OVRO, BBSO, BATSE, AND YOHKOH OBSERVATIONS OF A TWIN SOLAR-FLARE

We present the results of studies on two solar flares that occurred on 1993 February 11: an M1.1 hare at 18:07 UT and an M2.7 flare at 18:31 UT. Our study was based on comprehensive observations by the followin g observatories: Owens Valley Radio Observatory, which obtains 1-18 GH z microwave images; Big Bear Solar Observatory, which obtains magnetog rams, H alpha and He D3 filtergrams; BATSE on board Compton Gamma Ray Observatory, which obtains high-resolution hard X-ray spectra; and the Japanese satellite Yohkoh, which obtains high-resolution soft and har d X-ray images. We find the following. (1) While the optical and hard X-ray emissions are confined to a small loop near the leading spot of the active region for both flares, a large-scale soft X-ray loop conne cts from the leading to the following spot 160 '' away. In low-frequen cy microwaves (<4 GHz), sources appear at each end of the big loop, an d the source near the following spot (away from the H alpha flare site ) dominates at frequencies <2.8 GHz. For both flares, as frequency inc reases, the source near the leading spot becomes dominant, and the sou rce near the following spot vanishes gradually. (2) As frequency incre ases, the centroid of the leading microwave source moves progressively downward until it reaches the footpoint at high frequencies. (3) For the M2.7 event, in the compact loop near the leading spot, two footpoi nts are seen in both soft and hard X-rays. The dominant hard X-ray sou rce has a softer spectrum than the weaker one, suggesting that the wea ker one may become dominant at the higher energies (>100 keV) responsi ble for the microwave emission. The high-frequency microwave emission is better associated with this latter footpoint. (4) The large soft X- ray loop in the M2.7 flare is the postflare loop of the M1.1 hare. Thi s flare is associated with a different compact loop which is 40 '' awa y from the main hare. (5) For the M2.7 flare, the microwave brightness temperature spectra in the sources at the two ends of the big loop re quire very different source parameters. The primary source near the le ading spot can be explained by nonthermal gyrosynchrotron emission fro m electrons with a power-law energy index (delta) of 5.3. The same gro up of electrons can explain the observed BATSE hard X-ray spectra. The low-frequency radio source near the following spot is due to either a thermal component, or a nonthermal component with a steep energy inde x (delta = 9.4). Based on the available information, we cannot disting uish these two possibilities.