Bow shock specularly reflected ions in the presence of low-frequency electromagnetic waves: a case study

• Main Authors: K. Meziane, C. Mazelle, M. Wilber, D. LeQuéau, J. P. Eastwood, H. Rème, I. Dandouras, J. A. Sauvaud, J. M. Bosqued, G. K. Parks, L. M. Kistler, M. McCarthy, B. Klecker, A. Korth, M.-B. Bavassano-Cattaneo, R. Lundin, A. Balogh
• Format: Article
• Language: English
• Published: Copernicus Publications 2004-07-01
• Series: Annales Geophysicae
• Online Access: https://www.ann-geophys.net/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_e</i> from the shock and the averaged bow shock &theta;_<i>Bn0</i> 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;₀~&theta;_<i>Bn0</i>, 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;_<i>Bn</i> 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;_<i>Bn</i> angles are less than ~40&deg;, consistent with the specular reflection hypothesis as the production mechanism of ions.