Modulation of magnetic components of ULF Alfve´n wave (f=0.02–2 Hz), which is incident from the magnetosphere to the Earth’s surface, is considered. This modulation is due to variations of electron and ion concentrations within the ionosphere F-layer caused by acoustic-gravity waves or internal gravity waves of ULF (0.0005–0.02 Hz) range. The maximum modulation of the magnetic field at the Earth’s surface takes place when maximum variations of the electron concentration are observed at altitudes 225–275 km. The modulation is more essential when the inclination angle of the geomagnetic field is Z501. The 20% modulation of the electron concentration results in the magnetic field modulation at the Earth’s surface of 30–40%. At the frequencies f1 Hz there exists the resonant modulation when in the narrow (0.1 Hz) frequency range the modulation increases sharply. The results can explain the ground-based observations of generation of the seismo-related ULF magnetic perturbations.