NONLINEAR STANDING SHEAR ALFVEN WAVES IN THE EARTHS MAGNETOSPHERE

We present theory and numerical simulations of strong nonlinear effect s in standing shear Alfven waves (SAWs) in the Earth's magnetosphere, which is modeled as a finite size box with straight magnetic lines and (partially) reflecting boundaries. In a low beta plasma it is shown t hat the ponderomotive force can lead to large-amplitude SAW spatial ha rmonic generation due to nonlinear coupling between the SAW and a slow magnetosonic wave. The nonlinear coupling leads to secularly growing frequency shifts, and in the case of driven systems, nonlinear dephasi ng can lead to saturation of the driven wave fields. The results are d iscussed in the context of their possible relevance to the theory of s tanding ionospheric cavity wave modes and field line resonances in the high-latitude magnetosphere,