Microwave enhancement and variability in the elephant's trunk coronal hole: Comparison with SOHO observations

We report on an investigation of the microwave enhancement and its variability in the elephant's trunk coronal hole observed during the Whole Sun Month campaign (August 10 to September 9, 1996). The microwave images from the Nobeyama radioheliograph were compared with magnetograms and EUV images obtained simultaneously by the Michelson Doppler imager and the extreme ultraviolet imaging telescope (EIT) on board the SOHO spacecraft. The combined data set allowed us to understand the detailed structure of the microwave enhancement in the spatial and temporal domains. We find that the radio enhancement is closely associated with the enhanced unipolar magnetic regions underlying the coronal hole. The radio enhancement consists of a smooth component originating from network cell interiors and a compact component associated with network magnetic elements. When a minority polarity is present near a majority polarity element, within the coronal hole, the resulting mixed polarity region is associated with a bright-point-like emission in coronal EUV lines such as the Fe XII 195 Angstrom. These coronal bright points are also observed distinctly in the EIT 304 Angstrom band, but not in microwaves, On the other hand, the lower-temperature line emission (304 Angstrom) and the microwave enhancement are associated with the unipolar magnetic flux elements in the network. We found strong time variability of the radio enhancement over multiple timescales, consistent with the initial results obtained by SOHO instruments. The microwave enhancement is most probably due to temperature enhancement in the chromosphere and may be related to the origin of solar wind.