Abstract

There is a long-standing discrepancy between the interpretations of the radio and EUV data of coronal holes. The electron density in coronal holes derived from EUV observations is three times larger than that derived from metric and decimetric radio observations. We address this discrepancy using microwave images of a large low-latitude coronal hole observed during the Whole Sun Month campaign of August-September 1996. Unlike metric radio emission, the microwave emission originates physically closer to the EUV emission and hence we can make a detailed morphological and quantitative comparison between the radio and EUV observations. In particular, we compare the densities derived from CDS and SUMER observations on the SOHO spacecraft with those derived from microwave observations. This way, we can determine whether the discrepancy exists at microwaves or not. The microwave observations were obtained by the Nobeyama Radioheliograph at 17 GHz with a spatial resolution of about 10 arcsec. Detailed structure of the chromosphere in the coronal hole region may have to be considered in explaining the discrepancy. We also compare the radio observations with soft X-ray data obtained by Yohkoh spacecraft and EUV data obtained by SOHO/EIT to determine the vertical structure of the coronal hole.