Compton Scattering of Deexcitation-Line and Continuum Gamma Rays in Solar Flares

Measurements of solar-flare electron-bremsstrahlung X-rays are affected by Compton scattering in the solar atmosphere of the downward-directed radiation. Here we study how Compton-scattered and energy-degraded radiation from nuclear-deexcitation gamma-ray lines and continua affect the measurements of the gamma-ray radiation. Deexcitation-line photons with trajectories directed away from the Sun escape without significant interactions even for flares at the limb. We calculate the Compton-scattered component spectrum from downward-directed deexcitation lines for typical solar-flare accelerated-ion kinetic-energy spectra. The scattered component only a makes a significant contribution to the emerging spectrum at energies below ≈ 600 keV and is most prominent for flares occurring near the center of the solar disk. We study Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spectra obtained from the 28 October 2003 disk-centered flare when the electron-bremsstrahlung contribution was relatively weak. We find that inclusion of the scattered component does not significantly affect any of the derived flare parameters. This is true, in part, because the scattered component is not detectable over the significant RHESSI detector count continuum due to partial energy depositions of higher-energy solar photons. The scattered component may affect flare spectral measurements obtained with gamma-ray detectors having a more “diagonal” response.