EDP Sciences
Free access
Volume 488, Number 3, September IV 2008
Page(s) 1069 - 1077
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:200809355
Published online 09 July 2008

A&A 488, 1069-1077 (2008)
DOI: 10.1051/0004-6361:200809355

The flaring and quiescent components of the solar corona

C. Argiroffi1, 2, G. Peres1, 2, S. Orlando2, and F. Reale1, 2

1  Dipartimento di Scienze Fisiche ed Astronomiche, Sezione di Astronomia, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
    e-mail: [argi;peres;reale]@astropa.unipa.it
2  INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
    e-mail: orlando@astropa.inaf.it

Received 5 January 2008 / Accepted 30 April 2008

Context. The solar corona is a template to understand stellar activity. The Sun is a moderately active star, and its corona differs from that of active stars: for instance, active stellar coronae have a double-peaked emission measure distribution EM(T) with a hot peak at 8-20 MK, while the non-flaring solar corona has one peak at 1-2 MK and, typically, much cooler plasma.
Aims. We study the average contribution of flares to the solar emission measure distribution to investigate indirectly the hypothesis that the hot peak in the EM(T) of active stellar coronae is due to a large number of unresolved solar-like flares, and to infer properties about the flare distribution from nano- to macro-flares.
Methods. We measure the disk-integrated time-averaged emission measure, $EM_{\rm F}(T)$, of an unbiased sample of solar flares, analyzing uninterrupted GOES/XRS light curves over time intervals of one month. We obtain the $EM_{\rm Q}(T)$ of quiescent corona for the same time intervals from Yohkoh/SXT data. To investigate how $EM_{\rm F}(T)$ and $EM_{\rm Q}(T)$ vary during the solar cycle, we evaluate them at different phases of the cycle between December 1991 and April 1998.
Results. Irrespective of the solar cycle phase, $EM_{\rm F}(T)$ appears as a peak in the distribution, and it is significantly larger than the values of $EM_{\rm Q}(T)$ for $T\sim5{-}10$ MK. As a result, the time-averaged EM(T) of the entire solar corona 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. The $EM_{\rm F}(T)$ 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. In the assumption that the heating of the corona is entirely due to flares, from nano- to macro-flares, then either the flare distribution or the confined plasma response to flares, or both, are bimodal.

Key words: stars: activity -- stars: coronae -- Sun: activity -- Sun: corona -- Sun: flares -- Sun: X-rays, gamma rays

© ESO 2008