Gemma Bower
Transpolar arcs (TPAs) are auroral features that occur polewards of the main auroral oval, at latitudes where auroras rarely form, 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 method has been developed to detect the occurrence of TPAs in UV images captured from the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument on board the Defense Meteorological Satellite Program (DMSP) spacecraft, in order to further study their occurrence in more detail. Via this detection method 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.
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 (F16, F17 and F18) this leads to a preferential observation of the Northern Hemisphere with the detection method missing TPAs in the southern hemisphere between approximately 0 - 9 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 suggest a seasonal dependence, with more TPAs occurring during June in the southern hemisphere and December in the northern hemisphere. This therefore suggests, contradictory to initial expectations that more TPAs occur in the winter hemisphere than the summer hemisphere.
An automated detection method has been developed to detect the occurrence of TPAs in UV images captured from the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument on board the Defense Meteorological Satellite Program (DMSP) spacecraft F16, F17 and F18. The three spacecraft together provide nearly continuous observation. SSUSI is a scanning instrument which takes approximately 20 minutes to build up an image of a swath of the auroral region by scanning anti-sunward along its orbit and operates at five wavelength bands.
11448 potential TPA images where identified for the period 2010-2016; of these 5698 were real TPA images and 360 were bending arcs giving a success rate of approximately 53%. Plotting the number of TPAs per month and per UT for each hemisphere (figure 5a and b respectively) a clear dependence is seen such that more TPAs are identified in the winter hemisphere and no TPAs are identified between approximately 1 and 6 UT in the southern hemisphere. Looking at the spacecraft tracks (figure 6 for F16) it is clear that the spacecraft track moves with respect to UT such that at 5 UT in the southern hemisphere the area above 12.5 ° colatitude is not scanned so no TPAs can be identified. This effect is more prominent in the southern hemisphere. Plotting the average percent of the area above 12.5 ° colatitude for each month and UT (figure 5c and d respectively) the UT dependence is clear and matches that of the number of TPA showing that this is a bias in the data. However no seasonal dependence is seen thus suggesting there is a seasonal dependence on the occurrence of TPAs.
Following the Milan et al. (2005) model we expect the closed magnetic flux to be present in both hemispheres simultaneously. Possibilities for the seasonal dependence are then: a) a seasonal change in visibility of TPAs, b) the auroral signature of TPAs is seasonally dependant, or c) the mapping of closed flux is different in the two hemispheres.
If an open field line model is correct however then there is no need for the TPAs to be in both hemispheres simultaneously. The seasonal dependence could be explained if polar cap flow shears that produce field aligned currents (FACs) and hence TPAs are preferentially found in the winter hemisphere.