Occurrence Statistics of Horse Collar Aurora

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 interplanetary magnetic field (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 instrument on‐board the Defense Meteorological Satellite Program 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. 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. We relate this to the expected field aligned current 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.

The Question of Transpolar Arc Conjugacy: New Results From Comparing Solar Wind Data and Dipole Tilt Distribution of Five Different Datasets

“…The difference between the first three datasets, which are based on low-resolution global images, and the last two DMSP datasets where even faint or thin TPAs can be detected, strongly suggest that IMF magnitude, and especially the magnetic energy flux of the solar wind do not influence the occurrence of TPAs, but rather the brightness of the arcs. This is in agreement with results from a recent DMSP study by Bower et al (2022) who found no correlation between TPA occurrence frequency and solar cycle. During solar maximum, CMEs (and thus periods with a high solar wind magnetic energy flux) are much more common than during solar minimum.…”

“…Most of those conjugate dawn-dawn TPA events form during rather weak IMF 𝐴𝐴 𝐴𝐴y . The latter results are similar to the observation from a recently published study by Bower et al (2022). Also, Bower et al (2022) report conjugate TPAs where both arcs appear at dawn during low IMF 𝐴𝐴 𝐴𝐴y values.…”

“…Thus, the results by Bower et al. (2022) are a further indication that the magnetic energy flux does not influence the formation of a TPA itself.…”

“…During solar maximum, CMEs (and thus periods with a high solar wind magnetic energy flux) are much more common than during solar minimum. Thus, the results by Bower et al (2022) are a further indication that the magnetic energy flux does not influence the formation of a TPA itself.…”

Universal Time Variations in the Magnetosphere and the Effect of CME Arrival Time: Analysis of the February 2022 Event that Led to the Loss of Starlink Satellites

“…In such cases, a "horse-collar" auroral form is often seen, indicating a teardrop-shaped open flux region (Hones et al, 1989;Elphinstone et al, 1993;Imber et al, 2006;Milan et al, 2020;Bower et al, 2022;. Nevertheless, the assumption of a circular polar cap has been successfully used many times with the ECPC model (e.g., Lockwood et al, 1990;Milan et al, 2017), including an analysis of a full year of data (Milan et al, 2021) and has two major advantages for the present study.…”