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Ann. Geophys., 23, 3351-3364, 2005
www.ann-geophys.net/23/3351/2005/
© European Geosciences Union 2005
Dawn-dusk asymmetries and sub-Alfvénic flow in the high and low latitude magnetosheath
M. Longmore1, S. J. Schwartz2, J. Geach3, B. M. A. Cooling1, I. Dandouras4, E. A. Lucek2, and A. N. Fazakerley5
1Astronomy Unit, Queen Mary, University of London, Mile End Road, London, E1 4NS, UK
2Blackett Laboratory, Imperial College London, Prince Consort Road, London, SW7 2BW, UK
3Department of Physics, University of Durham, South Road, Durham, DH1 3LE, UK
4CESR-CNRS, 31028 Toulouse Cedex 4, France
5Mullard Space Science Laboratory, Univ. College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, London, UK
Abstract. We present the results of a statistical survey of the magnetosheath using
four years of Cluster orbital coverage. Moments of the plasma distribution
obtained from the electron and ion instruments together with magnetic field
data are used to characterise the flow and density in the magnetosheath. We
note two important differences between our survey and the gasdynamic model
predictions: a deceleration of the flow at higher latitudes close to the
magnetopause, resulting in sub-Alfvénic flow near the cusp, and a
dawn-dusk asymmetry with higher velocity magnitudes and lower densities
measured on the dusk side of the magnetosheath in the Northern Hemisphere.
The latter observation is in agreement with studies carried out by Paularena et al. (2001), Němeček
et al. (2000), and Šafránková et al. (2004). In equations of hydrodynamics for a single-component additon to
this we observe a reverse of this asymmetry for the Southern Hemisphere.
High-latitude sub-Alfvénic flow is thought to be a necessary condition
for steady state reconnection pole-ward of the cusp.
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