Polar-Cap and Coronal-Hole-Associated Brightenings of the Sun at Millimeter Wavelengths

Abstract

Abstract Observations of the Sun at 36 GHz (8.3 mm) and 98 GHz (3.1 mm) were made on July 16 to 22, 1984 with the Nobeyama 45-m telescope. High-quality maps were obtained from 240 radial scans across the disk. Maps taken at 36 GHz provided evidence for 3–7 % brightness enhancement near the poles at latitudes greater than 65°, although no limb brightenings above 1 % were observed in the equatorial region. Also a ~ 5 % brightness enhancement was observed at 36 GHz in at least one equatorial coronal hole region. On the other hand, no such brightenings were detected at 98 GHz. It is suggested that the two types of brightenings at 36 GHz are two aspects of one phenomenon, associated with polar and equatorial coronal holes, respectively. Also, it is suggested that the temperature and density structure of the upper chromosphere in coronal holes differs from that outside holes.