- Magnetoacoustic waves are investigated analytically for two frequency bands: the fast magnetic sound (low-frequency) band and the whistler (high-frequency) band. Nonlinear equations are derived in a consistent manner with allowance for thermal corrections. It is shown that fast magnetic sound is described by the shallow water equation, with negative dispersion over a wide range of angles relative to the external magnetic field. The behavior of fields during the reversal of magnetoacoustic waves is investigated by using the catastrophe theory. Nonlinear equations for a whistler in a plasma with beta (e) of about 1 is obtained, and it is shown that the nonlinear terms are proportional to temperature.