Production of γ-ray lines by energetic ion interactions (determination of solar atmospheric abundances)

Abstract

Acceleration of energetic ions in solar flares produces narrow and broad nuclear γ-ray de-excitation lines, by nuclear interactions with the ambient solar atmosphere. Observations of several solar flare γ-ray spectra have been made since 1972 by satellite-borne γ-ray spectrometers on OSO 7, HEAO 3, SMM, HINOTORI, GRANAT, YOHKOH and CGRO. The line intensities depend directly on the ambient and accelerated particle abundances. In addition, production of neutrons in several reactions leads to emission of the neutron–proton capture γ-ray line, from the photosphere, at an energy of 2.223 MeV. The rate of decay in intensity of this line depends on the photospheric 3He/H abundance ratio. Current analysis gives a photospheric 3He/H abundance ratio of (2.3±1.2)×10 5. In addition, a study of narrow γ-ray line fluxes from several flares suggests that the ratio of (Mg + Si + Fe) to (C + N + O) abundances changes with time during the flare. Since this ratio measures the abundance ratio of low first ionization potential (FIP) elements to high FIP elements, the γ-ray production site may start in the lower chromosphere and move, as the flare progresses, into the corona where the low/high FIP abundance ratio increases. A study of broad γ-ray line fluxes also permits a determination of the abundance of flare-accelerated heavy ions.