A Statistical Study of Microwave Flare Morphologies

Using Nobeyama Radioheliograph (NoRH) high spatial resolution images at 17 and 34 GHz, we study the morphology of several flare events that occured relatively close to the limb. Our study has been motivated by the recent detection of a small number of optically thin flares whose maximum emission peaks close to the loop top. These events may show significant anisotropy of the density and pitch angle distribution of the nonthermal electrons. However, we do not know how often they occur. Our sample consists of 104 flare events. Using data from the Nobeyama Polarimeter we were able to determine whether the 17 and 34 GHz emissions are optically thin or thick. Almost half of our events appear unresolved in the NoRH images. Among the resolved events, special attention is paid to those with circular polarization (V) showing two distinct sources. Such V sources are considered proxies to the flaring loop footpoints if the total intensity (I) morphology is consistent with a single flaring loop configuration. A small number of events, as expected at such high frequencies, are optically thick with I maximum between the loop footpoints. The number of optically thin events showing I maxima co-spatial with the V sources is approximately the same as the number of those optically thin resolved events showing emission peak between the V sources. Our future plans concerning the study of our database are also outlined.