Heating and acceleration of fast solar wind ions - a simulation study

We performed a set of one-dimensional hybrid simulations to investigate the preferential heating and acceleration of ions via the parametric decay of a large-amplitude Alfven-cyclotron wave. The electrons are treated as a charge- and current-neutralizing massless fluid, whereas a particle-in-cell method is used to follow the dynamics of the ion distribution functions under low-plasma-beta solar wind conditions. In the course of its evolution, the initial pump wave gets parametrically unstable and generates a broad spectrum of longitudinal ion-acoustic (IAW) and transverse ion-cyclotron waves (ICW), which destroy the coherent fluid motion of the ions and drive non-thermal processes. Trapping in, and consequent Landau damping of, the daughter IAWs lead to ion beam formation and generate or amplify differential streaming between the different ion species, whereas diffusion of the initial coherent ion bulk motion and pitch-angle scattering by the ICWs heat the ions in the perpendicular direction. Due to their lower mass densities and higher gyration rates, alpha particles and heavy ions get preferentially heated and accelerated. Depending on their initial drifts the core of the protons and alpha particles change their anisotropies, so that at low drifts the particles have larger temperatures in perpendicular direction, whereas for higher drifts strong parallel heating is observed.