The cleft ion fountain

Low-energy ionospheric ions, injected into the magnetosphere at the dayside cleft, are studied using data for the retarding ion mass spectrometer experiment on the Dynamics Explorer 1 satellite. It is concluded that the upwelling ion events identified in the vicinity of the cleft may be regarded as an ion fountain, supplying low-energy ions to the entire polar magnetosphere when convection is antisunward and strong. It is also shown that heavy ion flows can be downward in the polar cap, consistent with 'parabolic' trajectories of heavy ions from this cleft ion fountain.