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Ann. Geophys., 22, 2413-2429, 2004
www.ann-geophys.net/22/2413/2004/
© European Geosciences Union 2004
Cluster observations of magnetic field fluctuations in the high-altitude cusp
K. Nykyri1, P. J. Cargill1, E. Lucek1, T. Horbury1, B. Lavraud4, A. Balogh1, M. W. Dunlop2, Y. Bogdanova3, A. Fazakerley3, I. Dandouras4, and H. Rème4
1The Blackett Laboratory, Imperial College, London, UK
2Rutherford Appleton Laboratory, UK
3Mullard Space science laboratory, University College London, UK
4Centre d’Etude Spatiale des Rayonnements, Toulouse, France
Abstract. High-resolution (22 vector/s) magnetic field data from Cluster FGM instrument
are presented for the high-altitude cusp crossing on 17 March 2001.
Despite the quiet solar wind conditions, the cusp was filled with magnetic
field turbulence for much of the crossing. Large-scale fluctuations show some
correlation between spacecraft but the higher frequency fluctuations show no
correlation, indicating that the length scales of these waves are smaller
than the spacecraft separation (500km). In many intervals, there are clear
peaks in the wave power around the ion cyclotron frequency (~1Hz), and
there is some evidence for waves at the first harmonic of this frequency.
Both left- and right-hand polarised waves are found, with angles of
propagation with respect to the ambient magnetic field that range from
parallel to perpendicular. The regions of enhanced magnetic field
fluctuations appear to be associated with plasma flows possibly originating
from a lobe reconnection site. The most coherent, long lasting wave trains
with frequencies close to local ion cyclotron frequency occur at a boundary
between a sheared flow and a stagnant plasma.
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