Microwave Spectral Evolution of Solar Flares

We analyze the time evolution of microwave spectral indices for 103 solar flares observed by Nobeyama Radio Polarimeters (NoRP) between 1998 and 2005. The microwave spectral index, .t/, is derived from the flux spectral index, ı.t/. We assume a flux frequency dependence of the form S. ;t/=F0 ı.t/ in the optically thin part, i.e., above the turnover frequency, fmax. Here, fmax is defined as the frequency with the maximum flux among six discrete frequencies (i.e., 1, 2, 3.75, 9.4, 17, and 35 GHz). Based on NoRP observations, we find two typical types of solar flares with various microwave spectral behavior. Type I displays a soft–hard–soft pattern, while the type II flare follows a soft–hard–harder behavior in the rise–peak–decay phases at 35 GHz. Our statistical results of microwave flares confirm previous findings. Most microwave flares appear to be of type II (91 out of the present samples), which is different from the hard X-ray flares dominated by the soft–hard–soft property. Such a difference is an observational constraint for electron acceleration and transport theories in flare models.