University of Leicester
2023BowerGPhD.pdf (31.58 MB)

Statistical analysis of northward IMF phenomena, transpolar arcs and horse collar aurora

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posted on 2023-08-09, 09:15 authored by Gemma E. Bower

Some auroral phenomena occur primarily during quiet geomagnetic conditions when the interplanetary magnetic field (IMF) is directed northward; these auroral features tend to occur poleward of the main auroral oval. The Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument on-board Defense Meteorological Satellite Program (DMSP) provides a way to observe the auroral regions on the Earth almost simultaneously in northern and southern hemispheres which allows these phenomena to be studied in detail.

This thesis presents statistical analyses of transpolar arcs (TPAs) and horse collar aurora (HCA). Firstly, a detection algorithm is created to identify TPAs in the DMSP/SSUSI data based on peaks in the average radiance intensity poleward of 12.5° colatitude. TPAs are found to be observed more frequently in the winter hemisphere. A preference for TPAs to be detected at dawn is also seen. These findings are discussed in relation to current TPA formation models.

Similar statistical analysis is performed on HCA identified visually in the DMSP/SSUSI data. As expected, HCA form when the interplanetary magnetic field (IMF) clock angle is small. The occurrence of HCA does not appear to be dependent on season or the IMF Bx component. It is found that the dawn arc of the HCA is usually brighter than the dusk arc. It is suggested that this could lead to the unexpected predominance of TPAs at dawn, and that these are possibly misidentified HCA.

Finally, 11 HCA events are studied in further detail with the use of all-sky cameras (ASC) to track the formation and motion of the HCA. The arcs follow the predicted theory and move dawnward (duskward) in the northern hemisphere if the IMF becomes dominated by a negative (positive) By. TPAs and HCA are also seen to occur simultaneously.



Stephen Milan; Suzanne Imber

Date of award


Author affiliation

School of Physics and Astronomy

Awarding institution

University of Leicester

Qualification level

  • Doctoral

Qualification name

  • PhD



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