Effects of alpha-proton differential flow on proton temperature anisotropy instabilities in the solar wind: Wind observations

TL;DR

This study analyzes Wind spacecraft data from 2005-2015 to explore how alpha-proton differential flow influences electromagnetic cyclotron waves and proton temperature anisotropy instabilities in the solar wind, revealing flow direction-dependent wave polarization and energy correlations.

Contribution

It provides observational evidence linking alpha-proton differential flow direction and energy to wave polarization and occurrence rates, supporting theoretical models of plasma instabilities.

Findings

ECW occurrence rate depends on alpha-proton flow direction

Wave polarization shifts from LH to right-handed with flow rotation

High differential flow energy correlates with increased ECW occurrence

Abstract

Plasma kinetic waves and alpha-proton differential flow are two important subjects on the topic of evolution of the solar wind. 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 Vd. As Vd rotates from anti-Sunward direction to 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 Vd further illustrates that large kinetic energies of alpha-proton differential flow correspond to high occurrence rates of ECWs. These results are well consistent with theoretical predictions for effects of alpha-proton differential flow on proton temperature anisotropy instabilities.