  Magnetosonic waves propagating perpendicular to an external magnetic field
are studied with attention to the effect of multiple ion species. First, power
spectra of magnetic field fluctuations and autocorrelation functions in thermal
equilibrium plasmas are numerically obtained. In a multi-ion-species plasma,
besides $\omega \simeq kv_{\rm A}$ mode, numerous waves are present near many
different ion cyclotron frequencies. The autocorrelation function of the
quasi-mode consisting of these waves is not recovered to its initial value,
owing to the phase mixing of these waves. Next, with particle simulations,
evolution of a macroscopic perpendicular disturbance is investigated. In a
multi-ion-species plasma, this disturbance is damped. The energy is transferred
to from the magnetic field to the ions.
