Saturation of transpolar potential for large y component interplanetary magnetic field

This study examines the response of the transpolar potential to a large Y component interplanetary magnetic field (IMF B_y). The transpolar potential responds nonlinearly and saturates for large IMF B_y in the Lyon-Fedder-Mobarry (LFM) global MHD simulation, just as it does for large southward IMF (-B_z). Data from Defense Meteorological Satellite Program (DMSP) satellites and Assimilative Mapping of Ionospheric Electrodynamics (AMIE) results confirm the saturation of the transpolar potential during large IMF B_y. The magnitude of the saturated transpolar potential is significantly smaller for large IMF B_y than for large negative IMF B_z. This indicates that transpolar potential saturation does not depend on the strength of the Region 1 current. However, the magnitude of the IMF at which the transpolar potential becomes nonlinear and begins to exhibit saturation behavior is the same for large B_y as it is for large B_z. Furthermore, when the IMF (B_y or B _z) reaches the value that produces saturation, the magnetosheath becomes magnetically dominated, with β < 1. This suggests that the saturation of the transpolar potential is related to a change in the force balance in the magnetosheath from a plasma-pressure-dominated magnetosheath to a magnetically-dominated magnetosheath.