Reduction of viscous potential for northward interplanetary magnetic field as seen in the LFM simulation

The two primary methods responsible for solar wind magnetosphere coupling are magnetic reconnection and the viscous interaction. The viscous interaction is generated due to the antisunward dragging of plasma inside the magnetopause by the plasma flowing in the magnetosheath, creating a return flow deeper inside the magnetosphere and producing a circulation pattern. This viscous circulation pattern is mapped into the ionosphere via magnetic field lines, which results in ionospheric electric field in the nonrotating Earth's frame. We measure this interaction in terms of an electric potential, the viscous potential. In this paper, we use the results obtained from the Lyon‐Fedder‐Mobarry (LFM) simulation model during periods of purely northward interplanetary magnetic field (IMF) for different solar wind velocity and ionospheric conductivity, showing a reduction of the viscous potential with increasing magnitude of northward IMF. The viscous potential is found to settle around 5–10 kV for large + B z values. The decrease in viscous potential was found to be associated with a weak or nonexistent sunward plasma flow in the nightside plasmasheet. Instead, the return flow to the dayside occurs at high latitudes and is associated with the reconnection topology and dynamics that occur during northward IMF periods. We also show that the magnetosphere remains closed during purely northward IMF, except for two small regions—one on each hemisphere, where the magnetic reconnection occur. We argue that the reduction of the viscous potential is due to a reduction of the velocity shear across the magnetopause and the lack of sunward convection in the equatorial tail.