Mechanism for the formation of the daytime trough in the winter in the Southern Hemisphere

Latitudinal profiles of electron density determined by the Cosmos-900 Satellite in the Southern Hemisphere are compared with model calculations. It is found that the formation of the subauroral daytime trough in the winter is associated with the absence of an effective ionization source in the 'polar shadow' region which forms in the daytime sector (14-18 UT) in the course of the earth's diurnal rotation.