Ion cyclotron instability due to the thermal anisotropy of drifting ion species

In a recent study of ion cyclotron waves generated by the thermal anisotropy of oxygen ions, it was shown that the heavy ion drift velocity and a large thermal anisotropy of the heavy ions can destabilize proton-cyclotron waves [Gomberoff and Valdivia, 2003]. Here this study is extended to alpha particles in order to show that a much smaller thermal anisotropy is required to trigger strong proton-cyclotron waves. It is also shown that under some conditions, the alpha particle branch of the dispersion relation becomes unstable for frequency values beyond the proton gyrofrequency. This instability occurs for very large alpha particle thermal anisotropy and very low beta||alpha = vth2/vA2, where vth and vA are the thermal and Alfvén velocity, respectively. The maximum growth rate of this branch of the dispersion relation occurs for drift velocities of the alpha particles larger than those that drive the maximum growth rate of the proton-cyclotron instability. Finally, it is demonstrated that the combined effect of oxygen ions and alpha particles lead to a complex unstable spectrum, and to an enhancement of the proton-cyclotron instability. This mechanism is like a cascade effect in which low-frequency ion cyclotron waves can drive unstable high-frequency ion cyclotron waves through anisotropic heating and acceleration of heavy ions. These results may be relevant to the understanding of the heating process of the fast solar wind in coronal holes