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Four primary plasma instability processes have been proposed in the literature to explain the generation of phase scintillation associated with polar-cap plasma patches. These are the Gradient Drift, Current Convective and Kelvin-Helmholtz instabilities and a small-scale “Turbulence” process. In this paper the range of possible values of the linear growth-rates for each of these processes is explored using Dynamics Explorer 2 satellite observations. It is found that the inertial Turbulence instability is the dominant process, followed by inertial Gradient Drift, collisional Turbulence and collisional shortwave Current Convective instabilities. The other processes, such as Kelvin-Helmhotz, collisional Gradient Drift and inertial shortwave Current Convective instabilities very rarely (

This thesis aims to answer the question, “What physical process dominates theformation of plasma irregularities, capable of directly or indirectly causing GPS L1band scintillation, in polar cap plasma patches during magnetic storm conditions?.” A novel modelling technique utilising an ionospheric imaging algorithm is developed and used to elucidate the relative importance of the two most commonly discussed processes.These are the Gradient Drift Instability (GDI) and turbulence induced by electric field mapping to the ionosphere from the magnetosphere. The results show that in magnetic storm conditions, at times the GDI process is dominant, but that at other times turbulence may be as significant as the GDI in determining how the plasma within a polar cap patch behaves, possibly more so. This in turn suggests that further study of the turbulence process is necessary in order to fully understand how big a role it plays in causing GPS L1 band scintillation in the polar cap. The success of the modelling technique developed here shows the utility of ionospheric imaging as a tool for understanding physical problems of the ionosphere; efforts to improve it and to apply it in other contexts would be worthwhile.