Explosion-type dynamics of the magnetotail current sheet: Role of mode coupling in substorm onsets

Slow variation of magnetotail equilibrium on the growth phase cannot bring the system to a strongly unstable state that could be responsible for the substorm onset. The clue to the problem lies in the nonlinear dynamics of the system. A catastrophe of equilibrium takes place when the system gets close to the marginal stability point, during its quasi-static evolution on the growth phase. The particular dynamical path of the catastrophe, and the corresponding short time scales, can vary. A "rigid excitation" of a tearing-mode-type disturbance can result of a feedback coupling of that mode with coherently modulated cross-field current driven instability (CCI) of high-frequency (HF) plasma turbulence. That produces a momentum exchange of the latter with the electron component, thus suppressing the electron compressibility stabilizing effect for tearing. The thinner becomes the current sheet (CS), due to its quasi-static evolution, the lower is the threshold amplitude of the needed initial, seed disturbance. A "soft excitation" path is also possible. An HF mode existing due to CCI, can develop in a nonlinear fashion, with a feature of explosive instability. The nonlinearity is provided here by existence of a specific feedback with the tearing type disturbance near the marginally stable state of the latter. In another scenario, the existence of the HF turbulence is insignificant; the CS thinning develops up to the point of marginal stability for the kinetic, "laminar" tearing mode; then that mode itself grows up in a nonlinear manner.