Event-to-event variations in the isotopic composition of neon in solar energetic particle events

Using the Solar Isotope Spectrometer on the Advanced Composition Explorer, we have measured the isotopic composition of neon at 24–72 MeV/nucleon in nine solar energetic particle events between 4 November 1997 and 16 November 1998. The ^(22)Ne/^(20)Ne ratio varies from ∼0.7 to ∼2 times the solar wind value from event to event, indicating significant mass fractionation in the acceleration and/or transport of these particles. The mass fractionation is strongly correlated with the Fe/O and Na/Mg element abundance ratios, suggesting that elemental and isotopic fractionation are both governed by the ionic charge to mass ratio.