Isotopic abundances in the solar corona as inferred from ACE measurements of solar energetic particles

The isotopic composition of solar energetic particles (SEPs) has been measured using the Solar Isotope Spectrometer on the Advanced Composition Explorer. The measurements include up to 12 isotope abundance ratios for ten elements from C through Ni at energies of tens of MeV/nucleon in 18 large SEP events that have occurred since November 1997. These measurements clearly establish that SEP isotopic composition can vary widely (by factors of >3) from event to event, presumably due to mass fractionation processes during particle acceleration and/or transport. Elemental and isotopic abundance ratios are strongly correlated, suggesting that elemental and isotopic fractionation relative to the coronal source are largely governed by the same processes. Using empirical correlations to correct for the fractionation and obtain the coronal isotopic composition yields preliminary abundance values in good agreement with those found in the solar wind, with comparable accuracy.