Simultaneous measurements of the Total Electron Content (TEC) and the peak electron density NmF2 at vertical sounding sites enable the estimation of the equivalent slab thickness of the ionosphere. This shape parameter, defined as the ratio of variables TEC and NmF2 of the vertical electron density profile, is closely related to the neutral gas scale height under undisturbed conditions. Under disturbed conditions the equivalent slab thickness is very sensitive to the competition of plasma driving forces lifting up or moving down the plasma.

To get near real time information on these perturbation processes, the equivalent slab thickness is continuously monitored by combining vertical sounding and corresponding TEC data over Juliusruh/Germany, Pruhonice/Czech Republic and Tromsoe/Norway via the space weather service of DLR (http://swaciweb.dlr.de).

In this paper we discuss long-term trends of the equivalent slab thickness over several European ionosonde stations within the entire solar cycle 23 and beyond. The long-term trend of the equivalent slab thickness indicates a cooling process in the thermosphere during solar cycle 23. To get reliable conclusions on these long-term trends a discussion of the diurnal behaviour of the slab thickness is required. Ionospheric storms may change the equivalent slab thickness dramatically.