The height change of the ionosphere in the day-night transition region leads to coupling and energy transfer between modes in propagating VLF and LF fields. This effect has often been evaluated on the basis of the ‘slab approximation,’ suppressing at the same time any reflected energy. In the present study, we treat this problem by the method of Pierce, in which reflections are properly taken into account as well as the gradual nature of the transition, hence leading to a more accurate evaluation of mode coupling. The details of the treatment are patterned after our previous application of the same method, and of our subsequent evaluation of mode coupling, for the problem of acoustic propagation in an under-ocean sound channel.
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