Mean fields and fluctuations in compressible magnetohydrodynamic flows

Hollins, James F.; Sarson, Graeme R.; Evirgen, C. C.; Shukurov, Anvar; Fletcher, Andrew; Gent, Frederick A.

doi:10.1080/03091929.2022.2032022

Cited by 3 publications

(2 citation statements)

References 52 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…( 2013b ), which would be a more consistent choice when comparing with observations (see, e.g., the discussion in Hollins et al. 2022 ). The major difficulty with these alternative averaging techniques is that neither of them obeys the Reynolds rules, although this is no hindrance to deriving alternative, mathematically sound formalisms for the mean quantities (e.g., Germano 1992 ).…”

Section: Introductionmentioning

confidence: 93%

Computational approaches to modeling dynamos in galaxies

Korpi-Lagg,

Mac Low,

Gent

2024

Living Rev Comput Astrophys

Self Cite

View full text Add to dashboard Cite

Galaxies are observed to host magnetic fields with a typical total strength of around 15 $$\upmu $$ μ G. A coherent large-scale field constitutes up to a few microgauss of the total, while the rest is built from strong magnetic fluctuations over a wide range of spatial scales. This represents sufficient magnetic energy for it to be dynamically significant. Several questions immediately arise: What is the physical mechanism that gives rise to such magnetic fields? How do these magnetic fields affect the formation and evolution of galaxies? In which physical processes do magnetic fields play a role, and how can that role be characterized? Numerical modelling of magnetized flows in galaxies is playing an ever-increasing role in finding those answers. We review major techniques used for these models. Current results strongly support the conclusion that field growth occurs during the formation of the first galaxies on timescales shorter than their accretion timescales due to small-scale turbulent dynamos. The saturated small-scale dynamo maintains field strengths at only a few percent of equipartition with turbulence. This is in contradiction with the observed magnitude of turbulent fields, but may be reconciled by the further contribution to the turbulent field of the large-scale dynamo. The subsequent action of large-scale dynamos in differentially rotating discs produces field strengths observed in low redshift galaxies, where it reaches equipartition with the turbulence and has substantial power at large scales. The field structure resulting appears consistent with observations including Faraday rotation and polarisation from synchrotron and dust thermal emission. Major remaining challenges include scaling numerical models toward realistic scale separations and Prandtl and Reynolds numbers.

show abstract

Section: Introductionmentioning

confidence: 93%

Computational approaches to modeling dynamos in galaxies

Korpi-Lagg,

Mac Low,

Gent

2024

Living Rev Comput Astrophys

Self Cite

View full text Add to dashboard Cite

show abstract

“…As alluded to above, an important feature of the four Roberts flow dynamos is that all of them are examples of large-scale dynamos; that is, one can define an average (here an xy average) under which the magnetic field retains most of its energy and still captures its essential spatiotemporal evolution. The most suitable type of averaging depends on the type of mean magnetic field that can emerge in certain geometries and in certain parameter regimes (for a discussion, see Gent et al 2013 andHollins et al 2022). For example, in the context of disk galaxies, the azimuthally averaged magnetic field plays an important role.…”

Section: Large-scale Dynamos and Averagingmentioning

confidence: 99%

Galactic Dynamos

Brandenburg

Ntormousi

2023

Annu. Rev. Astron. Astrophys.

View full text Add to dashboard Cite

Spiral galaxies, including the Milky Way, have large-scale magnetic fields with significant energy densities. The dominant theory attributes these magnetic fields to a large-scale dynamo. We review the current status of dynamo theory and discuss various numerical simulations designed either to explain particular aspects of the problem or to reproduce galactic magnetic fields globally. Our main conclusions can be summarized as follows: ▪ Idealized direct numerical simulations produce mean magnetic fields, whose saturation energy density tends to decline with increasing magnetic Reynolds number. This is still an unsolved problem. ▪ Large-scale galactic magnetic fields of microgauss strengths can probably be explained only if helical magnetic fields of small or moderate length scales can be rapidly ejected or destroyed. ▪ Small-scale dynamos are important throughout a galaxy's life and probably provide strong seed fields at early stages. ▪ The circumgalactic medium (CGM) may play an important role in driving dynamo action at small and large length scales. These interactions between the galactic disk and the CGM may provide important insights into our understanding of galactic dynamos. We expect future research in galactic dynamos to focus on the cosmological history of galaxies and the interaction with the CGM as means of replacing the idealized boundary conditions used in earlier work. Expected final online publication date for the Annual Review of Astronomy and Astrophysics, Volume 61 is August 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

show abstract

Magnetic field amplification in cosmological zoom simulations from dwarf galaxies to galaxy groups

Pakmor,

Bieri,

van de Voort

et al. 2024

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

Magnetic fields are ubiquitous in the Universe. Recently, cosmological simulations of galaxies have successfully begun to incorporate magnetic fields and their evolution in galaxies and their haloes. However, so far they have mostly focused on Milky Way-like galaxies. Here we analyse a sample of high resolution cosmological zoom simulations of disc galaxies in haloes with mass ${M}_\rm {200c}$ from $10^{10}\, \rm {M}_\odot$ to $10^{13}\, \rm {M}_\odot$, simulated with the Auriga galaxy formation model. We show that with sufficient numerical resolution the magnetic field amplification and saturation is converged. The magnetic field strength reaches equipartition with turbulent energy density for galaxies in haloes with ${M}_\rm {200c}\gtrsim 10^{11.5}\, \mathrm{M_\odot }$. For galaxies in less massive haloes, the magnetic field strength saturates at a fraction of equipartition that decreases with decreasing halo mass. For our lowest mass haloes, the magnetic field saturates significantly below 10 % of equipartition. We quantify the resolution we need to obtain converged magnetic field strengths and discuss our resolution requirements also in the context of the IllustrisTNG cosmological box simulations. We show that, at z = 0, rotation-dominated galaxies in our sample exhibit for the most part an ordered large scale magnetic field, with fewer field reversals in more massive galaxies. Finally, we compare the magnetic fields in our cosmological galaxies at z = 0 with simulations of isolated galaxies in a collapsing halo setup. Our results pave the way for detailed studies of cosmic rays and other physical processes in similar cosmological galaxy simulations that crucially depend on the strength and structure of magnetic fields.

show abstract

Mean fields and fluctuations in compressible magnetohydrodynamic flows

Cited by 3 publications

References 52 publications

Computational approaches to modeling dynamos in galaxies

Computational approaches to modeling dynamos in galaxies

Galactic Dynamos

Magnetic field amplification in cosmological zoom simulations from dwarf galaxies to galaxy groups

Contact Info

Product

Resources

About