Reconciling Extreme-ultraviolet and Radio Observations of the Sunïs Corona

The SunÏs corona, which is composed of plasma at a temperature of a few millions of degrees, can be best viewed in two electromagnetic domains, one from wavelengths of a few angstroms to hundreds of angstroms (in the soft X-ray and EUV domain), the other from wavelengths of a few centimeters to several tens of centimeters (in the radio domain). In this paper, we present a quantitative comparison of coronal observations made in these two domains with high spatial resolution over the full disk of the Sun. The EUV observations were taken with the EIT (Extreme-Ultraviolet Imaging Telescope) on board SOHO (Solar and Heliospheric Observatory), and the radio observations were taken with the VLA (Very Large Array). The two sets of images show very similar morphologies, indicating that the di†erent wavelengths originate from common solar features. We predict radio Ñuxes using the temperature and emission measure of the corona calculated from EIT observations, adopting MeyerÏs table of coronal abundances for the calculations. In each of the seven observations investigated, there always exists a good linear correlation in the pixel-by-pixel correlation plot between the predicted and the observed radio Ñux for coronal features over a wide range of Ñux variation. Nevertheless, the predicted radio Ñux is systematically larger than that observed by a factor of 2.0 ^ 0.2, on average. We attribute the di†erence to the underestimation of the abundance of Fe relative to H in the abundances adopted by Meyer. On this basis, we place the absolute Fe abundance in the corona at 7.8] 10~5, which has an enrichment factor of 2.4 relative to the accepted photospheric Fe abundance. Subject headings : Sun: abundances È Sun: atmosphere È Sun: corona È Sun: radio radiation È Sun: UV radiation