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Ann. Geophys., 19, 1449-1460, 2001
www.ann-geophys.net/19/1449/2001/
© European Geosciences Union 2001
Cluster observes the Earth’s magnetopause: coordinated four-point magnetic field measurements
M. W. Dunlop1, A. Balogh1, P. Cargill1, R. C. Elphic2, K.-H. Fornaçon3, E. Georgescu4, F. Sedgemore-Schulthess5, and the FGM team
1Space and Atmospheric Physics Group, Imperial College, London SW7 2BZ, UK
2Los Alamos National Laboratory, Los Alamos, NM 87545, USA
3Institut für Geophysik und Meteorologie, TUB, Braunschweig, Germany
4Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
5DSRI, Copenhagen, Denmark
Abstract. The four-spacecraft
Cluster mission has provided high-time resolution measurements of the magnetic
field from closely maintained separation distances (200–600 km). Four-point
coverage of the Earth’s magnetopause began on the 9 and 10 November 2000 when
all spacecraft first exited the dusk-side magnetosphere at about 19:00 LT,
providing extensive coverage of the near flank magnetosheath and magnetopause
boundary layer on re-entry to the magnetosphere. The traversals on this
occasion were caused by the arrival of an intense CME at the Earth, which
produced a large compression of the magnetopause and high magnetic activity.
The magnetopause traversals represent an unprecedented data set, allowing
detailed analysis of the local magnetic structure (gradients) and dynamics of
the magnetopause boundary. By performing minimum variance analysis (MVA) on the
magnetic field data from all four spacecraft, we demonstrate that the
magnetopause was planar on the scale of the spacecraft separation scales and
that the transverse scale size of the magnetopause boundary layer was 1000–1100
km. We also show that the motion of the boundary (defined by the magnetic shear
at the current layer), is changing over the sequence of spacecraft crossings so
that acceleration of the magnetopause can be very high in this region of the
magnetosphere. Indeed, the magnetopause speed reaches the order of 300 km/s in
response to the arrival of the interplanetary shock. Using MVA coordinates, we
have identified a number of magnetospheric and magnetosheath FTE signatures,
which are sampled simultaneously by all spacecraft at different distances from
and on either side of the magnetopause. The signatures show a variation of
scale with distance from the boundary.
Key words. Magnetospheric physics
(magnetopause, cusp and boundary layers) Space plasma physics (discontinuities;
magnetic reconnection)
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