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

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.