A Statistical Study of Microwave Flare Morphologies

V. Tzatzakis$^1$, A. Nindos$^1$, C.E. Alissandrakis$^1$, K. Shibasaki$^2$

$^1$Section of Astrogeophysics, Physics Department, University of Ioannina, Ioannina GR-45110, Greece
$^2$Nobeyama Radio Observatory, National Astronomical Observatory of Japan, Minamimaki, Minamisaku, Nagano 384-1305, Japan

Using Nobeyama Radioheliograph (NoRH) high spatial resolution images at 17 and 34 GHz, we study the morphology of several flare events that occurred relatively close to the limb. Our study has been motivated by the recent detection of a small number of optically thin sources 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 are able to determine whether the 17 and 34 GHz emissions are optically thin or thick. More than 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 ($I$) morphologies are consistent with a single flaring loop configuration. The number of optically thin events with $I$ maxima co-spatial with the $V$ sources is at least twice larger than the number of optically thin events with maximum emission between the $V$ sources. The number of optically thick events is approximately equal to the number of optically thin loop-top events. Our future plans concerning the study of our database are also outlined.