Nonthermal Spectral Flattening in Early Impulsive Flares

Details Nonthermal Spectral Flattening in Early Impulsive Flares Nonthermal Spectral Flattening in Early Impulsive Flares

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsh13a0291s&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #SH13A-0291

Other

7513 Coronal Mass Ejections (2101), 7519 Flares, 7526 Magnetic Reconnection (2723, 7835)

Scientific paper

Early impulsive flares are those flares in which impulsive hard X-ray emission is seen early in the flare before the soft X-ray emission has risen significantly. Compared to the more normal flares where the soft X-ray emission rises well before the hard X-ray emission, the nonthermal flux dominates RHESSI flare spectra even down to very low energies. Consequently, because of the relatively weak thermal emission, they offer the advantage that evidence for a low-energy cutoff in the nonthermal electron distribution can be identified at lower energies in the X-ray spectrum. From the RHESSI 2002 flare catalog we have found 137 early impulsive flares, from which 35 flares with significant >25 keV hard X-ray flux were selected for this analysis. In 12 out of the 35 flares, the nonthermal component of the photon spectra was found to flatten below ~20 to 30 keV from its power-law distribution at higher energies. Spectral fitting with a bremsstrahlung thick-target model gives low-energy cutoffs in the electron distribution ranging from ~20 to 45 keV. Most interestingly, the cutoff energy correlates with the hard X-ray flux for all the flares. The implications of spectral flattening for particle acceleration and transport will be discussed. Other possible causes such as albedo and return current losses will also be explored. This work was supported in part by NASA SEC Guest Investigator Grant 370-16-20-16 and by the RHESSI project.

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