Solar Gamma-Ray Evidence for a Distinct Population of >1 MeV Flare-accelerated Electrons

Abstract

Significant improvements in our understanding of nuclear γ-ray line production and instrument performance allow us to better characterize the continuum emission from electrons at energies ≳300 keV during solar flares. We represent this emission by the sum of a power-law (PL) extension of hard X-rays and a power law times an exponential function (PLexp). We fit the γ-ray spectra in 25 large flares observed over 40 yr with this continuum and the calculated spectra of all known nuclear components. The PLexp is separated spectroscopically from the other components, and its presence is required with >99% confidence in 18 of the flares. Its distinct origin is suggested by significant differences between its time histories and those of the PL and nuclear components in 18 of the flares. RHESSI imaging/spectroscopy of the 2005 January 20 flare reveals that the PL and nuclear components come from the footpoints, while the PLexp component comes from the corona. While the index and flux of the anisotropic PL component are dependent on the flare’s heliocentric angle, the PLexp parameters do not show comparable dependences with 99.5% confidence. The PLexp spectrum is flat at low energies and rolls over at a few megaelectronvolts (MeV). Such a shape can be produced by thin-target bremsstrahlung from electrons with a spectrum that peaks between 3 and 5 MeV and by inverse Compton scattering of soft X-rays by 10–20 MeV electrons, or by a combination of the two. These electrons can produce radiation detectable at other wavelengths.