Transpolar auroras, their particle precipitation, and IMF B_y component

Abstract

Transpolar auroras, their associated particle precipitation, and their occurrence with respect to the IMF B_y polarity are examined on the basis of DMSP F6 auroral images and the corresponding particle data. It is found that the transpolar arcs are located in the poleward edge of the soft particle precipitation region extending from either the dawn or dusk part of the auroral oval precipitation; they are not embedded in the polar rain region. This finding suggests that the transpolar arcs are located along the poleward boundary of the closed field line region (or the equatorward boundary of the open region) as suggested by Meng (1981). Further, the appearance of the extended precipitation region from the oval depends on the polarity of the IMF B_y, in the northern hemisphere morning sector for IMF B_y < 0 or in the evening sector for IMF B_y > 0. In general, the precipitating particle flux in the extended precipitation region is not high enough to produce appreciable luminosity. Thus only the transpolar arcs (associated with relatively intense precipitation) near the poleward boundary tend to become much more luminous, forming the so-called “theta aurora.”