Origin of the interhemispheric potential mismatch of merging cells for interplanetary magnetic field B Y ‐dominated periods

When the dawn‐to‐dusk component of the interplanetary magnetic field (IMF B Y ) is dominant, ionospheric convection exhibits a basic two‐cell pattern with significant dawn‐dusk and interhemispheric asymmetries. For IMF B Y > 0 the duskside merging cell potential in the Northern Hemisphere is much higher than that in the Southern Hemisphere, and the dawnside merging cell potential in the Southern Hemisphere is much higher than that in the Northern Hemisphere. The situation is reversed for IMF B Y < 0. This interhemispheric potential mismatch originates from reconnection of overdraped lobe field lines and closed flankside field lines. This type of north‐south asymmetric reconnection does not affect the merging cell potentials in the same hemisphere as the reconnection point, whereas in the opposite hemisphere, it diminishes the potential of the dawnside (or duskside) Dungey‐type merging cell. Thus the total dawnside (or duskside) merging cell potential in one hemisphere is smaller than that in the other hemisphere by the reconnection voltage associated with the asymmetric reconnection.