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

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 (phi(pc)), so that little attention has been given to the effect of SW density, especially under northward IMF conditions. Previous studies have shown that phi(pc) increases with SW density as a response to the changes in magnetosheath force balance, while our study shows that phi(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 phi(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 phi(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.