Parametric interactions between Alfven waves and sonic waves

Parametric interactions between a sonic pump wave and a weak Alfven wave are studied. It is shown that if the Alfven-wave velocity V_A is small relative to the sonic-wave velocity V_s there is a time-growing instability leading to the increase of the Alfven wave at the expense of the sonic wave. This phenomenon can be of importance in solar and stellar physics. For V_A large relative to V_s, the interaction is of the stop-band type