We have examined the properties of low-frequency electromagnetic waves in multicomponent ionospheric plasma in the 1–30 Hz band, using the magnetoionic theory. Complex permittivity tensor components and refractive indices of normal waves (ordinary and extraordinary) were calculated at altitudes from 80 to 750 km. The calculations show that the refractive indices are highly dependent on frequency and height. Polarization of ordinary and extraordinary waves is elliptical over the entire range of the frequencies investigated. The refractive index and the polarization of normal waves are demonstrated to tend to magnetohydrodynamic (MHD) values only at frequencies lower than 1 Hz. The group velocity vector of an extraordinary wave is not directed along the magnetic field, as follows from the MHD approximation, but it lies inside a cone within ±(5–10) degrees, depending on frequency. The group velocity vector of an ordinary wave is practically independent of the angle with the geomagnetic field as in the MHD approximation. The proposed method for calculating the characteristics of normal waves in the ionosphere can be used to study ULF wave propagation from both natural and artificial ionospheric sources, which arise under the action of powerful HF radio waves in the lower and upper ionosphere.
ULF waves, ionosphere, refractive index, polarization, Alfvén wave, fast magnetosonic wave
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