A holistic approach to understand Helium enrichment in Interplanetary coronal mass ejections: New insights

TL;DR

This study analyzes 275 ICME events to understand helium enrichment, revealing that chromospheric evaporation and gravitational settling, influenced by flare timing, are key factors, while other potential influences are less significant.

Contribution

It provides a comprehensive analysis showing that chromospheric evaporation and gravitational settling, modulated by flare timing, explain helium enrichment in ICMEs, surpassing previous partial explanations.

Findings

AHE varies with solar activity levels.

Chromospheric evaporation and gravitational settling are primary factors.

Flare timing influences helium enrichment in ICMEs.

Abstract

Despite helium abundance (AHe = nH/nHe) is ~ 8 % at the solar photospheric/chromospheric heights, AHe can be found to exceed 8% in interplanetary coronal mass ejections (ICMEs) on many occasions. Although various factors like interplanetary shocks, chromospheric evaporation and "sludge removal" have been separately invoked in the past to address the AHe enhancements in ICMEs, none of these processes could explain the variability of AHe in ICMEs comprehensively. Based on extensive analysis of 275 ICME events, we show that there is a solar activity variation of ICME averaged AHe values. The investigation also reveals that the first ionization potential effect as well as coronal temperature are not the major contributing factors for AHe enhancements in ICMEs. Investigation on concurrent solar flares and ICME events for 63 cases reveals that chromospheric evaporation in tandem with gravitational settling determine the AHe enhancements and variabilities beyond 8% in ICMEs. While chromospheric evaporation releases the helium from chromosphere into the corona, the gravitationally settled helium is thrown out during the ICMEs.We show that the intensity and timing of the preceding flares from the same active region from where the CME erupts are important factors to understand the AHe enhancements in ICMEs.