The role of Birkeland currents in the Dungey cycle

The interaction of the solar wind with the near-Earth environment forms Earth's magnetosphere and drives a process called the Dungey Cycle. Birkeland currents are required to transmit stress within the system. This thesis uses large-scale, statistical analysis (both temporal and spatial) to examine their magnitude and spatial extent in the context of the Dungey Cycle. Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) data are processed; the methodology is described and the success rate examined before the regions 1 and 2 Birkeland current magnitudes are explicitly compared to dayside and nightside reconnection rate for the first time. The magnitudes of the currents are well-correlated with both, suggesting that magnetic reconnection on day and nightside is driving higher Birkeland current magnitudes. The behaviour of the R1 and R2 currents is examined in a superposed epoch analysis of 2900 substorms identified by SuperMAG. Both current systems increase in magnitude and spatial extent during the growth phase of a substorm, peaking shortly after expansion phase onset. This analysis yields new information about how the currents react to the substorm cycle. A seasonal and a diurnal variation in the Birkeland current magnitudes is described and linked to the effect of ionospheric conductance; this is explored further, and it is found that currents are well-described by multiplying the dayside reconnection rate by an number representing the global variation of conductance with UT. This thesis presents evidence that Birkeland current magnitudes are consistent with driving by ionospheric convection, which is in turn driven by magnetic reconnection on both the dayside and the nightside. It is also demonstrated that the current ovals measured by AMPERE expand and contract with magnetic reconnection as open flux is added to and removed from the polar cap. These insights are expanded upon with ideas for future research.