Helium Variation across Two Solar Cycles Reveals a Speed-dependent Phase Lag

Abstract

Abstract We study the relationship between the solar wind helium-to-hydrogen abundance ratio (A He), solar wind speed (v 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 He and SSN, we show that the phase delay between A He and SSN is a monotonic increasing function of v sw. Correcting for this lag, A 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 He from its fast wind value during solar wind formation.