Substorm onsets and single ion motion

The equations of motion are solved for ions moving in a model electric field which corresponds to the nightside equatorial region of the magnetosphere. Features of the substorm onset such as its association with the Harang discontinuity (HD), the presence of enhanced O+ in the plasma sheet, the creation of a substorm current wedge, and the local weakening of the magnetic field are treated. The divergence of inertia currents carried by the oxygen ions at the HD requires the formation of a substorm current wedge. A spontaneous onset occurs where the current wedge is driven to the point when sufficient east-west current is diverted to the ionosphere. A triggered onset occurs when IMFBz turns form south to north causing the cross-tail electric field to pass through zero. Under those conditions an analytic treatment in terms of elliptic functions shows that the ions may go chaotic thereby producing a current interruption