Analysis of high latitude ULF waves and HF radar performance for African equatorial latitude SuperDARN

This thesis studies the phase evolution in ultra low frequency (ULF) waves driven by substorm-injected energetic particles drifting azimuthally around the Earth using measurements from a Super Dual Auroral Radar Network (SuperDARN) radar at high latitude. The characterisation of a case-study Pc5 ULF wave event observed by the Hankasalmi SuperDARN radar suggests an equatorward latitudinal and eastward longitudinal phase propagation with an effective azimuthal wave number of 17 ± 1, in the intermediate range of those observed in ULF waves. This wave, also detected in nearby magnetometer datasets, has been interpreted as resulting from drifting electrons of energies of 35 ± 5 keV in a drift resonance condition linked to particle populations from a magnetospheric substorm. The latitudinal phase characteristics of this wave experienced temporal evolution. Statistical analysis of 18 similar ULF wave events also linked to substorminjected particles suggested that the phase evolution in the case-study is caused by additional injected particle populations associated with the same substorm driving the wave. This resulted in an observed backscatter "bite-out". This analysis enhances our current understanding of this species of ULF waves.

The thesis also presents the predicted performance of such SuperDARN radars at African equatorial latitudes where they do not currently exist. A high frequency ray tracing simulation using realistic ionospheres and magnetic field geometries indicates that the eastwest azimuth (~ 25° range) has a significant likelihood of achieving SuperDARN-type backscatter in this sector. This is mainly due to the magnetic field geometry, as these azimuths allow the HF waves to propagate orthogonal to the geomagnetic field in the ionospheric region where irregularities are expected. These results are central to the planning of a SuperDARN-style radar for studying the equatorial ionosphere, for instance, in determining the radar characteristics such as operating frequencies, antenna boresight and azimuth coverage.