Abstract A modified form of the ionospheric servo-model is used to describe the night-time F2-layer above St. Santin. Data taken by the incoherent scatter radar on nine nights in 1974–1977 were used to determine the height profiles of electron density, electron and ion temperature and electric field. The servo-model was then used to compute the theoretical variation through the night of the height of the F2 peak and the field-aligned plasma velocity, using gas concentrations and horizontal pressure gradients derived from the MSIS79 atmospheric model. On magnetically quiet nights these calculated values agreed closely with the observations. On disturbed nights, however, the calculations and observations began to diverge an hour or so after the onset of a substorm. The divergence could be explained by an enhanced southward wind.