A THERMAL NONTHERMAL MODEL FOR SOLAR MICROWAVE-BURSTS

High-resolution spectra of microwave bursts from Owens Valley Radio Observatory show numerous departures from expectations based on simple thermal or nonthermal models. In particular, (1) approximately 80% of the events show more than one spectral peak; (2) many burst have a low-side spectral index steeper than the maximum expected slope; and (3) the peak frequency stays relatively constant, and changes intensity in concert with the secondary peaks throughout a given event's evolution. We develop a theoretical formalism allowing both thermal and nonthermal particles to coexist in the flaring plasma. Within this formalism, we show that the observed spectral features can be accounted for through gyrosynchrotron radiation. Specifically: 1. The "secondary" components seen on the low-frequency side of many spectra are nonthermal enhancements superposed upon thermal radiation, occurring between the thermal harmonics. 2. A steep optically thick slope is accounted for by the thermal absorption of nonthermal radiation. 3. If the coexistence of thermal and nonthermal particles is interpreted in terms of electron heating and acceleration in current sheets, a changing electric field strength can account for the gross evolution of the microwave spectra. We present our model distribution function, use it to calculate gyrosynchrotron spectra, and systematically analyze these spectra. We demonstrate the applicability of this approach by fitting an observed Owens Valley spectrum. Last, we offer our physical interpretation of the thermal/nonthermal distribution function as it relates to solar flares.