Helium energetics in the high‐latitude solar wind: Ulysses observations

We present a study of the interplanetary evolution of solar wind helium (alpha particle) energetics. The analysis of Ulysses observations of the fast high‐latitude solar wind concentrates on the radial evolution of the alpha‐proton differential streaming v α p , the alpha temperature, and the alpha temperature anisotropy. Ulysses observations show that the average v α p steadily decreases with radius, ranging from ∼40 km s −1 at 1.5 AU to ∼15 km s −1 at 4.2 AU. In addition, observations indicate that the alphas cool more slowly than what would be expected from adiabatic expansion. The radial increase in the nonadiabatic heat content of the alphas matches the free energy liberated as v α p decreases with distance, suggesting that the dissipated energy acts to heat the alpha particles. The alphas also exhibit a temperature anisotropy of T ⊥α / T ‖α = 0.87, which is essentially constant with distance. These and other observations reported here place stringent constraints on recent plasma microinstability models that attempt to explain the evolution of alpha‐proton differential streaming and ion heating in the heliosphere.