187 research outputs found
The divergence of neighboring magnetic field lines and fast-particle diffusion in strong magnetohydrodynamic turbulence, with application to thermal conduction in galaxy clusters
We investigate field-line separation in strong MHD turbulence using direct
numerical simulations. We find that in the static-magnetic-field approximation
the thermal conductivity in galaxy clusters is reduced by a factor of about 50
relative to the Spitzer thermal conductivity of a non-magnetized plasma. This
value is too small for heat conduction to balance radiative cooling in
clusters.Comment: Major revision: higher resolution simulations lead to significantly
different conclusions. 4 pages, 4 figures, submitted to Physical Review
Letter
Thermal conduction and particle transport in strong MHD turbulence, with application to galaxy-cluster plasmas
We investigate field-line separation in strong MHD turbulence analytically
and with direct numerical simulations. We find that in the
static-magnetic-field approximation the thermal conductivity in galaxy clusters
is reduced by a factor of about 5-10 relative to the Spitzer thermal
conductivity of a non-magnetized plasma. We also estimate how the thermal
conductivity would be affected by efficient turbulent resistivity.Comment: Major revision: higher resolution simulations lead to significantly
different conclusions. 26 pages, 10 figure
Acceleration of energetic particles by large-scale compressible magnetohydrodynamic turbulence
Fast particles diffusing along magnetic field lines in a turbulent plasma can
diffuse through and then return to the same eddy many times before the eddy is
randomized in the turbulent flow. This leads to an enhancement of particle
acceleration by large-scale compressible turbulence relative to previous
estimates in which isotropic particle diffusion is assumed.Comment: 13 pages, 3 figures, accepted for publication in Ap
Gradient Particle Magnetohydrodynamics
We introduce Gradient Particle Magnetohydrodynamics (GPM), a new Lagrangian
method for magnetohydrodynamics based on gradients corrected for the locally
disordered particle distribution. The development of a numerical code for MHD
simulation using the GPM algorithm is outlined. Validation tests simulating
linear and nonlinear sound waves, linear MHD waves, advection of magnetic
fields in a magnetized vortex, hydrodynamical shocks, and three-dimensional
collapse are presented, demonstrating the viability of an MHD code using GPM.
The characteristics of a GPM code are discussed and possible avenues for
further development and refinement are mentioned. We conclude with a view of
how GPM may complement other methods currently in development for the next
generation of computational astrophysics.Comment: 26 pages, 11 figure
Phurbas: An Adaptive, Lagrangian, Meshless, Magnetohydrodynamics Code. II. Implementation and Tests
We present an algorithm for simulating the equations of ideal
magnetohydrodynamics and other systems of differential equations on an
unstructured set of points represented by sample particles. The particles move
with the fluid, so the time step is not limited by the Eulerian
Courant-Friedrichs-Lewy condition. Full spatial adaptivity is required to
ensure the particles fill the computational volume, and gives the algorithm
substantial flexibility and power. A target resolution is specified for each
point in space, with particles being added and deleted as needed to meet this
target. We have parallelized the code by adapting the framework provided by
GADGET-2. A set of standard test problems, including 1e-6 amplitude linear MHD
waves, magnetized shock tubes, and Kelvin-Helmholtz instabilities is presented.
Finally we demonstrate good agreement with analytic predictions of linear
growth rates for magnetorotational instability in a cylindrical geometry. This
paper documents the Phurbas algorithm as implemented in Phurbas version 1.1.Comment: 14 pages, 14 figures, ApJS accepted, revised in accordance with
changes to paper I (arXiv:1110.0835
Self-similar turbulent dynamo
The 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 to a commonly held view, intermittency of the folded field
does not increase indefinitely throughout the growth stage if diffusion is
present. Instead, as we show, the probability-density function (PDF) of the
field strength becomes self-similar. The normalized moments increase with
magnetic Prandtl number in a powerlike fashion. We argue that the
self-similarity is to be expected with a finite flow scale and system size. In
the nonlinear saturated state, intermittency is reduced and the PDF is
exponential. Parallels are noted with self-similar behavior recently observed
for passive-scalar mixing and for map dynamos.Comment: revtex, 4 pages, 5 figures; minor changes to match published versio
Simulations of small-scale turbulent dynamo
We 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 levels, the structure of the field lines, and the
field-strength distribution. The main results are (1) the folded structure
(direction reversals at the resistive scale, field lines curved at the scale of
the flow) persists from the kinematic to the nonlinear regime; (2) the field
distribution is self-similar and appears to be lognormal during the kinematic
regime and exponential in the saturated state; and (3) the bulk of the magnetic
energy is at the resistive scale in the kinematic regime and remains there
after saturation, although the spectrum becomes much shallower. We propose an
analytical model of saturation based on the idea of partial
two-dimensionalization of the velocity gradients with respect to the local
direction of the magnetic folds. The model-predicted spectra are in excellent
agreement with numerical results. Comparisons with large-Re, moderate-Pm runs
are carried out to confirm the relevance of these results. New features at
large Re are elongation of the folds in the nonlinear regime from the viscous
scale to the box scale and the presence of an intermediate nonlinear stage of
slower-than-exponential magnetic-energy growth accompanied by an increase of
the resistive scale and partial suppression of the kinetic-energy spectrum in
the inertial range. Numerical results for the saturated state do not support
scale-by-scale equipartition between magnetic and kinetic energies, with a
definite excess of magnetic energy at small scales. A physical picture of the
saturated state is proposed.Comment: aastex using emulateapj; 32 pages, final published version; a pdf
file (4Mb) of the paper containing better-quality versions of figs. 5, 8, 12,
15, 17 is available from http://www.damtp.cam.ac.uk/user/as629 or by email
upon request
Oxo-Functionalization and Reduction of the Uranyl Ion through Lanthanide-Element Bond Homolysis:Synthetic, Structural, and Bonding Analysis of a Series of Singly Reduced Uranyl-Rare Earth 5f<sup>1</sup>-4f<sup><em>n</em></sup> Complexes
The heterobimetallic complexes [{UO2Ln-(py)2(L)}2], combining a singly reduced uranyl cation and a rare-earth trication in a binucleating polypyrrole Schiff-base macrocycle (Pacman) and bridged through a uranyl oxo-group, have been prepared for Ln = Sc, Y, Ce, Sm, Eu, Gd, Dy, Er, Yb, and Lu. These compounds are formed by the single-electron reduction of the Pacman uranyl complex [UO2(py)(H2L)] by the rare-earth complexes LnIII(A)3 (A = N(SiMe3)2, OC6H3But 2-2,6) via homolysis of a LnâA bond. The complexes are dimeric through mutual uranyl exo-oxo coordination but can be cleaved to form the trimetallic, monouranyl âateâ complexes [(py)3LiOUO(ÎŒ-X)Ln(py)(L)] by the addition of lithium halides. X-ray crystallographic structural characterization of many examples reveals very similar features for monomeric and dimeric series, the dimers containing an asymmetric U2O2 diamond core with shorter uranyl U=O distances than in the monomeric complexes. The synthesis by LnIIIâA homolysis allows [5f1-4fn]2 and Li[5f1-4fn] complexes with oxobridged metal cations to be made for all possible 4fn configurations. Variable-temperature SQUID magnetometry and IR, NIR, and EPR spectroscopies on the complexes are utilized to provide a basis for the better understanding of the electronic structure of f-block complexes and their f-electron exchange interactions. Furthermore, the structures, calculated by restricted-core or allelectron methods, are compared along with the proposed mechanism of formation of the complexes. A strong antiferromagnetic coupling between the metal centers, mediated by the oxo groups, exists in the UVSmIII monomer, whereas the dimeric UVDyIII complex was found to show magnetic bistability at 3 K, a property required for the development of single-molecule magnets.JRC.E.6-Actinide researc
Theory and Applications of Non-Relativistic and Relativistic Turbulent Reconnection
Realistic astrophysical environments are turbulent due to the extremely high
Reynolds numbers. Therefore, the theories of reconnection intended for
describing astrophysical reconnection should not ignore the effects of
turbulence on magnetic reconnection. Turbulence is known to change the nature
of many physical processes dramatically and in this review we claim that
magnetic reconnection is not an exception. We stress that not only
astrophysical turbulence is ubiquitous, but also magnetic reconnection itself
induces turbulence. Thus turbulence must be accounted for in any realistic
astrophysical reconnection setup. We argue that due to the similarities of MHD
turbulence in relativistic and non-relativistic cases the theory of magnetic
reconnection developed for the non-relativistic case can be extended to the
relativistic case and we provide numerical simulations that support this
conjecture. We also provide quantitative comparisons of the theoretical
predictions and results of numerical experiments, including the situations when
turbulent reconnection is self-driven, i.e. the turbulence in the system is
generated by the reconnection process itself. We show how turbulent
reconnection entails the violation of magnetic flux freezing, the conclusion
that has really far reaching consequences for many realistically turbulent
astrophysical environments. In addition, we consider observational testing of
turbulent reconnection as well as numerous implications of the theory. The
former includes the Sun and solar wind reconnection, while the latter include
the process of reconnection diffusion induced by turbulent reconnection, the
acceleration of energetic particles, bursts of turbulent reconnection related
to black hole sources as well as gamma ray bursts. Finally, we explain why
turbulent reconnection cannot be explained by turbulent resistivity or derived
through the mean field approach.Comment: 66 pages, 24 figures, a chapter of the book "Magnetic Reconnection -
Concepts and Applications", editors W. Gonzalez, E. N. Parke
Uranyl oxo activation and functionalization by metal cation coordination
International audienceThe oxo groups in the uranyl ion [UO] , one of many oxo cations formed by metals from across the periodic tableâare particularly inert, which explains the dominance of this ion in the laboratory and its persistence as an environmental contaminant. In contrast, transition metal oxo (M=O) compounds can be highly reactive and carry out difficult reactions such as the oxygenation of hydrocarbons. Here we show how the sequential addition of a lithium metal base to the uranyl ion constrained in a âPacmanâ environment results in lithium coordination to the U=O bonds and single-electron reduction. This reaction depends on the nature and stoichiometry of the lithium reagent and suggests that competing reduction and CâH bond activation reactions are occurring
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