Differential kinetic behavior of protons and alpha particles in turbulent solar wind: hybrid Vlasov simulations

The general picture of turbulence in plasmas becomes more complicated in the solar wind because of its multi-component nature. In fact, although the solar wind is predominantly constituted of protons, is also made up of a finite amount of alpha particles, together with a few percent of heavier ions. `In situ' observations have shown that heavy ions (alpha particles in particular) seem to be preferentially heated and accelerated with respect to protons. However, due to very scarce measurements of heavy ions at time resolutions comparable with their kinetic scales, energy partition between species in turbulent plasma dissipation is basically unexplored. THOR measurements of ions at high temporal resolution, together with high energy and angular resolutions, could allow solving this key issue. For the moment, most of the information comes from numerical simulations and a crucial support is given by self-consistent, fully nonlinear Vlasov models. Here, hybrid Vlasov-Maxwell simulations are used to investigate the role of kinetic effects in a two-dimensional turbulent multi-ion plasma, composed of protons, alpha particles, and fluid electrons. In particular, the differential kinetic dynamics of ions are analyzed in terms of anisotropy e non-gyrotropy of the ion distribution functions. The stronger deviations of the ion velocity distributions from the thermodynamic equilibrium (Maxwellian configuration) appear localized near the peaks of the current density, associates also to a significant increase of the ion temperature. This behavior is found to be more important for alpha particles than for protons.