MAGNETOSPHERIC STRUCTURE WITH A SOUTHWARD INTERPLANETARY MAGNETIC-FIELD

A magnetospheric magnetic field that has a topological structure with one pair of neutral points and one segmental neutral line is studied. Such a magnetic field forms when the interplanetary field is southward . There is a surface current on an open-ended complete flux surface in this magnetospheric magnetic field. This current sheet on which the i nduced Chapman-Ferraro current flows is the outer separatrix, which se parates field lines attached to the Earth and field lines detached fro m the Earth. In addition, there is an inner separatrix, which separate s closed and open field Lines. The two neutral points are at the north and south apices of the common wall shared by the two separatrices, o n which the surface current is intense. The surface current is weak or even insignificant on other parts of the outer separatrix. The neutra l line is on the equatorial plane with two end points at the east and west apices of the common wall shared by the two separatrices. It is a separator at which closed, open, and detached field lines meet. The m agnetic topology indicates that field line reconnection, which can tak e place only at a separator, must occur away from the subsolar part of the dayside magnetopause in order for an open magnetosphere to remain in a quasisteady state.