The influence of the ionosphere on low frequency radio wave propagation.

Low frequency (LF: 30 - 300 KHz) radio waves can be employed for communications purposes and as a means of probing the earth's lower ionosphere. In this thesis theoretical and experimental techniques are employed to investigate the propagation characteristics of LF waves in the spherical earth - ionosphere duct, with particular emphasis on high latitude conditions. The "wave-hop" theory of LF propagation, in conjunction with previously measured electron density and collision frequency profiles of the ionospheric D region, is used in a study of the behaviour of LF radio links under realistic operating conditions. The choice of frequency and receiver height is shown to have a critical effect on the received signal strengths during day - and night-time conditions. The influence of the geomagnetic field, ground conductivity, and ionospheric parameters on the occurence of regions of low signal strength are also assessed. The results of a series of airborne experiments are presented. Interference effects arising at a discontinuity in ground conductivity are demonstrated. Propagation data obtained at VLF and LF are interpreted in terms of D region models; the LF data are found to be considerably more sensitive to D region parameters than those obtained at VLF. Profiles providing a best fit for daytime conditions are derived; their accuracy and resolution and their photochemical implications are discussed. LF data are also employed to select a best fit nighttime electron density profile from a set of published profiles. An overall assessment of the theoretical and experimental aspects of this study concludes that an acceptable degree of consistency has been obtained.