Ion convection velocity reversals in the dayside cleft

Nearly simultaneous measurements of energetic particle precipitation and vector ion velocity have been made for the first time on Atmosphere Explorer C. These data have revealed the existence of both shear and rotational reversals in the ion convection velocity within the region of dayside cleft particle precipitation. The ion velocity data have shown that substantial regions of the dayside polar cap boundary are nearly electric equipotentials at which cleft particle precipitation is also observed but which cannot be identified with a merging region producing open field lines. Asymmetries in the polar cap flow, first implied by Heppner, may be determined by the extent and location of the equipotential boundaries.