Periodic Fading of VLF Signals Received over a Polar Path During Sunset and Sunrise

  • S. Westerlund


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.


Order Mode Solar Zenith Angle Cycle Slippage Phase Step Evening Step 
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    D.D. Crombie, Periodic fading of VLF signals received over long paths during sunrise and sunset. Radio Science 68D, 27, 1964.Google Scholar
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    D.D. Crombie, Further observations of sunrise and sunset fading of very-low-frequency signals. Radio Science 1 (New series), 47, 1966.Google Scholar
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    A.B. Kaiser, VLF propagation over long paths, J. Atm. Terr. Phys. 29, 73, 1967.ADSCrossRefGoogle Scholar
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    D. Walker, Phase steps and amplitude fading of VLF signals at dawn and dusk, Radio Science 69D, 1435, 1965.Google Scholar

Copyright information

© Springer Science+Business Media New York 1968

Authors and Affiliations

  • S. Westerlund
    • 1
  1. 1.Kiruna Geophysical ObservatoryKiruna CSweden

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