Dependence of Solar Wind Proton Temperature on the Polarisation Properties of Alfv\'enic Fluctuations at Ion-kinetic Scales

Abstract

We use fluctuating magnetic helicity to investigate the polarisation properties of Alfv\'enic fluctuations at ion-kinetic scales in the solar wind as a function of $\beta_p$, the ratio of proton thermal pressure to magnetic pressure, and $\theta_{vB}$, the angle between the proton flow and local mean magnetic field, $\mathbf{B}_0$. Using almost 15 years of \textit{Wind} observations, we separate the contributions to helicity from fluctuations with wave-vectors, $\textbf{k}$, quasi-parallel and oblique to $\mathbf{B}_0$, finding that the helicity of Alfv\'enic fluctuations is consistent with predictions from linear Vlasov theory. This result suggests that the non-linear turbulent fluctuations at these scales share at least some polarisation properties with Alfv\'en waves. We also investigate the dependence of proton temperature in the $\beta_p$-$\theta_{vB}$ plane to probe for possible signatures of turbulent dissipation, finding that it correlates with $\theta_{vB}$. The proton temperature parallel to $\mathbf{B}_0$ is higher in the parameter space where we measure the helicity of right-handed Alfv\'enic fluctuations, and the temperature perpendicular to $\mathbf{B}_0$ is higher where we measure left-handed fluctuations. This finding is inconsistent with the general assumption that by sampling different $\theta_{vB}$ in the solar wind we can analyse the dependence of the turbulence distribution on $\theta_{kB}$, the angle between $\textbf{k}$ and $\mathbf{B}_0$. After ruling out both instrumental and expansion effects, we conclude that our results provide new evidence for the importance of local kinetic processes that depend on $\theta_{vB}$ in determining proton temperature in the solar wind.