Directionality of Solar Flare–accelerated Protons and α‐Particles from γ‐Ray Line Measurements

Abstract

The energies and widths of gamma-ray lines emitted by ambient nuclei excited by flare-accelerated protons and alpha-particles provide information on the ions' directionality and spectra and on the characteristics of the interaction region. We have measured the energies and widths of strong lines from de-excitations of 12C, 16O, and 20Ne in solar flares as a function of the heliocentric angle. The line energies from all three nuclei exhibit ~1% redshifts for flares at small heliocentric angles but are not shifted near the limb. The lines have widths of ~3% FWHM. We compare the 12C line measurements for flares at five different heliocentric angles with calculations for different interacting-particle distributions. A downward isotropic distribution (or one with a small upward component) provides a good fit to the line measurements. An angular distribution derived for particles that undergo significant pitch-angle scattering by MHD turbulence in coronal magnetic loops provides comparably good fits.