The helium isotopic ratio in the solar wind and ion fractionation in the corona by inefficient Coulomb drag

Abstract

Using data obtained between 1991 and 1996 with the SWICS instrument (Solar Wind Ion Composition Spectrometer) aboard the Ulysses spacecraft, a long time average of the ((4) He / (3) He ) isotopic ratio of 2450 +/- 460 in coronal hole dominated solar wind is derived. To assess the influence of inefficient Coulomb friction in the inner corona and to infer the solar photospheric abundance ratio from the solar wind flux ratio, the variation of the fluxes with different solar wind regimes is investigated and limits for the long time fractionation effects are given. Finally a present-day (4) He / (3) He abundance ratio in the outer convective zone of ((4) He / (3) He)_OCZ = 2670 +/- 500) is derived.