A Method for Determining the Photospheric 4He Abundance

The photospheric 4 He/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 based on solar wind isotopic He abundances and-ray line measurements. Recent Ulysses measurements of the 4 He/ 3 He isotopic ratio in the solar wind indicate that it is constant over a large range in velocities; long-term averages in the ratio from diierent experiments are also consistent with one another, with values ranging from 2050 to 2350 and uncertainties ranging from 120 to 200. 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 ares provides the only direct measurement of the 3 He/H ratio in the photosphere. This ratio has only been determined for a few ares. The most reliable ratio determined to date is 2:3 1:2 10 ?5 , where the uncertainty is statistical at the 90% conndence level. By multiplying these solar wind 4 He/ 3 He and-ray derived 3 He/H ratios we obtain estimates of the photospheric 4 He/H ratio ranging from 0.02 to 0.10. Improved observations of the solar wind 4 He/ 3 He ratio and modeling of its origin can connrm that it measures the photospheric ratio. Recent improvements in our understanding of are accelerated particles, based on-ray line data, can be applied to models of 2.223 MeV line production. Application of these corrected models to high-quality-ray line data obtained from instruments on the Compton Gamma Ray Observatory and the Solar Maximum Mission will result in a more accurate measurement of the photospheric 3 He/H ratio and thus the 4 He/H ratio.