The main mechanisms of ionospheric storms are thoroughly studied, but ionospheric perturbations often are unsatisfactorily predicted, maybe because of insufficient allowance for the ionospheric background. In particular, according to data on the electron concentration Ne in the maximum of the F2 region and the total electron content, it has been shown that the amplitude of the positive phase for similar atmospheric storms can differ by about 1.5 times. Such a difference can be caused by changes in thermospheric conditions that are not reflected by known activity indices. For further investigation of the ionosphere, we used the data of an incoherent scattering radar of the Institute of Ionosphere (Kharkiv, Ukraine) for the altitude range from 200 to 1000 km in very quiet periods ending by geomagnetic perturbations (Кр = 4-5). Stable periodic perturbations of the electron concentration Ne were found in the entire altitude range. These perturbations can be associated with the tidal mode m = 6. The wave amplitude is ~ 15%, and the phase varies with altitude. The beginning of a storm leads to amplitude doubling with the phase unchanged. The presence of perturbations in the ionosphere under very quiet conditions may be an extra factor complicating the ionospheric reaction to a magnetic storm.