Structure of the solar wind and compositional variations

The composition of the solar wind is largely determined by the composition of the source material, i.e. the present-day composition of the outer conve ctive zone. It is then modified by the processes which operate in the trans ition region and in the inner corona. In situ measurements of the solar win d composition give a unique opportunity to obtain information on the isotop ic and elemental composition of the Sun. However, elemental - and to some d egree also isotopic - fractionation can occur in the flow of matter from th e outer convective zone into the interplanetary space. The most important e xamples of elemental fractionation are the well-known FIP/FIT effect (First Ionization Potential/Time) and the sometimes dramatic variations of the he lium abundance relative to hydrogen in the solar wind. A thorough investiga tion of fractionation processes which cause compositional variations in dif ferent solar wind regimes is necessary to make inferences about the solar s ource composition from solar wind observations. Our understanding of these processes is presently improving thanks to the detailed diagnostics offered by the optical instrumentation on SOHO. Correlated observations of particl e instruments on Ulysses, WIND, and SOHO, together with optical observation s will help to make inferences for the solar composition. Continuous in sit u observations of several isotopic species with the particle instruments on WIND and SOHO are currently incorporated into an experimental database to infer isotopic fractionation processes which operate in different solar win d regimes between the solar surface and the interplanetary medium. Except for the relatively minor effects of secular gravitational sedimentat ion which works at the boundary between the outer convective zone and the r adiative zone, refractory elements such as Mg can be used as faithful witne sses to monitor the magnitude of these processes. With theoretical consider ations it is possible to make inferences about the importance of isotopic f ractionation in the solar wind from a comparison of optical and in situ obs ervations of elemental fractionation with the corresponding models. Theoretical models and preliminary results from particle observations indic ate that the combined isotope effects do not exceed a few percent per mass unit. In the worst case, which concerns the astrophysically important He-3/ (4) He ratio, we expect an overall effect of at most several percent in the sense of a systematic depletion of the heavier isotope. Continued observat ions with WIND, SOHO, and ACE, and, with the revival of the foil technique, with the upcoming Genesis mission will further consolidate our knowledge a bout the relation between solar wind dynamics and solar wind composition.