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Ann. Geophys., 22, 971-983, 2004
www.ann-geophys.net/22/971/2004/
© European Geosciences Union 2004
Cluster observations of surface waves on the dawn flank magnetopause
C. J. Owen1, M. G. G. T. Taylor1,4, I. C. Krauklis1, A. N. Fazakerley1, M. W. Dunlop2,5, and J. M. Bosqued3
1Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
2Blackett Laboratory, Imperial College of Science Technology and Medicine, Prince Consort Road, London, SW7 2BZ, UK
3CESR/CNRS, BP 4346 9, Avenue Colonel Roche, 31028 Toulouse Cedex, France
4Now at Los Alamos National Laboratory, Los Alamos, NM, USA
5Now at Rutherford Appleton Laboratory, Didcot, Oxon, UK
Abstract. On 14 June 2001 the four Cluster spacecraft recorded multiple encounters of the dawn-side
flank magnetopause. The characteristics of the observed electron populations varied between
a cold, dense magnetosheath population and warmer, more rarified boundary layer population
on a quasi-periodic basis. The demarcation between these two populations can be readily
identified by gradients in the scalar temperature of the electrons. An analysis of the
differences in the observed timings of the boundary at each spacecraft indicates that these
magnetopause crossings are consistent with a surface wave moving across the flank
magnetopause. When compared to the orientation of the magnetopause expected from
models, we find that the leading edges of these waves are approximately 45° steeper than the
trailing edges, consistent with the Kelvin-Helmholtz (KH) driving mechanism. A stability
analysis of this interval suggests that the magnetopause is marginally stable to this mechanism
during this event. Periods in which the analysis predicts that the magnetopause is unstable
correspond to observations of greater wave steepening. Analysis of the pulses suggests that
the waves have an average wavelength of approximately 3.4 RE and move at an average speed
of ~65km s-1 in an anti-sunward and northward direction, despite the spacecraft location
somewhat south of the GSE Z=0 plane. This wave propagation direction lies close to
perpendicular to the average magnetic field direction in the external magnetosheath,
suggesting that these waves may preferentially propagate in the direction that requires no
bending of these external field lines
Key words. Magnetospheric physics (magnetospheric configuration
and dynamics; MHD waves and unstabilities; solar
wind-magnetosphere interactions)
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