Alfvén wings at Earth’s magnetosphere under strong interplanetary magnetic fields

A number of recent studies have shown that the upstream Mach number may play a significant role in the energy transfer between the solar wind and the magnetosphere. Magnetohydrodynamic (MHD) simulation results of the magnetosphere-ionosphere system are presented that show the transition from nominal solar wind and interplanetary magnetic field driving to extremely strong driving. One of the predominant features of the magnetosphere that becomes apparent during low Mach number conditions is the formation of Alfvén wings above and below the magnetosphere. Alfvén wing are cavities of low flow, and have been observed at Io and Ganymede, both of which reside in regions of sub-Alfvénic flow. It is shown that Alfvén wings exist even during nominal Mach number time periods – the wings fold over to form what has been classically viewed as the magnetotail. The regions of low flow within the Alfvén wing limit the electric field applied across the ionosphere, hence causing the ionospheric cross polar cap potential to be dependent upon the Mach number, and in turn, causing the saturation of the potential.