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.