Observations of HF radiowaves propagated over high latitude paths.
2015-11-19T09:17:15Z (GMT) by
This thesis presents results from the NONCENTRIC experiment, conducted by the University of Leicester, in which five trans-auroral and polar cap HF propagation paths were monitored during two one-month campaigns. Signal strength, noise level, Doppler spreading and signal recognition were determined each hour on fourteen frequencies in the range 3 to 23 MHz. The diurnal variations in signal recognition and signal strength are consistent with the ionospheric changes produced by solar illumination. This behaviour is modified by processes occurring within the auroral zone and polar cap during disturbed geomagnetic conditions. Electron densities within the auroral ionosphere increase as a consequence of particle precipitation during disturbed conditions. Auroral enhancement of the D region attenuates signals on trans-auroral paths. This occurs in bursts with durations of tens of minutes, simultaneous with the occurrence of substorms. The magnitude of the absorption can be correlated with changes in the geomagnetic field at geosynchronous orbit. In contrast, enhanced electron densities within the auroral zone E and F regions increase the maximum frequency of propagation, especially at night, thus extending the propagation bandwidth. Within the winter polar cap ionosphere, sporadic high frequency propagation becomes possible from ionization patches convected from the dayside ionosphere. Prolonged periods of geomagnetic disturbance result in global changes in F region electron density, known as ionospheric storms. Four storms studied produce a decrease in the maximum usable frequency, a degradation of propagation reliability and, within the polar cap, an increase in the lowest usable frequency. Such periods of propagation degradation typically have a duration of five days, even during mild storm conditions. This comprehensive study of high latitude HF propagation has produced an improved understanding of the changes in the electron density of the polar ionosphere during both quiet and geomagnetically disturbed conditions.