The Solar Helium Abundance in the Outer Corona Determined from Observations with SUMER/SOHO

Abstract

At altitudes of about 1.05 R_solar or more, the corona above quiet solar regions becomes essentially isothermal. This obviates many of the difficulties associated with the inverse problem of determining emission measure distributions and allows for fairly straightforward relative element abundance measurements. We present new values for the He abundance. The first is based on a reanalysis of the He/O ratio studied previously using data acquired by SUMER. A more thorough evaluation of the atomic physics for He II, including a detailed treatment of radiative recombination, increases the predicted emission in the He II Balmer series compared with earlier analyses. We use a recently revised value of the O abundance to derive an He/H abundance ratio of 0.038 (mass fraction, Y=0.13), with an error of ~17% coming mainly from the O abundance uncertainty. We demonstrate that this result may be affected by gravitational settling of O relative to He. We also derive an abundance for He by direct comparison with emission lines of the H I Lyman series, with the result He/H=0.052+/-0.005 (Y=0.17), a value similar to He abundances determined in the slow-speed solar wind.