2000/08/28 by J. M. Laming, U. Feldman · 2 citations
Physics and Astronomy · #Ionosphere and magnetosphere dynamics #Solar and Space Plasma Dynamics #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/318238
16 pages, 2 figures, accepted by ApJ
arxiv created 2000/08/28 · openalex publication_date 2001/01/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
At altitudes of about 1.05 R ☉ 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.