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

TL;DR

This study analyzes gamma-ray spectra from solar flares, identifying a distinct coronal electron population likely produced by inverse Compton scattering or bremsstrahlung, revealing new insights into flare-accelerated electrons above 1 MeV.

Contribution

It introduces a new spectral component (PLexp) for flare-accelerated electrons, with detailed spectral and spatial analysis across multiple flares, highlighting its coronal origin and distinct properties.

Findings

The PLexp component originates from the corona, unlike the footpoint origin of other components.

The PLexp spectrum is flat at low energies and peaks around a few MeV, consistent with specific electron acceleration processes.

The spectral parameters of the PLexp are isotropic and independent of heliocentric angle.

Abstract

Significant improvements in our understanding of nuclear $\gamma$-ray line production and instrument performance allow us to better characterize the continuum emission from electrons at energies $\gtrsim$ 300 keV during solar flares. We represent this emission by the sum of a power-law extension of hard X-rays (PL) and a power law times an exponential function (PLexp). We fit the $\gamma$-ray spectra in 25 large flares observed by SMM, RHESSI, and Fermi with this summed continuum along with calculated spectra of all known nuclear components. The PL, PLexp, and nuclear components are separated spectroscopically. A distinct origin of the PLexp is suggested by significant differences between its time histories and those of the PL and nuclear components. 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 strongly dependent on the flares' heliocentric angle, the PLexp parameters show no such dependency and are consistent with a component that is isotropic. The PLexp spectrum is flat at low energies and rolls over at a few MeV. Such a shape can be produced by inverse Compton scattering of soft X-rays by 10--20 MeV electrons and by thin-target bremsstrahlung from electrons with a spectrum that peaks between 3 -- 5 MeV, or by a combination of the two processes. These electrons can produce radiation detectable at other wavelengths.