  The solar wind plasma is a fully ionized and turbulent gas ejected by the
outer layers of the solar corona at very high speed, mainly composed by protons
and electrons, with a small percentage of helium nuclei and a significantly
lower abundance of heavier ions. Since particle collisions are practically
negligible, the solar wind is typically not in a state of thermodynamic
equilibrium. Such a complex system must be described through self-consistent
and fully nonlinear models, taking into account its multi-species composition
and turbulence. We use a kinetic hybrid Vlasov-Maxwell numerical code to
reproduce the turbulent energy cascade down to ion kinetic scales, in typical
conditions of the uncontaminated solar wind plasma, with the aim of exploring
the differential kinetic dynamics of the dominant ion species, namely protons
and alpha particles. We show that the response of different species to the
fluctuating electromagnetic fields is different. In particular, a significant
differential heating of alphas with respect to protons is observed.
Interestingly, the preferential heating process occurs in spatial regions
nearby the peaks of ion vorticity and where strong deviations from
thermodynamic equilibrium are recovered. Moreover, by feeding a simulator of a
top-hat ion spectrometer with the output of the kinetic simulations, we show
that measurements by such spectrometer planned on board the Turbulence Heating
ObserveR (THOR mission), a candidate for the next M4 space mission of the
European Space Agency, can provide detailed three-dimensional ion velocity
distributions, highlighting important non-Maxwellian features. These results
support the idea that future space missions will allow a deeper understanding
of the physics of the interplanetary medium.
