Abstract

Field-aligned enhancements of plasma density act as waveguides for whistler-mode waves. Calculations are presented of mode characteristics for a planar structure, where the enhancement has a sech-squared profile. Particular attention is paid to the group velocity for different waveguide modes. Application is then made to the magnetosphere by calculating the group delay per unit length for a given mode at different latitudes along a duct (taken to be centred on a dipole field line) and then integrating to obtain the total group delay over the magnetospheric part of the whistler path. Compared with strictly longitudinal propagation, group travel times are less at low frequencies, but greater at frequencies near to one-half the equatorial electron gyrofrequency, the magnitude of the residuals at the extremes of the frequency range being greater for higher-order modes. Differences between modes can be of the order of milliseconds. It therefore seems possible that recently reported fine structure seen in whistler spectrograms could be due in part to multi-mode propagation.

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