Cleft contribution to ring current formation

The storm time transport of ionospheric plasma from the ``cleft fountain'' to the plasma sheet and ring current is investigated by means of three-dimensional trajectory codes. Using observations to define the source location and flow rate, we trace test particles during a ``taillike'' to ``dipolelike'' reconfiguration of the geomagnetic field. Emphasis is placed on the behavior of heavy ions of low charge state (O⁺). As a result of their long periods of gyration, these ions are highly sensitive to rapid field variations and possibly display transient nonadiabatic motions. It is demonstrated that O⁺ which have originated in the high-latitude ionosphere but which find themselves in the vicinity of the equator at substorm onset can experience considerable energization (from several keV up to several hundred keV) and pitch angle increase leading to trapping, as an effect of the induced electric field. These effects accompany a rapid earthward transport from midtail (~10-15 R_E) to geosynchronous altitudes. The average particle energy and density involved suggest a substantial contribution of the cleft O⁺ fountain to the ring current during substorms.