The equations of motion are solved for ions moving in a model electric field that corresponds to the nightside equatorial region of the magnetosphere. The model represents the poleward region of the Harang discontinuity mapped to the magnetosphere. Within this region the model electric field has a constant earthward gradient superimposed on a constant dawn-to-dusk electric field. In combination with the earthward drift motion due to the dawn-to-dusk field, the electric field gradient introduces an earthward inertia drift, which is proportional to the ion mass and therefore faster for O+ ions than for H+ ions or electrons. It is also found that the entry of the ions into the gradient region causes phase bunching and as a result ion density striations form. The striations are enhanced for more abrupt changes in the electric field gradient, a weaker magnetic field, a stronger cross-tail electric field and colder O+ ions. The first two conditions apply during the growth phase of a substorm. Using the Tsyganenko (1987) model a minimum electric field gradient value of 1¿10-9 V/m2 ((1 mV/m)/1000 km) at L=6--7 is found. Charge neutrality requires coupling with the ionosphere through electrons moving along magnetic field lines, and such electrons may be the cause of multiple auroral arcs. ¿ American Geophysical Union 1994