Remote analysis of cleft ion acceleration using thermal plasma measurements from Interball Auroral Probe

Three dimensional distributions of low energy (0-80 eV) ions have been obtained in the high-latitude dayside sector between 10,000 and 20,000 km by the Hyperboloid experiment onboard Interball-Auroral Probe. H+, He+ and O+ ions exhibit a latitude-energy dispersion characteristic of the cleft fountain. Test particle simulations are used to investigate the properties of the outflowing ion source region. Regardless of ion mass. it is shown that the bulk of the outflowing population originates from a narrow (< 2 degrees) latitudinal interval inside the dayside cleft. Ion acceleration in the direction perpendicular to the magnetic field is shown to occur at all altitudes at least up to 10,000 km, that is, higher than previously reported in cleft fountain studies. The simulations clearly display a gradual decrease of the heating efficiency with increasing altitude and suggest a weaker gradient for O+ than for H+. Parallel acceleration at low altitudes also appears to contribute to the net ion energization within the cleft.