Statistical characteristics and classification of ionospheric mid-latitude trough as revealed by the observations of DMSP-F18

Statistical characteristics and the classification of the topside ionospheric mid-latitude trough are systemically analyzed, using observations from the Defense Meteorological Satellite Program F18 (DMSP-F18) satellite. The data was obtained at an altitude of around 860 km in near polar orbit, throughout 2013. Our study identified the auroral boundary based on the in-situ electron density and electron spectrum, allowing us to precisely determine the location of the mid-latitude trough. This differs from most previous works, which only use Total Electron Content (TEC) or in-situ electron density. In our study, the troughs exhibited a higher occurrence rate in local winter than in summer, and extended to lower latitudes with increasing geomagnetic activity. It was found that the ionospheric mid-latitude trough, which is associated with temperature changes or enhanced ion drift, exhibited distinct characteristics. Specifically, the ionospheric mid-latitude troughs related to electron temperature (<italic>T</italic>_e) peak were located more equatorward of auroral oval boundary in winter than in summer. The ionospheric mid-latitude troughs related to <italic>T</italic>_e-maximum were less frequently observed at 60−70°S magnetic latitude and 90−240°E longitude. Furthermore, the troughs related to ion temperature (<italic>T</italic>_i) maximums were observed at relatively higher latitudes, occurring more frequently in winter. In addition, the troughs related to ion velocity (<italic>V</italic>_i) maximums could be observed in all seasons. The troughs with the maximum-<italic>T</italic>_i and maximum-<italic>V</italic>_i were located closer to the equatorward boundary of the auroral oval at the nightside, and in both hemispheres. This implies that enhanced ion drift velocity contributes to increased collisional frictional heating and enhanced ion temperatures, resulting in a density depletion within the trough region.