Monday

Transpolar arcs (TPAs) are auroral features that occur polewards of the main auroral oval, at latitudes where auroras are less common, suggesting that the magnetosphere has acquired a complicated magnetic topology. They are primarily a northward interplanetary magnetic field auroral phenomenon, and their formation and evolution have no single explanation that is unanimously agreed upon. An automated detection algorithm has been developed to detect the occurrence of TPAs in UV images captured from the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument onboard the Defense Meteorological Satellite Program (DMSP) spacecraft, in order to further study their occurrence. Via this detection algorithm TPAs are identified as a peak in the average radiance intensity above 12.5° colatitude, in two or more of the wavelengths/bands sensed by SSUSI.
Using the detection algorithm on observations from the years 2010 to 2016, over 5000 images containing TPAs are identified. The occurrence of these TPA images suggest a seasonal dependence, with more TPAs observed in the winter hemisphere. We also discover a 2-to-1 majority of TPAs in the dawn sector versus the dusk sector polar cap, which is unexpected in current TPA models.
Orbital biases in the data have been investigated and it has been found that each DMSP spacecraft has a different bias due to its orbit. For the spacecraft of interest this leads to a preferential observation of the northern hemisphere. No seasonal or dawn-dusk bias has been found for these spacecraft.
Comparing with previous statistical surveys, we note that the dawn-dusk asymmetry has been present but has not gained attention. We suggest that field-aligned current polarity may play a role in the observed asymmetry, and suggest that this requires a revision of our understanding of TPA formation.