The saturation of galactic dynamos

Moss, D.; Sokoloff, D.

doi:10.1002/asna.200911414

Cited by 32 publications

(7 citation statements)

References 27 publications

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“…It is also true that the saturation fields in our model are rather small compared to the equipartition field strength. This was also observed in the model of Shukurov et al (2006); see also Moss & Sokoloff (2011). One possibility, that we have not explored, is that the neglected inhomogeneity of the solar convection zone may be important.…”

Section: Discussionsupporting

confidence: 60%

“…It has been suggested that the quenching may be alleviated by the expulsion of magnetic helicity through open boundaries (Blackman & Field, 2000;Kleeorin et al, 2000). At least three different physical mechanisms may help in the expulsion of small scale magnetic helicity: (a) large scale shear (Vishniac & Cho, 2001;Subramanian & Brandenburg, 2004;Brandenburg & Sandin, 2004;Moss & Sokoloff, 2011); (b) turbulent diffusion of magnetic helicity (Mitra et al, 2010a); (c) non-zero mean flow out from a boundary of the domain, e.g. a wind.…”

Section: Introductionmentioning

confidence: 99%

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Alleviatingαquenching by solar wind and meridional flows

Mitra

Moss

Tavakol

et al. 2011

A&A

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Aims. We study the ability of magnetic helicity expulsion to alleviate catastrophic α-quenching in mean field dynamos in twodimensional spherical wedge domains. Methods. Motivated by the physical state of the outer regions of the Sun, we consider α 2 Ω mean field models with a dynamical α quenching. We include two mechanisms which have the potential to facilitate helicity expulsion, namely advection by a mean flow ("solar wind") and meridional circulation. Results. We find that a wind alone can prevent catastrophic quenching, with the field saturating at finite amplitude. In certain parameter ranges, the presence of a large-scale meridional circulation can reinforce this alleviation. However, the saturated field strengths are typically below the equipartition field strength. We discuss possible mechanisms that might increase the saturated field.

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Section: Discussionsupporting

confidence: 60%

Section: Introductionmentioning

confidence: 99%

Alleviatingαquenching by solar wind and meridional flows

Mitra

Moss

Tavakol

et al. 2011

A&A

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“…The regular field strength increases faster for higher dynamo numbers in the exponentially growing phase of a large-scale dynamo. The final field strength also depends on the dynamo saturation, which is a matter of heavy debate (e.g., Moss & Sokoloff 2011). The disk height and turbulent velocity are not independent.…”

Section: Integrated Galaxy Propertiesmentioning

confidence: 99%

Large-scale radio continuum properties of 19 Virgo cluster galaxies

Vollmer

Soida

Beck

et al. 2013

A&A

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Deep scaled array VLA 20 and 6 cm observations including polarization of 19 Virgo spiral galaxies are presented extending previous work on the influence of the cluster environment on the radio continuum properties of Virgo cluster spiral galaxies. This sample contains six galaxies with a global minimum of 20 cm polarized emission at the receding side of the galactic disk and quadrupolar type large-scale magnetic fields. In the new sample no additional case of a ram-pressure stripped spiral galaxy with an asymmetric ridge of polarized radio continuum emission was found. In the absence of a close companion, a truncated Hi disk, together with a ridge of polarized radio continuum emission at the outer edge of the Hi disk, is a signpost of ram pressure stripping. Six out of the 19 observed galaxies (NGC 4294, NGC 4298, NGC 4457, NGC 4532, NGC 4568, NGC 4808) display asymmetric 6 cm polarized emission distributions. Three galaxies belong to tidally interacting pairs (NGC 4294, NGC 4298, NGC 4568), two galaxies host huge accreting Hi envelopes (NGC 4532, NGC 4808), and one galaxy (NGC 4457) had a recent minor merger. Tidal interactions and accreting gas envelopes can lead to compression and shear motions which enhance the polarized radio continuum emission. The resulting asymmetries are located within the Hi distribution. The 6 cm average degree of polarization only correlates with the galaxy mass or rotation velocity. In addition, galaxies with low average star formation rate per unit area have a low average degree of polarization. Shear or compression motions can enhance the degree of polarization. The average degree of polarization of tidally interacting galaxies is generally lower than expected for a given rotation velocity and star formation activity. This low average degree of polarization is at least partly due to the absence of polarized emission from the thin disk. Ram pressure stripping can decrease whereas tidal interactions most frequently decreases the average degree of polarization of Virgo cluster spiral galaxies. We found that moderate active ram pressure stripping has no influence on the spectral index, but enhances the global radio continuum emission with respect to the FIR emission, while an accreting gas envelope can but not necessarily enhances the radio continuum emission with respect to the FIR emission.

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“…As the basic starting point, we assumed that this layer is not too thin, otherwise a magnetic field could not be generated (Phillips 2001;Arshakian et al 2009;Moss & Sokoloff 2011). Also, the observed half thickness of the layer of partially ionized warm HI in the solar vicinity is about 400 pc (Cox 2005).…”

Section: The Parameter Distributionsmentioning

confidence: 99%

“…Taking the uncertainties in the data into account, we consider two simple models with very different, however feasible, parameters for the layer of partially ionized gas (WIM+HI). As the basic starting point, we assumed that this layer is not too thin, otherwise a magnetic field could not be generated (Phillips 2001;Arshakian et al 2009;Moss & Sokoloff 2011). Also, the observed half thickness of the layer of partially ionized warm HI in the solar vicinity is about 400 pc (Cox 2005).…”

Section: The Parameter Distributionsmentioning

confidence: 99%

Magnetic fields near the peripheries of galactic discs

Михайлов

Kasparova

Moss

et al. 2014

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Context. Magnetic fields are observed beyond the peripheries of optically detected galactic discs, while numerical models of their origin and the typical magnitudes are still absent. Previously, studies of galactic dynamo have avoided considering the peripheries of galactic discs because of the very limited (though gradually growing) knowledge about the local properties of the interstellar medium. Aims. Here we investigate the possibility that magnetic fields can be generated in the outskirts of discs, taking the Milky Way as an example. Methods. We consider a simple evolving galactic dynamo model in the "no-z" formulation, applicable to peripheral regions of galaxies, for various assumptions about the radial and vertical profiles of the ionized gas disc.Results. The magnetic field may grow as galaxies evolve, even in the more remote parts of the galactic disc, out to radii of 15 to 30 kpc, becoming substantial after times of about 10 Gyr. This result depends weakly on the adopted distributions of the half thickness and surface density of the ionized gas component. The model is robust to changes in the amplitude of the initial field and the position of its maximum strength. The magnetic field in the remote parts of the galactic disc could be generated in situ from a seed field by local dynamo action. Another possibility is field production in the central regions of a galaxy, followed by transport to the disc's periphery by the joint action of the dynamo and turbulent diffusivity.Conclusions. Our results demonstrate the possibilities for the appearance and strengthening of magnetic fields at the peripheries of disc galaxies and emphasize the need for observational tests with new and anticipated radio telescopes (LOFAR, MWA, and SKA).

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The saturation of galactic dynamos

Cited by 32 publications

References 27 publications

Alleviatingαquenching by solar wind and meridional flows

Alleviatingαquenching by solar wind and meridional flows

Large-scale radio continuum properties of 19 Virgo cluster galaxies

Magnetic fields near the peripheries of galactic discs

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