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Ann. Geophys., 25, 533-542, 2007
www.ann-geophys.net/25/533/2007/
© European Geosciences Union 2007
Alfvén wings at Earth's magnetosphere under strong interplanetary magnetic fields
A. J. Ridley
University of Michigan, Ann Arbor, MI, USA
Abstract. 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.
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