A numerical simulation of the geomagnetic sudden commencement: 3. A sudden commencement in the magnetosphere‐ionosphere compound system

Global plasma flow convections in the magnetosphere and in the ionosphere, field‐aligned current systems, ionospheric currents, conversion between plasma internal energy and electromagnetic energy, and plasma population regimes are in a self‐consistent state in the steady magnetosphere‐ionosphere region. This system is called the magnetosphere‐ionosphere compound system (Tanaka, 2003). In the context of the compound system, a geomagnetic sudden commencement (SC) is interpreted in terms of transition from the state in the pre‐SC period to that in the post‐SC period. The state transition is featured with the SC transient cell convection and the transient, small‐scale pressure enhancement in the magnetosphere. It is noted that destroy and recovery of the global self‐consistency are associated with formation and extinction of these transient phenomena. The transition phase is also characterized with transiently enhanced field‐aligned current in the ionosphere. These features commonly appear in the state transition phenomena like the theta aurora that is invoked by a sudden change of the By component of the solar wind magnetic field. This paper also discusses that a SC is terminated with recovery of the Region 2 current.