Abstract
The polar cap can become teardrop shaped through the poleward expansion
of the dusk and dawn sectors of the auroral oval, to form what is called
horse collar aurora (HCA). The formation of HCA has been linked to
dual-lobe reconnection (DLR) where magnetic flux is closed at the
dayside magnetopause. A prolonged period of northward IMF is required
for the formation of HCA. HCA have previously been identified in UV
images captured by the Special Sensor Ultraviolet Spectrographic Imager
(SSUSI) instrument on-board the Defense Meteorological Satellite Program
(DMSP) spacecraft F16, F17 and F18. Events that have concurrent 630.0 nm
all-sky camera (ASC) data from the Redline Geospace Observatory (REGO)
Resolute Bay site are now studied in more detail, making use of the
higher cadence of the ASC images compared to DMSP/SSUSI. 11 HCA events
are studied and classified based on the IMF conditions at the end of the
event. Five of the events were found to end via a southward turning of
the IMF, two end with positive By dominated IMF and four with negative
By dominance. Under positive (negative) By the arcs move duskward
(dawnward) in the northern hemisphere with the opposite true in the
southern hemisphere. Under a southward turning the arcs move
equatorward. One event is of particular interest as it occurred while
there was a transpolar arc (TPA) also present. Understanding the
evolution of HCA will allow DLR to be studied in more detail.