The Sun as an X-Ray Star. I. Deriving the Emission Measure Distribution versus Temperature of the Whole Solar Corona from theYohkoh/Soft X-Ray Telescope Data

The scope of this work is to obtain the emission measure distributions versus temperature, EM(T), of the whole solar corona from Yohkoh Soft X-ray Telescope images. As discussed in Paper II, the EM(T) is our starting point for studying the Sun as an X-ray star. To this purpose, we need to extract as much information as possible from the Yohkoh/SXT data covering the whole range of the Yohkoh/SXT temperature sensitivity, i.e., 5.5<logT(K)<8. In particular at low photon counts and temperatures below 10⁶ K, errors on the temperature and emission measure determination are expected to be large. To this end, we have made an extensive set of simulations to explore the nominal performance of the entire system (instrument and data analysis system) in the determination of the plasma temperature and emission measure at low, intermediate, and high photon counts per pixel. We have shown that low-count data with a number of photons per pixel n_phot<10 are affected by large errors and lead to the derivation of an unrealistic EM(T) characterized by a steep negative slope. As a result, we have devised an analysis method that minimizes the instrumental and statistical effects on the determination of EM(T) and allows us to determine the global coronal EM(T). As a first application to real SXT data, we have derived the EM(T) of the Sun close to the maximum of the solar cycle, a challenging case. The low-temperature part is in agreement with analogous studies made in the UV band, and it shows a well-defined maximum at T~2 MK.