Abstract
Horse collar aurora (HCA) are an auroral feature where the dawn and dusk
sector auroral oval moves polewards and the polar cap becomes teardrop
shaped. They form during prolonged periods of northward IMF, when the
IMF clock angle is small. Their formation has been linked to dual-lobe
reconnection (DLR) closing magnetic flux at the dayside magnetopause.
The conditions necessary for DLR are currently not well-understood
therefore understanding HCA statistics will allow DLR to be studied in
more detail. We have identified over 600 HCA events between 2010 and
2016 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. As
expected, there is a clear preference for HCA occurring during northward
IMF. We find no clear seasonal dependence in their occurrence, with an
average of 8 HCA events per month. The occurrence of HCA events does not
appear to depend on the Bx component of the IMF, suggesting that Bx does
not modulate the rate of lobe reconnection. Considering the average
radiance intensity across the dusk-dawn meridian shows the HCA as a
separate bulge inside the auroral oval and that the dawn side arc of the
HCA is usually brighter than the dusk in the Lyman-Birge-Hopfield short
band (LBHs). We relate this to the expected field aligned current (FAC)
pattern of HCA formation. We further suggest that transpolar arcs
observed in the dawn sector simultaneously in both northern and southern
hemispheres are misidentified HCA.