An MHD simulation of By‐dependent magnetospheric convection and field‐aligned currents during northward IMF

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Abstract
We have used a high‐resolution and time‐dependent three‐dimensional magnetohydrodynamic (MHD) model to study the interaction between the solar wind and the earth's magnetosphere during intervals when the interplanetary magnetic field (IMF) has both a By component and northward Bz component. When the IMF is northward, the plasma sheet thickens near the noon‐midnight meridian and extends projections into the northern and southern lobes. When projected onto the polar cap, this appears as a narrow channel extending from midnight toward noon. This plasma sheet extension from the nightside toward the dayside shifts toward dawn for By < 0 and toward dusk for By > 0. Two types of upward field‐aligned currents were found: an arc‐shaped current which extends around the auroral zone and a tail lobe region 1 current running almost parallel to the sun‐earth line. Like the plasma sheet extension the sun‐aligned tail lobe region 1 current moves across the northern polar cap from dusk to dawn when the IMF orientation changes from duskward to dawnward during northward IMF. At latitudes north of the region 1 current in the polar cap the currents have the same direction as the region 2 currents (i.e., upward at dawn and downward at dusk). As the IMF orientation changes from northward to dawnward, the currents rotate such that the upward current joins with the upward region 1 current, while the earthward current expands in the polar region. These magnetospheric features can be well explained by high‐latitude merging cells and tail lobe convection cells resulting from antiparallel merging between the northward IMF and the tail field. They also are consistent with several phenomena of theta aurora dynamics.