A5045220529- Solar and Space Plasma Dynamics
- Ionosphere and magnetosphere dynamics
- Magnetic confinement fusion research
- Geomagnetism and Paleomagnetism Studies
- Fluid Dynamics and Turbulent Flows
- Astrophysics and Star Formation Studies
- Laser-Plasma Interactions and Diagnostics
- Astro and Planetary Science
- Laser-induced spectroscopy and plasma
- Stellar, planetary, and galactic studies
- Astrophysics and Cosmic Phenomena
- Galaxies: Formation, Evolution, Phenomena
- Geophysics and Gravity Measurements
- Astrophysical Phenomena and Observations
- Advanced Thermodynamics and Statistical Mechanics
- Statistical Mechanics and Entropy
- Dust and Plasma Wave Phenomena
- Cosmology and Gravitation Theories
- Plasma Diagnostics and Applications
- Dark Matter and Cosmic Phenomena
- Gamma-ray bursts and supernovae
- Characterization and Applications of Magnetic Nanoparticles
- Fusion materials and technologies
- Meteorological Phenomena and Simulations
- High-pressure geophysics and materials
University of Oxford
2016-2025
Kingston College
2016-2025
Institute for Advanced Study
2023
Science Oxford
2012-2022
Princeton University
2000-2022
Imperial College London
2002-2019
Rutherford Appleton Laboratory
2019
Optical Sciences (United States)
2019
University of Michigan
2019
University of York
2019
This paper presents a theoretical framework for understanding plasma turbulence in astrophysical plasmas. It is motivated by observations of electromagnetic and density fluctuations the solar wind, interstellar medium galaxy clusters, as well models particle heating accretion disks. All these plasmas many others have turbulent motions at weakly collisional collisionless scales. The focuses on strong mean magnetic field. key assumptions are that small compared to field, spatially anisotropic...
Current sheets formed in magnetic reconnection events are found to be unstable high-wavenumber perturbations. The instability is very fast: its maximum growth rate scales as S1∕4vA∕LCS, where LCS the length of sheet, vA Alfvén speed, and S Lundquist number. As a result, chain plasmoids (secondary islands) formed, whose number S3∕8.
We report the results of an extensive numerical study small-scale turbulent dynamo. The primary focus is on case large magnetic Prandtl numbers Prm, which relevant for hot low-density astrophysical plasmas. A Prm parameter scan given model viscosity-dominated (low Reynolds number) turbulence. concentrate three topics: energy spectra and saturation levels, structure field lines, intermittency strength distribution. main are as follows: (1) folded (direction reversals at resistive scale, lines...
A conceptual model of resistive magnetic reconnection via a stochastic plasmoid chain is proposed. The global rate shown to be independent the Lundquist number. distribution fluxes in plasmoids an inverse square law. It argued that there finite probability emergence abnormally large plasmoids, which can disrupt (and may responsible for observable abrupt events solar flares and sawtooth crashes). criterion transition from magnetohydrodynamic collisionless regime provided.
This paper studies the turbulent cascade of magnetic energy in weakly collisional magnetized plasmas. A model is presented, based on assumptions local nonlinear transfer wavenumber space, critical balance between linear propagation and interaction times, applicability dissipation rates for nonlinearly plasma. The follows from driving scale MHD regime, through transition at ion Larmor radius into kinetic Alfven wave which turbulence dissipated by processes. fluctuations remain frequencies...
This Letter presents the first ab initio, fully electromagnetic, kinetic simulations of magnetized turbulence in a homogeneous, weakly collisional plasma at scale ion Larmor radius (ion gyroscale). Magnetic- and electric-field energy spectra show break gyroscale; spectral slopes are consistent with scaling predictions for critically balanced Alfvén waves above gyroscale (spectral index -5/3) below indices -7/3 magnetic -1/3 electric fluctuations). behavior is also qualitatively situ...
A three-dimensional, nonlinear gyrokinetic simulation of plasma turbulence resolving scales from the ion to electron gyroradius with a realistic mass ratio is presented, where all damping provided by resolved physical mechanisms. The resulting energy spectra are quantitatively consistent magnetic power spectrum scaling k(-2.8) as observed in situ spacecraft measurements "dissipation range" solar wind turbulence. Despite strongly nature turbulence, linear kinetic Alfvén wave mode describes...
Hybrid-kinetic numerical simulations of firehose and mirror instabilities in a collisionless plasma are performed which pressure anisotropy is driven as the magnetic field changed by persistent linear shear $S$. For decreasing field, it found that mostly oblique fluctuations grow at ion Larmor scales saturate with energies $\ensuremath{\propto}{S}^{1/2}$; pinned stability threshold particle scattering off microscale fluctuations. In contrast, nonlinear large compared to scale secularly time;...
Magnetohydrodynamic (MHD) turbulence is encountered in a wide variety of astrophysical plasmas, including accretion disks, the solar wind, and interstellar intracluster medium. On small scales, this often expected to consist highly anisotropic fluctuations with frequencies compared ion cyclotron frequency. For number applications, scales are also collisionless, so kinetic treatment necessary. We show that well described by low frequency expansion theory called gyrokinetics. This paper first...
A detailed numerical study of magnetic reconnection in resistive MHD for very large, previously inaccessible, Lundquist numbers (10(4) <or= S 10(8)) is reported. Large-aspect-ratio Sweet-Parker current sheets are shown to be unstable super-Alfvénically fast formation plasmoid (magnetic-island) chains. The number scales as S(3/8) and the instability growth rate linear stage S(1/4), agreement with theory by Loureiro et al. [Phys. Plasmas 14, 100703 (2007)]. In nonlinear regime, plasmoids...
We show that under very general conditions, cluster plasmas threaded by weak magnetic fields are subject to fast growing plasma instabilities driven the anisotropy of pressure (viscous stress) with respect local direction field. Such an will naturally arise in any weakly magnetized has low collisionality and is stirring. The field must be sufficiently for occur, viz., beta>Re^{1/2}. captured extended MHD model Braginskii viscosity. However, their growth rates proportional wavenumber down...
The anisotropy of turbulence in the fast solar wind, between ion and electron gyroscales, is directly observed using a multispacecraft analysis technique. Second order structure functions are calculated at different angles to local magnetic field, for fluctuations both perpendicular parallel mean field. In components, function value large field ${S}_{\ensuremath{\perp}}$ greater than small ${S}_{\ensuremath{\parallel}}$: component...
This paper is a detailed report on programme of simulations used to settle long-standing issue in the dynamo theory and demonstrate that fluctuation exists limit large magnetic Reynolds number Rm>>1 small Prandtl Pm<<1. The dependence critical Rm_c vs. hydrodynamic Re obtained for 1<Re<6700. In Pm<<1, ~3 times larger than Pm>1. stability curve Rm_c(Re) (and, it argued, nature dynamo) substantially different from case liquid-metal experiments with mean flow. It not as yet possible determine...
Observations of galaxy clusters show that the intracluster medium (ICM) is likely to be turbulent and certainly magnetized. The properties this magnetized turbulence are determined both by fundamental nonlinear magnetohydrodynamic interactions plasma physics ICM, which has very low collisionality. Cluster threaded weak magnetic fields subject firehose mirror instabilities. These saturate produce fluctuations at ion gyroscale, can scatter particles, increasing effective collision rate and,...
We investigate the anisotropy of Alfv\'enic turbulence in inertial range slow solar wind and both driven decaying reduced magnetohydrodynamic simulations. A direct comparison is made by measuring anisotropic second-order structure functions data sets. In wind, perpendicular spectral index magnetic field close to -5/3. forced simulation, it -5/3 for velocity -3/2 field. fields. The becomes steeper at small angles local direction all cases. also show that when using global rather than mean...
A new analytically and numerically manageable model collision operator is developed specifically for turbulence simulations. The like-particle includes both pitch-angle scattering energy diffusion satisfies the physical constraints required operators: it conserves particles, momentum, energy, obeys Boltzmann’s H-theorem (collisions cannot decrease entropy), vanishes on a Maxwellian, efficiently dissipates small-scale structure in velocity space. process of transforming this into gyroaveraged...
Abstract Magnetic fields are ubiquitous in the Universe. The energy density of these is typically comparable to fluid motions plasma which they embedded, making magnetic essential players dynamics luminous matter. standard theoretical model for origin strong through amplification tiny seed via turbulent dynamo level consistent with current observations. However, experimental demonstration mechanism has remained elusive, since it requires conditions that extremely hard re-create terrestrial...
A 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 Uzdensky {\it et al.} [Phys. Rev. Lett. {\bf 105}, 235002 (2010)] confirmed and partially amended. normalized rate $\normEeff\sim 0.02$ independently $S\gg10^4$. plasmoid flux ($\Psi$) half-width ($w_x$) distribution functions scale as $f(\Psi)\sim \Psi^{-2}$ $f(w_x)\sim w_x^{-2}$. joint $\Psi$ $w_x$ shows that plasmoids populate...
X-ray surface brightness fluctuations in the core ($650 \times 650$ kpc) region of Coma cluster observed with XMM-Newton and Chandra are analyzed using a 2D power spectrum approach. The resulting spectra converted to 3D gas density fluctuations. Our independent analyses observations excellent agreement provide most sensitive measurements for hot cluster. We find that characteristic amplitude volume filling relative smooth underlying distribution varies from 7-10% on scales $\sim$500 kpc down...
Does overall thermal equilibrium exist between ions and electrons in a weakly collisional, magnetised, turbulent plasma---and, if not, how is energy partitioned electrons? This fundamental question plasma physics, the answer to which also crucial for predicting properties of far-distant astronomical objects such as accretion discs around black holes. In context discs, this was posed nearly two decades ago has since generated sizeable literature. Here we provide case injected into via...
A scaling theory of long-wavelength electrostatic turbulence in a magnetised, weakly collisional plasma (e.g., ITG turbulence) is proposed, with account taken both the nonlinear advection perturbed particle distribution by fluctuating ExB flows and its phase mixing, which caused streaming particles along mean magnetic field and, linear problem, would lead to Landau damping. It found that it possible construct consistent very little free energy leaks into high velocity moments function,...
We develop an analytic model of intermittent, three-dimensional, strong, reduced magnetohydrodynamic (RMHD) turbulence with zero cross helicity. take the fluctuation amplitudes to have a log-Poisson distribution and incorporate into new phenomenology scale-dependent dynamic alignment between Elsässer variables . find that structure function scales as , where is variation in across distance perpendicular magnetic field. calculate value β be ≃ 0.69 based on our assumption most intense coherent...