  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.
