Solar wind dependence of energy coupling between solar wind and magnetosphere during intense northward IMFs

We statistically study the solar wind dependence of the magnetospheric activities during intense northward interplanetary magnetic fields (IMFs) events (Bz>10 nT, last over 3 h). It was found that the energy coupling between solar wind and magnetosphere during intense northward IMFs was mainly controlled by solar wind velocity and IMF clock angle (θ). A northward IMF coupling function was derived based on the dimensional analysis and quantitative analysis of the solar wind parameters and the geomagnetic indices. It was shown that when θ>∼45°, the energy input increases obviously as θ increases. Most importantly, it was also found that the relative importance between the viscous interaction and the IMF ∣By∣ effect in controlling the energy input into the magnetosphere during intense northward IMFs mainly depends on IMF clock angle. The viscous interaction will outweigh the IMF ∣By∣ effect in the northward IMF events with θ<∼75°. In contrast, for those northward IMF events with θ>∼75°, the IMF ∣By∣ effect may be more important than the viscous interaction.