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. The propagation of long radio waves in polar regions can be quite different from that at midlatitudes and low latitudes for two reasons, the existence of polar icecaps and the occurrence of certain ionospheric disturbances at polar latitudes. Much recent data [see below] indicate that, under ordinary conditions, signals on transmission paths crossing the Greenland icecap exhibit extraordinarily large attenuation as well as unusual phase phenomena and diurnal behavior. In addition, phase and amplitude anomalies on transpolar VLF (3-30 kHz) paths occur during polar-cap absorption (PCA) events during which energetic protons impinge on the polar ionosphere, and thus produce anomalous ionization. When a combination of the two effects takes place, the PCA-induced anomaly is greatly Now at R&D Associates, Santa Monica, California 90406. enhanced. Thus, during a PCA, much larger signal losses and phase advances are usually noted on paths that cross Greenland than on transpolar paths that do not. This report presents the results of both a theoretical study of the phenomena just described and an experimental study using a laboratory model that simulates polar VLF propagation. Previous relevant experimental and theoretical work is reviewed briefly. The ionosphere and ground models used in the theoretical analysis and a VLF model experiment are described, and some data that give insight into transpolar VLF propagation phenomenology are presented. The results of the full-wave...

The propagation of mixed polarization VLF (f ≤5 kHz) radio waves in the Antarctic Earth‐ionosphere waveguide

“…Modeled and recorded data were Mm-x over ) 1.5-km-thick ice. These results give higher attenuation rates than may have been expected from previously derived daytime rates for mode i[Field et al, 1972;Dowden and Holzworth, 1988] and provide the first experimental confirmation of the low attenuation of QTE modes over thick ice.…”

Modeling of temporal variation of very low frequency radio waves over long paths as observed from Indian Antarctic stations

“…Second, some portion of the signal propagation path lies over both continental ice (for Maitri station) and southern sea ice (for both Maitri and Bharati stations), which is responsible for high attenuation in signal amplitude. It has been found from previous existing literatures [ Field and Greifinger , ; Sasmal et al , ] that there is a significant loss of VLF signal amplitude for propagation over ice mass. It has also been pointed out that for a fixed upper boundary of the Earth‐ionosphere waveguide (say 60–70 km), the attenuation is higher (around 8 times) for propagation over ice mass than sea surface.…”