Axel Brandenburg's Publications List

The alpha effect in rotating convection with sinusoidal shear

2010-10-10T06:24:36Z

Using three-dimensional convection simulations, it is shown that a sinusoidal variation of horizontal shear leads to a kinematic alpha effect with a similar sinusoidal variation. The effect exists even for weak stratification and arises owing to the inhomogeneity of turbulence and the presence of impenetrable vertical boundaries. This system produces large-scale magnetic fields that also show a si...

P J Kapyla, M J Korpi, A Brandenburg (2010) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 402: 3 1458-1466

Homochirality and the Need for Energy

2010-10-10T06:24:36Z

The mechanisms for explaining how a stable asymmetric chemical system can be formed from a symmetric chemical system, in the absence of any asymmetric influence other than statistical fluctuations, have been developed during the last decades, focusing on the non-linear kinetic aspects. Besides the absolute necessity of self-amplification processes, the importance of energetic aspects is often unde...

R Plasson, A Brandenburg (2010) ORIGINS OF LIFE AND EVOLUTION OF BIOSPHERES 40: 1 93-110

Large-scale magnetic flux concentrations from turbulent stresses

2010-10-10T06:24:36Z

In this study we provide the first numerical demonstration of the effects of turbulence on the mean Lorentz force and the resulting formation of large-scale magnetic structures. Using three-dimensional direct numerical simulations (DNS) of forced turbulence we show that an imposed mean magnetic field leads to a decrease of the turbulent hydromagnetic pressure and tension. This phenomenon is quanti...

A Brandenburg, N Kleeorin, I Rogachevskii (2010) ASTRONOMISCHE NACHRICHTEN 331: 1 5-13

Magnetic-field decay of three interlocked flux rings with zero linking number

2010-10-10T06:24:36Z

The resistive decay of chains of three interlocked magnetic flux rings is considered. Depending on the relative orientation of the magnetic field in the three rings, the late-time decay can be either fast or slow. Thus, the qualitative degree of tangledness is less important than the actual value of the linking number or, equivalently, the net magnetic helicity. Our results do not suggest that inv...

F Del Sordo, S Candelaresi, A Brandenburg (2010) PHYSICAL REVIEW E 81: 3

Calibrating passive scalar transport in shear-flow turbulence

2010-10-10T06:24:36Z

The turbulent diffusivity tensor is determined for linear shear-flow turbulence using numerical simulations. For moderately strong shear, the diagonal components are found to increase quadratically with Peclet and Reynolds numbers below about 10 and then become constant. The diffusivity tensor is found to have components proportional to the symmetric and antisymmetric parts of the velocity gradien...

E J M Madarassy, A Brandenburg (2010) PHYSICAL REVIEW E 82: 1

OSCILLATORY MIGRATING MAGNETIC FIELDS IN HELICAL TURBULENCE IN SPHERICAL DOMAINS

2010-10-10T06:24:36Z

We present direct numerical simulations of the equations of compressible magnetohydrodynamics in a wedge-shaped spherical shell, without shear, but with random helical forcing which has negative (positive) helicity in the northern (southern) hemisphere. We find a large-scalemagnetic field that is nearly uniform in the azimuthal direction and approximately antisymmetric about the equator. Furthermo...

D Mitra, R Tavakol, P J Kapyla, A Brandenburg (2010) ASTROPHYSICAL JOURNAL LETTERS 719: 1 L1-L4

ANGULAR MOMENTUM TRANSPORT IN CONVECTIVELY UNSTABLE SHEAR FLOWS

2010-10-10T06:24:36Z

Angular momentum transport due to hydrodynamic turbulent convection is studied using local three-dimensional numerical simulations employing the shearing box approximation. We determine the turbulent viscosity from non-rotating runs over a range of values of the shear parameter and use a simple analytical model in order to extract the non-diffusive contribution (Lambda-effect) to the stress in run...

P J Kapyla, A Brandenburg, M J Korpi, J E Snellman, R Narayan (2010) ASTROPHYSICAL JOURNAL 719: 1 67-76

Mean electromotive force proportional to mean flow in MHD turbulence

2010-10-10T06:24:36Z

In mean-field magnetohydrodynamics the mean electromotive force due to velocity and magnetic-field fluctuations plays a crucial role. In general it consists of two parts, one independent of and another one proportional to the mean magnetic field. The first part may be nonzero only in the presence of mhd turbulence, maintained, e.g., by small-scale dynamo action. It corresponds to a battery, which ...

K H Radler, A Brandenburg (2010) ASTRONOMISCHE NACHRICHTEN 331: 1 14-21

Numerical simulations of the decay of primordial magnetic turbulence

2010-10-10T06:24:36Z

We perform direct numerical simulations of forced and freely decaying 3D magnetohydrodynamic turbulence in order to model magnetic field evolution during cosmological phase transitions in the early Universe. Our approach assumes the existence of a magnetic field generated either by a process during inflation or shortly thereafter, or by bubble collisions during a phase transition. We show that the...

T Kahniashvili, A Brandenburg, A G Tevzadze, B Ratra (2010) PHYSICAL REVIEW D 81: 12

Convective dynamos in spherical wedge geometry

2010-10-10T06:24:36Z

Self-consistent convective dynamo simulations in wedge-shaped spherical shells are presented. Differential rotation is generated by the interaction of convection with rotation. Equatorward acceleration and dynamo action are obtained only for sufficiently rapid rotation. The angular velocity tends to be constant along cylinders. Oscillatory large-scale fields are found to migrate in the poleward di...

P J Kapyla, M J Korpi, A Brandenburg, D Mitra, R Tavakol (2010) ASTRONOMISCHE NACHRICHTEN 331: 1 73-81

Magnetic field evolution in simulations with Euler potentials

2010-10-10T06:24:36Z

Using two- and three-dimensional hydromagnetic simulations for a range of different flows, including laminar and turbulent ones, it is shown that solutions expressing the field in terms of Euler potentials (EP) are in general incorrect if the EP are evolved with an artificial diffusion term. In three dimensions, standard methods using the magnetic vector potential are found to permit dynamo action...

A Brandenburg (2010) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 401: 1 347-354

Equatorial magnetic helicity flux in simulations with different gauges

2010-10-10T06:24:36Z

We use direct numerical simulations of forced MHD turbulence with a forcing function that produces two different signs of kinetic helicity in the upper and lower parts of the domain. We show that the mean flux of magnetic helicity from the small-scale field between the two parts of the domain can be described by a Fickian diffusion law with a diffusion coefficient that is approximately independent...

D Mitra, S Candelaresi, P Chatterjee, R Tavakol, A Brandenburg (2010) ASTRONOMISCHE NACHRICHTEN 331: 1 130-135

LARGE-SCALE DYNAMOS AT LOW MAGNETIC PRANDTL NUMBERS

2009-09-12T08:40:52Z

Using direct simulations of hydromagnetic turbulence driven by random polarized waves it is shown that dynamo action is possible over a wide range of magnetic Prandtl numbers from 10(-3) to 1. Triply periodic boundary conditions are being used. In the final saturated state the resulting magnetic field has a large-scale component of Beltrami type. For the kinematic phase, growth rates have been det...

A Brandenburg (2009) ASTROPHYSICAL JOURNAL 697: 2 1206-1213

Turbulent diffusion with rotation or magnetic fields

2009-09-12T08:40:52Z

The turbulent diffusion tensor describing the evolution of the mean concentration of a passive scalar is investigated for non-helically forced turbulence in the presence of rotation or a magnetic field. With rotation, the Coriolis force causes a sideways deflection of the flux of mean concentration. Within the magnetohydrodynamics approximation there is no analogous effect from the magnetic field ...

A Brandenburg, A Svedin, G M Vasil (2009) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 395: 3 1599-1606

TURBULENT DYNAMOS IN SPHERICAL SHELL SEGMENTS OF VARYING GEOMETRICAL EXTENT

2009-09-12T08:40:52Z

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-...

D Mitra, R Tavakol, A Brandenburg, D Moss (2009) ASTROPHYSICAL JOURNAL 697: 1 923-933

LARGE-SCALE DYNAMOS IN RIGIDLY ROTATING TURBULENT CONVECTION

2009-09-12T08:40:52Z

The existence of large-scale dynamos in rigidly rotating turbulent convection without shear is studied using three-dimensional numerical simulations of penetrative rotating compressible convection. We demonstrate that rotating convection in a Cartesian domain can drive a large-scale dynamo even in the absence of shear. The large-scale field contains a significant fraction of the total field in the...

P J Kapyla, M J Korpi, A Brandenburg (2009) ASTROPHYSICAL JOURNAL 697: 2 1153-1163

Alpha effect and turbulent diffusion from convection

2009-09-12T08:40:52Z

Aims. We study turbulent transport coefficients that describe the evolution of large-scale magnetic fields in turbulent convection. Methods. We use the test field method, together with three-dimensional numerical simulations of turbulent convection with shear and rotation, to compute turbulent transport coefficients describing the evolution of large-scale magnetic fields in mean-field theory in th...

P J Kapyla, M J Korpi, A Brandenburg (2009) ASTRONOMY & ASTROPHYSICS 500: 2 633-646

Emergence of Protometabolisms and the Self-Organization of Non-equilibrium Reaction Networks

2009-09-12T08:40:52Z

R Plasson, H Bersini, A Brandenburg (2009) ORIGINS OF LIFE AND EVOLUTION OF BIOSPHERES 39: 3-4 263-264

TURBULENT DYNAMOS WITH SHEAR AND FRACTIONAL HELICITY

2009-09-12T08:40:52Z

Dynamo action owing to helically forced turbulence and large-scale shear is studied using direct numerical simulations. The resulting magnetic field displays propagating wave-like behavior. This behavior can be modeled in terms of an alpha Omega dynamo. In most cases super-equipartition fields are generated. By varying the fraction of helicity of the turbulence the regeneration of poloidal fields ...

P J Kapyla, A Brandenburg (2009) ASTROPHYSICAL JOURNAL 699: 2 1059-1066

Advances in Theory and Simulations of Large-Scale Dynamos

2009-09-12T08:40:52Z

Recent analytical and computational advances in the theory of large-scale dynamos are reviewed. The importance of the magnetic helicity constraint is apparent even without invoking mean-field theory. The tau approximation yields expressions that show how the magnetic helicity gets incorporated into mean-field theory. The test-field method allows an accurate numerical determination of turbulent tra...

A Brandenburg (2009) SPACE SCIENCE REVIEWS 144: 1-4 87-104

Paradigm shifts in solar dynamo modeling

2009-09-12T08:40:52Z

Selected topics in solar dynamo theory are being highlighted. The possible relevance of the near-surface shear lawyer is discussed. The role of turbulent; downward pumping is mentioned in connection with earlier concerns that a dynamo-generated magnetic field would be rapidly lost, from the convection zone by magnetic buoyancy. It is argued that shear-mediated small-scale magnetic helicity fluxes ...

A Brandenburg (2009) COSMIC MAGNETIC FIELDS : FROM PLANETS, TO STARS AND GALAXIES 259: 159-166

Numerical study of large-scale vorticity generation in shear-flow turbulence

2009-09-12T08:40:52Z

Simulations of stochastically forced shear-flow turbulence in a shearing-periodic domain are used to study the spontaneous generation of large-scale flow patterns in the direction perpendicular to the plane of the shear. Based on an analysis of the resulting large-scale velocity correlations it is argued that the mechanism behind this phenomenon could be the mean-vorticity dynamo effect pioneered ...

P J Kapyla, D Mitra, A Brandenburg (2009) PHYSICAL REVIEW E 79: 1

Mean-field effects in the Galloway-Proctor flow

2009-09-12T08:45:01Z

In the framework of mean-field electrodynamics the coefficients defining the mean electromotive force in Galloway-Proctor flows are determined. These flows show a two-dimensional pattern and are helical. The pattern wobbles in its plane. Apart from one exception a circularly polarized Galloway-Proctor flow, i.e. a circular motion of the flow pattern is assumed. This corresponds to one of the cases...

K H Radler, A Brandenburg (2009) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 393: 1 113-125

Numerical study of large-scale vorticity generation in shear-flow turbulence

2009-09-12T08:45:01Z

Turbulent stresses as a function of shear rate in a local disk model

2009-09-12T08:40:52Z

WC Present local numerical models of accretion disk turbulence driven by the magnetorotational instability with varying shear rate. The resulting turbulent stresses are compared with predictions of a closure model in which triple correlations are modelled in terms of quadratic correlations. This local model uses live nondimensional parameters to describe the properties of the flow. We attempt to d...

A J Liljestrom, M J Korpi, P J Kapyla, A Brandenburg, W Lyra (2009) ASTRONOMISCHE NACHRICHTEN 330: 1 92-99

Alpha effect and diffusivity in helical turbulence with shear

2009-09-12T08:45:01Z

Aims. We study the dependence of turbulent transport coefficients, such as the components of the alpha tensor (alpha(ij)) and the turbulent magnetic diffusivity tensor (eta(ij)), on shear and magnetic Reynolds number in the presence of helical forcing. Methods. We use three-dimensional direct numerical simulations with periodic boundary conditions and measure the turbulent transport coefficients u...

D Mitra, P J Kapyla, R Tavakol, A Brandenburg (2009) ASTRONOMY & ASTROPHYSICS 495: 1 1-8

Advances in Theory and Simulations of Large-Scale Dynamos

2009-09-12T08:45:01Z

Mean-field effects in the Galloway-Proctor flow

2009-09-12T08:40:52Z

TURBULENT DYNAMOS IN SPHERICAL SHELL SEGMENTS OF VARYING GEOMETRICAL EXTENT

2009-09-12T08:45:01Z

Alpha effect and diffusivity in helical turbulence with shear

2009-09-12T08:40:52Z

Emergence of Protometabolisms and the Self-Organization of Non-equilibrium Reaction Networks

2009-09-12T08:45:01Z

R Plasson, H Bersini, A Brandenburg (2009) ORIGINS OF LIFE AND EVOLUTION OF BIOSPHERES 39: 3-4 263-264

The critical role of magnetic helicity in astrophysical large-scale dynamos

2010-10-10T06:24:36Z

The role of magnetic helicity in astrophysical large-scale dynamos is reviewed and compared with cases where there is no energy supply and an initial magnetic field can only decay. In both cases magnetic energy tends to get redistributed to larger scales. Depending on the efficiency of magnetic helicity fluxes the decay of a helical field can speed up. Likewise, the saturation of a helical dynamo ...

A Brandenburg (2009) PLASMA PHYSICS AND CONTROLLED FUSION 51: 12

Homochirality and the Need for Energy.

2009-12-25T07:55:41Z

The mechanisms for explaining how a stable asymmetric chemical system can be formed from a symmetric chemical system, in the absence of any asymmetric influence other than statistical fluctuations, have been developed during the last decades, focusing on the non-linear kinetic aspects. Besides the absolute necessity of self-amplification processes, the importance of energetic aspects is often unde...

Raphaël Plasson, Axel Brandenburg (2009) Orig Life Evol Biosph :

Numerical study of large-scale vorticity generation in shear-flow turbulence.

2009-09-12T01:14:48Z

Simulations of stochastically forced shear-flow turbulence in a shearing-periodic domain are used to study the spontaneous generation of large-scale flow patterns in the direction perpendicular to the plane of the shear. Based on an analysis of the resulting large-scale velocity correlations it is argued that the mechanism behind this phenomenon could be the mean-vorticity dynamo effect pioneered ...

Petri J Käpylä, Dhrubaditya Mitra, Axel Brandenburg (2009) Phys Rev E Stat Nonlin Soft Matter Phys 79: 1 Pt 2

MEMORY EFFECTS IN TURBULENT TRANSPORT

2010-10-10T06:24:36Z

In the mean-field theory of magnetic fields, turbulent transport, i.e., the turbulent electromotive force is described by a combination of the a effect and turbulent magnetic diffusion, which are usually assumed to be proportional, respectively, to the mean field and its spatial derivatives. For a passive scalar, there is just turbulent diffusion, where the mean flux of concentration depends on th...

A Hubbard, A Brandenburg (2009) ASTROPHYSICAL JOURNAL 706: 1 712-726

The role of the Yoshizawa effect in the Archontis dynamo

2010-10-10T06:24:36Z

The generation of mean magnetic fields is Studied for a simple non-helical flow where a net cross-helicity of either sign call emerge. This flow, which is also known as the Archontis flow, is a generalization of the Arnold-Beltrami-Childress flow, but with the cosine terms omitted. The presence of cross-helicity leads to a mean-field dynamo effect that is known as the Yoshizawa effect. Direct nume...

S Sur, A Brandenburg (2009) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 399: 1 273-280

The alpha effect with imposed and dynamo-generated magnetic fields

2010-10-10T06:24:36Z

Estimates for the non-linear alpha effect in helical turbulence with an applied magnetic field are presented using two different approaches: the imposed-field method where the electromotive force owing to the applied field is used, and the test-field method where separate evolution equations are solved for a set of different test fields. Both approaches agree for stronger fields, but there are app...

A Hubbard, F Del Sordo, P J Kapyla, A Brandenburg (2009) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 398: 4 1891-1899

Shear-driven magnetic buoyancy oscillations

2010-10-10T06:24:36Z

The effects of uniform horizontal shear on a stably stratified layer of gas is studied. The system is initially destabilized by a magnetically buoyant flux tube pointing in the cross-stream direction. The shear amplifies the initial field to Lundquist numbers of about 200-400, but then its value drops to about 100-300, depending on the value of the sub-adiabatic gradient. The larger values corresp...

V Vermersch, A Brandenburg (2009) ASTRONOMISCHE NACHRICHTEN 330: 8 797-806

Turbulent stresses as a function of shear rate in a local disk model

2009-09-12T08:45:01Z

Small-scale magnetic helicity losses from a mean-field dynamo

2010-10-10T06:24:36Z

Using mean-field models with a dynamical quenching formalism, we show that in finite domains magnetic helicity fluxes associated with small-scale magnetic fields are able to alleviate catastrophic quenching. We consider fluxes that result from advection by a mean flow, the turbulent mixing down the gradient of mean small-scale magnetic helicity density or the explicit removal which may be associat...

A Brandenburg, S Candelaresi, P Chatterjee (2009) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 398: 3 1414-1422

Paradigm shifts in solar dynamo modeling

2009-09-12T08:45:01Z

LARGE-SCALE DYNAMOS IN RIGIDLY ROTATING TURBULENT CONVECTION

2009-09-12T08:45:01Z

LARGE-SCALE DYNAMOS AT LOW MAGNETIC PRANDTL NUMBERS

2009-09-12T08:45:01Z

Alpha effect and turbulent diffusion from convection

2009-09-12T08:45:01Z

TURBULENT DYNAMOS WITH SHEAR AND FRACTIONAL HELICITY

2009-09-12T08:45:01Z

Turbulent diffusion with rotation or magnetic fields

2009-09-12T08:45:01Z

Magnetic diffusivity tensor and dynamo

2009-09-12T08:45:01Z

The turbulent magnetic diffusivity tensor is determined in the presence of rotation or shear. The question is addressed whether dynamo action from the shear-current effect can explain large-scale magnetic field generation found in simulations with shear. For this purpose a set of evolution equations for the response to imposed test fields is solved with turbulent and mean motions calculated from t...

A Brandenburg, K H Radler, M Rheinhardt, P J Kapyla (2008) ASTROPHYSICAL JOURNAL 676: 1 740-751

Core mass function : The role of gravity

2009-09-12T08:45:01Z

We analyze the mass distribution of cores formed in an isothermal, magnetized, turbulent, and self-gravitating nearly critical molecular cloud model. Cores are identified at two density threshold levels. Our main results are that the presence of self-gravity modifies the slopes of the core mass function (CMF) at the high-mass end. At low thresholds, the slope is shallower than the one predicted by...

S Dib, A Brandenburg, J Kim, M Gopinathan, P Andre (2008) ASTROPHYSICAL JOURNAL LETTERS 678: 2 L105-L108

Kinematic alpha-effect in isotropic turbulence simulations

2009-09-12T08:45:01Z

Using numerical simulations at moderate magnetic Reynolds numbers up to 220, it is shown that in the kinematic regime, isotropic helical turbulence leads to an alpha-effect and a turbulent diffusivity whose values are independent of the magnetic Reynolds number, R-m, provided R-m exceeds unity. These turbulent coefficients are also consistent with expectations from the first-order smoothing approx...

S Sur, A Brandenburg, K Subramanian (2008) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 385: 1 L15-L19

MAGNETIC QUENCHING OF alpha AND DIFFUSIVITY TENSORS IN HELICAL TURBULENCE

2009-09-12T08:45:01Z

The effect of a dynamo-generated mean magnetic field of Beltrami type on the mean electromotive force is studied. In the absence of the mean magnetic field the turbulence is assumed to be homogeneous and isotropic, but it becomes inhomogeneous and anisotropic with this field. Using the test-field method the dependence of the a and turbulent diffusivity tensors on the magnetic Reynolds number Re-M ...

A Brandenburg, K H Radler, M Rheinhardt, K Subramanian (2008) ASTROPHYSICAL JOURNAL LETTERS 687: 1 L49-L52

A solar mean field dynamo benchmark

2009-09-12T08:45:01Z

Context. The solar magnetic activity and cycle are linked to an internal dynamo. Numerical simulations are an efficient and accurate tool to investigate such intricate dynamical processes. Aims. We present the results of an international numerical benchmark study based on two-dimensional axisymmetric mean field solar dynamo models in spherical geometry. The purpose of this work is to provide refer...

L Jouve, A S Brun, R Arlt, A Brandenburg, M Dikpati, A Bonanno, P J Kapyla, D Moss, M Rempel, P Gilman, M J Korpi, A G Kosovichev (2008) ASTRONOMY & ASTROPHYSICS 483: 3 949-960

Modeling a Maunder minimum

2009-09-12T08:45:01Z

We introduce on/off intermittency into a mean field dynamo model by imposing stochastic fluctuations in either the alpha effect or through the inclusion of a fluctuating electromotive force. Sufficiently strong small scale fluctuations with time scales of the order of 0.3-3 years can produce long term variations in the system on time scales of the order of hundreds of years. However, global suppre...

A Brandenburg, E A Spiegel (2008) ASTRONOMISCHE NACHRICHTEN 329: 4 351-358

Scale dependence of alpha effect and turbulent diffusivity

2009-09-12T08:45:01Z

Aims. We determine the alpha effect and turbulent magnetic diffusivity for mean magnetic fields with profiles of different length scales from simulations of isotropic turbulence. We then relate these results to nonlocal formulations in which alpha and the turbulent magnetic diffusivity correspond to integral kernels. Methods. We solve evolution equations for magnetic fields that give the response ...

A Brandenburg, K H Radler, M Schrinner (2008) ASTRONOMY & ASTROPHYSICS 482: 3 739-746

Turbulent protostellar discs

2009-09-12T08:45:01Z

Aspects of turbulence in protostellar accretion discs are being reviewed. The emergence of dead zones due to poor ionization and alternatives to the magneto-rotational instability are discussed. The coupling between dust and gas in protostellar accretion discs is explained and the turbulent drag is compared with laminar drag in the Stokes and Epstein regimes. Finally, the significance of magnetic-...

A Brandenburg (2008) PHYSICA SCRIPTA T130:

Lambda effect from forced turbulence simulations

2009-09-12T08:45:01Z

Aims. We determine the components of the Lambda-effect tensor that quantifies the contributions to the turbulent momentum transport even for uniform rotation. Methods. Three-dimensional numerical simulations are used to study turbulent transport in triply periodic cubes under the influence of rotation and anisotropic forcing. Comparison is made with analytical results obtained via the so-called mi...

P J Kapyla, A Brandenburg (2008) ASTRONOMY & ASTROPHYSICS 488: 1 9-U24

A growing dynamo from a saturated Roberts flow dynamo

2009-09-12T08:45:01Z

Using direct simulations, weakly non-linear theory and non-linear mean-field theory, it is shown that the quenched velocity field of a saturated non-linear dynamo can itself act as a kinematic dynamo. The flow is driven by a forcing function that would produce a Roberts flow in the absence of a magnetic field. This result confirms an analogous finding by Cattaneo & Tobias for the more complicated ...

A Tilgner, A Brandenburg (2008) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 391: 3 1477-1481

Large-scale dynamos in turbulent convection with shear

2009-09-12T08:45:01Z

Aims. To study the existence of large-scale convective dynamos under the influence of shear and rotation. Methods. Three-dimensional numerical simulations of penetrative compressible convection with uniform horizontal shear are used to study dynamo action and the generation of large-scale magnetic fields. We consider cases where the magnetic Reynolds number is either marginal or moderately supercr...

P J Kapyla, M J Korpi, A Brandenburg (2008) ASTRONOMY & ASTROPHYSICS 491: 2 353-362

The dual role of shear in large-scale dynamos

2009-09-12T08:45:01Z

The role of shear in alleviating catastrophic quenching by shedding small-scale magnetic helicity through fluxes along contours of constant shear is discussed. The level of quenching of the dynamo effect depends on the quenched value of the turbulent magnetic diffusivity. Earlier estimates that might have suffered from the force-free degeneracy of Beltrami fields are now confirmed for shear flows ...

A Brandenburg (2008) ASTRONOMISCHE NACHRICHTEN 329: 7 725-731

alpha-effect dynamos with zero kinetic helicity

2009-09-12T08:45:01Z

A simple explicit example of a Roberts-type dynamo is given in which the alpha effect of mean-field electrodynamics exists in spite of pointwise vanishing kinetic helicity of the fluid flow. In this way, it is shown that alpha-effect dynamos do not necessarily require nonzero kinetic helicity. A mean-field theory of Roberts-type dynamos is established within the framework of the second-order corre...

K H Radler, A Brandenburg (2008) PHYSICAL REVIEW E 77: 2

A solar mean field dynamo benchmark

2009-09-12T08:40:52Z

Modeling a Maunder minimum

2009-09-12T08:40:52Z

Boundary layer on the surface of a neutron star

2009-09-12T08:45:01Z

In an attempt to model the accretion on to a neutron star in low-mass X-ray binaries, we present 2D hydrodynamical models of the gas flow in close vicinity of the stellar surface. First, we consider a gas pressure-dominated case, assuming that the star is non-rotating. For the stellar mass we take M-star = 1.4 x 10(-2) M-circle dot and for the gas temperature T = 5 x 10(6) K. Our results are quali...

N Babkovskaia, A Brandenburg, J Poutanen (2008) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 386: 2 1038-1044

Scale dependence of alpha effect and turbulent diffusivity

2009-09-12T08:40:52Z

Boundary layer on the surface of a neutron star

2009-09-12T08:40:52Z

Magnetic diffusivity tensor and dynamo

2009-09-12T08:40:52Z

Kinematic alpha-effect in isotropic turbulence simulations

2009-09-12T08:40:52Z

Turbulent protostellar discs

2009-09-12T08:40:52Z

Lambda effect from forced turbulence simulations

2009-09-12T08:40:52Z

MAGNETIC QUENCHING OF alpha AND DIFFUSIVITY TENSORS IN HELICAL TURBULENCE

2009-09-12T08:40:52Z

Alpha-effect dynamos with zero kinetic helicity.

2009-09-12T01:14:48Z

A simple explicit example of a Roberts-type dynamo is given in which the alpha effect of mean-field electrodynamics exists in spite of pointwise vanishing kinetic helicity of the fluid flow. In this way, it is shown that alpha-effect dynamos do not necessarily require nonzero kinetic helicity. A mean-field theory of Roberts-type dynamos is established within the framework of the second-order corre...

Karl-Heinz Rädler, Axel Brandenburg (2008) Phys Rev E Stat Nonlin Soft Matter Phys 77: 2 Pt 2

A growing dynamo from a saturated Roberts flow dynamo

2009-09-12T08:40:52Z

Large-scale dynamos in turbulent convection with shear

2009-09-12T08:40:52Z

The dual role of shear in large-scale dynamos

2009-09-12T08:40:52Z

alpha-effect dynamos with zero kinetic helicity

2009-09-12T08:40:52Z

Core mass function : The role of gravity

2009-09-12T08:40:52Z

The helicity constraint in spherical shell dynamos

2009-09-12T08:45:01Z

The motivation for considering distributed large scale dynamos in the solar context is reviewed in connection with the magnetic helicity constraint. Preliminary accounts of 3-dimensional direct numerical simulations (in spherical shell segments) and simulations of 2-dimensional mean field models (in spherical shells) are presented. Interesting similarities as well as some differences are noted. (C...

A Brandenburg, P J Kapyla, D Mitra, D Moss, R Tavakol (2007) ASTRONOMISCHE NACHRICHTEN 328: 10 1118-1121

Thermal instability in shearing and periodic turbulence

2009-09-12T08:40:52Z

The thermal instability with a piecewise power law cooling function is investigated using one- and three-dimensional simulations with periodic and shearing-periodic boundary conditions in the presence of constant thermal diffusion and kinematic viscosity coefficients. Consistent with earlier findings, the flow behavior depends on the average density, [rho]. When [rho] is in the range (1-5); 10(-24...

A Brandenburg, M J Korpi, A J Mee (2007) ASTROPHYSICAL JOURNAL 654: 2 945-954

Thermal instability in shearing and periodic turbulence

2009-09-12T08:45:01Z

Simulations of the anisotropic kinetic and magnetic alpha effects

2009-09-12T08:40:52Z

Using simulations of isotropically forced helical turbulence the contributions to kinetic and magnetic alpha effects are computed. It is shown that for the parameter regimes considered in an earlier publication (Brandenburg & Subramanian 2005), the expressions for isotropic and anisotropic alpha effects give quantitatively similar results. Both kinetic and magnetic alpha effects are proportional t...

A Brandenburg, K Subramanian (2007) ASTRONOMISCHE NACHRICHTEN 328: 6 507-512

Magnetic helicity effects in astrophysical and laboratory dynamos

2009-09-12T08:40:52Z

Magnetic helicity effects are discussed in laboratory and astrophysical settings. Firstly, dynamo action in Taylor-Green flows is discussed for different boundary conditions. However, because of the lack of scale separation with respect to the container, no large-scale field is being produced and there is no resistively slow saturation phase as otherwise expected. Secondly, the build-up of a large...

A Brandenburg, P J Kapyla (2007) NEW JOURNAL OF PHYSICS 9:

Homochirality in an early peptide world.

2009-09-12T01:23:09Z

A recently proposed model of non-autocatalytic reactions in dipeptide formation that leads to spontaneous symmetry breaking and homochirality was examined. The model is governed by activation, polymerization, epimerization, and depolymerization of amino acids. Symmetry breaking was determined to result primarily from the different rates of reactions that involve homodimers and heterodimers, i.e., ...

Axel Brandenburg, Harry J Lehto, Kirsi M Lehto (2007) Astrobiology 7: 5 725-732

Homochirality in an early peptide world

2009-09-12T08:40:52Z

A recently proposed model of non-autocatalytic reactions in dipeptide formation that leads to spontaneous symmetry breaking and homochirality was examined. The model is governed by activation, polymerization, epimerization, and depolymerization of amino acids. Symmetry breaking was determined to result primarily from the different rates of reactions that involve homodimers and heterodimers, i.e., ...

A Brandenburg, H J Lehto, K M Lehto (2007) ASTROBIOLOGY 7: 5 725-732

Kinetic and magnetic alpha-effects in non-linear dynamo theory

2009-09-12T08:45:01Z

The backreaction of the Lorentz force on the alpha-effect is studied in the limit of small magnetic and fluid Reynolds numbers, using the first-order smoothing approximation (FOSA) to solve both the induction and momentum equations. Both steady and time-dependent forcings are considered. In the low Reynolds number limit, the velocity and magnetic fields can be expressed explicitly in terms of the ...

S Sur, K Subramanian, A Brandenburg (2007) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 376: 3 1238-1250

Kinetic and magnetic alpha-effects in non-linear dynamo theory

2009-09-12T08:40:52Z

Turbulent viscosity and Lambda-effect from numerical turbulence models

2009-09-12T08:40:52Z

Homogeneous anisotropic turbulence simulations are used to determine off-diagonal components of the Reynolds stress tensor and its parameterization in terms of turbulent viscosity and A-effect. The turbulence is forced in an anisotropic fashion by enhancing the strength of the forcing in the vertical direction. The Coriolis force is included with a rotation axis inclined relative to the vertical d...

P J Kapyla, A Brandenburg (2007) ASTRONOMISCHE NACHRICHTEN 328: 10 1006-1008

Simulations of the anisotropic kinetic and magnetic alpha effects

2009-09-12T08:45:01Z

Magnetic helicity effects in astrophysical and laboratory dynamos

2009-09-12T08:45:01Z

Homochirality in an early peptide world

2009-09-12T08:45:01Z

A recently proposed model of non-autocatalytic reactions in dipeptide formation that leads to spontaneous symmetry breaking and homochirality was examined. The model is governed by activation, polymerization, epimerization, and depolymerization of amino acids. Symmetry breaking was determined to result primarily from the different rates of reactions that involve homodimers and heterodimers, i.e., ...

A Brandenburg, H J Lehto, K M Lehto (2007) ASTROBIOLOGY 7: 5 725-732

The helicity constraint in spherical shell dynamos

2009-09-12T08:40:52Z

Turbulent viscosity and Lambda-effect from numerical turbulence models

2009-09-12T08:45:01Z

Radiative transfer in decomposed domains

2009-09-12T08:45:01Z

Aims. An efficient algorithm for calculating radiative transfer on massively parallel computers using domain decomposition is presented. Methods. The integral formulation of the transfer equation is used to divide the problem into a local but compute-intensive part for calculating the intensity and optical depth integrals, and a nonlocal part for communicating the intensity between adjacent proces...

T Heinemann, W Dobler, A Nordlund, A Brandenburg (2006) ASTRONOMY & ASTROPHYSICS 448: 2 731-U134

Galactic dynamo and helicity losses through fountain flow

2009-09-12T08:45:01Z

Aims. Nonlinear behaviour of galactic dynamos is studied, allowing for magnetic helicity removal by the galactic fountain flow. Methods. A suitable advection speed is estimated, and a one-dimensional mean-field dynamo model with dynamic alpha-effect is explored. Results. It is shown that the galactic fountain flow is efficient in removing magnetic helicity from galactic discs. This alleviates the ...

A Shukurov, D Sokoloff, K Subramanian, A Brandenburg (2006) ASTRONOMY & ASTROPHYSICS 448: 2 L33-L36

Magnetic helicity density and its flux in weakly inhomogeneous turbulence

2009-09-12T08:45:01Z

A gauge-invariant and hence physically meaningful definition of magnetic helicity density for random fields is proposed, using the Gauss linking formula, as the density of correlated field line linkages. This definition is applied to the random small-scale field in weakly inhomogeneous turbulence, whose correlation length is small compared with the scale on which the turbulence varies. For inhomog...

K Subramanian, A Brandenburg (2006) ASTROPHYSICAL JOURNAL 648: 1 L71-L74

Stellar dynamo driven wind braking versus disc coupling

2009-09-12T08:45:01Z

Star-disc coupling is considered in numerical models where the stellar field is not an imposed perfect dipole, but instead a more irregular self-adjusting dynamo-generated field. Using axisymmetric simulations of the hydromagnetic mean-field equations, it is shown that the resulting stellar field configuration is more complex, but significantly better suited for driving a stellar wind. In agreemen...

B von Rekowski, A Brandenburg (2006) ASTRONOMISCHE NACHRICHTEN 327: 1 53-71

Nonaxisymmetric stability in the shearing sheet approximation

2009-09-12T08:45:01Z

Aims. To quantify the transient growth of nonaxisymmetric perturbations in unstratified magnetized and stratified non-magnetized rotating linear shear flows in the shearing sheet approximation of accretion disc flows. Methods. The Rayleigh quotient in modal approaches for the linearized equations (with time-dependent wavenumber) and the amplitudes from direct shearing sheet simulations using a fin...

A Brandenburg, B Dintrans (2006) ASTRONOMY & ASTROPHYSICS 450: 2 437-444

Magnetic field generation in fully convective rotating spheres

2009-09-12T08:45:01Z

Magnetohydrodynamic simulations of fully convective, rotating spheres with volume heating near the center and cooling at the surface are presented. The dynamo-generated magnetic field saturates at equipartition field strength near the surface. In the interior, the field is dominated by small-scale structures, but outside the sphere, by the global scale. Azimuthal averages of the field reveal a lar...

W Dobler, M Stix, A Brandenburg (2006) ASTROPHYSICAL JOURNAL 638: 1 336-347

Turbulence from localized random expansion waves

2009-09-12T08:45:01Z

In an attempt to determine the outer scale of turbulence driven by localized sources, such as supernova explosions in the interstellar medium, we consider a forcing function given by the gradient of Gaussian profiles localized at random positions. Different coherence times of the forcing function are considered. In order to isolate the effects specific to the nature of the forcing function, we con...

A J Mee, A Brandenburg (2006) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 370: 1 415-419

Simulating field-aligned diffusion of a cosmic ray gas

2009-09-12T08:45:01Z

The macroscopic behaviour of cosmic rays in turbulent magnetic fields is discussed. An implementation of anisotropic diffusion of cosmic rays with respect to the magnetic field in a non-conservative, high-order, finite-difference magnetohydrodynamic code is discussed. It is shown that the standard implementation fails near singular X-points of the magnetic field, which are common if the field is r...

A P Snodin, A Brandenburg, A J Mee, A Shukurov (2006) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 373: 2 643-652

Life up North : Meeting report: Nordic Astrobiology 2006: Origins & distribution of life in the universe

2009-09-12T08:45:01Z

A M Poole, T Hode, A Brandenburg, A Hjalmarson, N G Holm (2006) ASTROBIOLOGY 6: 6 815-818

Location of the solar dynamo and near-surface shear

2009-09-12T08:45:01Z

The location of the solar dynamo is discussed in the context of new insights into the theory of nonlinear turbulent dynamos. It is argued that, from a dynamo-theoretic point of view, the bottom of the convection zone is not a likely location for the solar dynamo, but that it may be distributed over the convection zone. The near surface shear layer produces not only east-west field alignment, but i...

A Brandenburg (2006) Solar MHD Theory and Observations : A High Spatial Resolution Perspective, In honor of Robert F. Stein 354: 121-126

Hydrodynamic and hydromagnetic energy spectra from large eddy simulations

2009-09-12T08:45:01Z

Direct and large eddy simulations of hydrodynamic and hydromagnetic turbulence have been performed in an attempt to isolate artifacts from real and possibly asymptotic features in the energy spectra. It is shown that in a hydrodynamic turbulence simulation with a Smagorinsky subgrid scale model using 512(3) mesh points, two important features of the 4096(3) simulation on the Earth simulator [Y. Ka...

N E L Haugen, A Brandenburg (2006) PHYSICS OF FLUIDS 18: 7

The nature of turbulence in OMC1 at the scale of star formation : observations and simulations

2009-09-12T08:45:01Z

Aims. To study turbulence in the Orion Molecular Cloud (OMC1) by comparing observed and simulated characteristics of the gas motions. Methods. Using a dataset of vibrationally excited H-2 emission in OMC1 containing radial velocity and brightness which covers scales from 70 AU to 30 000 AU, we present the structure functions and the scaling of the structure functions with their order. These are co...

M Gustafsson, A Brandenburg, J L Lemaire, D Field (2006) ASTRONOMY & ASTROPHYSICS 454: 3 815-825

Magnetic helicity in primordial and dynamo scenarios of galaxies

2009-09-12T08:45:01Z

Some common properties of helical magnetic fields in decaying and driven turbulence are discussed. These include mainly the inverse cascade that produces fields on progressively larger scales. Magnetic helicity also restricts the evolution of the large-scale field: the field decays less rapidly than a non-helical field, but it also saturates more slowly, i.e. on a resistive time scale if there are...

A Brandenburg (2006) ASTRONOMISCHE NACHRICHTEN 327: 5-6 461-468

Galactic dynamo and helicity losses through fountain flow

2009-09-12T08:40:52Z

Hydrodynamic and hydromagnetic energy spectra from large eddy simulations

2009-09-12T08:40:52Z

Turbulence from localized random expansion waves

2009-09-12T08:40:52Z

Magnetic helicity density and its flux in weakly inhomogeneous turbulence

2009-09-12T08:40:52Z

Magnetic field generation in fully convective rotating spheres

2009-09-12T08:40:52Z

Location of the solar dynamo and near-surface shear

2009-09-12T08:40:52Z

Life up North : Meeting report: Nordic Astrobiology 2006: Origins & distribution of life in the universe

2009-09-12T08:40:52Z

A M Poole, T Hode, A Brandenburg, A Hjalmarson, N G Holm (2006) ASTROBIOLOGY 6: 6 815-818

The nature of turbulence in OMC1 at the scale of star formation : observations and simulations

2009-09-12T08:40:52Z

Simulating field-aligned diffusion of a cosmic ray gas

2009-09-12T08:40:52Z

Stellar dynamo driven wind braking versus disc coupling

2009-09-12T08:40:52Z

Magnetic helicity in primordial and dynamo scenarios of galaxies

2009-09-12T08:40:52Z

Life up north: meeting report: nordic astrobiology 2006: origins & distribution of life in the universe.

2009-09-12T01:23:09Z

Anthony M Poole, Tomas Hode, Axel Brandenburg, Ake Hjalmarson, Nils G Holm (2006) Astrobiology 6: 6 815-818

Radiative transfer in decomposed domains

2009-09-12T08:40:52Z

Nonaxisymmetric stability in the shearing sheet approximation

2009-09-12T08:40:52Z

Effect of the radiative background flux in convection

2009-09-12T08:45:01Z

Numerical simulations of turbulent stratified convection are used to study models with approximately the same convective flux, but different radiative fluxes. As the radiative flux is decreased, for constant convective flux: the entropy jump at the top of the convection zone becomes steeper, the temperature fluctuations increase and the velocity fluctuations decrease in magnitude, and the distance...

A Brandenburg, K L Chan, A Nordlund, R F Stein (2005) ASTRONOMISCHE NACHRICHTEN 326: 8 681-692

Astrophysical magnetic fields and nonlinear dynamo theory

2009-09-12T08:45:01Z

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 self-excited 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 num...

A Brandenburg, K Subramanian (2005) PHYSICS REPORTS-REVIEW SECTION OF PHYSICS LETTERS 417: 1-4 1-209

Passive scalar diffusion as a damped wave

2009-09-12T08:45:01Z

Three-dimensional turbulence simulations are used to show that the turbulent root mean square velocity is an upper bound of the speed of turbulent diffusion. There is a close analogy to magnetic diffusion where the maximum diffusion speed is the speed of light. Mathematically, this is caused by the inclusion of the Faraday displacement current which ensures that causality is obeyed. In turbulent d...

A Brandenburg, P J Kapyla, A Mohammed (2005) Progress in Turbulence 101: 3-6

Minimal tau approximation and simulations of the alpha effect

2009-09-12T08:45:01Z

The validity of a closure called the minimal tau approximation (MTA), is tested in the context of dynamo theory, wherein triple correlations are assumed to provide relaxation of the turbulent electromotive force. Under MTA, the alpha effect in mean field dynamo theory becomes proportional to a relaxation time scale multiplied by the difference between kinetic and current helicities. It is shown th...

A Brandenburg, K Subramanian (2005) ASTRONOMY & ASTROPHYSICS 439: 3 835-U2

Homochiral growth through enantiomeric cross-inhibition

2009-09-12T08:45:01Z

The stability and conservation properties of a recently proposed polymerization model are studied. The achiral (racemic) solution is linearly unstable once the relevant control parameter (here the fidelity of the catalyst) exceeds a critical value. The growth rate is calculated for different fidelity parameters and cross-inhibition rates. A chirality parameter is defined and shown to be conserved ...

A Brandenburg, A C Andersen, S H R Hofner, M Nilsson (2005) ORIGINS OF LIFE AND EVOLUTION OF THE BIOSPHERE 35: 3 225-241

Turbulence and its parameterization in accretion discs

2009-09-12T08:45:01Z

Accretion disc turbulence is investigated in the framework of the shearing box approximation. The turbulence is either driven by the magneto-rotational instability or, in the non-magnetic case, by an explicit and artificial forcing term in the momentum equation. Unlike the magnetic case, where most of the dissipation occurs in the disc corona, in the forced hydrodynamic case most of the dissipatio...

A Brandenburg (2005) ASTRONOMISCHE NACHRICHTEN 326: 9 787-797

Spectrum and amplitudes of internal gravity waves excited by penetrative convection in solar-type stars

2009-09-12T08:45:01Z

The excitation of internal gravity waves by penetrative convective plumes is investigated using 2-D direct simulations of compressible convection. The wave generation is quantitatively studied from the linear response of the radiative zone to the plumes penetration, using projections onto the g-modes solutions of the associated linear eigenvalue problem for the perturbations. This allows an accura...

B Dintrans, A Brandenburg, A Nordlund, R F Stein (2005) ASTRONOMY & ASTROPHYSICS 438: 1 365-376

The problem of small and large scale fields in the solar dynamo

2009-09-12T08:45:01Z

Three closely related stumbling blocks of solar mean field dynamo theory are discussed: how dominant are the small scale fields, how is the alpha effect quenched, and whether magnetic and current helicity fluxes alleviate the quenching? It is shown that even at the largest currently available resolution there is no clear evidence of power law scaling of the magnetic and kinetic energy spectra in t...

A Brandenburg, N E L Haugen, P Kapyla, C Sandin (2005) ASTRONOMISCHE NACHRICHTEN 326: 3-4 174-185

The case for a distributed solar dynamo shaped by near-surface shear

2009-09-12T08:45:01Z

Arguments for and against the widely accepted picture of a solar dynamo being seated in the tachocline are reviewed, and alternative ideas concerning dynamos operating in the bulk of the convection zone, or perhaps even in the near-surface shear layer, are discussed. Based on the angular velocities of magnetic tracers, it is argued that the observations are compatible with a distributed dynamo tha...

A Brandenburg (2005) ASTROPHYSICAL JOURNAL 625: 1 539-547

The onset of a small-scale turbulent dynamo at low magnetic Prandtl numbers

2009-09-12T08:45:01Z

We study numerically the dependence of the critical magnetic Reynolds number for the turbulent small-scale scale dynamo on the hydrodynamic Reynolds number. The turbulence is statistically homogeneous, isotropic, Re and mirror-symmetric. We are interested in the regime of low magnetic Prandtl number, which Pm = Rm/Re < 1 is relevant for stellar convective zones, protostellar disks, and laboratory ...

A A Schekochihin, N E L Haugen, A Brandenburg, S C Cowley, J L Maron, J C McWilliams (2005) ASTROPHYSICAL JOURNAL 625: 2 L115-L118

Strong mean field dynamos require supercritical helicity fluxes

2009-09-12T08:45:01Z

Several one and two dimensional mean field models are analyzed where the effects of current helicity fluxes and boundaries are included within the framework of the dynamical quenching model. In contrast to the case with periodic boundary conditions, the final saturation energy of the mean field decreases inversely proportional to the magnetic Reynolds number. If a nondimensional scaling factor in ...

A Brandenburg, K Subramanian (2005) ASTRONOMISCHE NACHRICHTEN 326: 6 400-408

Scaling laws in decaying helical hydromagnetic turbulence

2009-09-12T08:45:01Z

We study the evolution of growth and decay laws for the magnetic field coherence length, energy EM and magnetic helicity H in freely decaying 3D MHD turbulence. We show that with certain assumptions, self-similarity of the magnetic power spectrum alone implies that xi similar to t(1/2). This in turn implies that magnetic helicity decays as H similar to t(-2s), where s = (xi(diff)/xi(H))(2), in ter...

M Christensson, M Hindmarsh, A Brandenburg (2005) ASTRONOMISCHE NACHRICHTEN 326: 6 393-399

Dissociation in a polymerization model of homochirality.

2009-09-12T01:23:09Z

A fully self-contained model of homochirality is presented that contains the effects of both polymerization and dissociation. The dissociation fragments are assumed to replenish the substrate from which new monomers can grow and undergo new polymerization. The mean length of isotactic polymers is found to grow slowly with the normalized total number of corresponding building blocks. Alternatively,...

A Brandenburg, A C Andersen, M Nilsson (2005) Orig Life Evol Biosph 35: 6 507-521

Ejection of bi-helical fields from the Sun

2009-09-12T08:45:01Z

It is argued that much of the observed magnetic helicity losses at the solar surface may represent a reduction of an otherwise more dominant nonlinearity of solar and stellar dynamos. This nonlinearity is proportional to the internal twist (as opposed to writhe) of helical and sigmoidal surface structures.

A Brandenburg, E G Blackman (2005) HIGHLIGHTS OF ASTRONOMY, VOL 13 13: 101-104

Ejection of bi-helical fields from the Sun

2009-09-12T08:40:52Z

Dissociation in a polymerization model of homochirality

2009-09-12T08:45:01Z

Dissociation in a polymerization model of homochirality

2009-09-12T08:40:52Z

Homochiral growth through enantiomeric cross-inhibition.

2009-09-12T01:23:09Z

The stability and conservation properties of a recently proposed polymerization model are studied. The achiral (racemic) solution is linearly unstable once the relevant control parameter (here the fidelity of the catalyst) exceeds a critical value. The growth rate is calculated for different fidelity parameters and cross-inhibition rates. A chirality parameter is defined and shown to be conserved ...

A Brandenburg, A C Andersen, S Höfner, M Nilsson (2005) Orig Life Evol Biosph 35: 3 225-241

Outflows from dynamo-active protostellar accretion discs

2009-09-12T08:45:02Z

An axisymmetric model of a cool, dynamo-active accretion disc is applied to protostellar discs. Thermally and magnetically driven outflows develop that are not collimated within 0.1 AU. In the presence of a central magnetic field from the protostar, accretion onto the protostar is highly episodic, which is in agreement with earlier work.

B Von Rekowski, A Brandenburg, W Dobler, A Shukurov (2004) ASTROPHYSICS AND SPACE SCIENCE 292: 1-4 493-500

Self-regulating supernova heating in interstellar medium simulations

2009-09-12T08:45:01Z

Numerical simulations of the multi-phase interstellar medium have been carried out, using a 3D, nonlinear, magnetohydrodynamic, shearing-box model, with random motions driven by supernova explosions. These calculations incorporate the effects of magnetic fields and rotation in 3D; these play important dynamical roles in the galaxy, but are neglected in many other simulations. The supernovae drivin...

G R Sarson, A Shukurov, A Nordlund, B Gudiksen, A Brandenburg (2004) ASTROPHYSICS AND SPACE SCIENCE 292: 1-4 267-272

High-resolution simulations of nonhelical MHD turbulence

2009-09-12T08:45:01Z

According to the kinematic theory of nonhelical dynamo action, the magnetic energy spectrum increases with wavenumber and peaks at the resistive cutoff wavenumber. It has previously been argued that even in the dynamical case, the magnetic energy peaks at the resistive scale. Using high resolution simulations ( up to 1024(3) meshpoints) with no large-scale imposed field, we show that the magnetic ...

N E L Haugen, A Brandenburg, W Dobler (2004) ASTROPHYSICS AND SPACE SCIENCE 292: 1-4 53-60

Suppression of small scale dynamo action by an imposed magnetic field

2009-09-12T08:45:02Z

Nonhelical hydromagnetic turbulence with an externally imposed magnetic field is investigated using direct numerical simulations. It is shown that the imposed magnetic field lowers the spectral magnetic energy in the inertial range. This is explained by a suppression of the small scale dynamo. At large scales, however, the spectral magnetic energy increases with increasing imposed field strength f...

N E L Haugen, A Brandenburg (2004) PHYSICAL REVIEW E 70: 3

Mach number dependence of the onset of dynamo action

2009-09-12T08:45:02Z

The effect of compressibility on the onset of non-helical turbulent dynamo action is investigated using both direct simulations as well as simulations with shock-capturing viscosities, keeping, however, the regular magnetic diffusivity. It is found that the critical magnetic Reynolds number increases from about 35 in the subsonic regime to about 70 in the supersonic regime. Although the shock stru...

N E L Haugen, A Brandenburg, A J Mee (2004) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 353: 3 947-952

Identification of gravity waves in hydrodynamical simulations

2009-09-12T08:45:02Z

The excitation of internal gravity waves by an entropy bubble oscillating in an isothermal atmosphere is investigated using direct two-dimensional numerical simulations. The oscillation field is measured by a projection of the simulated velocity field onto the anelastic solutions of the linear eigenvalue problem for the perturbations. This facilitates a quantitative study of both the spectrum and ...

B Dintrans, A Brandenburg (2004) ASTRONOMY & ASTROPHYSICS 421: 3 775-782

Simulations of nonhelical hydromagnetic turbulence

2009-09-12T08:45:02Z

Nonhelical hydromagnetic forced turbulence is investigated using large scale simulations on up to 256 processors and 1024(3) mesh points. The magnetic Prandtl number is varied between 1/8 and 30, although in most cases it is unity. When the magnetic Reynolds number is based on the inverse forcing wave number, the critical value for dynamo action is shown to be around 35 for magnetic Prandtl number...

N E L Haugen, A Brandenburg, W Dobler (2004) PHYSICAL REVIEW E 70: 1

Homochiral growth through enantiomeric cross-inhibition

2009-09-12T08:45:02Z

A Brandenburg, A C Andersen, S Hofner, M Nilsson (2004) GEOCHIMICA ET COSMOCHIMICA ACTA 68: 11 A792-A792

Inertial range scaling in numerical turbulence with hyperviscosity

2009-09-12T08:45:02Z

Numerical turbulence with hyperviscosity is studied and compared with direct simulations using ordinary viscosity and data from wind tunnel experiments. It is shown that the inertial range scaling is similar in all three cases. Furthermore, the bottleneck effect is approximately equally broad (about one order of magnitude) in these cases and only its height is increased in the hyperviscous case-pr...

N E L Haugen, A Brandenburg (2004) PHYSICAL REVIEW E 70: 2

Catastrophic alpha quenching alleviated by helicity flux and shear

2009-09-12T08:45:01Z

A new simulation set-up is proposed for studying mean field dynamo action. The model combines the computational advantages of local Cartesian geometry with the ability to include a shear profile that resembles the sun's differential rotation at low latitudes. It is shown that in a two-dimensional mean field model this geometry produces cyclic solutions with dynamo waves traveling away from the equ...

A Brandenburg, C Sandin (2004) ASTRONOMY & ASTROPHYSICS 427: 1 13-21

Nonlinear current helicity fluxes in turbulent dynamos and alpha quenching

2009-09-12T08:45:01Z

Large scale dynamos produce small scale current helicity as a waste product that quenches the large scale dynamo process (alpha effect). This quenching can be catastrophic (i.e., intensify with magnetic Reynolds number) unless one has fluxes of small scale magnetic (or current) helicity out of the system. We derive the form of helicity fluxes in turbulent dynamos, taking also into account the nonl...

K Subramanian, A Brandenburg (2004) PHYSICAL REVIEW LETTERS 93: 20

Simulations of nonhelical hydromagnetic turbulence.

2009-09-12T01:23:09Z

Nonhelical hydromagnetic forced turbulence is investigated using large scale simulations on up to 256 processors and 1024(3) mesh points. The magnetic Prandtl number is varied between 1/8 and 30, although in most cases it is unity. When the magnetic Reynolds number is based on the inverse forcing wave number, the critical value for dynamo action is shown to be around 35 for magnetic Prandtl number...

Nils Erland L Haugen, Axel Brandenburg, Wolfgang Dobler (2004) Phys Rev E Stat Nonlin Soft Matter Phys 70: 1 Pt 2

Inertial range scaling in numerical turbulence with hyperviscosity.

2009-09-12T01:23:09Z

Numerical turbulence with hyperviscosity is studied and compared with direct simulations using ordinary viscosity and data from wind tunnel experiments. It is shown that the inertial range scaling is similar in all three cases. Furthermore, the bottleneck effect is approximately equally broad (about one order of magnitude) in these cases and only its height is increased in the hyperviscous case-pr...

Nils Erland L Haugen, Axel Brandenburg (2004) Phys Rev E Stat Nonlin Soft Matter Phys 70: 2 Pt 2

Suppression of small scale dynamo action by an imposed magnetic field.

2009-09-12T01:23:09Z

Nonhelical hydromagnetic turbulence with an externally imposed magnetic field is investigated using direct numerical simulations. It is shown that the imposed magnetic field lowers the spectral magnetic energy in the inertial range. This is explained by a suppression of the small scale dynamo. At large scales, however, the spectral magnetic energy increases with increasing imposed field strength f...

Nils Erland L Haugen, Axel Brandenburg (2004) Phys Rev E Stat Nonlin Soft Matter Phys 70: 3 Pt 2

Magnetic helicity evolution in a periodic domain with imposed field.

2009-09-12T01:23:09Z

In helical hydromagnetic turbulence with an imposed magnetic field (which is constant in space and time) the magnetic helicity of the field within a periodic domain is no longer an invariant of the ideal equations. Alternatively, there is a generalized magnetic helicity that is an invariant of the ideal equations. It is shown that this quantity is not gauge invariant and that it can therefore not ...

Axel Brandenburg, William H Matthaeus (2004) Phys Rev E Stat Nonlin Soft Matter Phys 69: 5 Pt 2

Self-similar scaling in decaying numerical turbulence.

2009-09-12T01:23:09Z

Decaying turbulence is studied numerically using as initial condition a random flow whose shell-integrated energy spectrum increases with wave number k like k(q). Alternatively, initial conditions are generated from a driven turbulence simulation by simply stopping the driving. It is known that the dependence of the decaying energy spectrum on wave number, time, and viscosity can be collapsed onto...

Tarek A Yousef, Nils Erland L Haugen, Axel Brandenburg (2004) Phys Rev E Stat Nonlin Soft Matter Phys 69: 5 Pt 2

Delayed correlation between turbulent energy injection and dissipation

2009-09-12T08:45:01Z

The dimensionless kinetic energy dissipation rate C-epsilon is estimated from numerical Simulations of statistically stationary isotropic box turbulence that is slightly compressible. The Taylor microscale Reynolds number (Re-lambda) range is 20 less than or similar to Re-lambda less than or similar to220 and the statistical stationarity is achieved with a random phase forcing method. The strong R...

B R Pearson, T A Yousef, N E L Haugen, A Brandenburg, P A Krogstad (2004) PHYSICAL REVIEW E 70: 5

Shearing and embedding box simulations of the magnetorotational instability

2009-09-12T08:45:01Z

Two different computational approaches to the magnetorotational instability (MRI) are pursued: the shearing box approach which is suited for local simulations and the embedding box approach whereby a Taylor Couette flow is embedded in a box so that numerical problems with the coordinate singularity are avoided. New shearing box simulations are presented and differences between regular and hypervis...

A Brandenburg, B Dintrans, N E L Haugen (2004) MHD COUETTE FLOWS : EXPERIMENTS AND MODELS 733: 122-136

Nonlinear current helicity fluxes in turbulent dynamos and alpha quenching.

2009-09-12T01:23:09Z

Large scale dynamos produce small scale current helicity as a waste product that quenches the large scale dynamo process (alpha effect). This quenching can be catastrophic (i.e., intensify with magnetic Reynolds number) unless one has fluxes of small scale magnetic (or current) helicity out of the system. We derive the form of helicity fluxes in turbulent dynamos, taking also into account the nonl...

Kandaswamy Subramanian, Axel Brandenburg (2004) Phys Rev Lett 93: 20

Delayed correlation between turbulent energy injection and dissipation.

2009-09-12T01:23:09Z

The dimensionless kinetic energy dissipation rate C(epsilon) is estimated from numerical simulations of statistically stationary isotropic box turbulence that is slightly compressible. The Taylor microscale Reynolds number (Re(lambda)) range is 20< or approximately equal to Re(lambda) < or approximately equal to 220 and the statistical stationarity is achieved with a random phase forcing method. T...

Bruce R Pearson, Tarek A Yousef, Nils Erland L Haugen, Axel Brandenburg, Per-Age Krogstad (2004) Phys Rev E Stat Nonlin Soft Matter Phys 70: 5 Pt 2

Magnetic helicity evolution in a periodic domain with imposed field

2009-09-12T08:45:02Z

In helical hydromagnetic turbulence with an imposed magnetic field (which is constant in space and time) the magnetic helicity of the field within a periodic domain is no longer an invariant of the ideal equations. Alternatively, there is a generalized magnetic helicity that is an invariant of the ideal equations. It is shown that this quantity is not gauge invariant and that it can therefore not ...

A Brandenburg, W H Matthaeus (2004) PHYSICAL REVIEW E 69: 5

The effects of spiral arms on the multi-phase ISM

2009-09-12T08:45:02Z

Statistical parameters of the ISM driven by thermal energy injections from supernova explosions have been obtained from 3D, nonlinear, magnetohydrodynamic, shearing-box simulations for spiral arm and interarm regions. The density scale height obtained for the interarm regions is 50% larger than for the spiral arms because of the higher gas temperature. The filling factor of the hot gas is also sig...

A Shukurov, G R Sarson, A Nordlund, B Gudiksen, A Brandenburg (2004) ASTROPHYSICS AND SPACE SCIENCE 289: 3-4 319-322

Outflows and accretion in a star-disc system with stellar magnetosphere and disc dynamo

2009-09-12T08:45:02Z

The interaction between a protostellar magnetosphere and a surrounding dynamo-active accretion disc is investigated using an axisymmetric mean-field model. In all models investigated, the dynamo-generated magnetic field in the disc arranges itself such that in the corona, the field threading the disc is anti-aligned with the central dipole so that no X-point forms. When the magnetospheric field is...

B von Rekowski, A Brandenburg (2004) ASTRONOMY & ASTROPHYSICS 420: 1 17-32

Non-Fickian diffusion and tau approximation from numerical turbulence

2009-09-12T08:45:02Z

Evidence for non-Fickian diffusion of a passive scalar is presented using direct simulations of homogeneous isotropic turbulence. The results compare favorably with an explicitly time-dependent closure model based on the tau approximation. In the numerical experiments three different cases are considered: (i) zero mean concentration with finite initial concentration flux, (ii) an initial top hat p...

A Brandenburg, P J Kapyla, A Mohammed (2004) PHYSICS OF FLUIDS 16: 4 1020-1027

Simulations of dust-trapping vortices in protoplanetary discs

2009-09-12T08:45:02Z

Local three-dimensional shearing box simulations of the compressible coupled dust-gas equations are used in the fluid approximation to study the evolution of different initial vortex configurations in a protoplanetary disc and their dust-trapping capabilities. The initial conditions for the gas are derived from an analytic solution to the compressible Euler equation and the continuity equation. Th...

A Johansen, A C Andersen, A Brandenburg (2004) ASTRONOMY & ASTROPHYSICS 417: 1 361-374

Self-similar scaling in decaying numerical turbulence

2009-09-12T08:45:02Z

Decaying turbulence is studied numerically using as initial condition a random flow whose shell-integrated energy spectrum increases with wave number k like k(q). Alternatively, initial conditions are generated from a driven turbulence simulation by simply stopping the driving. It is known that the dependence of the decaying energy spectrum on wave number, time, and viscosity can be collapsed onto...

T A Yousef, N E L Haugen, A Brandenburg (2004) PHYSICAL REVIEW E 69: 5

Structured outflow from a dynamo active accretion disc

2009-09-12T08:45:02Z

We present an axisymmetric numerical model of a dynamo active accretion disc. If the dynamo-generated magnetic field in the disc is sufficiently strong (close to equipartition with thermal energy), a fast magneto-centrifugally driven outflow develops within a conical shell near the rotation axis, together with a slower pressure driven outflow from the outer parts of the disc as well as around the ...

B von Rekowski, A Brandenburg, W Dobler, A Shukurov (2003) ASTRONOMY & ASTROPHYSICS 398: 3 825-844

Contributions to the theory of a two-scale homogeneous dynamo experiment

2009-09-12T08:45:02Z

The principle of the two-scale dynamo experiment at the Forschungszentrum Karlsruhe is closely related to that of the Roberts dynamo working with a simple fluid flow which is, with respect to proper Cartesian coordinates x, y, and z, periodic in x and y and independent of z. A modified Roberts dynamo problem is considered with a flow more similar to that in the experimental device. Solutions are c...

K H Radler, A Brandenburg (2003) PHYSICAL REVIEW E 67: 2

Doubly helical coronal ejections from dynamos and their role in sustaining the solar cycle

2009-09-12T08:45:02Z

Two questions about the solar magnetic field might be answered together once their connection is identified. The first is important for large-scale dynamo theory: what prevents the magnetic back- reaction forces from shutting down the dynamo cycle? The second question is, what determines the handedness of twist and writhe in magnetized coronal ejecta? Magnetic helicity conservation is important fo...

E G Blackman, A Brandenburg (2003) ASTROPHYSICAL JOURNAL 584: 2 L99-L102

Bottleneck effect in three-dimensional turbulence simulations.

2009-09-12T01:23:09Z

At numerical resolutions around 512(3) and above, three-dimensional energy spectra from turbulence simulations begin to show noticeably shallower spectra than k(-5/3) near the dissipation wave number ("bottleneck effect"). This effect is shown to be significantly weaker in one-dimensional spectra such as those obtained in wind tunnel turbulence. The difference can be understood in terms of the tra...

Wolfgang Dobler, Nils Erland L Haugen, Tarek A Yousef, Axel Brandenburg (2003) Phys Rev E Stat Nonlin Soft Matter Phys 68: 2 Pt 2

Stochastic excitation of gravity waves by overshooting convection in solar-type stars

2009-09-12T08:45:02Z

The excitation of gravity waves by penetrative convective plumes is investigated using 2D direct simulations of compressible convection. The oscillation field is measured by a new technique based on the projection of our simulation data onto the theoretical g-modes solutions of the associated linear eigenvalue problem. This allows us to determine both the excited modes and their corresponding ampl...

B Dintrans, A Brandenburg, A Nordlund, R F Stein (2003) ASTROPHYSICS AND SPACE SCIENCE 284: 1 237-240

Contributions to the theory of a two-scale homogeneous dynamo experiment.

2009-09-12T01:23:09Z

The principle of the two-scale dynamo experiment at the Forschungszentrum Karlsruhe is closely related to that of the Roberts dynamo working with a simple fluid flow which is, with respect to proper Cartesian coordinates x, y, and z, periodic in x and y and independent of z. A modified Roberts dynamo problem is considered with a flow more similar to that in the experimental device. Solutions are c...

Karl-Heinz Rädler, Axel Brandenburg (2003) Phys Rev E Stat Nonlin Soft Matter Phys 67: 2 Pt 2

Helical surface structures

2009-09-12T08:45:01Z

Over the past few years there has been growing interest in helical magnetic field structures seen at the solar surface, in coronal mass ejections, as well as in the solar wind. Although there is a great deal of randomness in the data, on average the extended structures are mostly left-handed on the northern hemisphere and right-handed on the southern. Surface field structures are also classified a...

A Brandenburg, E G Blackman (2003) MODELLING OF STELLAR ATMOSPHERES 210: 233-242

Helical surface structures

2009-09-12T08:40:52Z

How magnetic helicity ejection helps large scale dynamos

2009-09-12T08:45:02Z

There is mounting evidence that the ejection of magnetic helicity from the solar surface is important for the solar dynamo. Observations suggest that in the northern hemisphere the magnetic helicity flux is negative. We propose that this magnetic helicity flux is mostly due to small scale magnetic fields; in contrast to the more systematic large scale field of the 11 year cycle, whose helicity flu...

A Brandenburg, E G Blackman, G R Sarson (2003) MAGNETIC HELICITY AT THE SUN, IN SOLAR WIND AND MAGNETOSPHERES : VISTAS FROM X-RAY OBSERVATORIES 32: 10 1835-1844

Relaxation of writhe and twist of a bi-helical magnetic field

2009-09-12T08:45:02Z

In the past few years suggestions have emerged that the solar magnetic field might have a bi-helical contribution with oppositely polarized magnetic fields at large and small scales, and that the shedding of such fields may be crucial for the operation of the dynamo. It is shown that, if a bi-helical field is shed into the solar wind, positive and negative contributions of the magnetic helicity sp...

T A Yousef, A Brandenburg (2003) ASTRONOMY & ASTROPHYSICS 407: 1 7-12

Turbulent magnetic Prandtl number and magnetic diffusivity quenching from simulations

2009-09-12T08:45:02Z

Forced turbulence simulations are used to determine the turbulent kinematic viscosity, v(t), from the decay rate of a large scale velocity field. Likewise, the turbulent magnetic diffusivity, eta(t), is determined from the decay of a large scale magnetic field. In the kinematic regime, when the field is weak, the turbulent magnetic Prandtl number, v(t)/eta(t), is about unity. When the field is non...

T A Yousef, A Brandenburg, G Rudiger (2003) ASTRONOMY & ASTROPHYSICS 411: 3 321-327

The helicity issue in large scale dynamos

2009-09-12T08:45:02Z

'The connection between helically isotropic MHD turbulence and mean-field dynamo theory is reviewed. The nonlinearity in the mean-field theory is not yet well established, but detailed comparison with simulations begin to help select viable forms of the nonlinearity. The crucial discriminant is the magnetic helicity; which is known to evolve only on a slow resistive time scale in the limit of larg...

A Brandenburg (2003) TURBULENCE AND MAGNETIC FIELDS IN ASTROPHYSICS 614: 402-431

MHD simulations of small and large scale dynamos

2009-09-12T08:45:01Z

Isotropic homogeneous hydromagnetic turbulence is studied using numerical simulations at resolutions of up to 1024(3) meshpoints. It is argued that, in contrast to the kinematic regime, the nonlinear regime is characterized by a spectral magnetic power that is decreasing with increasing wavenumber, regardless of whether or not the turbulence has helicity. This means that the root-mean-square field...

A Brandenburg, N E L Haugen, W Dobler (2003) TURBULENCE, WAVES AND INSTABILITIES IN THE SOLAR PLASMA 124: 33-53

Bottleneck effect in three-dimensional turbulence simulations

2009-09-12T08:45:02Z

At numerical resolutions around 512(3) and above, three-dimensional energy spectra from turbulence simulations begin to show noticeably shallower spectra than k(-5/3) near the dissipation wave number ("bottleneck effect"). This effect is shown to be significantly weaker in one-dimensional spectra such as those obtained in wind tunnel turbulence. The difference can be understood in terms of the tra...

W Dobler, N E L Haugen, T A Yousef, A Brandenburg (2003) PHYSICAL REVIEW E 68: 2

Is nonhelical hydromagnetic turbulence peaked at small scales?

2009-09-12T08:45:02Z

Nonhelical hydromagnetic turbulence without an imposed magnetic field is considered in the case where the magnetic Prandtl number is unity. The magnetic field is entirely due to dynamo action. The magnetic energy spectrum peaks at a wavenumber of about 5 times the minimum wavenumber in the domain, and not at the resistive scale, as has previously been argued. Throughout the inertial range, the spe...

N E L Haugen, A Brandenburg, W Dobler (2003) ASTROPHYSICAL JOURNAL 597: 2 L141-L144

Magnetic helicity in stellar dynamos : new numerical experiments

2009-09-12T08:45:02Z

The theory of large scale dynamos is reviewed with particular emphasis on the magnetic helicity constraint in the presence of closed and open boundaries. In the presence of closed or periodic boundaries, helical dynamos respond to the helicity constraint by developing small scale separation in the kinematic regime, and by showing long time scales in the nonlinear regime where the scale separation ...

A Brandenburg, W Dobler, K Subramanian (2002) ASTRONOMISCHE NACHRICHTEN 323: 2 99-122

Nonlinear states of the screw dynamo

2009-09-12T08:45:02Z

The self-excitation of magnetic field by a spiral Couette flow between two coaxial cylinders is considered. We solve numerically the fully nonlinear, three-dimensional magnetohydrodynamic (MHD) equations for magnetic Prandtl numbers P-m (ratio of kinematic viscosity to magnetic diffusivity! between 0.14 and 10 and kinematic and magnetic Reynolds numbers up to about 2000. In the initial stage of ex...

W Dobler, A Shukurov, A Brandenburg (2002) PHYSICAL REVIEW E 65: 3

Effect of hyperdiffusivity on turbulent dynamos with helicity.

2009-09-12T01:23:09Z

In numerical studies of turbulence, hyperviscosity is often used as a tool to extend the inertial subrange and to reduce the dissipative subrange. By analogy, hyperdiffusivity (or hyperresistivity) is sometimes used in magnetohydrodynamics. The underlying assumption is that only the small scales are affected by this manipulation. In the present paper, possible side effects on the evolution of the ...

Axel Brandenburg, Graeme R Sarson (2002) Phys Rev Lett 88: 5

Nonlinear states of the screw dynamo.

2009-09-12T01:23:09Z

The self-excitation of magnetic field by a spiral Couette flow between two coaxial cylinders is considered. We solve numerically the fully nonlinear, three-dimensional magnetohydrodynamic (MHD) equations for magnetic Prandtl numbers P(m) (ratio of kinematic viscosity to magnetic diffusivity) between 0.14 and 10 and kinematic and magnetic Reynolds numbers up to about 2000. In the initial stage of e...

Wolfgang Dobler, Anvar Shukurov, Axel Brandenburg (2002) Phys Rev E Stat Nonlin Soft Matter Phys 65: 3 Pt 2B

Solitons in the noisy Burgers equation

2009-09-12T08:45:02Z

We investigate numerically the coupled diffusion-advective type field equations originating from the canonical phase space approach to the noisy Burgers equation or the equivalent Kardar-Parisi-Zhang equation in one spatial dimension. The equations support stable right hand and left hand solitons and in the low viscosity limit a long-lived soliton pair excitation. We find that two identical pair e...

H C Fogedby, A Brandenburg (2002) PHYSICAL REVIEW E 66: 1

Solitons in the noisy Burgers equation.

2009-09-12T01:23:09Z

We investigate numerically the coupled diffusion-advective type field equations originating from the canonical phase space approach to the noisy Burgers equation or the equivalent Kardar-Parisi-Zhang equation in one spatial dimension. The equations support stable right hand and left hand solitons and in the low viscosity limit a long-lived soliton pair excitation. We find that two identical pair e...

Hans C Fogedby, Axel Brandenburg (2002) Phys Rev E Stat Nonlin Soft Matter Phys 66: 1 Pt 2

Effect of hyperdiffusivity on turbulent dynamos with helicity

2009-09-12T08:45:02Z

In numerical studies of turbulence, hyperviscosity is often used as a tool to extend the inertial subrange and to reduce the dissipative subrange. By analogy, hyperdiffusivity (or hyperresistivity) is sometimes used in magnetohydrodynamics. The underlying assumption is that only the small scales are affected by this manipulation. In the present paper, possible side effects on the evolution of the ...

A Brandenburg, G R Sarson (2002) PHYSICAL REVIEW LETTERS 88: 5

Hydromagnetic turbulence in computer simulations

2009-09-12T08:45:02Z

The usefulness of high-order schemes in astrophysical MHD turbulence simulations is discussed. Simple advection tests of hat profiles are used to compare schemes of different order. Higher order schemes generally need less explicit diffusion. In the case of a standing Burgers shock it is shown that the overall accuracy improves as the order of the scheme is increased. A memory efficient 3-step 2N-...

A Brandenburg, W Dobler (2002) COMPUTER PHYSICS COMMUNICATIONS 147: 1-2 471-475

Dynamic nonlinearity in large-scale dynamos with shear

2009-09-12T08:45:02Z

We supplement the mean field dynamo growth equation with the total magnetic helicity evolution equation. This provides an explicitly time-dependent model for alpha-quenching in dynamo theory. For dynamos without shear, this approach accounts for the observed large-scale field growth and saturation in numerical simulations. After a significant kinematic phase, the dynamo is resistively quenched, i....

E G Blackman, A Brandenburg (2002) ASTROPHYSICAL JOURNAL 579: 1 359-373

Local and nonlocal magnetic diffusion and alpha-effect tensors in shear flow turbulence

2009-09-12T08:45:02Z

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 three-dimensional simulation of a rotating turbulent shear flow in a slab and a simple one-dimensional alpha omega dynamo model is given. A direct determination of transport coeff...

A Brandenburg, D Sokoloff (2002) GEOPHYSICAL AND ASTROPHYSICAL FLUID DYNAMICS 96: 4 319-344

Magnetic helicity and the solar dynamo

2009-09-12T08:45:02Z

Solar and stellar dynamos - latest developments

2009-09-12T08:45:02Z

Recent progress in the theory of solar and stellar dynamos is reviewed. Particular emphasis is placed on the mean-field theory which tries to describe the collective behavior of the magnetic field. In order to understand solar and stellar activity, a quantitatively reliable theory is necessary. Much of the new developments center around magnetic helicity conservation which is seen to be important ...

A Brandenburg, W Dobler (2002) ASTRONOMISCHE NACHRICHTEN 323: 3-4 411-416

Numerical simulations of turbulent dynamos

2009-09-12T08:45:02Z

Using 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 with alpha and eta(t) quenchings that are equally strong and 'catastrophic'.

A Brandenburg (2002) HIGHLIGHTS OF ASTRONOMY, VOL 12 12: 742-744

Magnetoconvection and dynamo coefficients - II. Field-direction dependent pumping of magnetic field

2009-09-12T08:45:02Z

We study the pumping of magnetic flux in three-dimensional compressible magnetoconvection in the context of stellar dynamos. The simulation domain represents a rectangular section from the lower part of a stellar convection zone plus the underlying stably stratified layer, with a total depth of up to five pressure scale heights. Once convection has attained a statistically stationary state, a magn...

M Ossendrijver, M Stix, A Brandenburg, G Rudiger (2002) ASTRONOMY & ASTROPHYSICS 394: 2 735-745

A new look at dynamo cycle amplitudes

2009-09-12T08:45:02Z

We explore the dependence of the amplitude of stellar dynamo cycle variability (as seen in the Mount Wilson Ca II HK timeseries data) on other stellar parameters. We find that the fractional cycle amplitude A(cyc) (i.e. the ratio of the peak-to-peak variation to the average) decreases somewhat with mean activity, increases with decreasing effective temperature, but is not correlated with inverse R...

S H Saar, A Brandenburg (2002) ASTRONOMISCHE NACHRICHTEN 323: 3-4 357-360

Dynamical friction of bodies orbiting in a gaseous sphere

2009-09-12T08:45:02Z

The dynamical friction experienced by a body moving in a gaseous medium is different from the friction in the case of a collisionless stellar system. Here we consider the orbital evolution of a gravitational perturber inside a gaseous sphere using three-dimensional simulations, ignoring however self-gravity. The results are analysed in terms of a 'local' formula with the associated Coulomb logarit...

F J Sanchez-Salcedo, A Brandenburg (2001) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 322: 1 67-78

The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence

2009-09-12T08:45:02Z

A numerical model of isotropic homogeneous turbulence with helical forcing is investigated. The resulting flow, which is essentially the prototype of the alpha (2) dynamo of mean field dynamo theory, produces strong dynamo action with an additional large-scale field on the scale of the box (at wavenumber k = 1; forcing is at k = 5). This large-scale field is nearly force free and exceeds the equip...

A Brandenburg (2001) ASTROPHYSICAL JOURNAL 550: 2 824-840

Cosmology. Magnetic mysteries.

2009-09-12T01:23:09Z

A Brandenburg (2001) Science 292: 5526 2440-2441

Large scale dynamos with helicity loss through boundaries

2009-09-12T08:45:02Z

Dynamo action is investigated in simulations of locally isotropic and homogeneous turbulence in a slab between open boundaries. It is found that a "pseudo-vacuum" boundary condition (where the field is vertical) leads to strong helicity fluxes which significantly reduce the amplitude of the resulting large-scale field. On the other hand, if there is a conducting halo outside the dynamo-active regi...

A Brandenburg, W Dobler (2001) ASTRONOMY AND ASTROPHYSICS 369: 1 329-338

Inverse cascade in decaying three-dimensional magnetohydrodynamic turbulence.

2009-09-12T01:23:09Z

We perform direct numerical simulations of three-dimensional freely decaying magnetohydrodynamic turbulence. For helical magnetic fields, an inverse cascade effect is observed in which power is transferred from smaller scales to larger scales. The magnetic field reaches a scaling regime with self-similar evolution, and power-law behavior at high wave numbers. We also find power-law decay in the ma...

M Christensson, M Hindmarsh, A Brandenburg (2001) Phys Rev E Stat Nonlin Soft Matter Phys 64: 5 Pt 2

The helicity constraint in turbulent dynamos with shear

2009-09-12T08:45:02Z

The evolution of magnetic fields is studied using simulations of forced helical turbulence with strong imposed shear. After some initial exponential growth, the magnetic field develops a large-scale travelling wave pattern. The resulting field structure possesses magnetic helicity, which is conserved in a periodic box by the ideal magnetohydrodynamics equations and can hence only change on a resis...

A Brandenburg, A Bigazzi, K Subramanian (2001) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 325: 2 685-692

The effects of vertical outflows on disk dynamos

2009-09-12T08:45:02Z

We consider the effect of vertical outflows on the mean-field dynamo in a thin disk. These outflows could be due to winds or magnetic buoyancy. We analyse both two-dimensional finite-difference numerical solutions of the axisymmetric dynamo equations and a free-decay mode expansion using the thin-disk approximation. Contrary to expectations, a vertical velocity can enhance dynamo action! provided ...

A Bardou, B von Rekowski, W Dobler, A Brandenburg, A Shukurov (2001) ASTRONOMY & ASTROPHYSICS 370: 2 635-648

The solar dynamo : Old, recent, and new problems

2009-09-12T08:45:02Z

A number of problems of solar and stellar dynamo theory are briefly reviewed and the current status of possible solutions is discussed. Results of direct numerical simulations are described in view of mean-field dynamo theory and the relation between the alpha-effect and the inverse cascade of magnetic helicity is highlighted. The possibility of 'catastrophic' quenching of the alpha-effect is expl...

A Brandenburg (2001) RECENT INSIGHTS INTO THE PHYSICS OF THE SUN AND HELIOSPHERE : HIGHLIGHTS FROM SOHO AND OTHER SPACE MISSIONS : 203 144-151

Helicity in hydro and MHD reconnection

2009-09-12T08:45:02Z

Helicity, a measure of the linkage of flux lines, has subtle and largely unknown effects upon dynamics. Both magnetic and hydrodynamic helicity are conserved for ideal systems and could suppress nonlinear dynamics. What actually happens is not clear because in a fully three-dimensional system there are additional channels whereby intense, small-scale dynamics can occur. This contribution shows one...

A Brandenburg, R M Kerr (2001) QUANTIZED VORTEX DYNAMICS AND SUPERFLUID TURBULENCE 571: 358-365

Two-dimensional disk dynamos with vertical outflows into a halo

2009-09-12T08:45:02Z

We study the effects of vertical outflows on mean-field dynamos in disks. These outflows could be due to thermal winds or magnetic buoyancy. We analyse numerical solutions of the nonlinear mean-field dynamo equations using a two-dimensional finite-difference model. Contrary to expectations, a modest vertical velocity can enhance dynamo action. This can lead to super-exponential growth of the magne...

B Von Rekowski, W Dobler, A Shukurov, A Brandenburg (2001) DYNAMO AND DYNAMICS, A MATHEMATICAL CHALLENGE 26: 305-312

Magnetoconvection and dynamo coefficients : Dependence of the alpha effect on rotation and magnetic field

2009-09-12T08:45:02Z

We present numerical simulations of three-dimensional compressible magnetoconvection in a rotating rectangular box that represents a section of the solar convection zone. The box contains a convectively unstable layer, surrounded by stably stratified layers with overshooting convection. The magnetic Reynolds number, Rm, is chosen subcritical, thus excluding spontaneous growth of the magnetic field...

M Ossendrijver, M Stix, A Brandenburg (2001) ASTRONOMY & ASTROPHYSICS 376: 2 713-726

Astrophysical significance of the anisotropic kinetic alpha effect

2009-09-12T08:45:02Z

The generation of large scale flows by the anisotropic kinetic alpha (AKA) effect is investigated in simulations with a suitable time-dependent space- and time-periodic anisotropic forcing lacking parity invariance. The forcing pattern moves relative to the fluid, which leads to a breaking of the Galilean invariance as required for the AKA effect to exist. The AKA effect is found to produce a clea...

A Brandenburg, B von Rekowski (2001) ASTRONOMY & ASTROPHYSICS 379: 3 1153-1160

Evolution of highly buoyant thermals in a stratified layer

2009-09-12T08:45:02Z

The buoyant rise of thermals (i.e. bubbles of enhanced entropy, but initially in pressure equilibrium) is investigated numerically in three dimensions for the case of an adiabatically stratified layer covering 6-9 pressure scale heights. It is found that these bubbles can travel to large heights before being braked by the excess pressure that builds up in order to drive the gas sideways in the hea...

A Brandenburg, J Hazlehurst (2001) ASTRONOMY & ASTROPHYSICS 370: 3 1092-1102

Cosmology - Magnetic mysteries

2009-09-12T08:45:02Z

A Brandenburg (2001) SCIENCE 292: 5526 2440-2441

The inverse cascade in turbulent dynamos

2009-09-12T08:45:02Z

The emergence of a large scale magnetic field from randomly forced isotropic strongly helical flows is discussed in terms of the inverse cascade of magnetic helicity and the alpha-effect. In simulations of such flows the maximum field strength exceeds the equipartition field strength for large scale separation. However, helicity conservation controls the speed at which this final state is reached....

A Brandenburg (2001) DYNAMO AND DYNAMICS, A MATHEMATICAL CHALLENGE 26: 125-132

Inverse cascade in decaying three-dimensional magnetohydrodynamic turbulence

2009-09-12T08:45:02Z

We perform direct numerical simulations of three-dimensional freely decaying magnetohydrodynamic turbulence. For helical magnetic fields, an inverse cascade effect is observed in which power is transfered from smaller scales to larger scales. The magnetic field reaches a scaling regime with self-similar evolution, and power-law behavior at high wave numbers. We also find power-law decay in the mag...

M Christensson, M Hindmarsh, A Brandenburg (2001) PHYSICAL REVIEW E 64: 5 art. no.-056405

Search for non-helical disc dynamos in simulations

2009-09-12T08:45:02Z

The origin of large scale magnetic fields in accretion discs is investigated. Using global three-dimensional simulations of accretion disc turbulence, a recent suggestion of Vishniac & Cho (2001, ApJ 550, 752) is re-examined, according to which large scale fields in accretion discs could be understood without explicitly invoking the usual helicity effect. Particular emphasis is placed on a certain...

R Arlt, A Brandenburg (2001) ASTRONOMY & ASTROPHYSICS 380: 1 359-372

Magnetohydrodynamic turbulence in accretion discs

2009-09-12T08:45:02Z

We present results from numerical simulations of magnetohydrodynamic turbulence in accretion discs. Our simulations show that the turbulent stresses that drive the accretion are less stratified than the matter; thus, the surface layers are more strongly heated than the interior of the disc.

U Torkelsson, A Brandenburg, A Nordlund, R F Stein (2000) HIGHLY ENERGETIC PHYSICAL PROCESSES AND MECHANISMS FOR EMISSION FROM ASTROPHYSICAL PLASMAS : 195 241-242

Nonlocal transport of passive scalars in turbulent penetrative convection

2009-09-12T01:23:09Z

We present a Green's function approach for quantifying the transport of a passive scalar (tracer) field in three-dimensional simulations of turbulent convection. Nonlocal, nondiffusive behavior is described by a transilient matrix (the discretized Green's function), whose elements contain the fractional tracer concentrations moving from one subvolume to another as a function of time. The approach ...

Miesch, Brandenburg, Zweibel (2000) Phys Rev E Stat Phys Plasmas Fluids Relat Interdiscip Topics 61: 1 457-467

Dynamo-generated turbulence and outflows from accretion discs

2009-09-12T08:45:02Z

Local hydromagnetic simulations of accretion-disc turbulence currently provide the most convincing evidence that the origin of turbulence in discs could be the Balbus-Hawley magnetorotational instability. The main results of such calculations are highlighted with particular emphasis on the generation of large-scale magnetic fields. Comparison with mean-field dynamo theory is made. This theory is t...

A Brandenburg (2000) PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES 358: 1767 759-774

The response of a turbulent accretion disc to an imposed epicyclic shearing motion

2009-09-12T08:45:02Z

We excite an epicyclic motion, the amplitude of which depends on the vertical position, z, in a simulation of a turbulent accretion disc. An epicyclic motion of this kind may be caused by a warping of the disc. By studying how the epicyclic motion decays, we can obtain information about the interaction between the warp and the disc turbulence. A high-amplitude epicyclic motion decays first by exci...

U Torkelsson, G I Ogilvie, A Brandenburg, J E Pringle, A Nordlund, R F Stein (2000) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 318: 1 47-57

Large scale dynamos with ambipolar diffusion nonlinearity

2009-09-12T08:45:02Z

It is shown that ambipolar diffusion as a useful model for nonlinearity leads to similar behaviour of large scale turbulent dynamos as full MHD. This is demonstrated using both direct simulations in a periodic box and a closure model for the magnetic correlation functions applicable to infinite space. Large scale fields develop via a nonlocal inverse cascade as described by the ct-effect. However,...

A Brandenburg, K Subramanian (2000) ASTRONOMY AND ASTROPHYSICS 361: 2 L33-L36

Dynamo mechanisms

2009-09-12T08:45:02Z

Dynamo theory is reviewed with particular emphasis on recent developments. There now seems to be a strong case for dynamo effects that are driven by the magnetic field itself. This is linked to recent interpretations of the observed stellar cycle periods which suggest that the ratio of cycle frequency to rotational frequency increases, up to some point, with stellar chromospheric activity. This ra...

A Brandenburg, S H Saar (2000) STELLAR CLUSTERS AND ASSOCIATIONS : CONVECTION, ROTATION, AND DYNAMOS 198: 381-390

Non-linear magnetic diffusivity in mean-field electrodynamics

2009-09-12T08:45:02Z

We consider non-linear transport and drift processes caused by an inhomogeneous magnetic field in a turbulent fluid. The coefficients of magnetic diffusivity and drift velocity are calculated by making use of the second-order correlation approximation. Transport processes in the presence of a sufficiently strong magnetic field become anisotropic with larger diffusion rate and turbulent electrical ...

V Urpin, A Brandenburg (2000) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 316: 3 684-688

Magnetic fields of stars : using stars as tools for understanding the origins of cosmic magnetic fields - Discussion

2009-09-12T08:45:02Z

I W Roxburgh, R Rosner, A Brandenburg, N O Weiss (2000) PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES 358: 1767 708-709

Nonlocal transport of passive scalers in turbulent penetrative convection

2009-09-12T08:45:02Z

We present a Green's function approach for quantifying the transport of a passive scalar (tracer) field in three-dimensional simulations of turbulent convection. Nonlocal, nondiffusive behavior is described by a transilient matrix (the discretized Green's function), whose elements contain the fractional tracer concentrations moving from one subvolume to another as a function of time. The approach ...

M S Miesch, A Brandenburg, E G Zweibel (2000) PHYSICAL REVIEW E 61: 1 457-467

Jets from accretion discs - Discussion

2009-09-12T08:45:02Z

A Shukurov, R E Pudritz, I F Mirabel, Y Uchida, I W Roxburgh, K Barker, D Lynden-Bell, A Brandenburg, R D Blandford (2000) PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES 358: 1767 756-758

Vortex tube models for turbulent dynamo action

2009-09-12T08:45:02Z

The possibility of dynamo action resulting from a pair of elongated vortex structures immersed in an electrically conducting fluid is investigated. For elongated vortex structures, the critical magnetic Reynolds number for dynamo action is about half that for the spherical rotors that have been studied previously. When applied to Kolmogorov turbulence with vortex structures of scale comparable to ...

A Bigazzi, A Brandenburg, D Moss (1999) PHYSICS OF PLASMAS 6: 1 72-80

Deceleration by dynamical friction in a gaseous medium

2009-09-12T08:45:02Z

The drag force experienced by a gravitational body moving in a straight-line trajectory through a homogeneous isothermal gaseous medium of given sound speed is investigated numerically. For perturbers with constant velocity, linear theory describes successfully the temporal evolution and magnitude of the force. The result obtained recently by E. Ostriker-that for Mach numbers M = 1-2 the force is ...

F J Sanchez-Salcedo, A Brandenburg (1999) ASTROPHYSICAL JOURNAL 522: 1 L35-L38

Time evolution of the magnetic activity cycle period. II. Results for an expanded stellar sample

2009-09-12T08:45:02Z

We further explore nondimensional relationships between the magnetic dynamo cycle period P-eye, the rotational period P-rot, the activity level (as observed in Ca II HK), and other stellar properties by expanding the stellar sample studied in the first paper in this series. We do this by adding photometric and other cycles seen in active stars and the secondaries of CV systems and by selectively a...

S H Saar, A Brandenburg (1999) ASTROPHYSICAL JOURNAL 524: 1 295-310

Evolution of a superbubble in a turbulent, multi-phased and magnetized ISM

2009-09-12T08:45:02Z

The evolution of a superbubble is simulated using a local three-dimensional, non-ideal MHD model, which includes galactic differential rotation, an external gravitational potential, heating via supernova explosions and radiative cooling of the interstellar medium (TSM). In our model a superbubble is formed due to the clustering of supernova activity, mimicking an OB association. Supernovae are mod...

M J Korpi, A Brandenburg, A Shukurov, I Tuominen (1999) ASTRONOMY AND ASTROPHYSICS 350: 1 230-239

Simulations and observations of stellar dynamos : evidence for a magnetic alpha-effect

2009-09-12T08:45:02Z

There are now several simulations showing the generation of large scale magnetic fields whose energy exceeds the kinetic energy of the turbulence. Those simulations have in common that the large scale field generation is related to some kind of a magnetic instability (magnetorotational instability, magnetic buoyancy instability and in some cases even the kink instability). The large scale field ge...

A Brandenburg (1999) STELLAR DYNAMOS : NONLINEARITY AND CHAOTIC FLOWS 178: 13-21

A supernova-regulated interstellar medium : Simulations of the turbulent multiphase medium

2009-09-12T08:45:02Z

The dynamic state of the interstellar medium, heated and stirred by supernovae (SNe), is simulated using a three-dimensional, nonideal MHD model in a domain extended 0.5 x 0.5 kpc horizontally and 2 kpc vertically, with the gravitational field symmetric about the midplane of the domain, z = 0. We include both Type I and Type II SNe, allowing the latter to cluster in regions with enhanced gas densi...

M J Korpi, A Brandenburg, A Shukurov, I Tuominen, A Nordlund (1999) ASTROPHYSICAL JOURNAL 514: 2 L99-L102

A helicity proxy from horizontal solar flow patterns

2009-09-12T08:45:02Z

Motivated by new observations of solar surface how patterns of mesogranulation, theoretical computations of the horizontal divergence-vorticity correlation are presented. Because of its close relation to the helicity in rotating turbulence such observations and discussions are of particular importance for the conventional dynamo theory. For the northern hemisphere we find a small, but always negat...

G Rudiger, A Brandenburg, V V Pipin (1999) ASTRONOMISCHE NACHRICHTEN 320: 3 135-140

Evidence for a singularity in ideal magnetohydrodynamics : Implications for fast reconnection

2009-09-12T08:45:02Z

Numerical evidence for a finite-time singularity in ideal 3D magnetohydrodynamics is presented. The simulations start from two interlocking magnetic flux rings with no initial velocity. Curvature shrinks the rings until they touch and current sheets form between them. The evidence for a singularity in a finite time t(c) is that the peak current density behaves like \\J\\(infinity) similar to 1/(t(...

R M Kerr, A Brandenburg (1999) PHYSICAL REVIEW LETTERS 83: 6 1155-1158

Magnetic-drift processes in differentially rotating turbulence

2009-09-12T08:45:02Z

The mean electromotive force is considered in a differentially rotating fluid taking into account stretching of the turbulent magnetic field. Calculations are performed by making use of the second order correlation approximation. Nonuniformity of the angular velocity leads to specific drift processes in the azimuthal direction. Due to this drift the magnetic field can rotate with a somewhat differ...

V Urpin, A Brandenburg (1999) ASTRONOMY AND ASTROPHYSICS 345: 3 1054-1058

The influence of geometry and topology on axisymmetric mean-field dynamos

2009-09-12T08:45:02Z

We study the changes in the dynamical behaviour of axisymmetric spherical mean-field dynamo models produced by changes in their geometry and topology, by considering a two parameter family of models, ranging from a full sphere to spherical shell, tents and disc-like configurations, within a unified framework. We find that the two parameter space of the family of models considered here separates in...

E Covas, R Tavakol, A Tworkowski, A Brandenburg, J Brooke, D Moss (1999) ASTRONOMY & ASTROPHYSICS 345: 2 669-679

Comment on "The sunspot as a self-excited dynamo"

2009-09-12T08:45:02Z

A recent paper claims that the well known Cowling 'anti-dynamo' theorem is a "misconception", and that a simple axisymmetric sunspot model constitutes a counter example. We do not believe these claims to have been substantiated.

D Moss, A Brandenburg (1999) ASTRONOMY AND ASTROPHYSICS 346: 3 1009-1010

Magnetic and vertical shear instabilities in accretion discs

2009-09-12T08:45:02Z

The stability properties of magnetized discs rotating with angular velocity Omega = Omega(s,z), dependent on both the radial and the vertical coordinates s and z, are considered. Such a rotation law is adequate for many astrophysical discs (e.g., galactic and protoplanetary discs, as well as accretion discs in binaries). In general, the angular velocity depends on height, even in thin accretion di...

V Urpin, A Brandenburg (1998) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 294: 3 399-406

Axisymmetric mean field dynamos with dynamic and algebraic alpha-quenchings

2009-09-12T08:45:02Z

We study axisymmetric mean field spherical and spherical shell dynamo models, with both dynamic and algebraic alpha-quenchings. Our results show that there are qualitative as well as quantitative differences and similarities between these models. Regarding similarities, both groups of models exhibit symmetric, antisymmetric and mixed modes of behaviour. As regards differences, the important featur...

E Covas, R Tavakol, A Tworkowski, A Brandenburg (1998) ASTRONOMY & ASTROPHYSICS 329: 1 350-360

Turbulence effects in planetesimal formation

2009-09-12T08:45:02Z

The formation of planetesimals is investigated by studying the transport of dust particles in a local three-dimensional simulation of accretion disc turbulence. Heavy particles fall rapidly towards the midplane, whereas lighter particles are strongly advected by the flow. For light particles the turbulence leads to a rapid redistribution of particles such that their density per unit mass is approx...

L S Hodgson, A Brandenburg (1998) ASTRONOMY & ASTROPHYSICS 330: 3 1169-1174

Magnetic fields in young galaxies due to the cross-helicity effect

2009-09-12T08:45:02Z

It is shown that the cross-helicity effect facilitates rapid growth of the large scale magnetic field in young galaxies. This field then acts as a seed for the standard alpha Omega-type dynamo at later stages. This mechanism may be responsible for the relatively strong magnetic fields observed in young high redshift galaxies.

A Brandenburg, V Urpin (1998) ASTRONOMY AND ASTROPHYSICS 332: 3 L41-L44

Mean field dynamos with algebraic and dynamic alpha-quenchings

2009-09-12T08:45:02Z

Calculations for mean field dynamo models (in both full spheres and spherical shells), with both algebraic and dynamic alpha-quenchings, show qualitative as well as quantitative differences and similarities in the dynamical behaviour of these models. We summarise and enhance recent results with extra examples. Overall, the effect of using a dynamic a appears to be complicated and is affected by th...

A Tworkowski, E Covas, R Tavakol, A Brandenburg (1998) STUDIA GEOPHYSICA ET GEODAETICA 42: 3 350-355

The radial disc structure around a magnetic neutron star : analytic and semi-analytic solutions

2009-09-12T08:45:02Z

The radial structure of a thin accretion disc is calculated in the presence of a central dipole magnetic field aligned with the rotation axis. The problem is treated using a modified expression for the turbulent magnetic diffusion, which allows the angular momentum equation to be integrated analytically. The governing algebraic equations are solved iteratively between 1 and 10(4) stellar radii. An...

A Brandenburg, C G Campbell (1998) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 298: 1 223-230

Time evolution of the magnetic activity cycle period

2009-09-12T08:45:02Z

We propose a new interpretation of the relationships between the dynamo cycle period (P-cyc) as observed in Ca II H and K, the rotational period (P-rot), the activity level, and other stellar properties. Viewed within this framework, the data suggest that the dynamo alpha-parameter increases with magnetic field strength, contrary to the conventional idea of alpha-quenching. The data also suggest a...

A Brandenburg, S H Saar, C R Turpin (1998) ASTROPHYSICAL JOURNAL 498: 1 L51-L54

Simulations of an alpha-effect due to magnetic buoyancy

2009-09-12T08:45:02Z

Three-dimensional simulations of a thermally stably stratified gas with a localized layer of toroidal magnetic field are carried out. The magnetic field gives rise to a magnetic buoyancy instability. Due to the presence of rotation the resulting fluid motions are helical and lead to an alpha-effect, i.e. to a component of the electromotive force in the direction of the mean magnetic field. The val...

A Brandenburg, D Schmitt (1998) ASTRONOMY AND ASTROPHYSICS 338: 2 L55-L58

Turbulence and magnetic fields in clusters of galaxies

2009-09-12T08:45:02Z

We consider turbulence generated by galaxies moving transonically through the intracluster gas. We show that neither the gravitational drag nor the gas stripping from the galaxies are able, by themselves, to generate turbulence at a level required to feed the dynamo in the intracluster gas. Some implications for cluster radio halos are discussed.

F J Sanchez-Salcedo, A Brandenburg, A Shukurov (1998) ASTROPHYSICS AND SPACE SCIENCE 263: 1-4 87-90

New results for the Herzenberg dynamo : Steady and oscillatory solutions

2009-09-12T08:45:02Z

The Herzenberg dynamo, consisting of two rotating electrically conducting spheres with non-parallel spin axes, immersed in a finite spherical conducting medium, is simulated numerically for a variety of parameters not accessible to the original asymptotic theory. Our model places the spheres in a spatially periodic box. The largest growth rate is obtained when the angle, cp, between the spin axes ...

A Brandenburg, D Moss, A M Soward (1998) PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON SERIES A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES 454: 1973 1283-1300

Local models of small-scale dynamo action

2009-09-12T08:45:02Z

Coherent structures in turbulent flows consisting of vorticity filaments may generate small-scale magnetic fields by means of dynamo action with a mechanism similar to that known as the Herzenberg dynamo. In order to analyse the consequences of this assumption for the properties of the resulting magnetic field, we have performed numerical simulations for a Herzenberg-like model system with a presc...

A Bigazzi, A Brandenburg, D Moss (1998) JOURNAL DE PHYSIQUE IV 8: P6 183-187

Driving galactic turbulence by supernova explosions

2009-09-12T08:45:02Z

We investigate the general properties of supernova driven interstellar turbulence using local three-dimensional MHD simulations under Galactic conditions. Our model includes the effects of large-scale shear due to Galactic differential rotation, density stratification, compressibility, magnetic fields, heating via supernova explosions and parameterized radiative cooling of the interstellar medium....

M J Korpi, A Brandenburg, I Tuominen (1998) STUDIA GEOPHYSICA ET GEODAETICA 42: 3 410-418

Intermittent behaviour in axisymmetric mean-field dynamo models in spherical shells

2009-09-12T08:45:02Z

Axisymmetric mean-field dynamo models in spherical shells are shown to be capable of producing temporally intermittent behaviour. This is of potential importance since (i) it is, as far as we are aware, the first time such behaviour has been produced internally by a mean-field dynamo model in a spherical shell, without requiring any additional assumptions or truncations, and (ii) it may be charact...

A Tworkowski, R Tavakol, A Brandenburg, J M Brooke, D Moss, I Tuominen (1998) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 296: 2 287-295

Robustness of truncated alpha Omega dynamos with a dynamic alpha

2009-09-12T08:45:02Z

In a recent work (Covas et al., 1996), the behaviour and the robustness of truncated alpha Omega dynamos with a dynamic alpha were studied with respect to a number of changes in the driving term of the dynamic alpha equation, which was considered previously by Schmalz and Stir (1991) to be of the form similar to A phi B phi Here we review and extend our previous work and consider the effect of add...

E Covas, A Tworkowski, R Tavakol, A Brandenburg (1997) SOLAR PHYSICS 172: 1-2 3-9

Modelling magnetised accretion discs

2009-09-12T08:45:02Z

Some recent results are reviewed that lead us now to believe that accretion discs are basically always magnetised. The main components are Balbus-Hawley and Parker instabilities on the one hand and a dynamo process on the other. A mechanical model for the Balbus-Hawley instability is presented and analysed quantitatively. Three-dimensional simulations are discussed, especially the resulting magnet...

A Brandenburg, C Campbell (1997) ACCRETION DISKS - NEW ASPECTS 487: 109-124

Recent developments in the theory of large-scale dynamos

2009-09-12T08:45:02Z

A Brandenburg (1997) PAST AND PRESENT VARIABILITY OF THE SOLAR-TERRESTRIAL SYSTEM : MEASUREMENT, DATA ANALYSIS AND THEORETICAL MODELS 133: 359-388

The dependence of the dynamo alpha on vorticity

2009-09-12T08:45:02Z

We use data from numerical simulations of dynamo-generated turbulence in the shearing box approximation to determine the dynamo alpha-effect and its dependence on the rotation law Omega(r). The data suggest that the dynamo alpha is not simply proportional to the local angular velocity Omega(r), as is usually assumed, but rather is proportional to the local Vorticity omega(r) = r(-1)d/dr(Omega r(2)...

A Brandenburg, K J Donner (1997) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 288: 2 L29-L33

Current sheet formation in the interstellar medium

2009-09-12T08:45:02Z

There is phenomenological evidence that magnetic reconnection operates in the interstellar medium, and magnetic reconnection is also necessary for the operation of a galactic dynamo. The extremely long ohmic diffusion times of magnetic fields in typical interstellar structures suggest that reconnection occurs in two stages, with thin current layers that have relatively short resistive decay times ...

E G Zweibel, A Brandenburg (1997) ASTROPHYSICAL JOURNAL 478: 2 563-568

The effect of Silk damping on primordial magnetic fields

2009-09-12T08:45:02Z

We study the effects of plasma viscosity on the dynamics of primordial magnetic fields by simulating magnetohydrodynamics in the early universe by appropriate non-linear cascade models, We find numerically that even in the presence of large kinetic viscosity, magnetic energy is transferred to large length scales. There are indications, however, that the inverse cascade stops at a given time which ...

A Brandenburg, K Enqvist, P Olesen (1997) PHYSICS LETTERS B 392: 3-4 395-402

Dynamos with different formulations of a dynamic alpha-effect

2009-09-12T08:45:02Z

We investigate the behaviour of alpha Omega dynamos with a dynamic alpha, whose evolution is governed by the imbalance between a driving and a damping term. We focus on truncated versions of such dynamo models which are often studied in connection with solar and stellar variability. Given the approximate nature of such models, it is important to study how robust they are with respect to reasonable...

E Covas, A Tworkowski, A Brandenburg, R Tavakol (1997) ASTRONOMY AND ASTROPHYSICS 317: 2 610-617

An incoherent alpha-Omega dynamo in accretion disks

2009-09-12T08:45:02Z

We use the mean-held dynamo equations to show that spatially and temporally incoherent fluctuations in the helicity in mirror-symmetric turbulence in a shearing flow can generate a large-scale, coherent magnetic field. We illustrate this effect with simulations of a few simple systems. For statistically homogeneous turbulence, we find that the dynamo growth rate is roughly tau(eddy)(-1/3)tau(shear...

E T Vishniac, A Brandenburg (1997) ASTROPHYSICAL JOURNAL 475: 1 263-274

Large-scale magnetic fields from hydromagnetic turbulence in the very early universe.

2009-09-12T01:23:09Z

Brandenburg, Enqvist, Olesen (1996) Phys Rev D Part Fields 54: 2 1291-1300

Testing Cowling's antidynamo theorem near a rotating black hole

2009-09-12T08:45:02Z

The kinematic evolution of axisymmetric magnetic and electric fields is investigated numerically in Kerr geometry for a simplified Keplerian disk near a rotating black hole. In the cases investigated it is found that a magnetic field cannot be sustained against ohmic diffusion. In flat space this result is known as Cowling's antidynamo theorem. No support is found for the possibility that the grav...

A Brandenburg (1996) ASTROPHYSICAL JOURNAL 465: 2 L115-L118

The turbulent viscosity in accretion discs

2009-09-12T08:45:02Z

Keplerian sheer flows are unstable towards a magnetic sheer instability that generates turbulence. The turbulence is considered to be a likely source for viscosity in accretion discs. Recently several groups have simulated this turbulence in order to estimate the strength of the turbulent viscosity. There are however significant quantitative discrepancies between their results. Estimates of the ef...

U Torkelsson, A Brandenburg, A Nordlund, R F Stein (1996) ASTROPHYSICAL LETTERS & COMMUNICATIONS 34: 1-6 383-388

Galactic magnetism : Recent developments and perspectives

2009-09-12T08:45:02Z

We discuss current observational and theoretical knowledge of magnetic fields, especially the large-scale structure in the disks and halos of spiral galaxies. Among other topics, we consider the enhancement of global magnetic fields in the interarm regions, magnetic spiral arms, and representations as superpositions of azimuthal modes, emphasizing a number of unresolved questions. It is argued tha...

R Beck, A Brandenburg, D Moss, A Shukurov, D Sokoloff (1996) ANNUAL REVIEW OF ASTRONOMY AND ASTROPHYSICS 34: 155-206

The dependence of the viscosity in accretion discs on the shear/vorticity ratio

2009-09-12T08:45:02Z

We estimate the Shakura-Sunyaev viscosity parameter alpha for different values of the shear/vorticity ratio, sigma/omega, using local simulations of dynamo-generated turbulence. We find that the time average of alpha is approximately proportional to sigma/omega (at least for sigma/omega < 10). We point out that this result may have important implications for the properties of thick accretion discs...

M Abramowicz, A Brandenburg, J P Lasota (1996) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 281: 3 L21-L24

Large-scale magnetic fields from hydromagnetic turbulence in the very early universe

2009-09-12T08:45:02Z

We investigate hydromagnetic turbulence of primordial magnetic fields using magnetohydrodynamics (MHD) in an expanding universe. We present the basic, covariant MHD equations, find solutions for MHD waves in the early universe, and investigate the equations numerically for random magnetic fields in two spatial dimensions. We find the formation of magnetic structures at larger and larger scales as ...

A Brandenburg, K Enqvist, P Olesen (1996) PHYSICAL REVIEW D 54: 2 1291-1300

Magnetic structures in a dynamo simulation

2009-09-12T08:45:02Z

We use three-dimensional simulations to study compressible convection in a rotating frame with magnetic fields and overshoot into surrounding stable layers. The, initially weak, magnetic field is amplified and maintained by dynamo action and becomes organized into flux tubes that are wrapped around vortex tubes. We also observe vortex buoyancy which causes upward flows in the cores of extended dow...

A Brandenburg, R L Jennings, A Nordlund, M Rieutord, R F Stein, I Tuominen (1996) JOURNAL OF FLUID MECHANICS 306: 325-352

The disk accretion rate for dynamo-generated turbulence

2009-09-12T08:45:02Z

Dynamo-generated turbulence is simulated in a modified shearing box approximation that removes scale invariance and allows finite accretion rates for a given distance from the central object. The effective Shakura-Sunyaev viscosity parameter, alpha(ss), is estimated in three different ways using the resulting mass accretion rate, the heating rate, and the horizontal components of the Maxwell and R...

A Brandenburg, A Nordlund, R F Stein, U Torkelsson (1996) ASTROPHYSICAL JOURNAL 458: 1 L45-L48

GENERALIZED ENTROPIES IN A TURBULENT DYNAMO SIMULATION

2009-09-12T08:45:02Z

A simulation of hydromagnetic turbulence exhibiting dynamo action is employed to estimate the generalized entropies, H-q, from the distribution of moments of local expansion factors of material line elements. These generalized entropies can be used to characterize the dynamics of turbulence and of nonlinear dynamo action. The value of the metric entropy H-1, is comparable to the largest Lyapunov e...

A BRANDENBURG, I KLAPPER, J KURTHS (1995) PHYSICAL REVIEW E 52: 5 R4602-R4605

DYNAMO-GENERATED TURBULENCE AND LARGE-SCALE MAGNETIC-FIELDS IN A KEPLERIAN SHEAR-FLOW

2009-09-12T08:45:02Z

The nonlinear evolution of magnetized Keplerian shear flows is simulated in a local, three-dimensional model, including the effects of compressibility and stratification. Supersonic flows are initially generated by the Balbus-Hawley magnetic shear instability. The resulting flows regenerate a turbulent magnetic field which, in turn, reinforces the turbulence. Thus, the system acts like a dynamo th...

A BRANDENBURG, A NORDLUND, RF STEIN, U TORKELSSON (1995) ASTROPHYSICAL JOURNAL 446: 2 741-754

MULTIPLE FAR-SUPERCRITICAL SOLUTIONS FOR AN ALPHA-LAMBDA-DYNAMO

2009-09-12T08:45:02Z

We compute numerical solutions for axisymmetric, dynamically consistent mean-field dynamos in a spherical shell of conducting incompressible fluid. In the process of investigating the stability properties of solutions in the far-supercritical regime we found an unusual behaviour, with the magnetic energy decreasing discontinuously as the dynamo number is increased. A new stable solution with a mor...

P MUHLI, A BRANDENBURG, D MOSS, I TUOMINEN (1995) ASTRONOMY AND ASTROPHYSICS 296: 3 700-704

STRUCTURAL STABILITY OF AXISYMMETRICAL DYNAMO MODELS

2009-09-12T08:45:02Z

We examine the stability of the dynamical behaviour of axisymmetric alpha(2) omega dynamo models in rotating spherical shells as well as in spheres. Overall, our results show that the spherical dynamo models are more stable in the following senses: spherical models (i) do not seem to allow chaotic behaviour and (ii) are robust with respect to changes in the functional form of alpha. On the other h...

R TAVAKOL, AS TWORKOWSKI, A BRANDENBURG, D MOSS, I TUOMINEN (1995) ASTRONOMY AND ASTROPHYSICS 296: 1 269-274

NONAXISYMMETRIC DYNAMO SOLUTIONS AND EXTENDED STARSPOTS ON LATE-TYPE STARS

2009-09-12T08:45:02Z

We have computed mean field dynamo models in a deep spherical shell, without restriction on spatial symmetries, in which the growth of the magnetic field is limited solely by the back reaction of the large scale Lorentz force on the large scale motions. A parameterization of the Reynolds stress tenser is included to describe the generation of differential rotation. We find for moderate values of t...

D MOSS, DM BARKER, A BRANDENBURG, I TUOMINEN (1995) ASTRONOMY AND ASTROPHYSICS 294: 1 155-164

Generalized entropies in a turbulent dynamo simulation.

2009-09-12T01:23:09Z

Brandenburg, Klapper, Kurths (1995) Phys Rev E Stat Phys Plasmas Fluids Relat Interdiscip Topics 52: 5 R4602-R4605

REYNOLDS STRESSES AND DIFFERENTIAL ROTATION IN BOUSSINESQ CONVECTION IN A ROTATING SPHERICAL-SHELL

2009-09-12T08:45:02Z

We consider the problem of how numerical simulations of convection in a spherical shell can be used to estimate turbulent transport coefficients that may be used in mean field theory. For this purpose we analyse data from simulations of three dimensional Boussinesq convection. The rotational influence on convection is described in terms of the LAMBDA-effect and anisotropic eddy conductivity. When ...

M RIEUTORD, A BRANDENBURG, A MANGENEY, P DROSSART (1994) ASTRONOMY AND ASTROPHYSICS 286: 2 471-480

TURBULENT ACCRETION DISK DYNAMOS

2009-09-12T08:45:02Z

We present numerical results for mean-field alpha2OMEGA-dynamos in an accretion disk. We first study the linear case in both disks with constant thickness and disks with radially increasing thickness. The preferred mode is dipolar for a thick disk, but quadrupolar for a thin one. The quadrupolar mode generates a magnetic torque that transports angular momentum outwards. The role of the geometrical...

U TORKELSSON, A BRANDENBURG (1994) ASTRONOMY AND ASTROPHYSICS 283: 2 677-692

DOES SOLAR DIFFERENTIAL ROTATION ARISE FROM A LARGE-SCALE INSTABILITY

2009-09-12T08:45:02Z

The suggestion by several authors that the solar differential rotation is caused by a large scale instability of the basic convective state is examined. We find that the proposed mean-field models are unstable to a Rayleigh-Benard type instability, but argue that this cannot explain the differential rotation of the Sun, because such a flow would become nonaxisymmetric. We discuss the applicability...

I TUOMINEN, A BRANDENBURG, D MOSS, M RIEUTORD (1994) ASTRONOMY AND ASTROPHYSICS 284: 1 259-264

CHAOS IN NONLINEAR DYNAMO MODELS

2009-09-12T08:45:02Z

Two nonlinear dynamos have been analyzed by numerical means: 3D-simulation of the magneto-hydrodynamic equations and qualitative analysis of a simplified low-dimensional mean field model. It turns out that both are capable of deterministic chaos in a certain parameter range. As the basic tool the calculation of Lyapunov exponents has been used.

J KURTHS, A BRANDENBURG, U FEUDEL, W JANSEN (1993) COSMIC DYNAMO : 157 83-89

ROTATIONAL EFFECTS ON CONVECTION SIMULATED AT DIFFERENT LATITUDES

2009-09-12T08:45:02Z

We simulate numerically convection inside the solar convection zone under the influence of rotation at different latitudes. The computational domain is a small rectangular box with stress-free upper and lower boundaries, and with periodicity assumed in the lateral directions. We study the transport of angular momentum, which is important for the generation of differential rotation. The sign and th...

P PULKKINEN, I TUOMINEN, A BRANDENBURG, A NORDLUND, RF STEIN (1993) ASTRONOMY AND ASTROPHYSICS 267: 1 265-274

REYNOLDS STRESSES DERIVED FROM SIMULATIONS

2009-09-12T08:45:02Z

Three-dimensional simulations of convection are studied with specific attention to the effects of rotation and latitude on different quantities. The latitude-dependence of the strength of convection is found to decrease with increasing strength of the turbulence. The Lambda-effect is, however, still present also in a more turbulent case.

P PULKKINEN, I TUOMINEN, A BRANDENBURG, A NORDLUND, RF STEIN (1993) COSMIC DYNAMO : 157 123-127

VERTICAL MAGNETIC-FIELDS ABOVE THE DISKS OF SPIRAL GALAXIES

2009-09-12T08:45:02Z

We investigate the magnetic fields above the discs of spiral galaxies in the framework of axisymmetric nonlinear mean-field dynamo models for a disc surrounded by a spherical halo, using realistic rotation curves. We consider, in particular, NGC 4631 and NGC 89 1, and include turbulent diamagnetism, an anisotropy of the alpha effect and a galactic wind. In these models magnetic field is generated ...

A BRANDENBURG, KJ DONNER, D MOSS, A SHUKUROV, DD SOKOLOFF, I TUOMINEN (1993) ASTRONOMY AND ASTROPHYSICS 271: 1 36-50

GALACTIC DYNAMOS AND DYNAMICS

2009-09-12T08:45:02Z

We discuss some aspects of the interrelationship between the dynamo problem for galaxies and their dynamics. First, we consider the generation of magnetic fields in the presence of fountain hows and galactic winds. Next, we discuss the distortion of a steady magnetic field by tidal effects and other transient spiral features. Finally, we give an expression for the amplitude of density waves genera...

KJ DONNER, A BRANDENBURG, M THOMASSON (1993) COSMIC DYNAMO : 157 333-337

MODELS FOR THE MAGNETIC-FIELD OF M81

2009-09-12T08:45:02Z

We study several mean field dynamo models in disk geometry in an attempt to understand the origin of the nonaxisymmetric magnetic field present in M81. There appear to be three (at least) relevant mechanisms, which are not mutually exclusive. Because field growth times are not very short compared to galactic ages, a predominantly nonaxisymmetric seed field may still give a significantly nonaxisym...

D MOSS, A BRANDENBURG, KJ DONNER, M THOMASSON (1993) ASTROPHYSICAL JOURNAL 409: 1 179-189

TOWARDS THE MAGNETIC-FIELD OF M81

2009-09-12T08:45:02Z

We present calculations showing how both transient effects associated with nonaxisymmetric seed fields, and an interaction with a companion, can produce BSS type magnetic structure in a spiral galaxy.

D MOSS, A BRANDENBURG, KJ DONNER, M THOMASSON (1993) COSMIC DYNAMO : 157 339-343

Fractal level sets and multifractal fields in direct simulations of turbulence.

2009-09-12T01:23:09Z

Brandenburg, Procaccia, Segel, Vincent (1992) Phys Rev A 46: 8 4819-4828

Energy spectra in a model for convective turbulence.

2009-09-12T01:23:09Z

Brandenburg (1992) Phys Rev Lett 69: 4 605-608

THE FRACTAL DIMENSION OF ISO-VORTICITY STRUCTURES IN 3-DIMENSIONAL TURBULENCE

2009-09-12T08:45:02Z

The fractal dimension of iso-vorticity surfaces is estimated from a 3-dimensional simulation of homogeneous turbulence at moderate Reynolds numbers, performed by Vincent and Meneguzzi. The results are found to be compatible with a recently proposed theory which predicts a crossover from a 2-dimensional geometry at small scales to a fractal geometry at larger scales, with a dimension D = 2.5 + zeta...

I PROCACCIA, A BRANDENBURG, MH JENSEN, A VINCENT (1992) EUROPHYSICS LETTERS 19: 3 183-187

DYNAMOS IN DISKS AND HALOS OF GALAXIES

2009-09-12T08:45:02Z

We investigate linear and nonlinear dynamo models for a galactic disc embedded in a halo, assuming a relatively strong magnetic diffusivity and a non-vanishing alpha-effect in the halo. We take the halo to be spherical and embedded in a vacuum. The field is assumed to be axisymmetric, but we do not impose symmetry conditions at the equatorial plane. In one parameter regime we find mixed parity sol...

A BRANDENBURG, KJ DONNER, D MOSS, A SHUKUROV, DD SOKOLOFF, I TUOMINEN (1992) ASTRONOMY AND ASTROPHYSICS 259: 2 453-461

DYNAMO ACTION IN STRATIFIED CONVECTION WITH OVERSHOOT

2009-09-12T08:45:02Z

We present results from direct simulations of turbulent compressible hydromagnetic convection above a stable overshoot layer. Spontaneous dynamo action occurs followed by saturation, with most of the generated magnetic field appearing as coherent flux tubes in the vicinity of strong downdrafts, where both the generation and destruction of magnetic field is most vigorous. Whether or not this field ...

A NORDLUND, A BRANDENBURG, RL JENNINGS, M RIEUTORD, J RUOKOLAINEN, RF STEIN, I TUOMINEN (1992) ASTROPHYSICAL JOURNAL 392: 2 647-652

ENERGY-SPECTRA IN A MODEL FOR CONVECTIVE TURBULENCE

2009-09-12T08:45:02Z

The energy cascade in both hydrodynamic and hydromagnetic Boussinesq convection is investigated at large Rayleigh numbers, using a scalar model for turbulence. Depending on the relative importance of direct and inverse transfer, either we observe classical Kolmogorov k-5/3 spectra or, if there is a strong inverse transfer of kinetic energy, we find a k-7/5 spectrum for the temperature fluctuation ...

A BRANDENBURG (1992) PHYSICAL REVIEW LETTERS 69: 4 605-608

FRACTAL LEVEL SETS AND MULTIFRACTAL FIELDS IN DIRECT SIMULATIONS OF TURBULENCE

2009-09-12T08:45:02Z

The fractal nature of level sets and the multifractal nature of various scalar and vector fields in hydromagnetic and hydrodynamic turbulence are investigated using data of direct simulations. It turns out that fields whose evolution is governed by stretching terms (vortex stretching, magnetic-field line stretching) exhibit "near singularities" that result in a multifractal scaling. Such stretchin...

A BRANDENBURG, I PROCACCIA, D SEGEL, A VINCENT (1992) PHYSICAL REVIEW A 46: 8 4819-4828

EVOLUTION OF A MAGNETIC-FLUX TUBE IN 2-DIMENSIONAL PENETRATIVE CONVECTION

2009-09-12T08:45:02Z

Highly supercritical compressible convection is simulated in a two-dimensional domain in which the upper half is unstable to convection while the lower half is stably stratified. This configuration is an idealization of the layers near the base of the solar convection zone. Once the turbulent flow is well developed, a toroidal magnetic field B(tor) is introduced to the stable layer. The field's ev...

RL JENNINGS, A BRANDENBURG, A NORDLUND, RF STEIN (1992) MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 259: 3 465-473

STOCHASTIC EFFECTS IN MEAN-FIELD DYNAMOS

2009-09-12T08:45:02Z

Motivated by the observations that (i) the solar cycle is distinctly irregular on the long term, (ii) a proper treatment of the averaging processes of mean field theory yields stochastic terms that cannot be neglected in solar and stellar convection zones and (iii) the inclusion of parametrized nonlinearities in models with two spatial dimensions has not produced such irregular behaviour, we inves...

D MOSS, A BRANDENBURG, R TAVAKOL, I TUOMINEN (1992) ASTRONOMY AND ASTROPHYSICS 265: 2 843-849

THE INFLUENCE OF BOUNDARY-CONDITIONS ON THE EXCITATION OF DISK DYNAMO MODES

2009-09-12T08:45:02Z

Calculations of mean field dynamos for galaxies have largely been for two rather disparate models. The thin disk model treats the ratio of disk height to radius explicitly as a small parameter, and applies zero tangential field boundary conditions at the disk surface. In contrast, the embedded disk model calculates the magnetic field in a spherical volume, whose radius is the disk radius and wit...

D MOSS, A BRANDENBURG (1992) ASTRONOMY AND ASTROPHYSICS 256: 2 371-374

STRATIFICATION AND THERMODYNAMICS IN MEAN-FIELD DYNAMOS

2009-09-12T08:45:02Z

We extend previous investigations of axisymmetric, incompressible mean-field dynamos to the compressible case with strong stratification. We take thermodynamic effects into account using the anelastic approximation and show that the effects of stratification, compressibility and thermodynamics on the rotation law are small when we compare our results with those previously obtained for incompressib...

A BRANDENBURG, D MOSS, I TUOMINEN (1992) ASTRONOMY AND ASTROPHYSICS 265: 1 328-344

Lyapunov exponents for hydromagnetic convection.

2009-09-12T01:23:09Z

Kurths, Brandenburg (1991) Phys Rev A 44: 6 R3427-R3429

NONLINEAR NONAXISYMMETRIC DYNAMO MODELS FOR COOL STARS

2009-09-12T08:45:02Z

Observational evidence for long-lived nonaxisymmetric features on the surfaces of rapidly rotating late-type giant stars (e.g. FK Comae and RS CVn stars) is beginning to be found. By analogy with sunspots, these features may be associated with large scale nonaxisymmetric magnetic field structures, generated by a dynamo operating in the convective envelopes. We describe a nonlinear nonaxisymmetri...

D MOSS, I TUOMINEN, A BRANDENBURG (1991) ASTRONOMY AND ASTROPHYSICS 245: 1 129-135

PROPERTIES OF MEAN FIELD DYNAMOS WITH NON-AXISYMMETRICAL ALPHA-EFFECT

2009-09-12T08:45:02Z

We investigate the influence of an azimuthally dependent alpha-effect on the properties of alpha-2 and alpha-2-OMEGA-dynamos in spherical geometry, restricting ourselves to odd parity solutions in linear theory. For all our linear models we find an exponentially growing mode, consisting of locked axisymmetric and nonaxisymmetric parts. A strong nonaxismmetry in alpha-substantially increases the ...

D MOSS, A BRANDENBURG, I TUOMINEN (1991) ASTRONOMY AND ASTROPHYSICS 247: 2 576-579

HYDROMAGNETIC ALPHA-OMEGA-TYPE DYNAMOS WITH FEEDBACK FROM LARGE-SCALE MOTIONS

2009-09-12T08:45:02Z

A BRANDENBURG, D MOSS, G RUDIGER, I TUOMINEN (1991) GEOPHYSICAL AND ASTROPHYSICAL FLUID DYNAMICS 61: 1-4 179-198

LYAPUNOV EXPONENTS FOR HYDROMAGNETIC CONVECTION

2009-09-12T08:45:02Z

We estimate the two largest Lyapunov exponents in a three-dimensional simulation of hydromagnetic convection in which there is dynamo action. It turns out that these first two exponents (from a total of 8 x 63(3)) are positive and of similar magnitude. Thus we conclude that the dynamo is chaotic. Furthermore, the consideration of local exponents helps in our understanding of the relevant dynamic...

J KURTHS, A BRANDENBURG (1991) PHYSICAL REVIEW A 44: 6 R3427-R3429

3-D SIMULATION OF TURBULENT CYCLONIC MAGNETOCONVECTION

2009-09-12T08:45:02Z

A BRANDENBURG, A NORDLUND, P PULKKINEN, RF STEIN, I TUOMINEN (1990) ASTRONOMY AND ASTROPHYSICS 232: 1 277-291

THE NONLINEAR SOLAR DYNAMO AND DIFFERENTIAL ROTATION - A TAYLOR NUMBER PUZZLE

2009-09-12T08:45:02Z

A BRANDENBURG, D MOSS, G RUDIGER, I TUOMINEN (1990) SOLAR PHYSICS 128: 1 243-251

MAGNETIC-FIELD STRUCTURE IN DIFFERENTIALLY ROTATING-DISKS

2009-09-12T08:45:02Z

KJ DONNER, A BRANDENBURG (1990) GEOPHYSICAL AND ASTROPHYSICAL FLUID DYNAMICS 50: 1-3 121-129

NONLINEAR DYNAMOS WITH MAGNETIC BUOYANCY IN SPHERICAL GEOMETRY

2009-09-12T08:45:02Z

D MOSS, I TUOMINEN, A BRANDENBURG (1990) ASTRONOMY AND ASTROPHYSICS 228: 1 284-294

CAN STELLAR DYNAMOS BE MODELED IN LESS THAN 3 DIMENSIONS

2009-09-12T08:45:02Z

R JENNINGS, A BRANDENBURG, D MOSS, I TUOMINEN (1990) ASTRONOMY AND ASTROPHYSICS 230: 2 463-473

BEHAVIOR OF HIGHLY SUPERCRITICAL ALPHA-EFFECT DYNAMOS

2009-09-12T08:45:02Z

R MEINEL, A BRANDENBURG (1990) ASTRONOMY AND ASTROPHYSICS 238: 1-2 369-376

DYNAMOS WITH A FLAT ALPHA-EFFECT DISTRIBUTION

2009-09-12T08:45:02Z

A BRANDENBURG, I TUOMINEN, F KRAUSE (1990) GEOPHYSICAL AND ASTROPHYSICAL FLUID DYNAMICS 50: 1-3 95-112

NONLINEAR MEAN-FIELD DYNAMO MODELS - STABILITY AND EVOLUTION OF 3-DIMENSIONAL MAGNETIC-FIELD CONFIGURATIONS

2009-09-12T08:45:02Z

KH RADLER, E WIEDEMANN, A BRANDENBURG, R MEINEL, I TUOMINEN (1990) ASTRONOMY AND ASTROPHYSICS 239: 1-2 413-423

BUOYANCY-LIMITED THIN SHELL DYNAMOS

2009-09-12T08:45:02Z

D MOSS, I TUOMINEN, A BRANDENBURG (1990) ASTRONOMY AND ASTROPHYSICS 240: 1 142-149

VARIATION OF EVEN AND ODD PARITY IN THE SOLAR DYNAMO

2009-09-12T08:45:02Z

A BRANDENBURG, R MEINEL, D MOSS, I TUOMINEN (1990) SOLAR PHOTOSPHERE : STRUCTURE, CONVECTION AND MAGNETIC FIELDS : 379-382

GENERATION AND INTERPRETATION OF GALACTIC MAGNETIC-FIELDS

2009-09-12T08:45:02Z

We present kinematic mean-field dynamo models for galaxies consisting of a turbulent gas disc embedded in a low-conductivity spherical halo. In the cases investigated an axisymmetric mode is the dominant one. This mode can be of either even or odd parity (S0 or A0, respectively). The preference of S0 or A0 modes is governed mainly by the radial profiles of the alpha effect and the turbulent magnet...

KJ DONNER, A BRANDENBURG (1990) ASTRONOMY AND ASTROPHYSICS 240: 2 289-298

TORSIONAL OSCILLATIONS AND THE SOLAR DYNAMO REGIME

2009-09-12T08:45:02Z

I TUOMINEN, G RUDIGER, A BRANDENBURG (1990) SOLAR PHOTOSPHERE : STRUCTURE, CONVECTION AND MAGNETIC FIELDS : 387-390

TORSIONAL OSCILLATIONS AND THE SOLAR DYNAMO REGIME

2009-09-12T08:45:02Z

We discuss the observational results of cyclic variations of solar rotation and how these can be used as a means of probing the solar dynamo. We shortly describe two examples of dynamo models where the alpha-effect has been modified, and compare the resulting flows to the observations.

I TUOMINEN, G RUDIGER, A BRANDENBURG (1990) IAU SYMPOSIA : 138 387-390

VARIATION OF EVEN AND ODD PARITY IN THE SOLAR DYNAMO

2009-09-12T08:45:02Z

We have studied axisymmetric nonlinear alpha-omega-dynamo models taking the interaction between even and odd parities fully into account. It turns out that the dominating type of symmetry is not always determined uniquely, but it can vary on a very long time scale compared to the period of the magnetic cycle. In some cases the frequency of this long term variation is close to the beat frequency ...

A BRANDENBURG, R MEINEL, D MOSS, I TUOMINEN (1990) IAU SYMPOSIA : 138 379-382

THE STABILITY OF NONLINEAR DYNAMOS AND THE LIMITED ROLE OF KINEMATIC GROWTH-RATES

2009-09-12T08:45:02Z

A BRANDENBURG, F KRAUSE, R MEINEL, D MOSS, I TUOMINEN (1989) ASTRONOMY AND ASTROPHYSICS 213: 1-2 411-422

IUE OBSERVATIONS OF THE M-DWARFS CM-DRACONIS AND ROSSITER-137B - MAGNETIC ACTIVITY AT SATURATED LEVELS

2009-09-12T08:45:02Z

O VILHU, CW AMBRUSTER, JE NEFF, JL LINSKY, A BRANDENBURG, IV ILYIN, NI SHAKHOVSKAYA (1989) ASTRONOMY AND ASTROPHYSICS 222: 1-2 179-186

ON THE GENERATION OF NON-AXISYMMETRIC MAGNETIC-FIELDS IN MEAN-FIELD DYNAMOS

2009-09-12T08:45:02Z

A BRANDENBURG, I TUOMINEN, KH RADLER (1989) GEOPHYSICAL AND ASTROPHYSICAL FLUID DYNAMICS 49: 1-4 45-55

ON THE NONLINEAR STABILITY OF DYNAMO MODELS

2009-09-12T08:45:02Z

A BRANDENBURG, I TUOMINEN, D MOSS (1989) GEOPHYSICAL AND ASTROPHYSICAL FLUID DYNAMICS 49: 1-4 129-141