The role of Coulomb collisions in limiting differential flow and temperature differences in the solar wind

Analysis of data obtained by Ogo 5 confirms the Imp 6 observations of the inverse dependence of the helium to hydrogen temperature ratio THe/TH on τe/τc, the ratio of the solar wind expansion time to the Coulomb collision equipartition time. The difference between the absolute values of the averages of THe/TH as observed by Imp and Ogo is probably due to the difference of the weights given to high- and low-velocity periods by the two experiments. The magnitude of the differential flow vector Δυ = |vHe - vH| varies inversely with τe/τs (where τs is the Coulomb collision slowing-down time) when τe/τs is small and approaches zero when τe/τs is large. The data suggest a model of continuous preferential acceleration and heating of helium (or deceleration and cooling of hydrogen) which is opposed and limited by Coulomb collisions. Since the average dependence of Δυ on τe/τs cannot explain all of the systematic variations of Δυ observed in corotating high-velocity streams, extra helium acceleration (or hydrogen deceleration) probably occurs on the leading edge of such streams.