Energization of ions to low energies (≤10 eV) in the topside ionosphere (300--2000 km altitude) has been observed for a decade. These ionospheric ions have been invoked as the source population of keV ions energized in the auroral magnetosphere (≥1 RE altitude). If so, the importance of the topside ionosphere as a source of magnetospheric plasma must be assessed quantitatively. To determine the efficiencies of this topside auroral ionospheric source of magnetospheric plasma, Monte Carlo simulations of wave-particle ion heating under realistic topside conditions have been carried out. It is shown that the source efficiency is greatly affected by (1) copious charge exchange interactions between the ions and thermospheric neutrals at an early stage of heating and (2) the observed existence of downward parallel electric potential drops. The composition of warm ions escaping the topside is predicted to be drastically different from the cold ionospheric composition. This study also predicts the existence of a hot (<10 eV) population of thermospheric neutrals in the high latitude topside ionosphere. ¿ American Geophysical Union 1988