Based on Cluster satellite measurements, ionospheric satellite measurements, and a GGCM MHD simulation, we reconstruct the topology and convection of a northward interplanetary magnetic field reconnection event at the Earth's dayside magnetopause. We infer a reconnection parallel electric field of -4 mV/m from Cluster satellite measurements in the magnetopause. The simulation merging voltage varies spatially with values up to -4 mV/m but a value of -1.3 mV/m on average. The mapping to the magnetopause of the potential drop DMSP measures across sunward flows in the ionosphere agrees approximately with the merging length in the MHD simulation of the magnetopause. We observe a singular field line reconnection topology in both the Cluster data at the reconnection site and in the MHD simulation. Antiparallel reconnection occurs along the magnetix separator at the magnetic nulls and also on the open/closed and open/IMF magnetic separatrices. On the open/IMF separatrix, IMF field lines reconnect to open tail lobe field lines in the northern (summer) hemisphere and reclose onto closed tail field lines on the southern (winter) open/closed separatrix. Along the separator at the magnetic nulls, IMF field lines reconnect to closed tail field lines in the northern hemisphere and reclose onto open tail field lines on the southern open/closed separatrix. Overdraping leads to quasi-steady convection of tail lobe field lines that have reconnected on northern hemisphere field lines and reclose on southern hemisphere tail field lines. Separatrix reconnection and a tearing mode make for a significant diffusion region on the magnetopause.