The Sun as an X-ray star. II. Using the Yohkoh/soft X-ray telescope-derived solar emission measure versus temperature to interpret stellar X-ray observations

This paper is the second of a project dedicated to using solar Yohkoh/SXT d ata as a guide and a template to interpret data on stellar coronae. In the light of the large differences in scope and approach between solar and stel lar studies, we have developed a method to translate Yohkoh/SXT data of the whole solar corona into stellar-like data, i.e., to put them in the same f ormat and context as the stellar ones. First from the Yohkoh/SXT images we derive the whole-Sun X-ray emission measure versus temperature [EM(T)], in the range 10(5.5)-10(8) K, during the specific observation. Then, we synthe size the solar X-ray spectrum; finally, we fold the spectrum through the in strumental response of nonsolar X-ray observatories, for instance, ROSAT/PS PC and ASCA/SIS. Finally, we analyze such solar coronal data in the same ba nd and with the same methods used for stellar observations, allowing a dire ct and homogeneous comparison with them. In this paper we present in detail our method and, as an example of results, we show and discuss EM(T) and st ellar-like spectra for three phases of the solar cycle: maximum, intermedia te phase, and minimum. The total amount and the distribution of the emissio n measure change dramatically during the cycle, in particular at temperatur es above 10(6) K. We also show the EM(T) of the whole solar corona during a large flare. The ROSAT/PSPC- and ASCA/SIS-like X-ray spectra of the Sun as a star that we obtain are discussed in the context of stellar coronal phys ics. The Sun's coronal total luminosity in the ROSAT/PSPC band ranges from approximate to 2.7 x 10(26) ergs s(-1) (at minimum) to approximate to 4.7 x 10(27) ergs s(-1) (at maximum). We discuss future developments and possibl e applications of our method.