Annales Geophysicae www.ann-geophys.net 0992-7689 1432-0576 25 3 2007 10.5194/angeo-25-785-2007 http://www.ann-geophys.net/25/785/2007/ http://www.ann-geophys.net/25/785/2007/angeo-25-785-2007.html http://www.ann-geophys.net/25/785/2007/angeo-25-785-2007.pdf 785 799 2007-03-29 Scattering of field-aligned beam ions upstream of Earth's bow shock A. Kis M. Scholer mbs@mpe.mpg.de B. Klecker H. Kucharek E. A. Lucek H. Rème 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 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. 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