OBSERVATIONS AND MODELING OF PLASMA FLOWS DRIVEN BY SOLAR FLARES by Sean ...

206 Pages·2016·5.37 MB·English

OBSERVATIONS AND MODELING OF PLASMA
FLOWS DRIVEN BY SOLAR FLARES
by
Sean Robert Brannon
A dissertation submitted in partial fulfillment
of the requirements for the degree
of
Doctor of Philosophy
in
Physics
MONTANA STATE UNIVERSITY
Bozeman, Montana
January 2016
(cid:13)c COPYRIGHT
by
Sean Robert Brannon
2016
All Rights Reserved
ii
DEDICATION
To Molly Catherine Arrandale.
iii
ACKNOWLEDGEMENTS
I would like to begin by thanking my academic advisor and committee chair, Prof.
Dana Longcope. His knowledge of physics is without peer, and he was kind enough
to patiently bestow his time and advice to me time and again as I hammered my
way painfully through this process. I would also like to extend my gratitude to my
graduate committee, especially Profs. David McKenzie and Charles Kankelborg for
their invaluable support along the way and for listening when I had concerns. Of
course, I must thank the MSU Department of Physics and the MSU Solar Group,
for providing me with a community of peers to whom I could always turn when I
needed help. I am forever indebted to all of the staff who have tirelessly worked to
shield me from the horrors of bureaucracy; this goes double for Margaret Jarrett,
who was always there for me with a kind heart and sound advice when I didn’t know
where else to turn. My family, especially Mom and Dad, who always encouraged me
along the way even when they had no idea what solar physics is. All of my friends
and classmates, especially Ritoban and Nickolas, who made physics fun even as we
complained about it. And finally, most importantly, to my fianc´e Molly: you are the
love of my life, my confidant, and my best friend. I look forward every day to our
journey together, and to whatever life brings us next.
iv
TABLE OF CONTENTS
1. INTRODUCTION................................................. 1
Overture ......................................................... 1
Structuring in the Solar Atmosphere ................................. 5
Evolution of a Solar Flare .......................................... 11
Plasma Flows Driven by Solar Flares................................. 18
The Narrative of My Story ......................................... 26
2. MODELING PROPERTIES OF CHROMOSPHERIC
EVAPORATION DRIVEN BY THERMAL CONDUCTION
FRONTS FROM RECONNECTION SHOCKS........................ 30
Contribution of Authors and Co–Authors............................. 30
Manuscript Information Page ....................................... 31
Introduction ...................................................... 32
Flare Loop Model ................................................. 37
Simulation Setup .................................................. 41
1-D Fluid Equations .......................................... 41
Numerical Integration ......................................... 43
Viscosity and Conductivity .................................... 45
Initial Loop Atmosphere....................................... 48
Initial Piston Shock........................................... 51
Simulation Results ................................................ 53
Hydrodynamics .............................................. 53
Differential Emission Measure .................................. 61
Synthetic Doppler Velocities ................................... 64
Observational Data Fit........................................ 70
Scaling Laws ..................................................... 73
Discussion........................................................ 79
Acknowledgements ................................................ 86
3. SPECTROSCOPIC OBSERVATIONS OF EVOLVING
FLARE RIBBON SUBSTRUCTURE SUGGESTING ORI-
GIN IN CURRENT SHEET WAVES ................................ 87
Contribution of Authors and Co–Authors............................. 87
Manuscript Information Page ....................................... 88
Introduction ...................................................... 89
Observation ...................................................... 94
Instrument .................................................. 94
Flare ....................................................... 95
v
TABLE OF CONTENTS – CONTINUED
Wavelength Correction ........................................100
Results ..........................................................102
Ribbon Evolution ............................................102
Spectral Line Fitting..........................................107
Doppler Velocities ............................................113
Additional Spectral Lines......................................115
East Ribbon .................................................118
Interpretation.....................................................121
Discussion........................................................129
Acknowledgements ................................................136
4. OBSERVATIONSOFPSEUDO-BALLISTICDOWNFLOWS
IN A M-CLASS FLARE WITH THE INTERFACE REGION
IMAGING SPECTROGRAPH ......................................137
Contribution of Authors and Co–Authors.............................137
Manuscript Information Page .......................................138
Introduction ......................................................139
Instrument .......................................................144
Flare Details......................................................145
Results ..........................................................151
Flare Evolution ..............................................151
Spectral Intensity Evolution ...................................156
Doppler Velocities and Line Widths .............................159
Plasma Density ..............................................165
Cooling Time ................................................168
Synthetic Spectra .................................................171
Discussion........................................................176
Acknowledgements ................................................182
5. DISCUSSION.....................................................183
REFERENCES CITED .................................................190
vi
LIST OF TABLES
Table Page
2.1. Simulation labels and properties................................... 75
vii
LIST OF FIGURES
Figure Page
1.1. Large-scale structuring of the solar atmosphere ..................... 6
1.2. Small-scale structuring of the solar atmosphere ..................... 9
1.3. Schematic diagram of a solar flare ................................. 13
2.1. Schematic diagram of shock tube model............................ 39
2.2. Artificial loop atmosphere........................................ 50
2.3. Hydrodynamic simulation evolution ............................... 55
2.4. Conductive-to-freestreaming flux ratio ............................. 60
2.5. Differential emission measure evolution ............................ 62
2.6. Flow velocity and synthetic Doppler velocity........................ 66
2.7. Evolution of the flow reversal point................................ 69
2.8. Fit to observational data......................................... 71
2.9. Results of parameter survey ...................................... 77
3.1. SDO/AIA 1600 ˚A image of the flare ribbons ........................ 96
3.2. SDO/HMI magnetogram of the flare active region ................... 98
3.3. SDO/AIA 171 ˚A image of the post-flare loops and ribbons ........... 99
3.4. IRIS SJI 1400 ˚A image of the flare ribbons .........................101
3.5. Time series of SJI 1400 ˚A images .................................103
3.6. Time-distance stack plots of intensity evolution .....................106
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LIST OF FIGURES – CONTINUED
Figure Page
3.7. Selected spectral plots ...........................................109
3.8. Time-distance stack plot of Doppler velocity evolution ...............114
3.9. Time slices of Doppler velocities ..................................116
3.10. Intensity stack plots for East Ribbon ..............................119
3.11. Phase portrait of sawtooth velocity and position ....................122
3.12. Schematic diagram of proposed scenario............................126
4.1. HMI context magnetogram of AR 12297 before flare .................147
4.2. AIA 1600 ˚A context image of AR 12297 during flare .................148
4.3. AIA 171 ˚A context image of AR 12297 after flare....................149
4.4. IRIS SJI 1400 ˚A context image during flare.........................150
4.5. Time series of IRIS SJI and SG images ............................153
4.6. Time-distance stack plots for Fe xxi and Si iv intensity ..............157
4.7. Spectra from representative pixels within the bullet..................161
4.8. Time-distance stack plots for Doppler velocity and NTB .............163
4.9. Time-distance stack plot for O iv density analysis ...................166
4.10. Synthetic spectral Doppler shifts ..................................173
ix
ABSTRACT
One of the fundamental statements that can be made about the solar atmosphere
is that it is structured. This structuring is generally believed to be the result of both
the arrangement of the magnetic field in the corona and the distribution of plasma
along magnetic loops. The standard model of solar flares involves plasma transported
into coronal loops via a process known as chromospheric evaporation, and the result-
ing evolution of the flare loops is believed to be sensitive to the physical mechanism of
energy input into the chromosphere by the flare. We present here the results of three
investigations into chromospheric plasma flows driven by solar flare energy release
and transport. First, we develop a 1-D hydrodynamic code to simulate the response
of a simplified model chromosphere to energy input via thermal conduction from
reconnection-driven shocks. We use the results from a set of simulations spanning a
parameter space in both shock speed and chromospheric-to-coronal temperature ratio
to infer power-law relationships between these quantities and observable evaporation
properties. Second, we use imaging and spectral observations of a quasi-periodic os-
cillation of a flare ribbon to determine the phase relationship between Doppler shifts
of the ribbon plasma and the oscillation. The phase difference we find leads us to sug-
gest an origin in a current sheet instability. Finally, we use imaging and spectral data
of an on-disk flare event and resulting flare loop plasma flows to generally validate
the standard picture of flare loop evolution, including evaporation, cooling time, and
draining downflows, and we use a simple free-fall model to produce the first direct
comparison between observed and synthetic downflow spectra.