Solar wind isotopic abundance ratios of Ne, Mg and Si measured by SOHO/CELIAS/MTOF as diagnostic tool for the inner corona

Using the high-resolution mass spectrometer MTOF on board SOHO we have meas ured the solar wind isotopic abundance ratios of Ne, Mg, and Si in differen t solar wind regimes with bulk velocities ranging from 350 km/s to 650 km/s . Data indicate a systematic depletion of the heavier isotopes in the slow solar wind compared to their abundances in the fast solar wind from coronal holes. These variations in the solar wind isotopic composition represent a pure mass-dependent effect because the different isotopes of an element pa ss 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 observations with o ther SOHO instruments. (C) 1999 Elsevier Science Ltd. All rights reserved.