scholarly journals On the dynamical interaction between overshooting convection and an underlying dipole magnetic field – I. The non-dynamo regime

2021 ◽  
Vol 503 (1) ◽  
pp. 362-375
Author(s):  
L Korre ◽  
NH Brummell ◽  
P Garaud ◽  
C Guervilly
Keyword(s):  
Magnetic Field ◽  
Turbulent Diffusion ◽  
Large Scale ◽  
Molecular Diffusion ◽  
Mean Field ◽  
Stable Region ◽  
Turbulent Regime ◽  
Poloidal Magnetic Field ◽  
Dynamical Interaction ◽  
Large Scale Field

ABSTRACT Motivated by the dynamics in the deep interiors of many stars, we study the interaction between overshooting convection and the large-scale poloidal fields residing in radiative zones. We have run a suite of 3D Boussinesq numerical calculations in a spherical shell that consists of a convection zone with an underlying stable region that initially compactly contains a dipole field. By varying the strength of the convective driving, we find that, in the less turbulent regime, convection acts as turbulent diffusion that removes the field faster than solely molecular diffusion would do. However, in the more turbulent regime, turbulent pumping becomes more efficient and partially counteracts turbulent diffusion, leading to a local accumulation of the field below the overshoot region. These simulations suggest that dipole fields might be confined in underlying stable regions by highly turbulent convective motions at stellar parameters. The confinement is of large-scale field in an average sense and we show that it is reasonably modelled by mean-field ideas. Our findings are particularly interesting for certain models of the Sun, which require a large-scale, poloidal magnetic field to be confined in the solar radiative zone in order to explain simultaneously the uniform rotation of the latter and the thinness of the solar tachocline.

scholarly journals Characterizing the dynamo in a radiatively inefficient accretion flow

2020 ◽  
Vol 494 (4) ◽  
pp. 4854-4866 ◽  
Author(s):  
Prasun Dhang ◽  
Abhijit Bendre ◽  
Prateek Sharma ◽  
Kandaswamy Subramanian
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Mean Field ◽  
Compact Object ◽  
Rotational Instability ◽  
Test Field ◽  
Poloidal Magnetic Field ◽  
Accretion Flow ◽  
The Mean ◽  
Value Decomposition

ABSTRACT We explore the magneto-rotational instability (MRI)-driven dynamo in a radiatively inefficient accretion flow (RIAF) using the mean field dynamo paradigm. Using singular value decomposition (SVD) we obtain the least-squares fitting dynamo coefficients α and γ by comparing the time series of the turbulent electromotive force and the mean magnetic field. Our study is the first one to show the poloidal distribution of these dynamo coefficients in global accretion flow simulations. Surprisingly, we obtain a high value of the turbulent pumping coefficient γ, which transports the mean magnetic flux radially outwards. This would have implications for the launching of magnetized jets that are produced efficiently in presence a large-scale poloidal magnetic field close to the compact object. We present a scenario of a truncated disc beyond the RIAF where a large-scale dynamo-generated poloidal magnetic field can aid jet launching close to the black hole. Magnitude of all the calculated coefficients decreases with radius. Meridional variations of αϕϕ, responsible for toroidal to poloidal field conversion, is very similar to that found in shearing box simulations using the ‘test field’ (TF) method. By estimating the relative importance of α-effect and shear, we conclude that the MRI-driven large-scale dynamo, which operates at high latitudes beyond a disc scale height, is essentially of the α − Ω type.

scholarly journals The Magnetic Field Near the Local Bubble

1997 ◽  
Vol 166 ◽  
pp. 227-238
Author(s):  
Carl Heiles
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Zeeman Splitting ◽  
Relativistic Electrons ◽  
Local Bubble ◽  
Scale Field ◽  
Large Scale Field ◽  
Field Lines ◽  
Hi Shells ◽  
The Magnetic Field

AbstractThere are almost no direct observational indicators of the magnetic field inside the local bubble. Just outside the bubble, the best tracers are stellar polarization and HI Zeeman splitting. These show that the local field does not follow the large-scale Galactic field. Here we discuss whether the deformation of the large-scale field by the local HI shells is consistent with the observations. We concentrate on the Loop 1 region, and find that the field lines are well-explained by this idea; in addition, the bright radio filaments of Radio Loop 1 delineate particular field lines that are “lit up” by an excess of relativistic electrons.

scholarly journals Mean-Field Magnetohydrodynamics as a Basis of Solar Dynamo Theory

1976 ◽  
Vol 71 ◽  
pp. 323-344 ◽  
Author(s):  
K.-H. Rädler
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Mean Field ◽  
Strong Relationship ◽  
Solar Radius ◽  
Small Scale ◽  
Dynamo Theory ◽  
The Magnetic Field ◽  
Complicated Situation ◽  
Insight Into

One of the most striking features of both the magnetic field and the motions observed at the Sun is their highly irregular or random character which indicates the presence of rather complicated magnetohydrodynamic processes. Of great importance in this context is a comprehension of the behaviour of the large scale components of the magnetic field; large scales are understood here as length scales in the order of the solar radius and time scales of a few years. Since there is a strong relationship between these components and the solar 22-years cycle, an insight into the mechanism controlling these components also provides for an insight into the mechanism of the cycle. The large scale components of the magnetic field are determined not only by their interaction with the large scale components of motion. On the contrary, a very important part is played also by an interaction between the large and the small scale components of magnetic field and motion so that a very complicated situation has to be considered.

scholarly journals Large-scale dynamo action of magnetized Taylor–Couette flows

2020 ◽  
Vol 493 (1) ◽  
pp. 1249-1260
Author(s):  
G Rüdiger ◽  
M Schultz
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Mean Field ◽  
Single Mode ◽  
Eddy Diffusivity ◽  
Magnetic Reynolds Number ◽  
Dynamo Model ◽  
Turbulence Energy ◽  
Dynamo Action ◽  
Taylor Couette

ABSTRACT A conducting Taylor–Couette flow with quasi-Keplerian rotation law containing a toroidal magnetic field serves as a mean-field dynamo model of the Tayler–Spruit type. The flows are unstable against non-axisymmetric perturbations which form electromotive forces defining α effect and eddy diffusivity. If both degenerated modes with m = ±1 are excited with the same power then the global α effect vanishes and a dynamo cannot work. It is shown, however, that the Tayler instability produces finite α effects if only an isolated mode is considered but this intrinsic helicity of the single-mode is too low for an α2 dynamo. Moreover, an αΩ dynamo model with quasi-Keplerian rotation requires a minimum magnetic Reynolds number of rotation of Rm ≃ 2000 to work. Whether it really works depends on assumptions about the turbulence energy. For a steeper-than-quadratic dependence of the turbulence intensity on the magnetic field, however, dynamos are only excited if the resulting magnetic eddy diffusivity approximates its microscopic value, ηT ≃ η. By basically lower or larger eddy diffusivities the dynamo instability is suppressed.

scholarly journals Reconstructing solar magnetic fields from historical observations

2019 ◽  
Vol 627 ◽  
pp. A11
Author(s):  
I. O. I. Virtanen ◽  
I. I. Virtanen ◽  
A. A. Pevtsov ◽  
L. Bertello ◽  
A. Yeates ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Active Regions ◽  
Long Term Evolution ◽  
Photospheric Magnetic Field ◽  
Solar Magnetic Fields ◽  
Term Evolution ◽  
Large Scale Field

Aims. The evolution of the photospheric magnetic field has only been regularly observed since the 1970s. The absence of earlier observations severely limits our ability to understand the long-term evolution of solar magnetic fields, especially the polar fields that are important drivers of space weather. Here, we test the possibility to reconstruct the large-scale solar magnetic fields from Ca II K line observations and sunspot magnetic field observations, and to create synoptic maps of the photospheric magnetic field for times before modern-time magnetographic observations. Methods. We reconstructed active regions from Ca II K line synoptic maps and assigned them magnetic polarities using sunspot magnetic field observations. We used the reconstructed active regions as input in a surface flux transport simulation to produce synoptic maps of the photospheric magnetic field. We compared the simulated field with the observed field in 1975−1985 in order to test and validate our method. Results. The reconstruction very accurately reproduces the long-term evolution of the large-scale field, including the poleward flux surges and the strength of polar fields. The reconstruction has slightly less emerging flux because a few weak active regions are missing, but it includes the large active regions that are the most important for the large-scale evolution of the field. Although our reconstruction method is very robust, individual reconstructed active regions may be slightly inaccurate in terms of area, total flux, or polarity, which leads to some uncertainty in the simulation. However, due to the randomness of these inaccuracies and the lack of long-term memory in the simulation, these problems do not significantly affect the long-term evolution of the large-scale field.

scholarly journals The Toroidal Magnetic Field inside the Sun

1991 ◽  
Vol 130 ◽  
pp. 187-189
Author(s):  
V.N. Krivodubskij ◽  
A.E. Dudorov ◽  
A.A. Ruzmaikin ◽  
T.V. Ruzmaikina
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Convection Zone ◽  
The Sun ◽  
Poloidal Magnetic Field ◽  
Radial Gradient ◽  
The Core ◽  
Large Scale Magnetic Field ◽  
Magnetic Buoyancy ◽  
The Magnetic Field

Analysis of the fine structure of the solar oscillations has enabled us to determine the internal rotation of the Sun and to estimate the magnitude of the large-scale magnetic field inside the Sun. According to the data of Duvall et al. (1984), the core of the Sun rotates about twice as fast as the solar surface. Recently Dziembowski et al. (1989) have showed that there is a sharp radial gradient in the Sun’s rotation at the base of the convection zone, near the boundary with the radiative interior. It seems to us that the sharp radial gradients of the angular velocity near the core of the Sun and at the base of the convection zone, acting on the relict poloidal magnetic field Br, must excite an intense toroidal field Bф, that can compensate for the loss of the magnetic field due to magnetic buoyancy.

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

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

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

scholarly journals Large-Scale Internal Magnetic Field of the Sun

1990 ◽  
Vol 138 ◽  
pp. 391-394
Author(s):  
A.E. Dudorov ◽  
V.N. Krivodubskij ◽  
A.A. Ruzmaikin ◽  
T.V. Ruzmaikina
Keyword(s):  
Magnetic Field ◽  
Differential Rotation ◽  
Large Scale ◽  
The Sun ◽  
Poloidal Magnetic Field ◽  
The Core ◽  
Torsional Waves ◽  
Formation And Evolution ◽  
Field Lines ◽  
The Magnetic Field

The behaviour of the magnetic field during the formation and evolution of the Sun is investigated. It is shown that an internal poloidal magnetic field of the order of 104 − 105 G near the core of the Sun may be compatible with differential rotation and with torsional waves, travelling along the magnetic field lines (Dudorov et al., 1989).

The nonlinear properties of a large-scale dynamo driven by helical forcing

2002 ◽  
Vol 456 ◽  
pp. 219-237 ◽  
Author(s):  
FAUSTO CATTANEO ◽  
DAVID W. HUGHES ◽  
JEAN-CLAUDE THELEN
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Initial Conditions ◽  
Spatial Scales ◽  
Mean Field ◽  
Forced Flow ◽  
Small Scale ◽  
Dynamo Action ◽  
Initial State ◽  
The Magnetic Field

By considering an idealized model of helically forced flow in an extended domain that allows scale separation, we have investigated the interaction between dynamo action on different spatial scales. The evolution of the magnetic field is studied numerically, from an initial state of weak magnetization, through the kinematic and into the dynamic regime. We show how the choice of initial conditions is a crucial factor in determining the structure of the magnetic field at subsequent times. For a simulation with initial conditions chosen to favour the growth of the small-scale field, the evolution of the large-scale magnetic field can be described in terms of the α-effect of mean field magnetohydrodynamics. We have investigated this feature further by a series of related numerical simulations in smaller domains. Of particular significance is that the results are consistent with the existence of a nonlinearly driven α-effect that becomes saturated at very small amplitudes of the mean magnetic field.

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

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

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

Sign in / Sign up

Export Citation Format

Share Document