scholarly journals Large-scale dynamos in rapidly rotating plane layer convection

2018 ◽  
Vol 612 ◽  
pp. A97 ◽  
Author(s):  
P. J. Bushby ◽  
P. J. Käpylä ◽  
Y. Masada ◽  
A. Brandenburg ◽  
B. Favier ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Plane Layer ◽  
Cycle Period ◽  
Dynamo Action ◽  
Horizontal Magnetic Field ◽  
Vortex Instability ◽  
Large Scale Vortex ◽  
The Mean ◽  
The Magnetic Field

Context.Convectively driven flows play a crucial role in the dynamo processes that are responsible for producing magnetic activity in stars and planets. It is still not fully understood why many astrophysical magnetic fields have a significant large-scale component.Aims.Our aim is to investigate the dynamo properties of compressible convection in a rapidly rotating Cartesian domain, focusing upon a parameter regime in which the underlying hydrodynamic flow is known to be unstable to a large-scale vortex instability.Methods.The governing equations of three-dimensional non-linear magnetohydrodynamics (MHD) are solved numerically. Different numerical schemes are compared and we propose a possible benchmark case for other similar codes.Results.In keeping with previous related studies, we find that convection in this parameter regime can drive a large-scale dynamo. The components of the mean horizontal magnetic field oscillate, leading to a continuous overall rotation of the mean field. Whilst the large-scale vortex instability dominates the early evolution of the system, the large-scale vortex is suppressed by the magnetic field and makes a negligible contribution to the mean electromotive force that is responsible for driving the large-scale dynamo. The cycle period of the dynamo is comparable to the ohmic decay time, with longer cycles for dynamos in convective systems that are closer to onset. In these particular simulations, large-scale dynamo action is found only when vertical magnetic field boundary conditions are adopted at the upper and lower boundaries. Strongly modulated large-scale dynamos are found at higher Rayleigh numbers, with periods of reduced activity (grand minima-like events) occurring during transient phases in which the large-scale vortex temporarily re-establishes itself, before being suppressed again by the magnetic field.

scholarly journals Multifractal structure of the large-scale heliospheric magnetic field strength fluctuations near 85AU

2004 ◽  
Vol 11 (4) ◽  
pp. 441-445 ◽  
Author(s):  
L. F. Burlaga
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Large Scale ◽  
Solar Maximum ◽  
Multifractal Spectrum ◽  
The Sun ◽  
Heliographic Latitude ◽  
The Mean ◽  
The Magnetic Field

Abstract. During 2002, the Voyager 1 spacecraft was in the heliosphere between 83.4 and 85.9AU (1AU is the mean distance from the Sun to Earth) at 34° N heliographic latitude. The magnetic field strength profile observed in this region had a multifractal structure in the range of scales from 2 to 16 days. The multifractal spectrum observed near 85AU is similar to that observed near 40AU, indicating relatively little evolution of the multifractal structure of the magnetic field with increasing distance in the distant heliosphere in the epoch near solar maximum.

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 The effect of velocity shear on dynamo action due to rotating convection

2013 ◽  
Vol 717 ◽  
pp. 395-416 ◽  
Author(s):  
D. W. Hughes ◽  
M. R. E. Proctor
Keyword(s):  
Magnetic Field ◽  
Velocity Field ◽  
Large Scale ◽  
Mean Field ◽  
Magnetic Reynolds Number ◽  
Velocity Shear ◽  
Magnetic Field Generation ◽  
Dynamo Action ◽  
Rotating Convection ◽  
The Magnetic Field

AbstractRecent numerical simulations of dynamo action resulting from rotating convection have revealed some serious problems in applying the standard picture of mean field electrodynamics at high values of the magnetic Reynolds number, and have thereby underlined the difficulties in large-scale magnetic field generation in this regime. Here we consider kinematic dynamo processes in a rotating convective layer of Boussinesq fluid with the additional influence of a large-scale horizontal velocity shear. Incorporating the shear flow enhances the dynamo growth rate and also leads to the generation of significant magnetic fields on large scales. By the technique of spectral filtering, we analyse the modes in the velocity that are principally responsible for dynamo action, and show that the magnetic field resulting from the full flow relies crucially on a range of scales in the velocity field. Filtering the flow to provide a true separation of scales between the shear and the convective flow also leads to dynamo action; however, the magnetic field in this case has a very different structure from that generated by the full velocity field. We also show that the nature of the dynamo action is broadly similar irrespective of whether the flow in the absence of shear can support dynamo action.

scholarly journals The nature of mean-field generation in three classes of optimal dynamos

2020 ◽  
Vol 86 (1) ◽  
Author(s):  
Axel Brandenburg ◽  
Long Chen
Keyword(s):  
Magnetic Field ◽  
Growth Rate ◽  
Boundary Conditions ◽  
Magnetic Energy ◽  
Mean Field ◽  
Dynamo Action ◽  
Magnetic Diffusivity ◽  
The Mean ◽  
The Magnetic Field ◽  
Mean Field Dynamo

In recent years, several optimal dynamos have been discovered. They minimize the magnetic energy dissipation or, equivalently, maximize the growth rate at a fixed magnetic Reynolds number. In the optimal dynamo of Willis (Phys. Rev. Lett., vol. 109, 2012, 251101), we find mean-field dynamo action for planar averages. One component of the magnetic field grows exponentially while the other decays in an oscillatory fashion near onset. This behaviour is different from that of an $\unicode[STIX]{x1D6FC}^{2}$ dynamo, where the two non-vanishing components of the planar averages are coupled and have the same growth rate. For the Willis dynamo, we find that the mean field is excited by a negative turbulent magnetic diffusivity, which has a non-uniform spatial profile near onset. The temporal oscillations in the decaying component are caused by the corresponding component of the diffusivity tensor being complex when the mean field is decaying and, in this way, time dependent. The growing mean field can be modelled by a negative magnetic diffusivity combined with a positive magnetic hyperdiffusivity. In two other classes of optimal dynamos of Chen et al. (J. Fluid Mech., vol. 783, 2015, pp. 23–45), we find, to some extent, similar mean-field dynamo actions. When the magnetic boundary conditions are mixed, the two components of the planar averaged field grow at different rates when the dynamo is 15 % supercritical. When the mean magnetic field satisfies homogeneous boundary conditions (where the magnetic field is tangential to the boundary), mean-field dynamo action is found for one-dimensional averages, but not for planar averages. Despite having different spatial profiles, both dynamos show negative turbulent magnetic diffusivities. Our finding suggests that negative turbulent magnetic diffusivities may support a broader class of dynamos than previously thought, including these three optimal dynamos.

scholarly journals The supernova-regulated ISM – VI. Magnetic effects on the structure of the interstellar medium

2019 ◽  
Vol 488 (4) ◽  
pp. 5065-5074 ◽  
Author(s):  
C C Evirgen ◽  
F A Gent ◽  
A Shukurov ◽  
A Fletcher ◽  
P J Bushby
Keyword(s):  
Magnetic Field ◽  
Interstellar Medium ◽  
Vertical Distribution ◽  
Large Scale ◽  
Dynamo Action ◽  
Hot Gas ◽  
Downward Pressure ◽  
Large Scale Magnetic Field ◽  
The Vertical Distribution ◽  
The Magnetic Field

ABSTRACT We explore the effect of magnetic fields on the vertical distribution and multiphase structure of the supernova-driven interstellar medium in simulations that admit dynamo action. As the magnetic field is amplified to become dynamically significant, gas becomes cooler and its distribution in the disc becomes more homogeneous. We attribute this to magnetic quenching of vertical velocity, which leads to a decrease in the cooling length of hot gas. A non-monotonic vertical distribution of the large-scale magnetic field strength, with the maximum at |z| ≈ 300 pc causes a downward pressure gradient below the maximum which acts against outflow driven by SN explosions, while it provides pressure support above the maximum.

Mixed convection in a horizontal duct with bottom heating and strong transverse magnetic field

2014 ◽  
Vol 757 ◽  
pp. 33-56 ◽  
Author(s):  
Xuan Zhang ◽  
Oleg Zikanov
Keyword(s):  
Magnetic Field ◽  
Mixed Convection ◽  
Large Scale ◽  
Three Dimensional ◽  
Transverse Magnetic ◽  
Temperature Fields ◽  
Horizontal Magnetic Field ◽  
Bottom Heating ◽  
Field Lines ◽  
The Magnetic Field

AbstractMixed convection in a horizontal duct with imposed transverse horizontal magnetic field is studied using direct numerical simulations (DNS) and linear stability analysis. The duct’s walls are electrically insulated and thermally insulated with the exception of the bottom wall, at which constant-rate heating is applied. The focus of the study is on flows at high Hartmann ($\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\mathit{Ha}\le 800$) and Grashof ($\mathit{Gr}\le 10^9$) numbers. It is found that, while conventional turbulence is fully suppressed, the natural convection mechanism leads to the development of large-scale coherent structures. Two types of flows are found. One is the ‘low-$\mathit{Gr}$’ regime, in which the structures are rolls aligned with the magnetic field and velocity and temperature fields are nearly uniform along the magnetic field lines outside of the boundary layers. Another is the ‘high-$\mathit{Gr}$’ regime, in which the convection appears as a combination of similar rolls oriented along the magnetic field lines and streamwise-oriented rolls. In this case, velocity and temperature distributions are anisotropic, but three-dimensional.

scholarly journals Magnetic Fields and Halos in Spiral Galaxies

Galaxies ◽  
2019 ◽  
Vol 7 (2) ◽  
pp. 54 ◽  
Author(s):  
Marita Krause
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Spiral Galaxies ◽  
Cosmic Ray ◽  
Radio Continuum ◽  
Field Pattern ◽  
Dynamo Action ◽  
Disk Plane ◽  
The Magnetic Field

Radio continuum and polarization observations reveal best the magnetic field structure and strength in nearby spiral galaxies. They show a similar magnetic field pattern, which is of spiral shape along the disk plane and X-shaped in the halo, sometimes accompanied by strong vertical fields above and below the central region of the disk. The strength of the total halo field is comparable to that of the disk. The small- and large-scale dynamo action is discussed to explain the observations with special emphasis on the rôle of star formation on the α − Ω dynamo and the magnetic field strength and structure in the disk and halo. Recently, with RM-synthesis of the CHANG-ES observations, we obtained the first observational evidence for the existence of regular magnetic fields in the halo. The analysis of the radio scale heights indicate escape-dominated radio halos with convective cosmic ray propagation for many galaxies. These galactic winds may be essential for an effective dynamo action and may transport large-scale magnetic field from the disk into the halo.

scholarly journals Dynamo generated field emergence through recurrent plasmoid ejections

2010 ◽  
Vol 6 (S273) ◽  
pp. 256-260
Author(s):  
Jörn Warnecke ◽  
Axel Brandenburg
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Solar Atmosphere ◽  
Large Scale ◽  
Computational Domain ◽  
Dynamo Action ◽  
Layer System ◽  
Forcing Function ◽  
Magnetic Buoyancy ◽  
The Magnetic Field

AbstractMagnetic buoyancy is believed to drive the transport of magnetic flux tubes from the convection zone to the surface of the Sun. The magnetic fields form twisted loop-like structures in the solar atmosphere. In this paper we use helical forcing to produce a large-scale dynamo-generated magnetic field, which rises even without magnetic buoyancy. A two layer system is used as computational domain where the upper part represents the solar atmosphere. Here, the evolution of the magnetic field is solved with the stress–and–relax method. Below this region a magnetic field is produced by a helical forcing function in the momentum equation, which leads to dynamo action. We find twisted magnetic fields emerging frequently to the outer layer, forming arch-like structures. In addition, recurrent plasmoid ejections can be found by looking at space–time diagrams of the magnetic field. Recent simulations in spherical coordinates show similar results.

Large-scale dynamo action driven by forced beating waves in a highly conducting plasma

2018 ◽  
Vol 84 (4) ◽  
Author(s):  
Krzysztof A. Mizerski
Keyword(s):  
Magnetic Field ◽  
Lorentz Force ◽  
Galaxy Formation ◽  
Large Scale ◽  
Short Paper ◽  
Back Reaction ◽  
Dynamical Regime ◽  
Dynamo Action ◽  
Acoustic Effect ◽  
The Mean

The aim of this short paper is simply to demonstrate the effect of interactions between distinct waves induced by an oscillatory forcing on generation of the large-scale electromotive force (EMF) in a highly electrically conducting plasma in the dynamical regime, i.e. when the back reaction of the Lorentz force on the flow is included. The mean EMF is induced by waves with distinct but close phase speeds, which are called ‘beating waves’ by analogy with the acoustic effect of ‘beat’, in the presence of a locally uniform seed magnetic field. These waves may have either positive or negative helicity, and it is supposed that waves of a single sign of helicity are preferentially excited by a symmetry-breaking mechanism. The formula for the mean EMF in a highly conducting plasma is derived, conditions relevant to those of the early universe before and during galaxy formation. Within the scope of the weak magnetohydrodynamic turbulence dominated by the linear waves an$\unicode[STIX]{x1D6FC}$-effect, that is generation of the large-scale EMF is studied, which leads to amplification of the seed mean magnetic field. Possible subsequent equilibration due to quenching resulting from the presence of the Lorentz force is briefly discussed.

scholarly journals Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars

2018 ◽  
Vol 609 ◽  
pp. A51 ◽  
Author(s):  
J. Warnecke ◽  
M. Rheinhardt ◽  
S. Tuomisto ◽  
P. J. Käpylä ◽  
M. J. Käpylä ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Turbulent Transport ◽  
Transport Coefficients ◽  
Test Field ◽  
Nonlinear Feedback ◽  
Magnetic Field Generation ◽  
Dynamo Action ◽  
Flux Transport ◽  
The Magnetic Field

Aims.We investigate dynamo action in global compressible solar-like convective dynamos in the framework of mean-field theory.Methods.We simulate a solar-type star in a wedge-shaped spherical shell, where the interplay between convection and rotation self-consistently drives a large-scale dynamo. To analyze the dynamo mechanism we apply the test-field method for azimuthally (φ) averaged fields to determine the 27 turbulent transport coefficients of the electromotive force, of which six are related to theαtensor. This method has previously been used either in simulations in Cartesian coordinates or in the geodynamo context and is applied here for the first time to fully compressible simulations of solar-like dynamos.Results.We find that theφφ-component of theαtensor does not follow the profile expected from that of kinetic helicity. The turbulent pumping velocities significantly alter the effective mean flows acting on the magnetic field and therefore challenge the flux transport dynamo concept. All coefficients are significantly affected by dynamically important magnetic fields. Quenching as well as enhancement are being observed. This leads to a modulation of the coefficients with the activity cycle. The temporal variations are found to be comparable to the time-averaged values and seem to be responsible for a nonlinear feedback on the magnetic field generation. Furthermore, we quantify the validity of the Parker-Yoshimura rule for the equatorward propagation of the mean magnetic field in the present case.

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