Searching for a Downward Birkeland Current Associated with the Sunlight Terminator

Abstract

In space physics the relationship between the Sun and its effects on the space around Earth is studied. The magnetosphere, where the geomagnetic field dominates, is constantly interacting with the solar wind and interplanetary magnetic field (IMF). The magnetosphere and the high-latitude ionosphere are connected by geomagnetic field lines. The currents that flow along these magnetic field lines are called field-aligned currents (FACs) and their roles have already been well documented \citep{Ganushkina}. However, the FACs associated with the polar day-night boundary of Earth, the sunlight terminator, have not been thoroughly explored yet. Previous studies have reported on terminator currents. They found a relatively strong terminator current, derived from a data assimilation model, compared to the strong region 1 FAC. This thesis explores the relationship between the sunlight terminator and FACs based on vector magnetic field measurements from the Swarm Alpha satellite and using a smoother realistic model of the solar extreme ultraviolet-produced conductance. We compare the predicted terminator current with the total measured FAC and with a modelled total FAC. Here we show that terminator currents are small ($> -90$ nA/m$^2$) and that stronger solar wind conditions result in greater downward terminator currents. Our results show currents that are one to two orders of magnitude smaller than in previous studies. Furthermore, the terminator currents may not exist as an independent current system and simply are part of the steady state FAC equation. Our results demonstrate that the processes occurring at the sunlight terminator require further study as the ambiguous results in this thesis do not allow for definite conclusions without a comprehensive statistical evaluation.