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

• We examine solar wind dependence of energy coupling during intense northward IMFs.
• We derive a solar wind-magnetosphere coupling function during northward IMFs.
• We study the relative importance between viscous interaction and IMF |By| effect.

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.