Energy transfer across the magnetopause for northward and southward interplanetary magnetic fields

A three-dimensional adaptive magnetohydrodynamic (MHD) model is used to examine the energy flow from the solar wind to the magnetosphere. Using the model, we directly compute fluxes of mechanical and electromagnetic energy across the magnetopause surface. For northward IMF, most of the energy flux inflow occurs near the polar cusps on magnetopause. The viscous interaction leads the carrying energy plasma enter into high latitudes of the tail magnetopause and then divert to low-latitude regions tangentially, where the plasma gets cooler and denser near the flanks of plasma sheet. For southward IMF, the largest electromagnetic energy input into the magnetosphere occurs at the tail lobe behind the cusps, and largest mechanical energy input occurs at near-equatorial dayside magnetopause. Under southward IMF conditions, mechanical energy transfer is enhanced at the flanks of magnetopause in response to increased IMF magnitude, while more electromagnetic energy input can be identified as increasing solar wind density. Our results suggest that the mechanisms proposed to energy transfer are mainly due to reconnection and viscous interaction processes for northward IMF. For southward IMF, reconnection is the dominant factor in energy transfer. If the electromagnetic energy coupling between the solar wind and the magnetosphere can be interpreted as a proxy for the reconnection efficiency, the average efficiency during northward IMF is about 20% of that for southward IMF under the solar wind conditions we considered.