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Astron. Astrophys. 361, L33-L36 (2000)


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Letter to the Editor

Large scale dynamos with ambipolar diffusion nonlinearity

A. Brandenburg 1,2 and K. Subramanian 3

1 NORDITA, Blegdamsvej 17, 2100 Copenhagen O, Denmark
2 Department of Mathematics, University of Newcastle upon Tyne, NE1 7RU, UK
3 National Centre for Radio Astrophysics - TIFR, Poona University Campus, Ganeshkhind, Pune 411 007, India

Received 31 July 2000 / Accepted 17 August 2000

Abstract

It is shown that ambipolar diffusion as a useful model for nonlinearity leads to similar behaviour of large scale turbulent dynamos as full MHD. This is demonstrated using both direct simulations in a periodic box and a closure model for the magnetic correlation functions applicable to infinite space. Large scale fields develop via a nonlocal inverse cascade as described by the [FORMULA]-effect. However, magnetic helicity can only change on a resistive timescale, so the time it takes to organize the field into large scales increases with magnetic Reynolds number.

Key words: hydrodynamics – magnetic fields – Magnetohydrodynamics (MHD) – turbulence – stars: magnetic fields – galaxies: magnetic fields

Send offprint requests to: A. Brandenburg

© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000

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