Effect of ion composition on magnetosonic waves

The propagation of the two kinds of fast magnetosonic waves, i.e., low- and high-frequency modes, in a two-ion-species plasma is studied theoretically and numerically. It is analytically found that the KdV equation for the low-frequency mode is valid for amplitudes $\varepsilon < 2 \Delta_{\omega}$, where $\Delta_{\omega} = (\omega_{+0} - \omega_{- {\rm r}})/ \omega_{+0}$ with $\omega_{+0}$ the cutoff frequency of the high-frequency mode and $\omega_{-{\rm r}}$ the resonance frequency of the low-frequency mode; $\Delta_{\omega}$ is given as a function of the density ratio and cyclotron frequency ratio of two ion species. It is then suggested that nonlinear coupling between the two modes can occur if $\varepsilon > 2 \Delta_{\omega}$. With electromagnetic particle simulations, the evolution of the low- and high-frequency-mode pulses is investigated for various density and cyclotron frequency ratios and is compared with theoretical predictions. In particular, it is shown that high-frequency-mode pulses are generated from a long-wavelength low-frequency-mode pulse if its amplitude $\varepsilon$ exceeds $2 \Delta_{\omega}$.