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A&A 426, 1093-1101 (2004)
DOI: 10.1051/0004-6361:20041367
The spectral evolution of impulsive solar X-ray flares
P. C. Grigis and A. O. BenzInstitute for Astronomy, ETH Zürich, 8092 Zürich, Switzerland
e-mail: pgrigis@astro.phys.ethz.ch
(Received 28 May 2004 / Accepted 15 July 2004)
Abstract
The time evolution of the spectral index and the non-thermal flux in 24
impulsive solar hard X-ray flares of GOES class M was studied in RHESSI
observations. The high spectral resolution allows for a clean separation
of thermal and non-thermal components in the 10-30 keV range, where most
of the non-thermal photons are emitted. Spectral index and flux can thus
be determined with much better accuracy than before.
The spectral soft-hard-soft behavior in rise-peak-decay phases is discovered
not only in the general flare development, but even more pronounced in
subpeaks.
An empirically found power-law dependence between the spectral index and the
normalization of the non-thermal flux holds during the rise and decay phases
of the emission peaks. It is still present in the combined set of all flares.
We find an asymmetry in this dependence between rise and decay phases of
the non-thermal emission.
There is no delay between flux peak and spectral index minimum.
The soft-hard-soft behavior appears to be an intrinsic signature
of the elementary electron acceleration process.
Key words: Sun: flares -- Sun: X-rays, gamma rays -- acceleration of particles
© ESO 2004