Temperature anisotropy and differential streaming of solar wind ions -- Correlations with transverse fluctuations

TL;DR

This study investigates how transverse fluctuations in solar wind plasma correlate with ion temperature anisotropy and differential streaming, revealing thresholds where these relationships change, consistent with kinetic wave-particle interaction theories.

Contribution

It provides new insights into the correlation between transverse wave power and ion temperature anisotropy and differential flow, highlighting threshold effects in solar wind ion behavior.

Findings

Both ion differential speed and proton temperature anisotropy increase with wave power.

Alpha-particle temperature anisotropy correlates positively with wave power below a certain differential speed.

Above a certain differential speed, alpha-particle anisotropy decreases despite high wave amplitudes.

Abstract

We study correlations of the temperature ratio (which is an indicator for perpendicular ion heating) and the differential flow of the alpha particles with the power of transverse fluctuations that have wave numbers between 0.01 and 0.1 (normalized to $k_p=1/l_p$, where $l_p$ is the proton inertial length). We found that both the normalized differential ion speed, $V_{\alpha p}/V_\mathrm{A}$ (where $V_\mathrm{A}$ is the Alfv\'en speed) and the proton temperature anisotropy, $T_{\perp p}/T_{\parallel p}$, increase when the relative wave power is growing. Furthermore, if the normalized differential ion speed stays below 0.5, the alpha-particle temperature anisotropy, $T_{\perp \alpha}/T_{\parallel \alpha}$, correlates positively with the relative power of the transverse fluctuations. However, if $V_{\alpha p}/V_\mathrm{A}$ is higher than 0.6, then the alpha-particle temperature anisotropy tends to become lower and attain even values below unity despite the presence of transverse fluctuations of relatively high amplitudes. Our findings appear to be consistent with the expectations from kinetic theory for the resonant interaction of the ions with Alfv\'en/ion-cyclotron waves and the resulting wave dissipation.