Free access article

A&A 454, 797-810 (2006)
DOI: 10.1051/0004-6361:20053062

Stationary field-aligned MHD flows at astropauses and in astrotails

Principles of a counterflow configuration between a stellar wind and its interstellar medium wind

D. H. Nickeler^{1, 2, 3}, J. P. Goedbloed^{2, 4} and H.-J. Fahr<sup>3

1</sup>  Astronomical Institute, AV CR, Fricova 298, 25165 Ondrejov, Czech Republic
    e-mail: nickeler@asu.cas.cz
²  Astronomical Institute, University of Utrecht, Princetonplein 5, 3584 CC Utrecht, The Netherlands
³  Institute for Astrophysics and Extraterrestrial Research, Auf dem Hügel 71, 53121 Bonn, Germany
⁴  FOM-Institute for Plasma Physics, Edisonbaan 14, 3439 MN Nieuwegein, The Netherlands

(Received 14 March 2005 / Accepted 14 March 2006)

Abstract
Context.A stellar wind passing through the reverse shock is deflected into the astrospheric tail and leaves the stellar system either as a sub-Alfvénic or as a super-Alfvénic tail flow. An example is our own heliosphere and its heliotail.
Aims. We present an analytical method of calculating stationary, incompressible, and field-aligned plasma flows in the astrotail of a star. We present a recipe for constructing an astrosphere with the help of only a few governing parameters, like the inner Alfvén Mach number and the outer Alfvén Mach number, the magnetic field strength within and outside the stellar wind cavity, and the distribution of singular points (neutral points) of the magnetic field within these flows.
Methods. Within the framework of a one-fluid approximation, it is possible to obtain solutions of the governing MHD equations for stationary flows from corresponding static MHD equilibria, by using noncanonical mappings of the canonical variables. The canonical variables are the Euler potentials of the magnetic field of magnetohydrostatic equilibria. Thus we start from static equilibria determined by the distribution of magnetic neutral points, and assume that the Alfvén Mach number for the corresponding stationary equilibria is finite.
Results.The topological structure, i.e. the distribution of magnetic neutral points, determines the geometrical structure of the interstellar gas - stellar wind interface. Additional boundary conditions like the outer magnetic field and the jump of the magnetic field across the astropause allow determination of the noncanonical transformations. This delivers the strength of the magnetic field at every point in the astrotail/astrosheath region beyond the reverse shock.
Conclusions.The mathematical technique for describing such a scenario is applied to astrospheres in general, but is also relevant for the heliosphere. It shows the restrictions of the outer and the inner magnetic field strength in comparison with the corresponding Alfvén Mach numbers in the case of subalfvénic flows.

Key words: magnetohydrodynamics (MHD) -- plasmas -- methods: analytical -- stars: winds, outflows -- stars: magnetic fields

© ESO 2006

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