Ion composition in the solar wind in relation to solar abundances

Isotopic abundances in the outer convective zone of the sun can best be derived from measurements of solar wind ions and of solar energetic particles which stem from flares. However, the emission of solar matter from the solar surface and subsequent acceleration processes lead to changes of elemental as well as isotopic abundances. At the present time the largest uncertainties regarding the solar isotopic composition of He and Ne and probably other elements are due to our limited understanding of the fractionation mechanisms. The second process, probably operating in or directly above the chromosphere, has to be invoked for explaining the observed dependency on the first ionization potential (FIP) of corona, SEP and SW abundances relative to the photospheric composition. A specific mechanism, the critical velocity process, that would lead to this type of element fractionation, was proposed long ago by Alfven (1960). Recently, Geiss (1982) has explored the possibility of explaining the evidence by a diffusive ion-atom separation process in a solar plasma under non-LTE condition. We shall further discuss this mechanism after having summarized the results of element and isotope abundance measurements in the solar wind