Still searching Arxiv, refresh for possibly better results.

Results for "A. A. Schekochihin"

total 229431took 0.20s
On geometric properties of passive random advectionJul 27 1999We study geometric properties of a random Gaussian short-time correlated velocity field by considering statistics of a passively advected metric tensor. That describes universal properties of fluctuations of tensor objects frozen into the fluid and passively ... More
A statistical model of three-dimensional anisotropy and intermittency in strong Alfvénic turbulenceMay 24 2016We propose a simple statistical model of three-dimensionally anisotropic, intermittent, strong Alfv\'enic turbulence, incorporating both critical balance and dynamic alignment. Our model is based on log-Poisson statistics for Elsasser-field increments ... More
A statistical model of three-dimensional anisotropy and intermittency in strong Alfvénic turbulenceMay 24 2016Dec 14 2016We propose a simple statistical model of three-dimensionally anisotropic, intermittent, strong Alfv\'enic turbulence, incorporating both critical balance and dynamic alignment. Our model is based on log-Poisson statistics for Elsasser-field increments ... More
A solvable model of Vlasov-kinetic plasma turbulence in Fourier-Hermite phase spaceSep 11 2017Jan 07 2018A class of simple kinetic systems is considered, described by the 1D Vlasov-Landau equation with Poisson or Boltzmann electrostatic response and an energy source. Assuming a stochastic electric field, a solvable model is constructed for the phase-space ... More
Models of magnetic-field evolution and effective viscosity in weakly collisional extragalactic plasmasDec 12 2013Mar 04 2014In weakly collisional extragalactic plasmas such as the intracluster medium, viscous stress and the rate of change of the magnetic-field strength are proportional to the local pressure anisotropy, so subject to constraints imposed by the pressure-anisotropy-driven ... More
Constraints on dynamo action in plasmasJul 22 2016Upper bounds are derived on the amount of magnetic energy that can be generated by dynamo action in collisional and collisionless plasmas with and without external forcing. A hierarchy of mathematical descriptions is considered for the plasma dynamics: ... More
Critical balance in magnetohydrodynamic, rotating and stratified turbulence: towards a universal scaling conjectureApr 22 2009Jun 04 2011It is proposed that critical balance - a scale-by-scale balance between the linear propagation and nonlinear interaction time scales - can be used as a universal scaling conjecture for determining the spectra of strong turbulence in anisotropic wave systems. ... More
A stringent limit on the amplitude of Alfvénic perturbations in high-beta low-collisionality plasmasMay 09 2016Sep 20 2016It is shown that low-collisionality plasmas cannot support linearly polarized shear-Alfv\'en fluctuations above a critical amplitude $\delta B_{\perp}/B_{0} \sim \beta^{\,-1/2}$, where $\beta$ is the ratio of thermal to magnetic pressure. Above this cutoff, ... More
Turbulence, magnetic fields and plasma physics in clusters of galaxiesJan 11 2006Oct 26 2006Observations of galaxy clusters show that the intracluster medium (ICM) is likely to be turbulent and is certainly magnetized. The properties of this magnetized turbulence are determined both by fundamental nonlinear magnetohydrodynamic interactions and ... More
Fast growth of magnetic fields in galaxy clusters: a self-accelerating dynamoAug 25 2005We propose a model of magnetic-field growth in galaxy clusters whereby the field is amplified by a factor of about 10^8 over a cosmologically short time of ~10^8 yr. Our model is based on the idea that the viscosity of the intracluster medium during the ... More
Turbulence and Magnetic Fields in Astrophysical PlasmasJul 29 2005May 14 2007We discuss the current understanding of the most basic properties of astrophysical MHD turbulence and trace the origins of the modern views and theoretical uncertainties to the ideas set forth in 1950s and 1960s by Iroshnikov, Kraichnan, Batchelor, Schlueter ... More
Generation of Internal Waves by Buoyant Bubbles in Galaxy Clusters and Heating of Intracluster MediumFeb 25 2018Buoyant bubbles of relativistic plasma in cluster cores plausibly play a key role in conveying the energy from a supermassive black hole to the intracluster medium (ICM) - the process known as radio-mode AGN feedback. Energy conservation guarantees that ... More
Powering of cool filaments in cluster cores by buoyant bubbles. I. Qualitative modelApr 11 2013Sep 11 2013Cool-core clusters (e.g., Perseus or M87) often possess a network of bright gaseous filaments, observed in radio, infrared, optical and X-ray bands. We propose that these filaments are powered by the reconnection of the magnetic field in the wakes of ... More
Thermal disequilibration of ions and electrons by collisionless plasma turbulenceJul 20 2018Jan 15 2019Does overall thermal equilibrium exist between ions and electrons in a weakly collisional, magnetised, turbulent plasma---and, if not, how is thermal energy partitioned between ions and electrons? This is a fundamental question in plasma physics, the ... More
Probing magnetic turbulence by synchrotron polarimetry: statistics and structure of magnetic fields from Stokes correlatorsMar 17 2009Mar 25 2010We describe a technique for probing the statistical properties of cosmic magnetic fields based on radio polarimetry data. Second-order magnetic field statistics like the power spectrum cannot always distinguish between magnetic fields with essentially ... More
Fast magnetic reconnection in the plasmoid-dominated regimeAug 19 2010A conceptual model of resistive magnetic reconnection via a stochastic plasmoid chain is proposed. The global reconnection rate is shown to be independent of the Lundquist number. The distribution of fluxes in the plasmoids is shown to be an inverse square ... More
Disruption of Alfvénic turbulence by magnetic reconnection in a collisionless plasmaJul 19 2017Oct 12 2017We calculate the disruption scale $\lambda_{\rm D}$ at which sheet-like structures in dynamically aligned Alfv\'enic turbulence are destroyed by the onset of magnetic reconnection in a low-$\beta$ collisionless plasma. The scaling of $\lambda_{\rm D}$ ... More
Weak Alfven-Wave Turbulence RevisitedOct 31 2011Weak Alfvenic turbulence in a periodic domain is considered as a mixed state of Alfven waves interacting with the two-dimensional (2D) condensate. Unlike in standard treatments, no spectral continuity between the two is assumed and indeed none is found. ... More
Constraints on ion vs. electron heating by plasma turbulence at low betaDec 23 2018It is shown that in low-beta plasmas, such as the solar corona, some instances of the solar wind, the aurora, inner regions of accretion discs, their coronae, and some laboratory plasmas, Alfvenic fluctuations produce no ion heating within the gyrokinetic ... More
Refined critical balance in strong Alfvenic turbulenceJun 21 2014Aug 24 2015We present numerical evidence that in strong Alfvenic turbulence, the critical balance principle---equality of the nonlinear decorrelation and linear propagation times---is scale invariant, in the sense that the probability distribution of the ratio of ... More
Plasmoid and Kelvin-Helmholtz instabilities in Sweet-Parker current sheetsAug 04 2012A 2D linear theory of the instability of Sweet-Parker (SP) current sheets is developed in the framework of Reduced MHD. A local analysis is performed taking into account the dependence of a generic equilibrium profile on the outflow coordinate. The plasmoid ... More
Disruption of sheetlike structures in Alfvénic turbulence by magnetic reconnectionDec 22 2016Jul 26 2017We propose a mechanism whereby the intense, sheet-like structures naturally formed by dynamically aligning Alfv\'enic turbulence are destroyed by magnetic reconnection at a scale $\hat{\lambda}_{\rm D}$, larger than the dissipation scale predicted by ... More
Fluctuation-dissipation relations for a plasma-kinetic Langevin equationMar 25 2014Jul 30 2014A linearised kinetic equation describing electrostatic perturbations of a Maxwellian equilibrium in a weakly collisional plasma forced by a random source is considered. The problem is treated as a kinetic analogue of the Langevin equation and the corresponding ... More
Fast collisionless reconnection and electron heating in strongly magnetized plasmasJan 02 2013Magnetic reconnection in strongly magnetized (low-beta), weakly collisional plasmas is investigated using a novel fluid-kinetic model [Zocco & Schekochihin, Phys. Plasmas 18, 102309 (2011)] which retains non-isothermal electron kinetics. It is shown that ... More
Constraints on ion vs. electron heating by plasma turbulence at low betaDec 23 2018Apr 10 2019It is shown that in low-beta, weakly collisional plasmas, such as the solar corona, some instances of the solar wind, the aurora, inner regions of accretion discs, their coronae, and some laboratory plasmas, Alfv\'enic fluctuations produce no ion heating ... More
Pressure-anisotropy-driven microturbulence and magnetic-field evolution in shearing, collisionless plasmaDec 26 2015The nonlinear state of a high-beta collisionless plasma is investigated when an imposed linear shear amplifies or diminishes a uniform magnetic field, driving pressure anisotropies and hence firehose/mirror instabilities. The evolution of the resulting ... More
Supergranulation and multiscale flows in the solar photosphere: Global observations vs. a theory of anisotropic turbulent convectionSep 19 2016Mar 13 2017The Sun provides us with the only spatially well-resolved astrophysical example of turbulent thermal convection. While various aspects of solar photospheric turbulence, such as granulation (one-Megameter horizontal scale), are well understood, the questions ... More
Exact scaling laws and the local structure of isotropic magnetohydrodynamic turbulenceNov 21 2006This paper examines the consistency of the exact scaling laws for isotropic MHD turbulence in numerical simulations with large magnetic Prandtl numbers Pm and with Pm=1. The exact laws are used to elucidate the structure of the magnetic and velocity fields. ... More
Instability of current sheets and formation of plasmoid chainsMar 24 2007Nov 02 2007Current sheets formed in magnetic reconnection events are found to be unstable to high-wavenumber perturbations. The instability is very fast: its maximum growth rate scales as S^{1/4} v_A/L, where L is the length of the sheet, v_A the Alfven speed and ... More
Nonlinear mirror instabilityJul 17 2014Slow dynamical changes in magnetic-field strength and invariance of the particles' magnetic moments generate ubiquitous pressure anisotropies in weakly collisional, magnetized astrophysical plasmas. This renders them unstable to fast, small-scale mirror ... More
Suppression of turbulence and subcritical fluctuations in differentially rotating gyrokinetic plasmasNov 21 2011Differential rotation is known to suppress linear instabilities in fusion plasmas. However, even in the absence of growing eigenmodes, subcritical fluctuations that grow transiently can lead to sustained turbulence. Here transient growth of electrostatic ... More
Supergranulation and multiscale flows in the solar photosphere: Global observations vs. a theory of anisotropic turbulent convectionSep 19 2016The Sun provides us with the only spatially well-resolved astrophysical example of turbulent thermal convection. While various aspects of solar photospheric turbulence, such as granulation (one-Megameter horizontal scale), are well understood, the questions ... More
Self-inhibiting thermal conduction in high-beta, whistler-unstable plasmaNov 30 2017Apr 17 2018A heat flux in a high-$\beta$ plasma with low collisionality triggers the whistler instability. Quasilinear theory predicts saturation of the instability in a marginal state characterized by a heat flux that is fully controlled by electron scattering ... More
Large-Scale Magnetic-Field Generation by Randomly Forced Shearing WavesOct 13 2008Aug 29 2011A rigorous theory for the generation of a large-scale magnetic field by random non-helically forced motions of a conducting fluid combined with a linear shear is presented in the analytically tractable limit of low Rm and weak shear. The dynamo is kinematic ... More
Critically balanced ion temperature gradient turbulence in fusion plasmasApr 22 2011Sep 12 2011Scaling laws for ion temperature gradient driven turbulence in magnetized toroidal plasmas are derived and compared with direct numerical simulations. Predicted dependences of turbulence fluctuation amplitudes, spatial scales, and resulting heat fluxes ... More
Diffusion of passive scalar in a finite-scale random flowApr 08 2004Dec 01 2004We consider a solvable model of the decay of scalar variance in a single-scale random velocity field. We show that if there is a separation between the flow scale k_flow^{-1} and the box size k_box^{-1}, the decay rate lambda ~ (k_box/k_flow)^2 is determined ... More
Kinetic simulations of the interruption of large-amplitude shear-Alfvén waves in a high-beta plasmaMay 04 2017Dec 13 2017Using two-dimensional hybrid-kinetic simulations, we explore the nonlinear "interruption" of standing and traveling shear-Alfv\'en waves in collisionless plasmas. Interruption involves a self-generated pressure anisotropy removing the restoring force ... More
Future magnetic fields studies using the Planck Surveyor experimentNov 16 2005Jan 04 2006The Planck mission will permit measurements of the polarization of the cosmic microwave background and of polarized foregrounds such as our own Galaxy with an unprecedented combination of accuracy and completeness. This will provide information on cosmological ... More
Turbulent dynamo in a collisionless plasmaDec 20 2015Apr 01 2016Magnetic fields pervade the entire Universe and affect the formation and evolution of astrophysical systems from cosmological to planetary scales. The generation and dynamical amplification of extragalactic magnetic fields through cosmic times, up to ... More
Inefficient Driving of Bulk Turbulence by Active Galactic Nuclei in a Hydrodynamic Model of the Intracluster MediumNov 10 2015Central jetted active galactic nuclei (AGN) appear to heat the core regions of the intracluster medium (ICM) in cooling-core galaxy clusters and groups, thereby preventing a cooling catastrophe. However, the physical mechanism(s) by which the directed ... More
Interplanetary and interstellar plasma turbulenceOct 26 2006Theoretical approaches to low-frequency magnetized turbulence in collisionless and weakly collisional astrophysical plasmas are reviewed. The proper starting point for an analytical description of these plasmas is kinetic theory, not fluid equations. ... More
Suppression of local heat flux in a turbulent magnetized intracluster mediumApr 06 2013Apr 01 2014X-ray observations of hot gas in galaxy clusters often show steeper temperature gradients across cold fronts -- contact discontinuities, driven by the differential gas motions. These sharp (a few kpc wide) surface brightness/temperature discontinuities ... More
Polarization of thermal bremsstrahlung emission due to electron pressure anisotropyApr 29 2016Jun 03 2016Astrophysical plasmas are typically magnetized, with the Larmor radii of the charged particles many orders of magnitude smaller than their collisional mean free paths. The fundamental properties of such plasmas, e.g., conduction and viscosity, may depend ... More
Intermittency and Alignment in Strong RMHD TurbulenceMar 25 2014We develop an analytic model of intermittent, three-dimensional, strong, reduced magnetohydrodynamic turbulence with zero cross helicity. We take the fluctuation amplitudes to have a log-Poisson distribution and incorporate into the model a new phenomenology ... More
Fluidization of collisionless plasma turbulenceAug 13 2018In a collisionless, magnetized plasma, particles may stream freely along magnetic-field lines, leading to phase "mixing" of their distribution function and consequently to smoothing out of any "compressive" fluctuations (of density, pressure, etc.,). ... More
Polarization of Sunyaev-Zeldovich signal due to electron pressure anisotropy in galaxy clustersNov 08 2017We describe polarization of the Sunyaev-Zel'dovich (SZ) effect associated with electron pressure anisotropy likely present in the intracluster medium (ICM). The ICM is an astrophysical example of a weakly collisional plasma where the Larmor frequencies ... More
The small-scale structure of magnetohydrodynamic turbulence with large magnetic Prandtl numbersMar 14 2002Sep 06 2002We study the intermittency and field-line structure of the MHD turbulence in plasmas with very large magnetic Prandtl numbers. In this regime, which is realized in the interstellar medium, some accretion disks, protogalaxies, galaxy-cluster gas, early ... More
Firehose and Mirror Instabilities in a Collisionless Shearing PlasmaJan 31 2014May 02 2014Hybrid-kinetic numerical simulations of firehose and mirror instabilities in a collisionless plasma are performed in which pressure anisotropy is driven as the magnetic field is changed by a persistent linear shear $S$. For a decreasing field, it is found ... More
Magneto-immutable turbulence in weakly collisional plasmasNov 29 2018Jan 15 2019We propose that pressure anisotropy causes weakly collisional turbulent plasmas to self-organize so as to resist changes in magnetic-field strength. We term this effect "magneto-immutability" by analogy with incompressibility (resistance to changes in ... More
Anisotropy of Alfvénic Turbulence in the Solar Wind and Numerical SimulationsSep 03 2010Jun 08 2011We investigate the anisotropy of Alfv\'enic turbulence in the inertial range of slow solar wind and in both driven and decaying reduced magnetohydrodynamic simulations. A direct comparison is made by measuring the anisotropic second-order structure functions ... More
Turbulent Magnetic Reconnection in Two DimensionsApr 05 2009Aug 12 2009Two-dimensional numerical simulations of the effect of background turbulence on 2D resistive magnetic reconnection are presented. For sufficiently small values of the resistivity ($\eta$) and moderate values of the turbulent power ($\epsilon$), the reconnection ... More
Suppressed effective viscosity in the bulk intergalactic plasmaJun 14 2019Transport properties, such as viscosity and thermal conduction, of the hot intergalactic plasma in clusters of galaxies, are largely unknown. While for laboratory plasmas these characteristics are derived from the gas density and temperature, such recipes ... More
Formation of Plasmoid Chains in Magnetic ReconnectionMar 03 2009A detailed numerical study of magnetic reconnection in resistive MHD for very large, previously inaccessible, Lundquist numbers ($10^4\le S\le 10^8$) is reported. Large-aspect-ratio Sweet-Parker current sheets are shown to be unstable to super-Alfv\'enically ... More
Magnetic reconnection and stochastic plasmoid chains in high-Lundquist-number plasmasAug 19 2011A numerical study of magnetic reconnection in the large-Lundquist-number ($S$), plasmoid-dominated regime is carried out for $S$ up to $10^7$. The theoretical model of Uzdensky {\it et al.} [Phys. Rev. Lett. {\bf 105}, 235002 (2010)] is confirmed and ... More
Numerical demonstration of fluctuation dynamo at low magnetic Prandtl numbersFeb 11 2007May 14 2007Direct numerical simulations of incompressible nonhelical randomly forced MHD turbulence are used to demonstrate for the first time that the fluctuation dynamo exists in the limit of large magnetic Reynolds number Rm>>1 and small magnetic Prandtl number ... More
Power and spectral index anisotropy of the entire inertial range of turbulence in the fast solar windFeb 10 2010We measure the power and spectral index anisotropy of high speed solar wind turbulence from scales larger than the outer scale down to the ion gyroscale, thus covering the entire inertial range. We show that the power and spectral indices at the outer ... More
Suppression of phase mixing in drift-kinetic plasma turbulenceMar 22 2016Transfer of free energy from large to small velocity-space scales by phase mixing leads to Landau damping in a linear plasma. In a turbulent drift-kinetic plasma, this transfer is statistically nearly canceled by an inverse transfer from small to large ... More
A model of nonlinear evolution and saturation of the turbulent MHD dynamoJul 23 2002Oct 30 2002The growth and saturation of magnetic field in conducting turbulent media with large magnetic Prandtl numbers are investigated. This regime is very common in low-density hot astrophysical plasmas. During the early (kinematic) stage, weak magnetic fluctuations ... More
The nonlinear small-scale dynamo and isotropic MHD turbulenceJul 06 2002This is a brief review of the main results of our recent studies of the nonlinear evolution of the small-scale MHD dynamo in the high-Prandtl-number regime and of the structure of the resulting saturated state of the isotropic homogeneous MHD turbulence. ... More
Anisotropy of Imbalanced Alfvenic Turbulence in Fast Solar WindSep 13 2010Dec 09 2010We present the first measurement of the scale-dependent power anisotropy of Elsasser variables in imbalanced fast solar wind turbulence. The dominant Elsasser mode is isotropic at lower spacecraft frequencies but becomes increasingly anisotropic at higher ... More
Magnetofluid dynamics of magnetized cosmic plasma: firehose and gyrothermal instabilitiesDec 08 2009Jun 10 2010Both global dynamics and turbulence in magnetized weakly collisional cosmic plasmas are described by general magnetofluid equations that contain pressure anisotropies and heat fluxes that must be calculated from microscopic plasma kinetic theory. It is ... More
Fast and slow nonlinear tearing mode reconnectionJul 11 2004This is a brief account of our numerical study of the tearing mode reconnection. We demonstrate two main points. First, we show that, given sufficiently small resistivity, the Rutherford regime always exists; larger values of Delta' require smaller values ... More
A thermally stable heating mechanism for the intracluster medium: turbulence, magnetic fields and plasma instabilitiesMar 13 2010Aug 21 2010We consider the problem of self-regulated heating and cooling in galaxy clusters and the implications for cluster magnetic fields and turbulence. Viscous heating of a weakly collisional magnetised plasma is regulated by the pressure anisotropy with respect ... More
X-point collapse and saturation in the nonlinear tearing mode reconnectionJul 28 2005We study the nonlinear evolution of the resistive tearing mode in slab geometry in two dimensions. We show that, in the strongly driven regime (large Delta'), a collapse of the X-point occurs once the island width exceeds a certain critical value ~1/Delta'. ... More
Nonlinear growth of firehose and mirror fluctuations in turbulent galaxy-cluster plasmasSep 24 2007Feb 29 2008In turbulent high-beta astrophysical plasmas (exemplified by the galaxy cluster plasmas), pressure-anisotropy-driven firehose and mirror fluctuations grow nonlinearly to large amplitudes, dB/B ~ 1, on a timescale comparable to the turnover time of the ... More
Simulations of small-scale turbulent dynamoDec 01 2003Jan 16 2005We report an extensive numerical study of the small-scale turbulent dynamo at large magnetic Prandtl numbers Pm. A Pm scan is given for the model case of low-Reynolds-number turbulence. We concentrate on three topics: magnetic-energy spectra and saturation ... More
Arithmetic with X-ray images of galaxy clusters: effective equation of state for small-scale perturbations in the ICMMay 29 2016We discuss a novel technique of manipulating X-ray images of galaxy clusters to reveal the nature of small-scale density/temperature perturbations in the intra cluster medium (ICM). As we show, this technique can be used to differentiate between sound ... More
Inertial-Range Kinetic Turbulence in Pressure-Anisotropic Astrophysical PlasmasJan 27 2015Jun 05 2015A theoretical framework for low-frequency electromagnetic (drift-)kinetic turbulence in a collisionless, multi-species plasma is presented. The result generalises reduced magnetohydrodynamics (RMHD) and kinetic RMHD (Schekochihin et al. 2009) for pressure-anisotropic ... More
A nonlinear theory of the parallel firehose and gyrothermal instabilities in a weakly collisional plasmaFeb 21 2010May 07 2011Weakly collisional plasmas dynamically develop pressure anisotropies with respect to the magnetic field. These anisotropies trigger plasma instabilities at scales just above the ion Larmor radius \rho_i and much below the mean free path \lambda_{mfp}. ... More
Two-dimensional gyrokinetic turbulenceApr 01 2009Jul 10 2010Two-dimensional gyrokinetics is a simple paradigm for the study of kinetic magnetised plasma turbulence. In this paper, we present a comprehensive theoretical framework for this turbulence. We study both the inverse and direct cascades (the `dual cascade'), ... More
Momentum injection in tokamak plasmas and transitions to reduced transportSep 03 2010Mar 18 2011The effect of momentum injection on the temperature gradient in tokamak plasmas is studied. A plausible scenario for transitions to reduced transport regimes is proposed. The transition happens when there is sufficient momentum input so that the velocity ... More
Linearized model Fokker-Planck collision operators for gyrokinetic simulations. I. TheoryAug 11 2008Feb 02 2009A new analytically and numerically manageable model collision operator is developed specifically for turbulence simulations. The like-particle collision operator includes both pitch-angle scattering and energy diffusion and satisfies the physical constraints ... More
Critical Magnetic Prandtl Number for Small-Scale DynamoAug 20 2003Feb 05 2004We report a series of numerical simulations showing that the critical magnetic Reynolds number Rm_c for the nonhelical small-scale dynamo depends on the Reynolds number Re. Namely, the dynamo is shut down if the magnetic Prandtl number Pr=Rm/Re is less ... More
From Small-Scale Dynamo to Isotropic MHD TurbulenceJun 14 2003Dec 01 2004We consider the problem of incompressible, forced, nonhelical, homogeneous, isotropic MHD turbulence with no mean magnetic field. This problem is essentially different from the case with externally imposed uniform mean field. There is no scale-by-scale ... More
Interpreting Power Anisotropy Measurements in Plasma TurbulenceSep 14 2009Feb 22 2010A relationship is derived between power anisotropy and wavevector anisotropy in turbulent fluctuations. This can be used to interpret plasma turbulence measurements, for example in the solar wind. If fluctuations are anisotropic in shape then the ion ... More
Suppression of thermal conduction in a mirror-unstable plasmaMar 01 2016Jun 03 2016The ICM plasma is subject to firehose and mirror instabilities at scales of order the ion Larmor radius. The mirror instability generates fluctuations of magnetic-field strength $\delta B / B \sim 1$. These fluctuations act as magnetic traps for the heat-conducting ... More
Self-similar turbulent dynamoJun 28 2003Mar 04 2004The amplification of magnetic fields in a highly conducting fluid is studied numerically. During growth, the magnetic field is spatially intermittent: it does not uniformly fill the volume, but is concentrated in long thin folded structures. Contrary ... More
Plasma instabilities and magnetic-field growth in clusters of galaxiesJan 18 2005Aug 03 2005We show that under very general conditions, cluster plasmas threaded by weak magnetic fields are subject to very fast growing plasma instabilities driven by the anisotropy of the plasma pressure (viscous stress) with respect to the local direction of ... More
Magnetised plasma turbulence in clusters of galaxiesNov 30 2004Cluster plasmas are magnetised already at very low magnetic field strength. Low collisionality implies that conservation of the first adiabatic invariant results in an anisotropic viscous stress (Braginskii viscosity) or, equivalently, anisotropic plasma ... More
Linear Structures in the Core of the Coma Cluster of GalaxiesSep 19 2013The hot X-ray emitting plasma in galaxy clusters is predicted to have turbulent motions which can contribute around ten percent of the cluster's central energy density. We report deep Chandra X-ray Observatory observations of the Coma cluster core, showing ... More
A Model of Turbulence in Magnetized Plasmas: Implications for the Dissipation Range in the Solar WindJul 20 2007Dec 12 2008This paper studies the turbulent cascade of magnetic energy in weakly collisional magnetized plasmas. A cascade model is presented, based on the assumptions of local nonlinear energy transfer in wavenumber space, critical balance between linear propagation ... More
Astrophysical Gyrokinetics: Basic Equations and Linear TheoryNov 30 2005May 04 2006Magnetohydrodynamic (MHD) turbulence is encountered in a wide variety of astrophysical plasmas, including accretion disks, the solar wind, and the interstellar and intracluster medium. On small scales, this turbulence is often expected to consist of highly ... More
Astrophysical gyrokinetics: Turbulence in pressure-anisotropic plasmas at ion scales and beyondDec 06 2017Mar 20 2018We present a theoretical framework for describing electromagnetic kinetic turbulence in a multi-species, magnetized, pressure-anisotropic plasma. Turbulent fluctuations are assumed to be small compared to the mean field, to be spatially anisotropic with ... More
Proton imaging of stochastic magnetic fieldsAug 05 2017Recent laser-plasma experiments report the existence of dynamically significant magnetic fields, whose statistical characterisation is essential for understanding the physical processes these experiments are attempting to investigate. In this paper, we ... More
Dissipation-Scale Turbulence in the Solar WindJul 20 2007We present a cascade model for turbulence in weakly collisional plasmas that follows the nonlinear cascade of energy from the large scales of driving in the MHD regime to the small scales of the kinetic Alfven wave regime where the turbulence is dissipated ... More
Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbersApr 16 2007This paper is a detailed report on a programme of simulations used to settle a long-standing issue in the dynamo theory and demonstrate that the fluctuation dynamo exists in the limit of large magnetic Reynolds number Rm>>1 and small magnetic Prandtl ... More
Symmetry breaking in MAST plasma turbulence due to toroidal flow shearSep 28 2016The flow shear associated with the differential toroidal rotation of tokamak plasmas breaks an underlying symmetry of the turbulent fluctuations imposed by the up-down symmetry of the magnetic equilibrium. Using experimental Beam-Emission-Spectroscopy ... More
Three-Dimensional Structure of Solar Wind TurbulenceSep 12 2011Aug 27 2012We present a measurement of the scale-dependent, three-dimensional structure of the magnetic field fluctuations in inertial range solar wind turbulence with respect to a local, physically motivated coordinate system. The Alfvenic fluctuations are three-dimensionally ... More
Collisionality scaling of the electron heat flux in ETG turbulenceJul 22 2016In electrostatic simulations of MAST plasma at electron-gyroradius scales, using the local flux-tube gyrokinetic code GS2 with adiabatic ions, we find that the long-time saturated electron heat flux (the level most relevant to energy transport) decreases ... More
Local dependence of ion temperature gradient on magnetic configuration, rotational shear and turbulent heat flux in MASTNov 13 2012Experimental data from the Mega Amp Spherical Tokamak (MAST) is used to show that the inverse gradient scale length of the ion temperature R/LTi (normalized to the major radius R) has its strongest local correlation with the rotational shear and the pitch ... More
Symmetry breaking in MAST plasma turbulence due to toroidal flow shearSep 28 2016Oct 13 2016The flow shear associated with the differential toroidal rotation of tokamak plasmas breaks an underlying symmetry of the turbulent fluctuations imposed by the up-down symmetry of the magnetic equilibrium. Using experimental Beam-Emission-Spectroscopy ... More
Transition to subcritical turbulence in a tokamak plasmaJul 27 2016Nov 27 2016Unstable perturbations driven by the pressure gradient and other sources of free energy in tokamak plasmas can grow exponentially and eventually saturate nonlinearly, leading to turbulence. Recent work has shown that in the presence of sheared flows, ... More
Collisionality scaling of the electron heat flux in ETG turbulenceJul 22 2016Jan 14 2017In electrostatic simulations of MAST plasma at electron-gyroradius scales, using the local flux-tube gyrokinetic code GS2 with adiabatic ions, we find that the long-time saturated electron heat flux (the level most relevant to energy transport) decreases ... More
Numerical experiments on dynamo action in sheared and rotating turbulenceJul 07 2008Aug 25 2008Numerical simulations of forced turbulence in elongated shearing boxes are carried out to demonstrate that a nonhelical turbulence in conjunction with a linear shear can give rise to a mean-field dynamo. Exponential growth of magnetic field at scales ... More
Transition to subcritical turbulence in a tokamak plasmaJul 27 2016Sep 12 2016Unstable perturbations driven by the pressure gradient and other sources of free energy in tokamak plasmas can grow exponentially and eventually saturate nonlinearly, leading to turbulence. Recent work has shown that in the presence of sheared flows, ... More
Ion-scale turbulence in MAST: anomalous transport, subcritical transitions, and comparison to BES measurementsApr 10 2017Aug 01 2017We investigate the effect of varying the ion temperature gradient (ITG) and toroidal equilibrium scale sheared flow on ion-scale turbulence in the outer core of MAST by means of local gyrokinetic simulations. We show that nonlinear simulations reproduce ... More
Measurement and physical interpretation of the mean motion of turbulent density patterns detected by the BES system on MASTMar 20 2012The mean motion of turbulent patterns detected by a two-dimensional (2D) beam emission spectroscopy (BES) diagnostic on the Mega Amp Spherical Tokamak (MAST) is determined using a cross-correlation time delay (CCTD) method. Statistical reliability of ... More
Experimental determination of the correlation properties of plasma turbulence using 2D BES systemsSep 29 2016A procedure is presented to map from the spatial correlation parameters of a turbulent density field (the radial and binormal correlation lengths and wavenumbers, and the fluctuation amplitude) to correlation parameters that would be measured by a Beam ... More
Transition to subcritical turbulence in a tokamak plasmaJul 27 2016Jan 09 2017Tokamak turbulence, driven by the ion-temperature gradient and occurring in the presence of flow shear, is investigated by means of local, ion-scale, electrostatic gyrokinetic simulations (with both kinetic ions and electrons) of the conditions in the ... More
The onset of a small-scale turbulent dynamo at low magnetic Prandtl numbersDec 22 2004Jun 03 2005We study numerically the dependence of the critical magnetic Reynolds number Rmc for the turbulent small-scale dynamo on the hydrodynamic Reynolds number Re. The turbulence is statistically homogeneous, isotropic, and mirror--symmetric. We are interested ... More