The main ionospheric trough is a large-scale depletion in the ionospheric electron density. It forms at the interface between the high-latitude auroral region and the mid-latitude ionosphere, separating these two regimes. In Western Europe it occurs in the evening and night-time ionosphere and progresses equatorward during the course of the evening. It exhibits substantial day-to-day variability in both its structure and latitude, with the trough minimum generally being further equatorward under conditions of increased geomagnetic activity.

The Electron Density Assimilative Model (EDAM) images the ionosphere. The model assimilates ionospheric observations into the background International Reference Ionosphere (IRI) to provide images of the ionosphere on days of interest. EDAM observations are presented of vertical total electron content and ionospheric electron density in the vicinity of the main trough. The observed trough is compared with previously reported measurements of the feature by other experimental techniques, and its observed structure is discussed in the context of the assimilated GPS electron content measurements along the satellite-to-receiver paths.