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Session 19 - Flares II.
Oral session, Tuesday, July 01
Ballroom A, Chair: Ronald Murphy

[19.03] A Method for Determining the Photospheric Helium Abundance

G. H. Share, R. J. Murphy (NRL)

The photospheric ^4He/H ratio is not measured directly, but is inferred from stellar evolution models, helioseismology, and coronal observations. We suggest a procedure to determine the photospheric ratio directly based on solar wind isotopic He abundances and \gamma-ray line measurements. Recent measurements of the ^4He/^3He isotopic ratio, 2290 \pm 200, in the solar wind indicate that it is constant over a large range in velocities; this constancy supports models indicating that fractionation of these isotopes does not occur as they are transported from the photosphere to the solar wind. Comparison of the time histories of the 2.223 MeV neutron capture line and prompt de-excitation lines from solar flares provides the only direct measurement of the ^3He/H ratio in the photosphere. This ratio has only been determined for a few flares and the most reliable measurement to date is (2.3 \pm 0.8) \times 10^-5. By multiplying the two ratios we can determine the photospheric ^4He/H ratio. This suggests that the photospheric ^4He/H ratio is 0.053 \pm 0.019 (error is statistical), which is lower than previous estimates. Recent improvements in our understanding of flare accelerated particles, based on gamma-ray line data, need to be applied to models of 2.223 MeV line production. Application of these improved models to high-quality \gamma-ray line data obtained from instruments on the Compton Gamma Ray Observatory will result in a more accurate measurement of the photospheric ^4He/H ratio. This work is funded under NASA contracts DPR W18995 and S57769F.

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