Origin of heavy ions in upstream events near the Earth's bow shock

Abstract

We have surveyed the ∼20–160 keV/nucleon heavy ion composition of 2102 upstream events observed by the Wind/STEP instrument near the Earth's bow shock over a complete solar cycle from day 325, 1994 through day 90, 2006. Our main results are: (1) During solar minimum conditions, both the C/O and Fe/O ratios are similar to the corresponding values measured in the solar wind and/or CIRs. (2) During solar maximum conditions, the C/O ratio is similar to that measured in SEP and CME‐driven IP shock events, while the Fe/O ratio lies between the values measured in IP shock and SEP events. We conclude that the source material for upstream events near the Earth's bow shock originates from a dynamic suprathermal ion population that is dominated by ions accelerated in SEP events during solar maximum conditions and by suprathermal solar wind ions and/or those accelerated in CIRs during solar minimum conditions.