Elemental Fractionation in Small Solar Energetic Particle Events

Abstract

Using the Solar Isotope Spectrometer on the Advanced Composition Explorer (ACE), we have measured the 11.0-21.8 MeV nucleon^(-1) heavy (Z ≥ 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) ^3He/^4He ratios. We find that their average heavy-element composition is similar to that of either large gradual events or ^3He-rich events, depending on their ^3He/^4He ratio. As seen in recent studies of small SEP events, we find that the heavy-element intensities relative to C increase with the ^3He/^4He 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 ~2 to 7 with no clear dependence on the ^3He/^4He ratio.