The generation and evolution of multi-band EMIC waves in the magnetosphere: Hybrid simulations

Electromagnetic ion cyclotron (EMIC) waves have played an important role in loss and acceleration of charged particles in the magnetosphere. In this paper, with a 1-D hybrid simulation model, we have studied the generation and evolution of multi-band EMIC waves in a homogenous multi-ion (protons and helium ions) plasma, where the waves are excited by the anisotropic hot protons, and the effects of the anisotropy of hot protons, concentration, and temperature of helium ions on the excited EMIC wave spectrum are considered. In the early phase of the cyclotron instability, the multi-band EMIC waves with a clear stop band around the helium ion gyrofrequency are preferentially generated under the condition of a lower anisotropy of hot protons, smaller concentration of helium ions, and colder helium ions, which is consistent with the linear theory. What's more, it is found that both the frequencies and wave numbers of EMIC waves will decrease with time, which is then proved to be a quasi-linear process caused by the decrease of anisotropy of hot protons. Meanwhile, the standing density structures will be generated in the system, which is due to the coupling between counter-propagating EMIC waves. Our simulations suggest that the linear theory should be valid to describe both generation and evolution of EMIC waves in the Earth's magnetosphere.