On the possibility of using an electromagnetic ionosphere
in global MHD simulations
P. Janhunen
Global magnetohydrodynamic (MHD) simulations of Earth's magnetosphere
must be coupled with a dynamical ionospheric module in order to give
realistic results. The usual approach is to compute the field-aligned
current (FAC) from the magnetospheric MHD variables at the ionospheric
boundary. The ionospheric potential is solved from an elliptic
equation using the FAC as a source term. The plasma velocity at the
boundary is the ${\bf E}\times{\bf B}$ velocity associated with the
ionospheric potential. Contemporary global MHD simulations which
include a serious ionospheric model use this method, which we call the
electrostatic approach in this paper.
We study the possibility to reverse the flow of information through
the ionosphere: The magnetosphere gives the electric field to the
ionosphere. The field is not necessarily electrostatic, thus we will
call this scheme electromagnetic. The electric field determines the
horizontal ionospheric current. The divergence of the horizontal
current gives the FAC, which is used as a boundary condition for MHD
equations. We derive the necessary formulas and discuss the validity
of the approximations necessarily involved. It is concluded that the
electromagnetic ionosphere-magnetosphere coupling scheme is a serious
candidate for future global MHD simulators, although a few problem
areas still remain. At minimum, it should be investigated further to
discover if there are any differences in the simulation using the
electrostatic or the electromagnetic ionospheric coupling.