A study of ULF propagation at high southern latitudes using sferics

Sferics are electromagnetic pulses generated by lightning events. Their maximum spectrum energy is in the frequency range below 15 kHz. These powerful natural VLF transmitters can be used to determine the propagation characteristics of the atmospheric wave guide between earth and ionospheric D layer along virtually every propagation path. A VLF-sferics-analyzer was operating at the German antarctic Von Neumayer station from January to July 1983. This analyzer records sferics from distant ligntning events in the frequency range between 5 and 9 kHz. The method of measurement is described. The data of June 1983 are evaluated, and the distances of the sources are determined by a comparison with rainfall records during the same month and, in addition, with sferics recordings from a station in Pretoria, South Africa. The data evaluation of the propagation conditions is compared with theoretical calculations. The main result is that the virtual reflection height of the ionospheric D layer is between about 78 km at noon and about 84 km during midnight in reasonable agreement with the theory. The difference between west-to-east and east-to-west propagation is smaller than the theory predicts indicating that the ionospheric D layer at high souther latitudes behaves more isotropic with respect to VLF propagation than at mid-latitudes.