Collisional Thermalization of Hydrogen and Helium in Solar-Wind Plasma

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 AU from the Sun, the observed values of the alpha-proton temperature ratio, theta(alpha p) equivalent to 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 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 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.