The equatorward wall of the subauroral trough in the afternoon/evening sector

Although ionospheric troughs are a very important feature of the subauroral ionosphere, many of their properties remain incompletely documented and understood. Here Dynamics Explorer-2 satellite data are used to investigate one specific part of this complex phenomenon, namely its equatorward wall. We find that in the afternoon/evening sector of the Northern Hemisphere the location of this density drop depends primarily on the level of geomagnetic activity and magnetic local time. Longitudinal variations are only of secondary importance. A formula is derived which summarizes these variations. The magnitude of the density drop in the trough wall depends primarily on altitude and longitude, and to a lesser degree on local time and geomagnetic activity. These variations are also described quantitatively. Using a superposed epoch type of averaging procedure, a mean latitudinal profile of the trough wall is derived. No anomalous increase in the density at the equatorward edge of the trough is observed. There is, however, a significant increase in the electron temperature at the location of the density drop. Our results are important for the empirical description and numerical simulation of ionospheric troughs. They also may be used to define the boundary between middle and subauroral latitudes.