The theta aurora over Antarctica. Auroral imaging with Dynamics Explorer 1 has identified a unique auroral configuration that can occur when the interplanetary magnetic field is directed northward. This image obtained at 0022 UT on 11 May 1983 shows one of the finest examples available to date of this spatial distribution comprising the auroral oval and a transpolar arc. The arc extends into the polar cap from local midnight, across the polar cap and joins with the auroral oval at local noon. The term theta aurora derives from the striking similarity of this spatial distribution to the Greek letter . Motion of the transpolar arc in the dawn-dusk direction appears to be controlled by the direction of the interplanetary magnetic field along the dawn-dusk direction. In the Northern Hemisphere, the arc moves in the direction of the IMF By component.