scholarly journals A physical approach to modelling large-scale galactic magnetic fields

2019 ◽  
Vol 623 ◽  
pp. A113 ◽  
Author(s):  
Anvar Shukurov ◽  
Luiz Felippe S. Rodrigues ◽  
Paul J. Bushby ◽  
James Hollins ◽  
Jörg P. Rachen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Milky Way ◽  
Mean Field ◽  
Axially Symmetric ◽  
Dynamo Action ◽  
Disc Galaxy ◽  
Large Scale Magnetic Field ◽  
Dynamo Equation

Context. A convenient representation of the structure of the large-scale galactic magnetic field is required for the interpretation of polarization data in the sub-mm and radio ranges, in both the Milky Way and external galaxies. Aims. We develop a simple and flexible approach to construct parametrised models of the large-scale magnetic field of the Milky Way and other disc galaxies, based on physically justifiable models of magnetic field structure. The resulting models are designed to be optimised against available observational data. Methods. Representations for the large-scale magnetic fields in the flared disc and spherical halo of a disc galaxy were obtained in the form of series expansions whose coefficients can be calculated from observable or theoretically known galactic properties. The functional basis for the expansions is derived as eigenfunctions of the mean-field dynamo equation or of the vectorial magnetic diffusion equation. Results. The solutions presented are axially symmetric but the approach can be extended straightforwardly to non-axisymmetric cases. The magnetic fields are solenoidal by construction, can be helical, and are parametrised in terms of observable properties of the host object, such as the rotation curve and the shape of the gaseous disc. The magnetic field in the disc can have a prescribed number of field reversals at any specified radii. Both the disc and halo magnetic fields can separately have either dipolar or quadrupolar symmetry. The model is implemented as a publicly available software package GALMAG which allows, in particular, the computation of the synchrotron emission and Faraday rotation produced by the model’s magnetic field. Conclusions. The model can be used in interpretations of observations of magnetic fields in the Milky Way and other spiral galaxies, in particular as a prior in Bayesian analyses. It can also be used for a simple simulation of a time-dependent magnetic field generated by dynamo action.

scholarly journals On the combined role of cosmic rays and supernova-driven turbulence for galactic dynamos

2020 ◽  
Vol 500 (3) ◽  
pp. 3527-3535
Author(s):  
Abhijit B Bendre ◽  
Detlef Elstner ◽  
Oliver Gressel
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Mean Field ◽  
Cosmic Ray ◽  
Field Method ◽  
Dynamo Model ◽  
Test Field ◽  
Dynamo Action ◽  
Magnetohydrodynamic Simulations

ABSTRACT Large-scale coherent magnetic fields observed in the nearby galaxies are thought to originate by a mean-field dynamo. This is governed via the turbulent electromotive force (EMF, $\overline{{\boldsymbol {\cal E}} {}}$) generated by the helical turbulence driven by supernova (SN) explosions in the differentially rotating interstellar medium (ISM). In this paper, we aim to investigate the possibility of dynamo action by the virtue of buoyancy due to a cosmic ray (CR) component injected through the SN explosions. We do this by analysing the magnetohydrodynamic simulations of local shearing box of ISM in which the turbulence is driven via random SN explosions and the energy of the explosion is distributed in the CR and/or thermal energy components. We use the magnetic field aligned diffusion prescription for the propagation of CR. We compare the evolution of magnetic fields in the models with the CR component to our previous models that did not involve the CR. We demonstrate that the inclusion of CR component enhances the growth of dynamo slightly. We further compute the underlying dynamo coefficients using the test-field method and argue that the entire evolution of the large-scale mean magnetic field can be reproduced with an α − Ω dynamo model. We also show that the inclusion of CR component leads to an unbalanced turbulent pumping between magnetic field components and additional dynamo action by the Rädler effect.

scholarly journals Generation of large-scale magnetic fields due to fluctuating in shearing systems

2018 ◽  
Vol 84 (6) ◽  
Author(s):  
Naveen Jingade ◽  
Nishant K. Singh ◽  
S. Sridhar
Keyword(s):  
Magnetic Field ◽  
Differential Equation ◽  
Growth Rate ◽  
Magnetic Fields ◽  
Correlation Time ◽  
Large Scale ◽  
Dynamo Theory ◽  
Dynamo Action ◽  
Stochastic Fluctuations ◽  
Large Scale Magnetic Field

We explore the growth of large-scale magnetic fields in a shear flow, due to helicity fluctuations with a finite correlation time, through a study of the Kraichnan–Moffatt model of zero-mean stochastic fluctuations of the$\unicode[STIX]{x1D6FC}$parameter of dynamo theory. We derive a linear integro-differential equation for the evolution of the large-scale magnetic field, using the first-order smoothing approximation and the Galilean invariance of the$\unicode[STIX]{x1D6FC}$-statistics. This enables construction of a model that is non-perturbative in the shearing rate$S$and the$\unicode[STIX]{x1D6FC}$-correlation time$\unicode[STIX]{x1D70F}_{\unicode[STIX]{x1D6FC}}$. After a brief review of the salient features of the exactly solvable white-noise limit, we consider the case of small but non-zero$\unicode[STIX]{x1D70F}_{\unicode[STIX]{x1D6FC}}$. When the large-scale magnetic field varies slowly, the evolution is governed by a partial differential equation. We present modal solutions and conditions for the exponential growth rate of the large-scale magnetic field, whose drivers are the Kraichnan diffusivity, Moffatt drift, shear and a non-zero correlation time. Of particular interest is dynamo action when the$\unicode[STIX]{x1D6FC}$-fluctuations are weak; i.e. when the Kraichnan diffusivity is positive. We show that in the absence of Moffatt drift, shear does not give rise to growing solutions. But shear and Moffatt drift acting together can drive large-scale dynamo action with growth rate$\unicode[STIX]{x1D6FE}\propto |S|$.

scholarly journals Probing Magnetic Fields With SNRs

2012 ◽  
Vol 10 (H16) ◽  
pp. 395-395
Author(s):  
Roland Kothes
Keyword(s):  
Magnetic Field ◽  
Simple Model ◽  
Magnetic Fields ◽  
Supernova Remnants ◽  
Large Scale ◽  
Milky Way ◽  
Magnetic Energy ◽  
Large Scale Magnetic Field ◽  
Rotation Measure ◽  
Field Lines

AbstractAs supernova remnants (SNRs) expand, their shock waves freeze in and compress magnetic field lines they encounter; consequently we can use SNRs as magnifying glasses for interstellar magnetic fields. A simple model is used to derive polarization and rotation measure (RM) signatures of SNRs. This model is exploited to gain knowledge about the large-scale magnetic field in the Milky Way. Three examples are given which indicate a magnetic anomaly, an azimuthal large-scale magnetic field towards the anti-centre, and a chimney that releases magnetic energy from the plane into the halo.

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 Sun-like Stars: magnetic fields, cycles and exoplanets

2015 ◽  
Vol 11 (A29A) ◽  
pp. 360-364
Author(s):  
Rim Fares
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Observational Constraints ◽  
The Sun ◽  
Field Generation ◽  
Large Scale Magnetic Field ◽  
Planetary Orbits ◽  
Activity Cycles ◽  
Magnetic Cycles

AbstractIn Sun-like stars, magnetic fields are generated in the outer convective layers. They shape the stellar environment, from the photosphere to planetary orbits. Studying the large-scale magnetic field of those stars enlightens our understanding of the field properties and gives us observational constraints for field generation dynamo models. It also sheds light on how “normal” the Sun is among Sun-like stars. In this contribution, I will review the field properties of Sun-like stars, focusing on solar twins and planet hosting stars. I will discuss the observed large-scale magnetic cycles, compare them to stellar activity cycles, and link that to what we know about the Sun. I will also discuss the effect of large-scale stellar fields on exoplanets, exoplanetary emissions (e.g. radio), and habitability.

scholarly journals Probing interstellar magnetic fields with Supernova remnants

2008 ◽  
Vol 4 (S259) ◽  
pp. 75-80 ◽  
Author(s):  
Roland Kothes ◽  
Jo-Anne Brown
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Supernova Remnants ◽  
Large Scale ◽  
Galactic Plane ◽  
Field Configuration ◽  
Large Scale Magnetic Field ◽  
Rotation Measure ◽  
Field Lines ◽  
The Magnetic Field

AbstractAs Supernova remnants expand, their shock waves are freezing in and compressing the magnetic field lines they encounter; consequently we can use Supernova remnants as magnifying glasses for their ambient magnetic fields. We will describe a simple model to determine emission, polarization, and rotation measure characteristics of adiabatically expanding Supernova remnants and how we can exploit this model to gain information about the large scale magnetic field in our Galaxy. We will give two examples: The SNR DA530, which is located high above the Galactic plane, reveals information about the magnetic field in the halo of our Galaxy. The SNR G182.4+4.3 is located close to the anti-centre of our Galaxy and reveals the most probable direction where the large-scale magnetic field is perpendicular to the line of sight. This may help to decide on the large-scale magnetic field configuration of our Galaxy. But more observations of SNRs are needed.

Dynamo action in rotating convection

2009 ◽  
Vol 5 (H15) ◽  
pp. 347-347
Author(s):  
Gustavo Guerrero ◽  
Elisabete M. de Gouveia Dal Pino
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Hydrostatic Equilibrium ◽  
Computational Domain ◽  
Dynamo Action ◽  
Rotating Convection ◽  
Helical Turbulence ◽  
Large Scale Magnetic Field ◽  
Unstable Layer ◽  
Scale Shear

AbstractWe present MHD numerical simulations of a rotating turbulent convection system in a 3D domain (we have used the finite volume, Goudunov type MHD code PLUTO (Mignone et al. 2007)). Rotating convection is the natural scenario for the study of the dynamo action which is able to generate a large scale magnetic field, like the observed in the sun. Though we have neglected in the present approach the Ω effect, due to a large scale shear, our model is appropriate to test the controversial existence of the so called α effect that arises from helical turbulence (e.g. Cattaneo & Hughes 2006, Käpylä et al. 2009). We start with a two-layer piece-wise polytropic region in hydrostatic equilibrium (e.g. Ziegler 2002), considering one stable overshoot layer at the bottom and a convectively unstable layer at the top of the computational domain. We have allowed this hydrodynamic system to evolve up to the steady state, i.e., after about 10 turnover times (τ). Then, we introduced a seed magnetic field and let the system evolve for more ~40 τ. Our preliminary results are summarized below in Figure 2.

scholarly journals Rotation Measures of Extragalactic Sources behind the Southern Galactic Plane: New Insights into the Large‐Scale Magnetic Field of the Inner Milky Way

2007 ◽  
Vol 663 (1) ◽  
pp. 258-266 ◽  
Author(s):  
J. C. Brown ◽  
M. Haverkorn ◽  
B. M. Gaensler ◽  
A. R. Taylor ◽  
N. S. Bizunok ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Milky Way ◽  
Galactic Plane ◽  
Large Scale Magnetic Field

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.

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