Transpolar arcs: Seasonal dependence identified by an automated
detection algorithm
Abstract
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.
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