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

The propagation and trapping of whistler waves in two or more ducts are investigated by ray tracing in a magnetospheric model in which multiple and complex duct structures are superimposed on a smooth plasma distribution. Wave propagation through three ducts is analyzed, as are two-duct trapping and trapping in a duct with fine structure. It is found that: (1) when two or more ducts are present, propagation through a duct in which a ray does not become trapped results in little deviation of the ray path; (2) a double-duct trapping mode becomes possible when two ducts are in the same meridian plane and close together in L value; and (3) in the case of a duct with fine structure, rays that are first trapped in the main duct at low altitude can be further trapped in fine-structure enhancements at a higher altitude. It is concluded that the existence of two or more ducts or of ducts with fine structure may explain some features of ground and satellite VLF observations, e.g., the observation of whistlers with a particularly high cutoff/nose frequency ratio.