The High-Latitude Trough in the Southern Hemisphere Observed by Swarm-A Satellite

Based on the electron density, field-aligned currents (FACs) and ion drift velocity data from the Swarm-A satellite during 2013-2018, we investigate the features of the high-latitude trough in the Southern Hemisphere. The results show that the high-latitude trough in the Southern Hemisphere is a persistent postmidnight feature in winter. It is observed mainly in the eastern longitudes under low solar activity conditions and in a smaller longitudinal range under high solar activity conditions. The high-latitude trough moves to higher latitudes at later local times, which is more obvious under low solar activity conditions. We also find that the features of FACs and ion drift velocity distribution in the high-latitude trough region depend on longitude. In the longitude sector of 0 degrees-70 degrees E, the high-latitude trough positions are always located at the poleward boundary of the downward FAC region, and collocated with the convection reversal boundary in the postmidnight region. In the longitude sector of 80 degrees-130 degrees E, the high-latitude trough positions are located at the poleward boundary of the downward FAC region in the midnight region and at the strong westward or antisunward ion flow region in the postmidnight region. It is suggested that the high-latitude trough formation and evolution is possibly associated with downward FACs, flow stagnation in the convection reversal boundary, and westward or antisunward ion flow.