On the Regulation of the Solar Wind Helium Abundance by the Hydrogen Compressibility
B. L. Alterman, R. D'Amicis
TL;DR
This study demonstrates that the solar wind helium abundance $A_{He}$ is not set solely by wind speed but is regulated by hydrogen compressibility, as quantified by $|\delta n|/n$, and by the Alfvénic content $|\sigma_c|$. Using Wind SWE data, the authors identify two helium-rich populations: an incompressible, Alfvénic subset with a saturation at $A_s \approx 4.2\%$ occurring near $v_s \approx 428$ km s$^{-1}$, and a compressible, non-Alfvénic subset that exhibits larger $m_s$ and upshifts in $v_s$. Through quantile analyses across $|\delta n|/n$ and $|\sigma_c|$, they show that compressibility can shift saturation parameters and even produce helium enhancements beyond traditional saturation values, suggesting a link to transients and PBS/slow-mode dynamics. The work provides a framework for mapping solar wind observations to solar-source regions by jointly considering $A_{He}$, $|\sigma_c|$, and $|\delta n|/n$, revealing that compressibility is a crucial, previously underappreciated control on helium enrichment in interplanetary space.
Abstract
Traditionally, fast solar wind is considered to originate in solar source regions that are continuously open to the heliosphere and slow wind originates in regions that are intermittently open to it. In fast wind, the gradient of the solar wind helium abundance ($A_\mathrm{He}$) with increasing solar wind speed ($v_\mathrm{sw}$) is $\sim0$ and $A_\mathrm{He}$ is fixed at $\sim50\%$ of the photospheric value. In slow wind, this gradient is large, $A_\mathrm{He}$ is highly variable, and it doesn't exceed this $\sim50\%$ value. Although the normalized cross helicity in fast wind typically approaches 1, this is not universally true and Alterman & D'Amicis (2025) show that $\nabla_{v_\mathrm{sw}} \! A_\mathrm{He}$ in fast wind unexpectedly increases with decreasing $\left|σ_c\right|$. We show that these large gradients are due to the presence of compressive fluctuations. Accounting for the solar wind's compressibility ($\left|δn/n\right|$), there are two subsets of enhanced $A_\mathrm{He}$ in excess of typical fast wind values. The subset with a large compressibility is likely from neither continuously nor intermittently open sources. The portion of the solar wind speed distribution over which these fluctuations are most significant corresponds to the range of Alfvén wave-poor solar wind from continuously open source regions, which is likely analogous to the Alfvénic slow wind. Mapping the results of this work to Alterman & D'Amicis (2025) and vice versa shows that, in any given $\left|δn/n\right|$ quantile, $\left|σ_c\right| \lesssim 0.65$, an upper bound on non-Alfvénic cross helicity. Similarly, $\left|δn/n\right| \lesssim 0.15$ in any given $\left|σ_c\right|$ quantile, is an upper bound on incompressible fluctuations. We conclude that $\left|δn/n\right|$ is essential for characterizing the solar wind helium abundance and possibly regulating it.