ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-22-2325-2004 Bow shock specularly reflected ions in the presence of low-frequency electromagnetic waves: a case study Meziane K. 1 Mazelle C. 2 Wilber M. 3 LeQuéau D. 2 Eastwood J. P. 4 Rème H. 2 Dandouras I. 2 Sauvaud J. A. 2 Bosqued J. M. 2 Parks G. K. 3 Kistler L. M. 5 McCarthy M. 6 Klecker B. 7 Korth A. 8 Bavassano-Cattaneo M.-B. 9 Lundin R. 10 Balogh A. 4 Physics Department, University of New Brunswick, Canada Centre d’Etudes Spatiales des Rayonnements, 9 Avenue du Colonel Roche, Toulouse, 31028, France Space Sciences Laboratory, University of California, Berkeley 94720, USA Space and Atmospheric Physics, The Blackett Laboratory, Imperial College, London, SW7 2BW, UK Space Science Center, University of New Hamshire, Durham, 03824, USA Earth and Space Sciences, University of Washington, Seattle, 98195, USA MPI für Extraterrestrische Physik, Garching, 85741, Germany MPI für Aeronomie, Katlenburg-Lindau, 37191, Germany CNR-IFSI, Roma, 00133, Italy Swedish Institute of Space Physics, Kiruna, 98128, Sweden 14 07 2004 22 7 2325 2335 Copyright: © 2004 K. Meziane et al. 2004 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/22/2325/2004/angeo-22-2325-2004.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/22/2325/2004/angeo-22-2325-2004.pdf

An energetic ion (<i>E</i>&le;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 <i>R<sub>e</sub></i> from the shock and the averaged bow shock &theta;<sub><i>Bn0</i></sub> is about ~30&deg;. 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 &alpha;<sub>0</sub>~&theta;<sub><i>Bn0</i></sub>, 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 &theta;<sub><i>Bn</i></sub> 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 &theta;<sub><i>Bn</i></sub> angles are less than ~40&deg;, consistent with the specular reflection hypothesis as the production mechanism of ions.