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 ofO+does not involve any additional heating or 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 Reif the cavity is maintained. This time reduces to<1 hour for cavity heights<3000 km. During the transient state (107. An analysis for the steady state shows that outflow is controlled by the density scale height in