Energetic particle composition at 1 au during periods of moderate interplanetary particle fluxes

It has recently been proposed that residual energetic ions from preceding impulsive solar energetic particle (SEP) events may contribute to the source population for the larger gradual events accelerated by CME-driven shocks (Mason et al., 1999). This suggests that there may be impulsive SEP material left in interplanetary space during periods relatively free of energetic particle activity. To test this idea we have studied the heavy ions and ^3He present at L1 during periods of low—to—moderate energetic particle fluxes between February 1998 and April 2000. Using the Solar Isotope Spectrometer on the Advanced Composition Explorer, we have measured the energy spectra of heavy elements (Z≥6) and ^3He over the ranges ∼8–20 MeV/nucleon and ∼4–15 MeV/nucleon, respectively. In these energy spectra, we have looked for spectral and compositional signatures typically characteristic of impulsive or gradual SEP events. We discuss the implications of these observations for the origin of particles accelerated in gradual SEP events.