Conditions of High Helium Concentrations in Coronal Mass Ejections

Abstract

We analyze the structure and composition of 248 ICMEs observed by the Wind spacecraft from February 1995 to December 2015. We group each event based on their doubly ionized Helium concentration within the magnetic cloud region into either the high-concentration (i.e., >8% for at least 25% of the magnetic cloud) or nominal-concentration (i.e., <8% for at least 75% of the magnetic cloud) group. We then perform an epoch analysis on plasma and magnetic field parameters within each distinct region of the ICME structure to identify similarities and/or differences associated with each group of events. Results show that the sheath region of ICMEs with high Helium concentration exhibits a two-part structure, which is indicative of the compression and expansion of the sheath as it propagates. We further investigate the effect of high Helium concentration within ICMEs on resulting geomagnetic activity by comparing the average Sym-H geomagnetic index between the two groups. We find that the Sym-H achieves a greater minimum for high-concentration events by a factor of ∼1.6. We suggest that this effect is related to the increase in dynamic pressure within the sheath region for ICMEs with high Helium content. Results indicate that high Helium content in ICMEs may play a non-trivial role in both the evolution of the ICME sheath throughout interplanetary space and the resulting geomagnetic activity induced by Earthward-directed ICMEs.