VLA AND YOHKOH OBSERVATIONS OF AN M1.5 FLARE

A major solar flare (X-ray importance M1.5 and optical importance SB) was fully observed by the Very Large Array and the Yohkoh mission on 1993 April 22. Both thermal and nonthermal emissions were observed in radio. In soft X-rays, the flare was confined to a compact region in an arcade. In hard X-rays, there were two prominent footpoints, coincident in projection with the soft X-ray footpoints and located on either side of the magnetic neutral line inferred from photospheric magnetograms. The Yohkoh Bent Crystal Spectrometer (BCS) data provided important context information which was helpful in cross-checking the quantitative agreement between the radio and X-ray data. The microwave spectrum peaked around 10 GHz and showed Razin suppression in the beginning. Later on, the low-frequency spectral index dropped to a value of 2, suggesting thermal emission. The VLA images of the flare at 1.5 GHz show that the flare emission started as a single source above one footpoint; later on, the emission centroid moved toward the soft X-ray structure to finally become cospatial with the latter. The two locations of the 20 cm source corresponded to nonthermal (footpoint source) and thermal (source cospatial with the soft X-ray structure) emissions. We performed temperature and emission measure analysis of the X-ray data (SXT, BCS, and HXT) and used them as input to determine the expected radio emission. While there is morphological agreement between the radio and soft X-ray structures in the thermal phase, the 20 cm brightness temperature shows quantitative agreement with temperature derived from the BCS data. We were able to identify three emission mechanisms contributing to the 20 cm radio emission at different times without any ad hoc assumption regarding emission mechanisms. Razin-suppressed nonthermal gyroresonance emission, plasma emission, and thermal free-free emission seem to be operating and are found to be consistent with the plasma parameters derived from the X-ray data. The magnetic field structure in the flaring region showed differences before and after the flare as traced by soft X-ray structures in the flaring region and confirmed by 20 cm radio images. The superhot component with a temperature of 32 MK was observed in hard X-ray images and in light curves during the impulsive phase of the flare with possible radio signatures at 20 cm wavelength. We derived the physical parameters of the flaring plasma, the magnetic field, and the characteristics of nonthermal particles in the flaring region.