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Ann. Geophys., 23, 2605-2620, 2005
www.ann-geophys.net/23/2605/2005/
© European Geosciences Union 2005
Cluster observations of bounday layer structure and a flux transfer event near the cusp
R. C. Fear1, A. N. Fazakerley1, C. J. Owen1, A. D. Lahiff1, E. A. Lucek2, A. Balogh2, L. M. Kistler3, C. Mouikis3, and H. Rème4
1Mullard Space Science Laboratory, 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
3Space Science Center, University of New Hampshire, Durham, NH 03824, USA
4CESR/CNRS, 9 Avenue du Colonel Roche, B.P. 4346, F-31028, Toulouse Cedex 4, France
Abstract. On the 25th January 2002 between 10:00 and 12:00 UT, the four Cluster
spacecraft passed through the northern high-latitude cusp, the dayside
magnetosphere and into the magnetosheath in a linear formation. In the
magnetosphere the PEACE electron spectrometers on the four spacecraft all
observed a series of transient bursts of magnetosheath-like plasma, but
without bipolar magnetic signatures in the magnetopause normal component as
might be expected if the plasma had been injected by transient reconnection
(flux transfer events – FTEs). Reordering the data using the magnetopause
transition parameter reveals that these plasma observations, the related
variations in the magnetic field and the balance of magnetic and thermal gas
pressures are consistent with transient entries into a stable high-latitude
boundary layer structure. However, once some of the spacecraft entered the
magnetosheath, FTE signatures were observed outside the magnetopause at the
same time as some of the boundary layer entries occurred at the other
spacecraft inside. Thus, (a) the lack of a bipolar BN signature is
inconsistent with the traditional picture of a magnetospheric FTE, and (b)
the cause of the observed entry of the spacecraft into the boundary layer
(pressure pulse or passing magnetosheath FTE) can only be determined by
spacecraft observations in the magnetosheath.
Keywords. Magnetospheric physics (Magnetopause, cusp
and bondary layers; Solar wind- magnetosphere interactions;
Magnetosheath)
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