D ata on t h e p e riodi c changes in am p litude and ph ase of VLF s ig na ls rece ived ove r lon g VLF paths is examin ed. It is conc luded th at t he va ri ations a rc d ue to mul t imodp propagat ion in the ni ghtt ime portion of t he paL h .
An earlier expl~nation of sunrise and sunset fading on long VLF paths is confirmed by examining data t~ken over a ~1der range of frequency and directions of propagation. It is found. however. that _there IS an app~ec1able dependence of the fadinl! period on the magnetic direction of propagation. It IS shown that this dependence is qualitatively in agreement with what would be expected from a sharply bounded ionosphere and a transverse magnetic field.
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