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

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.