The role of the Y-component of the interplanetary magnetic field in transpolar saturation and ring current response as found in data and simulation

Energy is transferred into the magnetosphere of Earth through merging of magnetic fields (Dungey cycle and lobe cell convection) and through momentum transfer across the magnetopause (viscous interactions). These processes cause enhanced plasma convection in the inner magnetosphere and ionosphere, which in turn form the ring current and the transpolar potential, respectively. These processes have been studied extensively with regard to the role of the north-south component of the interplanetary magnetic field (IMF). The role of the dawn-dusk IMF is less well known.;We use the LFM global MHD simulation, the CRCM, and data to quantify the transpolar potential and ring current responses to the dawn-dusk IMF (B y). As with the north-south IMF (Bz), we find the transpolar potential saturates for large values of B y. Furthermore, the transpolar potential saturates at approximately the same magnitude of the magnetic field, but with a much smaller transpolar potential value. This suggests the saturation of the transpolar potential depends on the balance of the forces in the magnetosheath rather than the region 1 current. Unlike with Bz, we find the ring current does not respond to changes in By. This indicates a decoupling of the plasma convection in the ionosphere from convection in the inner magnetosphere. We discuss the implications of these results for space physics.