A new excitation mechanism of H e + band electromagnetic ion cyclotron wave: Hybrid simulation study

A new excitation mechanism of the H e + 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 H e + 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 H e + band EMIC wave. Hybrid simulation results show that the distribution of H e + is changed to be X-type during this process. Further investigation confirms that X-type distributed H e + may be caused via the pitch angle scattering by the H + band EMIC wave, which stimulates the H e + band EMIC wave. Beyond that, further research shows that the free energy to excite the H e + band EMIC wave comes from higher energy H e + with a butterfly wing shaped part of X type distribution.