Dynamo–generated turbulence and outflows from accretion discs

Brandenburg, Axel

doi:10.1098/rsta.2000.0557

Cited by 15 publications

(13 citation statements)

References 25 publications

(29 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Hodapp & Ladd 1995), as one might expect from jet models that start off with a prescribed large scale field. Brandenburg (2000). ]…”

Section: Relevance To Outflows and Jetsmentioning

confidence: 99%

Computational aspects of astrophysical MHD and turbulence

Brandenburg¹

2019

Advances in Nonlinear Dynamos

Self Cite

View full text Add to dashboard Cite

The advantages of high-order finite difference scheme for astrophysical MHD and turbulence simulations are highlighted. A number of one-dimensional test cases are presented ranging from various shock tests to Parker-type wind solutions. Applications to magnetized accretion discs and their associated outflows are discussed. Particular emphasis is placed on the possibility of dynamo action in three-dimensional turbulent convection and shear flows, which is relevant to stars and astrophysical discs. The generation of large scale fields is discussed in terms of an inverse magnetic cascade and the consequences imposed by magnetic helicity conservation are reviewed with particular emphasis on the issue of α-quenching.

show abstract

“…Hodapp & Ladd 1995), as one might expect from jet models that start off with a prescribed large scale field. Brandenburg (2000). ]…”

Section: Relevance To Outflows and Jetsmentioning

confidence: 99%

Computational aspects of astrophysical MHD and turbulence

Brandenburg¹

2019

Advances in Nonlinear Dynamos

Self Cite

View full text Add to dashboard Cite

show abstract

“…The strength of this effect was computed in recent analytical and numerical simulations for both convectively unstable as well as stable stratifications. While Brandenburg et al (1990) worked with a box heated from below, Brandenburg and Schmitt (1998) considered magnetic buoyancy in the transition region between the radiative solar core and the convection zone, Brandenburg (2000) probed the Balbus-Hawley instability for dynamo-α production. Ferrière (1993), Kaisig et al (1993), and Ziegler et al (1996) used random supernova explosions to drive the galactic turbulence.…”

Section: Basic Theorymentioning

confidence: 99%

Physics of the solar cycle

Rüdiger¹,

Arlt²

2019

Advances in Nonlinear Dynamos

View full text Add to dashboard Cite

The theory of the solar/stellar activity cycles is presented, based on the mean-field concept in magnetohydrodynamics. A new approach to the formulation of the electromotive force as well as the theory of differential rotation and meridional circulation is described for use in dynamo theory. Activity cycles of dynamos in the overshoot layer (BL-dynamo) and distributed dynamos are compared, with the latter including the influence of meridional flow. The overshoot layer dynamo is able to reproduce the solar cycle periods and the butterfly diagram only if α = 0 in the convection zone. The problems of too many magnetic belts and too short cycle times emerge if the overshoot layer is too thin. The distributed dynamo including meridional flows with a magnetic Reynolds number Rm > ∼ 20 (low magnetic Prandtl number) reproduces the observed butterfly diagram even with a positive dynamo-α in the bulk of the convection zone. The nonlinear feedback of strong magnetic fields on differential rotation in the mean-field conservation law of angular momentum leads to grand minima in the cyclic activity similar to those observed. The 2D model described here contains the large-scale interactions as well as the small-scale feedback of magnetic fields on differential rotation and induction in terms of a mean-field formulation (Λquenching, α-quenching). Grand minima may also occur if a dynamo occasionally falls below its critical eigenvalue. We expressed this idea by an on-off α function which is non-zero only in a certain range of magnetic fields near the equipartition value. We never found any indication that the dynamo collapses by this effect after it had once been excited. The full quenching of turbulence by strong magnetic fields in terms of reduced induction (α) and reduced turbulent diffusivity (η T ) is studied with a 1D model. The full quenching results in a stronger dependence of cycle period on dynamo number compared with the model with α-quenching alone giving a very weak cycle period dependence. Also the temporal fluctuations of α and η T from a random-vortex simulation were applied to a dynamo model. Then the low 'quality' of the solar cycle can be explained with a relatively small number of giant cells acting as the dynamo-active turbulence. The simulation contains the transition from almost regular magnetic oscillations (many vortices) to a more or less chaotic time series (very few vortices).

show abstract

“…In order to estimate the resulting field strength, let us here reproduce an earlier estimate by Brandenburg (2000). The basic idea is that outflows (both on stellar and on galactic scales) tend to be magnetized.…”

Section: Outflows From Agns or Ysos For Seeding Galaxiesmentioning

confidence: 99%

“…AGNs might provide more coherent fields because their scale is larger. The general idea of outflows seeding the interstellar medium has been around for some time (see, e.g., Goldshmidt & Rephaeli 1993, 1994Völk & Atoyan 1999;Brandenburg 2000;Kronberg et al 2001). On the average the coherence scale of the field in clusters of galaxies is 5 kpc (Clarke et al 2001), but locally it can be much larger.…”

Section: Outflows From Agns or Ysos For Seeding Galaxiesmentioning

confidence: 99%

Magnetic helicity in primordial and dynamo scenarios of galaxies

Brandenburg¹

2006

Astron Nachr

View full text Add to dashboard Cite

Abstract. 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 no magnetic helicity fluxes. The former effect is utilized in primordial field scenarios, while the latter is important for successfully explaining astrophysical dynamos that saturate faster than resistively. Dynamo action is argued to be important not only in the galactic dynamo, but also in accretion discs in active galactic nuclei and around protostars, both of which contribute to producing a strong enough seed magnetic field. Although primordial magnetic fields may be too weak to compete with these astrophysical mechanisms, such fields could perhaps still be important in producing polarization effects in the cosmic background radiation.

show abstract

Dynamo–generated turbulence and outflows from accretion discs

Cited by 15 publications

References 25 publications

Computational aspects of astrophysical MHD and turbulence

Computational aspects of astrophysical MHD and turbulence

Physics of the solar cycle

Magnetic helicity in primordial and dynamo scenarios of galaxies

Contact Info

Product

Resources

About