Microwave enhancement in low-latitude coronal holes Original Research Article

We present results of a systematic analysis of microwave brightness distributions in low-latitude coronal holes during July 1992 and June 1996. The microwave images from the Nobeyama Radioheliograph at 17 GHz were compared with Kitt Peak magnetograms and soft X-ray images from the Yohkoh satellite. We show that: (a) Some of coronal holes are accompanied by weak excess emission at 17 GHz as compared with the neighboring quiet regions, while the others are not. (b) The microwave enhancement consists of a diffuse component and bright patches with excess brightness temperatures up to 5000 K, assuming the brightness temperature at 17 GHz of 10000 K for the disk. (c) The enhanced microwave emission is associated with enhanced unipolar magnetic field regions accompanied by magnetic field of more than about 70 Gauss. (d) The microwave enhancement shows a recurrent property for most of the cases. (e) The microwave enhancement in low-latitude coronal holes shows solarcycle dependence, becoming less prominent toward the solar minimum.