scholarly journals Mean Field Dynamo Saturation: Toward Understanding Conflicting Results

2002 ◽  
Vol 12 ◽  
pp. 736-738
Author(s):  
Eric G. Blackman ◽  
George B. Field
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Magnetic Energy ◽  
Mean Field ◽  
Magnetic Helicity ◽  
Boundary Terms ◽  
Helicity Conservation ◽  
The Galaxy ◽  
Mean Field Dynamo

AbstractMean field dynamos may explain the origin of large scale magnetic fields of galaxies, but controversy arises over the extent of dynamo quenching by the growing field. Here we explain how apparently conflicting results may be mutually consistent, by showing the role of magnetic helicity conservation and boundary terms usually neglected. We estimate the associated magnetic energy flowing out of the Galaxy but emphasize that the mechanism of field escape needs to be addressed.

scholarly journals Application of a helicity proxy to edge-on galaxies

2020 ◽  
Vol 496 (4) ◽  
pp. 4749-4759
Author(s):  
Axel Brandenburg ◽  
Ray S Furuya
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Mean Field ◽  
Numerical Results ◽  
Magnetic Helicity ◽  
The Galaxy ◽  
Rotationally Invariant ◽  
The Magnetic Field ◽  
Mean Field Dynamo

ABSTRACT We study the prospects of detecting magnetic helicity in galaxies by observing the dust polarization of the edge-on galaxy NGC 891. Our numerical results of mean-field dynamo calculations show that there should be a large-scale component of the rotationally invariant parity-odd B polarization that we predict to be negative in the first and third quadrants, and positive in the second and fourth quadrants. The large-scale parity-even E polarization is predicted to be negative near the axis and positive further away in the outskirts. These properties are shown to be mostly a consequence of the magnetic field being azimuthal and the polarized intensity being maximum at the centre of the galaxy and are not a signature of magnetic helicity.

scholarly journals Current helicity constraints in solar dynamo models

2012 ◽  
Vol 8 (S294) ◽  
pp. 313-318
Author(s):  
D. Sokoloff ◽  
H. Zhang ◽  
D. Moss ◽  
N. Kleeorin ◽  
K. Kuzanyan ◽  
...  
Keyword(s):  
Large Scale ◽  
Numerical Models ◽  
Mean Field ◽  
Active Regions ◽  
Solar Dynamo ◽  
Magnetic Helicity ◽  
Dynamo Model ◽  
Small Scale ◽  
Mean Field Dynamo ◽  
Current Helicity

AbstractWe investigate to what extent the current helicity distribution observed in solar active regions is compatible with solar dynamo models. We use an advanced 2D mean-field dynamo model with dynamo action largely concentrated near the bottom of the convective zone, and dynamo saturation based on the evolution of the magnetic helicity and algebraic quenching. For comparison, we also studied a more basic 2D mean-field dynamo model with simple algebraic alpha quenching only. Using these numerical models we obtain butterfly diagrams for both the small-scale current helicity and the large-scale magnetic helicity, and compare them with the butterfly diagram for the current helicity in active regions obtained from observations. This comparison shows that the current helicity of active regions, as estimated by −A·B evaluated at the depth from which the active region arises, resembles the observational data much better than the small-scale current helicity calculated directly from the helicity evolution equation. Here B and A are respectively the dynamo generated mean magnetic field and its vector potential.

scholarly journals Alpha-Quenched Alpha-Lambda Dynamos and the Excitation of Nonaxisymmetric Magnetic Fields

1993 ◽  
Vol 157 ◽  
pp. 147-151
Author(s):  
Dale M. Barker ◽  
David Moss
Keyword(s):  
Magnetic Fields ◽  
Differential Rotation ◽  
Large Scale ◽  
Meridional Circulation ◽  
Mean Field ◽  
Velocity Fields ◽  
Navier Stokes ◽  
Reynolds Stresses ◽  
Navier Stokes Equation ◽  
Mean Field Dynamo

We present calculations showing how stable nonaxisymmetric magnetic fields may be excited in an alpha-quenched mean field dynamo in a deep spherical shell. The large scale velocity fields (differential rotation, meridional circulation) are determined by solving the axisymmetric Navier-Stokes equation, neglecting the Lorentz force but including a term parameterizing the turbulent Reynolds stresses.

scholarly journals From convective to stellar dynamos

2010 ◽  
Vol 6 (S271) ◽  
pp. 279-287 ◽  
Author(s):  
Axel Brandenburg ◽  
Petri J. Käpylä ◽  
Maarit J. Korpi
Keyword(s):  
Magnetic Fields ◽  
Time Scale ◽  
Large Scale ◽  
Magnetic Helicity ◽  
Turbulent Eddies ◽  
Helicity Conservation

AbstractConvectively driven dynamos with rotation generating magnetic fields on scales large compared with the scale of the turbulent eddies are being reviewed. It is argued that such fields can be understood as the result of an α effect. Simulations in Cartesian domains show that such large-scale magnetic fields saturate on a time scale compatible with the resistive one, suggesting that the magnitude of the α effect is here still constrained by approximate magnetic helicity conservation. It is argued that, in the absence of shear and/or any other known large-scale dynamo effects, these simulations prove the existence of turbulent α2-type dynamos. Finally, recent results are discussed in the context of solar and stellar dynamos.

LARGE-SCALE COSMOLOGICAL MAGNETIC FIELDS AND MAGNETIC HELICITY

2005 ◽  
Vol 14 (11) ◽  
pp. 1839-1854 ◽  
Author(s):  
V. B. SEMIKOZ ◽  
D. D. SOKOLOFF
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Early Universe ◽  
Galaxy Formation ◽  
Large Scale ◽  
Magnetic Helicity ◽  
Observational Estimate ◽  
Field Production

Role of cosmological magnetic field and cosmological magnetic helicity for astrophysics is considered. We discuss possible mechanisms for cosmological magnetic field production in the early universe as well as upper observational estimate for such field. The general conclusion is that a substantial cosmological field with a non-vanishing magnetic helicity can be generated in the early universe and survive up to the epoch of galaxy formation.

scholarly journals Numerical Simulations of Turbulent Dynamos

2002 ◽  
Vol 12 ◽  
pp. 742-744
Author(s):  
Axel Brandenburg
Keyword(s):  
Numerical Simulations ◽  
Large Scale ◽  
Mean Field ◽  
Helicity Conservation ◽  
Scale Field ◽  
Large Scale Field ◽  
Mean Field Dynamo

AbstractUsing a periodic box calculation it is shown that, owing to helicity conservation, a large scale field can only develop on a resistive timescale. This behaviour can be reproduced by a mean-field dynamo withαandƞtquenchings that are equally strong and ‘catastrophic’.

scholarly journals Magnetic helicity fluxes in αΩ dynamos

2010 ◽  
Vol 6 (S274) ◽  
pp. 464-466
Author(s):  
Simon Candelaresi ◽  
Axel Brandenburg
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Field Model ◽  
Mean Field ◽  
Reynolds Numbers ◽  
Magnetic Helicity ◽  
Small Scale ◽  
Mean Field Model ◽  
One Dimensional

AbstractIn turbulent dynamos the production of large-scale magnetic fields is accompanied by a separation of magnetic helicity in scale. The large- and small-scale parts increase in magnitude. The small-scale part can eventually work against the dynamo and quench it, especially at high magnetic Reynolds numbers. A one-dimensional mean-field model of a dynamo is presented where diffusive magnetic helicity fluxes within the domain are important. It turns out that this effect helps to alleviate the quenching. Here we show that internal magnetic helicity fluxes, even within one hemisphere, can be important for alleviating catastrophic quenching.

Magnetic helicity in non-axisymmetric mean-field dynamo

2016 ◽  
Vol 52 (1) ◽  
pp. 145-154
Author(s):  
V. V. Pipin ◽  
Keyword(s):  
Mean Field ◽  
Magnetic Helicity ◽  
Mean Field Dynamo

scholarly journals The magnetic helicity density patterns from non-axisymmetric solar dynamo

2021 ◽  
Vol 87 (1) ◽  
Author(s):  
Valery V. Pipin
Keyword(s):  
Magnetic Field ◽  
Differential Rotation ◽  
Conservation Law ◽  
Large Scale ◽  
Solar Dynamo ◽  
Magnetic Helicity ◽  
Dynamo Model ◽  
Density Distributions ◽  
Large Scale Magnetic Field ◽  
Helicity Conservation

We study the helicity density patterns which can result from the emerging bipolar regions. Using the relevant dynamo model and the magnetic helicity conservation law we find that the helicity density patterns around the bipolar regions depend on the configuration of the ambient large-scale magnetic field, and in general they show a quadrupole distribution. The position of this pattern relative to the equator can depend on the tilt of the bipolar region. We compute the time–latitude diagrams of the helicity density evolution. The longitudinally averaged effect of the bipolar regions shows two bands of sign for the density distributions in each hemisphere. Similar helicity density patterns are provided by the helicity density flux from the emerging bipolar regions subjected to surface differential rotation.

scholarly journals Global MHD phenomena and their importance for stellar surfaces

2010 ◽  
Vol 6 (S273) ◽  
pp. 141-147
Author(s):  
Rainer Arlt
Keyword(s):  
Magnetic Fields ◽  
Mean Field ◽  
Momentum Equation ◽  
Small Scale ◽  
Dynamo Action ◽  
Complex Activity ◽  
Mhd Instabilities ◽  
Stellar Magnetic Fields ◽  
Mean Field Dynamo

AbstractThis review is an attempt to elucidate MHD phenomena relevant for stellar magnetic fields. The full MHD treatment of a star is a problem which is numerically too demanding. Mean-field dynamo models use an approximation of the dynamo action from the small-scale motions and deliver global magnetic modes which can be cyclic, stationary, axisymmetric, and non-axisymmetric. Due to the lack of a momentum equation, MHD instabilities are not visible in this picture. However, magnetic instabilities must set in as a result of growing magnetic fields and/or buoyancy. Instabilities deliver new timescales, saturation limits and topologies to the system probably providing a key to the complex activity features observed on stars.

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