Using a hydrodynamic model for the plasma, it is demonstrated that the auroral plasma cavity is capable of drawing an appreciably large flux of oxygen ions, which are normally gravitationally bound. This escape mechanism of O+ does not involve any additional heating of acceleration of the plasma in the ionosphere. The temporal evolution of the outflow shows that it starts near the cavity and penetrates into the ionosphere with a flux front moving down with the ion-acoustic speed. A steady outflow is reached in a few hours for a cavity at a height of one Re if the cavity is maintained. This time reduces to &Ggr;+0(cm-2 s-1)<107. An analysis for the steady state shows that outflow is controlled by the density scale height in the cavity. ¿ American Geophysical Union 1989