TRANSIENT-RESPONSE OF UNIFORM IONOSPHERE AND PRELIMINARY REVERSE IMPULSE OF GEOMAGNETIC STORM SUDDEN COMMENCEMENT

Ionospheric propagation of magnetic disturbance due to a suddenly impressed source field is studied by the use of the Laplace transform method. In the E-region, assumed to be an anisotropic metallic medium, the mode is a diffusion or a Schrödinger type wave according to whether the propagation is perpendicular or parallel to the static magnetic field. Analytical solutions are used to examine whether the preliminary reverse impulse (PRI) of geomagnetic storm sudden commencement (SSC) is produced by the direct incidence of hydromagnetic waves on the ionosphere. No PRI's are obtained under both the polar and equatorial condition, while a small PRI-like impulse appears transiently in the polar case. From a viewpoint that the equatorial PRI is caused by the polar electric field communicated from the magnetosphere, the solutions are also used to find a possible mode for the instantaneous transmission of the polar electric field to the equator. On the assumption that the transmission is made one-dimensionally in the E-region, it takes more than 1 h for the steplike source field to make an observable effect at the equator and if the source field varies with time scale of SSC, no fields are observed at the equator because of great attenuation. It is suggested that the instantaneous horizontal transmission is possible in the Earth-ionosphere waveguide. © 1979.