Fluctuation dynamo in a weakly collisional plasma

St-Onge, Denis A.; Kunz, Matthew W.; Squire, Jonathan; Schekochihin, A. A.

doi:10.1017/s0022377820000860

Cited by 37 publications

(42 citation statements)

References 93 publications

(205 reference statements)

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“…This may also be a step towards simulations becoming more consistent with Faraday-rotation observations of magnetic fields in galaxy clusters suggesting kpc-scale coherence [61,62], as well as recent laboratory laser-plasma experiments exhibiting a Pm 1 fluctuation dynamo [63,64]. Determining whether or not this result holds when using the pressure-anisotropic MHD [39,65] or kinetic [66,67] descriptions more appropriate to the weakly collisional intracluster medium awaits future work.…”

Section: Discussionmentioning

confidence: 85%

“…Once the mean magnetic energy becomes comparable to the energy of the viscous-scale motions (viz., B 2 rms ∼ U 2 Re −1/2 ), the kinematic stage ends and the dynamo becomes nonlinear. Namely, the Lorentz force due to the spatially coherent magnetic folds back-reacts on the viscous-scale motions and suppresses their ability to amplify the magnetic field [17,[33][34][35][36][37][38][39]. As a result, progressively larger (and slower) eddies are responsible for amplifying the field, while the eddies whose energies are lower,…”

Section: A Generation and Persistence Of Magnetic Foldsmentioning

confidence: 99%

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Tearing instability and current-sheet disruption in the turbulent dynamo

Galishnikova¹,

Kunz²,

Schekochihin³

2022

Preprint

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Turbulence in a conducting plasma can amplify seed magnetic fields in what is known as the turbulent, or small-scale, dynamo. The associated growth rate and emergent magnetic-field geometry depend sensitively on the material properties of the plasma, in particular on the Reynolds number Re, the magnetic Reynolds number Rm, and their ratio Pm ≡ Rm/Re. For Pm > 1, the amplified magnetic field is gradually arranged into a folded structure, with direction reversals at the resistive scale and field lines curved at the larger scale of the flow. As the mean magnetic energy grows to come into approximate equipartition with the fluid motions, this folded structure is thought to persist. Using analytical theory and high-resolution MHD simulations with the Athena++ code, we show that these magnetic folds become unstable to tearing during the nonlinear stage of the dynamo for Rm 10 4 and Re 10 3 . An Rm-and Pm-dependent tearing scale, at and below which folds are disrupted, is predicted theoretically and found to match well the characteristic fieldreversal scale measured in the simulations. The disruption of folds by tearing increases the ratio of viscous-to-resistive dissipation. In the saturated state, the magnetic-energy spectrum exhibits a sub-tearing-scale steepening to a slope consistent with that predicted for tearing-mediated Alfvénic turbulence. Its spectral peak appears to be independent of the resistive scale and comparable to the driving scale of the flow, while the magnetic energy resides in a broad range of scales extending down to the field-reversal scale set by tearing. Emergence of a degree of large-scale magnetic coherence in the saturated state of the turbulent dynamo may be consistent with observations of magnetic-field fluctuations in galaxy clusters and recent laboratory experiments.

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Section: Discussionmentioning

confidence: 85%

Section: A Generation and Persistence Of Magnetic Foldsmentioning

confidence: 99%

Tearing instability and current-sheet disruption in the turbulent dynamo

Galishnikova¹,

Kunz²,

Schekochihin³

2022

Preprint

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“…This would be the case when hard-coded "anisotropy limiters" are employed to bound the values of the pressure anisotropy within a physically "sensible" range, something usually adopted when modeling (nearly) collisionless plasmas as fluids. For example, to mimic microphysical instabilities that "feed" on the pressure anisotropy, limiters were added to collisionless MHD-like models to perform numerical studies of magneto-rotational instability (Sharma et al 2006), to perform numerical studies of the turbulent dynamo in the intracluster medium (Santos-Lima et al 2014;St-Onge et al 2020), and to derive a heating mechanism able to stably balance radiative cooling in the central regions of cold-core clusters of galaxies (Kunz et al 2011). In all these examples the inclusion (via the limiters) of microphysical effects significantly changes the saturation level of macroscopic quantities.…”

Section: Hvlf: Landau-fluid Electronsmentioning

confidence: 99%

Bridging hybrid- and full-kinetic models with Landau-fluid electrons

Finelli

Cerri

Califano

et al. 2021

A&A

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Context. Magnetic reconnection plays a fundamental role in plasma dynamics under many different conditions, from space and astrophysical environments to laboratory devices. High-resolution in situ measurements from space missions allow naturally occurring reconnection processes to be studied in great detail. Alongside direct measurements, numerical simulations play a key role in the investigation of the fundamental physics underlying magnetic reconnection, also providing a testing ground for current models and theory. The choice of an adequate plasma model to be employed in numerical simulations, while also compromising with computational cost, is crucial for efficiently addressing the problem under study. Aims. We consider a new plasma model that includes a refined electron response within the “hybrid-kinetic framework” (fully kinetic protons and fluid electrons). The extent to which this new model can reproduce a full-kinetic description of 2D reconnection, with particular focus on its robustness during the nonlinear stage, is evaluated. Methods. We perform 2D simulations of magnetic reconnection with moderate guide field by means of three different plasma models: (i) a hybrid-Vlasov-Maxwell model with isotropic, isothermal electrons, (ii) a hybrid-Vlasov-Landau-fluid (HVLF) model where an anisotropic electron fluid is equipped with a Landau-fluid closure, and (iii) a full-kinetic model. Results. When compared to the full-kinetic case, the HVLF model effectively reproduces the main features of magnetic reconnection, as well as several aspects of the associated electron microphysics and its feedback onto proton dynamics. This includes the global evolution of magnetic reconnection and the local physics occurring within the so-called electron-diffusion region, as well as the evolution of species’ pressure anisotropy. In particular, anisotropy-driven instabilities (such as fire-hose, mirror, and cyclotron instabilities) play a relevant role in regulating electrons’ anisotropy during the nonlinear stage of magnetic reconnection. As expected, the HVLF model captures all these features, except for the electron-cyclotron instability.

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“…𝑘 rms measures the overall variation of magnetic fields. For a subsonic fluctuation dynamo with Pm 1, it is shown that the magnetic field organises itself into folded structures [42,68]. Furthermore, the structures are folded sheets in the kinematic stage (identified by the condition, 𝑘 𝑘 b•j 𝑘 b×j ∼ 𝑘 rms ) and folded ribbons in the saturated stage (identified by the condition, 𝑘 𝑘 b•j 𝑘 b×j ∼ 𝑘 rms ).…”

Section: B Characteristic Magnetic Scalesmentioning

confidence: 99%

Saturation mechanism of the fluctuation dynamo in supersonic turbulent plasmas

Seta¹,

Federrath²

2021

Preprint

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Magnetic fields in several astrophysical objects are amplified and maintained by a dynamo mechanism, which is the conversion of the turbulent kinetic energy to magnetic energy. A dynamo that amplifies magnetic fields at scales less than the driving scale of turbulence is known as the fluctuation dynamo. We aim to study the properties of the fluctuation dynamo in supersonic turbulent plasmas, which is of relevance to the interstellar medium of star-forming galaxies, structure formation in the Universe, and laboratory experiments of laser-plasma turbulence. Using numerical simulations of driven turbulence, we explore the global and local properties of the exponentially growing and saturated (statistically steady) state of the fluctuation dynamo for subsonic and supersonic turbulent flows. First, we confirm that the fluctuation dynamo efficiency decreases with compressibility. Then, we show that the fluctuation dynamo generated magnetic fields are spatially intermittent and the intermittency is higher for supersonic turbulence, but in both cases, the level of intermittency decreases as the field saturates. We also find a stronger back reaction of the magnetic field on the velocity for the subsonic case as compared to the supersonic case. Locally, we find that the level of alignment between vorticity and velocity, velocity and magnetic field, and current density and magnetic field in the saturated stage is enhanced in comparison to the exponentially growing phase for the subsonic case, but only the current density and magnetic field alignment is enhanced for the supersonic case. Finally, we show that both the magnetic field amplification (mainly due to weaker stretching of magnetic field lines) and diffusion decreases when the field saturates, but the diffusion is enhanced relative to amplification. This occurs throughout the volume in the subsonic turbulence, but primarily in the strong-field regions for the supersonic case. This leads to the saturation of the fluctuation dynamo. Overall, both the amplification and diffusion of magnetic fields are affected by the exponentially growing magnetic fields and thus a drastic change in either of them is not required for the saturation of the fluctuation dynamo.

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Fluctuation dynamo in a weakly collisional plasma

Cited by 37 publications

References 93 publications

Tearing instability and current-sheet disruption in the turbulent dynamo

Tearing instability and current-sheet disruption in the turbulent dynamo

Bridging hybrid- and full-kinetic models with Landau-fluid electrons

Saturation mechanism of the fluctuation dynamo in supersonic turbulent plasmas

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