Studies on the propagation of ULF waves in the magnetosphere-ionosphere coupled system

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The propagation of IJLF waves in the magnetosphere-ionosphere coupled system was investigated and the following main results were obtained : 1. Both in the cases of the ground magnetic field being due to compressional MHD waves directly incident on the ionosphere and to the ionospheric current of polar origin, its phase at the dip equator lags behind those in the off-equatorial region. Such a phase lag arises from an enhancement of the ionospheric conductivity at the dip equator. 2. When the localized Alfv_n mode impulse is incident on the polar ionosphere, there are two types of horizontal transmission of electromagnetic signals in the Earth-ionosphere waveguide. One is the electromagnetic transmission and the other is the electrostatic transmission. The former is associated with the poloidal mode wave induced from the Alfven mode impulse through the ionospheric Hall current. The latter is a horizontal spread of the electric field represented with a scalar potential, where the electric field is directly associated with the Alfv_n mode impulse. The electrostatic transmission occurs. only when the ionosphere is non-uniform. 3. The field-aligned current is closed in the ionosphere via the divergent Hall current driven by the rotational electric field as well as the ordinary Pedersen current. Such a closing brings about an effective control of eigenmodes of the field line oscillation by the height-integrated Pedersen and Hall conductivities.