Hybrid simulation of ion cyclotron resonance in the solar wind: evolution of velocity distribution functions

Resonant interaction between ions (oxygen ions O+5 and protons) and ion cyclotron waves is investigated using a one dimensional hybrid code. Ion cyclotron waves are self-consistently generated by an ion cyclotron anisotropy instability. We focus on the detailed acceleration process of ions. The energization of oxygen ions due to waves is found to have two stages. During the first stage, oxygen ions are energized by ion cyclotron waves in the direction perpendicular to the background magnetic field and can develop extreme high temperature anisotropies with T O /T O ≈ 22 in an initially low beta plasma (beta value at 0.01) with very little parallel heating. During this stage, oxygen ions do not show an appreciable bulk acceleration along the background magnetic field. In the second stage, a large bulk acceleration of oxygen ions as large as 0.3v A , where v A is the Alfvén speed, is observed. Ion cyclotron waves are not able to maintain a high temperature anisotropy as inferred from observations. The nonlinear nature of wave particle interaction produces highly complex velocity distribution functions in the oxygen ions. In contrast, the heating and acceleration behavior of the major species, namely protons, is quite different. The velocity distribution functions of protons are less complex than the oxygen velocity distributions. Protons can also develop a large temperature anisotropy with preferential heating in the perpendicular direction. A bulk acceleration of protons (much smaller than the acceleration of oxygen ions) along the background magnetic field is observed to develop simultaneously with the development of a proton temperature anisotropy.