On the modulation of intensity of Alfvén resonances before earthquakes: Observations and model

Modulation of magnetic components of ULF Alfvén 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 ≥50°. The 20% modulation of the electron concentration results in the magnetic field modulation at the Earth's surface of 30-40%. At the frequencies f∼1 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.