Date of Award
Doctor of Philosophy in Applied Physics - (Ph.D.)
Dale E. Gary
Gregory D. Fleishman
Jason T. L. Wang
As the most energetic explosive events in the solar system and a major driver for space weather, solar flares need to be thoroughly understood. However, where and how the free magnetic energy stored in the corona is released to power the solar flares remains not well understood. This lack of understanding is, in part, due to the paucity of coronal magnetic field measurements and the lack of comprehensive understanding of nonthermal particles produced by solar flares. This dissertation focuses on studies that utilize microwave imaging spectroscopy observations made by the Expanded Owens Valley Solar Array (EOVSA) to diagnose the nonthermal electrons and coronal magnetic field in solar flares.
In the first study, a partial eruption of a twisted solar filament is observed in Ha and extreme ultraviolet (EUV) wavelengths during an M1.4-class solar flare on September 6, 2017. The microwave counterpart of the filament is observed by EOVSA. The spectral properties of the microwave source are consistent with nonthermal gyrosynchrotron radiation. Using spatially resolved microwave spectral analysis, the magnetic field strength along the filament spine is derived, which ranges from 600-1400 Gauss from its apex to the legs. The results agree well with the non-linear force-free magnetic model extrapolated from the pre-flare photospheric magnetogram. The existence of the microwave counterpart also suggests that the newly reconnected magnetic field lines have the flare-accelerated electrons injected into the filament-hosting magnetic flux rope cavity.
The second study focuses on another eruptive solar flare event that features three post-impulsive X-ray and microwave bursts immediately following its main impulsive phase. A tight positive correlation between the flux rope acceleration and electron energization is found during the post-impulsive phase bursts, conforming to the standard flare?coronal-mass-ejection scenario, in which positive feedback between flare reconnection and flux rope acceleration is expected. In contrast, such a correlation does not seem to hold during its main impulsive phase. The lack of flux rope acceleration during the main impulsive phase, as interpreted in this dissertation, is mainly attributed to the tether-cutting reconnection scenario when the flux rope eruption has not been fully underway. In addition, observations suggest a weakening guide field may contribute to the hardening of the nonthermal electron spectrum throughout the main- and post-impulsive phase of the event, shedding new light on understanding the electron acceleration mechanisms in solar flares.
Wei, Yuqian, "Coronal magnetometry and energy release in solar flares" (2023). Dissertations. 1672.