Differential kinetic dynamics and heating of ions in the turbulent solar wind

The solar wind plasma is a fully ionized and turbulent gas ejected by the outer layers of the solar coronaat very high speed, mainly composed by protons and electrons, with a small percentage of heliumnuclei and a significantly lower abundance of heavier ions. Since particle collisions are practicallynegligible, the solar wind is typically not in a state of thermodynamic equilibrium. Such a complexsystem must be described through self-consistent and fully nonlinear models, taking into account itsmulti-species composition and turbulence.Weuse a kinetic hybrid Vlasov–Maxwell numerical codeto reproduce the turbulent energy cascade down to ion kinetic scales, in typical conditions of theuncontaminated solar wind plasma, with the aim of exploring the differential kinetic dynamics of thedominant ion species, namely protons and alpha particles.Weshow that the response of differentspecies to the fluctuating electromagnetic fields is different. In particular, a significant differentialheating of alphas with respect to protons is observed. Interestingly, the preferential heating processoccurs in spatial regions nearby the peaks of ion vorticity and where strong deviations fromthermodynamic equilibrium are recovered. Moreover, by feeding a simulator of a top-hat ionspectrometer with the output of the kinetic simulations, we show that measurements by suchspectrometer planned on board the Turbulence Heating ObserveR (THORmission), a candidate forthe nextM4space mission of the European Space Agency, can provide detailed three-dimensional ionvelocity distributions, highlighting important non-Maxwellian features. These results support the ideathat future space missions will allow a deeper understanding of the physics of the interplanetarymedium.