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

A new excitation mechanism of theHe+ band electromagnetic ion cyclotron (EMIC) wave is proposed in this paper. Through 1D hybrid simulation, we find that the frequency of theH+ band EMIC wave triggered by anisotropic hotH+ drops quickly in the initial stage, and then, a narrowHe+ 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 theHe+ band EMIC wave. Hybrid simulation results show that the distribution ofHe+ is changed to be X-type during this process. Further investigation confirms that X-type distributedHe+ may be caused via the pitch angle scattering by theH+ band EMIC wave, which stimulates theHe+ band EMIC wave. Beyond that, further research shows that the free energy to excite theHe+ band EMIC wave comes from higher energyHe+ with a butterfly wing shaped part of X type distribution.