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 By). The transpolar potential responds nonlinearly and saturates for large IMF By in the Lyon-Fedder-Mobarry (LFM) global MHD simulation, just as it does for large southward IMF (-Bz). 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 By. The magnitude of the saturated transpolar potential is significantly smaller for large IMF By than for large negative IMF Bz. 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 By as it is for large Bz. Furthermore, when the IMF (By or Bz) 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.