The results presented in this paper provide an explanation for electromagnetic oscillations with frequencies much less than the fundamental eigenfrequency of the magnetosphere measured in the regions where the ionospheric conductivity is low and a small-amplitude, large-scale electric field in the ionosphere exists. This study is based on numerical simulations of a reduced two-fluid MHD model describing propagation of dispersive Alfv¿n waves in the highly inhomogeneous magnetospheric plasma and interaction between these waves and the active ionosphere. Simulations show that electromagnetic oscillations with frequency below 10 mHz can be generated by a strongly nonlinear interaction between magnetic field-aligned currents and the ionosphere called ionospheric feedback instability. To produce oscillations in ULF frequency range, this mechanism requires neither very stretched geometry of the ambient magnetic field nor coupling between shear and slow MHD waves. These features of the ionospheric feedback mechanism makes it particularly suitable for the explanation of the ULF oscillations detected on midlatitude and low-latitude, nightside field lines when the background ionospheric conductivity is low and the magnitude of the perpendicular electric field in the ionosphere is small.