Longitudinal Gradients in Field-Aligned Currents as Observed by Swarm

Field-aligned currents (FACs) are closely related to aurora and a key component of the magnetosphere-ionosphere-thermosphere system. Large scale FAC structures, like Region 1 / Region 2, threading the whole auroral oval, as well as smaller scale FACs, associated with auroral arcs, are often assumed to consists of upward / downward current sheet pairs, uniform in longitudinal direction. While such a uniformity is consistent with the prevalent 1D symmetry of the auroral arcs and oval, longitudinal gradients may develop at times, for example when the 1D symmetry prepares to break, during the growth phase of auroral substorms. The Swarm mission provides optimum conditions to explore systematically longitudinal gradients in FACs, namely a proper spacecraft configuration, with the Swarm A / Swarm C pair lining up periodically with Swarm B at auroral latitudes, and high quality magnetic field data. The present report concentrates on a set of auroral events observed by the Swarm satellites, in this suitable configuration, during the first six months of the mission operational phase. At that time, the distance between Swarm A / Swarm C and Swarm B was in the range of a few 100 km, comparable to the length scale of electrojet currents associated with auroral arcs. Not surprising, longitudinal gradients in FACs are occasionally significant, a feature which is discussed with respect to the location, activity level, and substorm phase of the event.