Solar Wind Helium Abundance Heralds Solar Cycle Onset

TL;DR

This study reveals that the helium-to-hydrogen abundance in the solar wind increases before sunspot minima and exhibits a rapid depletion and recovery at cycle onset, indicating a new marker for solar cycle transitions.

Contribution

It identifies a distinct helium abundance shutoff prior to solar cycle onset, linked to cycle overlap and flux cancelation, providing new insights into solar cycle indicators.

Findings

$A_\mathrm{He}$ increases before SSN minima

Rapid $A_\mathrm{He}$ depletion occurs before cycle onset

$A_\mathrm{He}$ shutoff is consistent across solar wind speeds

Abstract

We study the solar wind helium-to-hydrogen abundance's ($A_\mathrm{He}$) relationship to solar cycle onset. Using OMNI/Lo data, we show that $A_\mathrm{He}$ increases prior to sunspot number (SSN) minima. We also identify a rapid depletion and recovery in $A_\mathrm{He}$ that occurs directly prior to cycle onset. This $A_\mathrm{He}$ Shutoff happens at approximately the same time across solar wind speeds ($v_\mathrm{sw}$), implying that it is formed by a mechanism distinct from the one that drives $A_\mathrm{He}$'s solar cycle scale variation and $v_\mathrm{sw}$-dependent phase offset with respect to SSN. The time between successive $A_\mathrm{He}$ shutoffs is typically on the order of the corresponding solar cycle length. Using Brightpoint (BP) measurements to provide context, we infer that this shutoff is likely related to the overlap of adjacent solar cycles and the equatorial flux cancelation of the older, extended solar cycle during Solar Minima.