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Ann. Geophys., 19, 1533-1543, 2001
www.ann-geophys.net/19/1533/2001/
© European Geosciences Union 2001
First Cluster results of the magnetic field structure of the mid- and high-altitude cusps
P. J. Cargill, M. W. Dunlop, A. Balogh, and the FGM team
Space and Atmospheric Physics, The Blackett Laboratory, Imperial College, London SW7 2BZ, UK
Abstract. Magnetic field
measurements from the four Cluster spacecraft from the mid- and high-altitude
cusp are presented. Cluster underwent two encounters with the mid-altitude cusp
during its commissioning phase (24 August 2000). Evidence for field-aligned
currents (FACs) was seen in the data from all three operating spacecraft from
northern and southern cusps. The extent of the FACs was of the order of 1 RE
in the X-direction, and at least 300 km in the Y-direction.
However, fine-scale field structures with scales of the order of the spacecraft
separation (300 km) were observed within the FACs. In the northern crossing,
two of the spacecraft appeared to lie along the same magnetic field line, and
observed very well matched signals. However, the third spacecraft showed
evidence for structuring transverse to the field on scales of a few hundred km.
A crossing of the high-altitude cusp from 13 February 2001 is presented. It is
revealed to be a highly dynamic structure with the boundaries moving with
velocities ranging from a few km/s to tens of km/s, and having structure on
timescales ranging from less than one minute up to several minutes. The cusp
proper is associated with the presence of a very disordered magnetic field,
which is entirely different from the magnetosheath turbulence.
Key words. Magnetospheric physics
(current systems; magnetopause, cusp, and boundary layers) – Space plasma
physics (discontinuities)
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