Necessity of substorm expansions in the initiation of steady magnetospheric convection

Steady magnetospheric convection (SMC) events occur during enhanced solar wind driving of the magnetosphere and are characterized by quasi‐stable convection, without any substorm expansions, for several hours. Previous research has hinted that a substorm onset may be required before the magnetosphere can enter the SMC mode of response. For the first time, we show statistically that nearly all SMC events are preceded by a substorm. Only 1% of SMC events occur without any preceding substorm signatures. The typical magnetospheric reaction to enhanced solar wind driving is a substorm. After this occurs, we find that the duration and stability of southward IMF determines whether a particular substorm will transition into an SMC event or not. The initial substorm sets up a high pressure region in the inner magnetosphere that causes flux diversion to the flanks, which allows closed flux from the nightside x‐line to return to balance the dayside reconnection rate.