Galactic dynamo and helicity losses through fountain flow

Shukurov, Anvar; Sokoloff, Dmitry; Subramanian, Kandaswamy; Brandenburg, Axel

doi:10.1051/0004-6361:200600011

Cited by 145 publications

(206 citation statements)

References 30 publications

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“…However, there has been no such hint from Hα observations of edge-on spiral galaxies so far (Hoopes et al 1999). Alternatively, the magnetic pitch angle may be affected by gas flows, for instance, by outflows that become weaker towards the outer disk and can decrease the effective dynamo number (Shukurov et al 2006;Sur et al 2007). …”

Section: Pitch Angles Of the Spiral Magnetic Field And The Spiral Strmentioning

confidence: 99%

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

Beck

2015

A&A

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Context. Magnetic fields play an important role in the formation and stabilization of spiral structures in galaxies, but the interaction between interstellar gas and magnetic fields has not yet been understood. In particular, the phenomenon of "magnetic arms" located between material arms is a mystery. Aims. The strength and structure of interstellar magnetic fields and their relation to spiral arms in gas and dust are investigated in the nearby and almost face-on spiral galaxy IC 342. Methods. The total and polarized radio continuum emission of IC 342 was observed with high spatial resolution in four wavelength bands with the Effelsberg and VLA telescopes. At λ6.2 cm the data from both telescopes were combined. I separated thermal and nonthermal (synchrotron) emission components with the help of the spectral index distribution and derived maps of the magnetic field strength, degree of magnetic field order, magnetic pitch angle, Faraday rotation measure, and Faraday depolarization. Results. IC 342 hosts a diffuse radio disk with an intensity that decreases exponentially with increasing radius. The frequency dependence of the scalelength of synchrotron emission indicates energy-dependent propagation of the cosmic-ray electrons, probably via the streaming instability. The equipartition strength of the total field in the main spiral arms is typically 15 μG, that of the ordered field about 5 μG. The total radio emission, observed with the VLA's high resolution, closely follows the dust emission in the infrared at 8 μm (Spitzer telescope) and 22 μm (WISE telescope). The polarized emission is not diffuse, but concentrated in spiral arms of various types: (1) a narrow arm of about 300 pc width, displaced inwards with respect to the eastern arm by about 200 pc, indicating magnetic fields compressed by a density wave; (2) a broad arm of 300-500 pc width around the northern arm with systematic variations in polarized emission, polarization angles, and Faraday rotation measures on a scale of about 2 kpc, indicative of a helically twisted flux tube generated by the Parker instability; (3) a rudimentary magnetic arm in an interarm region in the north-west; (4) several broad spiral arms in the outer galaxy, related to spiral arms in the total neutral gas; (5) short features in the outer south-western galaxy, probably distorted by tidal interaction. Faraday rotation of the polarization angles reveals an underlying regular field of only 0.5 μG strength with a large-scale axisymmetric spiral pattern, probably a signature of a mean-field α − Ω dynamo, and an about 10× stronger field that fluctuates on scales of a few 100 pc. The magnetic field around the bar in the central region of IC 342 resembles that of large barred galaxies; it has a regular spiral pattern with a large pitch angle, is directed outwards, and is opposite to the large-scale regular field in the disk. Polarized emission at λ20.1 cm is strongly affected by Faraday depolarization in the western and northern parts of the galaxy. Helical fields extending fro...

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Section: Pitch Angles Of the Spiral Magnetic Field And The Spiral Strmentioning

confidence: 99%

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

Beck

2015

A&A

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“…3 Dynamo models with helicity fluxes Shukurov et al (2006) small scale helicity from the disc region. They showed that in a simple local galactic dynamo model, the field can grow to saturation levels in a turbulent diffusion time, i.e.…”

Section: Magnetic Helicity Conservationmentioning

confidence: 99%

The saturation of galactic dynamos

Moss

Sokoloff

2011

Astron Nachr

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We summarize the current state of the long term discussion about the saturation mechanisms associated with rapid growth of small-scale magnetic field, that operate in large-scale galactic dynamos, and related problems with magnetic helicity conservation. Our general conclusion is that, taking into account magnetic helicity fluxes, large-scale magnetic field can be amplified up to about the equipartition level. In contrast, models without helicity fluxes give an initial temporal magnetic field growth, but then decay. In our opinion, it is more appropriate to refer to the situation as a "potentially catastrophic scenario" rather than as "catastrophic α-quenching".

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“…Nevertheless this constraint may be lifted if the system is able to rid itself of small-scale helicity through at least one of several ways such as open boundaries, advective, diffusive and shear-driven fluxes (Shukurov et al 2006;Zhang et al 2006;Sur et al 2007;Käpylä et al 2008;Brandenburg et al 2009;Guerrero et al 2010). Even though the helicity constraint in direct numerical simulations (DNS) of dynamos with strong shear have been clearly identified, the results can be matched with mean field models with a weaker algebraic quenching than α 2 dynamos .…”

Section: Introductionmentioning

confidence: 99%

Magnetic helicity fluxes in interface and flux transport dynamos

Chatterjee

Gómez

Brandenburg

2010

A&A

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Context. Dynamos in the Sun and other bodies tend to produce magnetic fields that possess magnetic helicity of opposite sign at large and small scales, respectively. The build-up of magnetic helicity at small scales provides an important saturation mechanism. Aims. In order to understand the nature of the solar dynamo we need to understand the details of the saturation mechanism in spherical geometry. In particular, we aim to understand the effects of magnetic helicity fluxes from turbulence and meridional circulation. Methods. We consider a model with only radial shear confined to a thin layer (tachocline) at the bottom of the convection zone. The kinetic α owing to helical turbulence is assumed to be localized in a region above the convection zone. The dynamical quenching formalism is used to describe the build-up of mean magnetic helicity in the model, which results in a magnetic α effect that feeds back on the kinetic α effect. In some cases we compare these results with those obtained from a model with a simple algebraic α quenching formula.Results. In agreement with earlier findings, the magnetic α effect has the opposite sign compared with the kinetic α effect and leads to a catastrophic decrease of the saturation field strength proportional to the inverse magnetic Reynolds number. At high latitudes this quenching effect can lead to secondary dynamo waves that propagate poleward because of the opposite sign of α. These secondary dynamo waves are driven by small-scale magnetic helicity instead of the small-scale kinetic helicity. Magnetic helicity fluxes both from turbulent mixing and from meridional circulation alleviate catastrophic quenching. Interestingly, supercritical diffusive helicity fluxes also give rise to secondary dynamo waves and grand minima-like episodes.

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Galactic dynamo and helicity losses through fountain flow

Cited by 145 publications

References 30 publications

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

The saturation of galactic dynamos

Magnetic helicity fluxes in interface and flux transport dynamos

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