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 EISP does not saturate under extreme interplanetary electric field (IEF) of ∼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 EISP. 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 EISP as well as the CPCP. Further analysis shows the difference in the magnetosheath plasma β, 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. Plain Language Summary The cross polar cap potential is the electrical potential imposed on the Earth by solar wind driver. It measures the electrodynamic coupling between the Earth and the solar wind. Typically, this potential is thought to depend mostly on the electric field in the solar wind and would saturate when the solar wind electric field exceeds certain threshold. Recently, a rare case was found when the potential did not saturate under very large solar wind electric field. We use global magnetosphere‐ionosphere model to investigate this event and found the solar wind density plays a critical role in affecting the potential response under the extreme driving conditions. Key Points A rare event of linear CPCP response to very large interplanetary electric field is investigated Global magnetosphere‐ionosphere simulations show that the high solar wind density contributes to the linear CPCP response in the presence of strong IEF The magnetosheath force balance theory is applicable under extreme driving conditions