The effective reflection coefficient of the D-region for VLF radio waves during sunrise is calculated by applying a full-wave technique to a model of the temporal distribution of free electrons in the lower ionosphere. Estimations are made of the field strength of signals propagated from. a distant transmitter by a process of multiple reflections between the Earth and the ionosphere as the propagation path progressively comes under solar influence. The calculated fields are compared with observations made in the U.S.A. of transmissions from Panama (24 kHz) and Hawaii (23.4 kHz). The results indicate that variations in the field strength of VLF transmissions may be interpreted satisfactorily in terms of changes in the free electron content of the D-region during sunrise. © 1974.
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