The sun as an X-ray star. III. Flares

In previous works we have developed a method to convert solar X-ray data, c ollected with the Yohkoh/SXT, into templates of stellar coronal observation s. Here we apply the method to several solar flares, for comparison with st ellar X-ray flares. Eight flares, from weak (GOES class C5.8) to very inten se ones (X9) am selected as representative of the flaring Sun. The emission measure distribution versus temperature, EM(T), of the flaring regions is derived from Yohkoh/SXT observations in the rise, peak, and decay of the fl ares. The EM(T) is rather peaked and centered around T approximate to 10(7) K for most of the time. Typically, it grows during the rise phase of the f lare, and then it decreases and shifts toward lower temperatures during the decay, more slowly if there is sustained heating. The most intense flare w e studied shows emission measure even at very high temperatures (T approxim ate to 10(8) K). Time-resolved X-ray spectra both unfiltered and filtered t hrough the instrumental responses of the nonsolar instruments ASCA/SIS and ROSAT/PSPC are then derived. Synthesized ASCA/SIS and ROSAT/PSPC spectra ar e generally well fitted with single thermal components at temperatures clos e to that of the EM(T) maximum, albeit two thermal components are needed to fit some flare decays. ROSAT/PSPC spectra show that solar flares are in a 2 orders of magnitude flux range (10(6)-10(8) ergs cm(-2) s(-1)) and a narr ow PSPC hardness ratio range, however, higher than that of typical nonflari ng solar-like stars.