Nonlinear Response of the Cross Polar Cap Potential to Solar Wind Density Under Northward Interplanetary Magnetic Field

Abstract

It is commonly believed that the magnitude and orientation of interplanetary magnetic field (IMF) together with the solar wind (SW) velocity have the most important impact on the cross polar cap potential ( pc ), so that little attention has been given to the effect of SW density, especially under northward IMF conditions. Previous studies have shown that pc increases with SW density as a response to the changes in magnetosheath force balance, while our study shows that pc has more complicated responses to the SW density depending on the magnitude of IMF rather than a simple linear response as reported previously. The pc may be insensitive to SW density increasing at moderate IMF B z (cf. 8 nT) and at intense B z (20 nT) under large-density conditions. The different behavior of SW density in regulating pc is mainly due to the competing effects originated from viscous interaction and magnetic reconnection. Further, the physical mechanisms are explored, including the driving sources of the viscous potential and affecting factors of reconnection potential. These results pave the way for better understanding of the SW density effects on solar wind-magnetosphere-ionosphere (SW-M-I) interactions.Plain Language Summary Ionospheric electric potential in the polar region is primarily generated due to the interaction between solar wind (SW) and magnetosphere. The cross polar cap potential ( pc ), which represents difference between the maximum and minimum electric potentials in the polar cap, is closely related to the upstream interplanetary magnetic field and SW driving. SW density is one of the important dominant factors of pc , while its physical mechanism has not been fully understood. A systematic research is conducted by using the LFM magnetosphere model to clarify the mechanism of SW density affecting pc under purely northward interplanetary magnetic field (IMF). The simulation results demonstrate that pc responses differently to the increasing SW density under different IMF B z . Under small IMF B z , pc increases linearly with SW density, and as B z increases, pc becomes stable and changes little with SW density increasing. Under large IMF, pc first increases and then becomes insensitive. These results pave the way for better understanding of the SW density effects on SW-M-I interactions.