Modification of magnetic signals of short-period pulsations by the ionosphere
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The transmission, mode conversion, and reflection of HM-waves associated with short-period geomagnetic pulsations through and by the ionosphere are comparatively examined for three different model cases, bearing in mind the localized injection of the shear Alfven wave at high latitude and the consequent horizontal spread of disturbances to low-latitude regions through the ducted propagation in the upper ionosphere. In the first model case composed of two semiinfinite layers (the magnetosphere and the neutral atmosphere) with a separating interface (the ionospheric E-layer with anisotropic conductivities), we have studied conductivity dependences of the transmission of magnetic signals of the shear Alfven and the fast magnetosonic waves respectively, propagating down along vertical field lines and having a periodic horizontal structure. For the Alfven wave incidence with the static electric field the transmitted magnetic signal intensity has a peak against the Hall conductivity variation, while it rather decreases as the Pedersen conductivity increases. We have also shown that the ionosphere behaves as a shielding metalic conductor for the incidence of the fast magnetosonic wave with the inductive electric field. In the second model case consisting of five layers, i.e. a magnetospheric region, an upper ionosphere as a trapping region, an interface (E-region), an atmospheric region, and a semi-infinite solid earth, we have dealt with ionospheric effects on a localized incidence of the shear Alfven wave. A comparison of magnetic signal intensities at the levels just above and below the E-layer leads us to conclude that the behaviour of magnetic signals in a near-source region is similar to that obtained in the first model case without a trapping region, and that the dominating direct source of ground magnetic fields is the induced eddy Hall current in the ionosphere. Finally, the ionospheric shielding effect in a distant region from the injection source has been estimated by employing the third model of the ducted propagation of the fast magnetosonic wave in the upper ionosphere. We conclude that the ionospheric screening is rather significant in the source region when the Pedersen to Hall conductivity ratio is much larger than unity while it is not so significant in the intermediate and distant regions.
- 国立極地研究所の論文
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関連論文
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- Modification of magnetic signals of short-period pulsations by the ionosphere