Potential Detection of Dual Lobe Reconnection Associated with Horse-Collar Auroras via Near-Magnetopause Cluster Observations

The presence of horse-collar auroras (HCAs) during northward interplanetary magnetic field periods offers opportunities to study unique dynamics in the Earth’s high-latitude magnetosphere. Horse-collar auroras have been linked to closure of the polar cap and dual lobe magnetic reconnection, which also results in closure of the dayside terrestrial magnetic field. However, this reconnection event has not been observed in situ. On 14 April 2007, the Defense Meteorological Satellite Program (DMSP) orbiter F17 ultraviolet imager (SSUSI) and spectrometer (SSJ) captured visual and particle flux evidence of an HCA extending from nightside to cusp where an emission spot was present. During this event, the Cluster satellite crossed the high-latitude southern magnetopause, observing bidirectional plasma motion in the magnetosheath and significant plasma populations in the magnetosphere. The former is likely consistent with recently closed dayside magnetic field lines, while the latter is likely consistent with closed magnetotail field lines characteristic of HCAs. Supported by a positive Walén test, we conclude that dual lobe reconnection associated with an HCA may have been detected directly for the first time. Analysis of this event is ongoing and may produce further insights into magnetic flux transport during HCA events.