TRANSIENT MICROWAVE BRIGHTENINGS IN SOLAR ACTIVE REGIONS - COMPARISONBETWEEN VLA AND YOHKOH OBSERVATIONS

We report observations of transient microwave (2 cm) brightenings and their relationship with brightenings in soft X-rays. The peak flux of the microwave brightenings observed by the Very Large Array is smaller than the previously reported fluxes by two orders of magnitude. The m icrowave sources were highly polarized (up to 100%) and were situated on the periphery of a sunspot umbra. Among the many transients observe d in X-rays by Yohkoh, two were observed simultaneously in microwaves. The microwave sources were found to be closer to the umbra of the sun spot than were the X-ray loops. It seems that the microwave sources ar e located at the footpoints of the looplike X-ray transients. Using th e combined VLA, Yohkoh, and Mees data set, we determine the physical p arameters of the loop in which the brightenings occur. We find that an increase in emission measure accompanied by small-scale heating can a ccount for the X-ray brightening. The microwave emission can be interp reted as thermal gyroresonance or nonthermal gyrosynchrotron processes during the X-ray brightening. The magnetic field in the microwave-sou rce region is found to be 1200-1800 G. The observations also provide e vidence for temperature gradient in the coronal loops.