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Ann. Geophys., 19, 1399-1409, 2001
www.ann-geophys.net/19/1399/2001/
© European Geosciences Union 2001
Cluster magnetic field observations of the bowshock: Orientation, motion and structure
T. S. Horbury1, P. J. Cargill1, E. A. Lucek1, A. Balogh1, M. W. Dunlop1, T. M. Oddy1, C. Carr1, P. Brown1, A. Szabo2, and K.-H. Fornaçon3
1The Blackett Laboratory, Imperial College, London, UK
2Goddard Space Flight Center, Greenbelt, MD, USA
3Technische Universität Braunschweig, Braunschweig, Germany
Abstract. Four spacecraft Cluster
magnetic field observations of the low <beta> quasi-perpendicular
terrestrial bowshock are presented for the first time. Multiple
quasi-perpendicular crossings on 25 December 2000 are analysed. By combining
data from the four spacecraft, bowshock orientations and velocities can be
calculated. It is shown that, even while in rapid motion, the bowshock normal
direction remains remarkably constant, and that coplanarity estimates are
accurate to, typically, around 20°. Magnetic field magnitude profiles are
shown to be very well correlated between spacecraft although downstream waves
with fluctuations perpendicular to the local field, while statistically similar
at all four spacecraft, are poorly correlated on separation scales of several
hundred km. Examples are shown of a number of bowshock phenomena, including
non-standing fluctuations in the shock foot and the shock interacting with
changing solar wind conditions.
Key words. Interplanetary physics
(planetary bow shocks) Space plasma physics (shock waves; waves and
instabilities)
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