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15
Astrophysical magnetic fields and nonlinear dynamo theory
, 2005
"... The current understanding of astrophysical magnetic fields is reviewed, focusing on their generation and maintenance by turbulence. In the astrophysical context this generation is usually explained by a selfexcited dynamo, which involves flows that can amplify a weak ‘seed ’ magnetic field exponen ..."
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The current understanding of astrophysical magnetic fields is reviewed, focusing on their generation and maintenance by turbulence. In the astrophysical context this generation is usually explained by a selfexcited dynamo, which involves flows that can amplify a weak ‘seed ’ magnetic field exponentially fast. Particular emphasis is placed on the nonlinear saturation of the dynamo. Analytic and numerical results are discussed both for small scale dynamos, which are completely isotropic, and for large scale dynamos, where some form of parity breaking is crucial. Central to the discussion of large scale dynamos is the socalled alpha effect which explains the generation of a mean field if the turbulence lacks mirror symmetry, i.e. if the flow has kinetic helicity. Large scale dynamos produce small scale helical fields as a waste product that quench the large scale dynamo and hence the alpha effect. With this in mind, the microscopic theory of the alpha effect is revisited in full detail and recent results for the loss of helical magnetic fields are reviewed.
LOCAL AND NONLOCAL MAGNETIC DIFFUSION AND ALPHAEFFECT TENSORS IN SHEAR FLOW TURBULENCE
, 2002
"... Various approaches to estimate turbulent transport coefficients from numerical simulations of hydromagnetic turbulence are discussed. A quantitative comparison between the averaged magnetic field obtained from a specific threedimensional simulation of a rotating turbulent shear flow in a slab and a ..."
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Cited by 6 (4 self)
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Various approaches to estimate turbulent transport coefficients from numerical simulations of hydromagnetic turbulence are discussed. A quantitative comparison between the averaged magnetic field obtained from a specific threedimensional simulation of a rotating turbulent shear flow in a slab and a simple onedimensional alpha–omega dynamo model is given. A direct determination of transport coefficients is attempted by calculating the correlation matrix of different components of the field and its derivatives. This matrix relates the electromotive force to physically relevant parameters like the tensor components of theeffect and the turbulent diffusivity. Theeffect operating on the toroidal field is found to be negative and of similar magnitude as the value obtained in previous work by correlating the electromotive force with the mean magnetic field. The turbulent diffusion of the toroidal field is comparable to the kinematic viscosity that was determined earlier by comparing the stress with the shear. However, the turbulent diffusion of the radial field component is smaller and can even be formally negative. The method is then modified to obtain the spectral dependence of the turbulent transport coefficients on the wavenumber. There is evidence for nonlocal behaviour in that most of the response comes from the smallest wavenumbers corresponding to the largest scale possible in the simulation. Again, the turbulent diffusion coefficient for the radial field component is small, or even negative, which is considered unphysical. However, when the diffusion tensor is assumed to be diagonal the radial component
Advances in Theory and Simulations of LargeScale Dynamos
, 2009
"... Recent analytical and computational advances in the theory of largescale dynamos are reviewed. The importance of the magnetic helicity constraint is apparent even without invoking meanfield theory. The tau approximation yields expressions that show how the magnetic helicity gets incorporated into ..."
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Cited by 4 (1 self)
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Recent analytical and computational advances in the theory of largescale dynamos are reviewed. The importance of the magnetic helicity constraint is apparent even without invoking meanfield theory. The tau approximation yields expressions that show how the magnetic helicity gets incorporated into meanfield theory. The testfield method allows an accurate numerical determination of turbulent transport coefficients in linear and nonlinear regimes. Finally, some critical views on the solar dynamo are being offered and targets for future research are highlighted.
Nonlinear shearcurrent dynamo and magnetic helicity transport in sheared turbulence. Geophys. Astrophys. Fluid Dynam
"... The nonlinear meanfield dynamo due to a shearcurrent effect in a nonhelical homogeneous turbulence with a mean velocity shear is discussed. The transport of magnetic helicity as a dynamical nonlinearity is taken into account. The shearcurrent effect is associated with the WTJ term in the mean el ..."
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Cited by 2 (1 self)
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The nonlinear meanfield dynamo due to a shearcurrent effect in a nonhelical homogeneous turbulence with a mean velocity shear is discussed. The transport of magnetic helicity as a dynamical nonlinearity is taken into account. The shearcurrent effect is associated with the WTJ term in the mean electromotive force, whereW is the mean vorticity due to the largescale shear motions and J is the mean electric current. This effect causes the generation of largescale magnetic field in a turbulence with large hydrodynamic and magnetic Reynolds numbers. The dynamo action due to the shearcurrent effect depends on the spatial scaling of the correlation time (k) of the background turbulence, where k is the wave number. For Kolmogorov scaling, ðkÞ / k2=3, the dynamo instability occurs, while when ðkÞ / k2 (small hydrodynamic and magnetic Reynolds numbers) there is no the dynamo action in a sheared nonhelical turbulence. The magnetic helicity flux strongly affects the magnetic field dynamics in the nonlinear stage of the dynamo action. Numerical solutions of the nonlinear meanfield dynamo equations which take into account the shearcurrent effect, show that if the magnetic helicity flux is not small, the saturated level of the mean magnetic field is of the order of the equipartition field determined by the turbulent kinetic energy. Turbulence with a largescale velocity shear is a universal feature in astrophysics, and the obtained results can be important for elucidation of origin of the largescale magnetic fields in astrophysical sheared turbulence.
The α Dynamo Effects in Laboratory Plasmas
, 2001
"... A concise review of observations of the α dynamo effect in laboratory plasmas is given. Unlike many astrophysical systems, the laboratory pinch plasmas are driven magnetically. When the system is overdriven, the resultant instabilities cause magnetic and flow fields to fluctuate, and their correlati ..."
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A concise review of observations of the α dynamo effect in laboratory plasmas is given. Unlike many astrophysical systems, the laboratory pinch plasmas are driven magnetically. When the system is overdriven, the resultant instabilities cause magnetic and flow fields to fluctuate, and their correlation induces electromotive forces along the mean magnetic field. This αeffect drives mean parallel electric current, which, in turn, modifies the initial background mean magnetic structure towards the stable regime. This driveandrelax cycle, or the socalled selforganization process, happens in magnetized plasmas in a time scale much shorter than resistive diffusion time, thus it is a fast and unquenched dynamo process. The observed αeffect redistributes magnetic helicity (a measure of twistedness and knottedness of magnetic field lines) but conserves its total value. It can be shown that fast and unquenched dynamos are natural consequences of a driven system where fluctuations are statistically either not stationary in time or not homogeneous in space, or both. Implications to astrophysical phenomena will be discussed. 1. Introduction Phenomena
Magnetic helicity fluxes in an α2 dynamo embedded in a halo. Geophys
 Astrophys. Fluid Dyn
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TURBULENT DYNAMOS DRIVEN BY THE MAGNETIC HELICITY FLUX by
, 2010
"... The problem of origin and maintenance of cosmic magnetic fields is a longstanding one. In this thesis we present results obtained by author in the field of magnetic dynamos. Using the incompressible MHD framework and direct numerical simulations, we contributed to answer the following questions: – w ..."
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Cited by 1 (0 self)
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The problem of origin and maintenance of cosmic magnetic fields is a longstanding one. In this thesis we present results obtained by author in the field of magnetic dynamos. Using the incompressible MHD framework and direct numerical simulations, we contributed to answer the following questions: – what are the necessary and the sufficient conditions for the large scale dynamo to exist – whether kinematic αeffect plays any role in the realistic dynamos – whether dynamo process is governed by the magnetic helicity transfer between scales and which components of helicity flux are responsible for this. Based on numerical simulations of driven MHD turbulence with an imposed large scale (LS) sinusoidal shear and small scale (SS) random forcing, we produced strong LS magnetic field through the dynamo action. The dynamo action is highly dependent on the spectral properties of the forcing function, producing either dynamo or antidynamo by choosing particular forcing for the SS turbulence. This dynamo does not require a continuous injection of fluid helicity of particular sign to exist. We suppose the LS dyna
Konkoly Observatory Hungarian Academy of Sciences
"... Helsinki University Print (Unigrafia) ..."
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Nonlinear
, 2006
"... shearcurrent dynamo and magnetic helicity transport in sheared turbulence ..."
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shearcurrent dynamo and magnetic helicity transport in sheared turbulence