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 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 .