Diversion of plasma due to high pressure in the inner magnetosphere during steady magnetospheric convection

Abstract

Steady magnetospheric convection (SMC) events in the Earth's magnetosphere are thought to result from balancing the rate of opening flux through solar wind‐magnetosphere reconnection at the dayside magnetopause to the rate of closing flux through reconnection in the magnetotail. For this to occur, reconnected flux in the tail must return to the dayside to balance the dayside reconnection rate. Using Geotail and THEMIS data over a span of 14 years, we examine the average plasma conditions and fast Earthward flows during SMC intervals and compare them to other types of geomagnetic activity, such as quiet intervals, isolated substorm phases, and the two hours before an SMC (Pre‐SMC intervals). We show that the average total pressure in the inner magnetosphere is higher during SMC events than for other types of activity. This higher pressure region extends to larger radial distances, and causes fast Earthward flows to divert toward the dawn or dusk flanks and continue to the dayside. This pattern is contrasted to substorms, during which flows are directed toward the inner magnetosphere and flux remains there in the “pile‐up region.” We suggest that the SMC pattern of flow deflection carries enough flux from the tail to the dayside to allow for balanced reconnection. Finally, the Pre‐SMC intervals have plasma conditions that are similar to, but slightly weaker than, SMC events. Since most SMCs begin with a substorm, this indicates that preconditioning of the magnetosphere by prior geomagnetic activity is important in setting up the magnetotail for an SMC state.