Emission measure distribution for an active region using coordinated SERTS and Yohkoh SXT observations

Often the derived temperature of an active region reflects the method and the nature of the instrument used in its measurement. The emission measure (i.e., the amount of emitting material) derived from spectroscopic observations usually depends on assumptions about the absolute elemental abundances and ionization fractions of the emitting ions. Yet establishing the distribution of emission measure with temperature is the first step needed to proceed with most of the interesting physics of active regions-including heating processes, cooling timescales, and loop stability. Accurately characterizing the thermal distribution of the coronal plasma requires data which can resolve multithermal features and constrain both low- and high-temperature emission. To model the temperature distribution of NOAA Active Region 7563, we have combined broadband filter data from the Yohkoh Soft X-Ray Telescope (SXT) with simultaneous spectral line data from the Goddard Solar EUV Rocket Telescope and Spectrograph (SERTS) taken during its flight on 1993 August 17. We have used a forward-folding technique to determine the emission measure distribution of the active region loops. We have found that (1) the SXT response functions are sensitive to both the elemental abundances and the ionization fractions assumed to compute the solar spectrum that is folded through the instrument effective area; (2) the relative calibration between the SERTS and the SXT instruments must be adjusted by a factor of 2 (a value consistent with the absolute measurement uncertainty of the 1993 SERTS flight) no matter which abundances or iron ionization fractions are used; (3) the two-peaked differential emission measure previously determined using SERTS data alone is not consistent with the SXT data: including the SXT data as a high-temperature constraint in the analysis requires that the emission above about 3 MK drop off steeply rather than extending out to 6 MK. The sensitivity of the SXT filter response functions to elemental abundance and iron ionization fraction could have a major impact on many routine analyses of SXT data. The emission measures can be greatly affected (up to a factor of 7) and temperatures derived from filter ratios can be significantly altered (up to at least 40%) by adopting different sets of commonly used elemental and ionic abundances. The results of our multithermal analysis imply that using broadband SXT data or a comparable high-temperature constraint in conjunction with high-resolution spectra covering a wide lower temperature range to study solar active regions can significantly improve the information derived from either data set alone. In this study, the revised multithermal distribution reduces the thermal energy content of the region by about a factor of 2 and the required heating by about a factor of 5, which in turn relaxes some constraints on possible heating models.