Two-temperature Coronal Models from Soho/eit Observations

We present a method for deriving a twoÈthermal-component approximation to the di†erential emission measure distribution of plasma in the SunÏs corona in the temperature range to which the ExtremeUltraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) spacecraft is sensitive. EIT takes high-resolution full-disk coronal images in three of its four optimized channels by observing emission lines of highly ionized Fe whose formation temperatures overlap and cover the range from 0.7 to 2.8 MK. It is straightforward to show that the traditional single-temperature models based on the ratio of a pair of EIT images at di†erent wavelengths are not able adequately to represent the plasma contributing to all three wavelength ranges. In this paper, we develop a modiÐed image-ratio method that results in a twoÈthermal-component model for the plasma producing the coronal emission observed by EIT. The products of this method are two temperature and two emission measure full-disk maps of the SunÏs corona, with the full resolution of the EIT telescope, in two temperature regimes : one from 0.8 to 1.6 MK and the other from 1.6 to 2.6 MK. The two-component solutions are tested using a series of model di†erential emission measures (DEMs) from the CHIANTI atomic database package. This method appears to produce realistic results in all regions of the SunÏs atmosphere with the exception of coronal holes, where very cool Si VII/Mg VII lines (\0.7 MK) contribute more to the EIT 284 AŽ image than the otherwise dominant hot Fe XV lines and result in unrealistically high temperatures for the hot component there. We demonstrate that while the raw EIT images are dominated by the spatial distribution of emission measure in the corona, the temperature maps often emphasize Ðne structure, which is less visible in the Ñux images. The emission measure of the hot component is always larger than that of the cool component. On the disk there appears to be a Ðrm lower limit to the integrated column emission measure along any line of sight, including toward coronal holes. There is no overall correlation between temperature and emission measure. Subject headings : Sun: atmosphere È Sun: corona È Sun: UV radiation