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

• Published in: Geophysical research letters 2020-04, Vol.47 (8), p.n/a
• Main Authors: Yang, Ziyi, Zhang, Binzheng, Lei, Jiuhou, Dang, Tong
• Format: Article
• Language: English
• Subjects: Computer simulation; Density; Electric potential; Interplanetary magnetic field; Ionosphere; Magnetic field; Magnetic fields; Magnetic reconnection; Magnetosheath; Magnetospheres; Magnetospheric-solar wind relationships; Nonlinear response; Orientation; polar cap potential; Polar caps; Polar environments; Polar regions; Saturn; Solar magnetic field; Solar wind; Solar wind density; Solar wind effects; Solar wind velocity; Wind effects
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL087559

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 (φpc), so that little attention has been given to the effect of SW density, especially under northward IMF conditions. Previous studies have shown that φpc increases with SW density as a response to the changes in magnetosheath force balance, while our study shows that φpc has more complicated responses to the SW density depending on the magnitude of IMF rather than a simple linear response as reported previously. The φpc may be insensitive to SW density increasing at moderate IMF Bz (cf. 8 nT) and at intense Bz (20 nT) under large‐density conditions. The different behavior of SW density in regulating φpc 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 (φpc), 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 φpc, 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 φpc under purely northward interplanetary magnetic field (IMF). The simulation results demonstrate that φpc responses differently to the increasing SW density under different IMF Bz. Under small IMF Bz, φpc increases linearly with SW density, and as Bz increases, φpc becomes stable and changes little with SW density increasing. Under large IMF, φpc first increases and then becomes insensitive. These results pave the way for better understanding of the SW density effects on SW‐M‐I interactions. Key Points φpc shows different responses to solar wind (SW) density under different interplanetary magnetic field (IMF) A combined parameter of IMF and solar wind density