Spectra of the critical frequency of the F2 ionospheric layer in the range of periods 0.5 – 40 days are analyzed using the data of measurements with the ionosonde DPS-4 at the Peterhof scientific station of Saint Petersburg State University (60°N, 30°E). Spectral analysis is executed using the Lomb –Skargle method for 60-day running intervals. Spectra show maxima at periods of 1 day and 0.5 day corresponding to diurnal and semidiurnal changes, and maxima in the range of
periods 2 – 40 days. These waves have frequently maximum amplitudes in spring and summer, which is opposite to the PW amplitudes observed in the lower and middle atmosphere. This may be caused by different mechanisms including possible PW propagation from the middle atmosphere of the winter hemisphere to the summer thermosphere along the waveguides crossing the equator at altitudes above 60 km.