A study of VLF wave propagation characteristics in the earth-ionosphere waveguide

In the light of newly found applications for very low frequency (VLF) (3–30 kHz) measurements in the prediction of earthquakes and the detection of lightning discharges and gamma ray bursts, there has been a revival of interest in the study of VLF propagation in the earth-ionosphere waveguide. The propagation characteristics of VLF radiowaves in the earth-ionosphere waveguide critically depend upon the lower ionospheric ionization, which determines the conductivity profile of the upper boundary of the waveguide. In this context, it is potentially worthwhile to revisit the waveguide mode analysis to compute propagation parameters of long-distance VLF transmissions while taking into account different approximations to the ionospheric conductivity. We have carried out a waveguide mode analysis of 16-kHz VLF waves traveling great distances, assuming three different models for conductivity. The computational results show that at heights of less than 70 km, the change in the phase of the VLF waves due to changes in phase velocity is smaller when the ionosphere is sharply bound and assumed to have finite conductivity rather than infinite conductivity. The effect of a non-sharp or diffuse boundary at the top of the earth-ionosphere waveguide is found to cause a lowering of the height of reflection.