Vm. Sorokin et al., Ultralow-frequency magnetic-field oscillations on the Earth's surface generated by horizontal inhomogeneities of the ionospheric conductivity, GEOMAG AERO, 41(3), 2001, pp. 315-319
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.