Galactic Dynamos

2014 ◽  
pp. 151-179
Author(s):  
Ulrich Klein ◽  
Andrew Fletcher
Keyword(s):  
Galactic Dynamos

scholarly journals Magnetic Fields in Galaxies

2010 ◽  
Vol 6 (S271) ◽  
pp. 135-144
Author(s):  
Ellen G. Zweibel
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Cosmic Time ◽  
Origin And Evolution ◽  
Cosmological Problem ◽  
Background Turbulence ◽  
Multicomponent Gas ◽  
Magnetic Buoyancy ◽  
Galactic Dynamos

AbstractThe origin and evolution of magnetic fields in the Universe is a cosmological problem. Although exotic mechanisms for magneotgenesis cannot be ruled out, galactic magnetic fields could have been seeded by magnetic fields from stars and accretion disks, and must be continuously regenerated due to the ongoing replacement of the interstellar medium. Unlike stellar dynamos, galactic dynamos operate in a multicomponent gas at low collisionality and high magnetic Prandtl number. Their background turbulence is highly compressible, the plasma β ~ 1, and there has been time for only a few large exponentiation times at large scale over cosmic time. Points of similarity include the importance of magnetic buoyancy, the large range of turbulent scales and tiny microscopic scales, and the coupling between the magnetic field and certain properties of the flow. Understanding the origin and maintenance of the large scale galactic magnetic field is the most challenging aspect of the problem.

scholarly journals Galactic Dynamos — a Challenge for Observers

1993 ◽  
Vol 157 ◽  
pp. 283-297
Author(s):  
Rainer Beck
Keyword(s):  
Large Scale ◽  
General Distribution ◽  
Future Research ◽  
Small Scale ◽  
Dynamo Theory ◽  
Galactic Winds ◽  
Scale Field ◽  
Field Lines ◽  
Galactic Dynamos ◽  
The Magnetic Field

Results of linear αΩ-dynamo models are confronted with radio polarization observations of spiral galaxies. The general distribution of polarized emission and the magnetic field pitch angle can be described with sufficient accuracy. The occurrance of systematic large-scale variations in Faraday rotation (RM) is the strongest argument in favour of dynamo theory. However, the predominance of axisymmetric SO modes could not be confirmed by observations; S1 modes are about equally frequent. The azimuthal variations of field pitch angles and, in two cases, the phases of the RM variations are inconsistent with a classical αΩ-dynamo. Locally deviating RM values indicate field lines bending out of the plane. There is increasing evidence that galactic fields cannot be described by simple dynamo modes. This calls for more realistic dynamo models, taking into account non-axisymmetric velocity fields and galactic winds.Interpretation of radio observations is difficult because Faraday depolarization can seriously affect the data. Observations of small-scale field structures are summarized which show the path for future research. Instrumental needs for such investigations are discussed.

scholarly journals Enhancement of Galactic Dynamos by Density Waves

1990 ◽  
Vol 140 ◽  
pp. 127-130 ◽  
Author(s):  
Makoto Tosa ◽  
Masashi Chiba
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Density Wave ◽  
Wave Perturbation ◽  
Density Waves ◽  
Dynamo Action ◽  
Galactic Dynamos ◽  
The Magnetic Field ◽  
Effects Of Density

We examine effects of density waves on the local galactic αω-dynamo. Oscillations of the magnetic field and the dynamo parameters due to the density wave perturbation irreversibly couple with the dynamo action to enhance the growth of the magnetic fields.

scholarly journals Galactic Dynamos Without Sharp Boundaries: Non-Axisymmetric Fields in Axisymmetric Disks?

1990 ◽  
Vol 140 ◽  
pp. 115-116
Author(s):  
R. Meinel ◽  
D. Elstner ◽  
G. Rüdiger ◽  
F. Krause
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Spiral Galaxies ◽  
Galactic Magnetic Fields ◽  
Axisymmetric Structure ◽  
Galactic Dynamos

Radio polarization observations of spiral galaxies suggest the existence of large-scale galactic magnetic fields which are of either axisymmetric -spiral (ASS) or bisymmetric-spiral (BSS), i.e. non-axisymmetric, structure (cf. Beck, 1939). Clear evidence for a BSS field was indicated for M31 by M. Krause et al. (1989).

Structure of magnetic fields generated by galactic dynamos

1990 ◽  
Vol 50 (1-3) ◽  
pp. 147-157 ◽  
Author(s):  
Makoto Tosa ◽  
Masashi Chiba
Keyword(s):  
Magnetic Fields ◽  
Galactic Dynamos

scholarly journals Nonlinear magnetic diffusion and magnetic helicity transport in galactic dynamos

2003 ◽  
Vol 400 (1) ◽  
pp. 9-18 ◽  
Author(s):  
N. Kleeorin ◽  
D. Moss ◽  
I. Rogachevskii ◽  
D. Sokoloff
Keyword(s):  
Magnetic Helicity ◽  
Magnetic Diffusion ◽  
Galactic Dynamos

scholarly journals What can we say about seed fields for galactic dynamos?

2012 ◽  
Vol 107 (1-2) ◽  
pp. 3-10 ◽  
Author(s):  
Dmitry Sokoloff ◽  
David Moss
Keyword(s):  
Galactic Dynamos

scholarly journals Derivation and precision of mean field electrodynamics with mesoscale fluctuations

2018 ◽  
Vol 84 (3) ◽  
Author(s):  
Hongzhe Zhou ◽  
Eric G. Blackman ◽  
Luke Chamandy
Keyword(s):  
Large Scale ◽  
Mean Field ◽  
Scale Separation ◽  
Precision Error ◽  
Theoretical Predictions ◽  
Wide Scale ◽  
The Mean ◽  
Scale Properties ◽  
Galactic Dynamos ◽  
Correction Terms

Mean field electrodynamics (MFE) facilitates practical modelling of secular, large scale properties of astrophysical or laboratory systems with fluctuations. Practitioners commonly assume wide scale separation between mean and fluctuating quantities, to justify equality of ensemble and spatial or temporal averages. Often however, real systems do not exhibit such scale separation. This raises two questions: (I) What are the appropriate generalized equations of MFE in the presence of mesoscale fluctuations? (II) How precise are theoretical predictions from MFE? We address both by first deriving the equations of MFE for different types of averaging, along with mesoscale correction terms that depend on the ratio of averaging scale to variation scale of the mean. We then show that even if these terms are small, predictions of MFE can still have a significant precision error. This error has an intrinsic contribution from the dynamo input parameters and a filtering contribution from differences in the way observations and theory are projected through the measurement kernel. Minimizing the sum of these contributions can produce an optimal scale of averaging that makes the theory maximally precise. The precision error is important to quantify when comparing to observations because it quantifies the resolution of predictive power. We exemplify these principles for galactic dynamos, comment on broader implications, and identify possibilities for further work.

scholarly journals The role of magnetic helicity transport in nonlinear galactic dynamos

2002 ◽  
Vol 387 (2) ◽  
pp. 453-462 ◽  
Author(s):  
N. Kleeorin ◽  
D. Moss ◽  
I. Rogachevskii ◽  
D. Sokoloff
Keyword(s):  
Magnetic Helicity ◽  
Galactic Dynamos
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