The parallel dynamics of the plasma along high-latitude field lines is a critical feature of the ionosphere-magnetosphere coupling problem. We have created a time-dependent, large scale simulation of these dynamics in a region from the ionosphere well into the magnetosphere. Including multiple electron species has allowed us to dynamically simulate the diode-like response of the field-line plasma to the parallel currents coupling the ionosphere and magnetosphere. Downward (return) currents flow with small resistance, while upward currents produce kilovolt-sized potential drops along the field due to the effect of the converging magnetic field on the high energy magnetospheric electrons. ¿ American Geophysical Union 1990