Stationary field-aligned MHD flows at astropauses and in astrotails. Principles of a counterflow configuration between a stellar wind and its interstellar medium wind

摘要

A stellar wind passing through the reverse shock is deflected into theastrospheric tail and leaves the stellar system either as a sub-Alfvenic or asa super-Alfvenic tail flow. An example is our own heliosphere and itsheliotail. We present an analytical method of calculating stationary,incompressible, and field-aligned plasma flows in the astrotail of a star. Wepresent a recipe for constructing an astrosphere with the help of only a fewparameters, like the inner Alfven Mach number and the outer Alfven Mach number,the magnetic field strength within and outside the stellar wind cavity, and thedistribution of singular points of the magnetic field within these flows.Within the framework of a one-fluid approximation, it is possible to obtainsolutions of the MHD equations for stationary flows from corresponding staticMHD equilibria, by using noncanonical mappings of the canonical variables. Thecanonical variables are the Euler potentials of the magnetic field ofmagnetohydrostatic equilibria. Thus we start from static equilibria determinedby the distribution of magnetic neutral points, and assume that the Alfven Machnumber for the corresponding stationary equilibria is finite. The topologicalstructure determines the geometrical structure of the interstellar gas -stellar wind interface. Additional boundary conditions like the outer magneticfield and the jump of the magnetic field across the astropause allowdetermination of the noncanonical transformations. This delivers the strengthof the magnetic field at every point in the astrotail region beyond the reverseshock. The mathematical technique for describing such a scenario is applied toastrospheres in general, but is also relevant for the heliosphere. It shows therestrictions of the outer and the inner magnetic field strength in comparisonwith the corresponding Alfven Mach numbers in the case of subalfvenic flows.