Abstract
In a cold homogeneous plasma, fast mode and transverse mode MHD waves can be independently excited. Inhomogeneities couple the modes and allow the excitation of field line resonances by compressional (fast mode) perturbations of the magnetosphere. Past discussions have focused on inhomogeneities in the radial direction (or its equivalent in simpler geometries), ignoring gradients of the Alfvén velocity along the unperturbed field direction. The nature of the coupling and the effectiveness of excitation are significantly modified by the presence of such gradients. One essential change is that the compressional and transverse field perturbations differ in their structure along the field. This reduces the effectiveness of the coupling between the fast (global) mode and standing (localized) field line resonances for identical harmonics of the parallel structure but allows different harmonics to couple. A fast mode wave propagating in from the outer boundary can, therefore, drive field line resonances in the region exterior to its turning point, or effective reflection point, where its amplitude has not yet decayed.
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