Dependence of solar wind proton temperature on the polarization properties of alfvenic fluctuations at ion-kinetic scales

Abstract

We use fluctuating magnetic helicity to investigate the polarization properties of Alfvénic fluctuations at ion-kinetic scales in the solar wind as a function of βp, the ratio of proton thermal pressure to magnetic pressure, and θvB, the angle between the proton flow and local mean magnetic field, B0. Using almost 15 yr of Wind observations, we separate the contributions to helicity from fluctuations with wavevectors, k, quasi-parallel and oblique to B0, finding that the helicity of Alfvénic fluctuations is consistent with predictions from linear Vlasov theory. This result suggests that the nonlinear turbulent fluctuations at these scales share at least some polarization properties with Alfvén waves. We also investigate the dependence of proton temperature in the βp–θvB plane to probe for possible signatures of turbulent dissipation, finding that it correlates with θvB. The proton temperature parallel to B0 is higher in the parameter space where we measure the helicity of right-handed Alfvénic fluctuations, and the temperature perpendicular to B0 is higher where we measure left-handed fluctuations. This finding is inconsistent with the general assumption that by sampling different θvB in the solar wind we can analyze the dependence of the turbulence distribution on θkB, the angle between k and B0. 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 θvB in determining proton temperature in the solar wind.