AMulti-Instrument Analysis of Bifurcating Region 2 Field-Aligned Currents and Sub-Auroral Polarization Streams

The solar wind-magnetosphere-ionosphere-thermosphere coupled system is complex, and even after several decades of research, a lot remains unknown. One method of observing this relationship is via field-aligned currents (FACs) that electrodynamically couple the regions. The Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) provides global and continuous data of these FACs. This thesis reports on a unique phenomenon, previously undetected by these means: the generation of a new, second pair of Region 2 (R2) FACs, denoted as Region 2 Bifurcations (R2Bs). Appearing equatorward of the standard R2 current and predominantly at dusk, suggests a connection with fast ionospheric flows, known as Sub-Auroral Polarization Streams (SAPS). A mechanism is proposed explaining how R2Bs can form: multiple particle injections into the inner magnetosphere, from separate substorm occurrences, creating independent partial ring currents and R2 currents.

The large-scale behaviour of the R2Bs is studied with statistical analyses. A study of the R2B-local time dependence compares the northern hemisphere solstice and southern hemisphere solstice. Dawn-dusk and interhemispheric asymmetries are observed, with R2Bs favouring the dusk and northern hemisphere. A seasonal analysis shows themajority of events have a signature in both hemispheres simultaneously, of the others a preference for the summer hemisphere is seen. A longitudinal study shows two peaks in a UT-occurrence distribution of the R2Bs, the cause of which is unknown. The connection between R2Bs and SAPS is analysed further. Contingency tables and superposed epoch analyses show that both R2Bs and SAPS are associated with substorms. Whilst work into the R2Bs and solar wind/geomagnetic activity relationship shows R2Bs occur during more strongly driven substorms, on more expanded auroral ovals. There is also evidence of a BY dependence for the northern and southern R2Bs detected in the noon/midnight regions, with no corresponding dependence for dawn/dusk R2Bs.