Tomographic images of the high latitude F region, covering one year of routinely based observations, have been used to investigate the morphology of the ionospheric trough. The density plots are reconstructed by means of statistical inversion using the differential phase of the 150 and 400 MHz signals observed by the Finnish chain of receivers from Tromsø (69.66°N, 18.94°E), Kilpisjärvi (69.02°N, 20.86°E), Kiruna (67.84°N, 20.41°E), Luleå (65.58°N, 22.17°E) and Kokkola (63.83°N, 23.06°E). Since our results are established using a large database of F electron density plots, they can be used to test various ionospheric models attempting to predict the location of the main ionospheric trough. An important finding of our study is that the orientation of the By and Bz components of the interplanetary magnetic field play an important, though different, role in the occurrence of the trough for different levels of the geomagnetic activity. Our results are contributing to the understanding of the contribution of the large scale plasma transport induced by the convection electric field to the formation of the trough.