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Ann. Geophys., 21, 1289-1294, 2003
www.ann-geophys.net/21/1289/2003/
© European Geosciences Union 2003
The charge-exchange induced coupling between plasma-gas counterflows in the heliosheath
H. J. Fahr
Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
Abstract. Many hydrodynamic models
have been presented which give similar views of the interaction of the solar
wind plasma bubble with the counterstreaming partially ionized interstellar
medium. In the more recent of these models it is taken into account that the
solar and interstellar hydrodynamic flows of neutral atoms and protons are
coupled by mass-, momentum-, and energy-exchange terms due to charge exchange
processes. We shall reinvestigate the theoretical basis of this coupling here
by use of a simplified description of the heliospheric interface and describe
the main physics of the H-atom penetration through the more or less standing
well-known plasma wall ahead of the heliopause. Thereby we can show that the
type of charge exchange coupling terms used in up-to-now hydrodynamic
treatments unavoidably leads to an O-type critical point at the sonic point of
the H-atom flow, thus not allowing for a continuation of the integration of the
hydrodynamic set of differential equations. The remedy for this problem is
given by a more accurate formulation of the momentum exchange term for
quasi-and sub-sonic H-atom flows. With a refined momentum exchange term derived
from basic kinetic Boltzmann principles, we instead arrive at a characteristic
equation with an X-type critical point, allowing for a continuous solution from
supersonic to subsonic flow conditions. This necessitates that the often
treated problem of the propagation of inter-stellar H-atoms through the
heliosheath has to be solved using these newly derived, differently effective
plasma – gas friction forces. Substantially different results are to be
expected from this context for the filtration efficiency of the heliospheric
interface.
Key words. Interplanetary physics (heliopause
and solar wind termination; interstellar gas) – Ionosphere (plasma
temperature and density)
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