Authors: P. Bochsler
Reference: Space Sci. Rev., 85, 291-302, 1998
Abstract:
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.