A&A 439, 835-843 (2005)
DOI: 10.1051/0004-6361:20053221
Minimal tau approximation and simulations of the alpha effect
A. Brandenburg1 and K. Subramanian21 NORDITA, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark
2 IUCAA, Post Bag 4, Pune University Campus, Ganeshkhind, Pune 411 007, India
(Received 10 April 2005 / Accepted 30 May 2005)
Abstract
The validity of a closure called
the minimal tau approximation (MTA),
is tested in the context of dynamo theory,
wherein triple correlations are assumed to
provide relaxation of the turbulent electromotive force.
Under MTA, the alpha effect in mean field dynamo theory becomes
proportional to a relaxation time scale multiplied by
the difference between kinetic and current helicities.
It is shown that the value of the relaxation time is positive and,
in units of the turnover time at the forcing wavenumber,
it is of the order of unity.
It is quenched by the magnetic field - roughly independently of the
magnetic Reynolds number.
However, this independence becomes uncertain
at large magnetic Reynolds number.
Kinetic and current helicities are shown to be dominated by
large scale properties of the flow.
Key words: magnetohydrodynamics (MHD) -- turbulence
© ESO 2005
BibSonomy
CiteUlike
Connotea
Del.icio.us
Digg
Facebook