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- Herausgeber: John Wiley & Sons
- Kategorie: Lebensstil
- Serie: Geophysical Monograph Series
- Sprache: Englisch
- Veröffentlichungsjahr: 2024
Alfvén Waves Across Heliophysics Progress, Challenges, and Opportunities Alfvén waves are fundamental to the dynamics of space plasmas. Recent advances in our knowledge about Alfvén waves have come from several directions, including new space missions to unexplored heliospheric regions, sophisticated rocket campaigns in the auroral zone, enlarged magnetometer arrays and radar networks, and significant advances in computer modeling. Alfvén Waves Across Heliophysics: Progress, Challenges, and Opportunities is an interdisciplinary collaboration from different space science communities to review recent and current Alfvén wave research. Volume highlights include: * Alfvén waves in the solar atmosphere * Alfvén waves at the giant planets * Alfvén waves at Mars * Alfvén waves in moon-magnetosphere systems * Alfvén waves in geospace * Alfvén waves in the laboratory The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
List of Contributors
PREFACE
Part I: Sun
1 Introduction to Alfvén Waves Across Heliophysics: Progress, Challenges, and Opportunities
ACKNOWLEDGMENTS
2 Alfvén Waves in the Solar Atmosphere
2.1. INTRODUCTION
2.2. ALFVÉN, ALFVÉNIC, TRANSVERSE, KINK, FAST KINK? NOMEN EST OMEN
2.3. OBSERVATIONS OF ALFVÉN WAVES IN THE SOLAR ATMOSPHERE
2.4. ADVANCES IN THE THEORY OF ALFVÉN WAVES
2.5. SUMMARY AND OUTLOOK
ACKNOWLEDGMENTS
REFERENCES
NOTE
3 Alfvén Waves in Solar Flares
3.1. INTRODUCTION
3.2. SOLAR FLARES BACKGROUND
3.3. TERMINOLOGY: ALFVÉN AND ALFVÉNIC
3.4. NONTHERMAL LINE WIDTHS
3.5. WAVE ENERGY DENSITY AND POYNTING FLUX
3.6. GENERATION OF ALFVÉNIC WAVES/TURBULENCE
3.7. TRANSPORT TO THE LOWER SOLAR ATMOSPHERE
3.8. FUTURE OUTLOOK
3.9. SUMMARY
ACKNOWLEDGMENTS
REFERENCES
4 Excitation and Absorption of Alfvén Waves on the Sun and at Earth: A Comparison
4.1. INTRODUCTION
4.2. ALFVÉN WAVES IN THE MAGNETOSPHERE
4.3. ALFVÉN WAVES IN THE SOLAR ATMOSPHERE
4.4. A COMPARISON
4.5. CONCLUSIONS
REFERENCES
Part II: Planets
5 Alfvén Waves at Mars
5.1. INTRODUCTION
5.2. ALFVÉN WAVES UPSTREAM FROM THE MARTIAN BOW SHOCK
5.3. ALFVÉN WAVES IN THE MARTIAN MAGNETOSHEATH
5.4. ALFVÉN WAVES INSIDE THE MPB AND THE IONOSPHERE
5.5. CONCLUSIONS
ACKNOWLEDGMENTS
REFERENCES
6 Alfvén Waves at the Giant Planets
6.1. INTRODUCTION
6.2. GIANT PLANET MAGNETOSPHERES: DYNAMICAL BACKGROUND
6.3. THE PLASMA SOURCES AND ALFVÉN WAVES
6.4. LESSONS TO DRAW
6.5. IMPACT OF PLASMA ROTATION ON THEORY
6.6. THE CORIOLIS EFFECT ON ALFVÉN WAVES IN A ROTATING MAGNETOSPHERE
6.7. WAVES NEAR THE PLANETARY ROTATION PERIOD
6.8. THE HYDROMAGNETIC SURFACE WAVE IN A ROTATING SYSTEM
6.9. BACKGROUND PLASMA DIFFUSION: SOME OPEN ISSUES
6.10. CONCLUDING REMARKS
AVAILABILITY STATEMENT
ACKNOWLEDGMENTS
REFERENCES
7 Alfvén Waves Related to Moon–Magnetosphere Interactions
7.1. HISTORY OF THE MOON–MAGNETOSPHERE INTERACTIONS
7.2. MOON–MAGNETOSPHERE AND SIMILAR INTERACTIONS THROUGHOUT THE SOLAR SYSTEM
7.3. SUMMARY
REFERENCES
8 Alfvén Waves and Aurora at Earth and Jupiter: Comparative Analysis
8.1. INTRODUCTION
8.2. BROADBAND ACCELERATION: THE “ALFVÉNIC AURORA”
8.3. QUASI‐STATIC AURORAL ACCELERATION
8.4. DISCUSSION: SIMILARITIES AND DIFFERENCES BETWEEN EARTH AND JUPITER
ACKNOWLEDGMENTS
REFERENCES
Part III: Earth
9 Role of Alfvén Waves in Dynamic Magnetosphere–Ionosphere Coupling: New Perspectives From Satellite and Ground Observations
9.1. INTRODUCTION
9.2. ROLE OF ALFVÉN WAVES IN FIELD‐ALIGNED CURRENT DYNAMICS
9.3. INTERFERING ALFVÉN WAVES
9.4. GLOBAL MIC AND THE ACTIVE IONOSPHERE
9.5. SMALL‐SCALE ALFVÉN WAVES
9.6. CONCLUSIONS
ACKNOWLEDGMENTS
AVAILABILITY STATEMENT
REFERENCES
10 Alfvén Waves in the Earth's Magnetosphere
10.1. INTRODUCTION
10.2. THEORETICAL CONCEPTS AND SIMULATIONS
10.3. EXTERNALLY DRIVEN (LOW‐
m
) ALFVÉN WAVES
10.4. INTERNALLY DRIVEN (HIGH‐
m
) ALFVÉN WAVES
10.5. ROLE OF ALFVÉN WAVES IN SPACE WEATHER
10.6. CLOSING REMARKS
ACKNOWLEDGMENTS
REFERENCES
11 Alfvén Waves at the Interface of Solar Wind and Magnetosphere
11.1. INTRODUCTION
11.2. MODE CONVERSION AT THE ALFVÉN AND PERPENDICULAR ION CYCLOTRON RESONANCES
11.3. COUPLING BETWEEN ALFVÉN AND KELVIN‐HELMHOLTZ WAVES
11.4. SUMMARY
ACKNOWLEDGMENTS
AVAILABILITY STATEMENT
REFERENCES
12 Laboratory‐Space Comparisons of Alfvén Waves
12.1. INTRODUCTION
12.2. LINEAR ALFVÉN WAVE THEORY IN IDEAL MHD, INERTIAL, AND KINETIC LIMITS
12.3. LINEAR ALFVÉN WAVE PHYSICS
12.4. NONLINEAR ALFVÉN WAVE PHYSICS
12.5. DISCUSSION AND CONCLUSION
ACKNOWLEDGMENTS
REFERENCES
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 A selection of spacecraft missions (launched since 2000) relevant ...
List of Illustrations
Chapter 2
Figure 2.1 Schematic representation of the perturbation of straight and unif...
Figure 2.2 Results of a 2.5D (two spatial dimensions,
x
and
z
, and three vec...
Figure 2.3 Replicating the 2.5D MHD simulation shown in Figure 2.2 in a full...
Figure 2.4 Illustration representing a coronal hole with Alfvén waves driven...
Figure 2.5 Schematic representation of a surface Alfvén wave on a planar int...
Figure 2.6 Left: Schematic representation of the
m
= 1 antisymmetric torsion...
Figure 2.7 Images illustrating the omnipresence of transverse swaying of spi...
Figure 2.8 Alfvénic waves in the solar atmosphere as seen by CoMP. (a) CoMP ...
Chapter 3
Figure 3.1 CSHKP model of a two‐ribbon flare. Magnetic field lines sketch th...
Figure 3.2 Evolution of flare nonthermal velocities (top panels, markers, le...
Figure 3.3 Evolution of nonthermal velocities in the above‐the‐loop region o...
Figure 3.4 Map of nonthermal velocity in the corona during an X1.2 flare. Or...
Figure 3.5 Line‐of‐sight Doppler velocity (top panel) and nonthermal velocit...
Figure 3.6 Evolution of Si IV in the footpoint of a B‐class flare. The top p...
Figure 3.7 Frayed flux ropes within 3D self‐generated magnetic reconnection....
Figure 3.8 MHD simulation demonstrating generation of small‐scale motions by...
Figure 3.9 Evolution of model solar atmospheres in three scenarios. The top ...
Chapter 4
Figure 4.1 Focusing of a shear Alfvén wave on a density cavity. Refraction o...
Figure 4.2 High‐beta plasma impinging on a magnetic boundary proceeds by dir...
Figure 4.3 (Right) A flow burst simulated as low‐entropy flux tube arriving ...
Figure 4.4 “Dying” flow bursts stopped at the outer boundary of the near‐dip...
Figure 4.5 Slightly oblique Alfvén wave fronts attached to the auroral accel...
Figure 4.6 Fourier spectrum of the Doppler velocities in the corona fitted b...
Figure 4.7 Phase speeds Alfvénic motions derived from cross‐correlation of s...
Figure 4.8 Coordinated IRIS and Hinode/SOT observations of an active promine...
Figure 4.9 Ribbons displaced along the PIL indicating sheared arcade fields ...
Figure 4.10 A reconfiguring coronal field launches a torsional Alfvén wave p...
Figure 4.11 Chromospheric heating by Alfvén waves. (a) Looptop plasma layer ...
Figure 4.12 (Left) TRACE 195 Å images of the confined filament eruption on 2...
Figure 4.13 Brightness maps in SDO/AIA 335 Å filter (left) and maps of verti...
Figure 4.14 Reconnection site generated by a sheared erupting flux rope. The...
Chapter 5
Figure 5.1 Schematic representation of the Martian magnetosphere. The bow sh...
Figure 5.2 Solar wind properties computed from 30‐min time intervals upstrea...
Figure 5.3 The normalized Alfvén ratio for solar wind measurements between O...
Figure 5.4 (a) Histogram of the number of analyzed 512 s time intervals () ...
Figure 5.5 Magnetic field power spectra density as a function of the observe...
Figure 5.6 Probability distribution function of log for different Martian o...
Figure 5.7 Statistical magnetic field strength wave power for several freque...
Figure 5.8 Occurrence rate of several low‐frequency wave modes. (a) Alfvén a...
Figure 5.9 Median magnetic field power spectra density as a function of the ...
Figure 5.10 (Upper) Magnetic field power spectral density for a Martian magn...
Figure 5.11 Compressional ratio based on MAVEN MAG and SWIA (panel a) and ...
Chapter 6
Figure 6.1 The phase fronts of an Alfvén wave attached to the Jovian moon Io...
Figure 6.2 From Manners and Masters (2019) / John Wiley & Sons, Galileo spac...
Figure 6.3 From Rusaitis et al. (2021) / John Wiley & Sons, panels (a) and (...
Figure 6.4 A composite of two sketches from Southwood and Cowley (2014) / Jo...
Chapter 7
Figure 7.1 Outputs from magneto‐hydrodynamic simulations of the interaction ...
Figure 7.2 (a) Schematic of the local interaction in the case of immediate f...
Figure 7.3 (a) Image of the northern FUV aurora of Jupiter acquired with the...
Figure 7.4 Schematic presentation of the Alfvén wing reflection pattern and ...
Figure 7.5 (a) Calibrated dynamic spectra of the electric field (top) and ma...
Figure 7.6 Schematic of the chain of processes arising from the Io–Jupiter e...
Figure 7.7 (a‐d) Electric and magnetic field frequency‐time spectrograms mea...
Figure 7.8 (a) Image of the Extreme‐UV northern aurora at Saturn acquired on...
Figure 7.9 Plot of the Cassini trajectory during the distant Rhea encounter ...
Figure 7.10 Sketches of the interaction between the solar wind and the Earth...
Figure 7.11 Distribution of the average ARTEMIS measurements for the density...
Chapter 8
Figure 8.1 Kinetic Alfvén wave parameters for a model auroral field line at ...
Figure 8.2 The logarithm of the energy for which the electron thermal speed ...
Chapter 9
Figure 9.1 Electric and magnetic fields, and related derived field‐aligned c...
Figure 9.2 The local time and latitude dependence of the statistical field‐a...
Figure 9.3 Upward and downward R1/R2 field‐aligned currents and related elec...
Figure 9.4 Global FAC systems observed by AMPERE on 31 May 2014 during a per...
Figure 9.5 Magnetic‐field perturbations observed in mean field‐aligned coord...
Figure 9.6 Correlation between magnetic‐field time series between different ...
Figure 9.7 e‐POP and Swarm observations of discrete auroral arc dynamics. (a...
Figure 9.8 E‐ and B‐field time series and dynamics spectra and cross‐spectra...
Figure 9.9 IAR signatures visualized via (a) wave ellipticity and (b) azimut...
Figure 9.10 Scale‐dependence of Poynting flux during an auroral‐zone crossin...
Figure 9.11 Swarm‐satellite‐derived statistical dayside and nightside interh...
Figure 9.12 Example of upward Poynting flux event during an auroral‐zone cro...
Figure 9.13 Hodograms and polarization of EMIC waves observed at multiple gr...
Figure 9.14 EMIC wave observations by Swarm during a mid‐latitude traversal ...
Figure 9.15 EMIC wave‐propagation simulation of the observational event show...
Chapter 10
Figure 10.1 (a) The structure of the Earth's magnetosphere (Nakariakov et al...
Figure 10.2 Schematic of MHD waves in the magnetotail waveguide (Wright & Al...
Figure 10.3 Simulation results (Wright & Allan (2020) /John Wiley & Sons). (...
Figure 10.4 The orthogonal field‐aligned coordinates for a dipole field (W...
Figure 10.5 Ratio of the toroidal () to poloidal () Alfvén frequencies for...
Figure 10.6 A close‐up view in the equatorial plane of time‐averaged energy ...
Figure 10.7 (a) The variation of Alfvén frequency, , with polarization angl...
Figure 10.8 (a) Resonance Map in the equatorial plane for the compressed dip...
Figure 10.9 Reproduction of figure 1 from Elsden & Wright (2020) showing the...
Figure 10.10
Figure 10.11 Simulation results of auroral arcs and satellite data. (a) Pred...
Figure 10.12 Magnetoseismic analysis of the torus at Arase (Nosé et al., 2...
Figure 10.13 Data from the Van Allen Probe (RBSP)—A spacecraft for a fundame...
Figure 10.14 Analysis of particle data for a second‐harmonic poloidal wave e...
Figure 10.15 Data from the THEMIS‐D spacecraft for a second harmonic poloida...
Chapter 11
Figure 11.1 (a) Magnetic field (
B
y
) with local Alfvén speed (red line). (b) ...
Figure 11.2 The wave dispersion relation calculated using Equation (7) for (...
Figure 11.3 (a) Density and Alfvén velocity profile in
x
. (b) Compressional ...
Figure 11.4 (a) Adopted Alfvén velocity (
V
A
) and electron temperature (
T
e
). ...
Figure 11.5 Spatial cuts of the transverse component (
B
y
) of a hybrid simula...
Figure 11.6 (a) Illustration of KH vortex. The gray‐shaded area is the magne...
Figure 11.7 Alfvén resonance (
ω
A
) from Equation (8), perpendicular ion ...
Figure 11.8 (a) Illustration of the adopted background plasma profiles near ...
Figure 11.9 Real frequencies and growth rates at the dayside magnetopause fo...
Figure 11.10 (a) and (b) Friedrichs phase speed (normalized by
V
AIII
) diagra...
Figure 11.11 Unstable wave frequency (solid black line) and growth rate (das...
Figure 11.12 Time‐dependent simulation results showing the primary and secon...
Chapter 12
Figure 12.1 Drawing of Earth's magnetosphere with Alfvén waves generated by ...
Figure 12.2 Plasma devices that have been used for Alfvén wave experiments. ...
Figure 12.3 A pileup of energy density is caused by dispersive Alfvén waves ...
Figure 12.4 Dispersive Alfvén wave parallel phase speed and damping measured...
Figure 12.5 The first report of Alfvén waves generated by magnetic reconnect...
Figure 12.6 An ion‐ion Alfvén wave resonator in the LAPD. (a,b) The upper an...
Figure 12.7 Alfvén wave dispersion in the Auburn Linear Experiment for Space...
Figure 12.8 Counterpropagating Alfvén waves in the LAPD generate an ion acou...
Figure 12.9 Parametric instability of a single large‐amplitude Alfvén wave i...
Figure 12.10 Predicted and measured of the lowest‐order nonlinear interact...
Figure 12.11 Nonresonant beat wave interactions between counterpropagating A...
Figure 12.12 The parallel suprathermal electron velocity distribution measur...
Figure 12.13 Application of the field‐particle correlation technique to Alfv...
Guide
Cover
Table of Contents
Geophysical Monograph Series
Title Page
Copyright
List of Contributors
Preface
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Index
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Andreas Keiling (Ed.)
Alfvén Waves Across Heliophysics
Progress, Challenges, and Opportunities
Andreas Keiling
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List of Contributors
Bertrand Bonfond
STAR Institute
University of Liège
Liège, Belgium
Peter A. Damiano
Geophysical Institute
University of Alaska Fairbanks
Fairbanks, AK, USA
Gina A. DiBraccio
Planetary Magnetospheres Laboratory
NASA Goddard Space Flight Center
Greenbelt, MD, USA
Tom Elsden
Department of Mathematics and Statistics
University of St Andrews
St Andrews, UK
Jared R. Espley
Planetary Magnetospheres Laboratory
NASA Goddard Space Flight Center
Greenbelt, MD, USA
Christopher M. Fowler
Department of Physics and Astronomy
West Virginia University
Morgantown, WV, USA
Gerhard Haerendel
Max Planck Institute for Extraterrestrial Physics
Garching near Munich, Germany
Jasper S. Halekas
Department of Physics and Astronomy
University of Iowa
Iowa City, IA, USA
Michael D. Hartinger
Department of Earth, Planetary, and Space Sciences
University of California Los Angeles
Los Angeles, CA, USA
and
Space Science Institute
Boulder, CO, USA
Jay R. Johnson
School of Engineering
Andrews University
Berrien Springs, MI, USA
Andreas Keiling
Space Sciences Laboratory
University of California Berkeley
Berkeley, CA, USA
Eun‐Hwa Kim
Princeton Plasma Physics Laboratory
Princeton University
Princeton, NJ, USA
Robert L. Lysak
School of Physics and Astronomy
University of Minnesota
Minneapolis, MN, USA
Norbert Magyar
Centre for Mathematical Plasma‐Astrophysics Department of Mathematics
Katholieke Universiteit Leuven
Leuven, Belgium
Ian R. Mann
Department of Physics
University of Alberta
Edmonton, Canada
Ivan P. Pakhotin
Department of Physics and Astronomy
University of Calgary
Calgary, Canada
Norberto Romanelli
Department of Astronomy
University of Maryland
College Park, MD, USA
and
Planetary Magnetospheres Laboratory
NASA Goddard Space Flight Center
Greenbelt, MD, USA
Alexander J. B. Russell
School of Mathematics and Statistics
University of St Andrews
St Andrews, UK
James W. R. Schroeder
Department of Physics and Engineering
Wheaton College
Wheaton, IL, USA
David Southwood
Department of Physics
Imperial College
London, UK
Ali Sulaiman
School of Physics and Astronomy
University of Minnesota
Minneapolis, MN, USA
Kazue Takahashi
The Johns Hopkins University Applied Physics Laboratory
Laurel, MD, USA
Andrew N. Wright
Department of Mathematics and Statistics
University of St Andrews
St Andrews, UK
PREFACE
Heliophysics encompasses all kinds of physics with a focus on the interaction between the Sun and other bodies in the solar system. Given the enormous space, space scientists specialize in phenomena associated with a certain region in space, each region being associated with a specific plasma regime. Accordingly, they typically share and discuss their results in separate conferences and workshops. However, phenomena can occur across different plasma regimes, which provides opportunities to extend our understanding of the influence of different plasma parameters. There has been a growing trend to acknowledge this interdisciplinary connection in conferences and workshops.
Personally, I started 13 years ago to bring different space science communities together. The chosen format was the Chapman conference series, which are small, topically focused meetings under the umbrella of the AGU. Not initially planned, it turned into a pursuit for more. Between 2011 and 2023, together with colleagues, we organized five Chapman conferences, all of which aimed to promote interdisciplinary scientific discussions among solar/heliospheric scientists and magnetospheric/ionospheric scientists. Each conference touched upon a different phenomenon or region in space, but all can be seen as part of the bigger picture: solar–terrestrial–planetary coupling and its consequences on space weather.
In connection with each Chapman conference, we published books in AGU's Geophysical Monograph Series: Auroral Phenomenology and Magnetospheric Processes: Earth and Other Planets (2012, vol. 197), Magnetotails in the Solar System (2015, vol. 207), Low‐Frequency Waves in Space Plasmas (2016, vol. 216), and Electric Currents in Geospace and Beyond (2018, vol. 235). This book, Alfvén Waves Across Heliophysics: Progress, Challenges, and Opportunities, is the fifth addition to this miniseries, reviewing recent advances in Alfvén wave research. It is a topic dearest to my heart, being the foundation of my research career, starting with my Ph.D. It is still a very active research field, whose beginning can easily be traced to Hannes Alfvén's pioneering research almost a century ago. Starting with observations in the laboratory, observations in geospace, other planets, and the Sun followed, which took many decades to accomplish. It has unequivocally been proven that the Alfvén wave is a universal phenomenon, and its physics and applications continue to be intensely investigated across the heliosphere.
Perhaps, it is the solar and planetary science communities that have recently produced the most significant advances in our knowledge thanks to new space missions, but it is the geospace community that has the longest history of direct observations of Alfvén waves. Thus, solar and planetary wave researchers may be able to lean on the geospace community in understanding how these waves operate and under what conditions to learn how processes may scale with environmental conditions. One way to facilitate this is by bringing them all together in one meeting. We did this in my hometown Berlin in May 2023, as another Chapman conference.
In preparation for this meeting, 11 reviews were written for this book and presented as tutorials at the conference. The reviews cover three domains, corresponding to three parts in the book: Alfvén waves in the Sun; Alfvén waves at planets (excluding Earth); and Alfvén waves in geospace (Earth). In addition, some reviews take comparative approaches across different domains, and one review looks at Alfvén waves in the laboratory setting. While the solar wind is an invaluable part of Alfvén wave research, and it was initially planned as a contribution to this book, due to unpreventable and unfortunate circumstances, the contributions on this subject were not able to be included.
I trust the readers of this book will find it well worth the read, and I hope it can serve as a reference book for today's and future generations of space scientists. Because of its interdisciplinary nature, it should appeal to a broad community of space scientists and should also be of interest to astronomers and astrophysicists studying space plasmas beyond our solar system.
This latest Chapman conference and book—as well as the four preceding—have been a great journey. Beverly, my wife with a doctorate in microbiology (an expertise that came in handy, unexpectedly, when we had to deal with an Ebola wave during one conference, and a COVID‐19 wave during the planning of the most recent conference), helped with all the conferences with regard to logistics, local matters, and cultural engagement. I am grateful to her and so many others (too many to list individually) who helped in organizing the conferences. Similarly, I am grateful to my earlier book co‐editors, the chapter authors, and reviewers. Importantly, I heartily thank AGU Meetings staff, AGU Publications staff, and the team at Wiley for providing support throughout. Lastly, my editorial work on this book was supported by the National Science Foundation (AGS‐2016788, AGS‐2116971).
Andreas Keiling
University of California at Berkeley, USA
Part ISun
1Introduction to Alfvén Waves Across Heliophysics: Progress, Challenges, and Opportunities
Andreas Keiling
Space Sciences Laboratory, University of California Berkeley, Berkeley, CA, USA
ABSTRACT
Alfvén Waves Across Heliophysics: Progress, Challenges, and Opportunities is an interdisciplinary collaboration among different space science communities. In the 12 chapters of this book, advances of our knowledge about Alfvén waves across heliophysics are reviewed. Here, a small selection of examples is given to illustrate the range of Alfvén waves in various space plasmas of our solar system, including a selection of relevant space missions launched in the 21st century. Because of its interdisciplinary nature, this book should appeal to a broad community of space scientists, and it should also be of interest to astronomers/astrophysicists who are studying space plasmas beyond our solar system.
Alfvén waves are fundamental to the dynamics of space plasmas. This recognition is one of the great achievements in space research, going back to the last century, when Hannes Alfvén published his groundbreaking paper in 1942, which ultimately earned him the Nobel Prize in 1970. Since entering the 21st century, great advances in Alfvén wave research have been made across the entire field of heliophysics, coming from several directions: new space missions (Table 1.1) to unexplored heliospheric regions, sophisticated rocket campaigns in the auroral zone, enlarged magnetometer arrays and radar networks, and significant advances in the capabilities of simulations to allow modeling many of the relevant processes. This book compiles this new understanding of Alfvén waves with contributions from different space science communities. Overall, the book is divided into three main parts, dealing separately with Alfvén waves at the Sun (Part I), the inner and outer planets (Part II, which excludes Earth), and Earth (Part III). In addition, they include chapters that take a comparative approach across plasma regimes. To whet the appetite, a small selection of examples from each part is given next. While the solar wind is an invaluable part of Alfvén wave research, and it was initially planned as a contribution to this book, due to unpreventable and unfortunate circumstances, we were not able to include the contribution on this subject.
In geospace, spacecraft missions, such as THEMIS, Reimi, and Van Allen Probes, have provided unprecedented observational links between Alfvén waves in space and auroral fine structure in the ionosphere. Recent studies found that Alfvén waves are not just a marker of dynamic processes in space plasma but also an active factor. For example, Alfvén wave activity is responsible, at least partly, for the acceleration of relativistic electrons of the radiation belt. Furthermore, Alfvén waves are important in the energization of ionospheric plasma. There is also a growing recognition of the global, hemispheric energy transport by Alfvén waves in the magnetosphere–ionosphere system. It is also important to understand the limitations of current Alfvén wave theory in explaining related phenomena in order to identify missing pieces of physics.
In addition to advances in understanding Alfvén waves in our own geospace, much knowledge has been gained about other planetary magnetospheres in the last decade. For example, the current Juno mission provides ample evidence of Alfvénic activity at Jupiter's polar regions, which led to the conclusion that Alfvénic activity plays a major role in auroral acceleration processes for Jupiter's aurora. In fact, Alfvén waves have been thought to play a key role in many facets, including but not limited to moon–magnetosphere interactions, auroral acceleration processes, turbulent heating in the middle magnetosphere, and more. Prior to the Juno mission, evidences for Alfvénic interactions were limited to the equatorial magnetosphere (e.g., from Galileo), but now we are starting to appreciate how the Alfvén waves are transported and dissipated at the high magnetic latitudes, so we can probe their role in magnetosphere–ionosphere coupling at Jupiter. Although Juno is revealing new and exciting results on the importance of Alfvén waves, there are many unresolved questions regarding how and where they are dissipated, the role of whistler‐mode versus Alfvén‐mode waves in auroral processes, and where Alfvén waves are dominant in the auroral region. The structure and dynamics of Jupiter's magnetosphere are qualitatively different than those of Earth's due to Jupiter's strong magnetic field, its rapid rotation, and internal plasma source from Io. Nonetheless, the Jovian community may be able to lean on the terrestrial community in understanding how and under what conditions these processes operate to learn how processes may scale with environmental conditions.
Table 1.1 A selection of spacecraft missions (launched since 2000) relevant to current Alfvén wave research.
Launch
Mission
2000
Cluster
2000
IMAGE
2000
CHAMP
2004
MESSENGER
2005
Venus Express
2005
Reimi
2007
THEMIS
2010
Solar Dynamics Observatory
2011
Juno
2012
Van Allen Probes
2013
MAVEN
2013
Swarm
2015
MMS
2016
Arase
2018
Parker Solar Probe
2018
BepiColombo
2020
Solar Orbiter
New solar missions, such as Solar Orbiter and Parker Solar Probe, have provided new insights into Alfvén wave research in the solar atmosphere and the nascent solar wind. In the solar corona, Alfvén waves are important, because they are now being observed regularly as decayless waves in many coronal structures. In the last decade, observational evidence for Alfvén waves in the photosphere and chromosphere has also been reported. During solar flares, Alfvén waves launched by reconnection may play a significant role in transporting flare energy through the solar atmosphere and dissipating it through heating and particle acceleration. However, there are many challenges in understanding theoretically how this would work, particularly in complex coronal magnetic fields, and in diagnosing it using only remote‐sensing flare observations. Currently, new simulation results are being obtained on how the waves potentially play a role in heating the corona. Further, Alfvén waves are being utilized for seismology of the solar atmosphere, yielding magnetic field maps. It is our opinion that Alfvén wave research related to the Sun provides some of the most exciting new results.
In the remainder of this book, many more examples with greater details are given to build up a comprehensive picture, which allows contrasting similarities and differences of Alfvén waves in different plasma regions/regimes. It is hoped that this book will serve as a reference book for today's and future generations of space scientists. Because of its interdisciplinary nature, this book should appeal to a broad community of space scientists, and it should also be of interest to astronomers/astrophysicists who are studying space plasmas beyond our solar system.
ACKNOWLEDGMENTS
A.K. acknowledges the National Science Foundation for their support (grants AGS‐2016788, AGS‐2116971) throughout the entire editorial process of this book. The author thanks the science program committee in helping formulate the proposal for the AGU Chapman Conference (Berlin, Germany, 2023), from which some of the text was used for this introduction chapter.
2Alfvén Waves in the Solar Atmosphere
Norbert Magyar
Centre for Mathematical Plasma‐Astrophysics, Department of Mathematics, Katholieke Universiteit Leuven, Leuven, Belgium
ABSTRACT
Alfvén waves are propagating disturbances of magnetic tension and are one of the fundamental waves in plasma at large scales. The study of Alfvén waves in the solar corona has a long and continuing tradition, started way before Alfvén waves were found to be ever‐present in the solar atmosphere. Their study is important for mainly two reasons. First, Alfvén waves might play a key role in the long‐standing chromospheric and coronal heating problems, as well as the solar‐wind acceleration problem, both for transmitting the energy in photospheric convection higher up in the atmosphere and more directly as a means of cascading and dissipating this energy through various mechanisms. Second, observed properties of Alfvén waves can be used to infer properties of the plasma through seismology. In this review, an overview of the basic theoretical understanding of magnetohydrodynamic (MHD) waves is presented, in an attempt to clarify the role that Alfvén waves play in the greater class of MHD waves encountered in the solar atmosphere. Furthermore, observational and theoretical findings on Alfvén waves in the solar atmosphere of the previous decade are reviewed, followed by a short summary of these results and still open questions.
2.1. INTRODUCTION
First derived by Alfvén (1942) as electromagnetic‐hydrodynamic waves, confirmed experimentally in liquid mercury by Lundquist (1949), and needing the approval of such authorities in physics as Fermi to be taken seriously after years of disapproval by other world‐renowned scientists (Dessler, 1970; Russell, 2018), the waves now bearing Hannes Alfvén's name were from their conception central to a better understanding of solar atmospheric processes. Alfvén himself first realized that such waves should in principle exist, through deep physical intuition, while thinking about the nature of sunspots (Alfvén & Lindblad, 1945), known to be strongly magnetized since Hale (1908). Other phenomena in which Alfvén waves might play a substantial role have been proposed shortly after, among the firsts being the then recently discovered coronal heating (Alfvén & Lindblad, 1947) and the existence of cosmic rays (Fermi, 1949). This interdisciplinary monograph, dedicated to reviewing recent advances in understanding Alfvén waves in the wide field of space physics, of which this chapter is part of, stands as proof of the everlasting impact that H. Alfvén made to the field, even 80 years on.
Although in his pioneering work Alfvén only derived a wave driven purely by the magnetic tension force, it had become increasingly crystallized in the 1950s that electroconducting fluids or plasmas whose dynamics can be described through the magnetohydrodynamic (MHD) equations admit various wave modes. This is caused by the interplay between various restoring forces, mostly magnetic and gas pressure forces. The different wave modes are clearly distinguishable under ideal and homogeneous conditions, leading to their usual identification as Alfvén, fast, and slow waves. Without entering too much into detail here, and instead allowing for an extended discussion on this subject in a dedicated section of this chapter, it is already important to point out the usually blurred lines between these adjectives, the properties assigned to these adjectives based on linear analysis under homogeneous conditions, and the actual wave properties, especially when accounting for inhomogeneities or other nonideal conditions present. It is important to stress that waves identified as “Alfvén” waves, either in laboratory experiments, observations, or numerical studies, would seldom share only the properties of a pure linear Alfvén wave encountered in an ideal and homogeneous plasma. Therefore, while going along with the usual broad interpretation of what an Alfvén wave is when selecting the studies to be included in this review, let us always keep in mind that pure Alfvén waves are a very special case of a plasma wave, probably seldom encountered in their pure form in nature, as will be argued later on.
Alfvén waves play multiple important roles in our quest for a better understanding of the physics of the solar atmosphere. First, they potentially act as means of transmitting the energy available in the convective buffeting of the photosphere, thought as the ultimate energy source for heating the corona and accelerating the solar wind, along magnetic field lines reaching into the solar atmosphere. Once inside the chromosphere or corona, Alfvén wave energy could be dissipated by various proposed mechanisms. Dissipation in the almost ideal conditions of the solar plasma presupposes that small enough scales are generated (large spatial gradients in Alfvén wave perturbation field), at which thermalization is supposedly taken over by kinetic processes. These small‐scale generation mechanisms range from linear processes such as phase mixing and resonant absorption to nonlinear shock dissipation and generation of Alfvén wave turbulence. Second, Alfvén waves could be the causing factor of various phenomena observed in the solar atmosphere, such as in increasing height from the photosphere, photospheric vortices, umbral and penumbral shocks, spicules, coronal jets, switchbacks, and so on, some of which phenomena will be discussed in detail later on. Last but not least, through the seismology of the observed waves, where the correct identification of the waves as outlined earlier is particularly important, physical properties of the local plasma through which the Alfvén waves are propagating could be inferred by comparing measured oscillation properties with the ones derived from MHD wave theory. These oscillation properties often depend on various properties of the plasma, such as magnetic field intensity, mass density, gravitational scale height, polytropic index, to name a few.
This chapter is dedicated to reviewing advances in the research and application of Alfvén waves in the solar atmosphere published in the last decade. According to the Smithsonian Astrophysical Observatory/National Aeronautics and Space Administration (SAO/NASA) Astrophysics Data System, there have been roughly 500 peer‐reviewed publications since 2012, uniformly distributed through the years, on the subject. The search criteria were based on words found in the abstract, specifically the combination of “Alfvén,” “wave,” “solar,” and “photosphere” or “chromosphere” or “corona.” The study of Alfvén waves in the extended solar atmosphere and inner heliosphere, such as the outer corona and solar wind, has a separate chapter dedicated to it within this monograph. It is also important to note that this chapter only considers plasma for which the fluid approximation is valid; that is, for which MHD is a good approximation, and does not touch upon kinetic dynamics, such as kinetic Alfvén waves (e.g., Chen et al., 2021). At the spatial and temporal scales that waves are observed in the solar atmosphere, the fluid approximation holds to a very good degree. Besides giving my perspective on the current state of research on Alfvén waves in the solar atmosphere and guiding the reader through a summary of these advances, the task is to present the most important findings in a hopefully comprehensive manner in the following text.
Many helpful reviews of Alfvén waves in the solar atmosphere have been published in the preceding years. These discuss in detail some of the milestone observational, numerical, and theoretical findings of the previous decades, such as the theoretical breakthroughs of phase mixing, mode conversion, and resonant absorption (Goossens et al., 2011; Roberts, 2000), Alfvén wave turbulence (Schekochihin, 2022), the observational firsts of directly imaged large‐amplitude transverse waves in coronal loops (Aschwanden et al., 1999; Nakariakov et al., 1999), the ubiquitous propagating waves first observed using the Coronal Multi‐Channel Polarimeter (CoMP, Mathioudakis et al., 2012; Tomczyk et al., 2007), torsional Alfvén waves in the chromosphere (Jess et al., 2009, 2012), and advances in the field of coronal seismology (De Moortel & Nakariakov, 2012), among other related areas, such as decayless kink waves and the effects of thermal misbalance (Nakariakov et al., 2016; Nakariakov & Kolotkov, 2020). Additionally, the dynamics of Alfvén waves have been a part of several textbooks on MHD and plasma dynamics (Aschwanden, 2005; Goedbloed & Poedts, 2004; Priest, 2014; Somov, 2006), with even a dedicated book on the subject published decades ago but still remaining relevant (Hasegawa & Uberoi, 1982).
The first section of this review deals with the often overlooked but particularly important aspects of nomenclature and semantics of MHD waves, a problem already mentioned above, with some theoretical background to support the narrative. The second section is dedicated to recent observational identifications of Alfvén waves in the solar atmosphere, categorized according to the different atmospheric layers. In the third section, recent theoretical and numerical advances, particularly focusing on the ones applied to solar atmospheric physics, are reviewed. Finally, I conclude this review by summarizing some of the key recent findings and offering some future perspectives on further advancing our understanding of Alfvén waves and implications for solar atmospheric dynamics.
2.2. ALFVÉN, ALFVÉNIC, TRANSVERSE, KINK, FAST KINK? NOMEN EST OMEN
There is a great deal of disagreement within the solar physics community about the usage of specific adjectives to designate waves, with multiple adjectives in use to refer even to the same observation or numerical result, with examples in the title of this section. This is particularly true for waves that manifest as transverse oscillations or displacements of magnetic field lines, with their group speed vector aligned to the local mean magnetic field. As Alfvén waves are in fact transverse displacements of magnetic field lines (see Figure 2.1), it is important to dive deeper into this problem.
Figure 2.1 Schematic representation of the perturbation of straight and uniform magnetic field lines, represented by parallel arrows, by a plasma flow transverse to the magnetic field lines, with this initial condition shown in the image on the left. Considering an ideal plasma, magnetic field lines are frozen‐in; therefore, the plasma flow drags the magnetic field lines, building up magnetic tension through the bending of the field lines as in the image in the center. This combined velocity and magnetic field perturbation splits into two propagating perturbations of magnetic tension and associated velocity perturbations, representing parallel and antiparallel propagating Alfvén waves propagating at the Alfvén speed VA.
Let us recollect here the timeline of a still ongoing debate on the nature of the CoMP‐like omnipresent propagating transverse waves in the solar corona. “Transverse” is the least controversial, if not descriptive, and most widely accepted way to refer to these waves. First reported by Tomczyk et al. (2007), these waves were referred to by the authors as Alfvén waves. This was quickly dismissed by Van Doorsselaere et al. (2008), stating that the waves seen by CoMP were instead fast kink waves of flux tubes, the propagating counterpart of standing waves in coronal loops first observed and identified as fast kink waves by Aschwanden et al. (1999) and Nakariakov et al. (1999). Under their (critical) assumption that the observed waves are propagating on flux tubes, Van Doorsselaere et al. (2008) stated that Alfvén waves are unable to transversely displace these flux tubes. Waves in a flux tube or magnetic cylinder were first investigated theoretically by Zaitsev and Stepanov (1975) and Edwin and Roberts (1983). Based on their analysis, fast kink waves are the only wave solutions that, in a low beta plasma as the solar corona, have group speeds close to the average Alfvén speed and could cause transverse displacements of the tube axis. However, the story does not stop there, as Goossens et al. (2009) first pointed out that “fast” is not the correct adjective to characterize kink waves and suggested using “Alfvénic” instead. Nevertheless, the authors still cautioned that waves propagating on an inhomogeneity or nonuniformity, such as kink waves on a flux tube, should not be characterized based on the distinct categories found in a homogeneous plasma, as waves in inhomogeneous plasma usually have mixed properties. That is, waves could display Alfvén, fast, or slow properties to varying degrees, depending on the local inhomogeneity through which they propagate. This important fact will be expanded upon later in this section. Goossens et al. (2012) doubled down on the problem of characterizing the transverse displacement of flux tubes. They suggested that kink waves, specifically the fundamental radial mode that survives even in an incompressible plasma under the conditions in which the fast waves vanish, behave like surface Alfvén waves, restricting the usage of the “fast” adjective to radial overtones, which require the plasma to be compressible. The term surface Alfvén waves has previously been used to refer to incompressible waves existing on a Cartesian plasma discontinuity, whose amplitude decreases exponentially with distance from the discontinuity (e.g., Roberts, 1981). Spurred by the diversity of proposed terms, it is not surprising that there are various preferred ways to refer to observed transverse waves in the solar atmosphere. Still, it is important to correctly characterize these waves, as terms used to describe them imply different properties, with crucial implications for their dynamics, as we will see in the following text, where the theoretical background of the aforementioned concepts will be gradually built up.
2.2.1. Waves in a Homogeneous Plasma of Infinite Extent: Alfvén, Fast, and Slow
Let us briefly present the clear distinction that can be made between different linear wave modes in a homogeneous plasma. For this, I largely follow the analysis presented in Goossens et al. (2012, 2019). We start by listing the equations of ideal MHD:
where the symbols have their usual meanings. Next, let us consider perturbations around some homogeneous equilibrium with . Motions atop this equilibrium displace plasma elements from r → r + ξ, where ξ is the Lagrangian displacement, v = dξ/dt. Linear perturbations, denoted with a prime, of Equations (2.1)–(2.5), expressed in terms of the linearized Lagrangian displacement v = ∂ξ/∂t, read:
It is clear that the equation of motion can be written only in terms of ξ. Looking for wavelike solutions, we assume that the displacement has the form . Next, a new set of variables is defined for a clear demonstration of distinct wave properties:
Then, the equations governing wavelike solutions admitted by Equations (2.6)–(2.9) expressed in terms of the new variables are:
where is the sound speed, is the Alfvén speed, and ωA = kzvA is the Alfvén frequency. We obtained two sets of uncoupled equations. Equation (2.15) describes pure Alfvén waves, which are exclusively and completely characterized by parallel vorticity1. They cause no compression (Y = 0), and do not perturb the parallel component of velocity (X = 0), or equivalently, they do not cause total pressure variations, P′ = p′ + B′2/2μ0, a quantity that will be of crucial importance later on for the analysis of inhomogeneous plasma waves. Equations (2.13) and (2.14) describe the magnetosonic waves, fast and slow, which cause compression, perturb total pressure, but do not propagate parallel vorticity. The analysis above demonstrates that linear Alfvén waves in a homogeneous plasma of infinite extent are in their own class, with unique properties not shared with magnetosonic waves. It follows from this that given any superposition of linear perturbations atop a homogeneous plasma, the subset of Alfvén waves is always completely identifiable and quantifiable. The appearance of fast, slow, and Alfvén waves as a result of a localized perturbation in a homogeneous plasma with a straight and uniform magnetic field can be seen in Figure 2.2.
Figure 2.2 Results of a 2.5D (two spatial dimensions, x and z, and three vector components) MHD numerical simulation with a straight and uniform magnetic field along the z axis, showing pressure, density, and two components of plasma velocity. The initial condition is a Gaussian perturbation in pressure and out‐of‐plane velocity (Vy) at (x,z) = (0,0). The Alfvén waves propagating parallel and antiparallel to the magnetic field appear only in out‐of‐plane velocity. The isotropic waves are the fast waves, while the three‐lobed perturbations lagging behind are the slow waves. The remnant density perturbation at the position of the initial pulse is the entropy wave, of zero frequency and phase speed. The value of plasma‐β is 0.5. Units are arbitrary.
It must be emphasized again that the result above is valid only for linear perturbations. Once nonlinear terms of these perturbations are taken into account, couplings between the X, Y, and Z variables appear, allowing for wavelike solutions without linear counterparts, and, more importantly, for various couplings between Alfvén, fast, and slow modes. A relevant example of such a coupling in this case is the nonlinear Alfvén wave. While in an incompressible plasma (Y = 0) nonlinear Alfvén waves are exact solutions propagating without suffering distortion, in compressible plasma these drive, through ponderomotive forces, parallel displacement, and pressure and density perturbations. The coupling strength is proportional to the magnetic pressure gradient, . The finite amplitude Alfvén wave induces flows along the magnetic field direction only for noncircular polarization, that is, z‐dependent total magnetic field perturbation. However, not even circularly polarized Alfvén waves in a homogeneous compressible plasma are stable: these can get distorted and decay over time due to the parametric decay instability (PDI, Galeev & Oraevskii, 1963; Goldstein, 1978), generating reflection and slow wave perturbations. PDI appears due to any preexisting perturbation or noise of the background plasma, already hinting at the impact of plasma nonuniformity on the coupling of wave modes and spoiling of distinct wave types, the subject of the next subsection.
Given the observations above, let us draw some conclusions on the observability of pure Alfvén waves in nature. Obviously, if by pure Alfvén waves one means waves having strictly the properties of the linear Alfvén wave in a homogeneous and infinite plasma, then almost certainly these do not exist outside of the idealized conditions of our model. Nonlinearities and plasma noise couple the dynamics that are characteristic of the separate classes of waves existing under ideal conditions, that of Alfvén and magnetoacoustic modes. Therefore the term “Alfvén wave,” when used to refer to waves observed in the solar atmosphere, necessarily carries an approximative meaning, denoting instead MHD waves with properties that are dominantly those of pure Alfvén waves.
2.2.2. Waves in an Inhomogeneous Plasma
Inhomogeneity Perpendicular to the Magnetic Field
The solar atmosphere appears inhomogeneous and highly structured. This structuring, either of the chromosphere or corona, referred to as fibrils, filaments, strands, or threads, largely outlines the local mean magnetic field, and is identifiable down to the resolution limit of observing instruments (Aschwanden & Peter, 2017; Williams et al., 2020). The highly filamentary appearance of the corona revealed by the Sun‐grazing comet Lovejoy (Raymond et al., 2014) within hundreds of Megameters from the solar surface is yet another proof of the permeating inhomogeneity. The outer corona is also observed to be highly nonuniform, with a “wood‐grain” structure as revealed through processed coronagraph images (DeForest et al., 2018).
Motivated by these observations, let us continue with the linear harmonic analysis of inhomogeneous plasma. We switch to cylindrical coordinates, motivated by the popularity of the flux tube model, and allow for inhomogeneity only in the radial direction, , with the condition that total pressure is constant throughout the domain, . We seek solutions of Equations (2.1)–(2.5) that satisfy harmonic Lagrangian displacements of the form . The azimuthal wavenumber is an integer m ≥ 0, such that the solution is periodic in . As shown by Appert et al. (1974), Sakurai et al. (1991), and Goossens et al. (1992, 1995), the resulting linear MHD waves can be described by equations for Eulerian total pressure perturbations P′, and the radial component of displacement , with the hat dropped:
where the coefficient functions D, C2 are given by
where is called the cusp frequency. Equations (2.16) and (2.17) can also be rewritten as a second‐order ordinary differential equation for P′ as:
where Γ(ω2) is given by
As we will see more clearly in the following text, the linear coupling of total pressure perturbations and radial displacement in Equations (2.16) and (2.17) is a harbinger of the mixed Alfvén–magnetosonic properties that waves generally possess in an inhomogeneous plasma, as already noted by Hasegawa and Uberoi (1982). To better illustrate why that is the case, let us investigate the connection between the total pressure, radial displacement, and the X, Y, and Z variables employed in the homogeneous analysis. These were derived by Sakurai et al. (1991) and Goossens et al. (2019):
The system above shows that, in general, all wave variables that for a homogeneous plasma represented the separate classes of Alfvén and magnetosonic waves are coupled by the total pressure and radial displacement, which in turn are coupled on their own. This is the theoretical basis for the statement made by Goossens et al. (2019) that, in general, MHD waves in an inhomogeneous plasma present mixed properties, and cannot be clearly divided into Alfvén, fast, and slow modes any longer, already in the linear regime. In fact, the coupling between compression and parallel vorticity, the signatures of magnetosonic and Alfvén waves, respectively, can be written in an elegant way, for m ≠ 0
