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Ann. Geophys., 19, 1421-1428, 2001
www.ann-geophys.net/19/1421/2001/
© European Geosciences Union 2001
Cluster magnetic field observations in the magnetosheath: four-point measurements of mirror structures
E. A. Lucek1, M. W. Dunlop1, T. S. Horbury1, A. Balogh1, P. Brown1, P. Cargill1, C. Carr1, K.-H. Fornaçon2, E. Georgescu3,4, and T. Oddy1
1Space and Atmospheric Physics, The Blackett Laboratory, Imperial College, London, UK
2Institut für Geophysik und Meteorologie, Technische Universität Braunschweig, Germany
3Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
4Institut of Space Sciences, Bucharest, Romania
Abstract. The Cluster spacecraft
have returned the first simultaneous four-point measurements of the
magnetosheath. We present an analysis of data recorded on 10 November 2000,
when the four spacecrafts observed an interval of strong mirrorlike activity.
Correlation analysis between spacecraft pairs is used to examine the scale size
of the mirror structures in three dimensions. Two examples are presented which
suggest that the scale size of mirror structures is ~ 1500–3000 km along the
flow direction, and shortest along the magnetopause normal (< 600 km),
which, in this case, is approximately perpendicular to both the mean magnetic
field and the magnetosheath flow vector. Variations on scales of ~ 750–1000
km are found along the maximum variance direction. The level of correlation in
this direction, however, and the time lag observed, are found to be variable.
These first results suggest that variations occur on scales of the order of the
spacecraft separation ( ~ 1000 km) in at least two directions, but analysis of
further examples and a statistical survey of structures observed with different
magnetic field orientations and tetrahedral configurations will enable us to
describe more fully the size and orientation of mirror structures.
Key words. Magnetosphenic physics (magnetosheath;
plasma waves and instabilities)
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