Polar cap Bifurcation During Steady-State Northward Interplanetary Magnetic Field With |BY|~BZ

We propose a polar cap configuration model that occurs for steady-sate northward interplanetary magnetic field (IMF) with |B_Y|~B_Z. When the IMF reconnects with the closed geomagnetic field, two types of open geomagnetic field lines are created. For the first type, the neutral point and the foot point are in the same hemisphere; for the second type the neutral point and the foot point are in opposite hemispheres. The latter type of field lines are draped against the magnetopause and subsequently slip through the magnetopause while undergoing antisunward motion. This field line motion results in intrusion of open magnetic flux into the flankside plasma sheet, and its ionospheric signature is the bifurcation of the polar cap. For B_Y>0 the intrusion occurs on the duskside (dawnside) in the northern (southern) ionosphere, and conversely for B_Y<0. The field lines which have slipped through the magnetopause into the plasma sheet then reconnect with closed geomagnetic field lines to complete a steady-state magnetic flux circulation.