Earth Planets Space, Vol. 64 (No. 2), pp. 61-71, 2012
M. Yamauchi1, Y. Futaana1, A. Fedorov2, R. A. Frahm3, E. Dubinin4, R. Lundin1, J.-A. Sauvaud2, J. D. Winningham3, S. Barabash1, and M. Holmström1
1Swedish Institute of Space Physics, Box 812, S-98128 Kiruna, Sweden
2Centre d'Etude Spatiale des Rayonnements, Université de Toulouse, F-31038 Toulouse, France
3Southwest Research Institute, San Antonio, TX 78228-0510, USA
4Max-Planck-Institut für Sonnensystemforschung, D-37191 Katlenburg-Lindau, Germany
(Received February 4, 2011; Revised May 27, 2011; Accepted July 12, 2011; Online published March 8, 2012)
The ion mass analyzer (IMA) on board Mars Express revealed bundled structures of ions in the energy domain within a distance of a proton gyroradius from the Martian bow shock. Seven prominent traversals during 2005 were examined when the energy-bunched structure was observed together with pick-up ions of exospheric origin, the latter of which is used to determine the local magnetic field orientation from its circular trajectory in velocity space. These seven traversals include different bow shock configurations: (a) quasi-perpendicular shock with its specular direction of the solar wind more perpendicular to the magnetic field (QT), (b) quasi-perpendicular shock with its specular reflection direction of the solar wind more along the magnetic field (FS), and (c) quasi-parallel (QL) shock. In all seven cases, the velocity components of the energy-bunched structure are consistent with multiple specular reflections of the solar wind at the bow shock up to at least two reflections. The accelerated solar wind ions after two specular reflections have large parallel components with respect to the magnetic field for both QL cases whereas the field-aligned speed is much smaller than the perpendicular speed for all QT cases.
Key words: Mars, bow shock, acceleration.