  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
$\textit{Wind}$ spacecraft's Faraday cups reveal that, at $r=1.0\ \textrm{AU}$
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 $r=0.1\ \textrm{AU}$. These inferred $\theta_{\alpha p}$-values have no
trace of the bimodality seen in the $\theta_{\alpha p}$-values measured at
$r=1.0\ \textrm{AU}$ 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.
