Substorm convection and current system deduced from the global simulation

The substorm mechanism is investigated by analyzing the numerical results obtained through the use of the recently refined magnetohydrodynamic model. After showing the growth phase features, the numerical solution reproduces the observed signatures of a substorm onset, including the formation of a near‐Earth neutral line (NENL), earthward directed flow in the plasma sheet, a dipolarization, a geosynchronous D deflection, the development of the nightside field aligned currents (FACs), and electrojets in the ionosphere. The onset is triggered by a sudden collapse of the plasma sheet and a successive formation of a high‐pressure region in the inner magnetosphere. The energy source of this high‐pressure region is the magnetic tension released from the NENL. The tail disturbance is primary transmitted to the ionosphere by the region 2 FAC accompanying the high‐pressure region formed in the inner magnetosphere. The associated nightside region 1 FAC is not connected to the traditional current wedge but to the cusp region. The region 1 FAC path to the ionosphere develops from dayside to nightside, so as to construct a grand loop with the region 2 FAC from the partial ring current. In the grand loop, the region 1 FAC provides a short circuit in the ionosphere for the region 2 FAC. In the expansion phase, convection enhances to relax the distorted pressure distribution formed at the onset, accompanied by a further development of dipolarization and a thickening of the plasma sheet in the magnetosphere and increases in the westward and eastward electrojets in the ionosphere. The present model can explain the explosive growth phase and the thinning of the midtail plasma sheet just after the onset.