Origins of Very Low Helium Abundance Streams Detected in the Solar Wind Plasma

TL;DR

This study investigates the origins of extremely low helium abundance events in the solar wind, linking them to coronal streamer boundaries and gravitational settling processes near the Sun.

Contribution

It identifies the source regions of very low helium abundance in the solar wind as coronal streamer boundaries, supported by modeling and spacecraft observations.

Findings

Low-$A_{He}$ events originate near coronal streamer boundaries.

Streamer core regions facilitate gravitational settling of helium.

Reconnection near streamer cusps releases helium-depleted plasma.

Abstract

The abundance of helium ($A_{He}$) in the solar wind exhibits variations typically in the range from 2-5% with respect to solar cycle activity and solar wind velocity. However, there are instances where the observed $A_{He}$ is exceptionally low ($<$ 1%). These low-$A_{He}$ occurrences are detected both near the Sun and at 1 AU. The low $A_{He}$ events are generally observed near the heliospheric current sheet. We analyzed 28 low-$A_{He}$ events observed by the Wind spacecraft and 4 by Parker Solar Probe (PSP) to understand their origin. In this work, we make use of the ADAPT-WSA model to derive the sources of our events at the base of the solar corona. The modeling suggests that the low-$A_{He}$ events originated from the boundaries of coronal holes, primarily from large quiescent helmet streamers. We argue that the cusp above the core of the streamer can produce such very low helium abundance events. The streamer core serves as an ideal location for gravitational settling to occur as demonstrated by previous models, leading to the release of this plasma through reconnection near the cusp, resulting in low $A_{He}$ events. Furthermore, observations from Ulysses provide direct evidence that these events originated from coronal streamers.