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Ann. Geophys., 22, 2301-2308, 2004
www.ann-geophys.net/22/2301/2004/
© European Geosciences Union 2004
On the origin of field-aligned beams at the quasi-perpendicular bow shock: multi-spacecraft observations by Cluster
H. Kucharek1, E. Möbius1, M. Scholer2, C. Mouikis1, L. M. Kistler1, T. Horbury3, A. Balogh3, H. Réme4, and J. M. Bosqued4
1Department of Physics and Institute for the Study of Earth, Oceans and Space (EOS) University of New Hampshire, Durham, NH 03824, USA
2MPI für extraterrestrische Physik, Garching, Germany
3Imperial College, London, UK
4CESR, Toulouse, France
Abstract. Two distinct populations of reflected and accelerated ions are known to
originate from quasi-perpendicular shocks, gyrating ions and reflected ion
beams. Recent observations under such bow shock conditions with Cluster have
shown strong evidence that both particle distributions appear to emerge from
the same reflection process. In this paper the basic production mechanism of
field-aligned beams has been investigated by using CLUSTER multi-spacecraft
measurements. We have analyzed several quasi-perpendicular shocks with the
Cluster Ion Spectrometry experiment (CIS) and followed the spatial and
temporal evolution of the reflected and transmitted ion populations across
the shock. These observations show that the field-aligned beams most likely
result from effective scattering in pitch angle during reflection in the
shock ramp. Investigating a low Mach number shock, leakage of a fraction of
the thermalized ion distribution in the downstream region does not appear to
be the source as the volume in phase space occupied by beam ions is empty
downstream of the shock ramp.
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