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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.

This dissertation explores novel techniques for observing the ionosphere using natural signals. The ionosphere is a region of plasma hundreds of kilometers above the Earth that affects communication and remote sensing applications across the world. Traditional techniques for observing the ionosphere involve using man made radio signals, either to reflect the signal at HF frequencies or to pass several signals through the ionosphere and compare the difference the ionosphere makes in the signals. However, such techniques are limited by the ability of equipment to produce these signals and by the numerous laws and regulations governing transmission of signals in the radio spectrum. Natural signals on the other hand are many orders of magnitude more powerful than man made signals, and are produced in locations unavailable to man made transmitters. How these natural signals interact with the ionosphere provides a wealth of new information about the ionosphere. Once such natural signal source is pulsars, which are the remnant cores of massive stars after they go supernova. The result of the supernova and core collapse is a rapidly spinning object that produces bright radio emission from it's magnetic poles. If the magnetic poles are misaligned with the rotational axis, and the pole at some point in its rotation points near Earth, a bright radio pulse will be seen here. These pulses are extremely regular, and often are polarized. A polarized pulse traveling through a magnetized plasma will under go Faraday Rotation, or a rotation of the angle of the linear polarization. This effect is frequency dependent, so it is possible to measure the frequency independent Faraday Rotation, known as Rotation Measure, by sampling a large enough spread in frequencies. The Rotation Measure imparts information about the density of the plasma as well as the strength of the magnetic field as the signal passes through it. If one of the quantities is known, such as the magnetic field of the Earth, the distribution of the electrons can be reconstructed from the passing signal. Lightning is another natural source of radio emission, producing bright radio pulses as the lightning ionizes the air and as the voltage distribution within a cloud equalizes. These radio pulses travel isotropically from the point of the flash, and interact with the plasma of the ionosphere. Emission that is at lower frequencies than the plasma frequency of a layer of the ionosphere reflects at that altitude, and returns to earth. By comparing the direct line of sight information of the lightning signal to the reflected signal across the range of HF frequencies, the height density profile of the ionosphere can be determined. Moreover, by observing these signals with an array of antennas, the direction of the reflected signal can be determined. This provides not just the height profile, but also spatially where the reflected point comes from. This can show dynamic ionospheric features such as traveling waves in the ionosphere. Lightning emission above the plasma frequency of the ionosphere can also be used to detect ionospheric structures. If the ionosphere contains irregularities and local densities of plasma, these structures can reflect signals at frequencies higher than the plasma frequency. By cross correlating the direct line of sight signal of the lightning with radio emission from all other points in the sky, the signals returning from the many lightning pulses can be summed, illuminating structures that reflect the lightning. These techniques demonstrate the ability of natural signals to passively specify the ionosphere and open a new regime for examining the complex structures of this important region of the atmosphere.