Authors: P. Bochsler
Reference: Space Sci. Rev., 85, 291-302, 1998
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 convective zone. It is then modi?ed by the processes which feed the transition region and the corona, and ?nally by the transport mechanisms which operate in the corona, and in the interplanetary plas- ma. In situ measurements of the solar wind composition are interesting because they give a unique opportunity to obtain information on the isotopic and elemental composition of the Sun, although elemental - and to some degree also isotopic - fractionation can occur on the way of matter from the outer convective zone into the interplanetary space. The most important examples of elemental fractionation are the well-known FIP/FIT eect (First Ionization Potential/Time) and the sometimes dramatic variations of helium abun- dances relative to hydrogen in the solar wind. A thorough investigation of fractionation processes which cause compositional variations in dierent solar wind regimes is necessary in order to make inferences about the solar composition from solar wind observations. Our understanding of these processes is presently improving thanks to the detailed diagnos- tics oered by the optical instrumentation on SOHO. Correlated observations of particle instruments on Ulysses, WIND, SOHO together with optical observations will help to make inferences for the solar composition. First continuous in-situ observations of several isotopic species with the particle instruments on WIND and SOHO are presently establish- ing an experimental database for isotopic fractionation processes which operate between the solar surface and the interplanetary medium in dierent solar wind regimes. Except for the relatively minor eects of secular gravitational sedimentation, which works at the boundary between the outer convective zone and the radiative zone, refractory elements such as Mg can be used as faithful witnesses to monitor the magnitude of these processes. On the theoretical side it is possible to make inferences about the importance of isotopic fractionation in the solar wind from a comparison of optical and in situ obser- vations of elemental fractionation with the corresponding theoretical models. Theoretical models and preliminary results from particle observations indicate that the combined isotope eects do not exceed a few percent per mass unit. For the case of the astrophysically important 3 He= 4 He ratio, we expect an overall eect of 10% in the sense of a systematic depletion of the heavier isotope. Continued observations with WIND, SOHO, and ACE and the revival of the foil technique with the upcoming GENESIS mission will further consolidate our knowledge about the relation between solar wind dynamics and solar wind composition.