scholarly journals Electrodynamics of turbulent fluids with fluctuating electric conductivity

2020 ◽  
Vol 86 (3) ◽  
Author(s):  
G. Rüdiger ◽  
M. Küker ◽  
P. J. Käpylä
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Mean Field ◽  
Outer Core ◽  
Cycle Times ◽  
Solar Convection ◽  
Magnetic Diffusivity ◽  
Large Scale Magnetic Field ◽  
Alpha Effect ◽  
Turbulent Fluids

Consequences of fluctuating microscopic conductivity in mean-field electrodynamics of turbulent fluids are formulated and discussed. If the conductivity fluctuations are assumed to be uncorrelated with the velocity fluctuations then only the turbulence-originated magnetic diffusivity of the fluid is reduced and the decay time of a large-scale magnetic field or the cycle times of oscillating turbulent dynamo models are increased. If, however, the fluctuations of conductivity and flow in a certain well-defined direction are correlated, an additional diamagnetic pumping effect results, transporting the magnetic field in the opposite direction to the diffusivity flux vector $\langle \unicode[STIX]{x1D702}^{\prime }\boldsymbol{u}^{\prime }\rangle$ . In the presence of global rotation, even for homogeneous turbulence fields, an alpha effect appears. If the characteristic values of the outer core of the Earth or the solar convection zone are applied, the dynamo number of the new alpha effect does not reach supercritical values to operate as an $\unicode[STIX]{x1D6FC}^{2}$ -dynamo but oscillating $\unicode[STIX]{x1D6FC}\unicode[STIX]{x1D6FA}$ -dynamos with differential rotation are not excluded.

scholarly journals Oscillatory migratory large-scale fields in mean-field and direct simulations

2009 ◽  
Vol 5 (S264) ◽  
pp. 197-201
Author(s):  
Dhrubaditya Mitra ◽  
Reza Tavakol ◽  
Axel Brandenburg ◽  
Petri J. Käpylä
Keyword(s):  
Magnetic Field ◽  
Numerical Simulations ◽  
Large Scale ◽  
Mean Field ◽  
Direct Numerical Simulations ◽  
Mhd Equations ◽  
Perfect Conductor ◽  
Large Scale Magnetic Field ◽  
Wedge Shape ◽  
Polarity Reversals

AbstractWe summarise recent results form direct numerical simulations of both non-rotating helically forced and rotating convection driven MHD equations in spherical wedge-shape domains. In the former, using perfect-conductor boundary conditions along the latitudinal boundaries we observe oscillations, polarity reversals and equatorward migration of the large-scale magnetic fields. In the latter we obtain angular velocity with cylindrical contours and large-scale magnetic field which shows oscillations, polarity reversals but poleward migration. The occurrence of these behviours in direct numerical simulations is clearly of interest. However the present models as they stand are not directly applicable to the solar dynamo problem. Nevertheless, they provide general insights into the operation of turbulent dynamos.

scholarly journals Advection/diffusion of large scale magnetic field in accretion disks

2009 ◽  
Vol 16 (1) ◽  
pp. 77-81 ◽  
Author(s):  
R. V. E. Lovelace ◽  
G. S. Bisnovatyi-Kogan ◽  
D. M. Rothstein
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Turbulent Viscosity ◽  
Pressure Ratio ◽  
Rotational Instability ◽  
Small Scale ◽  
Magnetic Diffusivity ◽  
Large Scale Magnetic Field ◽  
Scale Field ◽  
Large Scale Field

Abstract. Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magneto-rotational instability or MRI) which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity which acts to prevent the build up of a significant large-scale field. Recent work has pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical (z) profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. We derive a sixth-order differential equation for the radial flow velocity vr(z) which depends mainly on the midplane thermal to magnetic pressure ratio β>1 and the Prandtl number of the turbulence P=viscosity/diffusivity. Boundary conditions at the disk surface take into account a possible magnetic wind or jet and allow for a surface current in the highly conducting surface layer. The stationary solutions we find indicate that a weak (β>1) large-scale field does not diffuse away as suggested by earlier work.

scholarly journals Helicity–vorticity turbulent pumping of magnetic fields in the solar dynamo

2012 ◽  
Vol 8 (S294) ◽  
pp. 367-368
Author(s):  
V. V. Pipin
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Differential Rotation ◽  
Large Scale ◽  
Convection Zone ◽  
Solar Dynamo ◽  
Solar Convection Zone ◽  
Solar Convection ◽  
Large Scale Magnetic Field ◽  
Convective Motions

AbstractThe interaction of helical convective motions and differential rotation in the solar convection zone results in turbulent drift of a large-scale magnetic field. We discuss the pumping mechanism and its impact on the solar dynamo.

scholarly journals A physical approach to modelling large-scale galactic magnetic fields

2019 ◽  
Vol 623 ◽  
pp. A113 ◽  
Author(s):  
Anvar Shukurov ◽  
Luiz Felippe S. Rodrigues ◽  
Paul J. Bushby ◽  
James Hollins ◽  
Jörg P. Rachen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Milky Way ◽  
Mean Field ◽  
Axially Symmetric ◽  
Dynamo Action ◽  
Disc Galaxy ◽  
Large Scale Magnetic Field ◽  
Dynamo Equation

Context. A convenient representation of the structure of the large-scale galactic magnetic field is required for the interpretation of polarization data in the sub-mm and radio ranges, in both the Milky Way and external galaxies. Aims. We develop a simple and flexible approach to construct parametrised models of the large-scale magnetic field of the Milky Way and other disc galaxies, based on physically justifiable models of magnetic field structure. The resulting models are designed to be optimised against available observational data. Methods. Representations for the large-scale magnetic fields in the flared disc and spherical halo of a disc galaxy were obtained in the form of series expansions whose coefficients can be calculated from observable or theoretically known galactic properties. The functional basis for the expansions is derived as eigenfunctions of the mean-field dynamo equation or of the vectorial magnetic diffusion equation. Results. The solutions presented are axially symmetric but the approach can be extended straightforwardly to non-axisymmetric cases. The magnetic fields are solenoidal by construction, can be helical, and are parametrised in terms of observable properties of the host object, such as the rotation curve and the shape of the gaseous disc. The magnetic field in the disc can have a prescribed number of field reversals at any specified radii. Both the disc and halo magnetic fields can separately have either dipolar or quadrupolar symmetry. The model is implemented as a publicly available software package GALMAG which allows, in particular, the computation of the synchrotron emission and Faraday rotation produced by the model’s magnetic field. Conclusions. The model can be used in interpretations of observations of magnetic fields in the Milky Way and other spiral galaxies, in particular as a prior in Bayesian analyses. It can also be used for a simple simulation of a time-dependent magnetic field generated by dynamo action.

scholarly journals Self-similar structure of resistive ADAFs with outflow and large-scale magnetic field

2020 ◽  
Vol 37 ◽  
Author(s):  
S. M. Ghoreyshi
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Field Component ◽  
Gas Temperature ◽  
Vertical Field ◽  
Magnetic Diffusivity ◽  
Large Scale Magnetic Field ◽  
Self Similar ◽  
Sgr A ◽  
Ideal Magnetohydrodynamic

Abstract The observations and simulations have revealed that large-scale magnetic field and outflows can exist in the inner regions of an advection-dominated accretion disc where the resistive diffusion may also be important. In the present paper, the roles of large-scale magnetic field and outflows in the structure of resistive advection-dominated accretion discs are explored by assuming that the accretion flow is radially self-similar. In the non-ideal magnetohydrodynamic (MHD) approximation, the results show that the angular velocity is always sub-Keplerian when both the outflow and the large-scale magnetic field are taken into account. A stronger toroidal field component leads to faster rotation, while the disc rotates with faster rate if the vertical field component is weaker. The increase of magnetic diffusivity causes the infall velocity to be close to Keplerian velocity. Although the previous studies in the ideal MHD approximation have shown that the disc temperature decreases due to the vertical field component, we find that the effect of vertical field component on the temperature of a resistive disc depends on the magnetic diffusivity. When the magnetic diffusivity is high, the more efficient mechanism for decreasing the disc temperature can be the outflows, and not the large-scale magnetic field. In such a limit of the magnetic diffusivity, the components of the large-scale magnetic field enhance the gas temperature. The increase of temperature can lead to heating and acceleration of the electrons and help us to explain the origin of phenomena such as the flares in Sgr A*. On the other hand, the infall velocity in such a limit rises as the temperature increases, and therefore the surface density falls to too low values. Any change in the density profile can alter the structure and the emitted spectrum of disc.

Lagrangian approach to the mean-field electrodynamics for turbulent fluids with arbitrary conductivities

1989 ◽  
Vol 208 ◽  
pp. 115-126 ◽  
Author(s):  
L. L. Kichatinov
Keyword(s):  
Turbulent Flow ◽  
Mean Field ◽  
Reynolds Numbers ◽  
Convective Transport ◽  
Lagrangian Approach ◽  
Compressible Turbulence ◽  
Magnetic Diffusivity ◽  
Alpha Effect ◽  
The Mean ◽  
Turbulent Fluids

A modification is made to the traditional Lagrangian approach to the derivation of the mean EMF of turbulent fluids which allows for finite conductivities. Consideration is confined to the case of homogeneous, isotropic but generally mirrornon-invariant and compressible turbulence. The eddy magnetic diffusivity and the coefficient α of the alpha-effect are expressed in terms of statistical moments of displacements of adjacent particles which undergo convective transport and microscopic diffusion in a turbulent flow. These expressions, being valid for arbitrary conductivities, reproduce known results in the cases of both very large and very small magnetic Reynolds numbers. Difficulties and advantages of the use of the results obtained for evaluations of the mean EMF are discussed.

scholarly journals Non-Linear Diamagnetic Transport of the Large-Scale Magnetic Field in the Solar Convection Zone

1993 ◽  
Vol 157 ◽  
pp. 49-50
Author(s):  
V.N. Krivodubskij
Keyword(s):  
Magnetic Field ◽  
Turbulence Intensity ◽  
Large Scale ◽  
Convection Zone ◽  
Back Reaction ◽  
Solar Convection Zone ◽  
Solar Convection ◽  
Large Scale Magnetic Field ◽  
Magnetic Buoyancy ◽  
The Mean

The mean magnetic field transport due to inhomogeneity of the turbulence intensity is considered taking the field back reaction on motion into account. In spite of the magnetic quenching, the downward diamagnetic pumping is still powerful enough to keep the fields of 3 to 4 kG strength near the SCZ base against the magnetic buoyancy.

scholarly journals The supernova-regulated ISM

2018 ◽  
Vol 614 ◽  
pp. A101 ◽  
Author(s):  
M. S. Väisälä ◽  
F. A. Gent ◽  
M. Juvela ◽  
M. J. Käpylä
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Dust Emission ◽  
Mean Field ◽  
Small Scale ◽  
Polarization Angle ◽  
Magnetic Fluctuations ◽  
Field Orientation ◽  
Mhd Simulations ◽  
Large Scale Magnetic Field

Context.Efforts to compare polarization measurements with synthetic observations from magnetohydrodynamics (MHD) models have previously concentrated on the scale of molecular clouds.Aims.We extend the model comparisons to kiloparsec scales, taking into account hot shocked gas generated by supernovae and a non-uniform dynamo-generated magnetic field at both large and small scales down to 4 pc spatial resolution.Methods.We used radiative transfer calculations to model dust emission and polarization on top of MHD simulations. We computed synthetic maps of column densityNH, polarization fractionp, and polarization angle dispersionS, and studied their dependencies on important properties of MHD simulations. These include the large-scale magnetic field and its orientation, the small-scale magnetic field, and supernova-driven shocks.Results.Similar filament-like structures ofSas seen in thePlanckall-sky maps are visible in our synthetic results, although the smallest scale structures are absent from our maps. Supernova-driven shock fronts andSdo not show significant correlation. Instead,Scan clearly be attributed to the distribution of the small-scale magnetic field. We also find that the large-scale magnetic field influences the polarization properties, such that, for a given strength of magnetic fluctuation, a strong plane of the sky mean field weakens the observedS, while strengtheningp. The anticorrelation ofpandS, and decreasingpas a function ofNHare consistent across all synthetic observations. The magnetic fluctuations follow an exponential distribution, rather than Gaussian characteristic of flows with intermittent repetitive shocks.Conclusions.The observed polarization properties and column densities are sensitive to the line-of-sight distance over which the emission is integrated. Studying synthetic maps as the function of maximum integration length will further help with the interpretation of observations. The effects of the large-scale magnetic field orientation on the polarization properties are difficult to be quantified from observations solely, but MHD models might turn out to be useful for separating the effect of the large-scale mean field.

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