Transfer of magnetosheath momentum and energy to the ionosphere along open field lines

A framework is established for describing the transfer of magnetosheath energy along open field lines to the ionosphere in terms of MHD (magnetohydrodynamic) waves. The evolution of field lines crossing an initial velocity shear is shown to extract momentum and energy from the faster how (magnetosheath) and deposit it in the slower how. If the slower plasma is taken to be at rest and have density rho(0) and Alfven speed V-A, while the faster plasma has flow speed V-0 and Alfven speed V-A' then the rate of energy density exchange is rho(0)V(0)(2)V(A)(3)/(V-A + V-A')(2), while that of momentum density is rho(0)V(0)V(A)(2)(V-A + V-A'). The effect of an ionosphere is included by introducing a height-integrated Pedersen conductivity (Sigma(p)), and we find that field lines slip with a velocity V-0/(1 + mu(0) Sigma(p)V(A)) just above the ionosphere. By considering the accelerated hows associated with newly opened field lines, it may be possible to understand transient features seen in data such as initial azimuthal surges.