Collisional Thermalization of Hydrogen and Helium in Solar Wind Plasma

Abstract

In situ observations of the solar wind frequently show the temperature of $\alpha$-particles (fully ionized helium), $T_{\alpha }$, to significantly differ from that of protons (ionized hydrogen), $T_p$. Many heating processes in the plasma act preferentially on $\alpha$-particles, even as collisions among ions act to gradually establish thermal equilibrium. Measurements from the $\text{Wind}$ spacecraft's Faraday cups reveal that, at $r=1.0\text{AU}$ from the Sun, the observed values of the $\alpha$-proton temperature ratio, ${\theta }_{\alpha p}\equiv T_{\alpha }/T_p$ has a complex, bimodal distribution. This study applied a simple model for the radial evolution of ${\theta }_{\alpha p}$ to these data to compute expected values of ${\theta }_{\alpha p}$ at $r=0.1\text{AU}$. These inferred ${\theta }_{\alpha p}$-values have no trace of the bimodality seen in the ${\theta }_{\alpha p}$-values measured at $r=1.0\text{AU}$ but are instead consistent with the actions of the known mechanisms for $\alpha$-particle preferential heating. This result underscores the importance of collisional processes in the dynamics of the solar wind and suggests that similar mechanisms may lead to preferential $\alpha$-particle heating in both slow and fast wind.

5 pages, 2 figures, accepted to Physical Review Letters

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