Swarm Survey of Alfvenic Fluctuations and Their Relation to Nightside Field-Aligned Current and Auroral Arc Systems

By comparing data from 530 nightside auroral zone crossings of the three Swarm satellites with images from THEMIS ground-based all-sky imagers (ASIs), we find that 154 arc systems, containing two or more arcs, host regions of fluctuating magnetic and electric fields (Alfven waves) up to 8 Hz. Many cases exhibit a standing wave pattern as seen by comparing the amplitude ratio and phase difference of electric and magnetic field Fourier components. The largest peak of Alfvenic cross-power spectral density (CPSD, which is similar to Poynting flux) within a large-scale current system tends to be narrow in latitude (about 50-100 km) and its magnitude increases with an increasing number of current sheets in the field-aligned current (FAC) system. The peak of CPSD tends to be located on the poleward side of the current system but can extend as equatorward as the boundary between the Region 1 and Region 2 FACs. CPSD tends to peak either within downward current sheets (40% of cases) or on the boundary between downward and upward FACs (37%). Only 23% of peaks occur within upward current sheets, indicating that there is only a weak association with discrete auroral arcs, a finding which is confirmed by comparison with ASI images. These findings demonstrate that Alfvenic fluctuations occur over a much wider range of locations and conditions than previously reported.