Periodic Fading of VLF Signals Received over a Polar Path During Sunset and Sunrise
VLF radio signals from the transmitter NPM/26.1 kHz have been recorded in Kiruna, Sweden. The signals show very strong mode interference with frequent cycle slippage. From the data it is concluded that by night the first and second order modes have almost exactly the same attenuation and a phase velocity difference of 0.51%, which is in agreement with Wait and Spies’ calculations for a nighttime ionosphere of height 90 km and a lower waveguide boundary of infinite conductivity (sea water). When the same model is used for the excitation factors the mode conversion factor is determined and is found to depend strongly on the angle between the shadow line and the great circle path.
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