Document Type

Dissertation

Date of Award

Summer 2017

Degree Name

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

Department

Physics

First Advisor

Haimin Wang

Second Advisor

Dale E. Gary

Third Advisor

Gregory D. Fleishman

Fourth Advisor

Andrew Gerrard

Fifth Advisor

Lucia Kleint

Abstract

Solar flares are one of the most violent and energetic space weather events that are known to cause various adverse effects on the Earth. One of the major problems that must be solved to understand flares and to be able to predict their magnitudes is how the particles in the solar atmosphere are accelerated after the magnetic reconnection. One way to help solve this problem is to investigate the properties of the high energy electrons produced during the flare impulsive phase, observed in the hard X-ray (HXR) and microwave (MW). The two emissions are considered to be produced by a “common population” of the electrons, but some studies have also reported temporal, spatial, and energy discrepancies between them, challenging the widely-used notion. In order to truly understand the relationship between the two emissions, high-cadence observations must be made simultaneously in two wavelengths, both temporally and spatially, and the spectral inversion must also be spatially-resolved and done in a realistic magnetic field geometry.

In this dissertation, the properties of the high energy electrons produced in two major solar flares, the 2011-02-15 X2.2 flare and the 2015-06-22 M6.5 flare, are investigated, using the high-cadence HXR and MW observations, and the advanced three-dimensional modeling tools. For the 2011-02-15 X2.2 flare, the time delays, source locations, spectral indices, and their temporal evolution of the HXR and the MW emissions during the impulsive phase are investigated using observations made by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radio Observatory (NoRO). For the 2015-06-22 M6.5 flare, the realistic three-dimensional, forward-fit modeling of the HXR and the MW emission is conducted at one point in time during flare, using the Non-Linear Force Free Field (NLFFF) model extrapolated from the observed photospheric magnetic field, the three-dimensional multi-wavelength modeling tool GX Simulator, and the observational constraints provided by the RHESSI and the new Expanded Owens Valley Solar Array (EOVSA).

The major results in the 2011-02-15 M2.2 flare study are: (1) the multiple peaks simultaneously observed in the HXR and the MW during the impulsive phase came predominantly from two locations that suggest two separate episodes of magnetic reconnection, which can be interpreted in terms of tether-cutting flare scenario, (2) the transition between these two episodes occur more slowly in MW, suggesting the trapped nature of the MW-emitting electrons, and (3) the asymmetry in the HXR and MW emission intensity is observed, which can be explained by the asymmetry in the magnetic field strengths discussed in several previous studies. The major results in the 2015-06-22 M6.5 flare study are: (1) the low frequency part of the observed MW spectrum is modeled to be dominated by the emission from a “HXR invisible” source containing a non-negligible number of nonthermal electrons in a large volume of weak magnetic field, (2) the modeled electron populations in the “HXR visible” sources fit the standard flare model, with the thermal population interpreted as the result of chromospheric evaporation and the nonthermal population having an upward break in its power-law energy spectrum, producing HXR and MW emission in different energy range, and (3) the model can be successfully made with a post-reconnection magnetic field configuration. The results from this work motivate further modeling efforts, which have the potential to contribute to the prediction of the intensity of flare soft X-ray (SXR) emission and the Solar Energetic Particles (SEPs).

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