Paper detail

Dissipation of parallel and oblique Alfvén-cyclotron waves: implications for minor ion heating in the solar wind

We perform 2.5D hybrid simulations with massless fluid electrons and kinetic particle-in-cell ions to study the temporal evolution of ion temperatures, temperature anisotropies and velocity distribution functions in relation to the dissipation and turbulent evolution of a broad-band spectrum of parallel and obliquely propagating Alfvén-cyclotron waves. The purpose of this paper is to study the relative role of parallel versus oblique Alfvén-cyclotron waves in the observed heating and acceleration of minor ions in the fast solar wind. We consider collisionless homogeneous multi-species plasma, consisting of isothermal electrons, isotropic protons and a minor component of drifting $α$ particles in a finite-$β$ fast stream near the Earth. The kinetic ions are modeled by initially isotropic Maxwellian velocity distribution functions, which develop non-thermal features and temperature anisotropies when a broad-band spectrum of low-frequency non-resonant, $ω\leq 0.34 Ω_p$, Alfvén-cyclotron waves is imposed at the beginning of the simulations. The initial plasma parameter values, such as ion density, temperatures and relative drift speeds, are supplied by fast solar wind observations made by the \textit{Wind} spacecraft at 1AU. The imposed broad-band wave spectra is left-hand polarized and resembles \textit{Wind} measurements of Alfvénic turbulence in the solar wind. The imposed magnetic field fluctuations for all cases are within the inertial range of the solar wind turbulence and have a Kraichnan-type spectral slope $α=-3/2$. We vary the propagation angle from $θ= 0^\circ$ to $θ=30^\circ$ and $θ=60^\circ$, and find that the minor ion heating is most efficient for the highly-oblique waves propagating at $60^\circ$, whereas the protons exhibit perpendicular cooling at all propagation angles.