Thermal anisotropies in the solar wind: Evidence of heating by interstellar pickup ions?

A recent paper by Gray et al. [1996] shows that the Alfven ion cyclotron instability is generated by newly created pickup ions and heats the thermal solar wind protons in the direction perpendicular to the magnetic field. This instability operates most effectively in regions where the plasma β is low, so this mechanism predicts that the ratio of the temperatures perpendicular and parallel to the magnetic field should be larger in low-β regions of the solar wind. We look for this effect in ISEE-3 and Voyager 2 data. The near-Earth ISEE-3 data show no evidence for greater thermal anisotropies at low β. Voyager 2 data obtained between 1 and 8 AU show that the ambient proton thermal anisotropy is a function of β, with parallel temperatures generally greater than perpendicular temperatures except when β is small. For Voyager 2 data, the average ratio of perpendicular to parallel temperature is about 0.9, but this ratio is ∼2 when β is less than 0.1 and ∼4 when β is less than 0.01.