Experimental studies of the lower ionosphere using very low frequency radiowaves.

During the period 1967--1970 the phase and amplitude of twelve LF and VLF radiowave transmissions were monitored at Leicester for periods ranging from three years to a few weeks; the path lengths varied between 27km and 8,600km. The records taken were analyzed to extract the diurnal and seasonal behaviour of the signals. The horizontally polarized skywave of six local (U.K.) transmissions were isolated using loop aerials designed and built during the project, and the value of the ionospheric conversion coefficient R at noon and midnight throughout the year was obtained from the records. The apparent height of reflection was also measured. Full-wave calculations were performed to test several published models of the ionospheric D-region for consistency with the measurements. Modified models were also tested and new models for winter days, and for nights throughout the year were derived. TM mode propagation of VLF transmissions on eight long paths was examined, and TE mode propagation on three long paths was also examined at two receiving sites using orthogonal loop aerials to isolate the TE and TM modes. Measurements were found to be consistent with published predicted values of relevant ionospheric parameters. Disturbances resulting from solar flare activity were observed on all the paths and the anomalies present in the records were examined in detail. The contrasting response of the different paths was noted. Comparison of selected anomalies with the incident flux of X-radiation enabled an explanation for the different response of the various paths to be found. The effect of Polar Cap Disturbances and the influx of solar cosmic rays on transmission paths at different magnetic latitudes was also examined, and the anomalous response of a low latitude path during these geophysical disturbances was discovered.