The Sun as a benchmark of flaring activity in stellar coronae

Abstract

The solar corona is a template to study and understand stellar activity. However the solar corona differs from that of active stars: the Sun has lower X-ray luminosity, and on average cooler plasma temperatures. Active stellar coronae have a hot peak in their emission measure distribution, EM(T), at 8-20 MK, while the non-flaring solar corona has a peak at 1-2 MK. In the solar corona significant amounts of plasma at temperature ~10 MK are observed only during flares. To investigate what is the time-averaged effect of solar flares we measure the disk-integrated time-averaged emission measure, EMF(T), of an unbiased sample of solar flares. To this aim we analyze uninterrupted GOES/XRS light curves over time intervals of one month. We also obtain the EMQ(T) of the quiescent corona for the same time intervals from Yohkoh/SXT data. To investigate variations due to the solar cycle we evaluate EMF(T) and EMQ(T) at different phases of the cycle between December 1991 and April 1998. Irrespective of the solar cycle phase, EMF(T) appears as a peak in the distribution, and it is significantly larger than the values of EMQ(T) for T~5-10 MK. Adding EMF(T) and EMQ(T) we obtain for the first time a time average EM(T) of the entire solar corona: it is double-peaked, with the hot peak, due to time-averaged flares, being located at temperatures similar to those of active stars, but less enhanced. In the assumption that the heating of the corona is entirely due to flares, from nano- to macro-flares, a two peak EM(T) distribution suggests that then either the flare distribution or the confined plasma response to flares, or both, are bimodal. Moreover the EMF(T) shape supports the hypothesis that the hot EM(T) peak of active coronae is due to unresolved solar-like flares. If this is the case, quiescent and flare components should follow different scaling laws for increasing stellar activity.