Free-free absorption of gyrosynchrotron radiation in solar microwave bursts.

Abstract : A model for solar microwave bursts is considered in which over a broad frequency band the flux density is a slowly varying function of frequency. It is proposed that such an essentially flat spectrum could result from free-free absorption of gyrosynchrotron emission of nonthermal electrons accelerated in solar flares. The theory of gyrosynchrotron radiation is reviewed, and simplified formulae for the Razin effect, selfabsorption and gyroresonance absorption are given and their range of validity is discussed. Using these results the thermal electron density, temperature, emission measure, magnetic fields and number of nonthermal electrons in the emitting region are evaluated for a flare with a flat microwave spectrum. (Author)