Authors: B. L. Alterman (NASA/GSFC), Yogesh (UIowa), F. Carcaboso (NASA/GSFC), A. Weiss (NASA/GSFC), M. Martinovic (UArizona), J. M. Laming (NRL), B. Maruca (UDel), M. Stevens (CfA), S. Chabbra (NASA/GSFC), E. Mason (PredSci), L. B. Wilson III (NASA/GSFC)

The solar wind helium abundance (A_He) is governed by the magnetic topology of its coronal source regions. In fast wind from continuously open coronal holes, A_He stays fixed near 50% of its photospheric value across the solar cycle, while in slow wind and interplanetary coronal mass ejections (ICMEs) from intermittently open closed loops it is highly variable and tracks solar activity. Combining three studies of Wind observations across solar cycles 23–25, we show that this variability is set by processing within active regions (ARs).

Slow and fast wind A_He are drawn from statistically distinct parent distributions. Slow wind A_He correlates equivalently with sunspot number and with AR open magnetic flux, with an indistinguishable functional dependence on either, in sharp contrast to the nearly activity-independent fast wind. We therefore infer that slow wind helium is released with AR-processed plasma, and interpret its depletion below fast wind values, in excess of the FIP effect, as gravitational settling in long-lived closed loops.

Slow wind A_He also has a fixed amplitude across cycles 23, 24, and 25, making it an amplitude-independent activity indicator. We report the first in situ detection of the Gnevyshev gap in slow wind A_He and argue that both the gap and the helium shutoff are quasi-biennial oscillation signatures, enabling early identification of solar minimum and maximum for space weather forecasting. Finally, ICME A_He resembles slow wind but is commonly enhanced, which we attribute to chromospheric evaporation during eruption.