Table Of ContentGuangli Huang · Victor F. Melnikov
Haisheng Ji · Zongjun Ning
Solar Flare
Loops:
Observations and
Interpretations
Solar Flare Loops: Observations and Interpretations
Guangli Huang Victor F. Melnikov
(cid:129)
Haisheng Ji Zongjun Ning
(cid:129)
Solar Flare Loops:
Observations
and Interpretations
123
GuangliHuang Haisheng Ji
PurpleMountain Observatory PurpleMountain Observatory
Nanjing Nanjing
China China
Victor F. Melnikov ZongjunNing
PulkovoObservatory PurpleMountain Observatory
Russian Academy of Sciences Nanjing
Saint-Petersburg China
Russia
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Foreword
Solarflaresareamongthemostspectaculareruptivephenomenainthesolarsystem
as well as in the universe. Flares can produce a large number of high-energy
particles and intense emissions into the interplanetary space which may affect the
solar-terrestrial environment. Solar flares involve diverse physical processes like
magnetic reconnection, particle acceleration, bremsstrahlung, gyrosynchrotron
emission,radiativetransfer,andsoon,eachofwhichisanimportanttopicandhas
generic applications to other eruptive phenomena in the universe. Although solar
flares have been intensively studied for more than half a century, there are still
manyunresolvedquestionsregardingtheeruptionmechanisms,emissionprocesses
at various wavelengths, and dynamics of the flaring plasma. Therefore, studies on
solar flares have always been and will continue to be an attractive topic in
astrophysics.
Duringthepastdecades,peoplewitnessedamajorprogressintheunderstanding
of solar flares thanks to a series of spacecrafts like RHESSI, Hinode, SDO, and
IRIS, and ground-based solar telescopes and radio heliographs. Many new obser-
vationaldiscoveriesandtheoreticalmodelshavebeenachievedintherecentyears.
Therefore, it is highly desirable to publish a monograph which provides a com-
prehensive review on recent advances and prospects for future research. One year
ago, when Prof. Guangli Huang told me that he and his collaborators planned to
write a book on solar flares, I felt very excited and full of expectation. As a
researcher in this field, I strongly support their plan and hope this book can bring
readers a new horizon in the related research field.
This book focuses on a specific topic, flare loops, which are an important
component in the whole flare process. Flare loops are the creatures of magnetic
reconnectionandthesiteswheremostflareemissionsoriginate.Theplasmainflare
loops undergoes violent dynamic evolution with a rapid energy input from, most
likely, energetic electron beams. Thebookintroduces indetail therecent advances
inthestudyofthemicrowaveemissionsandhardX-rayemissionsfromflareloops,
bothofwhicharethoughttobeproducedbyenergeticelectrons.Moreimportantly,
the book describes the major topics from two aspects, both basic theories and
observations.Thismakesiteasyforthereaderstogetacomprehensiveviewofthe
v
vi Foreword
topics of interest. The book also introduces the diagnostics of physical parameters
and the dynamics offlare loops, which are also important research topics with the
advent of various high-resolution observations in recent years. The authors of the
book are all active researchers on solar flares. Therefore, the book provides the
frontier of the studies on flare loops with a unique depth.
I would like to recommend this book to researchers and graduate students
working on solar physics, space physics, and related fields.
Nanjing, China Mingde Ding
October 2016 Nanjing University
Preface
Regarding the publication of this book, I should mention Qian Jun, a sunny boy,
and the editor of Science Press for my first book in 2009, “Plasma kinetic theory
andapplicationinsolarphysics”,hevisitedmyinstitutein2013,andsuggestedme
toconsideranewpublication.Inthefirstbook,theplasmatheories(70%)arewell
combinedwithobservations(30%),andofcourse,Iwanttokeepthischaracteristic,
but to increase the proportion of observations properly to attract much attention of
astronomers. Thus, I should thank my coauthor, Victor F. Melnikov (in Central
Astronomical Observatory at Pulkovo of Russian Academy of Sciences,
Saint-Petersburg, Russia), to contribute greatly to the second book, “Physics in
flaring loops”, especially in the theoretical development of gyrosynchrotron radi-
ation and particle propagation, and distinctive studies on observations. The main
topic of this book that is different from the other numerous solar monographs is
concentrated in the microwave theories and observations offlaring loops, which is
expected to be attractive for the studies of solar physics. And I also would like to
thank the other two coauthors, Dr. Ji Hai-sheng and Dr. Ning Zong-jun (my col-
league in Purple Mountain Observatory (PMO) of Chinese Academy of Sciences,
Nanjing,China)toextendthisbookfrommicrowaveandX-raytoopticalandEUV
bands. Another characteristic of the second bookis that it covers all representative
studiesoftheauthorsinrecentyears,eventhemostpapersthathavebeenpublished
in English, but the authors have reorganized the material and deepened it to be a
systematical monograph (including theories and observations, case study and
statistics, forward and backward diagnostics, and micro and global processes), and
helpful particularly for the Chinese solar people and graduate students to under-
stand this book.
After the second book was published by Science Press in 2015, it was a bit
surprising that the editor of Springer, Li Jian suggested me to publish an English
version of this book, and which is actually cooperated with Science Press, and the
editor Qian Jun as well. Probably a specific monograph in the field of solar
microwavetheoriesandobservationswasrareinrecentyears,anddefinitely,itisa
great honor for Chinese scientists to publish a book by a famous publisher in the
world. Inthiscase,Idon'twanttosimplytranslatetheChineseversiontoEnglish,
vii
viii Preface
butkeepabout80%oftheoriginalmaterial,andincreasesomenewcontentsinthe
recent 2 years given by all of the authors, and here I should thank Prof. Mingde
Ding of Nanjing University, Nanjing, China and Dr. Zhang Qing-min of PMO
(jointed by Dr. Ning Zong-jun) to add the recent theoretical and observational
studies on chromospheric evaporation to this book, which make it to face more
solar physicists of the world, and Prof. Mingde Ding has also kindly provided a
gracefulforewordforthisbookasgivenbefore.Theotherauthorshavealsoadded
theirnewresultspublishedin2015–2016totheEnglishversion,tomakeanupdate
rate over 20% in comparison with the Chinese version.
The feature of English version is somehow similar to the Chinese one, both
focused in the microwave and relevant X-ray emissions in flaring loops. The
English version is composed of seven chapters. A general review is given in
Chap. 1 to summarize the main contents and new results of this book. Chapters 2
and 4 respectively introduce the radiation mechanisms in microwave (gyrosyn-
chrotron) and X-ray bands (bremsstrahlung), as the theoretical basis of this book,
while Chaps. 3 and 5 respectively show the observational studies in these two
bands, including intensity, time profiles, spectral evolution, polarization, etc., and
theoretical explanations for these observations. Chapter 6 pays particular attention
totheradioandX-raydiagnosticsforflaringparametersfromtheobservationsand
theories,especiallyforcoronalmagneticfieldandenergeticelectrons.Atlast,some
large-scaledorglobalvariationsofflaringloopsbasedonthemicrowaveandX-ray
data and relevant theories are discussed in Chap. 7.
Moreover,animportantmotivationofthisbookisbasedonthenewlydeveloped
radioheliographsinChina(InnerMongolia,MUSER),Russia(Irkutsk,SSRT),and
USA(Owens-Valley,FASR),etc.,fromwhichwecanexpecta3-Dradioimageof
solar corona, as well as coronal magnetic field and energetic particles with high
spatial, temporal, and spectral resolutions, to open a new window to explore and
answer the key mysteries of solar physics, as well as to develop and protect our
Earth. The content of this book is closely associated with the scientific objective
of the instruments as mentioned above, and it is expected to play a definite role to
analyze the released data from these instruments in near future. This is my dream
and expectation to the young generation of solar physicists as a retired person.
Finally,Ialso wouldlike tothank mycolleagues ofPMO,Dr.Li Jian-ping and
Dr. Song Qi-wu, for supporting the publication of this book effectively.
Furthermore, I should thank all of the coauthors who participated in the main
referencesoftheauthorsfor theircontribution tothisbook,especially,Dr.Hiroshi
Nakajima (Nobeyama Radio Observatory, NAOJ, Minamimaki, Minamisaku,
Nagano 384-1305, Japan), Dr. Gregory Fleishman (Center For Solar-Terrestrial
Research, New Jersey Institute of Technology, Newark, NJ 07102, USA), etc. I’d
like to share all my happiness and hardship about this book with my wife Gu
Li-min, as well as my Ph.D. supervisor Wang De-yu in PMO.
Nanjing, China Guangli Huang
February 2016
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Theory of MW Emissions of Solar Flaring Loops. . . . . . . . . . . . . . . 13
2.1 Observational Characteristics of Solar Microwave Emissions. . . . . 14
2.1.1 Intensity, Polarization, and Spectrum of MW
Emission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.2 Radiation Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.1.3 Thermal and Nonthermal (NT) Emission. . . . . . . . . . . . . . 17
2.2 Gyrosynchrotron (GS) Emission . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.1 Emissivity and Self-absorption Coefficient. . . . . . . . . . . . . 18
2.2.2 Formation of GS Spectrum. . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.3 Influence of Magnetic Field Strength . . . . . . . . . . . . . . . . 21
2.2.4 Influence of Self-Absorption. . . . . . . . . . . . . . . . . . . . . . . 22
2.2.5 Influence of High Plasma Density: Razin Effect . . . . . . . . 24
2.2.6 Razin Effect and Electron Power-Law Index . . . . . . . . . . . 26
2.2.7 Plasma Density Increase on the Late Decay Phase. . . . . . . 29
2.2.8 Influence of Plasma Inhomogeneity on GS Spectrum. . . . . 29
2.3 Effects of Electron Pitch-Angle Anisotropy . . . . . . . . . . . . . . . . . 32
2.3.1 Parameters of Numerical Simulation . . . . . . . . . . . . . . . . . 33
2.3.2 Pitch-Angle Distributions of SinN Type . . . . . . . . . . . . . . 33
2.3.3 Effect of Pitch-Angle Distribution Shape: Gaussian
Loss-Cone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.3.4 Effect of Pitch-Angle Distribution Shape: Beam-like
Distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.3.5 Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.4 Trapping and Transport Effects . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.4.1 Dependence on the Position of Acceleration/Injection
Site. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
ix
x Contents
2.4.2 Spectral an Polarization Responses to Specific Electron
Distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.4.3 Diagnostic Potential. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
2.5 Effects of Other Parameters on GS Emission . . . . . . . . . . . . . . . . 55
2.5.1 Spectral Shape. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
2.5.2 Radio Flux Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
2.5.3 Spectral Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
2.5.4 Turnover Frequency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
2.6 Numerical Codes for Fast GS Emission Calculations . . . . . . . . . . 59
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3 ObservationsandExplanationsofMWEmissionsinSolarFlaring
Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.1 Studies on Spatially Unresolvable Observations . . . . . . . . . . . . . . 65
3.1.1 Flattened Spectra in Solar Radio Bursts at Cm and Mm
Bands and Dynamics of Energetic Electrons in Flaring
Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.1.2 Dynamics of Peak Frequency in Solar MW Bursts:
Self-absorption and Razin Effect. . . . . . . . . . . . . . . . . . . . 75
3.1.3 Optically Thin Emission, Power-Law Distribution of
Flares, and Occurrence Rate of Flares. . . . . . . . . . . . . . . . 95
3.2 Spatial Distribution of Microwave Brightness. . . . . . . . . . . . . . . . 100
3.2.1 Nonthermal MW Source at the Top of Extended Flaring
Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.2.2 Time Delays Between MW Emissions from Different
Parts of Flaring Loop. . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
3.2.3 Time Delays of MW Emissions at Different
Frequencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
3.2.4 Redistribution of MW Brightness in Flaring Loops . . . . . . 104
3.2.5 Comparison of Observations and Model Predictions . . . . . 106
3.2.6 Spatial Distribution of Energetic Electrons in Flaring
Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
3.2.7 Dynamics of Electron Spatial Distribution. . . . . . . . . . . . . 110
3.2.8 Constraints on Particle Acceleration, Projection,
and Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
3.3 Statistical Studies on MW Brightness Distributions. . . . . . . . . . . . 112
3.3.1 Data Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
3.3.2 Comparison of MW Brightness in LT and FPs . . . . . . . . . 114
3.3.3 Relation of MW Brightness and Other Parameters. . . . . . . 114
3.3.4 Asymmetry of MW Brightness in Flaring Loop FPs . . . . . 117
3.4 Spectral Properties of MW Emissions . . . . . . . . . . . . . . . . . . . . . 126
3.4.1 Distribution of Spectral Slopes Along Flaring Loops . . . . . 127
3.4.2 Statistics of Optically Thin Spectral Indices of MW
Emissions in LT and FPs. . . . . . . . . . . . . . . . . . . . . . . . . 132
