Effects of Alpha-Proton Differential Flow on Proton Temperature Anisotropy Instabilities in the Solar Wind: Wind Observations

Plasma kinetic waves and alpha-proton differential flow are two important subjects on the topic of solar wind evolution. Based on the Wind data during 2005-2015, this paper reports that the occurrence of electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency significantly depends on the direction of alpha-proton differential flow {{\boldsymbol{V}}}_d. As {{\boldsymbol{V}}}_d rotates from the anti-Sunward direction to the Sunward direction, the occurrence rate of ECWs as well as the percentage of left-handed (LH) polarized ECWs decreases considerably. In particular, it is shown that the dominant polarization changes from LH polarization to right-handed polarization during the rotation. The investigation on proton and alpha particle parameters ordered by the direction of {{\boldsymbol{V}}}_d further illustrates that large kinetic energies of alpha-proton differential flow correspond to high occurrence rates of ECWs. These results are consistent with theoretical predictions for effects of alpha-proton differential flow on proton temperature anisotropy instabilities.