Fractionation Processes in the Solar Wind Revealed by Noble Gases Collected by Genesis Regime Targets

Introduction: Genesis collected solar wind (SW) ions with one major objective being to obtain abundances and isotopic composition of ultra-volatile elements. The SW is a proxy for the composition of the solar outer convective zone (OCZ) and, hence for the solar nebula. For most elements accurate values for the isotopic composition of sun can be inferred from meteorites. Carbonaceous chondrites also reflect solar composition for most elements. However, the solar isotopic and elemental composition of volatile elements is poorly constrained from meteorites. Moreover, due to the high first excitation potentials of noble gases, it is difficult to obtain solar noble gas abundances from spectroscopic. In situ measurements show that the SW from different regimes (e.g. fast and slow) differs in elemental and to some extent also in isotopic compostion [e.g. 1, 2]. Assuming a homogeneously mixed photosphere as the source for SW matter, this compositional variability is ascribed to fractionation processes during SW formation and acceleration. To infer solar abundances from SW abundances it is mandatory to understand and quantify these processes. In order to constrain the extent of the compositional variability, Genesis collected bulk SW and three major SW regimes separately: Fast, coronal hole associated SW (CH); slow, interstream SW (IS) (both also called “quasi-stationary”); and SW from coronal mass ejections (CMEs). Abundances and isotopic composition of He, Ne, Ar of the bulk SW and the different regimes were presented by [3]. We found significant differences in isotopic as well as elemental compositions among the different SW regimes. Here we discuss what the data of [3] may tell us about elemental and isotopic fractionation processes in the SW. Light noble gases are particularly well suited for such a study because their collection is least affected by contamination. Also, isotopic fractionation effects can be easily quantified because of the large relative mass differences of the stable isotopes. Isotope fractionation: Differences in the measured isotopic composition are particularly pronounced between IS and CH. The IS is depleted in the heavy isotopes compared to CH. This depletion is large for He (6.3±0.4%/amu) and decreases with atomic mass, i.e. is 0.4±0.1%/amu for Ne and 0.26±0.10%/amu for Ar. We thus confirm indications found by ISEE-3/ICI, Ulysses/SWICS and SOHO/CELIAS. Since isotopic fractionation in the solar wind is purely mass-related and does not depend on atomic properties, it is generally ascribed to inefficient Coulomb drag (CD) from protons [e.g. 4]. Whereas in CH related SW waveparticle interaction heats and accelerates heavy species indiscriminately, Coulomb collisions in the inner corona are important in accelerating species heavier than protons in IS regimes. It is well known from spacecraft observations that inefficient CD can produce SW strongly depleted in He and consequently, it has to be expected that it operates on all isotopes. The measured difference in the He, Ne, and Ar isotopic composition between CH and IS [3] is in general accordance with the systematics of Coulomb drag fractionation.