The dynamic processes in the plasma along high-latitude field lines plays an important role in ionosphere-magnetosphere coupling process. We have created a time-dependent, large-scale simulation of these dynamics parallel to the geomagnetic field lines from the ionosphere well into the magnetosphere. The plasma consists of hot e- and H+ of magnetospheric origin and low-energy e-, H+, and O+ of ionospheric origin. Including multiple electron species, a major improvement to the model, has allowed us for the first time to simulate the upward current region properly and to dynamically simulate the diodelike response of the field-line plasma to the parallel currents coupling the ionosphere and magnetosphere. It is shown that return currents flow with small resistance, while upward currents produce kilovolt-sized potential drops along the field, as concluded from satellite observations. The kilovolt potential drops are due to the effect of the converging magnetic field on the high-energy magnetospheric electrons. ¿ American Geophysical Union 1992