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

Abstract

Abstract 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 . As 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 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.