ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-25-785-2007 Scattering of field-aligned beam ions upstream of Earth's bow shock Kis A. 1 5 Scholer M. 1 Klecker B. 1 Kucharek H. 2 Lucek E. A. 3 Rème H. 4 Max-Planck-Institut für extraterrestrische Physik, Garching, Germany Dept. of Physics and Institute for the Study of Earth, Oceans and Space, Univ. of New Hampshire, Durham, USA Imperial College, London, UK CESR, Toulouse, France now at: Geodetic and Geophysical Research Institute of HAS, Sopron, Hungary 29 03 2007 25 3 785 799 Copyright: © 2007 A. Kis et al. 2007 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/25/785/2007/angeo-25-785-2007.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/25/785/2007/angeo-25-785-2007.pdf

Field-aligned beams are known to originate from the quasi-perpendicular side of the Earth's bow shock, while the diffuse ion population consists of accelerated ions at the quasi-parallel side of the bow shock. The two distinct ion populations show typical characteristics in their velocity space distributions. By using particle and magnetic field measurements from one Cluster spacecraft we present a case study when the two ion populations are observed simultaneously in the foreshock region during a high Mach number, high solar wind velocity event. We present the spatial-temporal evolution of the field-aligned beam ion distribution in front of the Earth's bow shock, focusing on the processes in the deep foreshock region, i.e. on the quasi-parallel side. Our analysis demonstrates that the scattering of field-aligned beam (FAB) ions combined with convection by the solar wind results in the presence of lower-energy, toroidal gyrating ions at positions deeper in the foreshock region which are magnetically connected to the quasi-parallel bow shock. The gyrating ions are superposed onto a higher energy diffuse ion population. It is suggested that the toroidal gyrating ion population observed deep in the foreshock region has its origins in the FAB and that its characteristics are correlated with its distance from the FAB, but is independent on distance to the bow shock along the magnetic field.

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