Spectral Properties of Heavy Ions Associated with the Passage of Interplanetary Shocks at 1 AU

We have surveyed the energy spectra of ~0.1–100 MeV nucleon^(-1) C, O, and Fe nuclei associated with the passage of 72 interplanetary (IP) shocks observed on board the ACE spacecraft during the period 1997 October–2002 October. Our main results are as follows: (1) The spectral fit parameters are independent of the local shock properties. (2) About 7% of the events exhibit increasing Fe/O ratios with energy; the remaining events have Fe/O ratios that either remain constant or decrease with energy. (3) The Fe/O ratio in the shock-associated particles is typically ~30% lower than in the ambient population. (4) The fractionation pattern of the elemental abundances, the O spectra, and the energy-dependence of Fe/O at the IP shocks are remarkably similar to those of the ambient interplanetary suprathermal ion population. We suggest that the IP shocks studied here reaccelerate energetic particle seed spectra composed of ions from impulsive and gradual solar energetic particle events by systematic rigidity-dependent mechanisms in which higher rigidity ions are accelerated less efficiently than lower rigidity ions.