Analysis of the diurnal variations of TEC (Total Electron Content) at midlatitude stations showed that in periods of geomagnetic disturbances and after them marked strengthening of the ionosphere variations with periods of 4-8 hours. Such perturbations can persist for several days after the storm. The analysis of perturbations of the diurnal TEC variations at midlatitude stations shows that the spectrum of TEC perturbations are determined by amplification of harmonics with periods of 4-5 h and 6-8 h. The periods such perturbations increasing with decreasing latitude. The slow variations of ionosphere(τ = 6-8 hours) have high sensitivity to the geomagnetic disturbances. It is suggested that observed disturbances of TEC variations are caused by the excitation of planetary Poincare waves in the atmosphere. Latitudinal dependences of the periods and amplitudes of the spectral harmonics have consistent with the dispersion relations for Poincare waves with zonal wavenumbers s = 1, 2 and meridional wavenumbers n = 1, 2 with a width of atmosphere channel L ~ 12000 km. The possibilities of forming such atmosphere channel and generation of Poincare waves during geomagnetic disturbances is discussed.