EFFECTS OF RING CURRENT DISTURBANCES ON GLOBAL MHD OSCILLATIONS

Abstract

Global compressional MHD wave properties are numerically studied in a three-dimensional box model. In this model, the effect of the ring current is introduced as a disturbance near the plasmapause. It is assumed to be constant in the azimuthal direction (axisymmetric) and have pressure gradients in the meridional plane. We investigate how global and transverse modes are affected by any small changes in the ring current in the ideal MHD regime. It is found that the spectrum of global cavity modes, produced by an impulsive source, may be significantly distorted due to the perturbations of the ring current. However field line resonances (FLR) are found to remain the same as long as the distribution of the ring current does not vary rapidly in the azimuthal direction and becomes almost axisymmetric. It is suggested that the interaction of the ring current with an axisymmetric global mode may help to explain the observed feature that transverse waves have relatively strong spectral harmonic peaks, while discrete global cavity modes have rarely been confirmed even when the source is presumed impulsive. In addition, we discuss the general characteristics of the magnetospheric global modes in the system with complicated boundaries and highly nonuniform plasmas.