Study of the substorm: Part 2: Development of magnetosphere-ionosphere convection to the substorm

Abstract

It has been believed that the substorm is a manifestation of extraordinary plasma processes in the magnetosphere, such as instability, anomalous resistivity, and reconnection. In this paper, we show that this belief is a misleading concept and that the substorm must be understood as the development and transition of the convection system. Major observed signatures of the substorm have all become reproducible by the recent magnetosphere-ionosphere (M-I) coupling simulation. In order to understand the substorm as a change in convection system, we first study from these numerical solutions the energy conversion driving the convection and field-aligned current (FAC), namely the formation process of the dynamo. The dynamos for the region 1 and region 2 FACs are formed in the cusp-mantle region and inside the plasma sheet, respectively, and are driven by the expanding slow mode. These structures are unchanged even in the substorm case. The substorm onset is attributed to the phase space transition in the convection system, caused by a change in force balance in the plasma sheet. This process results in the formation of high-pressure region in the inner magnetosphere and an accompanying rapid increase in the region 2 FAC to cause the onset.