OBSERVATIONS OF DOWNWARD MOVING O+ IN THE POLAR TOPSIDE IONOSPHERE

New investigations of the core ion motion within the high-latitude topside ionosphere and near-Earth magnetosphere, using data from the Dynamics Explorer retarding ion mass spectrometer (RIMS), reveal the existence of significant regions of downward moving O+. The occurrences of downgoing versus upgoing O+ are not clearly separable in terms of either solar zenith angle or Kp but are well distinguished by the direction of the z component of the interplanetary magnetic field (IMF). On average, downflow dominates when IMF B-z < 0, while upflow dominates for IMF B-z > 0. Combining cross-field convection velocities derived from an empirical convection electric field model with the observed parallel velocities yields a two-dimensional ion velocity field, This velocity field is consistent with a scenario which has O+ of cusp/cleft and auroral zone origin convecting into the polar cap and, because of the dominance of gravitational energy over the upward kinetic energy, falling back into the ionosphere. This provides additional confirmation of the results of studies of the cleft ion fountain. Estimates of the flux of O+ in the upflowing and downflowing regions for Lambda greater than or equal to 60 degrees give a total upflow of similar to 6 x 10(25) ions s(-1) for IMF B-z > 0 and total upflow and downflow of similar to 4 x 10(25) ions s(-1), and 1 X 10(25) ions s(-1), respectively, for IMF B-z < 0. In all cases the magnitude of the dayside outflow is consistent with previous work on upwelling ions. While the magnitudes vary for high and low Kp the ratios of upward to downward flow are roughly the same at similar to 1.7. The downflowing O+ shows a correlation with the magnitude of the outflow of light ions in the same region but the cause and effect of this relationship is not distinguishable.