Transpolar propagation of long radio waves

Abstract

This report presents the results of a theoretical analysis and a laboratory simulation of certain transpolar VLF/ELF propagation phenomena. The calculations are based on daytime ionospheric models representative of ambient conditions and of conditions that prevail during polar-cap absorption (PCA) events. The laboratory simulation utilized a wave guide that models VLF propagation in the earth-ionosphere cavity. The influence of the Greenland icecap is included in both the theoretical and experimental approaches. The calculations predict, in agreement with actual transpolar propagation data, that much larger signal losses will be suffered on paths that cross Greenland than on paths that do not. Furthermore, the calculations correctly predict that a given PCA will typically produce much larger amplitude degradations and phase advances on signals that cross Greenland than on ones that propagate only over sea water and/or relatively highly conducting ground. The data from the laboratory model is in good general agreement with actual transpolar propagation measurements and our theoretical results. In addition to causing sizable ground losses, the presence of thick ice at the lower boundary of the earth-ionosphere wave guide distorts the structure of the modes greatly from that which prevails for propagation over highly conducting ground. This distortion manifests itself in ambiguities in mode numbering and in an increase in the losses due to ionospheric heating. Ions contribute significantly to the propagation phenomena during moderate and intense PCA events.