On the bulk isotopic composition of magnesium and silicon during the May 1998 CME: ACE/SWIMS

The coronal mass ejection (CME) observed at the Advanced Composition Explor er (ACE) spacecraft on May 2 and 3, 1998, exhibited very high as well as ex ceptionally low charge states of all ions. In addition, He-3 was considerab ly enhanced by in the bulk material of the CME. High He-3/He-4 ratios in so lar energetic particles have recently been observed in some events to coinc ide with substantial enrichments in the heavy isotopes of heavy elements. W e use data from the Solar Wind Ion Mass Spectrometer (SWIMS) on ACE to inve stigate whether the enrichment of the heavy isotopes in solar energetic par ticles is mirrored in the isotopic composition of solar wind Mg and Si. We concentrate on the time period where the unusual mixture of charge states a nd unusually high He-3/He-4 was observed to test for isotopic fractionation in such extreme solar wind conditions. We find very little or no enrichmen t of the neutron-rich isotopes with respect to the main isotopes. Incidenta lly, this is also the first report on the isotopic composition of ions with Z > 2 in the bulk material in a CME.