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Deutsche Physikalische Gessellschaft IOP Institute of Physics

Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbers

Focus on Magnetohydrodynamics and the Dynamo Problem

A A Schekochihin1,2,3,6, A B Iskakov4, S C Cowley1,4, J C McWilliams5, M R E Proctor3 and T A Yousef3

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Part of Focus on Magnetohydrodynamics and the Dynamo Problem

This paper is a detailed report on a programme of direct numerical simulations of incompressible nonhelical randomly forced magnetohydrodynamic (MHD) turbulence that are used to settle a long-standing issue in the turbulent dynamo theory and demonstrate that the fluctuation dynamo exists in the limit of large magnetic Reynolds number Rm Gt 1 and small magnetic Prandtl number Pm Lt 1. The dependence of the critical Rmc for dynamo versus the hydrodynamic Reynolds number Re is obtained for 1 lesssim Re lesssim 6700. In the limit Pm Gt 1, Rmc is at most three times larger than for the previously well established dynamo at large and moderate Prandtl numbers: Rmc lesssim 200 for Re gtrsim 6000 compared to Rmc ~ 60 for Pm \geqslant 1. The stability curve Rmc(Re) (and, it is argued, the nature of the dynamo) is substantially different from the case of the simulations and liquid-metal experiments with a mean flow. It is not as yet possible to determine numerically whether the growth rate of the magnetic energy is {\propto} Rm^{1/2} in the limit Re Gt Rm Gt 1, as should be the case if the dynamo is driven by the inertial-range motions at the resistive scale, or tends to an Rm-independent value comparable to the turnover rate of the outer-scale motions. The magnetic-energy spectrum in the low-Pm regime is qualitatively different from the Pm ≥ 1 case and appears to develop a negative spectral slope, although current resolutions are insufficient to determine its asymptotic form. At Rm \in(1, Rm_{\rm c}), the magnetic fluctuations induced via the tangling by turbulence of a weak mean field are investigated and the possibility of a k−1 spectrum above the resistive scale is examined. At low Rm < 1, the induced fluctuations are well described by the quasistatic approximation; the k−11/3 spectrum is confirmed for the first time in direct numerical simulations. Applications of the results on turbulent induction to understanding the nonlocal energy transfer from the dynamo-generated magnetic field to smaller-scale magnetic fluctuations are discussed. The results reported here are of fundamental importance for understanding the genesis of small-scale magnetic fields in cosmic plasmas.


PACS

91.25.Cw Origins and models of the magnetic field; dynamo theories

95.30.Qd Magnetohydrodynamics and plasmas

96.60.Hv Electric and magnetic fields

96.50.Tf MHD waves; plasma waves, turbulence

Subjects

Plasma physics

Environmental and Earth science

Astrophysics and astroparticles

Dates

Issue 8 (August 2007)

Received 2 February 2007

Published 31 August 2007



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