Energetic heavy ions observed upstream of the Earth's bow shock by the STEP/EPACT instrument on WIND

The Supra-Thermal through Energetic Particle (STEP) subsystem of the EPACT experiment on the WIND spacecraft observed numerous short duration heavy ion enhancements during the approx.9 month period Nov. 1994-Sept. 1995. These enhancements were most frequent and intense when WIND was close to the magnetosphere, but were often observed also during the period when the spacecraft was >100 REupstream. The events occur in association with high speed solar wind streams that are signatures of corotating interaction regions (CIRs). A typical event observed on January 31, 1995 when WIND was 195 REupstream showed strong field aligned anisotropies and rapid time variations. Heavy ion abundances near 45 keV/nucleon (nuc) were He:C:O:Fe = 86:1.46:≅1:0.12, similar to those observed at higher energies in CIRs, and distinctly different from magnetospheric ring current abundances measured on the AMPTE spacecraft. We suggest that the CIRs provide the seed population for these heavy ion events, that are then further accelerated in association with the Earth's bow shock.