A low- (four-) dimensional state space model for the basic energy components of the night-side magnetosphere is developed based on truncated descriptions of the collisionless microscopic (Hamiltonian) energy transfer processes occurring in the quasi-neutral layer. The substorm trigger, due to bifurcation of the system either from magnetic reconnection or from ballooning-mirror modes, is modeled by a fast unloading above a critical current. For constant southward interplanetary magnetic field solar wind input the system performs oscillations with recurrence times in the range of 60--70 min. For northward IMF the system decays to a static MHD equilibrium. For the intermediate case of a weak solar wind input there occur uncorrelated, isolated substorms. The theoretical basis for a low-dimensional model may be supported within the framework of self-organized criticality since the geotail is a highly stressed global system during the growth phase of substorms. The new energy-conserving state space model eliminates the ''dripping faucet'' unloading role of the Klimas et al. [1992> model and includes the parallel streaming kinetic energy in the substorm dynamics. The new model shows a wide range of dynamical behavior according to the values of the system parameters and the form of the solar wind input signal. ¿ American Geophysical Union 1996