A New Excitation Mechanism of He+ Band Electromagnetic Ion Cyclotron Wave: Hybrid Simulation Study

A new excitation mechanism of the He+ band electromagnetic ion cyclotron (EMIC) wave is proposed in this paper. Through 1D hybrid simulation, we find that the frequency of the H+ band EMIC wave triggered by anisotropic hot H+ drops quickly in the initial stage, and then, a narrow He+ band EMIC wave is excited. Meanwhile, the temperature of ions also changes significantly. Using the linear kinetic theory, we have proved that the temperature variance cannot stimulate the He+ band EMIC wave. Hybrid simulation results show that the distribution of He+ is changed to be X-type during this process. Further investigation confirms that X-type distributed He+ may be caused via the pitch angle scattering by the H+ band EMIC wave, which stimulates the He+ band EMIC wave. Beyond that, further research shows that the free energy to excite the He+ band EMIC wave comes from higher energy He+ with a butterfly wing shaped part of X type distribution.