Daytime tropical D region parameters from short path VLF phase and amplitude

Observed phases and amplitudes of VLF radio signals, propagating on a short (similar to 300-km) path, are used to find improved parameters for the lowest edge of the (D region of the) Earth's ionosphere. The phases, relative to GPS 1-s pulses, and the amplitudes were measured both near (similar to 100 km from) the transmitter, where the direct ground wave is very dominant, and at distances of similar to 300 km near where the ionospherically reflected waves form a (modal) minimum with the (direct) ground wave. The signals came from the 19.8 kHz, 1 MW transmitter, NWC, on the North West Cape of Australia, propagating similar to 300 km ENE, mainly over the sea, to the vicinity of Karratha/Dampier on the N.W. coast of Australia. The bottom edge of the mid-day tropical/equatorial ionosphere was thus found to be well-modeled by H' = 70.5 +/- 0.5 km and beta = 0.47 +/- 0.03 km(-1) where H' and beta are the traditional height and sharpness parameters as used by Wait and by the U. S. Navy in their Earth-ionosphere VLF radio waveguide programs. U. S. Navy modal waveguide code calculations are also compared with those from the wave hop code of Berry and Herman (1971). At least for the vertical electric fields on the path studied here, the resulting phase and amplitude differences (between the similar to 100-km and similar to similar to 300-km sites) agree very well after just a small adjustment of similar to 0.2 km in H' between the two codes. Such short paths also allow more localization than the usual long paths; here this localization is to low latitudes.