Helium Variation Across Two Solar Cycles Reveals A Speed-Dependent Phase Lag

Abstract

We study the relationship between solar wind helium to hydrogen abundance ratio ($A_{\mathrm{He}}$), solar wind speed ($v_{\mathrm{SW}}$), and sunspot number (SSN) over solar cycles 23 and 24. This is the first full 22-year Hale cycle measured with the Wind spacecraft covering a full cycle of the solar dynamo with two polarity reversals. While previous studies have established a strong correlation between $A_{\mathrm{He}}$ and SSN, we show that the phase delay between $A_{\mathrm{He}}$ and SSN is a monotonic increasing function of $v_{\mathrm{SW}}$. Correcting for this lag, $A_{\mathrm{He}}$ returns to the same value at a given SSN over all rising and falling phases and across solar wind speeds. We infer that this speed-dependent lag is a consequence of the mechanism that depletes slow wind $A_{\mathrm{He}}$ from its fast wind value during solar wind formation.

9 pages, 4 figures

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