Determination of the Abundances of Subcoronal 4He and of Solar Flare-accelerated 3He and 4He from Gamma-Ray Spectroscopy

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Abstract
A series of narrow gamma-ray lines at 0.339, 0.429, 0.478, 0.937, 1.00, 1.04, 1.05, and 1.08 result almost exclusively from the bombardment of ambient He, O, and Fe by accelerated α-particles and 3He nuclei. Study of these lines, combined with the 56Fe line at 0.847 MeV and the 16O line at 6.129 MeV, allows the determination of the abundances of α-particles and 3He accelerated in solar flares and of the ambient He in the gamma-ray production region in the solar atmosphere. Using the Solar Maximum Mission/GRS and Compton Gamma Ray Observatory/OSSE data for 20 flares, we find that with significance not exceeding about 2.5 σ there are flares that exhibit α-particle abundance enhancements (α/p>0.1), show evidence for the presence of accelerated 3He, and indicate ambient He abundance enhancements. In some cases the accelerated 3He/4He~1 , and for essentially all of the flares 3He/4He could be 0.1, consistent with our earlier conclusion that in both impulsive and gradual flares the particles that interact and produce gamma rays are always accelerated by the same mechanism that operates in impulsive flares, namely, stochastic acceleration through gyroresonant wave-particle interactions. The ambient He abundance enhancements suggest that there are chromospheric regions where He/O exceeds its photospheric value, a result that could have important implications for solar atmospheric dynamics.