Inc. 13

14  DAYSIDE MAGNETOSPHERE INTERACTIONS

and cause particles to scatter into the loss cone and ­precipitate into the ionosphere, generate field‐aligned currents (FACs) in the magnetosphere that drive magnetic impulse events in the high‐latitude ionosphere (Eastwood et  al., 2008), and trigger transient auroral brightenings (Fillingim et al., 2011; Sibeck et al., 1999). Similarly, FBs and other transients may have a significant impact on the magnetosphere due to the dynamic pressure variations in these structures. Some specific outstanding science questions before the focus group are listed below. 1. What are the physical differences and relationships between different transient phenomena at the bow shock? 2. How do transient phenomena at the bow shock evolve with time? 3. How do the magnetosphere and ionosphere respond to transient phenomena generated at the bow shock? 4. What are the roles played by heavy ions (oxygen) and cold ions (plasmaspheric population) in magnetic recon­ nection and KH instability (KHI) at the magnetopause? 5. How does asymmetric reconnection differ from symmetric reconnection? 6. What are the formation conditions for transient phenomena at the bow shock and magnetopause? All of above questions have been addressed during this focus group. In the following, we start with an overview of transient phenomena at the bow shock (Section 2.2) con­ sidering the similarities among these transient ­phenomena, then followed by the progress mostly done during the focus group (Sections 2.3–2.5), and a discussion (Section  2.6). Please note that Geospace Environment Modeling (GEM) had many parallel sessions during this time and some of the relevant topics to the title of this manuscript were dis­ cussed during other GEM focus groups: magnetosheath, system science, and dayside kinetic processes, from which the publications came out at the same time as some of the later publications from the transient focus group. 2.2. OVERVIEW OF TRANSIENT PHENOMENA AT THE BOW SHOCK Several transient kinetic phenomena have been reported upstream from the Earth’s bow shock including HFAs, spontaneous hot flow anomalies (SHFAs), FBs, foreshock cavities, foreshock cavitons, foreshock compressional boundary (FCB), density holes, and short large‐amplitude magnetic structures (SLAMS). The kinetic processes asso­ ciated with these phenomena modify the solar wind just prior to its interaction with the Earth’s magnetosphere. Our focus group generated a table (Table 2.1) and a short description of many transient foreshock phe­ nomena, together with a list of HFA and FB events with summary plots and posted them on the GEM wiki page at https://gem.epss.ucla.edu/mediawiki/index.php/ FG:_Transient_Phenomena_at_the_Magnetopause_ and_Bow_Shock_and_Their_Ground_Signatures

2.2.1. Hot Flow Anomalies HFAs are marked by greatly heated plasmas and substan­ tial flow deflections, with durations of a few minutes (e.g., Facskó et  al., 2008; Lucek et  al., 2004; Schwartz, 1995; Schwartz et al., 1985; Thomsen et al., 1986; S. Wang et al., 2013c; Zhang et al., 2010). Figure 2.1 shows an example of an HFA observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) C upstream from the bow shock on 19 August 2008. HFAs are thought to be produced by the interaction of certain types of upstream discontinuities with the bow shock (Thomas et al., 1991). The ions reflected from the bow shock are ener­ gized and trapped in the vicinity of the discontinuities when the motional electric field points toward the discontinuity. 2.2.2. Spontaneous Hot Flow Anomalies SHFAs are similar to HFAs, though SHFAs occur independent of any discontinuity in the pristine, upstream magnetic fields. They form due to processes internal to the quasi‐parallel foreshock and display all of the same core and compression region characteristics as HFAs (Omidi et  al., 2013; Zhang et  al., 2013). Figure  2.2 shows an example of an SHFA observed by THEMIS A upstream from the bow shock at 0431 UT. 2.2.3. Foreshock Bubbles FBs can form when energetic foreshock ions upstream of quasi‐parallel planetary bow shocks interact with rotational discontinuities in the pristine, upstream inter­ planetary magnetic field (IMF). Unlike HFAs, FBs form independent of any connection between the discontinuity and the bow shock. FBs form just upstream of the responsible discontinuity and move antisunward with it. As an FB impinges upon a planetary bow shock and sweeps up more and more energetic ions in the foreshock, it grows in time, potentially reaching sizes on the same order as Earth’s entire dayside magnetosphere. Due to the building concentration of suprathermal ions in their cores, FBs exhibit very high core temperatures, resulting in the expulsion of thermal plasma, which drops the core density and field strength (see Figure 2.3). Core fields are highly distorted, and within the core, there are very strong and sometimes even sunward bulk flow deflections (­similar to HFAs). Compression regions form around the edges of the core, and provided sufficient conditions, the  upstream compression regions can evolve into fast ­magnetosonic shocks (i.e., if the difference between the upstream bulk velocity and the rate at which the FB grows back into the upstream plasma approaches and exceeds the fast magnetosonic speed). FBs should be par­ ticularly efficient particle accelerators since they involve two converging shocks (i.e., that of the FB at the upstream

Table 2.1  Comparison of transient phenomena at the bow shock

Depletion in the density and magnetic field strength Compressions at edges Presence of energetic (>30 keV) particles Significant flow deflection Significant plasma heating Associated with an IMF discontinuity Duration Scale size Generation mechanisms

0004494259.INDD 15

Foreshock compressional boundary

HFAs

SHFAs

Foreshock bubbles

Foreshock cavities

Foreshock cavitons

Density holes

SLAMS

Yes

Yes

Yes

Yes

Yes

Yes on the turbulent side

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Only on the upstream edge Yes

Yes

Yes

No

Yes

No

Yes

Yes

Yes

No

No

No

Yes

No

Yes

Yes

Yes

Modest

No

No

Yes

Yes

Yes

No

Yes

Sometimes

No

No

Yes

No

Minutes A few RE Interaction of IMF discontinuities with the bow shock

Minutes A few RE Interaction of foreshock cavitons with the bow shock

Minutes Up to 10 RE Kinetic interactions between suprathermal, backstreaming ions and incident solar wind plasma with embedded IMF discontinuities that move through and alter the ion foreshock

Minutes A few RE Antisunward‐moving slabs of magnetic field lines connected to the bow shock that are sandwiched between broader regions of magnetic field lines that remain unconnected to the bow shock

Minutes ~RE Nonlinear evolution of ultra‐low‐ frequency (ULF) waves

Minutes ~RE Backstreaming ions result in increased pressure within the foreshock region leading to its expansion against the pristine solar wind and the generation of foreshock compressional boundary (FCB).

Seconds Ion gyroradius Possibly due to backstreaming particles interacting with the original solar wind

~10 seconds Ion gyroradius Nonlinear wave steepening

04-12-2019 11:47:30

16  DAYSIDE MAGNETOSPHERE INTERACTIONS

Btotal (nT)

(b)

n (cm–3)

(c)

VGSM (km/s)

(d)

Ions (eV)

(e)

Electrons (eV)

(f)

10 5 0 –5 –10 –15 15

Bz By Bx

10 5 0 10 8 6 4 2 0 200

Vy Vz

0 –200 –400

Vx

–600 10 000

107

1000

106 105

100

104

10 10 000

108 107

1000

106

100

105

10 thc_X-GSM thc_Y-GSM thc_Z-GSM hhmm 2008 Aug 19

eV/(cm2-s-sr-eV)

BGSM (nT)

(a)

104 18.3 –3.6 –4.0 0750

18.4 –3.6 –4.0 0800

Figure 2.1  An overview plot of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) C observations of a mature hot flow anomaly (HFA) upstream from the bow shock. From top to bottom: (a) components of the magnetic field in geocentric solar magnetospheric (GSM) coordinate system, (b) magnetic field magnitude, (c) plasma ion density, (d) components of plasma flow, (e) plasma ion spectrum, and (f) plasma ­electron spectrum. Source: From Zhang et al. (2010).

edge and the bow shock itself) and increased wave activity in their cores (Omidi et al., 2010; Turner et al., 2013). 2.2.4. Foreshock Cavities Foreshock cavities, although more common, are less prominent than HFAs in the sense that the solar wind distributions within the cavities show little evidence of heating or significant flow deflection although a second, suprathermal population is present. Foreshock cavities can be identified based on enhanced magnetic field

strengths and densities bounding regions of depressed magnetic field strength and density containing a supra­ thermal ion component (e.g., Billingham et  al., 2008; Schwartz et  al., 2006; Sibeck et  al., 2002, 2004). An example of a foreshock cavity is shown in Figure  2.4. Foreshock cavities form due to antisunward‐moving slabs of magnetic field lines connected to the bow shock, that are sandwiched between broader regions of magnetic field lines that remain unconnected to the bow shock. The slabs are filled with reflected and energized particles from the bow shock. The corresponding

(a)

THEMIS A

BGSM (nT)

10 5

By Bz

0

Bx

–5 –10

Btotal (nT)

(b)

8 6 4 2 0 6 4 2 0 200

VGSM (km/s)

(d)

Ions (eV)

(e)

Electrons (eV)

(f)

(g)

Vz Vy

0 –200 –400

Vx

–600 105 104

107

103

106

102

105

101

104

100 105 104

108 107

103

106

102

eV/(cm2-s-sr-eV)

n (cm–3)

(c)

105

101 100

104

Ti (eV)

400 300 200 100 0 200 PTotal (pPa)

(h)

150 100 50

0 tha_X-GSM tha_Y-GSM tha_Z-GSM hhmm 2007 Aug 12

12.8 –5.5 –2.4 0430

12.8 –5.5 –2.4 0432

Figure 2.2  An overview plot of THEMIS A observations of an SHFA upstream from the bow shock. From top to bottom: (a) components of the magnetic field in GSM coordinate system, (b) magnetic field magnitude, (c) plasma ion density, (d) components of plasma flow in GSM coordinate system, (e) plasma ion spectrum, (f) plasma ­electron spectrum with 3‐second time resolution (g) ion temperature, (h) total (plasma plus magnetic) pressure. Source: From Zhang et al. (2013).

18  DAYSIDE MAGNETOSPHERE INTERACTIONS

(f)

(g)

Ion temp. (eV) Ion V-GSM (km/s)

(e)

i+ eFlux (eV)

(d)

e– eFlux (eV)

(c)

Density (cm–3)

BTot (nT)

(b)

10 0 –10 –20 –30 30 25 20 15 10 5 0 10.0

BZ

B-field discontinuity at downstream edge

Low field strength in core region

No downstream compression

BY

Core waves

BX

Upstream shock e–

Tenuous core

1.0

i+

0.1 2 500 2 000 1 500 1 000 500 0

Hot core

VZ VY VX VTot

500 0

Plasma deflections

–500 105 104 103 102

Dispersed ions

10 000

107 106 105 104

Energetic electrons

1 000

100 Seconds 00 2008 Jul 14 2157

107 106 105 104 103 102

30

00 2158

(ev/cm2-s-sr-eV)

B-GSM (nT)

TH-C

(a)

30

Figure 2.3  An overview plot of THEMIS C observations of a foreshock bubble upstream from the bow shock. From top to bottom: (a) components of the magnetic field in GSM coordinate system, (b) magnetic field magnitude, (c) plasma ion and electron density, (d) ion temperature, (e) components and magnitude of plasma flow in GSM coordinate system, (f) plasma ion spectrum, (g) plasma electron spectrum with 3‐second time resolution. Source: From Turner et al. (2013).

pressure enhancement causes the cavities to expand, depressing the internal densities and magnetic fields but enhancing these parameters at the expanding edge (Schwartz et al., 2006). 2.2.5. Foreshock Cavitons Foreshock cavitons are about an RE in size. Their cores exhibit drops in density and magnetic field, while their outer edges show plasma and magnetic field enhance­ ments (Figure 2.5). They form as a result of the nonlinear evolution of two types of waves: the parallel propagating sinusoidal waves and the highly oblique, linearly polar­ ized, fast magnetosonic waves (Lin, 2003; Lin & Wang, 2005; Omidi, 2007; Blanco‐Cano et  al., 2009, 2011). Foreshock cavitons are embedded in regions with ULF activity, which is in contrast with the isolated character of foreshock cavities.

2.2.6. Foreshock Compressional Boundary The FCB (Sibeck et al., 2008; Omidi et al., 2009) forms in the ion foreshock and is associated with enhanced densities and magnetic field strengths. Backstreaming ions result in increased pressure within the foreshock region leading to its expansion against the pristine solar wind and the genera­ tion of FCB (Figure 2.6). Although the FCB itself is asso­ ciated with increases in the magnetic field strength and density, these quantities are reduced on the turbulent side of the FCB as compared to the pristine solar wind. FCBs may be a steady‐state feature, but observed transiently because of slight changes in the IMF orientation. 2.2.7. Density Holes Similar to HFAs, density holes display significant bulk flow deflections and are filled with heated plasma.

TRANSIENT PHENOMENA AT THE MAGNETOPAUSE AND BOW SHOCK AND THEIR GROUND SIGNATURES  19

B(nT)

5 0 –∣B∣ –Bx –By –Bz –5 nHIA (cm−3)

4 3 2 1 Jp (cm2ssr−1)−1

6 × 104 4 × 104 2 × 104

Cluster rumba (SC 1) 27 January 2003

–8 10 5 0 Bz (nT) –5 –10 12 8 B (nT) 4 0 20 10 0 –400 Vx –440 (km/s) –480

Density (cm–3)

Vy (km/s)

40 0 –40

–Vz 19:56:40

20:00:00

20:03:20

20:06:40

UT ∣vHIA∣(km/s)

750 700 650

T(MK)

600 1.5 1.0 0.5 14h 10m

Bx (nT)

0 –4

12m

14m

16m

18m

20m

2005-12-28 (UT)

Figure 2.4 A foreshock cavity observed upstream from the bow shock. Source: From Billingham et al. (2008).

Density holes are accompanied by similarly shaped magnetic holes (Figure  2.7). They have enhanced den­ sities and compressed magnetic field at one or both edges. Density holes have durations of ~18 seconds, which are much shorter than those of HFAs, and scale sizes of an ion gyroradius, which are much smaller than those of HFAs (Parks et  al., 2006). Density holes are possibly formed by backstreaming particles interacting with the original solar wind (Parks et al., 2006). 2.2.8. Short Large‐Amplitude Magnetic Structures Short large‐amplitude magnetic structures (SLAMS) are large amplitude (the magnetic field magnitude is enhanced over the undisturbed field by at least a factor of 2.5) magnetic structures with durations of the order of 10 seconds (Schwartz, 1991; Lucek et al., 2002). They grow

Figure 2.5 Cluster C1 observations of a foreshock caviton upstream from the bow shock. Source: From Blanco‐Cano et al. (2009).

rapidly (~seconds) out of ULF waves in the foreshock region. Examples of SLAMS are shown in Figure 2.8. Below is a summary of some presentations presented in the oral sessions of the Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures focus group during GEM summer work­ shops. In a general sense, there has been continued progress in (i) transient processes in the foreshock, bow shock, and magnetosheath, (ii) dayside magnetopause processes and transport, and (iii) geoeffects of dayside transients. 2.3. TRANSIENT PROCESSES IN THE FORESHOCK, BOW SHOCK, AND MAGNETOSHEATH 2.3.1. Hot Flow Anomalies and Spontaneous Hot Flow Anomalies 2.3.1.1. HFAs are Universal Phenomena and They are Frequently Observed HFAs are universal phenomena. HFAs have been observed at the bow shock of the Earth (e.g., Schwartz et al., 1985), Mars (Øieroset et al., 2001; G. Collinson et al., 2015), Saturn (Masters et al., 2009), Jupiter (Valek et al., 2017), Venus (G. A. Collinson et al., 2012), and Mercury (Uritsky et  al., 2014). HFAs have been ­ frequently observed upstream from the Earth’s bow shock. S. Wang et  al. (2013c) identified 513 HFAs in Cluster satellite data  from 2003 to 2009 upstream from the bow shock. There are 213 (44%) HFAs associated with clear IMF

20  DAYSIDE MAGNETOSPHERE INTERACTIONS Total B

RUMBA (SC1) 03/Mar/2004

1200

M_A = 10

1000 100 10 (b) 10.0

6

1.0 0.1 200 (c) 0 –200 –400 –600 (d) 100.0 10.0 1.0 0.1 (e) 15

4

400

FCB

2

0 200

600

1000

Vx Vy Vz T TII T Bx

10

1400

By

5 0

X (c/ωp) Figure 2.6 Hybrid simulations showed that backstreaming ions result in increased pressure within the foreshock region leading to its expansion against the pristine solar wind and the generation of foreshock compressional boundaries. Source: From Omidi et al. (2009).

N

т

Y (c/ωp)

800

0

LogJE 7.3 lons 6.1 ϕAll 4.9 θAll 3.6

(a) 10 000

Bz B

–5 03:19:00

03:20:00

03:21:00

Figure 2.7 An overview plot of Cluster observations of a density hole upstream from the bow shock. From top to bottom: (a) ion energy spectrum (in eV), (b) ion density (in cm−3), (c) components of plasma flow (km/s) in geocentric solar ecliptic (GSE) coordinates, (d) ion temperature (in MK), (e) the magnitude and components of the magnetic field (in nT) in GSE coordinate system. Source: From Parks et al. (2006).

AMPTE UKS Day 304 1984 (30 October)

50 40 B (nT)

30 20 10 0 180

ϕB (deg)

90 0 –90

θB (deg)

–180 120 90 60 30 0 –30 –60 –90 10:40

10:44

10:48

10:52

10:56

11:00

UT

Figure 2.8  Short large‐amplitude magnetic structures (SLAMS) observed upstream from the Earth’s bow shock. Source: From Schwartz et al. (1992).

TRANSIENT PHENOMENA AT THE MAGNETOPAUSE AND BOW SHOCK AND THEIR GROUND SIGNATURES  21

­ iscontinuities (classical HFAs) and 300 (56%) HFAs not d associated with clear discontinuities (SHFAs). Chu et  al. (2017) identified 136 HFAs from THEMIS C data from June 2007 to December 2009. About half (49%) of the events are SHFAs. 2.3.1.2. Discovery of SHFAs and Their Parametric Dependencies Both in‐situ observations (Figure 2.2) by the THEMIS spacecraft and global hybrid simulations (Figure  2.9) demonstrate that HFAs can be generated spontaneously (in the absence of any current sheets) at quasi‐parallel bow shocks and are therefore a new category of HFAs which are called “spontaneous HFAs” (Omidi et al., 2013; Zhang et al., 2013). Omidi et al. (2013) showed that the simulated SHFAs form as a result of the interaction of foreshock cavitons with the bow shock. Simulations show the formation of large numbers of SHFAs and demon­ strate that they are an inherent part of quasi‐parallel shock dissipation processes. Using global hybrid simula­ tions, Omidi et  al. (2014a) investigated the parametric dependencies of SHFAs. They demonstrated that SHFAs are formed sporadically at MA = 3 and SHFAs are formed frequently at higher Mach numbers. They also found that the size of SHFAs and the level of ion heating increase with MA. They also showed that SHFAs can form at cone angles as large as 90° as long as MA > 3. Chu et al. (2017) showed that SHFAs were observed by the THEMIS spacecraft at almost all cone angles and the occurrence rate was low at cone angles close to 90°. Chu et al. (2017) found that no SHFAs were observed when the Mach number was less than 5, suggesting that there is a minimum threshold Mach number for SHFAs to form. Number density 6.0

900

Bow shock

Z (c/ωp)

4.8 800

3.6

SHFA X

2.4 700

1.2

700

800

900

.010

X (c/ωp)

Figure 2.9 Simulation results from a 2.5‐D electromagnetic hybrid code demonstrate the formation of spontaneous hot flow anomalies (SHFAs) upstream of quasi‐parallel bow shocks during steady solar wind conditions and in the absence of discontinuities. Source: From Omidi et al. (2013).

2.3.1.3 HFA Formation Conditions L. L. Zhao et al. (2017a) performed a statistical study to determine what kinds of discontinuities are more effi­ cient to generate HFAs. Their results show that magnetic field on at least one side of the interplanetary (IP) discon­ tinuities has to be connected to the bow shock in order to form HFAs. Discontinuities with large magnetic shear angles are more efficient to form HFAs. Current sheets with thickness from 1000 km to about 3162 km are more efficient to form HFAs. HFAs are more likely to form when the reflected flow from the bow shock is along the discontinuity. Chu et al. (2017) presented a statistical study of both HFAs and SHFAs using THEMIS data. They showed that HFAs are more prevalent when there is an approxi­ mately radial IP magnetic field. They also found that no HFAs or SHFAs were observed when the Mach number was less than 5, suggesting that there is a minimum threshold Mach number for HFAs and SHFAs to form. High solar wind speeds favor the formation of HFAs (Chu et  al., 2017; Facskó et al., 2008, 2009; S. Wang et al., 2012). Observational test of nine HFAs indicates that HFAs are associated with current sheets exhibiting the predicted inward electric field orientation on at least one side (Thomsen et al., 1993). However, S. Wang et al. (2013c) found that electric field on neither leading nor trailing edge points toward the discontinuity for 19 out of 144 (13%) HFAs. This result implies that the convective electric field pointing toward the discontinuity may help an HFA growing but its presence is not a necessary condition to generate an HFA. Simulations and observations show completely differ­ ent results on whether HFAs occur at quasi‐perpendic­ ular or quasi‐parallel shocks (Zhang et  al. 2010 and references therein). S. Wang et  al. (2013b) showed that HFAs can form at both quasi‐parallel and quasi‐perpen­ dicular shocks. 2.3.1.4. Hydramagnetic HFA Formation Mechanism Otto (presentation at GEM 2014 workshop) investi­ gated bow shock interaction with transient solar wind structures using a magnetohydrodynamic (MHD) simu­ lation. He found that the interaction of the bow shock with a density depletion structure in the solar wind results in a sunward flow, which may provide an alternate mech­ anism for the formation of HFAs. 2.3.1.5. Evolution of HFAs L. L. Zhao et  al. (2015) identified 199 classical HFAs (those associated with discontinuities) in Cluster data from 2001 to 2010. These HFAs were classified as young and mature according to the ion distributions. HFAs were classified into four categories (“‐+,” “+‐,” “M,” and “W,” where the symbols describe the profile of the dynamic

22  DAYSIDE MAGNETOSPHERE INTERACTIONS

pressure variations) according to the dynamic pressure profile. Then they determine the percentage of mature HFAs in each category. Most “W” and “M” type HFAs are mature HFAs and most “‐+” and “+‐” type HFAs are young HFAs, indicating that “M” and “W” type HFAs may be the later evolution stages of “‐+” and “+‐” type HFAs. They suggested that the four categories of HFAs may also be due to the fact that the spacecraft crossed an HFA structure along different paths. Superposed epoch analysis result shows that variations of plasma parameters and magnetic field of mature HFAs are more dramatic than those of young HFAs, except for temperature. S. Wang et al. (2013c) investigated the ion and electron spectra inside HFAs and found that both ion and electron spectra can be used to classify young and mature HFAs. Young HFAs have two distinct ion populations whereas mature HFAs have a single hot ion population. One‐ dimensional cut plots of the electron distributions show differences between young and mature HFAs. In addition, classifications according to ion and electron spectra are not absolutely consistent, which might be due to different heating mechanisms and efficiency for ions and electrons. 2.3.1.6. Ion Heating Inside HFAs S. Wang et al. (2013c) and Chu et al. (2017) determined the correlation between the thermal energy increase and the kinetic energy decrease from the background solar wind to the HFA center. A significant part of the thermal energy inside HFAs is converted from the kinetic energy of the solar wind, although additional heating process(es) is required to heat the plasma inside an HFA. 2.3.1.7. Flow Deflections Inside HFAs S. Wang et al. (2013a) investigated 87 HFAs with large flow deflections (with the magnitude of Vy or Vz in geo­ centric solar ecliptic [GSE] coordinates greater than 200 km/s) from Cluster‐C1 observations from 2003 to 2009 and found that the large flow deflections in HFAs are location dependent and that the ions are near‐specu­ larly reflected at the bow shock. 2.3.1.8. Propagation Characteristics of HFAs Xiao et al. (2015) investigated the propagation velocities of HFA boundaries upstream from the Earth’s bow shock. According to the difference in the propagation velocity of the leading and trailing edges, they categorized HFAs into three groups, namely, contracting, expanding, and stable events. They further investigated what causes them to contract, expand, or stay stable by calculating the thermal pressure of the HFAs. The contraction speed is a few tens of kilometers per second for the contracting HFAs, and the expansion speed is tens to more than a hundred kilometers per second for expanding events. For the stable events, the leading and trailing edges propagate at almost

the same speed within the error range. Xiao et al. (2015) showed that the magnetic pressure and thermal pressure play an important role in the evolution of HFAs. 2.3.1.9. Where Do HFAs Occur? THEMIS observations show that HFAs span a wide range of magnetic local times (MLTs) from approximately 7 to 16.5 MLT (Chu et al., 2017). Woolliscroft et al. (1986) inferred that one HFA extended no more than several RE upstream from the bow shock by noting the absence of this event further upstream despite an observation of the same IP current sheet. Zhang et  al. (2010) showed that proto‐HFAs were observed by THEMIS B located 14 RE upstream from the bow shock and later they developed into HFAs and were observed by THEMIS C located 5 RE upstream from the bow shock. Statistical study showed that HFAs were observed up to 6.3 RE upstream from the bow shock and their occurrence decreases with distance upstream from the bow shock (Chu et al., 2017). 2.3.1.10. HFA Size A statistical study base on Cluster data shows that HFA size varies a lot. The typical size of HFAs is between 0.3 and 10 RE (L. L. Zhao et al., 2017a). 2.3.1.11. Plasma Parameter and Magnetic Field Variations Inside HFAs and SHFAs S. Wang et  al. (2013c) presented superposed epoch analysis results of plasma parameter and magnetic field variations inside HFAs and SHFAs. Variations of plasma parameters inside HFAs, for example, velocity, tempera­ ture, and number density, are generally larger than those of SHFAs. 2.3.1.12. Waves and Turbulences Inside HFAs Electromagnetic waves in the Pc3 ULF (30 seconds) and the lower hybrid frequency (0.1–1 Hz) have often been observed inside HFAs (Zhang et al., 2010). Kovács et  al. (2014) presented a case study of the magnetic turbulent properties inside a mature HFA observed by the Cluster spacecraft. With high‐pass filtering, they showed the existence of magnetic turbulence inside the HFA cavity, while the low‐frequency part of the turbu­ lence might be hidden by wave activities. 2.3.2. Foreshock Bubbles Using multipoint in situ observations from the THEMIS mission, Turner et al. (2013) presented the first observations of FBs and compared them to observations of HFAs (Figure 2.3). They showed the enhanced fluxes of energetic particles up to ~10 keV seen in FBs and attributed the enhanced fluxes as evidence for acceleration via Fermi and shock‐drift acceleration processes.

TRANSIENT PHENOMENA AT THE MAGNETOPAUSE AND BOW SHOCK AND THEIR GROUND SIGNATURES  23

Rotational discontinuities are known to drive FBs (Omidi et al., 2010). Liu et al. (2015) presented a hypo­ thesis about the tangential discontinuity (TD)‐driven FB formation and supported their hypothesis by THEMIS observations. They suggested that a statistical study should be applied to compare RD‐ versus TD‐driven FBs in order to fully understand how FBs are formed. In addition, their observational results should be examined with global hybrid simulations to further validate the premise and the process of FB formation by TDs. To elucidate FB spatial structure, evolution, expansion, and formation conditions, Liu et  al. (2016a) calculated the expansion speed of multiple HFAs and FBs using multipoint THEMIS observations in which three or more spacecraft observed the same events. They found that FBs typically expand faster than HFAs and their expan­ sion speed is likely determined by the solar wind speed. They also discussed the different conditions leading to formation of FBs and HFAs. If the thickness of the discontinuity is thicker (thinner) than the foreshock ion gyroradius, it is more likely to form an HFA (FB). Liu et al. (2016b) showed THEMIS observations of a new ion and electron foreshock upstream of an FB’s shock. FB’s shock could be an additional accelerator and a particle source for the parent shock acceleration. 2.3.3. Foreshock Compressional Boundary Omidi et al. (2013) demonstrated the dynamic nature of the FCB and its relation to foreshock cavities. Using global hybrid simulations with steady and time‐varying IMF con­ ditions, they showed that even during steady IMF condi­ tions, the FCB is highly dynamic and in practice will likely not reach an equilibrium state. Simulations also show that FCBs are part of foreshock cavities and should be detected at their edges regardless of which mechanism is responsible for the formation of foreshock cavities. Rojas‐Castillo et al. (2013) presented Cluster observations of FCBs and showed that they either form downstream of the ion foreshock boundary or coincide with it. Observations show the presence of FCBs during steady or time‐varying IMF. They also show the presence of FCBs under a wide range of solar wind speeds and IMF cone angles. 2.3.4. ULF Wave Growth in the Foreshock at Lunar Distances Hietala et  al. (2016 GEM presentation) presented ARTEMIS observations of ULF wave growth in the fore­ shock at lunar distances. The growth rate obtained from the two spacecraft measurements, as well as the other properties of the waves, matches well the results of a dis­ persion solver that uses the observed ion beam distribu­ tion as an input.

2.3.5. Transient, Local Ion Foreshocks Pfau‐Kempf et  al. (2016) presented results obtained with the hybrid‐Vlasov model Vlasiator. Magnetosheath perturbations are found to deform the bow shock so that transient foreshock‐like field‐aligned ion beams form, a scenario supported by Geotail observations. 2.3.6. Incorporating Foreshock Effects in Global MHD Simulations Samsonov et al. (2017) presented a method for incor­ porating kinetic foreshock effects into a global MHD model. They simulated four events with very distant subsolar magnetopause crossings that occurred during nearly radial IMF intervals lasting from one to several hours. They changed the solar wind boundary con­ ditions for a global model assuming that the density and velocity in the foreshock cavity decrease to ~60% and ~94% of the respective ambient solar wind values during intervals with small IMF cone angles. They ­ demonstrate that the modified model predicts magneto­ pause distances during radial IMF intervals close to those observed by THEMIS. They also pointed out the limitation of their method: they changed the boundary conditions along the whole inflow boundary, although a more correct approach would be to vary parameters only in the foreshock. Therefore, a global modification of the boundary conditions better predicts the location of the magnetopause behind the foreshock but may fail  in predicting the location of other parts of the magnetopause. 2.3.7. Magnetosheath Filamentary Structures Omidi et  al. (2014b) showed the formation of field‐ aligned, filamentary plasma structures in the magne­ tosheath using 2.5‐D electromagnetic hybrid simulations. These structures begin at the quasi‐parallel bow shock and extend far into the magnetosheath. They exhibit anti­ correlation in plasma density and ion temperature. Magnetosheath filamentary structures (MFSs) are formed by the injection of energetic ions accelerated at the quasi‐parallel bow shock. Gutynska et al. (2015) investigated the density enhance­ ments in the magnetosheath using THEMIS observa­ tions and compared their results with those from global hybrid simulations. They found an anticorrelation bet­ ween the density and ion temperature within these struc­ tures which are consistent with the MFSs in hybrid simulations. The scale size of the density fluctuations is about 0.4 RE. They also found that plasma structures with density enhancement are mostly observed during radial IMF orientations and for small 𝜃BN.

24  DAYSIDE MAGNETOSPHERE INTERACTIONS

2.3.8. Mirror Instability in the Magnetosheath Linear theory predicts that the ion cyclotron instability should dominate over the mirror mode instability in an electron–proton plasma. However, mirror modes have been found to occur throughout the magnetosheath. Remya et al. (2013) examined the role of plasma electron temperature anisotropy and heavy ions on the ion cyclo­ tron and mirror mode instabilities and showed that the presence of a small amount of heavy ions and an electron temperature anisotropy of T⟂e∕ T∥e ≥ 1.2 will lead the ion mirror instability to have a higher linear growth rate than that for the proton cyclotron instability. Ahmadi et  al. (2016) investigated effects of electron anisotropy on proton mirror instability evolution in the magnetosheath using particle‐in‐cell (PIC) simulations and concluded that in the nonlinear regime, the electron whistler insta­ bility grows faster than the proton mirror instability and consumes the available electron‐free energy such that there is no electron temperature anisotropy left to fuel the proton mirror mode growth. Remya et al. (2017) pointed out that Ahmadi et al. (2016) uses a periodic boundary system (closed system) in their simulations, the whistler mode waves may lead to continuous scattering of elec­ trons to isotropization. However, in an open system like the magnetosheath, saturated whistler mode waves would escape the generation region quite rapidly leaving enough electron anisotropy to boost the proton mirror instability to dominate over the ion cycloton modes. Ahmadi et al. (2017) agree that the electron whistler instability will not lead to complete isotropization of the electrons but only lower it to the instability threshold and this limited isotro­ pization will still eliminate the dominance of mirror mode and restore the usual dominance of the proton cyclotron mode (in the absence of heavy ions). Ahmadi et  al. (2017) agree that combination of heavy ions and electron temperature anisotropy could make the mirror mode stronger than the proton cyclotron mode. 2.3.9. Energetic Ions in the Magnetosheath Lee et al. (2016b) showed that the inverse dispersions of energetic ions were observed by Magnetospheric Multiscale (MMS)/Energetic Ion Spectrometer (EIS) in the magnetosheath just outside the magnetopause and the observed ion structure can be explained as the effect of a transient solar wind dynamic pressure pulse. 2.3.10. Hot Electron Enhancement in Midtail Magnetosheath C.‐P. Wang et al. (2014) reported ARTEMIS observa­ tions of strong and bursty (about a few minutes) enhance­ ments of electron fluxes in the midtail magnetosheath at

energies from ~1 to 10 keV. Using 4 years of measure­ ments from ARTEMIS, Wang et al. (2015a) statistically investigated the dawn–dusk asymmetry of the hot electron enhancements in the midtail magnetosheath and their correlations with the solar wind/IMF conditions. They found that hot electron enhancements occur three to four times more often on the dawnside than on the duskside and fluxes of hot electron enhancements are twice larger on the dawnside than on the duskside. They sug­ gested that the dawn–dusk asymmetry may be caused by processes at the quasi‐parallel bow shock. 2.4. DAYSIDE MAGNETOPAUSE PROCESSES AND TRANSPORT 2.4.1. KH Instabilities 2.4.1.1. Occurrence Rate of KH Waves and Their Dependence on Solar Wind Conditions Kavosi and Raeder (2015) conducted a survey of KH waves using 7 years of THEMIS data. They found that KH waves are present at the magnetopause approximately 19% of the time regardless of the solar wind conditions. They also showed that the occurrence rate of KH waves increases with solar wind speed, Alfvén Mach number, and number density, but is mostly independent of IMF magni­ tude. They also found that although the occurrence rate of KH waves under southward IMF is significantly higher than previously detected, it is still approximately four times less than the occurrence rate under northward IMF. Most of the events during southward IMF are irregular, short, and polychromatic compared to regular, long lasting, and monochromatic waves under northward IMF. 2.4.1.2. KH Waves in the Midtail Chih‐Ping Wang et al. (2016 GEM workshop presenta­ tion) showed that during a prolonged (~5 hours) northward IMF interval with very steady southward IMF conditions, ARTEMIS at X  = 60 RE near the dusk magnetopause boundary layer observed quasi‐periodic (7–10 minutes) perturbations in the plasma and magnetic field propa­ gating tailward with a spatial scale of ~8 RE in the X direction. A simulation of this event using the LyonFedder-Mobarry (LFM) model shows that KH waves are formed in the near‐Earth flanks and propagate to the midtail, which qualitatively explain the observed perturbations. 2.4.1.3.  KH Instability Increases the  Magnetopause Losses of Energetic Hydrogen and Oxygen Ions Sasha Ukhorskiy (2016 GEM workshop presentation) showed that, for the first time, the high‐resolution LFM global MHD model was coupled with a symplectic test particle code and used to investigate the role of the KH

TRANSIENT PHENOMENA AT THE MAGNETOPAUSE AND BOW SHOCK AND THEIR GROUND SIGNATURES  25

instability in the magnetopause losses of energetic hydrogen and oxygen ions. They showed that the KH instability substantially increases the loss rates for both ion species at the dusk as well as the dawn magnetopause flanks and that after the magnetopause crossing and prior to the escape into the IP space, energetic oxygen ions remain in the magnetosheath much longer than hydrogen ions, which is consistent with recent MMS observations. 2.4.1.4. Effect of Plasmaspheric Plume on KHI Hwang et  al. (2014 GEM presentation) discussed the effect of plasmaspheric plume on magnetic reconnection and KHI. They pointed out that plasmaspheric plume may reduce the reconnection rate while it facilitates the excitation of the KHI, thus it may play an important role in controlling the competition between reconnection and KHI during southward IMF. They further pointed out that the plasmaspheric plume (mainly located on the duskside) may increase the KHI growth rate on the dusk flank magnetopause and lead to the dawn–dusk asym­ metry of the KHI. Walsh et al. (2015) showed observations of KH waves along the dayside terrestrial magnetopause measured from the THEMIS spacecraft and Saskatoon high‐frequency (HF) radar. These observations occur during a time period when a cold, dense plasmaspheric plume contacts the day­ side magnetopause in the dusk sector. Theoretical calcula­ tions and observations indicate that the dense plume plasma lowered the KH threshold and permitted the waves to form on a region of the magnetopause that is farther on the dayside than typically observed. 2.4.1.5. Magnetopause Surface Eigenmodes Hartinger et al. (2015) investigated the global structure and time evolution of dayside magnetopause surface eigenmodes. They found that magnetopause surface eigenmodes are a potential source of ULF waves below 2 mHz and magnetopause surface eigenmodes can seed tailward propagating surface waves via the KHI. 2.4.1.6. Interaction of Magnetic Reconnection and KH Modes Magnetic reconnection and the KH instability can occur simultaneously at the Earth’s ­magnetopause during southward IMF. Ma et  al. (2014a, 2014b, 2016a) dis­ cussed the interaction of KHI and reconnection for large magnetic shear. In particular, each strongly impacts the other, with KHI limiting the r­ econnected flux and modi­ fying the dissipation region structure. It is also demon­ strated that the reconnection rate maximizes for conditions that allow a strong nonlinear evolution of KH waves, that is, fast shear flow and limited guide magnetic field. Nakamura et al. (2013) showed results of 3D fully kinetic simulations of secondary reconnection occurring during

KHI, emphasizing the necessity of 3D. They demonstrated that the compressed current sheets generate magnetic flux ropes along the compressed current layer around the periphery of the vortex. These flux ropes propagate with the shear flow and merge with the vortex eventually. In the final stage, the vortices undergo a merging process that drives new compressed current sheets and flux ropes. 2.4.2. Magnetic Reconnection 2.4.2.1. Cold Ions at the Magnetopause and Their Effect on Reconnection Lee et  al. (2015, 2016a) presented case and statistical studies on the characteristics of the cold‐dense ions observed at the dayside magnetopause by using the Cluster spacecraft data sets. They found that the occur­ rence rate of plasmaspheric plume or ionospheric plasma strongly depends on the solar wind/IMF conditions. In particular, plasmaspheric plumes tend to occur during southward IMF, whereas ionospheric outflows tend to occur during northward IMF. The occurrence rate of the plasmaspheric plumes is significantly higher on the dusk­ side than that on the dawnside. Lee et al. (2014) showed that the motion of cold plas­ maspheric ions entering the reconnection region differs from that of warmer magnetosheath and magnetospheric ions. In contrast to the warmer ions, which are probably accelerated by reconnection in the diffusion region near the subsolar magnetopause, the colder ions are simply entrained by ExB drifts at high latitudes on the recently reconnected magnetic field lines. This indicates that plas­ maspheric ions can sometimes play only a very limited role in the reconnection process, in contrast to previous simulation studies. Wang et  al. (2015b) found that the reconnection rates are dominated by the magnetosheath parameters. Wang et  al. (2015b) further suggested that whether the cold ions affect the reconnection rate depends on where they entered the reconnection region. If the cold ions do not enter the cold ion diffusion region, which is much smaller than the hot ion diffusion region, they may not affect the reconnection rate. 2.4.2.2. Plasma Transport Through Reconnection Maimaiti et  al. (2017) showed a case study when Resolute Bay Incoherent Scatter Radar – North (RISR‐N) was located in the noon sector and directly measured reverse convection in the dayside throat region while the IMF was transitioning from strong positive By to strong positive Bz. Time‐lagged correlation analysis reveals that the IMF By influence acted on a lag time which was 10 minutes faster than that of the Bz component, and this was attributed to the occurrence of magnetic merging at two different magnetopause sites as determined by favored merging geometries for the two components of the IMF.

26  DAYSIDE MAGNETOSPHERE INTERACTIONS

Wilder et  al. (2012) showed Defense Meteorological Satellite Program (DMSP) observations of fast sunward flow channels on open field lines under northward and By‐dominant IMF conditions. They compared observed velocities with predicted reconnection jet speeds using magnetosheath and cusp parameters from an MHD sim­ ulation. Their results suggest that the fast ionospheric flow corresponds to portions of the reconnection jet pop­ ulated by low‐density plasma. Connor et  al. (2015) presented OpenGGCM‐LTPT simulation results of cusp ion signatures and their rela­ tion to dayside reconnection during four IMF clock angles. They found that during northward IMF, both northern and southern magnetic reconnection produce ion precipitation into the northern cusp. They also found that during 120° clock angle, the coexistence of compo­ nent and anti‐parallel reconnection produces a flat and dispersed signature. During 60° clock angle, repetitive FTE formation on the southern magnetopause causes double reserve dispersions. Wenhui Li (2012 GEM presentation) showed OpenGGCM‐CTIM simulations of magnetic reconnec­ tion in the dayside cusp region when IMF Bz ~ 0. The sim­ ulations reproduce both Poynting flux and neutral density “hot spots” which are consistent with observations. Shi et  al. (2013) presented Cluster observations of a transition layer equatorward of the cusp, which contains both magnetosheath and magnetospheric populations, during northward IMF conditions. This transition layer is possibly formed by dual‐lobe reconnection when the IMF is northward. 2.4.2.3. Asymmetric Reconnection Wilder et  al. (2014) showed Cluster observations of reconnection at the polar cusp, which is strongly asym­ metric and has a significant shear flow. They found the exhaust is predominantly on the magnetospheric side of the magnetopause, consistent with theoretical predictions of asymmetric reconnection. The speed of the X-line is also consistent with the asymmetric reconnection theory. Lee et al. (2014) presented Cluster observations of asym­ metric reconnection at the dayside magnetopause. They examined a controlling factor that leads to the asym­ metric reconnection geometry at the magnetopause. It is demonstrated that the separatrix and flow boundary angles are greater on the magnetosheath side than on the magnetospheric side of the magnetopause, probably due to the stronger density asymmetry rather than magnetic field asymmetry at this boundary. 2.4.2.4. The Reconnection Structure Denton et  al. (2016) used magnetic and particle data from MMS to find the motion of the MMS spacecraft through the reconnection structure described in Burch

et al. (2016). They located the position of the magnetic X point and found that MMS‐4 passed within about 1.4 km from the X point and that MMS‐3 and MMS‐2 passed within about 1.7 and 2.4 km, respectively, from the posi­ tion of maximum out of plane current. Ma et al. (2016b) showed that magnetic reconnection with a supercritical perpendicular sheared flow forms an expanding outflow region to maintain the total pressure balance and violates the Walén relation. Plausible obser­ vational signatures in the outflow region include decreased density and pressure and increased magnetic field strength. Using combined ground radar and MMS/EIS observa­ tions, Lee et al. (2016b) estimated a longitudinal extent of 1.5 RE for the reconnection line in a case study. Kitamura et al. (2016) reported that simultaneous observations bet­ ween Geotail and MMS on 18 November 2015 are con­ sistent with the hypothesis that the dayside magnetopause reconnection line shifts from the subsolar point toward the northern (winter) hemisphere due to the effect of geomagnetic dipole tilt. 2.4.2.5. The Reconnection Rate Hoilijoki et al. (2017) showed that reconnection rates at the dayside magnetopause in a global hybrid‐Vlasov simu­ lation correlate well with the analytical model by Cassak and Shay (2007). In addition, their results indicate that magnetosheath fluctuations affect the reconnection rate. Kris Maynard (2016 GEM workshop presentation) used OpenGGCM to show evidence that reconnection happens at two simultaneous X lines during FTE formation. They quantified the reconnection rate using the quasi‐potential. 2.4.3. Flux Transfer Events C. Zhao et  al. (2016) used magnetometer and fast plasma instrument measurement from four MMS space­ craft to calculate the magnetic and plasma forces in FTEs. The force analysis shows that some but not all FTEs are indeed force‐free structures in which the magnetic pressure force balances the magnetic curvature force. Collado‐Vega (2012 GEM workshop presentation) showed that the motion of FTEs observed by the Cluster spacecraft were found to be consistent with both compo­ nent and antiparallel merging, depending on the IMF conditions. Collado‐Vega (2013 GEM presentation) pre­ sented Cluster observations showing FTE motion is strongly dependent on conditions in IP space. Some FTEs move with a sunward component; most of these events had a strong By, which is consistent with predictions from Sibeck and Lin (2011). Trattner et al. (2012) analyzed several cusp crossings to search for the signature of FTEs in the energy distribu­ tion of downward precipitating ions. They found several

TRANSIENT PHENOMENA AT THE MAGNETOPAUSE AND BOW SHOCK AND THEIR GROUND SIGNATURES  27

cusp events which show an energy overlap for parallel‐ streaming precipitating ions. They suggested that this condition might be caused by reopening an already reconnected field line, forming a flux rope at the magnetopause. Collado‐Vega et al. (2013) employed MHD simulations to examine the creation and evolution of plasma vortices within the Earth’s magnetosphere for steady solar wind plasma conditions. They showed that the rotation axis of magnetopause vortices in MHD simulations during dynamic solar wind conditions was found to be mainly aligned with the z‐direction, whereas for fixed solar wind conditions it was in the x–y directions. They attribute vortices formed during steady conditions to the flows set up by reconnection on the high‐latitude magnetopause during intervals of northward IMF orientation. 2.4.4. Magnetopause Position Samsonov et al. (2016) calculated magnetopause posi­ tions for stationary cases with northward and south­ ward IMF orientations using a set of empirical and global MHD models. They found that axisymmetric magnetopause models cannot reproduce the cusp inden­ tations or the changes related to the dipole tilt effect, and most of them predict the magnetopause closer to the Earth than nonaxisymmetric models for typical solar wind conditions and zero tilt angle. They also found that predictions of two global nonaxisymmetric models do not match each other. MHD models often predict the magnetopause closer to the Earth than the nonaxisymmetric empirical models, indicating that MHD simulations may need corrections for the ring current effect and decreases of the dynamic pressure due to foreshock transients. 2.4.5. SMILE – New Mission to Image the Magnetosphere Sibeck (2015 GEM presentation) introduced a recently selected mission called Solar wind Magnetosphere Ionosphere Link Explorer (SMILE). This is a joint mission between European Space Agency and Chinese Academy of Sciences studying interaction between Earth’s magnetosphere and solar wind. SMILE will be able to simultaneously capture images and movies of the magnetopause, polar cusps, and aurora. 2.5. GEOEFFECTS OF DAYSIDE TRANSIENTS The foreshock phenomena may have significant impacts on the Earth’s Magnetosphere–Ionosphere system. A variety of space‐ and ground‐based measure­ ments has been used to examine the response of the

­ agnetosphere to solar wind transients and various m ­foreshock phenomena. 2.5.1. Bow Shock Motion and Magnetopause Surface Wave Using multipoint observations, Archer et  al. (2015) showed that FBs have a global impact on Earth’s ­magnetosphere. They showed that an FB resulted in fast, sunward flows as well as outward motion of the magne­ topause. Signatures of magnetopause motion were also shown in ground‐based magnetometers simultaneously across 7 hours of MLT (corresponding to a distance of 21.5 RE along the magnetopause). Zhang et al. (GEM presentation) presented THEMIS observations of an extreme HFA which lasted 17 minutes near the prenoon bow shock. They showed that this HFA deformed the magnetopause by at least 4 RE. Timing analysis shows that the bulge convected tailward with the magnetosheath flow at ~100 km/s. Observations of the IMAGE magnetometer network at 9 MLT show clear response to this extreme HFA. Cluster located in the dawnside magnetotail also observed a clear response to this extreme HFA. Turner et  al. (2012 GEM presentation) showed that 23  transient foreshock events (including HFAs, FCBs, and FBs) were identified from one day’s THEMIS C data (14 July 2008) when the solar wind is steady (~700 km/s). These foreshock events resulted in significant magneto­ pause disturbances observed by THEMIS D and THEMIS E just inside the magnetopause. Korotova et  al. (2012) presented THEMIS observa­ tions of an unusual bow shock motion attending a magnetospheric transient event. Plaschke et  al. (2013) used MHD theory to show that THEMIS observations of a magnetopause surface wave were inconsistent with the KHI. Using global hybrid simulations with a variety of solar wind Mach numbers, Omidi et  al. (2016) investigated impacts of SHFAs on the magnetosheath and magneto­ pause. They demonstrated that in addition to the formation of MFSs, SHFAs results in the formation of large‐scale cavities in the magnetosheath which are asso­ ciated with decreases in density and magnetic field strength and an increase in ion temperature. They also showed regions of high flow speeds form as a result of SHFAs which may correspond to magnetosheath jets observed by spacecraft. They also showed that SHFAs can cause in and out motion of the magnetopause. Omidi et al. (2013 GEM presentation) presented hybrid simulation results of interaction of corotating interaction region (CIR) shocks with the bow shock. They showed that this interaction can (i) cause waves that propagate through the magnetosheath and the plasma depletion

28  DAYSIDE MAGNETOSPHERE INTERACTIONS

layer; (ii) modify magnetopause reconnection and com­ press the magnetosphere; (iii) energize ions including those accelerated through reconnection to higher energies and result in their trapping in the magnetosphere; (iv) generate/amplify electromagnetic waves in the magneto­ sphere; and (v) enhance ion precipitation into the ionosphere. Plaschke et al. (2016) determined impact rates of mag­ netosheath high‐speed jets and their properties at the magnetopause, which can then be used as input to global magnetospheric models. The high‐speed jets are related to kinetic foreshock processes and drive significant local increases in dynamic pressure and ULF fluctuations at the magnetopause. The jets occur preferentially in radial IMF conditions, happening at rates as large as 9 per hour with typical perpendicular scales of 1.34 RE. 2.5.2. Trigger Reconnection Cluster observations show that magnetic flux rope can form within a magnetosheath HFA (Figure  2.10). The properties of the flux rope, such as its slow speed, magnetic field variations, and the absence of magneto­ spheric electrons, suggest that the flux rope was created in the magnetosheath through magnetic reconnection initi­ ated in the course of the HFA development (Hasegawa et al., 2012). 2.5.3. Generate ULF Waves Several studies have demonstrated that transient phe­ nomena near the bow shock (such as HFAs and FBs) can generate ULF waves in the Earth’s magnetosphere.

Quasi-parallel

IMF

nleading

GSE Z

E nTD E

Bow shock

Tan g

ent

ial d

Flux rope motion in HFA

isco

ntin

uity

Flux rope C3 path

IMF Quasi-perpendicular

ntrailing

GSE X

Figure 2.10  A schematic plot of a magnetosheath HFA and a flux rope inside the HFA observed by the Cluster spacecraft. Source: From Hasegawa et al. (2012).

(This is different from the low‐latitude Pc3 waves that are driven by upstream waves in the ion foreshock.) The ULF waves generated by transient phenomena near the bow shock in both Pc3 (Eastwood et  al., 2011; L. L. Zhao et al., 2017b) and Pc5 (Hartinger et al., 2013; Shen et  al., 2018) ranges have been reported. In addition, there may be considerable variation between ULF waves resulting from different transient features (e.g., Hartinger et  al. [2013] showed mostly compressional waves, Eastwood et  al. [2011] and L. L. Zhao et  al. [2017b] showed standing Alfvén waves, and Shen et  al. [2018] showed both compressional and Alfvén waves). L. L. Zhao et al. (2017b) presented HFA‐generated Pc3 ULF waves observed by multiple spacecraft and ground magnetometers. The ULF waves are standing Alfvén waves. The wave power of poloidal mode is stronger than that of toroidal mode. The Pc3 ULF waves were observed at dawn, noon, and dusk sectors, indicating the magneto­ spheric response to the HFA is global. On the other hand, using in‐situ and ground‐based observations, Shen et al. (2018) report localized Pc5 ULF waves generated by a foreshock transient (Figure  2.11). Both the foreshock transient and ULF waves were found on the duskside; while on the morning side of the magnetosphere, no clear wave signatures were captured. In addition to the FAC system traditionally used to identify events from the ground, Pc1–3 wave bursts are an important observable for identifying and cataloging events (particularly near noon, where the FAC system is generally weak). Dayside transients including solar wind  pressure pulses and foreshock transients can ­stimulate electromagnetic ion cyclotron (EMIC) waves (Engebretson et al., 2013) and modify the transmission of foreshock Pc3 waves to the ground. Hartinger et al. (2013) demonstrated that large, rapid magnetopause displacements are effective drivers of non‐ sinusoidal ULF waves in dayside magnetosphere. They pointed out that these perturbations are a substantial fraction of background values, therefore we cannot nec­ essarily assume linear ULF response of magnetosphere during periods with substantial magnetopause motion (e.g., caused by HFAs, FBs, and large solar wind pressure pulses). Frissell et al. (2012 GEM presentation) presented a new analysis technique for extracting ULF wave signatures from high time‐resolution Super Dual Auroral Radar Network (SuperDARN) “camping beam” data. Samsonov et  al. (2014) presented THEMIS observa­ tions of sudden impulses in the magnetosphere resulted from impact of IP shocks. They showed that compressional waves can be observed very deep (~1.8 RE) in the magne­ tosphere and amplitude of these waves will decrease closer to the Earth.

TRANSIENT PHENOMENA AT THE MAGNETOPAUSE AND BOW SHOCK AND THEIR GROUND SIGNATURES  29 Sun Dusk Dawn IMF discontinuities

n

Propagatio

1. Foreshock transient 2. M’pause deformation

J||

3. Localized ULF waves 4. Discrete/diffuse aurora brightening

–65

Later

Earlier

Dusk

Dawn

MLAT (°)

–70 –75 –80 –85

0

20

40

MLON (°)

0

20

40

MLON (°)

Figure 2.11 A schematic plot of magnetospheric and ionospheric response to a foreshock transient, from magnetopause deformation to field line resonance inside the magnetosphere and the aurora response in the ionosphere. See Shen et  al. (2018) and B. Wang et al. (2018) for details. Source: Revised from Shen et al. (2018) and Wang et al. (2018).

Halford et al. (2016) presented BARREL observations of a solar energetic electron event. There were ULF oscil­ lations observed with precipitation and it is yet unclear if this is due to the movement of the open‐closed boundary or processes within the magnetosheath as these same oscillations were not observed in the solar wind. 2.5.4. Ground Magnetic Response Kim et al. (2015, 2017) presented conjugate observa­ tions of traveling current vortices (TCVs) and EMIC waves associated with transient events at the magneto­ pause. They showed magnetic impulsive events (MIEs)/ TCVs observed by Greenland and Canadian magne­ tometers and their conjugate network in Antarctica in response to solar wind pressure impulse events. They found that EMIC waves (identified as Pc1–2 on the

ground) were also observed in conjunction with the TCV events from the ground network. Turner et  al. (2012 GEM presentation) showed that foreshock activity correlates with enhanced ionospheric convection, based on equivalent ionospheric currents (EICs) derived from GMAGs and SuperDARN signa­ tures. Murr et  al. (2012 GEM presentation) showed how transient features can be extracted from global positioning system (GPS)/total electron content (TEC) measurements. Murr et  al. (2013 GEM presentation) showed that not all foreshock transients cause ground signatures. Using observations and simulations, Hartinger et  al. (2017) demonstrated that the location of a ground mag­ netometer station relative to the dayside transient FAC and the auroral precipitation contribution to ionospheric conductivity need to be considered when interpreting ground magnetometer observations. Clauer et al. (GEM presentation) presented an unusual class of events in which a solar wind pressure increase produces a decrease in the low‐latitude magnetic field, rather than an increase. Oliveira and Raeder (2014, 2015) investigated the geoeffectiveness of IP shock impact angles using global MHD simulations and observations. They found that the Earth’s magnetosphere and ionosphere respond to IP shocks in different ways depending on the shock impact angle. In general, strong (high speed) and almost frontal (small impact angle) shocks are more geoeffective than inclined shocks with low speed. They attribute this result to the fact that frontal shocks compress the magneto­ sphere symmetrically from all sides, which is a favorable condition for the release of magnetic energy stored in the magnetotail, which can produce moderate‐to‐strong sub­ storms and magnetic field perturbations observed by ground‐based magnetometers. Chi et al. (GEM presentation) investigated the magne­ tospheric response to interplanetary field enhancements (IFE) using coordinated ground‐ and space‐based obser­ vations. They found that the IFE‐induced ionospheric current vortices are opposite to those induced by sudden impulses. Borovsky (2012) presented the effect of sudden wind shears on the Earth’s magnetosphere predicted by MHD simulations, including boundary‐layer motions, transient magnetosphere–ionosphere currents, transients in cross‐ polar‐cap potential, and magnetotail disconnections. 2.5.5. Aurora Signatures B. Wang et al. (2018) discussed the dayside auroral evo­ lution due to a foreshock transient in high‐resolution 2D imaging (Figure 2.11). The size (a few RE in Y) and prop­ agation (duskward) of the disturbance were determined

30  DAYSIDE MAGNETOSPHERE INTERACTIONS

by mapping the imaging to the magnetosphere. The dusk­ ward propagation of a pair of FACs is consistent with motion of the aurora. B. Wang et  al. (2016) discussed the driving mecha­ nisms of poleward‐moving auroral forms (PMAFs) with coordinated all sky imager and satellite observa­ tions, showing a strong statistical relationship with southward turnings of the IMF (72%), with a response time of ~8 minutes. Motoba et al. (2014) reported fine‐scale transient arcs at the initial stage of the shock aurora observed at South Pole Station. These arcs are optical signatures of transient FACs just equatorward of the open‐closed boundary. Mende (2014 GEM presentation) showed that persis­ tently occurring dayside transients in the aurora are PMAFs which occur regularly regardless of the direction of the IMF Bz component. Rodriguez et  al. (2012) presented a statistical study of “crewcuts” which are quiet‐time auroral features extending equatorward from the dayside oval during negative Bx and By‐dominated conditions. Han et  al. (2016, 2017) investigated throat aurora, north‐south‐ aligned discrete aurora arcs equatorward of the dis­ crete aurora oval. They believe that the crewcuts (Rodriguez et  al., 2012) are identical to the throat aurora. They showed that these auroral features relate to scales of ~3 RE in the equatorial plane and are the ionospheric signatures of the interaction of cold mag­ netospheric ions with dayside magnetopause reconnec­ tion. This implies that throat aurora may provide important information on studying the interaction of  cold magnetospheric plasma with magnetopause reconnection. Xiaoyan Zhou (GEM presentation) presented the aurora signature of the magnetopause reconnection. The red aurora emissions indicated an equatorward expan­ sion of the cusp due to the magnetopause erosion during the dayside magnetic reconnection. 2.5.6. Thermospheric Heating Connor et  al. (2014, 2016) presented OpenGGCM‐ CTIM simulation results of thermospheric heating in the high‐latitude dayside regions after the sudden enhance­ ment of solar wind pressure. They showed that the cou­ pled magnetosphere–ionosphere–thermosphere (MIT) model produces localized increase of electric field and aurora precipitation in the high‐latitude dayside region after the solar wind dynamic pressure impact, which in turn effectively heat the thermosphere and causes the neutral density increases at 400‐km altitude. Their model results demonstrate that the physics‐based magneto­ spheric energy input is critical to improve ionosphere– thermosphere model predictions.

2.6. DISCUSSION In this section, we discuss the great progress made dur­ ing this focus group toward answering some specific out­ standing science questions. 1. What are the physical differences and relationships between different transient phenomena at the bow shock? A zoo of transient phenomena at the bow shock have been identified and they exhibit both similarities and dif­ ferences (Table 2.1). Two significant contributions during this focus group are the discovery of SHFAs (Omidi et al., 2013; Zhang et al., 2013) and first observations of FBs (Turner et al., 2013). Hybrid simulations show that both tangential discontinuities and rotational discontinu­ ities can generate HFAs by interaction with the bow shock although it is easier for tangential discontinuities to generate HFAs (personal communication with Yu Lin). Hybrid simulations show that rotational disconti­ nuities can drive FBs (Omidi et al., 2010). Observations show that tangential discontinuities can also drive FBs (Liu et al., 2015). The major observational feature to dis­ tinguish FBs and HFAs is whether the structures have two compressional boundaries (HFAs) or only one shock on the trailing edge (foreshock bubbles) (Turner et  al., 2013). However, sometimes a compressional boundary can also be observed on the leading edge of rotational discontinuity‐driven FBs (Liu et  al., 2016a) and HFAs can also have only one compressional boundary on the trailing edge (Thomsen et al., 1988). Therefore, it is not easy to distinguish HFAs and FBs. Hybrid simulations show that SHFAs form as a result of the interaction of the foreshock cavitons with the bow shock (Omidi et al., 2013). The observed proto‐SHFA is very similar to foreshock cavitons (Zhang et  al., 2013). These results suggest that foreshock cavitons and SHFAs could be different evolution stages of the same phe­ nomena. Density holes (Parks et al., 2006) show similar characteristics as HFAs except that the typical duration of density holes is about 18 seconds which is shorter than that of HFAs. Are density holes small‐scale HFAs? Further studies are required to answer this question. 2. How do transient phenomena at the bow shock evolve with time? Hybrid simulations and observations show that fore­ shock cavitons can evolve into SHFAs (Omidi et al., 2013; Zhang et al., 2013). Most “W” and “M” (the profile of the dynamic pressure variations) type HFAs are mature HFAs and most “‐+” and “+‐” type HFAs are young HFAs, indicating that “M” and “W” type HFAs may be the later evolution stages of “‐+” and “+‐” type HFAs (L. L. Zhao et al., 2015). Both ion and electron spectra can be used to classify young and mature HFAs and classifications according to ion and electron spectra are not absolutely consistent, which might be due to different

TRANSIENT PHENOMENA AT THE MAGNETOPAUSE AND BOW SHOCK AND THEIR GROUND SIGNATURES  31

heating mechanisms and efficiency for ions and electrons (S. Wang et al., 2013c). Xiao et al. (2015) found that some observed HFAs are contracting, some are expanding, and others are stable. They showed that the magnetic pressure and thermal pressure play an important role in the evolu­ tion of HFAs. Liu et al. (2016a) found that FBs typically expand faster than HFAs and their expansion speed is likely determined by the solar wind speed. 3. How do the magnetosphere and ionosphere respond to transient phenomena generated at the bow shock? Transient phenomena at the bow shock can deform the bow shock and magnetopause and generate ULF waves in both Pc3 (Eastwood et  al., 2011; L. L. Zhao et  al., 2017b) and Pc5 (Hartinger et al., 2013; Shen et al., 2018) ranges. In addition, there may be considerable variation between ULF waves resulting from different transient features (e.g., Hartinger et  al. [2013] showed mostly compressional waves, Eastwood et  al. [2011] and L. L. Zhao et  al. [2017b] showed standing Alfvén waves, and Shen et al. [2018] showed both compressional and Alfvén waves). The magnetospheric response could be global (L. L. Zhao et al., 2017b) or localized (Shen et al., 2018). The different effects might be caused by the different pressure variation profiles associated with the transients, size of the transients, and the location where the waves were observed in the magnetosphere. The dayside auroral evo­ lution due to a foreshock transient in high‐resolution 2D imaging was reported (B. Wang et al., 2018). Foreshock activity correlates with enhanced ionospheric convection (Turner et al., 2012 GEM presentation). 4. What are the roles played by heavy ions (oxygen) and cold ions (plasmaspheric population) in magnetic recon­ nection and KHI at the magnetopause? Lee et al. (2014) showed that the motion of cold plasma­ spheric ions entering the reconnection region differs from that of warmer magnetosheath and magnetospheric ions. In contrast to the warmer ions, the colder ions are simply entrained by ExB drifts at high latitudes on the recently reconnected magnetic field lines, indicating that plasma­ spheric ions can sometimes play only a very limited role in the reconnection process, in contrast to previous simu­ lation studies. Wang et al. (2015b) suggested that whether the cold ions affect the reconnection rate depends on where they entered the reconnection region. If the cold ions do not enter the cold ion diffusion region, which is much smaller than the hot ion diffusion region, they may not affect the reconnection rate. Hwang et  al. (2014 GEM presentation) pointed out that plasmaspheric plume may reduce the reconnection rate while it facilitates the excitation of the KHI, thus it may play an important role in controlling the competi­ tion between reconnection and KHI during southward IMF. They further pointed out that the plasmaspheric plume (mainly located on the duskside) may increase

the KHI growth rate on the dusk flank magnetopause and lead to the dawn–dusk asymmetry of the KHI. Walsh et  al. (2015) showed observations of KH waves along the dayside terrestrial magnetopause during a time period when a cold, dense plasmaspheric plume contacts the dayside magnetopause in the dusk sector. Theoretical calculations and observations indicate that the dense plume plasma lowered the KHI threshold and permitted the waves to form on a region of the magne­ topause that is farther on the dayside than typically observed. 5. How does asymmetric reconnection differ from symmetric reconnection? Wilder et  al. (2014) showed Cluster observations of reconnection at the polar cusp, which is strongly asym­ metric and has a significant shear flow. They found the exhaust is predominantly on the magnetospheric side of the magnetopause, consistent with theoretical predictions of asymmetric reconnection. The speed of the X-line is also consistent with the asymmetric reconnection theory. Lee et al. (2014) presented Cluster observations of asym­ metric reconnection at the dayside magnetopause. They demonstrated that the separatrix and flow boundary angles are greater on the magnetosheath side than on the magnetospheric side of the magnetopause, consistent with the asymmetric reconnection theory. 6. What are the formation conditions for transient phe­ nomena at the bow shock and magnetopause? Statistical studies (Chu et  al. 2017; L. L. Zhao et  al., 2017a) showed that HFAs prefer to occur under the fol­ lowing conditions: high solar wind speed, radial IMF, Mach number greater than 5, discontinuities with large magnetic shear angles, magnetic field on at least one side of the IP discontinuities has to be connected to the bow shock, the reflected flow from the bow shock is along the discontinuity, and current sheets with thickness from 1000 to about 3162 km. Liu et al. (2016a) suggested that if the thickness of the discontinuity is thicker (thinner) than the foreshock ion gyroradius, it is more likely to form an HFA (foreshock bubble). However, HFAs have also been observed under nonpreferred conditions. For example, Thomsen et al. (1993) suggested that HFAs should be associated with current sheets exhibiting the predicted inward electric field orien­ tation on at least one side. However, S. Wang et al. (2013c) found that electric field on neither leading nor trailing edge points toward the discontinuity for 19 out of 144 (13%) HFAs. This result implies that the convective electric field pointing toward the discontinuity may help an HFA growing but its presence is not a necessary condition to generate an HFA. Simulations and observa­ tions show completely different results on whether HFAs occur at quasi‐perpendicular or quasi‐parallel shocks (Zhang et al. 2010 and references therein). S. Wang et al.

32  DAYSIDE MAGNETOSPHERE INTERACTIONS

(2013b) showed that HFAs can form at both quasi‐ parallel and quasi‐perpendicular shocks. Kavosi and Raeder (2015) found that KH waves are present at the magnetopause approximately 19% of the time regardless of the solar wind conditions. They also showed that the occurrence rate of KH waves increases with solar wind speed, Alfvén Mach number, and number density, but is mostly independent of IMF magnitude. They also found that although the occurrence rate of KH waves under southward IMF is significantly higher than previously detected, it is still approximately four times less than the occurrence rate under northward IMF. Although great progress has been made, there is still a lot to do in order to reach the ultimate goal: a quantitative understanding of these processes so that they could be parameterized for inclusion into space weather predic­ tion models, thereby improving forecast capability. A syn­ ergy of both modeling and experimental efforts is crucial to reach this goal. Observations and hybrid/particle sim­ ulations can help to understand the physical processes and formation condition of these transient phenomena. In addition to coordinated multipoint observations, inclusion of a localized pressure pulse in MHD simula­ tion can help us to understand the impact of these tran­ sients on the magnetosphere and ionosphere. ACKNOWLEDGMENTS This work was supported by NSFAGS 1303689 and 1352669, National Natural Science Foundation of China (41421003 and 41627805). We are grateful to the International Space Science Institute‐Beijing for supporting the interna­ tional team “Dayside Transient Phenomena and Their Impact on the Magnetosphere‐Ionosphere.” Below is a list of published papers related to presenta­ tions given at the Transient Phenomena at the Magnetopause and Bow Shock and Their Ground Signatures Focus Group sessions. PUBLICATIONS Ahmadi, N., Germaschewski, K., & Raeder, J. (2016). Effects of electron temperature anisotropy on proton mirror instability evolution. Journal of Geophysical Research: Space Physics, 121, 5350–5365. https://doi.org/10.1002/2016JA022429 Ahmadi, N., Germaschewski, K., & Raeder, J. (2017). Reply to comment by Remya et al. on “Effects of electron temperature anisotropy on proton mirror instability evolution”. Journal of Geophysical Research: Space Physics, 122, 748–752. https:// doi.org/10.1002/2016JA023452 Archer, M. O., Turner, D. L., Eastwood, J. P., Schwartz, S. J., & Horbury, T. S. (2015). Global impacts of a Foreshock Bubble: Magnetosheath, magnetopause and ground‐based observa­ tions. Planetary and Space Science, 106, 56–65. https:// doi.org/10.1016/j.pss.2014.11.026

Borovsky, J. E. (2012). The effect of sudden wind shear on the Earth’s magnetosphere: Statistics of wind shear events and CCMC simulations of magnetotail disconnections. Journal of Geophysical Research, 117, A06224. https://doi. org/10.1029/2012JA017623 Chu, C., Zhang, H., Sibeck, D., Otto, A., Zong, Q., Omidi, N., et  al. (2017). THEMIS satellite observations of hot flow anomalies at Earth’s bow shock. Annales de Geophysique, 35, 443–451. https://doi.org/10.5194/angeo‐35‐443‐2017 Collado‐Vega, Y. M., Kessel, R. L., Sibeck, D. G., Kalb, V. L., Boller, R. A., & Rastaetter, L. (2013). Comparison between vortices created and evolving during fixed and dynamic solar wind conditions. Annales de Geophysique, 31, 1463–1483. https://doi.org/10.5194/angeo‐31‐1463‐2013 Collinson, G., Halekas, J., Grebowsky, J., Connerney, J., Mitchell, D., Espley, J., et al. (2015). A hot flow anomaly at Mars. Geophysical Research Letters, 42. https://doi. org/10.1002/2015GL065079 Collinson, G. A., Sibeck, D. G., Masters, A., Shane, N., Slavin, J. A., Coates, A. J., et al. (2012). Hot flow anomalies at Venus. Journal of Geophysical Research, 117, A04204. https://doi. org/10.1029/2011JA017277 Connor, H. K., Raeder, J., Sibeck, D. G., & Trattner, K. J. (2015). Relation between cusp ion structures and dayside reconnection for four IMF clock angles: OpenGGCM‐LTPT results. Journal of Geophysical Research: Space Physics, 120, 4890–4906. https://doi.org/10.1002/2015JA021156 Connor, H. K., Zesta, E., Fedrizzi, M., Shi, Y., Raeder, J., Codrescu, M. V., & Fuller‐Rowell, T. J. (2016). Modeling the  ionosphere—Thermosphere response to a geomagnetic storm using physics‐based magnetospheric energy input: OpenGGCM‐CTIM results. Journal of Space Weather and Space Climate, 6, A25. https://doi.org/10.1051/swsc/2016019 Connor, H. K., Zesta, E., Ober, D. M., & Raeder, J. (2014). The relation between transpolar potential and reconnection rates during sudden enhancement of solar wind dynamic pressure: OpenGGCM‐CTIM results. Journal of Geophysical Research, 119. https://doi.org/10.1002/2013JA019728 Denton, R. E., Sonnerup, B. U. Ö., Hasegawa, H., Phan, T. D., Russell, C. T., Strangeway, R. J., et al. (2016). Motion of the MMS spacecraft relative to the magnetic reconnection ­structure observed on 16 October 2015 at 1307 UT. Geophysical Research Letters, 43, 5589–5596. https://doi.org/10.1002/2016GL069214 Engebretson, M. J., Yeoman, T. K., Oksavik, K., Søraas, F., Sigernes, F., Moen, J. I., et  al. (2013). Multi‐instrument observations from Svalbard of a traveling convection vortex, electromagnetic ion cyclotron wave burst, and proton precip­ itation associated with a bow shock instability. Journal of Geophysical Research: Space Physics, 118, 2975–2997. https://doi.org/10.1002/jgra.50291 Gutynska, O., Sibeck, D. G., & Omidi, N. (2015). Magnetosheath plasma structures and their relation to foreshock processes. Journal of Geophysical Research: Space Physics, 120, 7687– 7697. https://doi.org/10.1002/2014JA020880 Halford, A. J., McGregor, S. L., Hudson, M. K., Millan, R. M., & Kress, B. T. (2016). BARREL observations of a solar ener­ getic electron and solar energetic proton event. Journal of Geophysical Research: Space Physics, 121, 4205–4216. https:// doi.org/10.1002/2016JA022462

TRANSIENT PHENOMENA AT THE MAGNETOPAUSE AND BOW SHOCK AND THEIR GROUND SIGNATURES  33 Han, D.‐S., Hietala, H., Chen, X.‐C., Nishimura, Y., Lyons, L. R., Liu, J.‐J., et al. (2017). Observational properties of dayside throat aurora and implications on the possible ­generation mechanisms. Journal of Geophysical Research: Space Physics, 122, 1853–1870. https://doi.org/10.1002/ 2016JA023394 Han, D.‐S., Nishimura, Y., Lyons, L. R., Hu, H.‐Q., & Yang, H.‐G. (2016). Throat aurora: The ionospheric signature of magnetosheath particles penetrating into the magnetosphere. Geophysical Research Letters, 43, 1819–1827. https://doi. org/10.1002/2016GL068181 Hartinger, M. D., Plaschke, F., Archer, M. O., Welling, D. T., Moldwin, M. B., & Ridley, A. (2015). The global structure and time evolution of dayside magnetopause surface eigen­ modes. Geophysical Research Letters, 42, 2594–2602. https:// doi.org/10.1002/2015GL063623 Hartinger, M. D., Turner, D. L., Plaschke, F., Angelopoulos, V., & Singer, H. (2013). The role of transient ion foreshock phenomena in driving Pc5 ULF wave activity. Journal ­ of  Geophysical Research, 118, 299–312. https://doi.org/ 10.1029/2012JA018349 Hartinger, M. D., Xu, Z., Clauer, C. R., Yu, Y., Weimer, D. R., Kim, H., et  al. (2017). Associating ground magnetometer observations with current or voltage generators. Journal of Geophysical Research: Space Physics, 122, 7130–7141. https:// doi.org/10.1002/2017JA024140 Hasegawa, H., Zhang, H., Lin, Y., Sonnerup, B. U. Ö., Schwartz, S. J., Lavraud, B., & Zong, Q.‐G. (2012). Magnetic flux rope formation within a magnetosheath hot flow anomaly. Journal of Geophysical Research, 117, A09214. https://doi. org/10.1029/2012JA017920 Hoilijoki, S., Ganse, U., Pfau‐Kempf, Y., Cassak, P. A., Walsh, B. M., Hietala, H., et al. (2017). Reconnection rates and X line motion at the magnetopause: Global 2D‐3V hybrid‐Vlasov simulation results. Journal of Geophysical Research: Space Physics, 122, 2877–2888. https://doi.org/10.1002/2016JA023709 Kavosi, S., & Raeder, J. (2015). Ubiquity of Kelvin–Helmholtz waves at Earth’s magnetopause. Nature Communications, 6. https://doi.org/10.1038/ncomms8019 Kim, H., Clauer, C. R., Gerrard, A. J., Engebretson, M. J., Hartinger, M. D., Lessard, M. R., et  al. (2017). Conjugate observations of electromagnetic ion cyclotron waves associ­ ated with traveling convection vortex events. Journal of Geophysical Research: Space Physics, 122, 7336–7352. https:// doi.org/10.1002/2017JA024108 Kim, H., Clauer, C. R., Engebretson, M. J., Matzka, J., Sibeck, D. G., Singer, H. J., et al. (2015). Conjugate observations of traveling convection vortices associated with transient events at the magnetopause. Journal of Geophysical Research: Space Physics, 120, 2015–2035. https://doi.org/10.1002/2014JA020743 Kitamura, N., Hasegawa, H., Saito, Y., Shinohara, I., Yokota, S., Nagai, T., et al. (2016). Shift of the magnetopause recon­ nection line to the winter hemisphere under southward IMF conditions: Geotail and MMS observations. Geophysical Research Letters, 43, 5581–5588. https://doi.org/10.1002/ 2016GL069095 Korotova, G. I., Sibeck, D. G., Omidi, N., & Angelopoulos, V. (2012). THEMIS observations of unusual bow shock motion attending a transient magnetospheric event. Journal

of Geophysical Research, 117, A12207. https://doi. org/10.1029/2012JA017510 Kovács, P., Facskó, G., & Dandouras, I. (March 2014). Turbulent dynamics inside the cavity of hot flow anomaly. Planetary and Space Science, 92, 24–33. https://doi. org/10.1016/j.pss.2014.01.001 Lee, S. H., Sibeck, D. G., Hwang, K.‐. J., Wang, Y., Silveira, M. V. D., Fok, M.‐. C., et al. (2016b). Inverse energy disper­ sion of energetic ions observed in the magnetosheath. Geophysical Research Letters, 43, 7338–7347. https://doi. org/10.1002/2016GL069840 Lee, S. H., Zhang, H., Zong, Q.‐G., Otto, A., Rème, H., & Liebert, E. (2016a). A statistical study of plasmaspheric plumes and ionospheric outflows observed at the dayside magnetopause. Journal of Geophysical Research: Space Physics, 121, 492–506. https://doi.org/10.1002/ 2015JA021540 Lee, S. H., Zhang, H., Zong, Q.‐G., Otto, A., Sibeck, D. G., Wang, Y., et al. (2014). Plasma and energetic particle behav­ iors during asymmetric magnetic reconnection at the magne­ topause. Journal of Geophysical Research: Space Physics, 119. https://doi.org/10.1002/2013JA019168 Lee, S. H., Zhang, H., Zong, Q.‐G., Wang, Y., Otto, A., Rème, H., & Glassmeier, K.‐H. (2015). Asymmetric ionospheric outflow observed at the dayside magnetopause. Journal of Geophysical Research: Space Physics, 120. https://doi. org/10.1002/2014JA020943 Liu, T. Z., Hietala, H., Angelopoulos, V., & Turner, D. L. (2016b). Observations of a new foreshock region upstream of a foreshock bubble’s shock. Geophysical Research Letters, 43, 4708–4715. https://doi.org/10.1002/2016GL068984 Liu, T. Z., Turner, D. L., Angelopoulos, V., & Omidi, N. (2016a). Multipoint observations of the structure and evolution of foreshock bubbles and their relation to hot flow anomalies. Journal of Geophysical Research: Space Physics, 121, 5489– 5509. https://doi.org/10.1002/2016JA022461 Liu, T. Z., Turner, D. L., Angelopoulos, V., & Omidi, N. (2015). THEMIS observations of tangential discontinuity‐driven foreshock bubbles. Geophysical Research Letters, 42, 7860– 7866. https://doi.org/10.1002/2015GL065842 Ma, X., Otto, A., & Delamere, P. A. (2014a). Interaction of magnetic reconnection and Kelvin‐Helmholtz modes for large magnetic shear: 1. Kelvin‐Helmholtz trigger. Journal of Geophysical Research: Space Physics, 119, 781–797. https:// doi.org/10.1002/2013JA019224 Ma, X., Otto, A., & Delamere, P. A. (2014b). Interaction of magnetic reconnection and Kelvin‐Helmholtz modes for large magnetic shear: 2. Reconnection trigger. Journal of Geophysical Research: Space Physics, 119, 808–820. https:// doi.org/10.1002/2013JA019225 Ma, X., Otto, A., & Delamere, P. A. (2016b). Magnetic recon­ nection with a fast perpendicular sheared flow. Journal of Geophysical Research: Space Physics, 121, 9427–9442. https:// doi.org/10.1002/2016JA023107 Ma, X., Otto, A., Delamere, P. A., & Zhang, H. (2016a). Interaction between reconnection and Kelvin–Helmholtz at the high‐latitude magnetopause. Advances in Space Research, 58(2), 231–239. ISSN 0273‐1177, doi:https://doi.org/10.1016/j. asr.2016.02.025

34  DAYSIDE MAGNETOSPHERE INTERACTIONS Maimaiti, M., Ruohoniemi, J. M., Baker, J. B. H., Clauer, C. R., Nicolls, M. J., & Hairston, M. R. (2017). RISR‐N observations of the IMF By influence on reverse convection during extreme northward IMF. Journal of Geophysical Research: Space Physics, 122, 3707–3720. https://doi.org/10.1002/2016JA023612 Masters, A., McAndrews, H. J., Steinberg, J. T., Thomsen, M. F., Arridge, C. S., Dougherty, M. K., et al. (2009). Hot flow anomalies at Saturn’s bow shock. Journal of Geophysical Research, 114, A08217. https://doi.org/10.1029/2009JA014112 Motoba, T., Ebihara, Y., Kadokura, A., & Weatherwax, A. T. (2014). Fine‐scaletransient arcs seen in a shock aurora. Journal of Geophysical Research: Space Physics, 119, 6249– 6255. https://doi.org/10.1002/2014JA020229 Nakamura, T. K. M., Daughton, W., Karimabadi, H., & Eriksson, S. (2013). Three‐dimensional dynamics of vortex‐ induced reconnection and comparison with THEMIS obser­ vations. Journal of Geophysical Research: Space Physics, 118, 5742–5757. https://doi.org/10.1002/jgra.50547 Oliveira, D. M., & Raeder, J. (2014). Impact angle control of interplanetary shock geoeffectiveness. Journal of Geophysical Research: Space Physics, 119, 8188–8201. https://doi. org/10.1002/2014JA020275 Oliveira, D. M., & Raeder, J. (2015). Impact angle control of interplanetary shock geoeffectiveness: A statistical study. Journal of Geophysical Research: Space Physics, 120, 4313– 4323. https://doi.org/10.1002/2015JA021147 Omidi, N., Berchem, J., Sibeck, D., & Zhang, H. (2016). Impacts of spontaneous hot flow anomalies on the magnetosheath and magnetopause. Journal of Geophysical Research: Space Physics, 121, 3155–3169. https://doi.org/10.1002/2015JA022170 Omidi, N., Sibeck, D., Blanco‐Cano, X., Rojas‐Castillo, D., Turner, D., Zhang, H., & Kajdi c, P. (2013). Dynamics of the foreshock compressional boundary and its connection to foreshock cavities. Journal of Geophysical Research: Space Physics, 118, 823–831. https://doi.org/10.1002/jgra.50146 Omidi, N., Sibeck, D., Gutynska, O., & Trattner, K. J. (2014b). Magnetosheath filamentary structures formed by ion acceleration at the quasi‐parallel bow shock. Journal of Geophysical Research: Space Physics, 119, 2593–2604. https:// doi.org/10.1002/2013JA019587 Omidi, N., Zhang, H., Chu, C., Sibeck, D. G., & Turner, D. (2014a). Parametric dependencies of spontaneous hot flow anomalies. Journal of Geophysical Research: Space Physics, 119. https://doi.org/10.1002/2014JA020382 Omidi, N., Zhang, H., Sibeck, D., & Turner, D. (2013). Spontaneous hot flow anomalies at quasi‐parallel shocks: 2. Hybrid simulations. Journal of Geophysical Research: Space Physics, 118, 173–180. https://doi.org/10.1029/ 2012JA018099 Pfau‐Kempf, Y., Hietala, H., Milan, S. E., Juusola, L., Hoilijoki, S., Ganse, U., et al. (2016). Evidence for transient, local ion foreshocks caused by dayside magnetopause reconnection. Annales de Geophysique, 34, 943–959. https://doi.org/10.5194/ angeo‐34‐943‐2016 Plaschke, F., Angelopoulos, V., & Glassmeier, K.‐H. (2013). Magnetopause surface waves: THEMIS observations com­ pared to MHD theory. Journal of Geophysical Research: Space Physics, 118, 1483–1499. https://doi.org/10.1002/ jgra.50147

Plaschke, F., Hietala, H., Angelopoulos, V., & Nakamura, R. (2016). Geoeffective jets impacting the magnetopause are very common. Journal of Geophysical Research: Space Physics, 121, 3240–3253. https://doi.org/10.1002/2016JA022534 Remya, B., Reddy, R. V., Tsurutani, B. T., & Lakhina, G. S. (2017). Comment on “Effects of electron temperature anisot­ ropy on proton mirror instability evolution”. Journal of Geophysical Research: Space Physics, 122, 745–747. https:// doi.org/10.1002/2016JA023148 Remya, B., Reddy, R. V., Tsurutani, B. T., Lakhina, G. S., & Echer, E. (2013). Ion temperature anisotropy instabilities in planetary magnetosheaths. Journal of Geophysical Research: Space Physics, 118, 785–793. https://doi.org/10.1002/ jgra.50091 Rodriguez, J. V., Carlson, H. C., & Heelis, R. A. (2012). Auroral forms that extend equatorward from the persistent midday aurora during geomagnetically quiet periods. Journal of Atmospheric and Solar ‐ Terrestrial Physics, 86, 6–24. https:// doi.org/10.1016/j.jastp.2012.06.001 Rojas‐Castillo, D., Blanco‐Cano, X., Kajdi c, P., & Omidi, N. (2013). Foreshock compressional boundaries observed by Cluster. Journal of Geophysical Research: Space Physics, 118, 698–715. https://doi.org/10.1029/2011JA017385 Samsonov, A. A., Gordeev, E., Tsyganenko, N. A., Šafránková, J., Němeček, Z., Šimůnek, J., et al. (2016). Do we know the actual magnetopause position for typical solar wind condi­ tions? Journal of Geophysical Research: Space Physics, 121, 6493–6508. https://doi.org/10.1002/2016JA022471 Samsonov, A. A., Sibeck, D. G., Šafránková, J., Němeček, Z., & Shue, J.‐H. (2017). A method to predict magnetopause expan­ sion in radial IMF events by MHD simulations. Journal of Geophysical Research: Space Physics, 122, 3110–3126. https:// doi.org/10.1002/2016JA023301 Samsonov, A. A., Sibeck, D. G., Walsh, B. M., & Zolotova, N. V. (2014). Sudden impulse observations in the dayside magneto­ sphere by THEMIS. Journal of Geophysical Research: Space Physics, 119, 9476–9496. https://doi.org/10.1002/2014JA020012 Shen, X. C., Shi, Q. Q., Wang, B. Y., Zhang, H., Hudson, M. K., Nishimura, Y., et  al. (2018). Dayside magnetospheric and ionospheric responses to a foreshock transient on 25 June 2008: 1. FLR observed by satellite and ground‐based magne­ tometers. Journal of Geophysical Research: Space Physics, 123. https://doi.org/10.1029/2018JA025349 Shi, Q. Q., Zong, Q.‐G., Fu, S. Y., Dunlop, M. W., Pu, Z. Y., Parks, G. K., et al. (2013). Solar wind entry into the high‐lat­ itude terrestrial magnetosphere during geomagnetically quiet times. Nature Communications, 4, 1466. Trattner, K. J., Petrinec, S. M., Fuselier, S. A., Omidi, N., & Sibeck, D. G. (2012). Evidence of multiple reconnection lines at the magnetopause from cusp observations. Journal of Geophysical Research, 117, A01213. https://doi.org/10.1029/ 2011JA017080 Turner, D. L., Omidi, N., Sibeck, D. G., & Angelopoulos, V. (2013). First observations of foreshock bubbles upstream of Earth’s bow shock: Characteristics and comparisons to HFAs. Journal of Geophysical Research, 118, 1552–1570. https://doi.org/10.1002/jgra.50198 Walsh, B. M., Thomas, E. G., Hwang, K.‐J., Baker, J. B. H., Ruohoniemi, J. M., & Bonnell, J. W. (2015). Dense plasma

TRANSIENT PHENOMENA AT THE MAGNETOPAUSE AND BOW SHOCK AND THEIR GROUND SIGNATURES  35 and Kelvin‐Helmholtz waves at Earth’s dayside magneto­ pause. Journal of Geophysical Research: Space Physics, 120, 5560–5573. https://doi.org/10.1002/2015JA021014 Wang, B., Nishimura, Y., Zou, Y., Lyons, L. R., Angelopoulos, V., Frey, H., & Mende, S. (2016). Investigation of triggering of pole­ ward moving auroral forms using satellite‐imager coordinated observations. Journal of Geophysical Research: Space Physics, 121, 10,929–10,941. https://doi.org/10.1002/2016JA023128 Wang, B., Nishimura, Y., Hietala, T., Shen, H., Shi, X. C., Zhang, Q. Q., et al. (2018). Dayside magnetospheric and ion­ ospheric responses to a foreshock transient on June 25, 2008: 2.2‐D evolution based on dayside auroral imaging. Journal of  Geophysical Research: Space Physics, 123. https:// doi.org/10.1029/2017JA024846 Wang, C.‐P., Xing, X., Nakamura, T. K. M., & Lyons, L. R. (2015a). Dawn‐dusk asymmetry in bursty hot electron enhancements in the midtail magnetosheath. Journal of Geophysical Research: Space Physics, 120, 7228–7239. https:// doi.org/10.1002/2015JA021522 Wang, C.‐P., Xing, X., Nakamura, T. K. M., Lyons, L. R., & Angelopoulos, V. (2014). Source and structure of bursty hot electron enhancements in the tail magnetosheath: Simultaneous two‐probe observation by ARTEMIS. Journal of Geophysical Research: Space Physics, 119, 9900–9918. https://doi.org/10.1002/2014JA020603 Wang, S., Kistler, L. M., Mouikis, C. G., & Petrinec, S. M. (2015b). Dependence of the dayside magnetopause recon­ nection rate on local conditions. Journal of Geophysical Research: Space Physics, 120, 6386–6408. https://doi. org/10.1002/2015JA021524 Wang, S., Zong, Q.‐G., & Zhang, H. (2012). Case and statistical study on evolution of hot flow anomalies with Cluster data. Science China Technological Sciences, 55(5), 1402–1418. https://doi.org/10.1007/s11431‐012‐4767‐z Wang, S., Zong, Q.‐G., & Zhang, H. (2013a). Cluster observa­ tions of hot flow anomalies with large flow deflections: 1. Velocity deflections. Journal of Geophysical Research: Space Physics, 118, 732–743. https://doi.org/10.1002/jgra.50100 Wang, S., Zong, Q.‐G., & Zhang, H. (2013b). Cluster observa­ tions of hot flow anomalies with large flow deflections: 2. Bow shock geometry at HFA edges. Journal of Geophysical Research: Space Physics, 118, 418–433. https://doi. org/10.1029/2012JA018204 Wang, S., Zong, Q.‐G., & Zhang, H. (2013c). Hot flow anomaly formation and evolution: Cluster observations. Journal of Geophysical Research: Space Physics, 118, 4360–4380. https:// doi.org/10.1002/jgra.50424 Wilder, F. D., Crowley, G., Eriksson, S., Newell, P. T., & Hairston, M. R. (2012). Ionospheric Joule heating, fast flow channels, and magnetic field line topology for IMF By‐dominant condi­ tions: Observations and comparisons with predicted reconnec­ tion jet speeds. Journal of Geophysical Research, 117, A11311. https://doi.org/10.1029/2012JA017914 Wilder, F. D., Eriksson, S., Trattner, K. J., Cassak, P. A., Fuselier, S. A., & Lybekk, B. (2014). Observation of a retreat­ ing x line and magnetic islands poleward of the cusp during northward interplanetary magnetic field conditions. Journal of Geophysical Research: Space Physics, 119, 9643–9657. https://doi.org/10.1002/2014JA020453

Xiao, T., Zhang, H., Shi, Q. Q., Zong, Q.‐G., Fu, S. Y., Tian, A. M., et  al. (2015). Propagation characteristics of young hot flow anomalies near the bow shock: Cluster observations. Journal of Geophysical Research: Space Physics, 120. https:// doi.org/10.1002/2015JA021013 Zhang, H., Sibeck, D. G., Zong, Q.‐G., Omidi, N., Turner, D., & Clausen, L. B. N. (2013). Spontaneous hot flow anomalies at quasi‐parallel shocks: 1. Observations. Journal of Geophysical Research: Space Physics, 118, 3357–3363. https://doi. org/10.1002/jgra.50376 Zhao, C., Russell, C. T., Strangeway, R. J., Petrinec, S. M., Paterson, W. R., Zhou, M., et al. (2016). Force balance at the magnetopause determined with MMS: Application to flux transfer events. Geophysical Research Letters, 43, 11,941– 11,947. https://doi.org/10.1002/2016GL071568 Zhao, L. L., Zhang, H., & Zong, Q. G. (2017b). Global ULF waves generated by a hot flow anomaly. Geophysical Research Letters, 44, 5283–5291. https://doi.org/10.1002/2017GL073249 Zhao, L. L., Zhang, H., & Zong, Q.‐G. (2017a). A statistical study on hot flow anomaly current sheets. Journal of Geophysical Research: Space Physics, 122, 235–248. https:// doi.org/10.1002/2016JA023319 Zhao, L. L., Zong, Q. G., Zhang, H., & Wang, S. (2015). Case and statistical studies on the evolution of hot flow anomalies. Journal of Geophysical Research: Space Physics, 120, 6332– 6346. https://doi.org/10.1002/2014JA020862

REFERENCES Billingham, L., Schwartz, S. J., & Sibeck, D. G. (2008). The statistics of foreshock cavities: Results of a Cluster survey. Annales de Geophysique, 26, 3653–3667. https://doi. org/10.5194/angeo‐26‐3653‐2008 Blanco‐Cano, X., Kajdič, P., Omidi, N., & Russell, C. T. (2011). Foreshock cavitons for different interplanetary magnetic field geometries: Simulations and observations. J. Geophys. Res., 116, A09101. https://doi.org/10.1029/2010JA016413 Blanco‐Cano, X., Omidi, N., & Russell, C. T. (2009). Global hybrid simulations: Foreshock waves and cavitons under radial interplanetary magnetic field geometry. J. Geophys. Res., 114, A01216. https://doi.org/10.1029/2008JA013406 Burch, J. L., Torbert, R. B., Phan, T. D., Chen, L. J., Moore, T. E., Ergun, R. E., et al. (2016). Electron‐scale measure­ ments of magnetic reconnection in space. Science, 352(6290), aaf2939. https://doi.org/10.1126/science.aaf2939 Cassak, P. A., & Shay, M. A. (2007). Scaling of asymmetric magnetic reconnection: Genera theory and collisional simulations. Physics of Plasmas, 14(10), 102114. https://doi.org/10.1063/1.2795630 Eastwood, J. P., Sibeck, D. G., Angelopoulos, V., Phan, T. D., Bale, S. D., McFadden, J. P., et al. (2008). THEMIS observa­ tions of a hot flow anomaly: Solar wind, magnetosheath, and ground‐based measurements. Geophysical Research Letters, 35, L17S03. https://doi.org/10.1029/2008GL033475 Eastwood, J. P., Schwartz, S. J., Horbury, T. S., Carr, C. M., Glassmeier, K.‐H., Richter, I., et al. (2011). Transient Pc3 wave activity generated by a hot flow anomaly: Cluster, Rosetta, and ground‐based observations. Journal of Geophysical Research, 116, A08224. https://doi.org/10.1029/2011JA016467

36  DAYSIDE MAGNETOSPHERE INTERACTIONS Facskó, G., Kecskeméty, K., Erdős, G., Tátrallyay, M., Daly, P. W., & Dandouras, I. (2008). A statistical study of hot flow anomalies using Cluster data. Advances in Space Research, 41, 1286–1291. https://doi.org/10.1016/j.asr.2008.02.005 Facskó, G., Németh, Z., Erdős, G., Kis, A., & Dandouras, I. (2009). A global study of hot flow anomalies using Cluster multi‐spacecraft measurements. Annales Geophysicae, 27, 2057–2076. Fillingim, M. O., Eastwood, J. P., Parks, G. K., Angelopoulos, V., Mann, I. R., Mende, S. B., & Weatherwaxd, A. T. (2011). Polar UVI and THEMIS GMAG observations of the ionospheric response to a hot flow anomaly. Journal of ­ Atmospheric and Solar ‐ Terrestrial Physics, 73, 137–145. https://doi.org/10.1016/j.jastp.2010.03.001 Hasegawa, H., Fujimoto, M., Phan, T.‐D., Reme, H., Balogh, A., Dunlop, M. W., et al. (2004). Transport of solar wind into Earth’s magnetosphere through rolled‐up Kelvin‐Helmholtz vortices. Nature, 430(7001), 755–758. https://doi.org/10.1038/ nature02799 Lin, Y. (2003). Global‐scale simulation of foreshock structures at the quasi‐parallel bow shock. J. Geophys. Res., 108(A11), 1390. https://doi.org/10.1029/2003JA009991 Lin, Y., & Wang, X. Y. (2005). Three‐dimensional global hybrid simulation of dayside dynamics associated with the quasi‐ parallel bow shock. J. Geophys. Res., 110, A12216. https:// doi.org/10.1029/2005JA011243 Lucek, E. A., Horbury, T. S., Dunlop, M. W., Cargill, P. J., Schwartz, S. J., Balogh, A., et  al. (2002). Cluster magnetic field observations at a quasi‐parallel bow shock. Ann. Geophys., 20, 1699–1710. Lucek, E. A., Horbury, T. S., Balogh, A., Dandouras, I., & Réme, H. (2004). Cluster observations of hot flow anomalies. Journal of Geophysical Research, 109, A06207. https://doi. org/10.1029/2003JA010016 Masters, A., Achilleos, N., Cutler, J. C., Coates, A. J., Dougherty, M. K., & Jones, G. H. (2012). Surface waves on Saturn’s mag­ netopause. Planetary and Space Science, 65, 109–121. https:// doi.org/10.1016/j.pss.2012.02.007 Omidi, N. (2007). Formation of cavities in the foreshock. In D. Shaikh & G. P. Zank (Eds.), Turbulence and Nonlinear Processes in Astrophysical Plasmas: 6th Annual International Astrophysics Conference (Vol. 932, pp. 181–190). AIP Conference Proceedings. https://doi.org/10.1063/1.2778962. Omidi, N., Sibeck, D. G., & Blanco‐Cano, X. (2009). Foreshock compressional boundary. J. Geophys. Res., 114, A08205. https://doi.org/10.1029/2008JA013950 Omidi, N., Eastwood, J. P., & Sibeck, D. G. (2010). Foreshock bubbles and their global magnetospheric impacts. Journal of Geophysical Research, 115, A06204. https://doi.org/10.1029/ 2009JA014828 Øieroset, M., Mitchell, D. L., Phan, T.‐D., Lin, R. P., & Acuña, M. H. (2001). Hot diamagnetic cavities upstream of the Martian bow shock. Geophysical Research Letters, 28, 887–890. Parks, G. K., Lee, E., Mozer, F., Wilber, M., Lucek, E., Dandouras, I., et  al. (2006). Larmor radius size density holes discovered in the solar wind upstream of Earth’s

bow shock. Physics of Plasmas, 13, 050701. https://doi. org/10.1063/1.2201056 Schwartz, S. J. (1991). Magnetic field structures and related phe­ nomena at quasi‐parallel shocks. Adv. Space Res., 11(9), 231. Schwartz, S. J., Burgess, D., Wilkinson, W. P., Kessel, R., Dunlop, M., & Luhr, H. (1992). Observations of short‐large amplitude magnetic structures at a quasi‐parallel shock. J. Geophys. Res., 97, 4209–4227. Schwartz, S. J. (1995). Hot flow anomalies near the Earth’s bow shock. Advances in Space Research, 15, 107–116. Schwartz, S. J., Chaloner, C. P., Christiansen, P. J., Coates, A. J., Hall, D. S., Johnstone, A. D., et al. (1985). An active current sheet in the solar wind. Nature, 318, 269–271. Schwartz, S. J., Sibeck, D., Wilber, M., Meziane, K., & Horbury, T. S. (2006). Kinetic aspects of foreshock cavities. Geophysical Research Letters, 33(12), 103. https://doi.org/10.1029/ 2005GL025612 Sibeck, D. G., Borodkova, N. L., Schwartz, S. J., Owen, C. J., Kessel, R., Kokubun, S., et al. (1999). Comprehensive study of the magnetospheric response to a hot flow anomaly. Journal of Geophysical Research, 104, 4577–4593. Sibeck, D. G., Phan, T.‐D., Lin, R., Lepping, R. P., & Szabo, A. (2002). Wind observations of foreshock cavities: A case study. Journal of Geophysical Research, 107, 4–1. https://doi. org/10.1029/2001JA007539 Sibeck, D. G., Kudela, K., Mukai, T., Nemecek, Z., & Safrankova, J. (2004). Radial dependence of foreshock cav­ ities: A case study. Annales de Geophysique, 22, 4143–4151. http://www.ann‐geophys.net/22/4143/2004/ Sibeck, D. G., Omidi, N., Dandouras, I., & Lucek, E. (2008). On the edge of the foreshock: Model‐data comparisons. Ann. Geophys., 26, 1539–1544. Sibeck, D. G., & Lin, R.‐Q. (2011). Concerning the motion and orientation of flux transfer events produced by component and antiparallel reconnection. Journal of Geophysical Research, 116, A07206. https://doi.org/10.1029/2011JA016560 Sundberg, T., Boardsen, S. A., Slavin, J. A., Anderson, B. J., Korth, H., Zurbuchen, T. H., et  al. (2012). MESSENGER orbital observations of large‐amplitude Kelvin‐Helmholtz waves at Mercury’s magnetopause. Journal of Geophysical Research, 117, A04216. https://doi.org/10.1029/2011JA017268 Thomsen, M. F., Gosling, J. T., Bame, S. J., Quest, K. B., Russell, C. T., & Fuselier, S. A. (1988). On the origin of hot diamag­ netic cavities near the Earth’s bow shock. J. Geophys. Res., 93, 11,311. Thomsen, M. F., Thomas, V. A., Winske, D., Gosling, J. T., Farris, M. H., & Russell, C. T. (1993). Observational test of hot flow anomaly formation by the interaction of a magnetic discontinuity with the bow shock. J. Geophys. Res., 98, 15,319. Thomas, V. A., Winske, D., Thomsen, M. F., & Onsager, T. G. (1991). Hybrid simulation of the formation of a hot flow anomaly. Journal of Geophysical Research, 96, 11,625–11,632. Thomsen, M. F., Gosling, J. T., Fuselier, S. A., Bame, S. J., & Russell, C. T. (1986). Hot, diamagnetic cavities upstream from the earth’s bow shock. Journal of Geophysical Research, 91(A3), 2961–2973.

TRANSIENT PHENOMENA AT THE MAGNETOPAUSE AND BOW SHOCK AND THEIR GROUND SIGNATURES  37 Uritsky, V. M., Slavin, J. A., Boardsen, S. A., Sundberg, T., Raines, J. M., Gershman, D. J., et al. (2014). Active current sheets and candidate hot flow anomalies upstream of Mercury’s bow shock. Journal of Geophysical Research: Space Physics, 119, 853–876. https://doi.org/10.1002/ 2013JA019052 Valek, P. W., Thomsen, M. F., Allegrini, F., Bagenal, F., Bolton, S., Connerney, J., et al. (2017). Hot flow anomaly observed at Jupiter’s bow shock. Geophysical Research Letters, 44, 8107– 8112. https://doi.org/10.1002/2017GL073175

Woolliscroft, L. J. C., Brown, C. C., Schwartz, S. J., Chaloner, C. P., & Christiansen, P. J. (1986). AMPTE‐UKS observa­ tions of current sheets in the solar wind. Advances in Space Research, 6, 89–92. https://doi.org/10.1016/0273‐1177 (86)90017‐7 Zhang, H., Sibeck, D. G., Zong, Q.‐G., Gary, S. P., McFadden, J. P., Larson, D., et  al. (2010). Time history of events and macroscale interactions during substorms observations of a series of hot flow anomaly events. Journal of Geophysical Research, 115. https://doi.org/10.1029/2009JA015180

3 Transient Solar Wind–Magnetosphere–Ionosphere Interaction Associated with Foreshock and Magnetosheath Transients and Localized Magnetopause Reconnection Y. Nishimura1,2, B. Wang2,3, Y. Zou3,4, E. F. Donovan5, V. Angelopoulos6, J. I. Moen7, L. B. Clausen7, and T. Nagatsuma8

ABSTRACT This paper discusses recent findings of solar wind–magnetosphere–ionosphere interaction driven by localized transient features: (i) foreshock transients, (ii) magnetosheath high‐speed jets (HSJs), (iii) magnetosheath magnetic field excursions, and (iv) localized magnetopause reconnection. A variety of magnetosphere–ionosphere responses have been found to connect to those drivers, such as magnetopause compression and rarefaction, magnetopause reconnection, flux transfer events (FTEs), magnetopause surface waves, ultra‐low‐frequency (ULF) waves, traveling convection vortices (TCVs), and auroral brightening (diffuse and discrete aurora). The properties of Time History of Events and Macroscale Interactions during Substorms (THEMIS), magnetosphere multiscale (MMS), Super Dual Auroral Radar Network (SuperDARN), and imagers, are used to highlight structure and evolution of the magnetosphere–ionosphere system to the transient driving. 3.1. INTRODUCTION

Interactions between the solar wind and bow shock create foreshock and magnetosheath transients, and such disturbances could modify responses of the magnetosphere–ionosphere system that are not predicted by solar wind measurements far upstream. A growing number of studies have documented various types of foreshock transients, including hot flow anomalies (HFAs), ­ spontaneous HFAs (SHFAs), foreshock bubbles (FBs), foreshock cavities, foreshock cavitons, foreshock compressional boundary, density holes, and short large‐ amplitude magnetic structures (SLAMS) (Blanco‐Cano et al., 2011; Omidi et al., 2009; Parks et al., 2006, 2007; Schwartz, 1995; Schwartz et al., 1992; Sibeck et al., 2002, 2008; Turner et al., 2013; Zhang et al., 2010, 2013). Some of those are associated with interplanetary magnetic field (IMF) discontinuities, while others occur spontaneously. The spatial and temporal scales span a wide range from kinetic to magnetohydrodynamic (MHD, ~a few RE). In the magnetosheath, the dynamic pressure can occasionally be substantially enhanced associated with density or

In addition to large‐scale solar wind driving, solar wind‐dayside magnetosphere–ionosphere interaction often occurs in a localized and transient manner. 1  Department of Electrical and Computer Engineering and Center for Space Physics, Boston University, Boston, MA, USA 2  Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA 3  Department of Astronomy and Center for Space Physics, Boston University, Boston, MA, USA 4  Cooperative Programs for the Advancement of Earth System Science, University Corporation for Atmospheric Research, Boulder, CO, USA 5  Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada 6  Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA, USA 7  Department of Physics, University of Oslo, Oslo, Norway 8  National Institute of Information and Communications Technology, Tokyo, Japan

Dayside Magnetosphere Interactions, Geophysical Monograph 248, First Edition. Edited by Qiugang Zong, Philippe Escoubet, David Sibeck, Guan Le, and Hui Zhang. © 2020 American Geophysical Union. Published 2020 by John Wiley & Sons, Inc. 39

40  DAYSIDE MAGNETOSPHERE INTERACTIONS

flow enhancements, which are called magnetosheath high‐speed jets (HSJs) (Archer & Horbury, 2013; Hietala et al., 2009; Nemecek et al., 1998; Plaschke et al., 2013, 2016). IMF rotational discontinuities (Archer et al., 2012; Lin et al., 1996a, 1996b) and a rippled bow shock surface during low IMF cone angles (Hao et  al., 2016, 2017; Hietala et al., 2009, 2012) have been considered as sources of HSJs. Foreshock and magnetosheath transients modulate the dynamic pressure and can impact the magnetosphere– ionosphere system. Pressure variations associated with foreshock and magnetosheath transients can cause magnetopause compression and rarefaction and drive magnetopause reconnection, flux transfer events (FTEs), magnetopause surface waves, ultra‐low‐frequency (ULF) waves, traveling convection vortices (TCVs), and auroral brightening (Archer et al., 2012; Engebretson et al., 2013; Jacobsen et  al., 2009; Omidi et  al., 2010; Plaschke & Glassmeier, 2011; Plaschke et  al., 2009; Sibeck et  al., 1999; Sitar et  al., 1998; Turner et  al., 2011). Localized magnetopause compression increases the magnetopause current and field‐aligned currents (FACs) and induces flow vortices in the magnetosphere (Cowley, 2000). Ground magnetic field perturbations by this localized transient current system are measured as TCVs (Murr & Hughes, 2003), and enhanced electric fields associated with the current system are measured as fast flows (Hietala et al., 2012). Upward FACs can be highlighted by discrete auroral brightening in the dayside discrete auroral oval (Wang et al., 2018a, 2018b). Such magnetosphere–ionosphere responses are often localized, but can also occur globally (Omidi et al., 2010; Zhao et al., 2017). Localized magnetopause compressions launch fast‐ mode waves toward the Earth, which can be measured as ULF waves (Hartinger et al., 2013). Magnetosphere compressions increase the anisotropy of energetic particles by adiabatic acceleration and hence growths of whistler‐ mode waves. Energetic particles are scattered by waves and precipitate into the upper atmosphere. Enhanced precipitation drives diffuse aurora equatorward of the discrete auroral oval, which maps to the magnetosphere inward from the magnetopause (Wang et  al., 2018a, 2018b). The wave‐particle interaction process is a common process for driving dayside diffuse aurora (Ni et al., 2014; Nishimura et al., 2013). Recently, a new type of discrete aurora, the throat aurora, was identified as narrow discrete aurora extending equatorward of the dayside discrete auroral oval (Han et al., 2015; Rodriguez et al., 2012). Throat aurora occurs as azimuthally localized auroral brightening associated with magnetosheath‐type particle precipitation (Han et al., 2016) and tends to be observed under small cone angles and small IMF |Bz|. The fact that both throat aurora and

HSJs have similar occurrence patterns and limited extent suggests that HSJs drive‐throat aurora (Han et al., 2017, 2018). This inference, however, needs to be tested in conjunction with in‐situ measurements. Since throat aurora was not measured when discrete and diffuse auroral brightenings were found during HSJs (Wang et al., 2018a, 2018b), this opens a possibility that another type of magnetosheath transients is responsible for throat aurora. Localized and transient solar wind‐dayside magnetosphere–ionosphere interaction can also occur without a clear indication of solar wind variations. For example, magnetopause reconnection can occur over a localized dawn–dusk extent and spread azimuthally even under a steady southward IMF (Zelenyi & Artemyev, 2013; Zou et al., 2018a). FTEs and poleward‐moving auroral forms (PMAFs) are localized azimuthally and can also occur without modulation in the solar wind (Le et  al., 1993; Lockwood et al., 1989; Lockwood & Wild, 1993; Sandholt et al., 2003), although those can also be triggered by large‐ scale variations in the solar wind (Mende et  al., 2009; Wang et al., 2016). The extent and spreading of the reconnection X‐line have been extensively discussed (Lapenta et  al., 2006; Nakamura et  al., 2012; Shay et  al., 2003; Shepherd & Cassak, 2012), but it has been difficult to constrain the extent and spreading observationally because in‐situ measurements cannot address whether satellites are on a single continuous X‐line or on different X‐lines. Also, isolated in‐situ measurements cannot determine the extent of reconnection lines beyond satellite locations. Ground‐based observations have an advantage in revealing two‐dimensional structures and evolution of magnetosphere–ionosphere responses that are difficult to obtain from a limited number of in‐situ spacecraft observations. Optical imaging provides 2D coverage and high spatial and temporal resolution and, thus, offers opportunities to identify and trace magnetosphere–ionosphere responses more uniquely than other means of observations. Below we present case studies of dayside satellite‐ ground conjunctions to further address structure and evolution of solar wind–magnetosphere–ionosphere interaction. Particular attention is given to magnetosphere–ionosphere responses to (i) foreshock transients, (ii) HSJs, (iii) magnetosheath magnetic field excursions, and (iv) localized magnetopause reconnection. 3.2. FORESHOCK INTERACTION WITH THE DAYSIDE MAGNETOSPHERE AND IONOSPHERE As mentioned above, foreshock transients have large impacts on the magnetosphere and ionosphere. While the size and propagation of magnetosphere and ionosphere responses have been difficult to obtain from a limited

MAGNETOSPHERE–IONOSPHERE RESPONSES TO UPSTREAM TRANSIENTS AND MAGNETOPAUSE RECONNECTION  41

number of observation points in past studies, 2D observation capabilities of ground‐based instruments can overcome this limitation. Engebretson et al. (2013) showed a burst of electromagnetic ion‐cyclotron (EMIC) waves and proton precipitation which they suggested to correspond to a spontaneous HFA event under a steady solar wind. All‐sky imagers (ASIs) identified distinct auroral brightenings that resemble shock aurora (Liou et al., 2007; Zhou et  al., 2009) and aided in specifying the location and motion of enhanced precipitation. TCVs and vortical flows detected by ground magnetometers and radars indicate enhanced localized FACs and flows. Measured flows extended to the open‐closed magnetic field line boundary, suggesting enhanced magnetopause reconnection. A recent work by Wang et  al. (2018a) quantified the size and propagation of magnetosphere–ionosphere responses to a foreshock transient by utilizing a satellite‐ imager conjunction. In their event summarized in Figure  3.1, Time History of Events and Macroscale

Interactions during Substorms-B (THEMIS‐B) was in the solar wind where it detected two tangential discontinuities with foreshock ions (Figure 3.1a and b, indicated by the dashed lines). THEMIS‐A was initially in the dayside magnetosphere and then exited into the magnetosheath. The elevated magnetic field just before the magnetopause crossing and a large anti‐sunward (and duskward) flow in the magnetosheath indicate a magnetosphere compression by an enhanced magnetosheath flow speed, which is possibly an HSJ (Figure 3.1c–e). THEMIS‐D in the dayside magnetosphere detected a Bz enhancement and ULF oscillation, indicating magnetosphere compression by fast‐mode waves (Figure 3.1f and g). The reason for the earlier signal arrival at THEMIS‐D than THEMIS‐A is because the discontinuities and foreshock transient swept duskward as can be seen in Figure 3.1i and j. Evidence of azimuthal propagation of the foreshock–magnetosphere interaction can also be seen from auroral observations. Dayside auroral brightening at South Pole initiated at

V (km/s)

B (nT)

(b)

Bz By Bx Bt Vz Vy Vx VII 106

(c)

(d)

105 (e)

104

(f)

–50 (g)

UT 1530

IQA B (nT)

106 104 102

1535

1540

1545

1550

Bz By Bx Bt Vz Vy Vx VII 1555

Z D H 300

30 East–west imager keogram MLON(°)

THEMIS–A Ion E (eV) V⊥ (km/s) B (nT) THEMIS–D

(a)

2008–06–25 Ground 20 (h) 0 –20 –40 –60 40 (i) 630.0 nm

200

20

100

10 0

0 800

40 (j) 557.7 nm 30

600

20

400

10

200

0 THB MLT 14.4 THB R 30.0 THA MLT 15.4 THA R 10.3 THD MLT 14.0 THD R 8.2 SPA MLT 12.4 UT 1530

14.4 30.0 15.4 10.3 14.0 8.3 12.5 1535

14.4 30.0 15.4 10.4 14.1 8.3 12.6 1540

14.4 30.1 15.4 10.4 14.1 8.4 12.7 1545

14.4 30.1 15.5 10.4 14.2 8.5 12.8 1550

14.4 30.1 15.5 10.4 14.2 8.6 12.8 1555

0

Figure 3.1  A conjunction event between the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites and South Pole all‐sky imagers (ASI) showing a foreshock transient driving a magnetosheath high‐speed jet (HSJ), magnetopause compression, ultra‐low‐frequency (ULF), and dayside aurora. Panels shown are (a and b) interplanetary magnetic field (IMF) and ion energy flux by THEMIS‐B in the solar wind, (c–e) magnetic field, plasma flow, and ion energy flux by THEMIS‐A encountering magnetopause crossing, (f and g) magnetic field and plasma flow by THEMIS‐D in the magnetosphere, (h) ground magnetic field at Iqaluit (IQA) conjugate to South Pole, and (i and j) east–west keograms of 630.0 and 557.7 nm aurora at South Pole. Source: Adopted from Wang et al. (2018a). The vertical lines indicate timings of discontinuities, magnetopause crossing, fast‐mode waves, and ground signals.

(Rayleigh)

–20 50

Bz By Bx

(Rayleigh)

THEMIS–B Ion E (eV) B (nT)

2008–06–25 THEMIS 8 4 0 –4 104 103 102 101 40 0 –40 200 100 0 –100 105 104 103 102 101 20

42  DAYSIDE MAGNETOSPHERE INTERACTIONS

pre‐noon magnetic local time (MLT), and then propagated duskward to post‐noon, consistent with the magnetospheric responses seen by THEMIS (Figure 3.1i and j). The diffuse auroral brightening (at the 557.7‐nm wavelength) coincided with ULF oscillation in the northern hemisphere conjugate magnetometer data (at Iqaluit, Figure  3.1h), indicating a connection to a TCV (not shown). By mapping the auroral brightening to the magnetosphere and timing of its passage (see Wang et  al. (2018a) for details), the possible size and propagation speeds of the foreshock–magnetosphere interaction at the magnetopause are ~2–3 RE and ~50 km/s. The localized azimuthal extent is consistent with the typical size of foreshock transients and HSJs (Plaschke et  al., 2016). ULF wave excitation by a foreshock transient is consistent with an earlier finding by Hartinger et al. (2013), and the optical observations provide a means to precisely estimate the size and propagation of the magnetospheric response to the foreshock interaction with the magnetosphere. 3.3. MAGNETOSHEATH HSJ INTERACTION WITH THE DAYSIDE MAGNETOSPHERE AND IONOSPHERE Ground‐based measurements can also reveal magnetosphere–ionosphere responses to magnetosheath HSJs. Radar measurements show pulsed poleward flows around the cusp associated with HSJs (Archer et al., 2014; Hietala et al., 2012), indicating enhanced reconnection driven by HSJs. Wang et  al. (2018b) used THEMIS‐South Pole imager conjunctions and showed localized and transient diffuse and discrete auroral brightenings during HSJs, indicating that HSJs compress the magnetosphere and drive FACs and low‐energy precipitation. Figure 3.2 shows a conjunction event between the magnetosphere multiscale (MMS) satellite and Svalbard Longyearbyen ASI during HSJs, where HSJs were identified based on the definition given by Plaschke et  al. (2013). Both MMS and auroral signatures of interest occurred within 1‐hour MLT from noon. MMS was located at 13 MLT, near the eastern edge of the imager field of view. MMS detected several HSJs as indicated by the vertical black lines on the left panels. Each one of them correspond to a diffuse auroral brightening, which can be seen as a diffuse glow equatorward of discrete aurora following a ~1‐minute lag from the MMS observations (~10 RE in the magnetosphere (~1000 km in the ionosphere). Those correspond to what are often referred to as localized (or patchy) and extended reconnection. Zou et al. (2018b) postulated that the extent is controlled by the IMF orientation, although

further study is needed. While in‐situ measurements cannot reliably estimate the extent and spreading of reconnection due to the lack of measurements between satellites, ground‐based observations can overcome this limitation and highlight the dynamic nature of magnetopause reconnection, which can occur even under a steady IMF. Figure 3.6 shows another satellite‐ground conjunction event but during pulsed reconnection. As shown by Kitamura et al. (2016), MMS encountered multiple flow jets during partial and full magnetopause crossings associated with reconnection close to but northward of MMS. The partial crossings may involve FTEs. The flow jets have a roughly one‐to‐one correspondence with a series of fast poleward flows measured by SuperDARN near the satellite footprint as indicated by the vertical lines. MMS has an advantage of measuring ion and electron velocities at high time sampling, which allow for identifying a magnetopause current carrier speed. Figure 3.6j compares the Y‐component of the ion, electron, and E × B speed. While those three speeds overall agree with one another, the ion flow speed notably deviated positively from the electron and E × B speed at 02:12:33–45

48  DAYSIDE MAGNETOSPHERE INTERACTIONS

R MLT UT

60 40 20 0 –20 –40 200 100 0 –100 –200 –300 –400 104

(b)

1000 800 600 400 200 0 Bx By Bz Bt Vx

(c)

106

102

105

101

104

(e)

109

103

108

102

107

101 101

106

100

(f)

V⊥e (km/s)

Pi Pmag Pdyn Ptot

10–1 9.5 11.4 0150

(keV/(cm2 s sr keV))

107

9.6 11.5 0200

9.8 11.5 0210

9.9 11.6 0220

MMS-3 magnetopause crossing BL

(g)

BM BN

(h)

VL

0

VM

–100 –200

100

VN

(i)

VL

0 –100

VM

–200

VN

–300 V⊥y (km/s)

Vz

(d)

100

–300 200

Vy

103

104

V (m/s) B (nT)

(a)

V⊥i (km/s)

80 79 78 77 76 75 74

60 40 20 0 –20 –40 –60 200

150 100 50 0 –50 –100 –150 –200 3

P (nPa)

P (nPa)

Ele E (eV)

Ion E (eV)

Vi (km/s)

MMS3 B (nT)

SuperDARN Mlat (deg)

SuperDARN and MMS-3 2015 Nov 18

Seconds

2

(j)

ExB Vi Ve

Pi Pmag

(k)

Pdyn Ptot

1 0

30 0212

40

50

00 0213

10

20

Figure 3.6  Dayside reconnection and pulsed flows measured by MMS‐SuperDARN conjunction on 18 November 2015. (a) Time variation of King Salmon SuperDARN velocity (positive poleward and dawnward). (b–f) MMS‐3 observations of the magnetic field, ion velocity, ion and electron energy flux, and pressure. (g–k) Burst‐mode observations by MMS‐3 during the magnetopause crossing. (g–i) Magnetic field, and ion and electron perpendicular velocity in the LMN (L, tangential positive northward; M, tangential positive downward; N, normal to the magnetopause outward positive) coordinates. (j) A comparison of the Y‐component of E × B, ion, and electron velocity. (k) Pressure.

and 02:12:56–02:13:03 UT. The deviations tend to occur at BL magnetic field and thermal pressure gradients, indicating that deviations of ion drifts from the E × B speed carry the magnetopause current and those are likely due to diamagnetic drifts of ions. Since electrons can also be carriers of the magnetopause current under a thin magnetopause (Pritchett & Mozer, 2009), the present observation illustrate only one of the possible signatures of X‐line spreading but yet shows that X‐line spreading and propagation speeds are consistent with particle drift speed at the magnetopause. Nevertheless, the magnitude of the ion‐electron velocity difference is on average a few tens of kilometer per second (maximum difference reaching ~100 km/s), and this is comparable to the level of azimuthal spreading speed deduced from the radar observations (Zou et al., 2018a). This agreement supports the

inference that the current carrier speed is related to reconnection X‐line spreading. Figure 3.7 shows the spatial distribution of the line‐of‐ sight King Salmon SuperDARN radar velocity during the MMS magnetopause crossing. The enhanced flow appeared as a longitudinally localized region of fast flow. The region of fast flow propagated dawnward. The extent of the ionosphere flow channel (defined as full width half maximum) was mapped to the equatorial plane using the T89 (Tsyganenko, 1989) magnetic field model as shown in Figure 3.7d, as a possible extent of the magnetopause reconnection. The flow channel was seen to expand from 0.6 to 1.3 RE, and its dawnward edge propagated dawnward at ~10 km/s. These likely highlight the size and propagation of a region with enhanced reconnection in this event.

MAGNETOSPHERE–IONOSPHERE RESPONSES TO UPSTREAM TRANSIENTS AND MAGNETOPAUSE RECONNECTION  49 (a)

(b)

80

02:12 UT

02:08 UT

75

70 V [m/s] 65 0

200

400

600 800 1000

(c)

(d) 4

02:20 UT

Y (RE)

3

2 1 02:08 UT 02:20 UT

0 –1 8

9

10

11

12

13

X (RE)

Figure 3.7 (a–c) Line‐of‐sight velocity maps from King Salmon SuperDARN during the MMS magnetopause crossing in Figure 3.6. (d) Azimuthal extent of the flow channel mapped to the magnetosphere.

3.6. CONCLUSION We discussed key features of solar wind–magnetosphere–ionosphere interaction during four types of transient activities, taking advantages of space‐ground coordination. Those are summarized in Figure  3.8. Foreshock transients and magnetosheath HSJs modulate the dynamic pressure and drive a variety of magnetosphere–ionosphere responses: magnetopause compression/rarefaction, magnetopause reconnection, FTEs, magnetopause surface waves, ULF waves, TCVs, and auroral brightening. Such responses can be highlighted by optical imaging, revealing the size and propagation of the foreshock–magnetosphere interaction. Magnetosheath magnetic field excursions have been identified as a potentially new type of magnetosheath transients. Those magnetic field signatures suggest that

localized IMF structures or processes occurring at the foreshock or bow shock result in magnetosheath magnetic field orientation changes that are uncorrelated with features far upstream in the solar wind, for example, at the L1 point. Those are found to be correlated with throat aurora and ionosphere fast flows, indicating that localized magnetopause reconnection and erosion may be triggered by magnetosheath magnetic field excursions. HSJs were absent during throat aurora. This is another indication that magnetosheath magnetic field excursions and throat aurora are fundamentally different features from HSJs and diffuse auroral brightenings. These findings from case studies should be examined statistically to test if the connections occur in general. The dynamic nature of localized magnetopause reconnection has been discussed using ground‐based

50  DAYSIDE MAGNETOSPHERE INTERACTIONS X

15

Foreshock transients Bow shock ripple

HSJ

Magnetopause reconnection & compression

Magnetosheath magnetic field excursion

10 ep

J||

Magnetopause reconnection

ULF waves 5

Y

GEO

Ionospheric flows aurora, FAC

Figure 3.8  Schematic illustration of transient solar wind–magnetosphere–ionosphere coupling processes. The left side depicts foreshock transients and magnetosheath HSJs driving magnetopause compression and reconnection, ULF waves, and aurora. The right side illustrates magnetosheath magnetic field excursion that triggers azimuthally localized reconnection, field‐aligned currents (FACs), and aurora. Source: Modified from Plaschke et al. (2016) and Hietala et al. (2018).

radar observations. Such measurements are found critical to identify the azimuthal size of the X‐line (localized or extended). Reconnection often spread azimuthally, and the spreading has been suggested to reflect the drift speed of magnetopause current carriers. ACKNOWLEDGMENTS This work was supported by NASA grants NNX15AI62G, NNX16AG21G, and NNX17AD15G, NSF grants PLR‐1341359, AGS‐1451911, AGS‐1723342, and AGS‐1723588, and AFOSR FA9550‐15‐1‐0179 and FA9559‐16‐1‐0364. The work by YZ was supported by UCAR Jack Eddy Fellowship. The THEMIS mission is supported by NASA contract NAS5‐02099, NSF grant AGS‐1004736, and CSA contract 9F007‐046101. We thank support from the ISSI workshop “Multiple‐instrument observations and simulations of the dynamical processes associated with polar cap patches/aurora and their associated scintillations.” The LYR imagers are maintained by University of Oslo and University of Calgary. The THEMIS and MMS data were obtained through cdaweb.gsfc.nasa.gov as daily CDF files. Contact ED, JM, LBC, and TN for accessing the imager and radar data.

REFERENCES Archer, M. O., & Horbury, T. S. (2013). Magnetosheath dynamic pressure enhancements: Occurrence and typical properties. Annales Geophysicae, 31, 319–331. https://doi.org/10.5194/ angeo–31–319–2013 Archer, M. O., Horbury, T. S., & Eastwood, J. P. (2012). Magnetosheath pressure pulses: Generation downstream of the bow shock from solar wind discontinuities. Journal of Geophysical Research, 117, A05228. https://doi. org/10.1029/2011JA017468 Archer, M. O., Turner, D. L., Eastwood, J. P., Horbury, T. S., & Schwartz, S. J. (2014). The role of pressure gradients in driving sunward magnetosheath flows and magnetopause motion. Journal of Geophysical Research: Space Physics, 119, 8117–8125. https://doi.org/10.1002/2014JA020342 Blanco‐Cano, X., Kajdič, P., Omidi, N., & Russell, C. T. (2011). Foreshock cavitons for different interplanetary magnetic field geometries: Simulations and observations. Journal of Geophysical Research, 116, A09101. https://doi.org/10. 1029/2010JA016413 Cowley, S. W. H. (2000). Magnetosphere‐ionosphere interactions: A tutorial review. In S.‐I. Ohtani, R. Fujii, M. Hesse, & R. L. Lysak (Eds.), magnetospheric current systems. Washington, DC: American Geophysical Union. https://doi. org/10.1029/GM118p0091 Dimmock, A. P., Nykyri, K., & Pulkkinen, T. I. (2014). A statistical study of magnetic field fluctuations in the dayside

MAGNETOSPHERE–IONOSPHERE RESPONSES TO UPSTREAM TRANSIENTS AND MAGNETOPAUSE RECONNECTION  51 magnetosheath and their dependence on upstream solar wind conditions. Journal of Geophysical Research: Space Physics, 119, 6231–6248. https://doi.org/10.1002/2014JA020009 Engebretson, M. J., Yeoman, T. K., Oksavik, K., Søraas, F., Sigernes, F., Moen, J. I., et al. (2013). Multi‐instrument observations from Svalbard of a traveling convection vortex, electromagnetic ion cyclotron wave burst, and proton precipitation associated with a bow shock instability. Journal of Geophysical Research: Space Physics, 118, 2975–2997. https:// doi.org/10.1002/jgra.50291 Han, D.‐S., Chen, X.‐C., Liu, J.‐J., Qiu, Q., Keika, K., Hu, Z.‐J., et  al. (2015). An extensive survey of dayside diffuse aurora based on optical observations at Yellow River Station. Journal of Geophysical Research: Space Physics, 120, 7447– 7465. https://doi.org/10.1002/2015JA021699 Han, D.‐S., Hietala, H., Chen, X.‐C., Nishimura, Y., Lyons, L. R., Liu, J.‐J., et al. (2017). Observational properties of ­dayside throat aurora and implications on the possible generation mechanisms. Journal of Geophysical Research: Space Physics, 122, 1853–1870. https://doi.org/10. 1002/2016JA023394 Han, D.‐S., Liu, J.‐. J., Chen, X.‐. C., Xu, T., Li, B., Hu, Z.‐. J., et  al. (2018). Direct evidence for throat aurora being the ionospheric signature of magnetopause transient and reflecting localized magnetopause indentations. Journal of Geophysical Research: Space Physics, 132. https://doi. org/10.1002/2017JA024945 Han, D.‐S., Nishimura, Y., Lyons, L. R., Hu, H.‐Q., & Yang, H.‐G. (2016). Throat aurora: The ionospheric signature of magnetosheath particles penetrating into the magnetosphere. Geophysical Research Letters, 43, 1819–1827. https://doi. org/10.1002/2016GL068181 Hao, Y., Gao, X., Lu, Q., Huang, C., Wang, R., & Wang, S. (2017). Reformation of rippled quasi‐parallel shocks: 2‐D hybrid simulations. Journal of Geophysical Research: Space Physics, 122, 6385–6396. https://doi.org/10.1002/2017JA024234 Hao, Y., Lembege, B., Lu, Q., & Guo, F. (2016). Formation of downstream high‐speed jets by a rippled nonstationary quasi‐ parallel shock: 2‐D hybrid simulations. Journal of Geophysical Research: Space Physics, 121, 2080–2094. https://doi. org/10.1002/2015JA021419 Hartinger, M. D., Turner, D. L., Plaschke, F., Angelopoulos, V., & Singer, H. (2013). The role of transient ion foreshock phenomena in driving Pc5 ULF wave activity. Journal of Geophysical Research: Space Physics, 118, 299–312. https:// doi.org/10.1029/2012JA018349 Hietala, H., Laitinen, T. V., Andréeová, K., Vainio, R., Vaivads, A., Palmroth, M., et  al. (2009). Supermagnetosonic jets behind a collisionless quasiparallel shock. Physical Review Letters, 103, 245001. https://doi.org/10.1103/PhysRevLett. 103.245001 Hietala, H., Partamies, N., Laitinen, T. V., Clausen, L. B. N., Facskó, G., Vaivads, A., et  al. (2012). Supermagnetosonic subsolar magnetosheath jets and their effects: From the solar wind to the ionospheric convection. Annales Geophysicae, 30, 33–48. https://doi.org/10.5194/angeo‐30‐33‐2012 Hietala, H., Phan, T. D., Angelopoulos, V., Oieroset, M., Archer, M. O., Karlsson, T., & Plaschke, F. (2018). In situ observations of a magnetosheath high‐speed jet triggering

magnetopause reconnection. Geophysical Research Letters, 45. https://doi.org/10.1002/2017GL076525 Huang, S. Y., Du, J. W., Sahraoui, F., Yuan, Z. G., He, J. S., Zhao, J. S., et al. (2017). A statistical study of kinetic‐size magnetic holes in turbulent magnetosheath: MMS observations. Journal of Geophysical Research: Space Physics, 122, 8577–8588. https://doi.org/10.1002/2017JA024415 Jacobsen, K. S., Phan, T. D., Eastwood, J. P., Sibeck, D. G., Moen, J. I., Angelopoulos, V., et al. (2009). THEMIS observations of extreme magnetopause motion caused by a hot flow anomaly. Journal of Geophysical Research, 114, A08210. https://doi.org/10.1029/2008JA013873 Kitamura, N., Hasegawa, H., Saito, Y., Shinohara, I., Yokota, S., Nagai, T., et  al. (2016). Shift of the magnetopause reconnection line to the winter hemisphere under southward IMF conditions: Geotail and MMS observations. Geophysical Research Letters, 43, 5581–5588. https://doi. org/10.1002/2016GL069095 Lapenta, G., Krauss‐Varban, D., Karimabadi, H., Huba, J. D., Rudakov, L. I., & Ricci, P. (2006). Kinetic simulations of X‐line expansion in 3D reconnection. Geophysical Research Letters, 33, L10102. https://doi.org/10. 1029/2005GL025124 Le, G., Russell, C. T., & Kuo, H. (1993). Flux transfer events: Spontaneous or driven? Geophysical Research Letters, 20, 791–794. Lin, Y., Lee, L. C., & Yan, M. (1996a). Generation of dynamic pressure pulses downstream of the bow shock by variations in the interplanetary magnetic field orientation. Journal of Geophysical Research, 101, 479–493. https://doi.org/10. 1029/95JA02985 Lin, Y., Swift, D. W., & Lee, L. C. (1996b). Simulation of pressure pulses in the bow shock and magnetosheath driven by variations in interplanetary magnetic field direction. Journal of Geophysical Research, 101, 27251–27269. https:// doi.org/10.1029/96JA02733 Liou, K., Newell, P. T., Shue, J.‐H., Meng, C.‐I., Miyashita, Y., Kojima, H., & Matsumoto, H. (2007). “Compression aurora”: Particle precipitation driven by long‐duration high solar wind ram pressure. Journal of Geophysical Research, 112, A11216. https://doi.org/10.1029/2007JA012443 Lockwood, M., Sandholt, P. E., Cowley, S. W. H., & Oguti, T. (1989). Interplanetary magnetic field control of dayside auroral activity and the transfer of momentum across the dayside magnetopause. Planetary and Space Science, 37, 1347–1365. Lockwood, M., & Wild, M. N. (1993). On the quasi‐periodic nature of magnetopause flux transfer events. Journal of Geophysical Research, 98, 5935–5940. Mende, S. B., Frey, H. U., McFadden, J., Carlson, C. W., Angelopoulos, V., Glassmeier, K.‐H., et  al. (2009). Coordinated observation of the dayside magnetospheric entry and exit of the THEMIS satellites with ground‐based auroral imaging in Antarctica. Journal of Geophysical Research, 114, A00C23. https://doi.org/10.1029/ 2008JA013496 Murr, D. L., & Hughes, W. J. (2003). Solar wind drivers of traveling convection vortices. Geophysical Research Letters, 30(7), 1354. https://doi.org/10.1029/2002GL015498

52  DAYSIDE MAGNETOSPHERE INTERACTIONS Nakamura, T. K. M., Nakamura, R., Alexandrova, A., Kubota, Y., & Nagai, T. (2012). Hall magnetohydrodynamic effects for three‐dimensional magnetic reconnection with finite width along the direction of the current. Journal of Geophysical Research, 117, A03220. https://doi.org/10.1029/2011JA017006 Nemecek, Z., Safrankova, J., Prech, L., Sibeck, D. G., Kokubun, S., & Mukai, T. (1998). Transient flux enhancements in the magnetosheath. Geophysical Research Letters, 25, 1273–1276. https://doi.org/10.1029/98GL50873 Neudegg, D. A., Cowley, S. W. H., McWilliams, K. A., Lester, M., Yeoman, T. K., Sigwarth, J., et  al. (2001). The UV aurora and ionospheric flows during flux transfer events. Annales Geophysicae, 19, 179–188. https://doi.org/10.5194/ angeo‐19‐179‐2001 Ni, B., Bortnik, J., Nishimura, Y., Thorne, R. M., Li, W., Angelopoulos, V., et al. (2014). Chorus wave scattering responsible for the Earth’s dayside diffuse auroral precipitation: A detailed case study. Journal of Geophysical Research: Space Physics, 119, 897–908. https://doi.org/10.1002/2013JA019507 Nishimura, Y., Kikuchi, T., Ebihara, Y., Yoshikawa, A., Imajo, S., Li, W., & Utada, H. (2016). Evolution of the current system during solar wind pressure pulses based on aurora and magnetometer observations. Earth, Planets and Space, 68(1), 144. Nishimura, Y., et  al. (2013). Structures of dayside whistler‐ mode waves deduced from conjugate diffuse aurora. Journal of Geophysical Research: Space Physics, 118, 664–673. https:// doi.org/10.1029/2012JA018242 Omidi, N., Eastwood, J. P., & Sibeck, D. G. (2010). Foreshock bubbles and their global magnetospheric impacts. Journal of Geophysical Research, 115, A06204. https://doi.org/10. 1029/2009JA014828 Omidi, N., Sibeck, D. G., & Blanco‐Cano, X. (2009). Foreshock compressional boundary. Journal of Geophysical Research, 114, A08205. https://doi.org/10.1029/2008JA013950 Parks, G. K., Lee, E., Lin, N., Mozer, F., Wilber, M., Lucek, E., et  al. (2007). Density holes in the upstream solar wind. In Turbulence and Nonlinear Processes in Astrophysical Plasmas, 6th Annual International Astrophysics Conference, AIP Proceedings (Vol. 932, pp. 9–15). Parks, G. K., Lee, E., Mozer, F., & Wilber, M. (2006). Larmor radius size density holes discovered in the solar wind upstream of Earth’s bow shock. Physics of Plasmas, 13, 050701. Plaschke, F., & Glassmeier, K.‐H. (2011). Properties of standing Kruskal Schwarzschild‐modes at the magnetopause. Annales Geophysicae, 29(10), 1793–1807. https://doi.org/10.5194/ angeo‐29‐1793‐2011 Plaschke, F., Glassmeier, K.‐H., Auster, H. U., Constantinescu, O. D., Magnes, W., Angelopoulos, V., et al. (2009). Standing Alfvén waves at the magnetopause. Geophysical Research Letters, 36, L02104. https://doi. org/10.1029/2008GL036411 Plaschke, F., Hietala, H., & Angelopoulos, V. (2013). Anti–sunward high–speed jets in the subsolar magnetosheath. Annales Geophysicae, 31, 1877–1889. https://doi.org/10.5194/angeo– 31–1877–2013 Plaschke, F., Hietala, H., Angelopoulos, V., & Nakamura, R. (2016). Geoeffective jets impacting the magnetopause are very common. Journal of Geophysical Research: Space Physics, 121, 3240–3253. https://doi.org/10.1002/2016JA022534

Pritchett, P. L., & Mozer, F. S. (2009). Asymmetric magnetic reconnection in the presence of a guide field. Journal of Geophysical Research, 114, A11210. https://doi.org/10. 1029/2009JA014343 Rodriguez, J. V., Carlson, H. C., & Heelis, R. A. (2012). Auroral forms that extend equatorward from the persistent midday aurora during geomagnetically quiet periods. Journal of Atmospheric and Solar‐Terrestrial Physics, 86, 6–24. https:// doi.org/10.1016/j.jastp.2012.06.001 Roux, A., Robert, P., Fontaine, D., Contel, O. L., Canu, P., & Louarn, P. (2015). What is the nature of magnetosheath FTEs? Journal of Geophysical Research: Space Physics, 120, 4576–4595. https://doi.org/10.1002/2015JA020983 Sandholt, P. E., Farrugia, C. J., Denig, W. F., Cowley, S. W. H., & Lester, M. (2003). Spontaneous and driven cusp dynamics: Optical aurora, particle precipitation, and plasma convection. Planetary and Space Science, 51(12), 797–812. https://doi. org/10.1016/S0032‐0633(03)00114‐4 Schwartz, S. J. (1995). Hot flow anomalies near the Earth’s bow shock. Advances in Space Research, 15(8‐9), 107–116. Schwartz, S. J., Burgess, D., Wilkinson, W. P., Kessel, R. L., Dunlop, M., & Lühr, H. (1992). Observations of short large‐ amplitude magnetic structures at a quasi‐parallel shock. Journal of Geophysical Research, 97(A4), 4209–4227. https:// doi.org/10.1029/91JA02581 Shay, M. A., Drake, J. F., Swisdak, M., Dorland, W., & Rogers, B. N. (2003). Inherently three dimensional magnetic reconnection: A mechanism for bursty bulk flows? Geophysical Research Letters, 30(6), 1345. https://doi.org/10. 1029/2002GL016267 Shepherd, L. S., & Cassak, P. A. (2012). Guide field dependence of 3‐D X‐line spreading during collisionless magnetic reconnection. Journal of Geophysical Research, 117, A10101. https://doi.org/10.1029/2012JA017867 Sibeck, D. G., Borodkova, N. L., Schwartz, S. J., Owen, C. J., Kessel, R., Kokubun, S., et al. (1999). Comprehensive study of the magnetospheric response to a hot flow anomaly. Journal of Geophysical Research: Space Physics, 104(A3), 4577–4593. https://doi.org/10.1029/1998JA900021 Sibeck, D. G., Omidi, N., Dandouras, I., & Lucek, E. A. (2008). On the edge of the foreshock: Model‐data comparisons. Annales Geophysicae, 26, 1539–1544. Sibeck, D. G., Phan, T.‐D., Lin, R., Lepping, R. P., & Szabo, A. (2002). Wind observations of foreshock cavities: A case study. Journal of Geophysical Research, 107(A10), 1271. https://doi. org/10.1029/2001JA007539 Sitar, R. J., Baker, J. B., Clauer, C. R., Ridley, A. J., Cumnock, J. A., Papitashvili, V. O., et al. (1998). Multi‐instrument analysis of the ionospheric signatures of a hot flow anomaly occurring on July 24, 1996. Journal of Geophysical Research: Space Physics, 103(A10), 23357–23372. https://doi.org/10.1029/98JA01916 Tsyganenko, N. A. (1989). A magnetospheric magnetic field model with a warped tail current sheet. Planetary and Space Science, 37, 5–20. Turc, L., Fontaine, D., Escoubet, C. P., Kilpua, E. K. J., & Dimmock, A. P. (2017). Statistical study of the alteration of the magnetic structure of magnetic clouds in the Earth’s magnetosheath. Journal of Geophysical Research: Space Physics, 122, 2956–2972. https://doi.org/10.1002/2016JA023654

MAGNETOSPHERE–IONOSPHERE RESPONSES TO UPSTREAM TRANSIENTS AND MAGNETOPAUSE RECONNECTION  53 Turc, L., Fontaine, D., Savoini, P., & Kilpua, E. K. J. (2014). Magnetic clouds’ structure in the magnetosheath as observed by Cluster and Geotail: Four case studies. Annales Geophysicae, 32, 1247–1261. https://doi.org/10.5194/angeo‐ 32‐1247‐2014 Turner, D. L., Eriksson, S., Phan, T. D., Angelopoulos, V., Tu, W., Liu, W., et al. (2011). Multispacecraft observations of a foreshock‐induced magnetopause disturbance exhibiting distinct plasma flows and an intense density compression. Journal of Geophysical Research, 116, A04230. https://doi. org/10.1029/2010JA015668 Turner, D. L., Omidi, N., Sibeck, D. G., & Angelopoulos, V. (2013). First observations of foreshock bubbles upstream of Earth’s bow shock: Characteristics and comparisons to HFAs. Journal of Geophysical Research: Space Physics, 118, 1552–1570. https://doi.org/10.1002/jgra.50198 Vörös, Z., Yordanova, E., Varsani, A., Genestreti, K. J., Khotyaintsev, Y. V., Li, W., et al. (2017). MMS observation of magnetic reconnection in the turbulent magnetosheath. Journal of Geophysical Research: Space Physics, 122, 11,442– 11,467. https://doi.org/10.1002/2017JA024535 Wang, B., Nishimura, Y., Hietala, H., Lyons, L., Angelopoulos, V., Ebihara, Y., & Weatherwax, A. (2018b). Impacts of magnetosheath high‐speed jets on the magnetosphere and ionosphere measured by optical imaging and satellite observations. Journal of Geophysical Research: Space Physics, 123. https:// doi.org/10.1029/2017JA024954 Wang, B., Nishimura, Y., Hietala, H., Shen, X.‐C., Shi, Q., Zhang, H., et al. (2018a). Dayside magnetospheric and ionospheric responses to a foreshock transient on June 25, 2008: 2. 2‐D evolution based on dayside auroral imaging. Journal of Geophysical Research: Space Physics, 123, 6347–6359. Wang, B., Nishimura, Y., Zou, Y., Lyons, L. R., Angelopoulos, V., Frey, H., & Mende, S. (2016). Investigation of triggering of poleward moving auroral forms using satellite‐imager coordinated observations. Journal of Geophysical Research: Space Physics, 121, 10,929–10,941. https://doi.org/10.1002/2016JA023128 Wild, J. A., Cowley, S. W. H., Davies, J. A., Khan, H., Lester, M., Milan, S. E., et al. (2001). First simultaneous observations of flux transfer events at the high‐latitude magnetopause by

the Cluster spacecraft and pulsed radar signatures in the conjugate ionosphere by the CUTLASS and EISCAT radars. Annales Geophysicae, 19, 1491–1508. Zelenyi, L., & Artemyev, A. (2013). Mechanisms of spontaneous reconnection: From magnetospheric to fusion plasma. Space Science Reviews, 178, 441–457. https://doi.org/10.1007/ s11214‐013‐9959‐8 Zhang, H., Sibeck, D. G., Zong, Q.‐G., Gary, S. P., McFadden, J. P., Larson, D., et  al. (2010). Time history of events and macroscale interactions during substorms observations of a series of hot flow anomaly events. Journal of Geophysical Research, 115, A12235. https://doi.org/10.1029/2009JA015180 Zhang, H., Sibeck, D. G., Zong, Q.‐G., Omidi, N., Turner, D., & Clausen, L. B. N. (2013). Spontaneous hot flow anomalies at quasi‐parallel shocks: 1. Observations. Journal of Geophysical Research: Space Physics, 118, 3357–3363. https://doi.org/10. 1002/jgra.50376 Zhao, L. L., Zhang, H., & Zong, Q. G. (2017). Global ULF waves generated by a hot flow anomaly. Geophysical Research Letters, 44, 5283–5291. https://doi.org/10.1002/2017GL073249 Zhou, X.‐. Y., Fukui, K., Carlson, H. C., Moen, J. I., & Strangeway, R. J. (2009). Shock aurora: Ground‐based imager observations. Journal of Geophysical Research, 114, A12216. https://doi.org/10.1029/2009JA014186 Zhou, X.‐Y., Strangeway, R. J., Anderson, P. C., Sibeck, D. G., Tsurutani, B. T., Haerendel, G., et al. (2003). Shock aurora: FAST and DMSP observations. Journal of Geophysical Research: Space Physics, 108(A4), 8019. https://doi. org/10.1029/2002JA009701 Zou, Y., Walsh, B. M., Nishimura, Y., Angelopoulos, V., Ruohoniemi, J. M., McWilliams, K. A., & Nishitani, N. (2018a). Local time extent of magnetopause reconnection using dual‐satellite and ground coordination. Annales Geophysicae, 37(2), 215–234. Zou, Y., Walsh, B. M., Nishimura, Y., Angelopoulos, V., Ruohoniemi, J. M., McWilliams, K. A., & Nishitani, N. (2018b). Spreading speed of magnetopause reconnection X‐lines using ground‐satellite coordination. Geophysical Research Letters, 45, 80–89. https://doi.org/10.1002/ 2017GL075765

4 Dayside Magnetospheric Interactions Inferred from Dayside Diffuse Aurora and Throat Aurora De‐Sheng Han

ABSTRACT Optical aurora can be classified into two broad categories, that is, diffuse and discrete auroras. Based on optical observations obtained on the dayside, some recent studies on the dayside auroras are reviewed in this paper. It has been revealed that dayside diffuse auroras (DDAs) can be classified into unstructured and structured DDAs. The structured DDAs observed in the ionospheric convection throat region are often involved with a newly defined discrete auroral form named throat aurora. Generation of throat aurora shows dependence on factors either inside or outside the magnetosphere. A conceptual model has been proposed for explaining these ­observational results. 4.1. INTRODUCTION

observations in different colors, we will be able to acquire two‐dimensional and dynamical information about energy distribution and number flux of the precipitation electrons associated with aurora. Therefore, auroral observations are very useful for studying space physical processes, while the auroras observed on the dayside are much informative for understanding the dayside magnetospheric interactions. Here, we focus on reviewing some recent results about dayside diffuse aurora (DDA) and throat aurora based on optical observations obtained on the dayside. Optical auroras observed on the ground have been classified into two broad categories, namely discrete and diffuse auroras, which are different in both morphological and physical properties (Akasofu, 1974). Morphologically, the discrete auroras can be in forms of arc, ray, band, or curl that are characterized by intense luminosity and clear boundaries, whereas the representative properties of diffuse aurora are weak and homogeneous luminosity in the auroral region. Physically, the source particles for discrete auroras can be from either open or closed field line regions and are suffered by field‐aligned acceleration, whereas the source particles for diffuse auroras are from the central plasma sheet (CPS) and are involved in wave‐ particle interactions. Figure 4.1 presents an example for

Auroras are produced by precipitation of energetic particles into the ionosphere along the magnetic field lines. Upon collisions, the trapped electrons belonging to the ionospheric atoms or molecules can be excited from lower‐ to higher‐energy states. When an excited electron deexcites back to the lower state, a photon at characteristic wavelength will be emitted. If the photon flux is large enough, it will display as visible light, which is aurora. Based on this principle, we know that the auroral intensity reflects the magnitude of photon flux, which can be used to infer the number flux of precipitation electrons for producing the aurora, while frequencies, that is, colors, of the emitted photons are decided by the energy differences between the excited state and the ground state. How high‐energy state of the trapped electrons can be excited up to is statistically determined by the energy distribution of precipitation electrons, so the relative ­ intensity between different colors can be used to estimate the energy distribution of precipitation particles (Sandholt et al., 2002). Using continuous auroral imaging State Key Laboratory of Marine Geology, School of Ocean and Earth Science, Tongji University, Shanghai, China

Dayside Magnetosphere Interactions, Geophysical Monograph 248, First Edition. Edited by Qiugang Zong, Philippe Escoubet, David Sibeck, Guan Le, and Hui Zhang. © 2020 American Geophysical Union. Published 2020 by John Wiley & Sons, Inc. 55

56  DAYSIDE MAGNETOSPHERE INTERACTIONS 2006-12-10 07:51:00 DMSP orbit trace M.N

13 Electrons Log JE

12 IONS

F17

9 DMSP location 4 LOG E AVE

8 4

Diffuse aurora 2

2

M.S 4

M.N

8

7

7

6

6

5

5

4

3 2

2 IONS

Red line

Log energy (eV)

ELEC

Green line

Log E flux Elec ion 9 8

3

Discrete aurora 4

M.S

07:50:03 UT MLAT 76.6 GLAT 80.1 GLONG 29.2 MLT 12.004

07:50:31 76.4 79.2 21.2 11.5555

07:51:01 76.1 78.1 14 11.0936

07:51:29 75.6 77 8.4 10.6891

07:51:57 74.9 75.7 3.8 10.3159

07:52:27 74 74.3 359.6 9.9537

07:52:55 73.1 72.9 356.3 9.6507

3

Dec 10

Figure 4.1  An example for optical aurora observed in green (left‐top) and red (left‐bottom) lines and the associated precipitation properties observed by Defense Meteorological Satellite Program (DMSP) satellite. The right panels, from top to bottom, show the total energy flux (eV/cm2 s sr) of electrons (black dots) and ions (red dots), the average energy (eV) of electrons (black dots) and ions (red dots), and spectrograms of differential energy flux of electrons and ions. Please note that the discrete aurora corresponds to high‐energy flux, but unnecessarily corresponds to high energy, while the diffuse aurora corresponds to homogeneous energy flux with relatively higher energy.

the optical auroras observed on the ground and the associated particle precipitations from low‐altitude satellite. Figure 4.1 clearly presents some differences between discrete and diffuse auroras. For example, the diffuse aurora is predominantly observed in the green line (557.7 nm) but is almost absent in the red line (630. nm). It is also seen that the essential difference for discrete and diffuse auroras lies in the number flux of precipitation electron. The structured luminosity of discrete aurora corresponds to structured high number flux (but unnecessarily with high energy), while the homogeneous luminosity of diffuse aurora results from the homogeneous flux of the precipitation electrons that are normally with energy higher than 1.0 keV. In order to make optical auroral observation on the dayside on the ground, the observation site should be under the aurora oval, and, at the same time, be at high geographic latitudes so that there will be no sunlight

c­ontamination when the station turns to the dayside. Figure 4.2 schematically shows the auroral oval above the northern hemisphere, which indicates that Northern America is not fit to make dayside optical auroral observation due to the low geographic latitudes for the coverage region of auroral oval, while Svalbard Island located in north of Europe is one of a few places that can make longtime optical auroral observation at the cusp latitude on the dayside during the boreal winter season on the Earth. Based on observations obtained from Svalbard, the auroral forms observed on the dayside have been systematically studied and classified into seven types by considering their magnetic local time (MLT) distributions, morphological and optical spectral characteristics, and interplanetary magnetic field (IMF) dependences (Sandholt et  al., 1998), among which the type 3 is diffuse aurora and all other types are discrete

Dayside Magnetospheric Interactions Inferred from Dayside Diffuse Aurora and Throat Aurora  57

4.2. DAYSIDE DIFFUSE AURORAS 4.2.1. Observational Results About DDA

Figure 4.2  A schematic location for the auroral oval above the northern hemisphere. Please note that the geographic latitude of the auroral oval is rather low in Northern America. When Northern America turns to dayside, the auroral zone is exposed to the sunlight, so we cannot make optical aurora observation on the dayside in Northern America.

auroral forms. Sandholt et al. (1998) indicate that type 3 aurora is a zone of quasi‐steady green line emission extending over all dayside local times and is situated at the lowest magnetic latitudes. Sandholt et al. (2002) suggested that the type 3 aurora is caused by precipitation of energetic electrons which drift azimuthally from the plasma sheet from the midnight sector to the dayside magnetopause during magnetospheric substorms. Besides the optical observations, occurrence of DDA has been early noticed by using particle observations from low‐altitude satellites (Meng et  al., 1979; Newell et al., 2004). However, a detailed study on DDA has not been done until recent years. Throat aurora is a special discrete auroral form that is observed only near the ionospheric convection throat region and is newly noticed during an extensive study on DDA. That is why we discuss them together here. Chinese Yellow River Station (YRS) is situated at Ny‐ Alesund in Svalbard. Since November 2003, an optical observation system consisting of three identical all‐sky imagers supplied with the narrow band filters centered at 427.8, 557.5, and 630. nm has been installed at YRS (Hu et al., 2009). Most of the results summarized in this paper are achieved based on the observations from YRS.

4.2.1.1. Unstructured and Structured DDAs Although diffuse aurora is typically characterized as an auroral region with relatively homogenous luminosity in large area, it also often presents as clear structures (Lui et al., 1973). By using the longtime continuous observations at YRS, DDAs are classified into two broad classes according to their morphological properties, which are called unstructured and structured DDAs (Han et  al., 2015) and their typical examples are shown in Figures 4.3 and 4.4, respectively. Figure  4.3a illustrates a typical unstructured DDA, which appears like a veil of light blanketing the full of field of view and thus was called veiling diffuse aurora (Kimball & Hallinan, 1998). Sometimes black auroral structures can be observed on the background of veiling diffuse aurora, which are also regarded as unstructured DDAs and an example is shown in Figure 4.3b. The veiling DDAs can change their forms within a few minutes and are associated with slow drifting. Sometimes the veiling DDAs are also pulsating. Figure 4.3c presents another kind of unstructured DDA, which is often observed in afternoon and is called afternoon diffuse band by Han et  al. (2015). The afternoon diffuse band is normally adjacent to the equatorward of the discrete auroral oval and often keeps stable for rather long time (normally more than tens of minutes). Note that the time interval between two images in Figure  4.3c is 10 minutes, but those in Figure 4.3a and b are 1 minute. The afternoon diffuse bands are mostly observed 1400–1800 MLT and can be observed both in the green (557.7 nm) and red (630. nm) lines. They can slowly drift, but seldom show pulsating. Han et al. (2015) summarized that the structured DDAs include patchy, stripy, and irregular forms. Their typical cases are illustrated in Figure 4.4a, b, and c, respectively. It has been early noticed that the patchy diffuse aurora often appears at the postmidnight sector during the substorm recovery phase (Akasofu, 1974). It is also reported that the diffuse patches have spatial scales varying from 10 to ~200 km and are mostly accompanied by pulsations with a period of 0.3–30 seconds (Royrvik & Davis, 1977). Han et al. (2015) noticed that the diffuse patches can be often observed on the dayside, especially near the magnetic local noon (MLN). Figure  4.4a presents a typical patchy DDA observed from 08:05:40 to 08:06:40 UT on 15 November 2007, which is ~200‐km width in east‐west direction and drifts westward (as indicated by the white arrow) with a speed of ~5.7 km/s by assuming the auroral height of 150 km.

58  DAYSIDE MAGNETOSPHERE INTERACTIONS 05:58:50

M.N

05:59:50

Veiling DDA

M.N

06:00:50

Veiling DDA

(kR) 3.5

Veiling DDA

(a) Veiling

M.N 2.8

E

W E

Discrete aurora

W E

W

2.1 1.4 0.7

2006−11−29 M.S

2006−11−29 M.S

2006−11−29 M.S

04:55:50

04:56:50

04:57:50

(b) Veiling with black E

M.N

Black aurora

M.N

M.N

(kR)

W E

0.8 W

0.6 0.4 0.2

Veiling diffuse 2007−11−12 M.S 13:00:10

1

Black aurora

Black aurora W E

0

M.N

2007−11−12 M.S

2007−11−12 M.S

13:10:10

13:20:10

M.N

0

M.N

(kR) 0.8

(c) Diffuse E band

W E

W E

Discrete aurora

0.64 W 0.48 0.32

Diffuse band 2007−12−06 M.S

Diffuse band 2007−12−06 M.S

2007−12−06 M.S

Diffuse band

0.16 0

Figure 4.3  (a)–(c) Typical examples for unstructured dayside diffuse auroras (DDAs) observed at Yellow River Station (YRS). Magnetic local time (MLT) at YRS ~= UT+3 hours. Source: From Han et al. (2015).

Figure 4.4b shows an example of stripy DDA. The most outstanding feature of stripy DDA is the orientation. We know that the auroral oval is normally east‐west aligned due to the precipitation particles predominantly distributed along the same L shell. However, in Figure 4.4b, the stripy DDAs are approximately along the southwest‐to‐ northeast direction, namely crossing different L shells. If we magnetically map the stripy DDAs into magnetosphere, they should be correspondent to wedge‐like structures radially extending from inner to outer magnetosphere. How these structures can be produced is not clear, but they are very informative for understanding the dayside magnetospheric processes as discussed below. Figure 4.4c shows an example for irregular DDA, whose shape is neither in patchy nor in stripy. Han et al. (2015) suggested that the irregular DDA should be a special stripy DDA. 4.2.1.2. Statistical Properties of Unstructured and Structured DDAs Han et al. (2015) statistically examined the occurrence of different types of DDAs distributed with MLT by

using 7 continuous years of winter time operations and the results are shown in Figure 4.5. On doing the statistic, the data are divided into 10‐minute segments at first and  the DDA types are visually confirmed in each segment. In Figure 4.5, the occurrence of pulsating DDA is also considered. The blue, yellow, and red bars in panel (a) show the number of the total observation, of the all DDA events (including unstructured and structured DDAs), and of the pulsating DDA, respectively. Panel (b) shows the occurrence rates of DDA and pulsating DDA, which are obtained by the event number divided by the total observation. The distribution of the occurrence of unstructured, structured, patchy and irregular, and stripy DDAs are given in panels (c), (d), (e), and (f), respectively. Figure  4.5 presents a clear result that the unstructured DDA presents the minimum occurrence near MLN (panel (c)), where the structured DDA shows the maximum occurrence (panel (d)). Besides the clear dependence of occurrence on MLT, another interesting statistical result about DDA is the alignments of the stripy DDAs. Figure 4.6 indicates that the stripy DDAs are statistically aligned along

Dayside Magnetospheric Interactions Inferred from Dayside Diffuse Aurora and Throat Aurora  59 08:05:40

08:06:10

E

M.N

08:06:40

M.N

Patchy DDA

Patchy DDA

(a) Patchy

M.N

W E

(kR) 2

Patchy DDA W E

1.6 W

1.2 0.8 0.4 0

2007−11−15 M.S

2007−11−15 M.S

2007−11−15 M.S

08:41:01

08:42:01

08:43:01

(b) Stripy

M.N

Stripy DDA E

M.N

(kR) 2 1.6

Stripy DDA

Stripy DDA

W E

M.N

W E

W

1.2 0.8 0.4 0

2003−12−26 M.S

2003−12−26 M.S

07:50:10

07:51:10

E

M.N

07:52:10

W E

M.N Irregular DDA

Irregular DDA

Irregular DDA

(c) Irregular

M.N

2003−12−26 M.S

W E

W

(kR) 2 1.6 1.2 0.8 0.4 0

2007−12−06

M.S

2007−12−06

M.S

2007−12−06

M.S

Figure 4.4  (a)–(c) Typical examples for structured DDAs observed at YRS. Source: From Han et al. (2015).

southwest‐to‐northeast, southeast‐to‐northwest, and south‐to‐north directions before, after, and at MLN, respectively. Figure 4.7 presents an example observed on 2 December 2007 to show the change of the stripe’s alignment around MLN. In each image, the discrete auroral oval is at poleward of the field of view (FOV). In Figure  4.7, the diffuse stripes align along southwest‐to‐ northeast, south‐to‐north, and southeast‐to‐northwest directions around ~1135 MLT, ~1154 MLT, and ~1216 MLT, respectively, which notably conform to the statistical results presented in Figure 4.6. 4.2.1.3. Two Types of Structured DDAs Observed near Magnetic Local Noon Recently, a detailed study further reveals that there exist two types of structured DDAs near MLN with obviously different dynamical properties. Type 1 usually drifts from low to high latitude with higher speed and shows pulsation. These dynamic properties are consistent with the structured patchy DDA as defined in Han et  al. (2015). Type 2 is always adjacent to discrete aurora oval and drifts together with nearby discrete aurora with much lower speed. Figure 4.8 shows two examples for ­coexisting

of the two types of DDAs in the same FOV by two cases observed at YRS on 27 November 2008 (upper) and on 23 December 2003 (bottom), respectively. The auroral images have been mapped to geomagnetic coordinates at 150 km altitude. In Figure  4.8, type 1 DDAs are indicated by yellow arrows, which are normally in stripy or patchy forms and show fast drifting or pulsating. Type 2 DDAs are indicated by red arrows, which are always adjacent to the discrete aurora oval and generally drift together with the nearby discrete aurora with much lower speed compared with the drifting speed of type 1. In Figure 4.8, the directions of the arrows approximately indicate the drifting directions of the DDAs. Detailed differences in the dynamic properties for the two types of DDAs can refer to the auroral movies provided in Supporting Information (SI) in Han et al. (2017b). Using coordinated observations from magnetospheric multiscale (MMS) satellites near the magnetopause and from ground‐based all‐sky camera at YRS, Figure  4.9 shows that the two types of diffuse auroras are well correlated with number density increase of O+ and of He2+ ions, respectively, as shown in panels (e) and (g). Because

60  DAYSIDE MAGNETOSPHERE INTERACTIONS

Number

200 100 50

Percentage

30

Number

(c)

Unstructured

60 30 90

Number

Occurrance rate of DDA Occurrance rate of pulsating

(b)

60

90

(d)

Structured

(e)

Patchy and irregular

60 30 90

Number

All DDA Pulsating

150

90

60 30 90

Number

(a)

(f)

Stripy

60 30

UT 03 ~MLT 06

04 07

05 08

06 09

07 10

08 11

09 12

10 13

11 14

12 15

13 16

14 17

15 18

Figure 4.5  Statistical results for the DDAs, in which (a) shows distributions for all the observation, DDA, and pulsating DDA, (b) is occurrence rate of DDA and pulsating DDA, (c)–(f) are distributions of unstructured DDA, structured DDA, patchy and irregular DDAs, and stripy DDA, respectively. Source: From Han et al. (2015).

it is generally accepted that O+ ions are originated from the ionosphere whereas highly charged ion species, such as He2+ ions, are originated in the solar wind, Figure 4.9i implies that generation of the type 1 and type 2 DDAs may be related with particles originated from inside and outside the magnetosphere, respectively. In addition, Figure  4.9 also shows that electron cyclotron harmonic (ECH) waves are observed associated with the DDAs. Recently, using coordinated observations from Time History of Events and Macroscale Interactions During Substorms (THEMIS) and all‐sky camera at South Pole station, Wang et  al. (2018) showed that magnetosheath high‐speed jets (HSJs) were well associated with the localized diffuse auroral brightening observed at South Pole station near local noon. This provides further evidence for existence of the Type 2 diffuse aurora near local noon.

4.2.2. Understanding the Observational Properties of DDA On considering the information that can be inferred from the observational results of DDA, we need to refer the processes for generation of diffuse aurora. It has been generally accepted that diffuse aurora is caused by precipitation of CPS electrons (Meng et al., 1979) that are scattered by whistler mode chorus (Ni et al., 2011b; Nishimura et  al., 2010; Thorne et  al., 2010) or ECH (Horne & Thorne, 2000; Liang et  al., 2011; Ni et  al., 2011a; X. J. Zhang et al., 2014) waves into the loss cone. This means that there are two key factors for generation of diffuse auroras. One is the source particles, which are electrons from the CPS. Another is the waves, that is, chorus or ECH waves, which can scatter the electrons into loss cone.

Dayside Magnetospheric Interactions Inferred from Dayside Diffuse Aurora and Throat Aurora  61

Number

20

M. N.

15 10

E

W

5

M. S.

Number

20

M. N.

15 10

E M. S.

Number

20

M. N.

15 E

M. S.

Number

20

M. N.

15 E

East to west W

5

UT 03 ~MLT 06

South to north W

5

10

Southeast to northeast W

5

10

Southwest to northeast

M. S. 04 07

05 08

06 09

07 10

08 11

09 12

10 13

11 14

12 15

13 16

14 17

15 18

Figure 4.6  Statistical results on the alignments of the stripy DDAs, which indicate that stripy DDAs are aligned along the southwest‐to‐northeast, southeast‐to‐northwest, and south‐to‐north directions before, after, and at the magnetic local noon, respectively. Source: From Han et al. (2015).

08:33:10

M.N

08:35:10

M.N

08:37:10

(a) E MLT ~1135

Auroral oval

Stripy DDA

W E

M.N

1.5

(kR)

Stripy DDA

W E

W 1.2

Stripy DDA

2007-12-02 M.S

2007-12-02 M.S

2007-12-02 M.S

08:52:10

08:54:10

08:56:10

M.N

M.N

M.N

0.9

(b) Stripy DDA

Stripy DDA

MLT ~1154

E

W E

W E

Stripy DDA

W 0.6

2007-12-02 M.S 09:14:10

M.N

2007-12-02 M.S 09:16:10

M.N

2007-12-02 M.S 09:18:10

M.N 0.3

(c) Stripy DDA

MLT E ~1216

W E

Stripy DDA

Stripy DDA

W E

W 0

2007-12-02 M.S

2007-12-02 M.S

2007-12-02 M.S

Stripy DDA observed on 2 December 2007

Figure 4.7  (a)–(c) An example observed on 2 December 2007 to show that the stripy DDA’s alignment is consistent with the statistical results as shown in Figure 4.6. Source: From Han et al. (2015).

62  DAYSIDE MAGNETOSPHERE INTERACTIONS Case 1 on 27 November 2008

Case 2 on 23 December 2003

Figure 4.8  Two examples for coexisting of the two types of DDAs (as marked by red and yellow arrows) in the same field of view by two cases observed at YRS on 27 November 2008 (upper) and on 23 December 2003 (bottom), respectively. Source: From Han et al. (2017b).

Type 1

By GSM

1000

107

100

106

10 000

107

105

Sheath-like signature

106

1 000

105

(h)

O + density (cm–3)

10 000 100

1 000 100

10

10 1 0.10 0.08

1

O+ density increase

0.06

N (O+)

0.04 0.02 0.00 10 000

1000

1 000

100

100

10

10

1

1

0.5 0.4 0.3

He++ density increase

0.2

N (He++)

0.1 0.0 3 000 2 800 2 600 2400 2 200 2 000 10 000

HFEsp (Hz)

(i)

104 103

1 000

10–12

fce

10–14

0.5*fce

10–16

10 000

Bpsd (Hz)

(j)

2*fce fce

1000

0.5*fce

100 10 mms1 Xgsm 9.3 mms1 Ygsm

–2.7

10–10

2*fce

Wed Apr 26 14:47:57 2017

(g)

He++ energy (eV) ELEV 0-360

(f)

He++ density (cm–3)

(e)

Aurora relative intensity

(d)

O+ energy (eV) ELEV 0–360

100

He++ Flux (cm2 ssr eV)–1

Ion (eV)

(c)

(keV/ (cm2 s sr keV))

108

O+ Flux (cm2 ssr eV)–1

10 000

(keV/ (cm2 s sr keV))

Bx GSM

9.3

9.3

9.2

9.1

9.1

9.0

9.0

8.9

8.8

–2.7

–2.6

–2.5

–2.5

–2.4

–2.3

–2.2

–2.2

–2.1

mms1 Zgsm

–0.8

–0.8

–0.8

–0.8

–0.8

–0.8

–0.8

–0.8

–0.7

–0.7

hhmm

0845

0850

0855

0900

0905

0910

0915

0920

0925

0930

(V/m)2/Hz

Electron (eV)

(b)

Bz GSM

10–3 10–4 10–5 10–6 10–7 10–8 10–9 10–10

2016 Jan 08

Figure 4.9  Observations from magnetospheric multiscale 1 (MMS1) on 8 January 2016. Panels (a)–(j) show the magnetic field, averaged energy spectrograms of electron and ions, energy spectrogram and number density of O+ and of He2+, luminosity of aurora averaged by 4 × 4 pixels at footprint of MMS1, power spectrum density of electric field and of magnetic field, respectively. Frequency of electron cyclotron (fce), 0.5 × fce, and 2.0 × fce are superimposed. Source: From Han et al. (2017b).

nT2/Hz

MMS1 FGM (nT)

(a)

Type 2

80 60 40 20 0 –20 –40 –60

64  DAYSIDE MAGNETOSPHERE INTERACTIONS

Based on these two factors, we can well explain the observational properties of DDA. Firstly, the source particles for diffuse auroras, no matter for dayside or nightside ones, are hot electrons from CPS with substantial fluxes and with certain types of pitch angle anisotropy, so they can provide free energy for the growth of the chorus or ECH waves (Horne et al., 2003). At the same time, because these electrons are originated from the earthward particle injection during substorm on the nightside (Newell et  al., 2009), they are drifting around the Earth, electrons to the east and protons to the west. Associated with the electrons drifting from midnight toward dawn and up to noon sectors, some of them can be scattered into loss cone by wave‐ particle interaction and produce diffuse aurora, thus the electron density is gradually decreased. This is fundamental reason for the observational fact that diffuse aurora is the most intense in postmidnight and the intensity is gradually decreasing toward dawn and noon sectors (Newell et al., 2004). On considering the distribution of unstructured and structured DDAs shown in Figure 4.5, we should understand that the essential differences for the two categories of DDAs lie in the size of the diffuse auroral region. If the auroral region is large enough, it will be full of the FOV of camera and thus be observed as “unstructured.” Otherwise, if the diffuse auroral region is not so large and its shape can be identified in the FOV, it will be observed as “structured.” In Figure 4.5, the DDAs observed in the morning are predominantly “unstructured.” This can be simply understood as that although the density of source particle is decreasing from midnight toward noon, it is still high in the morning, so the precipitation can produce diffuse aurora in large area and is often observed as “unstructured.” As for why the DDAs near MLN that is often become “structured,” we need to refer some early studies on how the structured diffuse auroras are generated. On the night side, structured diffuse aurora, mainly in patchy forms, have been early noticed and proposed to be magnetically conjugated to enhanced cold plasma structures in the magnetosphere. The dimension and motion of the cold plasma structure control the dimension and motion of the diffuse auroral patches (Davidson & Chiu, 1986; Demekhov & Yu., 1994; Liang et al., 2015). On the dayside, it has been observed that structured diffuse auroras are conjugated with cold plasma density enhancements (Ebihara et  al., 2007; Han et  al., 2017b; Nishimura et al., 2013). The cold plasma structure plays a crucial role on generation of structured diffuse aurora by decreasing the energy threshold and increasing the growth rate of whistler cyclotron instability (Brice & Lucas, 2012; Li et al., 2011). This means that existence of the cold plasma structure will enable some particles, whose energy is initially lower than the resonance

threshold, to be able to take part in wave resonance and thus be scattered into the loss cone. Because auroral intensity is proportional to precipitation number flux, more particle precipitation means stronger auroral intensity. Therefore, we can understand the structured DDAs observed near MLN based on following facts. Because the number density of CPS electrons is decreasing toward MLN, although some of them still can be scattered into loss cone and produce diffuse aurora, the auroral intensity may be too weak to be visible in optical observation in general. Under such condition, if there is a cold plasma structure in the magnetosphere, more CPS electrons passing through this structure will be scattered into the loss cone due to decreasing the resonance threshold of energy by the cold plasmas. Thus, the auroral intensity in the region magnetically conjugate to the cold plasma structure will be increased and can be observed as “structured” DDA. This could be the most reasonable explanation for the fact that structured DDAs are predominantly observed near MLN. It has been early noticed that cold plasma structures are common in the dayside outer magnetosphere (Chappell, 1974; Chen & Moore, 2006). Based on the profile of low temperature and high density observed during satellites passing through the dayside outer magnetosphere, the cold plasma distributions are suggested to be lumps or blobs. Their origins are suggested to be from plasmaspheric drainage plumes (Borovsky & Denton, 2008; Lin et al., 2007) or from ionospheric outflows (Andersson et al., 2005; Lee et al., 2015). According to the above explanation and considering all the DDAs being generated in the closed field line region, we will be able to infer more information about the cold plasmas in the dayside outer magnetosphere based on the two‐ dimensional observations of DDAs. Figure 4.4 shows that structured DDAs are mainly in patchy, stripy, and irregular forms, while the irregular DDA can be regarded as deformed stripy DDA. Han et  al. (2015) suggested that the patchy DDAs are correspondent to cold plasma lumps or blobs, but the stripy DDAs should correspond to wedge‐like cold plasma structures. Because the stripy DDAs are approximately north‐south aligned, the wedge‐like structure is assumed to be extending from low to high L shells. Most importantly, because the orientation of stripy DDA is consistent with the ionospheric convection flow direction, generation of the wedge‐like cold plasma structures should be closely related with the large‐scale convection. This observational fact is very informative and Han et al. (2017a) proposed the wedge‐like cold plasma structures may be directly resulted from the ionospheric outflows. Because the solar extreme ultraviolet (EUV) can produce dense plasma in the midlatitude ionosphere, it can act as a reservoir of source plasma for generation of a tongue

Dayside Magnetospheric Interactions Inferred from Dayside Diffuse Aurora and Throat Aurora  65

of ionization (Foster et al., 2005) or a poleward‐moving plasma concentration enhancement (PMPCE) (Q. H. Zhang et  al., 2013) by convection, while the enhanced plasma concentration may associate with enhanced ionospheric outflows (Q. H. Zhang et al., 2016b). Therefore, a PMPCE is expected to produce a wedge‐like cold plasma structure in the magnetosphere. Han et  al. (2017a) suggest that this may be the most possible scenario for generation of the wedge‐like cold plasma structure, although it still needs to be experimentally confirmed. On considering the occurrence of DDA dependent on the magnetic activities, the results of Figure 4.8 in Han et al. (2015) only presented the event number of diffuse aurora depending on Kp‐index, but did not calculate the normalized occurrence rate. A study on investigating the normalized occurrence rate should be carried out in the future. Figure  4.8 presents examples for two types of structured DDAs, named type 1 and type 2, which have obviously different dynamic properties and are simultaneously observed in the same FOV of all‐sky camera near MLN. By integrating the coordinated in‐situ and ground observations as shown in Figure 4.9, Han et al. (2017b) suggested that the type 2 is associated with magnetosheath particles penetrated into the magnetosphere, because they can also play roles for decreasing the energy threshold and providing anisotropy for wave resonance due to their temperature being lower than that of CPS electrons. This study implies that the ground‐based optical auroral observation can be used to monitor the magnetosheath particles penetrating into the magnetosphere by providing two‐dimensional and dynamic information. 4.3. THROAT AURORA 4.3.1. Observational Properties of Throat Aurora 4.3.1.1. A Typical Example During study on DDAs, Han et al. (2015) noticed that when the north‐south‐aligned stripy diffuse aurora is contacting with the east‐west‐aligned discrete auroral oval, a discrete aurora structure can be often observed along the stripy diffuse aurora. Because this kind of auroral form was observed around the ionospheric convection throat region, it was named “throat aurora.” We should note that although throat aurora is initially observed together with stripy diffuse aurora, this cannot be used as a criterion for selecting throat aurora cases. The most representative observational feature of throat aurora is that a discrete aurora structure extends from the persistent east‐west‐ aligned auroral oval toward low latitude. Figure 4.10a shows a typical example of throat aurora observed by all‐sky camera. In Figure 4.10, the discrete auroral oval, a stripy diffuse aurora, and a black aurora

region are also observed. These observational features, as well as the relative processes revealed by low‐altitude satellite observations (Han et al., 2016), are schematically illustrated in Figure 4.10b. Throat auroras should be the same as the “crewcuts” defined by Rodriguez et al. (2012). However, because Rodriguez et al. only used the red line (630. nm) observation and they were not able to recognize how the occurrence of throat aurora (crewcuts) is related with diffuse auroras. North‐south‐aligned auroral forms have been previously observed inside the polar cap, such as, theta aurora (Frank et al., 1986) or polar cap arcs (Kullen, 2012). It is the property of equatorward extending from the equatorward edge of auroral oval that makes the throat aurora to be different from the previously defined north‐south‐ aligned auroral forms. 4.3.1.2. The Source Particles Discrete auroras observed near MLN have been confirmed to be caused by particles originated from the magnetosheath and are on the open field lines (Lockwood, 1997; Mende et al., 2016).The equatorward boundary of the discrete auroral oval has been taken as the open‐ closed field line boundary near MLN (Lockwood & Moen, 1996). Because throat aurora is discrete aurora and is appeared as deformation of the equatorward edge of the discrete auroral oval, throat aurora has been supposed to be the ionospheric signature of an indentation of the magnetopause open‐closed field line boundary. This assumption was indirectly confirmed by checking the source region of precipitation particles for throat aurora by Han et  al. (2016). Figure  4.11 shows two parallel throat auroras observed by all‐sky cameras in green (left‐top) and red (left‐bottom) lines when Defense Meteorological Satellite Program (DMSP) F17 was passing through the throat auroras as indicated by the yellow curve. Right panel of Figure 4.11 shows the IMF conditions, magnetic field components, and energy spectra for electron and ions. In Figure 4.11, the energy spectrogram for electron observed by DMSP F17 presents clear magnetosheath properties at the moments “a” and “b” when the satellite was just passing through the throat auroras and it shows clear CPS properties at other time period. Other cases in Han et al. (2016) also show that the precipitation particles for throat aurora are originated from the magnetosheath. These observational facts strongly imply that throat auroras may correspond to indentations on the subsolar magnetopause. 4.3.1.3. The Spatial Scale Figure  4.12 presents an example to show how large throat aurora can be identified in the ground‐based all‐ sky camera. In Figure 4.12a, the equatorward boundary of the discrete auroral oval and the throat aurora outline

66  DAYSIDE MAGNETOSPHERE INTERACTIONS (a)

(b) Poleward

Discrete auroral oval near magnetic Iocal noon: mapping to open LLBL or cusp

Equatorward

Dawn

Dusk Inferred flows Throat aurora: produced by precipitation of magnetosheath particles, mapping to magnetopause reconnection, and corresponding to an upward FAC sheet.

Stripy diffuse aurora: produced by interaction of cold plasma structure with plasma sheet electrons, and supposed to be mapped to a wedgelike cold plasma structure in the dayside outer magnetosphere.

Upward

and downward

FACs,

viewed from ground to the ionosphere

E Black auroral region: corresponding to a downward FAC sheet. Inferred diffuse aurora: expected to be generated by interaction of cold plasmas penetrated from the magnetosheath with the plasma sheet electrons.

Figure 4.10  (a) A typical example of throat aurora observed by all‐sky camera, and (b) the inferred processes based on low‐altitude satellite observations. Source: From Han et al. (2016).

are marked out by the red and black dots, respectively. After mapping these dots into the geomagnetic equatorial plane, as shown in Figure  4.12c, we see that the throat aurora correspond to a magnetopause indentation with ~3.0 RE in radial direction and ~2.0 RE in azimuthal direction. Apparently, this is a rather large indentation. When five THEMIS spacecraft consecutively traversed the dayside (13.5 MLT) magnetopause, Øieroset et  al. (2008) observed an extended region of nearly stagnant magnetosheath plasma attached to the magnetopause on closed field line region and explained the results as substantial solar wind entry across the dayside magnetopause. We believe that Øieroset et al. (2008) may present a typical case for a large magnetopause indentation observed by satellites.

4.3.1.4. The Occurrence Frequency Throat auroras are suggested to be correspondent to magnetopause indentations. The processes for generation of such indentations must be associated with mass and energy coupling between the solar wind and the magnetosphere. If the occurrence frequency of such processes is high, it is expected to have important space weather effects. Therefore, it is meaningful to check how often the throat aurora occurs. Han et  al. (2017a) statistically studied the occurrence of throat aurora and its dependence on MLT and IMF conditions by dividing the 7‐year continuous observations into 10‐minute segments to check if the throat aurora is observed in each segment or not. The occurrence rate in each segment distributed with MLT near MLN is shown in Figure 4.13. We can see

2

08:19:10

Green E(Dusk)

(kR)

1

1

IMF(nT)

2008-12-29

08/364

0

17

–1

M.N

–2

0.8

a

Bx By Bz

07:55

08:02

08:09

08:16

08:24

60

Stripy diffuse aurora

0.6

40

b

Log E Flux

Upward FAC

ELEC

ssm db(nT)

20 0.4

0 –20 –40

0.2

–60

M.S

13

0

Black auroral region

i

Electrons

ii

iii

8

8

7

7

6

6

5

5

4

Bdown Bvel Bazim

Downward FAC

Upward FAC

ION

9

12

iv

IONS

Log JE

W(Dawn) 2008-12-29 08:19:10

Red

4 a

(kR)

E(Dusk)

2

M.N

1.6

0.8

0.4

ELEC Log energy (EV)

1.2 b

0

Throat aurora

W(Dawn)

b

↓

2 CPS

4 3 2

3

Ion flux enhancement

2 IONS

Black region between the two arcs

↓

2

a

M.S

8

9 4 Log E AVE

Dec 29

3 4

UT MLAT GLAT GLONG MLT

08:18:30 77.1 80.3 23.5 12.1305

08:18:45 77.2 80 19.4 11.9172

08:19:00 77.1 79.4 14.5 11.6389

08:19:15 77 78.9 10.6 11.3961

08:19:30 76.9 78.4 7 11.1576

08:19:45 76.7 77.8 3.7 10.9251

08:20:00 76.5 77.2 0.8 10.7

Figure 4.11  Two parallel throat auroras observed in green (left‐top) and red (left‐bottom) lines and the associated precipitation properties observed by DMSP satellite. The right panels, from top to bottom, show the interplanetary magnetic field (IMF) components, the geomagnetic field observed by DMSP, the total energy flux (eV/cm2 s sr) of electrons (black dots) and ions (red dots), the average energy (eV) of electrons (black dots) and ions (red dots), and spectrograms of differential energy flux of electrons and ions. Source: From Han et al. (2016).

84

°N

2008-12-21 08:17:00

(kR) 2

80

M.North

°N

76 90 W(Dawn)

E(Dusk)

72

°N

1.6

°E

1.2

°N

2°E 0.8 16

68

°N 10 8

0.4

°E

144°E

M.South

126°E Mapping to geomagnetic coordinate

Original auroral image in red line 15

0

Magnetopause

GSM X (Re)

12 9 6 3 0 –10 –8 –6 –4 –2

0

2

4

6

8

10

GSM Y (Re) Mapping to geomagnetic equatorial plane

Figure 4.12  An example of throat aurora observed on 21 December 2008 in the red line emission. The left is the original auroral observation. The equatorward boundary of discrete aurora oval and the throat aurora are outlined by the red and black dots, respectively, which are mapped to the geomagnetic coordinates (middle) and to the geomagnetic equatorial plane (right) by T96 model. Source: From Han et al. (2017a). 120 All observations Throat aurora Occurrence rate

Event number (#)

100

80

60

40

20

UT ~MLT

07:30

08:00

08:30

09:00

09:30

10:00

10:30

10:30

11:00

11:30

12:00

12:30

13:00

13:30

Figure 4.13  Normalized occurrence rate of throat aurora distributed with MLT. Source: From Han et al. (2017a).

Dayside Magnetospheric Interactions Inferred from Dayside Diffuse Aurora and Throat Aurora  69

4.3.1.5. The IMF Dependence Han et al. (2017a) statistically examined the occurrence of throat aurora dependent on the IMF conditions in detail. Figure 4.14a–e shows the dependence on the IMF Bx, By, Bz, clock angle (atan(By/Bz)), and cone angle (acos(|Bx|/B)), respectively. Here, we focus on two results. One is shown in Figure 4.14c, which indicates that occurrence of throat aurora has little dependence on the IMF

400

40

300

30

200

20

100

10

–10

0

5

(d)

600

50

400

40

300

30

200

20

100

10

–5

0

5

10

By (nT) 400

50

30

300 20

200

10

–10

–5

0

5

10

250

40

200

30

150 20

100

10

50 45

20

150 100

10

0

45

90

135 180

Clock angle [atan(By/Bz)] 50

30

30

200

–180 –135 –90 –45

300

15

250

50

Bz (nT)

0

40

300

Occurrence rate (%)

400

Event Number

Event number

40

100

Event number

50

350 Bz0 Bz0

500

(e)

500

–10

10

Bx (nT)

Occurrence rate (%)

(c)

–5

(b)

Event Number

50

Occurrence rate (%)

500

60

75

Occurrence rate (%)

Event number

(a)

Bz. Another is shown in Figure 4.14e, which presents the clearest dependence that the normalized occurrence rate of throat aurora decreases with increasing of IMF cone angle. The orientation of throat aurora is found to be convection aligned. They are aligned along southwest‐to‐ northeast, south‐to‐north, and southeast‐to‐northwest before, just at, and after MLN, respectively (Han et al., 2017a). In Figure  4.15, Han et  al. (2017a) identified all the time periods when the south‐to‐north‐aligned throat auroras were observed and present the median value of the occurrence time distributed with the IMF By component. Figure 4.15 shows that the occurrence of S‐N throat aurora clearly prefers to occur in the prenoon under By > 0 and in post‐noon under By  0 (By