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Ann. Geophys., 22, 2325-2335, 2004
www.ann-geophys.net/22/2325/2004/
© European Geosciences Union 2004
Bow shock specularly reflected ions in the presence of low-frequency electromagnetic waves: a case study
K. Meziane1, C. Mazelle2, M. Wilber3, D. LeQuéau2, J. P. Eastwood4, H. Rème2, I. Dandouras2, J. A. Sauvaud2, J. M. Bosqued2, G. K. Parks3, L. M. Kistler5, M. McCarthy6, B. Klecker7, A. Korth8, M.-B. Bavassano-Cattaneo9, R. Lundin10, and A. Balogh4
1Physics Department, University of New Brunswick, Canada
2Centre d’Etudes Spatiales des Rayonnements, 9 Avenue du Colonel Roche, Toulouse, 31028, France
3Space Sciences Laboratory, University of California, Berkeley 94720, USA
4Space and Atmospheric Physics, The Blackett Laboratory, Imperial College, London, SW7 2BW, UK
5Space Science Center, University of New Hamshire, Durham, 03824, USA
6Earth and Space Sciences, University of Washington, Seattle, 98195, USA
7MPI für Extraterrestrische Physik, Garching, 85741, Germany
8MPI für Aeronomie, Katlenburg-Lindau, 37191, Germany
9CNR-IFSI, Roma, 00133, Italy
10Swedish Institute of Space Physics, Kiruna, 98128, Sweden
Abstract. An energetic ion (E≤40) event observed by the CLUSTER/CIS experiment
upstream of the Earth's bow shock is studied in detail.
The ion event is observed in association with quasi-monochromatic ULF MHD-like waves,
which we show modulate the ion fluxes.
According to three statistical bow shock position models, the Cluster spacecrafts are
located at ~0.5 Re from the shock and the averaged bow shock θBn0 is about
~30°.
The analysis of the three-dimensional angular distribution indicates that ions
propagating roughly along the magnetic field direction are observed at the onset of the
event.
Later on, the angular distribution is gyrophase-bunched and the pitch-angle
distribution is peaked at α0~θBn0, consistent with the specular reflection
production mechanism.
The analysis of the waves shows that they are left-handed in the spacecraft frame
of reference (right-handed in the solar wind frame) and
propagate roughly along the ambient magnetic field; we have found that they are in
cyclotron-resonance with the field-aligned beam observed just upstream.
Using properties of the waves and particles, we explain the observed particle
flux-modulation in the context of θBn changes at the shock caused by the convected
ULF waves.
We have found that the high count rates coincide with particles leaving the shock
when θBn angles are less than ~40°, consistent with the specular
reflection hypothesis as the production mechanism of ions.
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