The Sun as an X-ray star. I. Deriving the emission measure distribution versus temperature of the whole solar corona from the Yohkoh/Soft X-ray Telescope data

The scope of this work is to obtain the emission measure distributions vers us temperature, EM(T), of the whole solar corona from Yohkoh Soft X-ray Tel escope images. As discussed in Paper II, the EM(T) is our starting point fo r 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 r ange of the Yohkoh/SXT temperature sensitivity, i.e., 5.5 < log T(K) < 8. I n particular at low photon counts and temperatures below 10(6) K, errors on the temperature and emission measure determination are expected to be larg e. To this end, we have made an extensive set of simulations to explore the nominal performance of the entire system (instrument and data analysis sys tem) in the determination of the plasma temperature and emission measure at low, intermediate, and high photon counts per pixel. We have shown that lo w-count data with a number of photons per pixel n(phot) < 10 are affected b y large errors and lead to the derivation of an unrealistic EM(T) character ized by a steep negative slope. As a result, we have devised an analysis me thod that minimizes the instrumental and statistical effects on the determi nation of EM(T) and allows us to determine the global coronal EM(T). As a f irst application to real SXT data, we have derived the EM(T) of the Sun clo se to the maximum of the solar cycle, a challenging case. The low-temperatu re part is in agreement with analogous studies made in the UV band, and it shows a well-defined maximum at T similar to 2 MK.