A fully self-consistent numerical model of the polar ionosphere and the global thermosphere has been developed to study the dynamical and chemical interactions between the neutral and ionospheric constituents of the polar regions. In the polar F region, the seasonal anomaly in ionospheric density is caused by the neutral thermospheric compositional response to the interhemispheric circulation cell near solstice, including the effect of the self-consistent input from geomagnetic heating. A tongue of ionisation is carried by convection into the winter polar cap from the vicinity of the dayside (sunlit) polar cusp, while the auroral oval, at F region altitudes, is barely distinct from the surrounding lower-latitude and polar regions. The subauroral trough of the winter hemisphere is caused by stagnation, and is thus dominated by characteristics of the polar convection pattern. A deep plasma trough is cut through the summer (southern) polar cap aroung 0600 UT, resulting from upwelling by geomagnetic forcing and solar isolation enhancement of molecular nitrogen. ¿ American Geophysical Union