Ray focussing of whistler-mode waves in a magnetoplasma

The purpose of this paper is to investigate the ray focussing of whistler-mode waves which results in an enhanced wave-particle interaction. The critical frequency in a slightly inhomogeneous magnetoplasma is studied, at which the refractive index surface of whistler-mode waves indicates a zero curvature at a longitudinal wave normal angle. This critical frequency is also found to be consistent with the zero diffraction coefficient in the full-wave theory. It is further suggested that the critical frequency is also important in the wave-particle interaction for the inhomogeneous outer magnetosphere outside the plasmapause and in the vicinity of the plasmapause.