Comparison of Helium Abundance between ICMEs and Solar Wind near 1 AU

TL;DR

This study statistically compares Helium abundance in ICMEs and solar wind near 1 AU over solar cycles, revealing higher and cycle-dependent $A_{He}$ in ICMEs linked to active regions, with implications for solar wind origins.

Contribution

It provides the first comprehensive statistical analysis of $A_{He}$ in ICMEs versus solar wind near 1 AU across multiple solar cycles, highlighting solar cycle dependence and source region implications.

Findings

$A_{He}$ in ICMEs shows clear solar cycle dependence.

ICMEs have higher $A_{He}$ than solar wind, especially near solar maximum.

High $A_{He}$ ($>$8.5) mainly originates from active regions.

Abstract

The Helium abundance, defined as $A_{He}=n_{He}/n_{H}\times 100$, is $\sim$8.5 in the photosphere and seldom exceeds 5 in fast solar wind. Previous statistics have demonstrated that $A_{He}$ in slow solar wind correlates tightly with sunspot number. However, less attention is paid to the solar cycle dependence of $A_{He}$ within interplanetary coronal mass ejections (ICMEs) and comparing the $A_{He}$ characteristics of ICMEs and solar wind. In this paper we conduct a statistical comparison of Helium abundance between ICMEs and solar wind near 1 AU with observations of \textit{Advanced Composition Explorer} from 1998 to 2019, and find that the ICME $A_{He}$ also exhibits the obvious solar cycle dependence. Meanwhile, we find that the $A_{He}$ is obviously higher within ICMEs compared to solar wind, and the means within 37\% and 12\% of ICMEs exceed 5 and 8.5, respectively. It is interesting to answer where and how the high Helium abundance originates. Our statistics demonstrate that 21\% (3\%) of ICME (slow wind) $A_{He}$ data points exceed 8.5 around solar maximum, which decreases dramatically near minimum, while no such high $A_{He}$ values appear in the fast wind throughout the whole solar cycle. This indicates that the high $A_{He}$ (e.g., $>$8.5) emanates from active regions as more ICMEs and slow wind originates from active regions around maximum, and supports that both active regions and quiet-Sun regions are the sources of slow wind. We suggest that the high $A_{He}$ from active regions could be explained by means of the magnetic loop confinement model and/or photoionization effect.