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

Abstract

Abstract The helium abundance, defined as A He = n He/n H × 100, is ∼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 the 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 originate from active regions around maximum, and it 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.