Gyrotropic waves in the mid-latitude ionosphere

The propagation of electromagnetic ULF perturbations in the thin conductive ionospheric layer in the magnetic meridian plane is considered. The dispersion relation for the waves propagating in the horizontal direction is obtained accounting for the influence of the conjugate ionosphere. The dependence of the phase velocity and absorption coefficient as the function of the wave frequency and magnetic field inclination are found. It is shown that in the frequency range 0.001-1 Hz the phase velocity increases from a few units to a few tenths of km/s depending on the frequency and so far follows the approximate dependence omega(1/3). The phase velocity decreases with the increase in the magnetic field inclination. The propagation of quasi-harmonic and unipolar impulses of gyrotropic waves in the horizontal direction is analyzed. The dependence of their characteristics on the values of conductivity tensor components and on the magnetic field inclination is evaluated. The generation of gyrotropic waves by the electromagnetic field of atmospheric origin in the presence of ionospheric inhomogeneities is considered. The calculation of the power spectra of the geomagnetic field oscillations at the ground level is carried out. The dependence of the spectrum on the magnetic field inclination is obtained. Some features of geophysical phenomena, associated with propagation of gyrotropic waves in the low ionosphere are discussed. (c) 2005 Elsevier Ltd. All rights reserved.