Substorm onset caused by the two state transition of the magnetosphere

The substorm process is simulated by resistive magnetohydrodynamic (MHD) equations. Starting from a stationary solution under a northward interplanetary magnetic field (IMF) condition with non-zero IMF, B_y, changes in magnetospheric convection following the southward turning of the IMF are investigated. The simulation results first show the progress of plasma sheet thinning in the tail. This thinning is caused by the drain of closed flux from the near-earth plasma sheet coupled with the enhanced ionospheric convection. In this stage, the flux transported from the near-earth tail to the dayside cannot be replaced by the flux from the midtail because the reclosure process of open field lines in the midtail plasma sheet is still slow because of the control by the remnant of northward IMF. The substorm onset occurs as an abrupt shift of the pressure peak in the plasma sheet from -12 Re to -8 Re 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 formation of near-earth neutral line (NENL) at the midtail, together with plasma injection into the inner magnetosphere and an enhancement of the nightside field-aligned current (FAC). During the dipolarization process, the static magnetospheric force balance changes from the z-direction-dominated state to the x-direction-dominated state. Thus, the dipolarization is not a mere pile up of the flux ejected from the NENL. The pressure distribution in the near-earth plasma sheet changes so, as to self adjust the restored magnetic tension associated with the establishment of force balance in the x direction. The direct cause of the onset is the state (phase-space) transition of the convection system from a thinned state to a dipolarized state.