The effect of multi-duct structure on whistler-mode wave propagation

Ducting of whistler-mode waves is investigated by ray-tracing in a magnetospheric model in which multiple and complex duct structures are superimposed on a smooth magnetospheric plasma distribution. When two or more ducts are present, propagation through a duct in which the ray does not become trapped is found to result in little deviation of the ray path, showing that upgoing waves can traverse several ducts before becoming trapped in one that is suitably positioned.The presence of two ducts situated in the same meridian plane and close together in L-value (ΔL ~ 0.07) is found to enable a double-duct trapping mode. This has the special property of guiding waves with frequency above half the local electron gyrofrequency across the equatorial plane in such a way that they become retrapped in the duct from which they had previously escaped at its local detrapping frequency. This may explain the observation of whistlers with a particularly high ratio of cut-off frequency to nose frequency (Bernhardt, 1979).Ducting is also investigated for a more complex duct structure in which fine structure is superimposed on a broader larger enhancement main duct. Here, it is found that rays which are first trapped to propagate in the main duct at low altitude can be further trapped to be ducted inside fine structure enhancements at higher altitude. This can result in certain components of multipath whistlers always being excited together and also having a common exit-point in the lower ionosphere. This is shown to be consistent with experimental observations.