The role of magnetic reconnection in solar wind-magnetosphere coupling

Dungey's classic reconnecting magnetosphere model still provides the basic context for most theoretical and observational discussions of magnetopause reconnection. The two-dimensional version of Dungey's model seems to naturally explain the half-wave rectifier response of geomagnetic activity to the orientation of the Interplanetary Magnetic Field (IMF): When the IMF is southward, reconnection occurs near the subsolar magnetopause, generating a global magnetospheric convection pattern which is thought to be the ultimate cause of magnetospheric storms and substorms; when the IMF is northward, however, subsolar reconnection is "switched off", and reconnection occurs poleward of the cusps. Unfortunately, the two-dimensional version of Dungey's model does not generalize in a straightforward way to three dimensions. In three dimensions, nearly steady subsolar reconnection is topologically possible for all non-vanishing IMF clock angles (note that while the pure northward and pure southward orientations are topologically stable in two dimensions, they are unstable and, thus, not likely to ever be observed in three dimensions). Three-dimensional magnetohydrodynamics (MHD) simulations, on the other hand, demonstrate that the subsolar magnetopause stagnation flow becomes unstable as the IMF turns southward: Flux Transfer Events (FTEs) are generated, producing changes in the global magnetopause topology. This result suggests that the transition from steady to time-dependent reconnection may play an important role in changing the global convection pattern of the magnetosphere as the IMF turns southward. We explore this hypothesis using three-dimensional MHD simulations.