Auroral Surge Currents and Electrodynamics with FAST and VIS

We analyze data from two FAST satellite passes in the vicinity of auroral surges (as seen in the VIS auroral imager on the Polar satellite) to study the large scale ionospheric electrodynamics of the surge, focusing in particular on the configuration of field-aligned and ionospheric currents. While most previous ground and satellite observations have found the surge to be a region of predominantly upward field-aligned currents (FACs), some satellite observations have found evidence for equal downward and upward FACs near the surge, suggesting that the FACs close locally as opposed to globally through the substorm current wedge. Our observations show that while regions of both upward and downward FACs are present near the surge, there is a significant net upward FAC in the northern portion of meridians passing through the surge and just east of the surge. Current continuity requires that these upward currents be fed by downward currents traditionally thought to be in the eastern portion the auroral substorm bulge. Such a current configuration requires a westward ionospheric current to connect the two FAC regions, and this is supported by our observations of a significant southward electric field in the high Hall conductance surge region, which drives a strong westward Hall electrojet connecting the upward surge FACs and the presumed downward FACs east of the surge.