The ballooning mode instability, which can excite hydromagnetic waves at the plasmapause, is studied in the presence of azimuthal plasma flows induced during geomagnetically disturbed periods. A general sufficient criterion for the ballooning mode stability is derived as ΔW≥0 (where ΔW represents the change in the potential energy) which involves the integration over an entire field line. A local stability analysis at the equatorial plasmapause region shows that the ballooning modes could be spontaneously generated via instability under at least two conditions: one is similar to the unusal interchange condition, and the second to the quasi-interchange modes. Both of these local instability conditions can be derived from the general ΔW≥0 stability criterion. Finally an exact solution for the equilibrium state with flow is derived analytically, and ΔW is computed numerically. It is found that, in the cases studied, the flow does not spontaneously excite the ballooning modes; it only further stabilizes (or destabilizes) the ballooning spectrum if originally the system is stable (or unstable). The analysis would be useful for the interpretation of some of the low frequency modes observed at the ground and near the equatorial plasmapause. ¿ American Geophysical Union 1990