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Kiyohiko ISHIKAWA Katsumi HATTORI Masashi HAYAKAWA
The purpose of this paper is to investigate the ray focussing of whistler-mode waves in the outer magnetosphere which results in an enhanced wave-particle interactions. The critical frequency in a homogeneous plasma is first 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 for a slightly inhomogeneous plasma. The two-dimensional ray-tracing computations for varying the frequency and initial wave normal direction in an inhomogeneous realistic model of the outer magnetosphere, have yielded that although the critical frequency for the homogeneous case has its importance even in the inhomogeneous plasma, the strongest ray focussing seems to occur at a frequency slightly below the above critical frequency, and hence that an enhanced gyroresonance wave-particle interaction is anticipated at this frequency.