Statistical Analysis of Current Sheets in Three-Dimensional Magnetohydrodynamic Turbulence

Zhdankin, Vladimir; Uzdensky, Dmitri; Perez, Jean C.; Boldyrev, Stanislav

doi:10.1088/0004-637x/771/2/124

Cited by 181 publications

(232 citation statements)

References 55 publications

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“…Below this scale the energy is strongly dissipated in the current sheets. Currently available numerical simulations indicate that a considerable fraction of small-scale current sheets look like sites of magnetic reconnection [15][16][17][18][19][20]. We note that such a dissipation channel is not a feature of the GS95 model, which predicts filamentlike eddies at small scales.…”

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confidence: 71%

Role of Magnetic Reconnection in Magnetohydrodynamic Turbulence

2017

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The current understanding of magnetohydrodynamic (MHD) turbulence envisions turbulent eddies which are anisotropic in all three directions. In the plane perpendicular to the local mean magnetic field, this implies that such eddies become current-sheetlike structures at small scales. We analyze the role of magnetic reconnection in these structures and conclude that reconnection becomes important at a scale, where S L is the outer-scale (L) Lundquist number and λ is the smallest of the fieldperpendicular eddy dimensions. This scale is larger than the scale set by the resistive diffusion of eddies, therefore implying a fundamentally different route to energy dissipation than that predicted by the Kolmogorov-like phenomenology. In particular, our analysis predicts the existence of the subinertial, reconnection interval of MHD turbulence, with the estimated scaling of the Fourier energy spectrum⊥ , where k ⊥ is the wave number perpendicular to the local mean magnetic field. The same calculation is also performed for high (perpendicular) magnetic Prandtl number plasmas (Pm), where the reconnection scale is found to be λ=L ∼ S Introduction.-Turbulence is a defining feature of magnetized plasmas in space and astrophysical environments, which are almost invariably characterized by very large Reynolds numbers. The solar wind [1], the interstellar medium [2,3], and accretion disks [4,5] are prominent examples of plasmas dominated by turbulence, where its detailed understanding is almost certainly key to addressing long-standing puzzles such as electron-ion energy partition, cosmic ray acceleration, magnetic dynamo action, and momentum transport.Weak collisionality implies that kinetic plasma physics is required to fully describe turbulence in many such environments [6]. However, turbulent motions at scales ranging from the system size to the ion kinetic scales, an interval which spans many orders of magnitude, should be accurately described by magnetohydrodynamics (MHD).The current theoretical understanding of MHD turbulence largely rests on the ideas that were put forth by Kolmogorov and others to describe turbulence in neutral fluids (the K41 theory of turbulence [7]), and then adapted to magnetized plasmas by Iroshnikov and Kraichnan [8,9] and, later, Goldreich and Sridhar (GS95) [10]. Very briefly, one considers energy injection at some large scale L (the forcing, or outer, scale), which then cascades to smaller scales through the inertial range where, by definition, dissipation is negligible and throughout which, therefore, energy is conserved. At the bottom of the cascade is the dissipation range, where the gradients in the flow are sufficiently large for the dissipation to be efficient.Turbulence in magnetized plasmas fundamentally differs from that in neutral fluids due to the intrinsic anisotropy

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confidence: 71%

Role of Magnetic Reconnection in Magnetohydrodynamic Turbulence

2017

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“…Further exploration is required to determine whether the current sheet thickness is bounded by the simulation resolution or by the decoupling of ions from the electromagnetic fluctuations at rk 1 i . Note that plasma turbulence simulations ubiquitously show the development of current sheets (Matthaeus & Montgomery 1980;Meneguzzi et al 1981;Wan et al 2012;Karimabadi et al 2013;TenBarge & Howes 2013;Wu et al 2013;Zhdankin et al 2013), which sometimes appear to persist for a long time relative to other turbulent fluctuations on the scale of the thickness of the current sheet. This mechanism for current sheet generation may explain why these current sheets appear to persist for a long time because their lifetime is governed by the interaction of much larger-scale Alfvén waves.…”

Section: Current Sheet Formationmentioning

confidence: 99%

The Dynamical Generation of Current Sheets in Astrophysical Plasma Turbulence

Howes

2016

ApJL

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Turbulence profoundly affects particle transport and plasma heating in many astrophysical plasma environments, from galaxy clusters to the solar corona and solar wind to Earthʼs magnetosphere. Both fluid and kinetic simulations of plasma turbulence ubiquitously generate coherent structures, in the form of current sheets, at small scales, and the locations of these current sheets appear to be associated with enhanced rates of dissipation of the turbulent energy. Therefore, illuminating the origin and nature of these current sheets is critical to identifying the dominant physical mechanisms of dissipation, a primary aim at the forefront of plasma turbulence research. Here, we present evidence from nonlinear gyrokinetic simulations that strong nonlinear interactions between counterpropagating Alfvén waves, or strong Alfvén wave collisions, are a natural mechanism for the generation of current sheets in plasma turbulence. Furthermore, we conceptually explain this current sheet development in terms of the nonlinear dynamics of Alfvén wave collisions, showing that these current sheets arise through constructive interference among the initial Alfvén waves and nonlinearly generated modes. The properties of current sheets generated by strong Alfvén wave collisions are compared to published observations of current sheets in the Earthʼs magnetosheath and the solar wind, and the nature of these current sheets leads to the expectation that Landau damping of the constituent Alfvén waves plays a dominant role in the damping of turbulently generated current sheets.

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“…The algorithm of Ref. [16] identifies current sheets by finding peaks in the current density and identifying the surrounding volume where the current density is higher than a certain threshold. We take the threshold to be j thr /j rms = 4.0.…”

Section: Characterization Of Current Sheetsmentioning

confidence: 99%

“…Such structures can dissipate energy intermittently [15]. Some resistive MHD simulations have shown strong current sheets that occupy 1% of the simulation volume but account for 25% of the resistive dissipation [16].…”

Section: Introductionmentioning

confidence: 99%

Energy dynamics and current sheet structure in fluid and kinetic simulations of decaying magnetohydrodynamic turbulence

Makwana

Zhdankin

et al. 2015

Physics of Plasmas

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Simulations of decaying magnetohydrodynamic (MHD) turbulence are performed with a fluid and a kinetic code. The initial condition is an ensemble of long-wavelength, counterpropagating, shear-Alfvén waves, which interact and rapidly generate strong MHD turbulence. The total energy is conserved and the rate of turbulent energy decay is very similar in both codes, although the fluid code has numerical dissipation whereas the kinetic code has kinetic dissipation. The inertial range power spectrum index is similar in both the codes. The fluid code shows a perpendicular wavenumber spectral slope of k −1.3 ⊥ . The kinetic code shows a spectral slope of k −1.5 ⊥ for smaller simulation domain, and k −1.3 ⊥ for larger domain. We estimate that collisionless damping mechanisms in the kinetic code can account for the dissipation of the observed nonlinear energy cascade. Current sheets are geometrically characterized. Their lengths and widths are in good agreement between the two codes. The length scales linearly with the driving scale of the turbulence. In the fluid code, their thickness is determined by the grid resolution as there is no explicit diffusivity. In the kinetic code, their thickness is very close to the skin-depth, irrespective of the grid resolution. This work shows that kinetic codes can reproduce the MHD inertial range dynamics at large scales, while at the same time capturing important kinetic physics at small scales.

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Statistical Analysis of Current Sheets in Three-Dimensional Magnetohydrodynamic Turbulence

Cited by 181 publications

References 55 publications

Role of Magnetic Reconnection in Magnetohydrodynamic Turbulence

Role of Magnetic Reconnection in Magnetohydrodynamic Turbulence

The Dynamical Generation of Current Sheets in Astrophysical Plasma Turbulence

Energy dynamics and current sheet structure in fluid and kinetic simulations of decaying magnetohydrodynamic turbulence

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