Transpolar arcs (TPAs) are auroral features that occur polewards of the main auroral oval suggesting that the magnetosphere has acquired a complicated magnetic topology. They are primarily a northward interplanetary magnetic field (IMF) 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 poleward of 12.5 degrees colatitude, in two or more of the wavelengths/bands sensed by SSUSI.
Orbital biases in the data have been investigated and these differ from spacecraft to spacecraft. For the spacecraft of interest (F16, F17 and F18) this leads to a preferential observation of the northern hemisphere with the detection algorithm missing TPAs in the southern hemisphere between approximately 01 - 06 UT. No seasonal bias has been found for these spacecraft.
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 shows a seasonal dependence, with more arcs being visible in the winter hemisphere. We discuss the ramifications of these findings in terms of proposed TPA generation mechanisms.