The interchange cycle: A fundamental mode of magnetic flux circulation for northward interplanetary magnetic field

The interchange cycle is a mode of magnetic flux circulation that dominates when the interplanetary magnetic field (IMF) clock angle (θ_c ≡ atan2(B_Y, B_Z)) lies in the range of |θ_c| ≲ 30°. It consists of IMF-lobe reconnection in one hemisphere and lobe-closed reconnection in the other hemisphere, producing in the ionosphere an interchange-type merging cell in the former hemisphere and a reciprocal cell in the latter hemisphere. For a given IMF clock angle and a given dipole tilt angle, there usually exist two independent interchange cycles. Thus the two interchange cycles drive twin reverse cells in both hemispheres, but the category of each reverse cell (either an interchange-type merging cell or a reciprocal cell) depends on the given conditions. In this paper, we demonstrate how the reverse cell pattern changes systematically depending on the IMF orientation and the dipole tilt.