The effect of the convection electric field in accelerating ions that escape from the polar ionosphere is investigated. It is shown that at high altitudes the velocity component of the ions along the magnetic field may be increased by more than an order of magnitude. The highest velocities are acqured by ions that escape from the region of the ionosphere that is connected along magnetic field lines to the dayside cusps. During disturbed times, ions from that region intercept the center plane of the magnetotail in one to two hours at radial distances exceeding about 6 RE. Investigation of the resulting O+ properties in the center plane, viz., their locations, number densities, and energies, indicates that the polar ionosphere near the cusps is the principal source of the O+ observed in the plasma sheet. Moreover, a study of the ion motion at quiet and disturbed times indicates that the increase of O+ in the plasma sheet with increasing AE values [Lennartsson and Shelley, 1986> is due mainly to an increase in the source of O+ rather than alternation of its transport path.