ISOTOPE FRACTIONATION OF SOLAR WIND PARTICLES

The solar wind provides a unique source of information on the isotopic composition of the Sun. This information is relevant because the Sun represents the least biased sample of interstellar matter as it existed 4.6 Gy ago at about 6 to 8 kpc from the galactic center. At the same time, solar matter is a geochemical reference for the original composition of the protosolar nebula from which the solar system formed. The isotopic uniformity of moderately-volatile and refractory elements within the solar system (e.g. Ca, Si, Mg, and Fe) can be used to infer solar isotopic abundances from terrestrial and meteoritic samples, thus providing benchmarks for an experimental determination of isotope fractionation processes in different solar wind regimes. On the other hand, the solar isotopic composition of volatile elements (He, C, N, O, Ne, Ar, S) can only be reliably determined from solar particles. Theoretical investigations have covered potential isotope fractionation processes occurring due to gravitational settling in the outer solar convective zone, neutral-ion separation in the chromosphere, coronal acceleration due to Coulomb friction with protons and alpha particles, and coronal acceleration and heating due to wave particle interaction. From these studies it is expected that, except for the case of helium, typical fractionation effects will not exceed 1 to 3% per mass unit. Experimental determinations of these effects on refractory elements are most useful if performed in correlation with fluctuations of the solar wind He/H ratio and with solar wind speed. The experimental precision achieved so far is sufficient to confirm the above theoretical upper limits, however, it is not yet sufficient to identify solar wind isotopic fractionation processes unambiguously. On the other hand, the precision is sufficient to obtain some basic and cosmochemically relevant data on the isotopic composition of solar volatiles. We will present a summary of results and applications of recent work on the solar wind isotopic composition.