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Dayside Magnetosphere InteractionsQiugang Zong

Philippe EscoubetDavid Sibeck

Guan LeHui Zhang

Editors

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

Geophysical Monograph 248

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v

CONTENTS

Contributors .........................................................................................................................................................vii

Preface ..................................................................................................................................................................xi

1. A Brief History of Dayside Magnetospheric PhysicsA. 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 2016Hui Zhang and Qiugang Zong ......................................................................................................................13

3. Transient Solar Wind–Magnetosphere–Ionosphere Interaction Associated with Foreshock and Magnetosheath Transients and Localized Magnetopause ReconnectionY. 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 AuroraDe‐Sheng Han ..............................................................................................................................................55

5. Magnetosphere Response to Solar Wind Dynamic Pressure Change: Vortices, ULF Waves, and AuroraeQ. 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 BoundariesPhilippe Escoubet, A. Masson, H. Laakso, and M. L. Goldstein .....................................................................101

7. Structure and Dynamics of the MagnetosheathKatariina Nykyri ..........................................................................................................................................117

8. An Examination of the Magnetopause Position and Shape Based Upon New ObservationsZ. Němeček, J. Šafránková, and J. Šimůnek ..................................................................................................135

9. Methods for Finding Magnetic Nulls and Reconstructing Field Topology: A ReviewH. S. Fu, Z. Wang, Qiugang Zong, X. H. Chen, J. S. He, A. Vaivads, and V. Olshevsky ...................................153

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 MagnetosphereA. S. Leonovich and D. A. Kozlov ................................................................................................................175

11. Ultra-Low-Frequency Wave–Particle Interactions in Earth’s Outer Radiation BeltR. 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 InteractionsW. Li, Q. Ma, J. Bortnik, and R. M. Thorne ...................................................................................................207

13. Current Status of Inner Magnetosphere and Radiation Belt ModelingMei‐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 PlumeJ. 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 ParticlesQiugang Zong, Jie Ren, and X. Z. Zhou .......................................................................................................265

16. Formation and Evolution of Polar Cap Ionospheric Patches and Their Associated Upflows and Scintillations: A ReviewQing‐He Zhang, Zan‐Yang Xing, Yong Wang, and Yu‐Zhang Ma ..................................................................285

17. Dayside Magnetosphere Interactions: Progress in Our Understanding and Outstanding QuestionsQiugang Zong, Philippe Escoubet, David Sibeck, Guan Le, and Hui Zhang .................................................303

Index ..................................................................................................................................................................307

vii

V. AngelopoulosDepartment of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA, USA

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

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

L. B. ClausenDepartment of Physics, University of Oslo, Oslo, Norway

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

A. W. DegelingShandong 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

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

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

Philippe EscoubetESA European Space Research and Technology Centre, Noordwijk, The Netherlands

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

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

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

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

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

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

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

D. A. KozlovInstitute of Solar‐Terrestrial Physics SB RAS, Irkutsk, Russia

H. LaaksoESA European Space Astronomy Centre, Madrid, Spain

Guan LeNASA Goddard Space Flight Center, Greenbelt, MD, USA

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

W. LiCenter for Space Physics, Boston University, Boston, MA, USA

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

CONTRIBUTORS

viii Contributors

Yu‐Zhang MaShandong 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. MassonESA European Space Astronomy Centre, Madrid, Spain

J. I. MoenDepartment of Physics, University of Oslo, Oslo, Norway

T. NagatsumaNational Institute of Information and Communications Technology, Tokyo, Japan

Z. NěmečekFaculty of Mathematics and Physics, Charles University, Prague, Czech Republic

Y. NishimuraDepartment 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 NykyriCentre of Space and Atmospheric Research, Department of Physical Sciences, Embry‐Riddle Aeronautical University, Daytona Beach, FL, USA

V. OlshevskyCenter for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Leuven, Belgium

A. OttoGeophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA

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

R. RankinDepartment of Physics, University of Alberta, Edmonton, Alberta, Canada

Jie RenInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

J. ŠafránkováFaculty of Mathematics and Physics, Charles University, Prague, Czech Republic

X.‐C. ShenShandong 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. ShiShandong 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 SibeckNASA Goddard Space Flight Center, Greenbelt, MD, USA

J. ŠimůnekInstitute of Atmospheric Physics, Czech Academy of Science, Prague, Czech Republic

D. SydorenkoDepartment of Physics, University of Alberta, Edmonton, Alberta, Canada

R. M. ThorneDepartment of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA

A. M. TianShandong 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. VaivadsSwedish Institute of Space Physics, Uppsala, Sweden

B. M. WalshDepartment of Electrical and Computer Engineering, Boston University, Boston, MA, USA

B. WangDepartment of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA; andDepartment of Astronomy and Center for Space Physics, Boston University, Boston, MA, USA

Contributors ix

C. R. WangDepartment of Physics, University of Alberta, Edmonton, Alberta, Canada

Yong WangShandong 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. WangInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

Z. WangSchool of Space and Environment, Beihang University, Beijing, China

G. Whittall-ScherfeeDepartment of Physics, University of Alberta, Edmonton, Alberta, Canada

J. R. WygantDepartment of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA

Zan‐Yang XingShandong 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. YaoShandong 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

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

Qing‐He ZhangShandong 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

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

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

Qiugang ZongInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

Y. ZouDepartment 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

xi

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, magne-tosheath, magnetopause, and cusps; the dayside magneto-sphere; and both the dayside polar and equatorial ionosphere. The meeting was particularly timely due to the results expected from NASA’s magnetospheric multi-scale (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 con-stellation 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 mag-netospheric 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

wind–magnetosphere– ionosphere interactions driven by 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 genera-tions 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 inter-actions 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 magne-tosphere (Zong et al., Chapters 15 and 17).

Over 128 scientists from more than 20 countries partic-ipated 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

PREFACE

1

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.

A Brief History of Dayside Magnetospheric Physics

A. Otto

1

1.1. SETTING THE STAGE: THE PRE‐SATELLITE ERA

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

studying the paths of electrons in a dipole representing Earth, Birkeland was convinced that the aurora and asso-ciated magnetic perturbations were caused by precipi-tating 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 par-ticle 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

Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA

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.

2 DAYSIDE MAGNETOSPHERE INTERACTIONS

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 quali-tatively with most magnetic storm properties particularly for the initial increase of the magnetic field (sudden com-mencement), 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 sug-gested 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 local-ized 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 invis-ible 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 coinci-dent 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 pub-lish 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 partic-ularly 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 magneto-pause 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 magneto-sphere with a bow shock and discussed the stability of the magnetospheric boundary. He also provided the familiar estimate of the magnetopause standoff distance and

Stream

Stream

To the sunEarth

+ + + + +

+

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

A BRIEf HISTORY Of DAYSIDE MAGNETOSPHERIC PHYSICS 3

argued correctly that the magnetic boundary encoun-tered 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 observa-tions. Burlaga and Ogilvie (1968) carried out a detailed comparison of Explorer 34 observations with theoretical shock predictions and found good agreement. Some dif-fuse 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 hydrody-namic 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 frame-work. 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 indi-cations 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 parti-cles 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 merg-ing. 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) recon-nection model depended only weakly on the actual dissi-pation at the x line and reconnection was much faster

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).


with a normalized rate of Order 0.1 which agrees well with most current observations of magnetic reconnec-tion. 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 reconnec-tion, 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 differ-ent context. Both Axford and Parker had speculated

about Kelvin–Helmholtz (KH) waves at the magneto-spheric 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 reconnec-tion (Vasyliunas, 1975), and indirect evidence such as cusp latitude control of reconnection (Burch, 1973). However, the lack of actual in‐situ signatures also gener-ated increasing skepticism (Heikkila, 1975) and alternative models for the plasma entry termed “impulsive penetra-tion” (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 reconnec-tion 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 magneto-sphere with the magnetosheath (Figure 1.4). They pro-posed 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 magneto-pause 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.,

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).


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 simu-lations. 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 magneto-pause. 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 sig-natures 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 observa-tion 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 disrup-tion 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 enor-mous 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 simu-lations (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

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).


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 tan-gential discontinuity is present (Zhang et al., 2013).

1.4. FINAL REMARKS

In particular, the earlier history might cause the impres-sion 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 method-ological 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 astrophys-ical 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 con-ferences 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 crit-ical 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 tran-sients at the magnetopause play a major role for the plasma and magnetic flux transport at the magneto-pause. We have quantitative models for FTE formation and KH waves that are in good agreement with observa-tions and the required global transport. Observations confirm the occurrence of reconnection in KH waves. Similarly, it is well established that large HFAs play a

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


major role not only for the shock structure but also for the magnetosheath, magnetopause, and the magneto-sphere 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 follow-ing: 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 vig-orous 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.

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Physics of Dayside Magnetospheric Response to Solar Wind

Discontinuities

Part I

13

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.

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

2

2.1. INTRODUCTION

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 magnetosphere–ionosphere system. A zoo of transient phenomena 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

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 expansion 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)

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

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.


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 reconnection 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) considering 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 discussed 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 associated 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 phenomena, 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 substantial 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 energized 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 interplanetary 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 particularly efficient particle accelerators since they involve two converging shocks (i.e., that of the FB at the upstream

https://gem.epss.ucla.edu/mediawiki/index.php/FG:_Transient_Phenomena_at_the_Magnetopause_and_Bow_Shock_and_Their_Ground_Signatureshttps://gem.epss.ucla.edu/mediawiki/index.php/FG:_Transient_Phenomena_at_the_Magnetopause_and_Bow_Shock_and_Their_Ground_Signatureshttps://gem.epss.ucla.edu/mediawiki/index.php/FG:_Transient_Phenomena_at_the_Magnetopause_and_Bow_Shock_and_Their_Ground_Signatures

Table 2.1 Comparison of transient phenomena at the bow shock

HFAs SHFAs Foreshock bubbles Foreshock cavitiesForeshock cavitons

Foreshock compressional boundary Density holes SLAMS

Depletion in the density and magnetic field strength

Yes Yes Yes Yes Yes Yes on the turbulent side

Yes Yes

Compressions at edges

Yes Yes Only on the upstream edge

Yes Yes Yes Yes Yes

Presence of energetic (>30 keV) particles

Yes Yes Yes Yes Yes No Yes No

Significant flow deflection

Yes Yes Yes No No No Yes No

Significant plasma heating

Yes Yes Yes Modest No No Yes Yes

Associated with an IMF discontinuity

Yes No Yes Sometimes No No Yes No

Duration Minutes Minutes Minutes Minutes Minutes Minutes Seconds ~10 secondsScale size A few RE A few RE Up to 10 RE A few RE ~RE ~RE Ion gyroradius Ion gyroradiusGeneration

mechanismsInteraction of IMF

discontinuities with the bow shock

Interaction of foreshock cavitons with the bow shock

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

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

Nonlinear evolution of ultra‐low‐frequency (ULF) waves

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).

Possibly due to backstreaming particles interacting with the original solar wind

Nonlinear wave steepening


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 a

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