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The flaring and quiescent components of the solar corona

By C. Argiroffi, G. Peres, S. Orl and F. Reale


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 the 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 of the EM(T) of active stellar coronae is due to a large number of unresolved solar-like flares, and to infer properties on the flare distribution from nano- to macro-flares. Methods. We measure the disk-integrated time-averaged emission measure, EMF(T), of an unbiased sample of solar flares analyzing uninterrupted GOES/XRS light curves over time intervals of one month. We obtain the EMQ(T) of quiescent corona for the same time intervals from the Yohkoh/SXT data. To investigate how EMF(T) and EMQ(T) vary with 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, EMF(T) appears like a peak of the distribution significantly larger than the values of EMQ(T) for T ∼ 5−10 MK. As a result the time-averaged EM(T) of the whole solar corona is double-peaked, with the hot peak, due to time-averaged flares, located at temperature similar of that of active stars, but less enhanced. 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 flar

Year: 2008
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