Statistical analysis of flaring loops observed by Nobeyama Radioheliograph. I. Comparison of looptop and footpoints

Abstract

Twenty-four events with looplike structures at 17 and 34 GHz are selected from the flare list of Nobeyama Radioheliograph. We obtained the brightness temperatures at 17 and 34 GHz, the polarization degrees at 17 GHz, and the power-law spectral indices at the radio peak time for one looptop (LT) and two footpoints (FPs) of each event. We also calculated the magnetic field strengths and the column depths of nonthermal electrons in the LT and FPs of each event, using the equations modified from the gyrosynchrotron equations by Dulk. The main statistical results from those data are summarized as follows. (1) The spectral indices, the brightness temperatures at 17 and 34 GHz, the polarization degrees at 17 GHz, the calculated magnetic field strengths, and the calculated column densities of nonthermal electrons are always positively correlated between the LT and the two FPs of the selected events. (2) About one-half of the events have the brightest LT at 17 and 34 GHz. (3) The spectral indices in the two FPs are larger (softer) than those in the corresponding LT in most events. (4) The calculated magnetic field strengths in the two FPs are always larger than those in the corresponding LT. (5) Most of the events have the same positive or negative polarization sense in the LT and the two FPs. (6) The brightness temperatures at 17 and 34 GHz in each of the LT and the two FPs statistically decrease with their spectral indices and the calculated magnetic field strengths, but increase with their calculated column densities of nonthermal electrons. Moreover, we try to discuss the possible causes of the present statistical results. © 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.