On the self-consistent description of dynamic magnetosphere-ionosphere coupling phenomena with resolved ionosphere

A set of model equations is presented that allows for the self-consistent description of dynamic magnetosphere-ionosphere coupling phenomena with finite ionosphere. The model is very similar to magnetohydrodynamics (MHD), but the plasma-neutral gas interaction in the lower ionosphere is taken into account by a frictional force between,ions and neutrals and an ionization-recombination term in the plasma transport equation. Further, the Hall term and the electron pressure term are retained in Ohm's law. It is shown that for the collision-dominated E layer, the familiar Pedersen and Hall conductivities can be derived directly from the basic equations. The model is used to numerically simulate the dynamic formation of a magnetospheric-ionospheric current system as the response to prescribed localized magnetospheric convection. Apart from a pair of Birkeland current sheets that are closed by Pedersen currents, associated Hall currents are generated. In addition, density irregularities in the E layer form as a direct consequence of the current closure. They are, however, not related to electron precipitation. For small length scales (approximate to 10 km), these density perturbations result in considerably enhanced conductivities below the upward Birkeland currents which in turn lead to a spatial concentration of the latter compared to the downward currents. The timescale for the relaxation of the current system and the ionospheric convection toward a stationary state after the onset of magnetospheric convection is longer than the Alfven travel time and depends on the height-integrated Pedersen conductivity. This is in good agreement with earlier theoretical predictions [Southwood and Kivelson, 1991].