Open and closed magnetic topology at the Earth's magnetopause for northward IMF

When the Interplanetary Magnetic Field is northward, magnetic reconnection occurs at the Earth's high latitude magnetopause in regions poleward of the Earth's magnetospheric cusps. In a previous study of Cluster plasma and magnetic field data, signatures of reconnection poleward of both cusps were investigated as a function of latitude. It was found that reconnection in at least one hemisphere occurs essentially all the time for northward IMF, creating an open magnetic topology. However, this open topology appears to persist at the dayside magnetopause at low latitudes. Using events from the previous study, this persistent open topology is investigated to determine if there is time for field lines to reconnect in the opposite hemisphere and create a closed magnetic topology. Field lines that reconnect poleward of the cusp in one hemisphere require some time to drape across the dayside magnetopause and possibly encounter the region poleward of the cusp in the opposite hemisphere. No systematic difference is observed among events where the spacecraft is near or far from the field line that passes through the subsolar point (i.e., the field line that spends the longest time at the dayside magnetopause). In addition to the electron data used in the previous study, ion data at the magnetopause are used to determine if they show evidence of long field line draping times. These results strongly suggest that open field lines convect away from the dayside magnetopause before they reconnect and close in the opposite cusp.