R. A. Leske¹, R. A. Mewaldt¹, C. M. S. Cohen¹, E. R. Christian², A. C. Cummings¹, P. L. Slocum³, E. C. Stone¹, T. T. von Rosenvinge², and M. E. Wiedenbeck³

1 California Institute of Technology, Pasadena, CA 91125 USA.

2 NASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA.

3 Jet Propulsion Laboratory, Pasadena, CA 91109 USA.

Using the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE), we have measured the isotopic composition of C, O, Ne, Mg, Si, S, Ar, Ca, Fe, and Ni at energies of tens of MeV/nucleon in as many as nine solar energetic particle (SEP) events that have occurred since November 1997. We find that isotopic composition varies dramatically from event to event. For example, the ²²Ne/²⁰Ne ratio ranges from ~ 0.7 to 2 times the solar wind value, being lowest for the soft, iron-poor events, and highest for the hard, iron-rich events. We present the SIS SEP isotope measurements to date, and show that the strong correlation of the mass fractionation with the Fe/O and other element abundance ratios suggests that elemental and isotopic fractionation are governed by the same process.