Nonlinear mechanisms for the substorm explosion in the geomagnetic tail

An abrupt loss of equilibrium in the geomagnetic tail during activation of a substorm is mainly explained by the nonlinear dynamics of the system. A catastrophe of equilibrium takes place when the system approaches the marginal stability state during its quasistatic evolution at the growth phase. Several scenarios are possible for the particular form of a dynamical trajectory of this catastrophe as well as for corresponding short time scales. Two different mechanisms of the positive feedback of a tearing-mode disturbance with a high-frequency plasma turbulence, which is coherently modulated with this disturbance and generated by a cross-field current instability, are responsible for either "hard" or "soft" excitation of such a large-scale disturbance. In a different possible scenario of soft excitation, the existence of HF turbulence is of no importance; the magnetotail current sheet is thinned down to the point of marginal stability for the kinetic "laminar" tearing mode; then, this mode itself grows up in a nonlinear manner.