Diagnostics of energy release in solar flares with radio dynamic imaging spectroscopy

Studies of the magnetic energy release and conversion process lie at the core of solar flare physics. Radio observations serve as a unique diagnostic method. In this dissertation, taking advantage of broadband radio dynamic imaging spectroscopy observations made by the Karl G. Jansky Very Large Array (VLA), studies are carried out on the flare energy release processes using different types of radio emissions.The VLA is a general-purpose radio observatory located in New Mexico, which provides high-quality radio dynamic imaging spectroscopic observations with an ultra-fast time cadence. In the first study, stochastic decimetric radio spike bursts are observed by the VLA during the extended energy release phase of an M8.4-class eruptive flare on March 10, 2012. Such short-lived and narrow-band spike bursts have been suggested as the signature of interactions between nonthermal electrons and density fluctuations at the termination shock front. VLA radio dynamic spectroscopic imaging of spikes centroids reveals an extended structure that resembles a shape consistent with a shock surface in projection. Using extreme-ultraviolet (EUV) imaging of the flare event from two different vantage views, combined with flare ribbons features from hard X-ray (HXR) and ultraviolet (UV) observations, a three-dimensional flare arcade is reconstructed. The radio spikes are located to be present at above the flare arcade, where the diffuse supra-arcade fan structure and plasma downflows exist. The source morphology and the reconstructed location suggest that the observed radio spikes are produced at a solar flare termination shock driven by fast plasma outflows impinging upon the flare arcade.The second study focuses on flare-associated quasi-periodic pulsations (QPPs), observed during the impulsive phase of a C1.8 flare. QPPs with a nonthermal nature are excellent probes for the energy release and transport processes during flares. However, a further understanding of their nature is limited owing to the paucity of spatially resolved observations with high temporal resolution. In this study, such a QPP event is observed during the impulsive phase of a C1.8-class eruptive solar flare. Radio imaging spectroscopy offered by the VLA shows three spatially distinct radio sources with very different physical characteristics. Two radio sources are located near the conjugate footpoints of the erupting magnetic flux rope with opposite senses of polarization, one of which displays a QPP behavior with a short, ?5-s period. The third radio source, located at the top of the post-flare arcade, coincides with the location of a looptop X-ray source and shares a similar period of ?25–45 s. It is further demonstrated that the two oppositely polarized radio sources are likely due to coherent electron cyclotron maser (ECM) emission from nonthermal electrons trapped near the footpoints of the erupting flux rope. On the other hand, the looptop QPP source, observed in both radio and X-rays, is consistent with incoherent gyrosynchrotron and bremsstrahlung emission, respectively, from nonthermal electrons trapped in the looptop region. It is concluded that the concurrent but spatially distinct QPP sources cannot be explained by a single modulation mechanism but must involve multiple mechanisms which operate in different magnetic loop systems and at different periods.The studies carried out in the dissertation address outstanding questions of flare energy release and the subsequent energy conversion and transport processes. They are mainly based on observations provided by the general-purpose Jansky VLA, which offers the capability of high-cadence, high-resolution dynamic radio imaging spectroscopy. They are also complemented by multi-wavelength data in EUV and X-rays obtained by space-borne solar telescopes observing from different perspectives as well as advanced emission modeling. These case studies further demonstrate the power of using high-cadence radio dynamic imaging spectroscopy to achieve an improved understanding of solar flare energy release. However, owing to the nature of these proposal-based, infrequent observations from a general-purpose telescope like the VLA, a systematic study is not possible. Future advance on this ground calls for the development of more powerful radio facilities dedicated to solar observing.