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Ann. Geophys., 25, 99-115, 2007
www.ann-geophys.net/25/99/2007/
© European Geosciences Union 2007
Physics of the Ion Composition Boundary: a comparative 3-D hybrid simulation study of Mars and Titan
S. Simon1, A. Boesswetter1, T. Bagdonat1, and U. Motschmann1,2
1Institute for Theoretical Physics, TU Braunschweig, Germany
2Institute for Planetary Research, DLR, Berlin, Germany
Abstract. The plasma environments of Mars and Titan have been studied by means of a
3-D hybrid simulation code, treating the electrons as a
massless, charge-neutralizing fluid, whereas ion
dynamics are covered by a
kinetic approach. As neither Mars nor Titan possesses a significant intrinsic
magnetic field, the upstream plasma flow interacts directly with the
planetary ionosphere. The characteristic
features of the interaction region
are determined as a function of the alfvénic, sonic and magnetosonic Mach
number of the impinging plasma. For the Martian interaction with
the solar wind as well as for the case of Titan being located outside Saturn's
magnetosphere in times of high solar wind dynamic pressure, all three Mach
numbers are larger than 1. In such a scenario, the interaction
gives rise to a
so-called Ion Composition Boundary, separating the ionospheric plasma from the
ambient flow and being highly asymmetric with respect to the direction of the
convective electric field. The formation of these features is explained by
analyzing the Lorentz forces acting on ionospheric and ambient plasma
particles. Titan's plasma environment is highly variable and allows
various different combinations of the three Mach numbers. Therefore, the
Ion Composition Boundary may vanish under certain circumstances.
The relevant physical
mechanism is illustrated as a function of the Mach numbers in the upstream
plasma flow.
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