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One of the open questions about polar cap aurora, including transpolar arcs, is whether conjugate signatures are observed in both hemispheres. Past results suggest that transpolar arcs form on closed field lines. Any mechanism that puts polar cap aurora on closed field lines predicts that they will be seen in both hemispheres simultaneously; however observations presented by Ostgaard et al. [2003] showed two intervals with simultaneous northern and southern auroral observations, where a transpolar arc was seen in only one hemisphere.

This poster will present preliminary results from a statistical study using the Special Sensor Ultra-violet Spectrographic Imager (SSUSI) on board the DMSP (Defence Meteorological Satellite Program) spacecraft F16, F17 and F18. The DMSP spacecraft are in low earth polar orbits, with an orbital period of 90 minutes. Hence on every orbit from each spacecraft, images of the northern and southern auroral regions are available 45 minutes apart. Furthermore, there are often conjunctions with near-simultaneous observations of both hemispheres. The aim of this study is to determine how often polar cap auroras are seen in both hemispheres and hence investigate different formation mechanisms for polar cap aurora. Where possible, data from the DMSP particle spectrometer will also be used.