Document Type

Dissertation

Date of Award

8-31-2021

Degree Name

Doctor of Philosophy in Applied Physics - (Ph.D.)

Department

Physics

First Advisor

Dale E. Gary

Second Advisor

Haimin Wang

Third Advisor

Gregory D. Fleishman

Fourth Advisor

Michele Pavanello

Fifth Advisor

Bin Chen

Abstract

Solar flares involve the sudden catastrophic release of magnetic energy stored in the Sun’s corona. This dissertation focuses on investigating the low frequency, optically-thick gyrosynchrotron emission during solar flares for its spatial and spectral dynamics, characteristics, and role in the flare process.

The first part of this dissertation first addresses the spectral dynamics and characteristics of the source morphology. The high-resolution spectra of a set of microwave bursts observed by the Expanded Owens Valley Solar Array (EOVSA) during its commissioning phase in the 2.5-18 GHz frequency range with 1-s time resolution are presented here. Out of the 12 events analyzed in this study, nine bursts exhibit a direct decrease with time in the optically thick spectral index αl, an indicator of source morphology. Particularly, five bursts display "flat" spectrum (αl ≤ 1.0) compared to that expected for a homogeneous/uniform source (αl ≈2.9). These flat spectra at the low-frequencies (< 10 GHz) can be defined as the emission from a spatially inhomogeneous source with a large area and/or with multiple emission components. In a subset of six events with partial cross-correlation data, two events with flat spectra both show a source size of ~ 120 arcsec at 2.6 - 3 GHz. Modeling based on inhomogeneity supports the conclusion that only multiple discrete sources can reproduce a flat spectrum. These flat spectra appear predominantly in the decay phase and typically grow flatter over the duration in most of the bursts, which indicates the increasing inhomogeneity and complexity of the emitting volume as the flare progresses. This large volume of flare emission filled with the trapped energetic particles is often invisible in other wavelengths, like hard X-rays, presumably due to the collisionless conditions in these regions of low ambient density and magnetic field strength.

In the second study, imaging spectroscopy of gyrosynchrotron emission from C-class flare SOL2017-04-04 observed by EOVSA is presented. The microwave source observed at the low frequencies showed a large source at the peak emission almost ten times as large as the associated high-frequency and hard X-ray flare sources. The area of these low-frequency sources grow steeply by more than an order of magnitude as we move from high to low frequency. Unlike a single and straightforward loop "standard solar model" type flare, this event in the microwave emission also shows the contribution of the different sized multiple magnetic flux loops that undergo "loop-loop 3D interaction", resulting in the flare eruption. The flare emission at these low frequencies is observed at a large spatial extent of the active region, whereas the other wavelength emission barely shows any signature of particle transport to create the extended emission at the secondary site. The microwave images indicate that, after the main reconnection process, the accelerated particles have access to a much larger volume of the flaring region through the overlying loops.

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