On the importance of antiparallel reconnection when the dipole tilt and IMF B y are nonzero

Several parameters may influence reconnection at the dayside magnetopause. They include the relative orientation of the magnetic fields in the magnetosheath and magnetosphere, the relative perpendicular velocities of field lines both before and after reconnection, and the location of the minimum geomagnetic field. We have used a three‐dimensional global MHD simulation of the magnetosphere to evaluate the relative importance of these parameters at the dayside magnetopause, when the interplanetary magnetic field (IMF) has a B y component and the dipole tilt is nonzero. For a purely southward IMF and finite tilt, reconnection occurs near the magnetic equator. The reconnection rate at the magnetic equator is smaller than the case with zero tilt because of increased magnetosheath plasma flow. For finite IMF B y the reconnection sites move away from the subsolar point. When we include a positive dipole tilt, the reconnection site in the summer hemisphere shifts sunward and equatorward while the one in the winter hemisphere moves tailward and away from the equator. Reconnection near the magnetic equator becomes less effective because IMF field lines move rapidly past the magnetic equator for the finite tilt. We have evaluated the importance of antiparallel reconnection by calculating the electric field at the magnetopause and found that antiparallel reconnection is more important than component reconnection for cases with finite dipole tilt and an IMF B y component.