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Ann. Geophys., 23, 609-624, 2005
www.ann-geophys.net/23/609/2005/
© European Geosciences Union 2005
Cluster observations of sudden impulses in the magnetotail caused by interplanetary shocks and pressure increases
K. E. J. Huttunen1, J. Slavin2, M. Collier2, H. E. J. Koskinen1,3, A. Szabo2, E. Tanskanen2, A. Balogh4, E. Lucek4, and H. Rème5
1Department of Physical Sciences, P.O. Box 64, FIN-00014 University of Helsinki, Finland
2Laboratory for Extraterrestial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland
3Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
4Blackett Laboratory, Imperial College, London, England
5Centre d’Etude Spatiale des Rayonnements, Toulouse, France
Abstract. Sudden impulses (SI) in the tail lobe magnetic field associated with solar
wind pressure enhancements are investigated using measurements from Cluster.
The magnetic field components during the SIs change in a manner consistent
with the assumption that an antisunward moving lateral pressure enhancement
compresses the magnetotail axisymmetrically. We found that the maximum
variance SI unit vectors were nearly aligned with the associated
interplanetary shock normals. For two of the tail lobe SI events during which
Cluster was located close to the tail boundary, Cluster observed the inward
moving magnetopause. During both events, the spacecraft location changed from
the lobe to the magnetospheric boundary layer. During the event on 6 November
2001 the magnetopause was compressed past Cluster. We applied the
2-D Cartesian model developed by collier98 in which a
vacuum uniform tail lobe magnetic field is compressed by a step-like pressure
increase. The model underestimates the compression of the magnetic field, but
it fits the magnetic field maximum variance component well. For events for
which we could determine the shock normal orientation, the differences
between the observed and calculated shock propagation times from the location
of WIND/Geotail to the location of Cluster were small. The propagation speeds
of the SIs between the Cluster spacecraft were comparable to the solar wind
speed. Our results suggest that the observed tail lobe SIs are due to lateral
increases in solar wind dynamic pressure outside the magnetotail boundary.
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