Abstract
Observed phases and amplitudes of VLF radio signals, propagating on a short (âŒ300âkm) 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 1âs pulses, and the amplitudes were measured both near (âŒ100 km from) the transmitter, where the direct ground wave is very dominant, and at distances of âŒ300 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 âŒ300 km ENE, mainly over the sea, to the vicinity of Karratha/Dampier on the N.W. coast of Australia. The bottom edge of the midâday tropical/equatorial ionosphere was thus found to be wellâmodeled by HâČ = 70.5 ± 0.5 km and ÎČ = 0.47 ± 0.03 kmâ1 where HâČ and ÎČ are the traditional height and sharpness parameters as used by Wait and by the U.S. Navy in their Earthâionosphere 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 âŒ100âkm and âŒ300âkm sites) agree very well after just a small adjustment of âŒ0.2 km in HâČ between the two codes. Such short paths also allow more localization than the usual long paths; here this localization is to low latitudes.
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