FIRST DETERMINATION OF THE SILICON ISOTOPIC COMPOSITION OF THE SOLAR-WIND - WIND MASS RESULTS/

Silicon is a common material in the solar system. For instance, Si acc ounts for about 10% of the material in primitive meteorites (CI chondr ites). Since silicon is a refractory element, we expect the meteoritic isotopic composition to be very similar to that of the Sun. The isoto pic composition of Si in meteorites is well known and varies little. T hus the three stable isotopes of Si may serve as powerful indicators t o test fractionation of isotopes in the transition from the solar atmo sphere into the solar wind. We present, for the first time, measuremen ts of the isotopic composition of Si in the solar wind. The data were obtained with the MASS instrument aboard the WIND spacecraft and accum ulated in exceedingly cold and slow wind. Such wind is often associate d with large superradial expansion factors and with current sheet cros sings which in turn are associated with the most efficient isotopic fr actionation processes in the solar wind acceleration region. We detect little or no isotopic fractionation between the solar surface assumed to be of meteoritic composition and the solar wind. This constrains s olar wind acceleration models and puts stringent limits on possible se cular changes in the isotopic composition of the outer solar convectiv e zone, the solar atmosphere, and the solar wind.