An explanation for non-power-law behavior in the hard X-ray spectrum of the 2002 July 23 solar flare

High-resolution Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data reveal that solar flare hard X-ray spectra show systematic deviations from power-law behavior. Even for injection of a power-law electron spectrum, such deviations are expected because of a number of effects, including nonuniform target ionization and solar albedo backscattering of the primary hard X-ray flux. In this Letter, we examine 1 keV resolution hard X-ray spectra for the intense 2002 July 23 event, corrected for the effects of decimation, pulse pileup, and background. We find that the observed spectra indeed deviate from a power-law behavior in a manner consistent with the effects of nonuniform target ionization. Further, this interpretation of the observed deviations requires that the amount of coronal material increases during the initial phase of the flare. The implications of this discovery for models of atmospheric response to flare heating are discussed.