Ion motion in a spatially varying electric field

Summary form only given. The equations of motion have been solved for ions moving in a model electric field which corresponds to the nightside equatorial region of the magnetosphere. The model electric field has a constant earthward gradient, representing the equatorial counterpart of the Harang discontinuity in the ionosphere, superimposed on a constant dawn-to-dusk electric field. In combination with 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/sup +/ ions than for H/sup +/ 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 minimum value of the electric field gradient required for the striation to occur in lower for a weaker magnetic field, an enhanced cross-tail electric field, and colder O/sup +/ ions. The first two conditions apply during the growth phase of a substorm.