Variations of the energy spectra of solar X-ray flares: interconnection with the photospheric, chromospheric, and magnetic activity of the Sun (1972−2001)

Abstract

The problem of flare energy distribution of the Sun is strongly related to the power-law energy spectra of flares on the UV Ceti-type stars. In a preliminary study (1972–1993), the X-ray flare integral energy spectrum (IES) was represented by a unique power law of N ~ E−b with index b = 0.76. Later it was shown that the exponent index b varies with the cycle phase, that is with the Wolf number. The purpose of this paper is to investigate the temporal variations in the power-law spectrum of soft X-ray flares over the three solar cycles (1972–2001). A more extensive statistic (56 000 flares) allows us to revise earlier results and to derive new ones. Not only b-index undergoes variations associated with a 11-year cycle but the limiting energy of flares (log Em) as well. The three-cycle-averaged b is 0.666, with variations in the range 0.50 < b < 0.80. The results may be useful for study of flare activity on red dwarf stars.