The role of tearing instability in substorm expansion onset is examined. From a consideration of observational constraints on the onset mechanism we conclude that this instability must have a considerable initial stage when the equilibrium magnetic field topology (without neutral lines) is still conserved. Moreover, this instability is shown to have no linear stage. Instead, either explicitly nonlinear or pseudolinear instability of negative energy eigenmodes can develop. The latter possibility may arise from electron preferential acceleration. This acceleration occurs for the electrons near the equilibrium chaos/adiabatic boundary that interact with the perturbation electric field and provide a means of dissipation to restore positive feedback for the instability development. We compute a set of single-particle orbits with realistic pitch angle distributions in a given tearing field model and evaluate statistical averages over the ensemble of particles and wave phases to verify the preferential acceleration of electrons. The resulting instability is analyzed by using a quasi-hydrodynamical multifluid model. Its amplitude can be less than that required for neutral line formation. It has a fast growth rate well within the timescales for substorm expansion onset. Its excitation leads to earthward acceleration of electrons and ions as well as local current amplification in the near-Earth region for excitation of cross-field current instability. In the theme of substorm as a low-dimension catastrophe for the current sheet, this nonlinear tearing instability approximates a fold catastrophe near the upper energy state of the system, while the unstable whistler waves from the cross-field current instability correspond to positive energy oscillations near a new lower-energy state of the current sheet achieved in the catastrophe. The nonlinear saturation of these whistler waves provides the irreversible relaxation of this state.¿ 1997 American Geophysical Union