Fractionation of Si, Ne, and Mg isotopes in the solar wind as measured by SOHO/CELIAS/MTOF

Using the high-resolution mass spectrometer CELIAS/MTOF on board SOHO we ha ve measured the solar wind isotope abundance ratios of Si, Ne, and Mg and t heir variations in different solar wind regimes with bulk velocities rangin g from 330 km/s to 650 km/s. Data indicate a small systematic depletion of the heavier isotopes in the slow solar wind on the order of (1.4+/-1.3)% pe r amu (2 sigma-error) compared to their abundances in the fast solar wind f rom coronal holes. These variations in the solar wind isotopic composition represent a pure mass-dependent effect because the different isotopes of an element pass the inner corona with the same charge state distribution. The influence of particle mass on the acceleration of minor solar wind ions is discussed in the context of theoretical models and recent optical observat ions with other SOHO instruments.