The state transition model of the substorm onset

The substorm mechanism is investigated from a resistive magnetohydrodynamic simulation under the assumption that the magnetotail becomes more diffusive as it goes further downtail. The simulation uses the finite volume total-variation diminishing scheme on an unstructured grid system to evaluate the magnetosphere-ionosphere coupling effect more precisely and to reduce the numerical viscosity in the near-Earth plasma sheet. The calculation started from a stationary solution under a northward interplanetary magnetic field (IMF) condition with non-zero IMF By. After a southward turning of the IMF the simulation results show the progress of plasma sheet thinning in the magnetosphere, together with increases in the size of the auroral oval and in the magnitude of the field aligned current (FAC) in the polar ionosphere. This thinning is promoted by the drain of closed flux from the plasma sheet occurring under the enhanced convection. In this stage the reclosure process in the plasma sheet which determines the flux piling up from the midtail to the near-Earth plasma sheet is not so effective, since it is still controlled by the remnant of northward IMF. The substorm onset occurs as an abrupt change of pressure distribution in the near-Earth plasma sheet and an intrusion of convection flow into the inner magnetosphere. After the onset the simulation results reproduce both the dipolarization in the near-Earth tail and the near-Earth neutral line (NENL) at the midtail, together with plasma injection into the inner magnetosphere and an enhancement of the nightside FAC. Dipolarization is hastened by a northward re-turning of the IMF, indicating that it is triggered off through the breakdown of dynamic stress balance in the near-tail plasma sheet established under the convection-controlling tail thinning. It is concluded that the direct cause of the onset is the dipolarization, which is not a mere pileup of the flux ejected from the NENL but the state (phase space) transition of the convection system from a thinned state to a dipolarized state associated with a self-organizing criticality. In this idea the NENL is a topologically controlled diffusive process and not directly responsible for the onset, although it constitutes an important part in the convection system necessary for the state transition. After the onset the simulation results show a thickening of the plasma sheet and a tailward retreating of the NENL. Under a continuously southward IMF this state with a thickened plasma sheet shows a signature of the steady magnetospheric convection event.

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