SPATIAL STRUCTURE OF SOLAR CORONAL MAGNETIC LOOPS REVEALED BY TRANSIENT MICROWAVE BRIGHTENINGS

We present the measurement of magnetic field gradient in magnetic loop s in the solar corona, based on the multi-wavelength Very Large Array observations of two transient microwave brightenings (TMBs) in the sol ar active region 7135. The events were observed at 2 cm (spatial resol ution similar to 2 '') and 3.6 cm (spatial resolution similar to 3 '') with a temporal resolution of 3.3 a in a time-sharing mode. Soft X-ra y data (spatial resolution similar to 2.5 '') were available from the Soft X-ray Telescope on board the Yohkoh satellite. The three-dimensio nal structure of simple magnetic loops, where the transient brightenin gs occurred, were traced out by these observations. The 2-cm and 3.6-c m sources were very compact, located near the footpoint of the magneti c loops seen in the X-ray images. For the two events reported in this paper, the projected angular separation between the centroids of 2 and 3.6-cm sources is about 2.3 '' and 3.1 '', respectively. We interpret that the 2 and 3.6-cm sources come from thermal gyro-resonance emissi on. The 2-cm emission is at the 3rd harmonic originating from the gyro -resonance layer where the magnetic field is 1800 G. The 3.6-cm emissi on is at the 2nd harmonic, originating from the gyro-resonance layer w ith a magnetic field of 1500 G. The estimated magnetic field gradient near the footpoint of the magnetic loop is about 0.09 G km(-1) and 0.1 2 G km(-1) for the two events. These values are smaller than those obs erved in the photosphere and chromosphere by at least a factor of 2.