Saturation of the turbulent dynamo

Schober, Jennifer; Schleicher, Dominik R. G.; Federrath, Christoph; Bovino, S.; Klessen, Ralf S.

doi:10.1103/physreve.92.023010

Cited by 93 publications

(136 citation statements)

References 59 publications

(148 reference statements)

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“…In this section we presented a quantitative comparison of the turbulent dynamo in the analytical and semi-analytical Kazantsev models by Subramanian (1999), Schober et al (2012cSchober et al ( ,a, 2015 and Bovino et al (2013) with 3-D simulations of supersonic MHD turbulence. We found that the dynamo operates at low and high magnetic Prandtl numbers, but is significantly more efficient for Pm > 1 than for Pm < 1.…”

Section: Discussionmentioning

confidence: 99%

“…It increases with Pm similar to the growth rate and is also well converged with increasing numerical resolution. For Pm 10, (E m /E k ) sat seems to become independent of Pm (and thus independent of Rm, because we have constant Schober et al (2015) are shown in the low-Pm and high-Pm limits -as for the growth rate, ϑ = 0.45 is the most relevant case for this comparison. The analytic prediction agrees qualitatively with the results of the MHD simulations, but similar to the limitations of the theories for the growth rate, more work is needed to incorporate the mode mixture (solenoidal versus compressive) in the saturation models.…”

Section: Dependence On Magnetic Prandtl Numbermentioning

confidence: 99%

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Magnetic field amplification in turbulent astrophysical plasmas

2016

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Magnetic fields play an important role in astrophysical accretion discs and in the interstellar and intergalactic medium. They drive jets, suppress fragmentation in star-forming clouds and can have a significant impact on the accretion rate of stars. However, the exact amplification mechanisms of cosmic magnetic fields remain relatively poorly understood. Here, I start by reviewing recent advances in the numerical and theoretical modelling of the turbulent dynamo, which may explain the origin of galactic and intergalactic magnetic fields. While dynamo action was previously investigated in great detail for incompressible plasmas, I here place particular emphasis on highly compressible astrophysical plasmas, which are characterised by strong density fluctuations and shocks, such as the interstellar medium. I find that dynamo action works not only in subsonic plasmas, but also in highly supersonic, compressible plasmas, as well as for low and high magnetic Prandtl numbers. I further present new numerical simulations from which I determine the growth of the turbulent (un-ordered) magnetic field component (B turb ) in the presence of weak and strong guide fields (B 0 ). I vary B 0 over five orders of magnitude and find that the dependence of B turb on B 0 is relatively weak, and can be explained with a simple theoretical model in which the turbulence provides the energy to amplify B turb . Finally, I discuss some important implications of magnetic fields for the structure of accretion discs, the launching of jets and the star-formation rate of interstellar clouds.

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Section: Discussionmentioning

confidence: 99%

Section: Dependence On Magnetic Prandtl Numbermentioning

confidence: 99%

Magnetic field amplification in turbulent astrophysical plasmas

2016

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“…As pointed out by Murphy (2009), a potential difficulty is however the increasing strength of the cosmic microwave background at high redshift, enhancing the inverse Compton emission and providing an additional loss mechanism for the cosmic ray electrons. It is thus conceivable that the latter may lead to a modification or a breakdown of the correlation at very high redshift due to differences in the energy loss mechanisms of cosmic rays Schleicher & Beck 2013;Schober et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

Schleicher

Beck

2016

A&A

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The far-infrared -radio correlation connects star formation and magnetic fields in galaxies, and has been confirmed over a large range of far-infrared / radio luminosities, both in the local Universe and even at redshifts of z ∼ 2. Recent investigations indicate that it may even hold in the regime of local dwarf galaxies, and we therefore explore here the expected behavior in the regime of star formation surface densities below 0.1 M⊙ kpc −2 yr −1 . We derive two conditions that can be particularly relevant for inducing a change in the expected correlation: a critical star formation surface density to maintain the correlation between star formation rate and the magnetic field, and a critical star formation surface density below which cosmic ray diffusion losses dominate over their injection via supernova explosions. For rotation periods shorter than 1.5 × 10 7 (H/kpc) 2 yrs, with H the scale height of the disk, the first correlation will break down before diffusion losses are relevant, as higher star formation rates are required to maintain the correlation between star formation rate and magnetic field strength. For high star formation surface densities ΣSFR, we derive a characteristic scaling of the non-thermal radio to the far-infrared / infrared emission with Σ 1/3 SFR , corresponding to a scaling of the non-thermal radio luminosity Ls with the infrared luminosity L th as L 4/3 th . The latter is expected to change when the above processes are no longer steadily maintained. In the regime of long rotation periods, we expect a transition towards a steeper scaling with Σ 2/3 SFR , implying Ls ∝ L 5/3th , while the regime of fast rotation is expected to show a considerably enhanced scatter, as a well-defined relation between star formation and magnetic field strength is not maintained. The scaling relations above explain the increasing thermal fraction of the radio emission observed within local dwarfs, and can be tested with future observations by LOFAR as well as the SKA and its precursor radio telescopes.

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“…And the system was forced magnetically (Park and Blackman 2012b) with f k = 0.02 with the resolution of 288 3 . We have a magnetic Prandtl number of Pr M =ν/η = 750, which we take as a good approximation for the large values Pr M ≫ 1 expected in the early universe (Kulsrud 1999, Schekochihin et al 2002, Schober et al 2012, and references there in). Turning on and off the forcing function (0 < t < 1, simulation time unit), we imitate an ephemeral event which had driven a celestial (MHD) system in the past.…”

Section: Simulation and Methodsmentioning

confidence: 96%

“…Also Haugen et al (2004) reported a critical Re M, crit in the range between ∼35 and ∼70. On the other hand, Schober et al (2012) proposed values of Re M, crit ∼ 110 for incompressible gas and Re M, crit ∼ 2700 for extremely compressible gas, i.e., for Kolmogorov and Burger turbulence, respectively. Going further, Federrath et al (2011), Schober et al (2012), and Schleicher et al (2013) explored the influence of Mach number and Pr M on the amplification of magnetic field in the small scale regime for the formation of primeval stars.…”

Section: Introductionmentioning

confidence: 98%

On the inverse transfer of (non-)helical magnetic energy in a decaying magnetohydrodynamic turbulence

Park

2017

Monthly Notices of the Royal Astronomical Society

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In our conventional understanding, large-scale magnetic fields are thought to originate from an inverse cascade in the presence of magnetic helicity, differential rotation, or a magneto-rotational instability. However, as recent simulations have given strong indications that an inverse cascade (transfer) may occur even in the absence of magnetic helicity, the physical origin of this inverse cascade is still not fully understood. We here present two simulations of freely decaying helical & non-helical magnetohydrodynamic (MHD) turbulence. We verified the inverse transfer of helical and non-helical magnetic fields in both cases, but we found the underlying physical principles to be fundamentally different. In the former case, the helical magnetic component leads to an inverse cascade of magnetic energy. We derived a semi analytic formula for the evolution of large scale magnetic field using α coefficient and compared it with the simulation data. But in the latter case, the α effect, including other conventional dynamo theories, are not suitable to describe the inverse transfer of non-helical magnetic. To obtain a better understanding of the physics at work here, we introduced a 'field structure model' based on the magnetic induction equation in the presence of inhomogeneities. This model illustrates how the curl of the electromotiveforce (EMF) leads to the build up of a large-scale magnetic field without the requirement of magnetic helicity. And we applied a Quasi Normal approximation to the inverse transfer of magnetic energy.

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Saturation of the turbulent dynamo

Cited by 93 publications

References 59 publications

Magnetic field amplification in turbulent astrophysical plasmas

Magnetic field amplification in turbulent astrophysical plasmas

Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

On the inverse transfer of (non-)helical magnetic energy in a decaying magnetohydrodynamic turbulence

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