The solar wind He 2+ to H + temperature ratio

The Los Alamos Scientific Laboratory Imp 6 solar wind ion plasma data measured between March 18, 1971, and May 30, 1972, were searched for a dependence of the alpha particle to proton temperature ratio Tα/Tp on the ratio of the solar wind expansion scale time to the Coulomb collision energy transfer time τe/τc. For flow conditions characterized by nearly Maxwellian ion velocity distributions, Tα/Tp is observed to vary inversely with τe/τc: when τe/τc is small, Tα/Tp ≃ 4, and when τe/τc ≳ 1, Tα/Tp ≲ 2. This result is consistent with a simplified model of the solar wind ion thermal state in which the dominant ion heating mechanisms active close to the sun are capable of maintaining Tα/Tp in the neighborhood of 3.25 out to some radial distance Rc ≃ 0.2–0.5 AU. For flow conditions characterized by sufficiently high Np and/or sufficiently low Tp + Tα/4 the relative alpha particle proton heating effects of these mechanisms become dominated by Coulomb energy transfer collisions, which reduce Tα/Tp toward unity.