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Astron. Astrophys. 323, 999-1010 (1997)


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Differentially rotating relativistic magnetic jets

Asymptotic trans-field force-balance including differential rotation

Christian Fendt

1 Lund Observatory, Box 43, S-22100 Lund, Sweden (chris@astro.lu.se)
2 Landessternwarte, Königstuhl, D-69117 Heidelberg, Germany (cfendt@lsw.uni-heidelberg.de)

Received 15 October 1996 / Accepted 30 January 1997

Abstract

Highly collimated jets are observed in various astronomical objects, as active galactic nuclei, galactic high energy sources, and also young stellar objects. There is observational indication that these jets originate in accretion disks, and that magnetic fields play an important role for the jet collimation and plasma acceleration. The rapid disk rotation close to the central object leads to relativistic rotational velocities of the magnetic field lines.

The structure of these axisymmetric magnetic flux surfaces follows from the trans-field force-balance described by the Grad-Schlüter-Shafranov equation. In this paper, we investigate the asymptotic field structure of differentially rotating magnetic jets, widening the study by Appl & Camenzind (1993a, b).

In general, our results show that, with the same current distribution, differentially rotating jets are collimated to smaller jet radii as compared with jets with rigidly rotating field. Differentially rotating jets need a stronger net poloidal current in order to collimate to the same asymptotic radius. Current-free solutions are not possible for differentially rotating disk -jet magnetospheres with cylindrical asymptotics.

We present a simple analytical relation between the poloidal current distribution and magnetic field rotation law. A general relation is derived for the current strength for jets with maximum differential rotation and minimum differential rotation. Analytical solutions are also given in the case of a field rotation leading to a degeneration of the light cylinder.

By linking the asymptotic solution to a Keplerian accretion disk, 'total expansion rates' for the jets, and also the flux distribution at the foot points of the flux surfaces are derived. Large poloidal currents imply a strong opening of flux surfaces, a stronger gradient of field rotation leads to smaller expansion rates. There is indication that AGN jet expansion rates are less than in the case of protostellar jets. High mass AGN seem to have larger jet expansion rates than low mass AGN.

Key words: MHD – ISM: jets and outflows – galaxies: jets – stars: magnetic field – stars: mass loss – stars: pre-main sequence

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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