A two-dimensional, low-dissipation magnetohydrodynamic simulation is used as a model for magnetospheric substorm dynamics. The simulation begins from a Grad-Shafranov equilibrium of the magnetotail including the Earth's dipole field. This initial configuration is driven by a continuous electric field (of IMF origin) which induces equatorward flows. A thin current sheet develops algebraically in time, spanning Y-points that stretch from the mid-tail region (~30 RE) to the near-Earth region (~10 RE). In the late growth phase, rapid thinning of the sheet and near-explosive intensification of the current density are observed. The onset of a partial current disruption results from the formation of small islands in the near-Earth region. Numerical results are compared with observations. ¿ American Geophysical Union 1995