Transpolar arcs: Seasonal dependence identified by an automated detection algorithm

Bower, Gemma; Milan, S. E.; Paxton, L. J.

doi:10.1002/essoar.10507649.1

Cited by 1 publication

(10 citation statements)

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“…The yellow quadrant accounts for the preponderance of dawn TPAs reported by Bower et al. (2022) and others (Hosokawa et al., 2011; Rairden & Mende, 1989; Valladares et al., 1994). These we suspect are misidentified HCA, which we test below.…”

Section: Discussionmentioning

confidence: 77%

“…As shown in Figures 9d–9f of Bower et al. (2022) less of the central polar cap is scanned at certain UTs in both hemispheres, with approximately 22 to 8 UT in the northern and approximately 0–8 UT in the southern hemisphere not being scanned with enough coverage over the central polar cap. This bias is most prominent in the southern hemisphere.…”

Section: Discussionmentioning

confidence: 86%

“…Bower et al. (2022) undertook an automated search for transpolar arcs (TPAs) in SSUSI data and found a preponderance of TPAs at dawn as opposed to dusk; they also noted that such an asymmetry had been recorded in previous studies (Fear & Milan, 2012; Hosokawa et al., 2011; Rairden & Mende, 1989). We now propose that HCAs with sufficiently dim dusk emission could be misidentified as a lone TPA at dawn.…”

Section: Discussionmentioning

confidence: 94%

“…This bias is most prominent in the southern hemisphere. There are also fewer HCA observations between 17 and 22 UT which can be linked to the F18 observations as there are less HCA identified in the F18 observations and it has poorer coverage in the northern hemisphere compared to the other spacecraft (Bower et al., 2022).…”

Section: Discussionmentioning

confidence: 99%

“…As discussed by Bower et al. (2022) the orbits of the DMSP spacecraft introduce a viewing bias into the observations by SSUSI. This distribution here is consistent with the viewing bias modeled by Bower et al.…”

Section: Observationsmentioning

confidence: 98%

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Occurrence Statistics of Horse Collar Aurora

et al. 2022

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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.

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