Transpolar arcs observed simultaneously in both hemispheres

Two coexisting transpolar arcs are observed on 31 August 2005. We track the formation and motion of the arcs in both the Northern and Southern Hemispheres, using data from two independent satellites (Imager for Magnetopause to Aurora Global Exploration and a Defence Meteorological Satellite Program satellite). The observations are supported by supplementary ground-based ionospheric convection data from the Super Dual Auroral Radar Network. The two arcs form during a period of northward interplanetary magnetic field. Following a change in the direction of the interplanetary magnetic field BY component from negative to positive, the dawnside arc traverses the polar cap to the duskside in the Northern Hemisphere. Over the same time period and in the Southern Hemisphere, the duskside arc traverses the polar cap to the dawnside. A complex magnetic field line topology resulting in the coexistence of two tongues of closed field lines protruding into the otherwise open polar cap is implied. We discuss these observations in terms of magnetic conjugacy and a model of transpolar arcs formation. Plain Language Summary Aurora emissions are normally found in a large ring, the “auroral ovals” that encircle the globe at high magnetic latitudes around the northern and southern poles. The “polar cap” areas poleward of these rings are often devoid of auroras. “Transpolar arcs,” however, are large-scale spurs of emission that extend from the auroral oval into the dark polar cap. There is still controversy about how a transpolar arc can form in the polar cap region. We simultaneously observe two transpolar arcs in each polar cap, using two independent satellites. In addition, the arcs form at opposite sides of the polar cap in each hemisphere. One of the arcs is then seen to move across the polar cap, in opposite directions in each hemisphere. The formation and movement of the arcs is consistent with a model of transpolar arc formation, whereby arcs will form at mirror positions about the midnight line in each hemisphere. However, the arrangement of the magnetic field implied by seeing two simultaneous arcs must be complex.