Structure of the Solar Wind and Compositional Variations |
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Authors: | P Bochsler |
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Institution: | (1) Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland |
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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 modified by the processes which operate in the transition region and in the inner
corona. In situ measurements of the solar wind composition give a unique opportunity to obtain information on the isotopic
and elemental composition of the Sun. However, elemental — and to some degree also isotopic — fractionation can occur in the
flow of matter from the outer convective zone into the interplanetary space. The most important examples of elemental fractionation
are the well-known FIP/FIT effect (First Ionization Potential/Time) and the sometimes dramatic variations of the helium abundance
relative to hydrogen in the solar wind. A thorough investigation of fractionation processes which cause compositional variations
in different solar wind regimes is necessary to make inferences about the solar source 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 particle instruments on Ulysses, WIND, and SOHO, together with optical observations will
help to make inferences for the solar composition. Continuous in situ 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 wind regimes between the solar surface and the interplanetary medium.
Except for the relatively minor effects 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. With theoretical considerations it is possible to make inferences about the importance of isotopic fractionation
in the solar wind from a comparison of optical and in situ observations of elemental fractionation with the corresponding
models.
Theoretical models and preliminary results from particle observations indicate that the combined isotope effects do not exceed
a few percent per mass unit. In the worst case, which concerns the astrophysically important 3He/4He ratio, we expect an overall
effect of at most several percent in the sense of a systematic depletion of the heavier isotope. Continued observations with
WIND, SOHO, and ACE, and, with 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.
This revised version was published online in June 2006 with corrections to the Cover Date. |
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