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A. Nindos, M. R. Kundu, S. M. White (U. of Maryland), D. E. Gary (NJIT), K. Shibasaki (Nobeyama Radio Obs.), K. P. Dere (NRL)
The radio emission of solar poles is brighter than the rest of the quiet Sun's emission in a limited range of frequencies from 17 GHz to 87 GHz. We have studied microwave images of the quiet Sun made with the Nobeyama Radioheliograph at 17 GHz. They show that the so-called polar-cap brightening consists of two components: a diffuse component of 1500 K excess brightness, and patchy compact sources with localized excess brightness of about 3500 K. The total flux and the number of compact polar sources as well as the North-South extent of the diffuse polar emission are larger in the pole which is closest to the Earth. We compared the microwave polar emission with nearly simultaneous SoHO EIT images taken in the lines of He {\sc ii} at 304 Å\ and Fe {\sc xii} at 195 Å. No one-to-one correlation between the compact radio sources and the bright EUV features was found: most of the radio emission arises between the plumes visible to EIT. The boundaries of the polar-cap brightenings did not match exactly the boundaries of the coronal holes as seen in the Fe {\sc xii} 195 Å\ images. The temporal variations of the compact microwave sources did not correspond to any significant changes in EUV emission. On the other hand, most He {\sc ii} 304 Å\ changing features were associated with the diffuse polar microwave emission which was practically constant. Our data suggest that the origin of the polar brightening is not coronal; it seems that the bulk of the patchy radio emission comes from heights below the 80000 K layer.
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