Abundances of Suprathermal Heavy Ions in CIRs during the Minimum of Solar Cycle 23

TL;DR

This study analyzes the elemental composition of suprathermal heavy ions in corotating interaction regions during the solar minimum of cycle 23, revealing annual variations and potential SEP influence on ion abundances.

Contribution

It provides high-statistics observations of heavy ion abundances in CIRs during solar minimum, highlighting variability and possible SEP contributions.

Findings

Annual variations in ion abundance ratios during 2007-2008.

Helium focusing cone and heliolatitude affect ion composition.

2010 CIRs show SEP-like ion abundance ratios.

Abstract

In this paper we examine the elemental composition of the 0.1-1 MeV/nucleon interplanetary heavy ions from H to Fe in corotating interaction regions (CIRs) measured by the SIT (Suprathermal Ion Telescope) instrument. We use observations taken on board the STEREO spacecraft from January 2007 through December 2010, which included the unusually long solar minimum following solar cycle 23. During this period instruments on STEREO observed more than 50 CIR events making it possible to investigate CIR ion abundances during solar minimum conditions with unprecedented high statistics. The observations reveal annual variations of relative ion abundances in the CIRs during the 2007-2008 period as indicated by the He/H, He/O and Fe/O elemental ratios. We discuss possible causes of the variability in terms of the helium focusing cone passage and heliolatitude dependence. The year 2009 was very quiet in CIR event activity. In 2010 the elemental composition in CIRs were influenced by sporadic solar energetic particle (SEP) events. The 2010 He/H and He/O abundance ratios in CIRs show large event to event variations with values resembling the SEP-like composition. This finding points out that the suprathermal SEPs could be the source population for CIR acceleration.