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Ground‐based observational signature of a downward current channel in an active auroral arc

Ground‐based images of an active auroral arc over Kaktovik, Alaska on 6 March 2005 show the evolution of an event near magnetic midnight during the expansion phase of an auroral substorm. The auroral arc developed a distinct dark stripe embedded in a region of active aurora near the poleward edge of the activity. The dark stripe was observed to form and slowly widen from 15 km to 55 km (at an assumed altitude of 100 km), over a time period of 50 seconds. A linear fit to the data gives a widening rate of 750 m/s. We compare the ground‐based signatures of this event to in situ downward‐current‐region signatures of events observed by FAST and Cluster, and show that such an auroral feature is consistent with the predictions and observations of a downward‐current region.