Correlation of microwave and hard X-ray spectral parameters

We present the analysis of 27 solar flares with multiple peaks that were observed at hard X-ray and microwave wavelengths. A total of 57 simultaneous peaks were observed by BATSE (hard X-rays) and Owens Valley Radio Observatory (microwaves). Throughout the duration of a flare, its spectra at both wavelengths are fitted independently at all times. The hard X-ray spectra were fitted by a single power law in most cases, whereas the microwave spectra were fitted as gyrosynchrotron emission. For each individual peak, the parameters at both wavelengths (peak flux, turnover frequency, spectral indices, and delays between hard X-ray and microwave peak emission) were then compared and correlated. We have also studied impulsive and nonimpulsive bursts individually. The main results obtained were as follows. (1) In 75% of the bursts, the inferred index of the electron energy distribution of the microwave-emitting electrons, δ_r, is harder than that of the lower energy hard X-ray-emitting electrons, δ_X, on average by 0.5-2.0. This implies that there is a breakup in the energy spectra of the electrons, as is sometimes observed in the hard X-ray spectra of giant flares. (2) A soft-hard-harder spectral index temporal evolution is more commonly seen in the microwave spectra (47%) than in the hard X-ray observations (32%) and in nonimpulsive flares than in impulsive ones. (3) Delays larger than 2 s were observed between the radio and hard X-ray peaks in 65% of the bursts, with the delays decreasing as the hard X-ray energy increased. (4) Nonimpulsive flares are more microwave rich, have higher delays between their radio emission and the hard X-ray peaks, and display harder spectral indices than impulsive bursts.