Low Brightness Temperature in Microwaves at Periphery of Some Solar Active Regions

The microwave regions with low brightness temperature are found to overlap the regions of the depressed coronal emission and open field lines at the periphery of two solar active regions (ARs). The imaging microwave observations of the Sun with the Nobeyama Radio heliograph at 1.76 cm, the MRO-14 radio telescope of Metsähovi Radio Observatory at 0.8 cm, and the RT-32 of Ventspils International Radio Astronomy Centre in the range 3.2-4.7 cm are used. To reduce the noise in the intensity distribution of the RT-32 maps of the Sun, one wavelet plane of “à trous” wavelet space decomposition is subtracted from each map. To locate the open-field regions, the full-Sun coronal magnetic fields with the potential field source surface (PFSS) model for R_SS = 1.8 R_ʘ are simulated. We conclude that the revealed LTRs present narrow coronal hole-like regions near two ARs and imply an extra investigation on the plasma outflow.