Large-scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability

Bhat, Pallavi; Ebrahimi, F.; Blackman, Eric G.

doi:10.1093/mnras/stw1619

Cited by 25 publications

(22 citation statements)

References 36 publications

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“…4. While it contrasts with the classical view of mean-field effects being related to small-scale, "inertialrange" turbulence, this hypothesis is definitely supported by our numerical simulations and magnetic energy budget analysis 1 , as well as by other recent numerical results (Bhat et al 2016). However, another result of the analysis conducted in Sect.…”

Section: Roles Of Active and Passive Perturbationssupporting

confidence: 74%

Magnetorotational dynamo chimeras

Riols

Rincon

Cossu

et al. 2017

A&A

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In Keplerian accretion disks, turbulence and magnetic fields may be jointly excited through a subcritical dynamo mechanism involving magnetorotational instability (MRI). This dynamo may notably contribute to explaining the time-variability of various accreting systems, as high-resolution simulations of MRI dynamo turbulence exhibit statistical self-organization into large-scale cyclic dynamics. However, understanding the physics underlying these statistical states and assessing their exact astrophysical relevance is theoretically challenging. The study of simple periodic nonlinear MRI dynamo solutions has recently proven useful in this respect, and has highlighted the role of turbulent magnetic diffusion in the seeming impossibility of a dynamo at low magnetic Prandtl number (Pm), a common regime in disks. Arguably though, these simple laminar structures may not be fully representative of the complex, statistically self-organized states expected in astrophysical regimes. Here, we aim at closing this seeming discrepancy by reporting the numerical discovery of exactly periodic, yet semi-statistical "chimeral MRI dynamo states" which are the organized outcome of a succession of MRI-unstable, non-axisymmetric dynamical stages of different forms and amplitudes. Interestingly, these states, while reminiscent of the statistical complexity of turbulent simulations, involve the same physical principles as simpler laminar cycles, and their analysis further confirms the theory that subcritical turbulent magnetic diffusion impedes the sustainment of an MRI dynamo at low Pm. Overall, chimera dynamo cycles therefore offer an unprecedented dual physical and statistical perspective on dynamos in rotating shear flows, which may prove useful in devising more accurate, yet intuitive mean-field models of time-dependent turbulent disk dynamos.

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Section: Roles Of Active and Passive Perturbationssupporting

confidence: 74%

Magnetorotational dynamo chimeras

Riols

Rincon

Cossu

et al. 2017

A&A

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“…2011; Riols et al. 2015; Squire & Bhattacharjee 2015 d ; Bhat, Ebrahimi & Blackman 2016 a ). The cross-correlation between the active fluctuation components and must depend on the dynamo field that mediates the instability somehow, but there is no obvious reason why it should be directly proportional to in the transitional regime.…”

Section: The Diverse Challenging Complexity Of Large-scale Dynamosmentioning

confidence: 99%

Dynamo theories

2019

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These lecture notes are based on a tutorial given in 2017 at a plasma physics winter school in Les Houches. Their aim is to provide a self-contained graduate-student level introduction to the theory and modelling of the dynamo effect in turbulent fluids and plasmas, blended with a review of current research in the field. The primary focus is on the physical and mathematical concepts underlying different (turbulent) branches of dynamo theory, with some astrophysical, geophysical and experimental contexts disseminated throughout the document. The text begins with an introduction to the rationale, observational and historical roots of the subject, and to the basic concepts of magnetohydrodynamics relevant to dynamo theory. The next two sections discuss the fundamental phenomenological and mathematical aspects of (linear and nonlinear) small- and large-scale magnetohydrodynamic (MHD) dynamos. These sections are complemented by an overview of a selection of current active research topics in the field, including the numerical modelling of the geo- and solar dynamos, shear dynamos driven by turbulence with zero net helicity and MHD-instability-driven dynamos such as the magnetorotational dynamo. The difficult problem of a unified, self-consistent statistical treatment of small- and large-scale dynamos at large magnetic Reynolds numbers is also discussed throughout the text. Finally, an excursion is made into the relatively new but increasingly popular realm of magnetic-field generation in weakly collisional plasmas. A short discussion of the outlook and challenges for the future of the field concludes the presentation.

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“…MRI dynamos have been studied in a number of different contexts [15][16][17], but all except one focused on non-linear, usually turbulent, dynamos. The only exception we are aware of, [18], found exponential growth of mean magnetic fields during the linear growth phase of the MRI far…”

Section: (G) Mean Electromotive Forcesmentioning

confidence: 90%

The magnetorotational instability prefers three dimensions

et al. 2020

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The magnetorotational instability (MRI) occurs when a weak magnetic field destabilises a rotating, electrically conducting fluid with inwardly increasing angular velocity. The MRI is essential to astrophysical disk theory where the shear is typically Keplerian. Internal shear layers in stars may also be MRI unstable, and they take a wide range of profiles, including nearcritical. We show that the fastest growing modes of an ideal magnetofluid are three-dimensional provided the shear rate, S, is near the two-dimensional onset value, Sc. For a Keplerian shear, threedimensional modes are unstable above S ≈ 0.10Sc, and dominate the two-dimensional modes until S ≈ 2.05Sc. These three-dimensional modes dominate for shear profiles relevant to stars and at magnetic Prandtl numbers relevant to liquid-metal laboratory experiments. Significant numbers of rapidly growing three-dimensional modes remain well past 2.05Sc. These finding are significant in three ways. First, weakly nonlinear theory suggests that the MRI saturates by pushing the shear rate to its critical value. This can happen for systems, like stars and laboratory experiments, that can rearrange their angular velocity profiles. Second, the non-normal character and large transient growth of MRI modes should be important whenever three-dimensionality exists. Finally, threedimensional growth suggests direct dynamo action driven from the linear instability.

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Large-scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability

Cited by 25 publications

References 36 publications

Magnetorotational dynamo chimeras

Magnetorotational dynamo chimeras

Dynamo theories

The magnetorotational instability prefers three dimensions

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