Parallel and Perpendicular Heating of Solar Wind Protons by Kinetic Waves as Inferred from WIND Observations

Abstract:

The solar wind may be heated non-adiabatically by Joule dissipation of coherent current structures or by wave-particle interaction with kinetic waves. In high-speed solar wind, where current structures of tangential-discontinuity type are rare and Alfven-like waves are abundant, wave-particle interaction may be a promising candidate for the heating mechanism. Here we address how the solar wind protons are heated parallel and perpendicularly based on the observations of proton velocity distributions and kinetic wave fluctuations from the WIND spacecraft. It is shown that solar wind protons consist of anisotropic core and beam populations with a relative field-aligned drift speed of ~V_A between them. Both quasi-parallel left-handed Alfven-cyclotron waves (LH-ACWs) and quasi-perpendicular right-handed Alfven-cyclotron waves / kinetic Alfven waves (RH-ACWs/KAWs) are also identified. It seems that the proton velocity distribution contours may be shaped by left-cyclotron resonance with quasi-parallel LH-ACWs for its anisotropic core components, Landau resonance with quasi-perpendicular KAWs for its beam drift, and right-cyclotron resonance with quasi-perpendicular RH-ACWs for its anisotropic beam component. Plasma instability is also investigated from the data, which shows that the core component anisotropy is usually unstable and may be responsible for the observed LH-ACWs with enhanced fluctuations; whereas the beam drift is stable and no RH-fast/whistler waves are observed. Moreover, the solar wind protons are observed, with the unprecedented cadence of 3s, to be highly dynamic in their velocity distributions with an apparent alternation between the stretching and contracting of the drifted beam, which may be connected with amplitude intermittency of associated waves.