Time Evolution of Solar Microwave Bursts

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Abstract
Spectra of solar microwave bursts often have a steeper slope than expected on the low-frequency side, manifest a low brightness temperature, and maintain a constant frequency of maximum brightness temperature νpeak, despite the burst's evolution in peak brightness. We used a gyrosynchrotron code to model the emission from the flare of 1992 July 16, which exhibited these characteristics. We find that the Razin effect—the suppression of radiation from an electron in a medium in which the index of refraction is less than unity—accounts for the shape and evolution of the burst. Using data from the Owens Valley Solar Array we demonstrate that in a medium with density 2 × 1011 cm-3 and magnetic field 300 G, conditions not uncommon for solar microwave bursts, the gyrosynchrotron spectrum from mildly relativistic electrons can be suppressed for frequencies up to at least 10 GHz. We can then explain the constancy of νpeak, the burst's low brightness temperature, and the steep low-frequency slope of the spectrum using a simple model in which only the density of accelerated electrons varies during the flare. Our model implies values for the density of accelerated and ambient electrons which are in agreement with those derived from X-ray data for this flare.