HARD-SOFT-HARD FLARE SPECTRA AND THEIR ENERGY DEPENDENCE IN SPECTRAL EVOLUTION OF A SOLAR HARD X-RAY FLARE

In this Letter, we studied the time evolution of the energy-dependent spectral indices for the 2004 November 3 solar hard X-ray flare observed by RHESSI. The common soft-hard-soft (SHS) pattern spectra were found at the lower energies, while a new feature, hard-soft-hard (HSH), was found at higher energies for each subpeak. As the energy increases, the SHS pattern is gradually converted into the HSH pattern. Some possible explanations for the spectral evolution and its energy dependence are discussed, such as the return current.