Abstract: Based on GEOS-2 observations, a new substorm expansion phase model-the Near Earth Ballooning Instability Model (NEBIM) is proposed. It is shown in this model that during the growth phase, strong earthward energetic ion gradient is build up at the inner edge of the near Earth plasmasheet (IENEP) which arrives at (6-10) R_E on the nightside; the tail current is greatly enhanced; and the magnetic field lines there are stretched out to the tail. Ballooning instability can be generated at the IENEPwhen the plasmasheet becomes very thin, and the electron precipitation and hence the conductivity in the auroral ionosphere are greatly increased, leading to the disruption and diversion of the tail current to the ionosphere, and the formation of the substorm current wedge. The substorm expansion phase is thus triggered. Further comparison with GEOS-2 data indicates that all features of the expansion phase predicted by the NEBIMare in good agreement with those observed by the spacecraft.