Some differences in diurnal phase and amplitude variations for VLF signals

The daily change in illumination over a VLF propagation path produces a regular variation in the phase and amplitude of a received signal. Significant differences have been reported for such variations when measured at the same frequency over propagation paths of similar length but different orientation. Past attempts have been made to explain these observations in terms of stellar X-ray sources and a major variation in VLF propagation parameters at the magnetic equator. This paper shows that a minor azimuthal variation in the relative phase velocities of interfering waveguide modes propagating at night can account for the observations in some detail.