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

Resonant interaction between ions (oxygen ions O ⁺⁵ and protons) and ion cyclotron waves is investigated using a ID hybrid code. Ion cyclotron waves are self-consistently generated by an ion cyclotron anisotropy instability. It is found that finite amplitude of ion cyclotron waves can lead to a bulk acceleration of oxygen ions as large as 0.3v _A, where V _A is the Alfvén speed, is observed. However ion cyclotron waves are not able to maintain a high temperature anisotropy as inferred from observations. The non-linear nature of wave particle interaction produces highly complex velocity distribution functions (VDF) in the oxygen ions. In contrast, the heating and acceleration behavior of the major species, namely protons, is quite different. The VDFs of protons are less complex than that of oxygen ions. Protons can also develop a large temperature anisotropy with preferential heating in the perpendicular direction. A bulk acceleration of protons (about 0.06v _A) along the background magnetic field is observed to develop simultaneously with the development of a proton temperature anisotropy.