In this thesis I will define and investigate a phenomenon called Birkeland current boundary flows (BCBF). These narrow regions of intense ion flow mark the boundary between upward and downward field aligned current in the auroral ionosphere. I use measurements of electric and magnetic field, ion temperature, plasma density, and electron temperature from the Swarm satellites to characterize the plasma surrounding these events. Ground-based magnetic field and optical measurements coincident with Swarm measurements of BCBF provide large-scale context for my observations with respect to the auroral electrojets and the optical aurora respectively. In a series of three papers, I will present first observations of BCBF, establish the morphology and significance of the phenomenon near midnight, and contrast BCBF with a similar phenomenon, subauroral ion drifts. While large electric fields have been previously reported in the auroral zone, often associated either with auroral precipitation or the auroral electrojets, the studies in this thesis first show BCBF to be a ubiquitous and persistent characteristic of auroral ionospheric current closure near midnight.