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Two-Temperature Coronal Models from SOHO/EIT Observations

We present a method for deriving a two-thermal-component approximation to the differential emission measure distribution of plasma in the Sun's corona in the temperature range to which the Extreme-Ultraviolet 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 different wavelengths are not able adequately to represent the plasma contributing to all three wavelength ranges. In this paper, we develop a modified image-ratio method that results in a two-thermal-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 Sun's 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 differential emission measures (DEMs) from the CHIANTI atomic database package. This method appears to produce realistic results in all regions of the Sun's atmosphere with the exception of coronal holes, where very cool Si VII/Mg VII lines (<0.7 MK) contribute more to the EIT 284 Å 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 fine structure, which is less visible in the flux 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 firm 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.