Evidence for a Suprathermal Seed Population of Heavy Ions Accelerated by Interplanetary Shocks near 1 AU

Abstract

We have surveyed the 0.1-10 MeV nucleon-1 elemental abundances at 72 interplanetary (IP) shocks observed by the Ultra-Low-Energy Isotope Spectrometer on board the Advanced Composition Explorer from 1997 October through 2002 September. We find the following: (1) The C/O ratios in IP shocks were substantially depleted (by more than ~40%) relative to solar wind values. (2) The IP shock abundances were poorly correlated with those measured in the slow and fast solar wind. (3) Energetic ions above ~0.1 MeV nucleon-1 from impulsive and gradual solar energetic particle events (SEPs) were present upstream of all the IP shocks in our survey. (4) The ~1 MeV nucleon-1 Fe/O ratio in IP shocks was positively correlated with that measured upstream of the shocks. (5) The IP shock abundances were well correlated with the upstream abundances, with a negative dependence on mass/charge. (6) The mean Fe/O ratio in IP shocks exhibited a positive correlation with the level of solar activity, as measured by the occurrence rates of X-ray flares and sunspots. The above results are inconsistent with shock acceleration of ions originating mainly from the bulk solar wind or a suprathermal tail composed predominantly of solar wind ions. Instead, it appears that for the events surveyed here, the IP shocks accelerated a seed population predominantly comprising ions that were previously accelerated in impulsive and gradual SEPs and that the shock acceleration process accelerated higher rigidity ions less efficiently than lower rigidity ions.