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Astron. Astrophys. 323, 634-643 (1997)

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Boundary conditions and critical surfaces in astrophysical MHD winds

S.V. Bogovalov

Moscow Engineering Physics Institute, Kashirskoje Shosse 31, 115409 Moscow, Russia

Received 9 January 1996 / Accepted 22 July 1996

Abstract

Boundary conditions and the nature of the critical surfaces for the problem of stationary MHD outflows are formulated from the point of view of the causality principle. It is shown that the number of the boundary conditions which have to be specified on the surface of the object ejecting plasma is equal to the number of MHD waves outgoing normally from this surface. The boundary conditions for objects of different nature are formulated. It is shown that the spherically symmetric Parker solution of the pure hydrodynamical outflow of the plasma from the surface of the star with a given temperature and density of the plasma on the surface of the star is not the unique solution. To pick out the unique solution of this problem an additional condition defining the tangential components of the velocity of the plasma on the surface of the star must be specified. The nature and properties of the critical surfaces are studied and the rules to determine the critical surfaces in MHD flow are proposed. In the general case, the critical surfaces where the general solution of the problem is singular are placed on characteristics. The slow and the fast mode critical surfaces do not coincide strictly speaking with the classical critical surfaces.

Key words: MHD – solar wind – pulsars – ISM; jets and outflows – galaxies; jets

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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