Since 1960, several experiments have shown strong perturbations of the ionosphere after earthquakes. For the biggest quakes, Doppler ionospheric soundings have revealed displacements of several tens of meter of the ionized layers E and F. An accurate modelling of these phenomena was however never described. We present here synthetic displacements of such oscillations and compare it with experimental data. Realistic models of the atmosphere including viscosity are used, as well as realistic Earth and seismic source model. These ionospheric oscillations are computed by normal modes summation taking explicitly into account the coupling between the solid Earth and the open atmosphere. They are associated to the Rayleigh fundamental surface waves. We obtain a good agreement between synthetics and observations, so it should be possible to use this method to study the ionospheric postseismic perturbations and possibly the high altitude profile of the density and the viscosity of the atmosphere. ¿ 2001 American Geophysical Union