In situ observations have led to the notion of the explosive growth phase when the cross-tail current sheet inside L~10 suddenly thins in a time of ~1 min. A theoretical explanation of the current intensification envisages the ballooning instability of magnetospheric plasma. In this paper, this theoretical notion is reanalyzed mathematically. In contrast to some previous work, we find that for a high-plasma, the ballooning instability can be excited easily by an earthward pressure gradient of any magnitude and for perturbations having parallel wavenumbers comparable to the field line curvature. The instability is compounded by its own development, leading ultimately to the extreme condition of a thin current sheet of a few ion gyroradius thick.¿ 1997 American Geophysical Union