ИСТИНА

A characteristic feature of the upper ionosphere is the occurrence of the ionospheric Alfven resonator (IAR) and the ionospheric fast-mode waveguide (IFW), which can trap ULF electromagnetic wave energy in the Pc1 frequency range from fractions of a Hz to a few Hz. This wave trapping ensures the strong dependence of the ionospheric transmission/reflective properties on frequency. We have developed a numerical model of the magnetospheric Alfven wave interaction with the ionosphere and transmission to the ground based on the solution of multi-fluid magnetohydrodynamic full-wave equations in a realistic ionosphere, whose parameters were reconstructed from the International Reference Ionosphere (IRI) model. The spatial structure of an incident Alfven wave is modeled as a localized beam with a finite latitudinal scale and azimuthally propagating wave with wave vector k2. The IAR and IFW modes are coupled owing to the frequency-dependent Hall conductivity of the ionosphere and geomagnetic field line inclination. The ground spatial and spectral structures of the Pc1 wave have been calculated for the local summer day/night conditions at a middle latitude observatory. The model predicts several new features which may interpret some observational results. Beneath the incident beam the ground magnetic response "duplicates" the incident beam structure after accounting for a pi/2 rotation and some latitudinal shift. The upper part of the ULF spectrum (f>1 Hz) is severely attenuated upon wave transmission through the daytime ionosphere. At nighttime the transmission of Alfven waves has an oscillatory dependence on frequency, thus forming "transmission windows" at resonant frequencies. The spectra are different at various distances from an incidence point: close to the source the frequency of the fundamental IAR eigenmode is highlighted, but at larger distances the spectral peaks with f>1 Hz associated with the IFW modes emerge. A complicated interference pattern between IFW modes is revealed in the spatially-dependent and frequency-dependent character of amplitude variations, especially at higher frequencies.