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Title:
Turbulent Dynamos in Spherical Shell Segments of Varying Geometrical Extent
Authors:
Mitra, Dhrubaditya; Tavakol, Reza; Brandenburg, Axel; Moss, David
Affiliation:
AA(Astronomy Unit, School of Mathematical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK ), AB(Astronomy Unit, School of Mathematical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK ), AC(NORDITA, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden ), AD(School of Mathematics, University of Manchester, Oxford Road, Manchester M13 9PL, UK)
Publication:
The Astrophysical Journal, Volume 697, Issue 1, pp. 923-933 (2009). (ApJ Homepage)
Publication Date:
05/2009
Origin:
IOP
Astronomy Keywords:
MHD, turbulence
DOI:
10.1088/0004-637X/697/1/923
Bibliographic Code:
2009ApJ...697..923M

Abstract

We use three-dimensional direct numerical simulations of the helically forced magnetohydrodynamic equations in spherical shell segments in order to study the effects of changes in the geometrical shape and size of the domain on the growth and saturation of large-scale magnetic fields. We inject kinetic energy along with kinetic helicity in spherical domains via helical forcing using Chandrasekhar-Kendall functions. We take perfect conductor boundary conditions for the magnetic field to ensure that no magnetic helicity escapes the domain boundaries. We find dynamo action giving rise to magnetic fields at scales larger than the characteristic scale of the forcing. The magnetic energy exceeds the kinetic energy over dissipative timescales, similar to that seen earlier in Cartesian simulations in periodic boxes. As we increase the size of the domain in the azimuthal direction, we find that the nonlinearly saturated magnetic field organizes itself in long-lived cellular structures with aspect ratios close to unity. These structures tile the domain along the azimuthal direction, thus resulting in very small longitudinally averaged magnetic fields for large domain sizes. The scales of these structures are determined by the smallest scales of the domain, which in our simulations is usually the radial scale. We also find that increasing the meridional extent of the domains produces little qualitative change, except a marginal increase in the large-scale field. We obtain qualitatively similar results in Cartesian domains with similar aspect ratios.
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