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 1 993 April 22. Both thermal and nonthermal emissions were observed in r adio. In soft X-rays, the flare was confined to a compact region in an arcade. In hard X-rays, there were two prominent footpoints, coincide nt in projection with the soft X-ray footpoints and located on either side of the magnetic neutral line inferred from photospheric magnetogr ams. The Yohkoh Bent Crystal Spectrometer (BCS) data provided importan t context information which was helpful in cross-checking the quantita tive agreement between the radio and X-ray data. The microwave spectru m peaked around 10 GHz and showed Razin suppression in the beginning. Later on, the low-frequency spectral index dropped to a value of 2, su ggesting thermal emission. The VLA images of the flare at 1.5 GHz show that the flare emission started as a single source above one footpoin t; later on, the emission centroid moved toward the soft X-ray structu re to finally become cospatial with the latter. The two locations of t he 20 cm source corresponded to nonthermal (footpoint source) and ther mal (source cospatial with the soft X-ray structure) emissions. We per formed temperature and emission measure analysis of the X-ray data (SX T, BCS, and HXT) and used them as input to determine the expected radi o emission. While there is morphological agreement between the radio a nd soft X-ray structures in the thermal phase, the 20 cm brightness te mperature shows quantitative agreement with temperature derived from t he BCS data. We were able to identify three emission mechanisms contri buting to the 20 cm radio emission at different times without any ad h oc assumption regarding emission mechanisms. Razin-suppressed nontherm al gyroresonance emission, plasma emission, and thermal free-free emis sion seem to be operating and are found to be consistent with the plas ma parameters derived from the X-ray data. The magnetic field structur e 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 3 2 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 w avelength. We derived the physical parameters of the flaring plasma, t he magnetic field, and the characteristics of nonthermal particles in the flaring region.