Solar Wind-Magnetosphere-Ionosphere Coupling and the Electric Circuit ofGeomagnetic Storms

Intense solar wind driving of the terrestrial magnetosphere, can result in geomagnetic storms, where the morphology and behaviour of the magnetosphere undergoes complex changes. This thesis studies storm-time changes in the electric current systems which electromagnetically couple the solar wind-magnetosphere-ionosphere system. In the first chapter, the occurrence, intensity and onset category of geomagnetic storms is determined, with 140 storms identified. A superposed epoch analysis of the field-aligned currents (FAC) during storm sudden and storm gradual commencement events is performed. The region 1 FACs are found to dominate. An asymmetry in upward and downward FAC magnitudes in the midnight magnetic local time sector ,is observed, which we propose is associated with the Harang discontinuity. A ‘twostep’ evolution of the FACs is found in region 1 and region 2 currents. Rapid variations (spikes) in magnetic field can produce geomagnetically induced currents at the surface of the Earth. Employing SuperMAG 1-second data, instances of dB dt > 5 nT/s are identified, over 5 geomagnetic storm case studies. 131,447 spikes are identified, concentrated in three MLT hotspots: pre-midnight, dawn and noon. Correlations between the Bz < 0 interplanetary magnetic field component, pressure pulse impact and substorm activity with spike occurrence are observed. Spikes are most often found co-located with upward FACs (56%) rather than downward FACs (30%) or no FACs (14%). The cross polar cap potential has been documented to saturate at ∼250 kV. An analogous saturation within the current systems present is investigated by comparing increasing solar wind parameters and the resulting FACs. FAC magnitude is observed to increase monotonically with solar wind driving, with a distinct knee in the variation around IMF Bz = −10 nT, above which the increase slows.