NONLINEAR DYNAMICS OF STANDING SHEAR ALFVEN WAVES

The nonlinear evolution of driven standing shear Alfvén waves is investigated by virtue of a model which includes the interaction of the waves with density perturbations excited by the ponderomotive force. It is shown that the plasma density perturbations take the form of a slow magnetosonic wave which nonlinearly shifts the frequency of the shear Alfvén wave and decouples it from the external source. This results in a complicated and sometimes chaotic temporal behavior of the amplitude of the excited waves which depends strongly on the plasma pressure, driver strength, and the frequency mismatch between the driver and shear Alfvén eigenmode. The results are discussed in the context of ultra low frequency (ULF) field line resonances in the Earth's magnetosphere and the excitation of waves in the Alfvénic wave resonator near to the polar ionosphere. © 1995 American Institute of Physics.