Solar-wind data obtained by the OGO-5 and IMP-6-8 Earth satellites show a positive correlation between the ratio of helium to hydrogen temperatures, TJ Tp, and the velocity difference between the two ions, v~ -vp. Although this result disagrees with the Prognoz-1 results reported earlier in this journal, it is consistent with the expected control by Coulomb collisions when the solar-wind density is high.
There is a convincing body of evidence that Coulomb collisions sometimes play an important role in determining the temperature and velocity of helium ions relative to hydrogen in the solar wind. When collisions are negligible, the helium is much hotter and flows away from the Sun faster than the hydrogen. and found that the helium to hydrogen temperature ratio, TJTp, varies inversely with the ratio of the solar-wind expansion time re to the time zc required for Coulomb collisions to transfer energy between the two ion species. and Griinwaldt and Rosenbauer (1978) also found that the difference in flow velocities, Av = v~ -vp, varies inversely with the ratio of Te to the Coulomb collision slowing-down time rs and that Av is near zero for felts above 0.3. Since Coulomb collisions play a role in limiting both TJ Tp and Av, one would expect to find a positive correlation between these two parameters. However, have published a paper in this journal in which they report that, according to plasma data obtained with Prognoz-1, the temperature ratio decreases rather than increases with increasing Av. It is the purpose of this paper to present helium temperature and velocity data obtained by the Earth satellites OGO-5 and IMP-6-8. These data disagree with the Prognoz-1 data and are consistent with expectations based on the role of Coulomb collisions.
The OGO-5 data were obtained intermittently between March, 1968, and April, 1971. The method of combining the difference in speeds, As = [v~ [-]vpl, with hourly averages of the interplanetary magnetic field to obtain the vector velocity difference, Av, was described by . Figure shows values of T,/Tp averaged over intervals of Av. The vertical error bars show the probable error of each average. The difference between the OGO-5 and Prognoz-1 results is obvious when this figure is compared to Figure of Bosqued et aI. (1977). The straight line in Figure shows