Solar flares as natural particle accelerators : a high-energy view from x-ray observations and theoretical models

247 Pages·2008·6.99 MB·English

by  Wei Liu

SOLAR FLARES AS NATURAL PARTICLE ACCELERATORS:
A HIGH-ENERGY VIEW FROM X-RAY OBSERVATIONS AND
THEORETICAL MODELS
Wei Liu
NASA Goddard Space Flight Center
To my wife
Li Jin
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Figure 1: Artistic view of RHESSI observing the Sun from a near-earth orbit. RHESSI is
a NASA Small Explorer mission designed to investigate particle acceleration and explosive
energy release in solar (cid:176)ares. Since its launch on 2002 February 5, RHESSI has been
providing scientists with unprecedented data revealing new physics and challenging existing
theories. This mission inspired the work that eventually led to the production of this book
[Courtesy of RHESSI Team].
Front cover picture | A full Sun image showing a (cid:176)are near disk center and an eruptive
prominence on the west limb.
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Contents
Preface ix
Acknowledgements xi
1 Introduction 1
1.1 Solar Flare Observations and Models . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Stochastic Particle Acceleration Model . . . . . . . . . . . . . . . . . . . . . 2
1.3 Hard X-ray Observations and RHESSI Instruments . . . . . . . . . . . . . . 4
1.4 Introduction to This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4.1 RHESSI Observations . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4.2 Combining the Fokker-Planck and Hydrodynamic Codes . . . . . . . 6
2 Statistical Study of RHESSI Limb Flares 8
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Data Reduction and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1 Sample Selection Criteria . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.2 Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.3 Imaging Spectra and Light Curves . . . . . . . . . . . . . . . . . . . 11
2.3 Case Study Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3.1 Single Loop Flares . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3.2 Multiple Loop Flares . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.3 Miscellaneous Types . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4 Statistical Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4.1 Imaging Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4.2 Statistics of the Relative Fluxes: FPs vs. LTs . . . . . . . . . . . . . 20
2.5 Flare Statistics and Selection Biases . . . . . . . . . . . . . . . . . . . . . . 20
2.6 Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3 Flare Reconnection Model: 2003-11-03 X3.9 Flare 23
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2 Observations and Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.1 Source Structure and Motion . . . . . . . . . . . . . . . . . . . . . . 25
3.2.2 Imaging Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3 Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
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4 Double Coronal Source: 2002-04-30 M1.4 Flare 33
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.2 Observations and Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . 35
4.2.1 Source Structure: Energy Dependence . . . . . . . . . . . . . . . . . 37
4.2.2 Source Structure: Temporal Evolution . . . . . . . . . . . . . . . . . 39
4.2.3 Spectral Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.3 Interpretation and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.3.1 Energy Dependence of Source Structure . . . . . . . . . . . . . . . . 46
4.3.2 Temporal Evolution of Source Structure . . . . . . . . . . . . . . . . 48
4.3.3 Spectral Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.4 Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5 Conjugate HXR Footpoints: 2003-10-29 X10 Flare 53
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
5.2 Observations and Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . 55
5.2.1 RHESSI Light Curves and Images . . . . . . . . . . . . . . . . . . . 56
5.2.2 Imaging Spectroscopy of Footpoint and Loop-top Sources . . . . . . 59
5.2.3 Multiwavelength Images . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.3 Two-phase Unshearing Motions of HXR Footpoints . . . . . . . . . . . . . . 62
5.4 Temporal Correlations of Conjugate Footpoints . . . . . . . . . . . . . . . . 66
5.4.1 Spectral Correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.4.2 Spatial Correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.4.3 Magnetic Field Correlation . . . . . . . . . . . . . . . . . . . . . . . 70
5.4.4 Correlations Among Spectral, Spatial, and Magnetic Field Parameters 71
5.4.5 Implications of Various Correlations . . . . . . . . . . . . . . . . . . 72
5.5 HXR Footpoint Asymmetries . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.5.1 Magnetic Mirroring . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.5.2 Column Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.5.3 Magnetic Mirroring and Column Density Combined . . . . . . . . . 78
5.5.4 Other Transport Efiects and FP Asymmetries . . . . . . . . . . . . . 79
5.5.5 Acceleration-induced Asymmetry . . . . . . . . . . . . . . . . . . . . 80
5.6 Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
6 Chromospheric Evaporation: 2003-11-13 M1.7 Flare 85
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6.2 Observations and Data Analyses . . . . . . . . . . . . . . . . . . . . . . . . 86
6.2.1 Source Structure and Evolution . . . . . . . . . . . . . . . . . . . . . 89
6.2.2 Spectral Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.2.3 The Neupert Efiect . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.3 Loop Density Derivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
6.4 Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
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7 Modeling Impulsive Phase Solar Flares 109
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
7.2 Simulation Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
7.2.1 Stochastic Acceleration Model . . . . . . . . . . . . . . . . . . . . . 111
7.2.2 Particle Transport and Radiation Model . . . . . . . . . . . . . . . . 115
7.2.3 NRL Hydrodynamic Model . . . . . . . . . . . . . . . . . . . . . . . 117
7.2.4 Combining the Particle and Hydrodynamic Codes . . . . . . . . . . 118
7.3 Simulation Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
7.3.1 Case R: Reference Calculation . . . . . . . . . . . . . . . . . . . . . 123
7.3.2 Case A: Fiducial Run with SA Model . . . . . . . . . . . . . . . . . 125
7.3.3 Case B: Variable Electron Spectrum . . . . . . . . . . . . . . . . . . 134
7.3.4 Case C: Harder Electron Spectrum . . . . . . . . . . . . . . . . . . . 139
7.3.5 Case D: Smaller Normalization . . . . . . . . . . . . . . . . . . . . . 139
7.3.6 Comparing The Cases: A Summary . . . . . . . . . . . . . . . . . . 145
7.4 Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
8 Testing the Neupert Efiect 149
8.1 Energy Budget and the Neupert Efiect . . . . . . . . . . . . . . . . . . . . . 149
8.2 Case R: Reference Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . 151
8.2.1 History of Energy Budget . . . . . . . . . . . . . . . . . . . . . . . . 151
8.2.2 Neupert Efiect Test . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
8.3 Cases A-D: Combined HD & Particle Calculation . . . . . . . . . . . . . . . 157
8.3.1 Case A: Fiducial Run with SA Model . . . . . . . . . . . . . . . . . 157
8.3.2 Case B: Variable Electron Spectrum . . . . . . . . . . . . . . . . . . 159
8.3.3 Case C: Harder Electron Spectrum . . . . . . . . . . . . . . . . . . . 160
8.3.4 Case D: Smaller Normalization . . . . . . . . . . . . . . . . . . . . . 160
8.4 Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
9 Hydrodynamic Simulation of the Decay Phase 166
9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
9.2 Model of Suppression of Conduction and Plasma Heating . . . . . . . . . . 167
9.3 Numerical Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
9.3.1 Case A: No Heating or Suppression of Conduction . . . . . . . . . . 170
9.3.2 Case B: Heating Only . . . . . . . . . . . . . . . . . . . . . . . . . . 172
9.3.3 Case C: Suppression of Conduction Only . . . . . . . . . . . . . . . 173
9.3.4 Case D: Heating and Suppression of Conduction . . . . . . . . . . . 174
9.3.5 Comparing Cases A-D . . . . . . . . . . . . . . . . . . . . . . . . . . 176
9.4 Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
10 Concluding Remarks 180
10.1 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
10.1.1 Hard X-ray Observations . . . . . . . . . . . . . . . . . . . . . . . . 180
10.1.2 Combined Fokker-Planck and Hydrodynamic Modeling. . . . . . . . 183
10.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
Appendices 187
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A RHESSI Data Analysis Tools 187
A.1 Imaging Spectroscopy Flow Chart . . . . . . . . . . . . . . . . . . . . . . . 187
A.2 Notes for Imaging Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 190
A.3 Spectral Analysis for 2002-04-30 M1.4 Flare . . . . . . . . . . . . . . . . . . 192
A.3.1 Spatially Integrated Spectra . . . . . . . . . . . . . . . . . . . . . . . 192
A.3.2 Spatially Resolved (Imaged) Spectra . . . . . . . . . . . . . . . . . . 195
A.4 Efiects of Pulse Pileup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
A.4.1 Pileup Efiects for 2003-11-13 M1.7 Flare . . . . . . . . . . . . . . . . 197
A.4.2 Pileup Efiects on Imaging Spectroscopy for 2003-10-29 X10 Flare . . 198
A.5 RHESSI Simulation Tool and Its Applications . . . . . . . . . . . . . . . . . 200
B Notes for Analyzing 2003-10-29 X10 Flare 202
B.1 Coalignment of Images from Difierent Instruments . . . . . . . . . . . . . . 202
B.2 Derivation of Footpoint Fluxes from Asymmetric Column Densities . . . . . 204
B.3 Estimation of Column Densities in Loop Legs . . . . . . . . . . . . . . . . . 205
C Coulomb Loss and Difiusion in Warm Plasmas 208
C.1 Coulomb Loss in Warm Plasmas . . . . . . . . . . . . . . . . . . . . . . . . 208
C.2 Coulomb Difiusion in Warm Plasmas . . . . . . . . . . . . . . . . . . . . . . 210
C.3 Implementation of Coulomb Loss and Difiusion . . . . . . . . . . . . . . . . 211
C.4 Thermalization Test of Injected Distribution . . . . . . . . . . . . . . . . . . 214
List of 215
Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
Bibliography 219
Index 227
Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
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Preface
It is well-known that the Sun is vital to the Earth and human beings because it supplies all
the energy we need to survive. Another role that the Sun plays had escaped our attention
until recent decades and the advent of space exploration. The Sun not only afiects the
terrestrialclimate,butitalsocontrolstheconditionsinspace,theso-called\spaceweather",
through solar activity. Solar (cid:176)ares, discovered in 1859, are one of the most spectacular
phenomenaofsolaractivity. Theyarenaturalacceleratorsthatcanboostparticlestonearly
the speed of light. These energetic particles, when arriving in near-earth space, can damage
satellites and do harm to astronauts. Solar (cid:176)ares have thus stirred renewed interest of solar
and space physicists. They provide a unique laboratory for studying particle acceleration
mechanisms in general and investigating solar activity for space weather forecast purposes
in particular. Understanding solar (cid:176)ares has other far-reaching implications, and can shed
lighton(cid:176)aresoccurringelsewhereintheuniverse, suchasthoseonotherstarsandaccretion
disks and near black holes.
A rich literature exists describing various aspects of solar (cid:176)ares. This includes sev-
eral (cid:176)are-dedicated books: Solar Flares by S•vestka (1976), The Physics of Solar Flares
by Tandberg-Hanssen & Emslie (1988), and Particle Acceleration and Kinematics in Solar
Flares by Aschwanden (2002), and a few space mission motivated conference proceedings:
Solar Flares { A Monograph from Skylab Solar Workshop II edited by Sturrock (1980),
Energetic Phenomena on the Sun { The Solar Maximum Mission Flare Workshop Proceed-
ings edited by Kundu & Woodgate (1986), and the upcoming Solar Flares at High Energy
{ A RHESSI-inspired Monograph edited by Dennis, Emslie, Hudson, & Lin (2008). The
comprehensive textbook Physics of the Solar Corona by Aschwanden (2004) also includes
extensive material on solar (cid:176)ares.
Advances in our knowledge of solar (cid:176)ares have been driven by multiwavelength obser-
vations obtained by space-borne and ground-based instruments over decades, particularly
hard X-rays (HXRs) recorded by Solar Maximum Mission (SMM), Hinotori, Yohkoh, and
Compton Gamma Ray Observatory (CGRO). In February 2002 NASA’s Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI) was launched. Unprecedented data
poured in and new physics waited to be discovered. It was RHESSI’s scientiflc promise that
motivated me to devote my PhD research to high-energy physics of solar (cid:176)ares. This led to
the production of the materials presented in this work, which includes primarily my disser-
tation completed in late 2006 at Stanford University. In early 2008, after being approached
by the German publisher Verlag Dr. Mu˜ller who proposed to publish my dissertation as
a monograph, I made necessary revisions (Chapters 4 and 5) and additions (Appendices)
based on my postdoctoral research at NASA Goddard Space Flight Center.
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