Elemental Fractionation in Small Solar Energetic Particle Events

Using the Solar Isotope Spectrometer on the Advanced Composition Explorer ( ACE), we have measured the 11.0 - 21.8 MeV nucleon(-1) heavy (Z greater than or equal to 6) element abundances of 39 small solar energetic particle (SEP) events that occurred between 1998 April 3 and 2002 February 26. Using He isotope data from the Ultra Low-Energy Isotope Spectrometer on ACE, we have classified the events according to their 0.5 - 2.0 MeV nucleon(-1 3)He/He-4 ratios. We find that their average heavy-element composition is similar to that of either large gradual events or He-3-rich events, depending on their He-3/He-4 ratio. As seen in recent studies of small SEP events, we find that the heavy-element intensities relative to C increase with the He-3/He-4 ratios. The dependence of the heavy-element abundances on first ionization potential (FIP) has been derived, using a model consisting of a power law in Q/M (and Z) times a step function in FIP. We report the magnitude of the FIP factor in each event and find that it varies from similar to2 to 7 with no clear dependence on the He-3/He-4 ratio.