Collisional Thermalization of Hydrogen and Helium in Solar Wind Plasma

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