The solar helium abundance in the outer corona determined from observations with SUMER/SOHO

At altitudes of about 1.05 R. or more, the corona above quiet solar regions becomes essentially isothermal. This obviates many of the difficulties ass ociated with the inverse problem of determining emission measure distributi ons and allows for fairly straightforward relative element abundance measur ements. 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 SUM ER. A more thorough evaluation of the atomic physics for He II, including a detailed treatment of radiative recombination, increases the predicted emi ssion in the He II Balmer series compared with earlier analyses. We use a r ecently 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 similar to 17% coming mainly from the O abundance uncertainty. We demonstrate that this result ma y 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 Ly man series, with the result He/H = 0.052 +/- 0.005 (Y = 0.17), a value simi lar to He abundances determined in the slow-speed solar wind.