Dependence of Solar Wind Proton Temperature on the Polarisation Properties of Alfvénic Fluctuations at Ion-kinetic Scales

Abstract

We use fluctuating magnetic helicity to investigate the polarisation properties of Alfvénic 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, $\text{k}$, quasi-parallel and oblique to ${\mathbf{B}}_0$, finding that the helicity of Alfvénic 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 Alfvén 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 Alfvénic 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 $\text{k}$ 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.

13 pages, 8 figures. Accepted for publication in The Astrophysical Journal

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