Abstract
There are two low frequency, magnetised, cold plasma wave modes that propagate through the Earth’s magnetosphere. These are the compressional (fast) and the shear Alfven modes. The fast mode distributes energy throughout the magnetosphere with the ability to propagate across the magnetic field. Previous studies of coupling between these two modes have often focussed on conditions necessary for mode coupling to occur in the magnetosphere. However, Kato and Tamao (1956) predicted mode coupling would occur for non-zero Hall currents. Recently, the importance of the Hall conductance in the ionosphere for low frequency wave propagation has been studied using one dimensional (1-D) models. In this paper we describe effects of the ionosphere Hall conductance on field line resonance and higher frequency, 0.1–5 Hz waves associated with the Ionospheric Alfven Resonator (IAR). The Hall conductance reduces the damping time of field line resonances and Joule dissipation into the ionosphere. The Hall conductance also couples shear Alfven waves trapped in the IAR to fast mode waves that propagate across the ambient magnetic field in an ionospheric waveguide. This coupling leads to the production of low frequency magnetic fields on the ground that can be observed by magnetometers.
Highlights
Near-Earth space is a dynamic region energised by solar processes
In this paper we highlight the effects of ionospheric Hall conductance on ULF wave interaction with the magnetosphere-ionosphere-atmosphere-ground system, an important factor recognised by Kato and Tamao (1956)
While Tamao (1965) discussed ULF wave mode coupling in the magnetosphere, in this paper we focus on the insight of Kato and Tamao (1956) who, ten years earlier had recognised that the two cold plasma Alfven waves would couple through the Hall conductance of the ionosphere
Summary
Near-Earth space is a dynamic region energised by solar processes. The solar wind and magnetosphere of Earth interact to produce perturbations in the magnetised plasma with a wide range of spatial and temporal scales. Dungey (1963) developed analytic solutions by dividing the problem into toroidal (divergence free) and poloidal (curl free) magnetic field components, linking these with the shear and fast Alfven, ideal magnetohydrodynamic (MHD) wave modes that exist in the cold, magnetised plasma of the magnetosphere (Stix, 1962; Alfven and Falthammar, 1963) This appears to have been a common approach at the time, as Kato and Tamao (1956) contains a similar analysis while referring to the contemporary literature that used a similar mathematical development. The recently revived interest in Hall current effects on the transition of ULF waves from the magnetosphere through to the ground have been investigated using one-dimensional model approximations Both simplified analytic approximations and more realistic numerical calculations have revealed the basic effects of the inductive process involved with the Hall conductance. For an oblique geomagnetic field, the conductivity tensor is (Lysak, 2004)
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