Solar Atmospheric and Solar Flare Accelerated Helium Abundances from Gamma-Ray Spectroscopy

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Abstract
From the measured fluences of αα and narrow gamma-ray lines we derive He abundances (He/O) in the gamma-ray production region of solar flares, most likely subcoronal regions located above the photosphere. The αα line is a spectral feature due to de-excitations in 7Li and 7Be produced by the interactions of α-particles with He. We find that the derived values of He/O are consistent with the photospheric He/O only if the accelerated particle α/p 0.5. More conventional values (α/p 0.1) imply a higher He abundance in the subcoronal regions than in the photosphere. Indications for the existence of such He-enriched regions in the solar atmosphere are obtained from solar wind observations. We show how future high-resolution observations with Ge detectors could distinguish between the two possibilities of high α/p or high He/O (and hence high He/H) values by resolving an already observed spectral feature at ~1.02 MeV. This feature is dominated by lines produced exclusively in α-particle interactions with 56Fe and 3He interactions with 16O. In addition to the determination of α/p and the ambient He abundance, this will also determine the accelerated 3He/4He and its time development as the acceleration progresses. The relative intensities of the αα and narrow de-excitation lines confirm our previous finding that in the gamma-ray production region the abundance of Ne exceeds its measured coronal and generally accepted photospheric value. Thus, the subcoronal abundances of the two elements with the highest first ionization potential, He and Ne, are higher than their corresponding coronal abundances.