Multi-satellite observations indicate the development of thin current sheets and a rapid intensification of the cross-tail current density at near-Earth distances during a short interval (<1 min) just before onset, after a period of sluggish growth (~0.5--1.5 hr). These observational features are described by a Hall MHD simulation in which we include the effects of the electron pressure gradient and the Hall current. These Hall MHD effects decouple the spatial scales of the parallel electric field and current density. The thin current sheet exhibits an impulsive pre-onset enhancement at near-Earth distances on a near-Alfv¿nic time scale, weakly dependent on the value of the Lundquist number. The amplitude of the pre-onset current sheet is larger, and its subsequent disruption faster than in a resistive MHD simulation. Agreement and discrepancies between theory and observation are discussed. ¿ 1998 American Geophysical Union