Document Type

Article

Publication Title

The Astrophysical Journal

Abstract

We have surveyed the 0.1-10 MeV nucleonˉ¹ 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 nucleonˉ¹ from impulsive and gradual solar energetic particle events (SEPs) were present upstream of all the IP shocks in our survey. (4) The ∼1 MeV nucleonˉ¹ 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.

DOI

10.1086/374310

Publication Date

5-10-2003

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