Swing coupling of sound and Alfven waves

The weakly nonlinear interaction of sound and linearly polarized Alfven waves propagating along an applied magnetic field is studied in the case when the sound and Alfven speeds are equal. The sound wave is coupled to an Alfven wave with double period and wavelength. The Alfven wave drives the sound wave due to the ponderomotive force, while the sound wave returns the energy back to the Alfven wave through the swing wave-wave interaction (Za-qarashvili, 2001). As a result, the waves alternately exchange their energy during the propagation. The process of energy exchange is faster for waves with stronger amplitudes. The phenomenon can be of importance in the solar atmosphere, solar wind and other astrophysical situations.