Ultralow-frequency magnetic-field oscillations on the Earth's surface generated by horizontal inhomogeneities of the ionospheric conductivity

It is shown that, in the presence of quasi-periodic horizontal inhomogeneit ies of the conductivity in the lower ionosphere, the electromagnetic backgr ound noise gives rise to polarization currents, which generate gyrotropic w aves with a narrow-band spectrum. For inhomogeneities with a characteristic spatial scale of the order of 10 km, the spectral maximum lies in the rang e 1-10 Hz. The spectral distribution of the amplitudes of electromagnetic d isturbances on the Earth's surface is calculated for the case when the gyro tropic waves propagate along the horizontal geomagnetic field in the thin c onductive layer of the lower ionosphere. The results obtained are used to i nterpret the electromagnetic disturbances observed near the epicenters of s ome impending earthquakes.