The role of solar wind density in cross polar cap potential saturation under northward interplanetary magnetic field

The role of solar wind density in the cross polar cap potential (CPCP) response under northward interplanetary magnetic field is investigated with observation-based global simulations. A rare event was reported by Clauer et al. (2016) during which the ionospheric electric field E-ISP does not saturate under extreme interplanetary electric field (IEF) of similar to 15 mV/m. While commonly utilized coupling functions based on IEF fail to provide an unambiguous explanation for the linear response, the Lyon-Fedder-Mobarry-Magnetosphere-Ionosphere Coupler/Solver model is used to explore the mechanisms in this study. The model first reproduces the observed linear features of the E-ISP. The simulated CPCP also responds linearly to IEF variations. A controlled simulation is designed with solar wind density artificially reduced to 10% of the observed value while all other parameters such as the IEF are kept the same. The controlled simulation shows saturation of the E-ISP as well as the CPCP. Further analysis shows the difference in the magnetosheath plasma beta, implying the distinct dominant forces between the two simulations. The Lopez magnetosheath force balance theory is used to explain the CPCP responses under different solar wind densities. This comparison study highlights the role of solar wind density in determining the magnetosphere-ionosphere response to extreme interplanetary drivings.