Interaction between Hydromagnetic Waves and the Anisotropically Conducting Ionosphere.

Abstract

The self-consistent ionospheric boundary conditions, which describe the interaction between hydromagnetic (HM) waves and the anisotropically conducting ionosphere, have been derived in the form convenient for the eigenmode analysis of coupled HM waves in the magnetosphere-ionosphere system. These conditions consist of (a) the continuity of the horizontal perturbation electric field across the ionosphere, (b) the divergence of a vector formula relating a jump of the horizontal perturbation magnetic field across the ionosphere to the sheet current flowing therein and (c) the normal component of the rotation of the vector formula to the ionosphere. In order to emphasize a significant role of the derived self-consistent conditions, under the assumption that there exists no parallel inhomogeneity but a radial one of the Alfven velocity VA and the ionospheric conductivities in both hemispheres are uniform and symmetric, the coupling between the axisymmetric fast and Alfven waves via the ionospheric Hall current has been investigated in case of no energy dissipation (or zero Pedersen conductivity). It is suggested that for a given fundamental eigenfrequency, the Alfven mode perturbation is sharply enhanced with a finite amplitude at a field line position which is deviated from the position predicted by the classical field line resonance theory that the resonant frequency is determined only from the distribution of VA along the field line and its length l|| between the northern and southern ionospheres. Such a deviation is due to the divergent Hall current and occurs toward the direction in which VA increases. Further, the Alfven mode perturbation has no phase shift across the enhancement position. It is also suggested that the parallel wavelength of the fast mode perturbation is slightly greater than 2l|| in the region with smaller VA and 2l||/3 in the region with larger VA, respectively, and it changes rapidly from ∼2l|| to r 2l||/3 through l|| as VA grows large, where the wavelength l|| appears just at the enhancement position.