ALFVEN WAVES IN NONSTATIONARY AND NONUNIFORM MEDIA - AN EXACTLY SOLVABLE MODEL

The modulation of Alfven waves interacting with a non-uniform and non- stationary plasma is considered. The waveforms are allowed to change r apidly. We examine our phenomena by means of exact analytical solution s of the MHD equations in the presence of large amplitude disturbances of the magnetic field and plasma density. In contrast to the WKB appr oach, we do not have to use limiting assumptions regarding the variati ons of the background medium, We show that the large amplitude time an d space disturbances lead to a new cut-off frequency for Alfven wave p ropagation. A rapid reshaping of the Alfven waveform can also obstruct the resonant interactions between the waves and the plasma particles.