The magnetohydrodynamic description of Earth's ionosphere

TL;DR

This paper explores the magnetohydrodynamic behavior of Earth's ionosphere, focusing on wave propagation, ion-neutral interactions, and their role in magnetosphere-ionosphere coupling, especially for low-frequency Alfven waves.

Contribution

It provides a detailed MHD framework for understanding wave dynamics in the partially ionized ionosphere, highlighting the effects of neutral collisions and plasma inertia.

Findings

Low-frequency Alfven waves can propagate with minimal damping.

Neutral inertia influences wave excitation and coupling to the magnetosphere.

Waves below a few Hz are significant for ionosphere-magnetosphere interactions.

Abstract

The wave propagation in the partially ionized ionosphere plays an important role in the magnetosphere ionosphere coupling. For example, the ionosphere may supports very low-frequency Alfven wave which can be caused by a balance between the bulk fluid inertia (mostly due to neutrals in the lower and middle E region) and the deformation of the magnetic field. The plasma neutral collisional momentum exchange facilitates the transfer of the magnetic stress (felt directly by the ions) to the neutrals. Therefore, in the low-frequency (with respect to the neutral ion collision frequency) limit, waves through the ionosphere can propagate with very little damping. In the vanishing plasma inertia limit, waves can be excited due to the loading of neutral inertia on the field lines and thus may have very long wavelength and can easily couple to the magnetosphere. The frequency of these waves are below few Hz.