Nonlinear waves along a magnetic field in a multi-ion-species plasma

Propagation of low frequency waves along a uniform external magnetic field in a cold plasma with multiple ion species is studied theoretically and numerically. First, linear dispersion relations are calculated, and nonlinear evolution equations are derived. The right circularly polarized wave does not suffer significant change by the presence of multiple ion species. However, the left circularly polarized wave greatly changes its properties. The Alfven and ion cyclotron waves are split into two modes in a two-ion-species plasma. These waves are unstable against modulational instability except for a small frequency domain; the growth rates are especially large near the ion cyclotron frequencies. Then, the development of the instability is further investigated with simulations based on a three-fluid model. The growth rates measured in the simulations are in good agreement with the theoretical prediction. Also, it is shown that the growth rates can be greater than cyclotron damping rates for some astrophysical plasmas.