A Method for Determining the Photospheric [TSUP]4[/TSUP]H[CLC]e[/CLC] Abundance

Abstract

The photospheric 4He/H ratio is not measured directly but is inferred from stellar evolution models, helioseismology, and coronal observations. We suggest a procedure for determining the photospheric ratio on the basis of solar wind isotopic He abundances and γ-ray line measurements. Recent Ulysses measurements of the 4He/3He isotopic ratio in the solar wind indicate that it is constant over a large range of velocities; long-term averages in the ratio from different 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 substantial 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 been determined for only a few flares. The most reliable ratio determined to date is 2.3 ± 1.2 × 10-5, where the uncertainty is statistical at the 90% confidence level. By multiplying these solar wind 4He/3He and γ-ray-derived 3He/H ratios, we obtain estimates of the photospheric 4He/H ratio ranging from ~0.02 to 0.10. Improved observations of the solar wind 4He/3He ratio and modeling of its origin can confirm that it measures the photospheric ratio. Recent improvements in our understanding of flare-accelerated particles, based on γ-ray line data, can be applied to models of 2.223 MeV line production. Application of these corrected models to high-quality γ-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 3He/H ratio and thus the 4He/H ratio.