The cleft ion fountain

Low‐energy (below approximately 50 eV) ionospheric ions, injected into the magnetosphere at the dayside cleft, are studied using data from the retarding ion mass spectrometer (RIMS) experiment on the Dynamics Explorer 1 satellite. It is concluded that upwelling ions at the cleft form an ion fountain and are blown into the polar cap by antisunward convection. At high K p (>4), convection is generally strong enough to fill the entire polar magnetosphere with low‐energy O + ions, whereas at low k p (<2) they are largely restricted to the dayside half of the cap. Using a two‐dimensional kinetic ion trajectory model, the locations where RIMS detected O + within the cap are shown to be consistent with the spatial distributions of O + density, predicted for an upwelling ion source at the cleft and various dawn‐dusk convection electric fields. A detailed study is made of one polar pass of DE 1, during which RIMS detected He + , N + , O + , and O ++ ions, the ion trajectory model being used to trace all these ions back to a common source at an observed upwelling ion event near the cleft. All observed species are deduced to be falling earthward in the nightside of the cap, as predicted from the model, indicating the dominance of gravity over upward field‐aligned acceleration (such as by the ambipolar electric field). Comparison of field‐aligned velocities observed for O + and O ++ ions defines a maximum limit to the upward electrostatic acceleration present within the cap which was only sufficient to eject ionospheric H + ions, all heavier ions being supplied from the dayside by the cleft ion fountain.