EDP Sciences
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Volume 387, Number 2, May IV 2002
Page(s) 453 - 462
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20020383

A&A 387, 453-462 (2002)
DOI: 10.1051/0004-6361:20020383

The role of magnetic helicity transport in nonlinear galactic dynamos

N. Kleeorin1, D. Moss2, I. Rogachevskii1 and D. Sokoloff3

1  Department of Mechanical Engineering, Ben-Gurion University of Negev, POB 653, 84105 Beer-Sheva, Israel
    e-mail: nat@menix.bgu.ac.il
2  Department of Mathematics, University of Manchester, Manchester M13 9PL, UK
    e-mail: moss@maths.man.ac.uk
3  Department of Physics, Moscow State University, Moscow 119899, Russia
    e-mail: sokoloff@dds.srcc.msu.su

(Received 8 January 2002 / Accepted 8 March 2002)

We consider the magnetic helicity balance for the galactic dynamo in the framework of the local dynamo problem, as well as in the no- z model (which includes explicitly the radial distribution of the magnetic fields). When calculating the magnetic helicity balance we take into account the redistribution of the small-scale and large-scale magnetic fields between the magnetic helicities, as well as magnetic helicity transport and diffusion due to small-scale turbulence. We demonstrate that the magnetic helicity flux through the galactic disc boundaries leads to a steady-state magnetic field with magnetic energy comparable to the equipartition energy of the turbulent motions of the interstellar medium. If such flux is ignored, the steady-state magnetic field is found to be much smaller than the equipartition field. The total magnetic helicity flux through the boundaries consists of both an advective flux and a diffusive flux. The exact ratio of these contributions seems not to be crucial for determining the strength of the steady-state magnetic field and its structure. However at least some diffusive contribution is needed to smooth the magnetic helicity profile near to the disc boundaries. The roles of various transport coefficients for magnetic helicity are investigated, and the values which lead to magnetic field configurations comparable with those observed are determined.

Key words: galaxies: magnetic fields

Offprint request: I. Rogachevskii, gary@menix.bgu.ac.il

© ESO 2002