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## DIFFUSION OF MAGNETIC FIELD AND REMOVAL OF MAGNETIC FLUX FROM CLOUDS VIA TURBULENT RECONNECTION

SANTOS-LIMA, R.; LAZARIAN, A.; PINO, E. M. de Gouveia Dal; CHO, J.
Fonte: IOP PUBLISHING LTD Publicador: IOP PUBLISHING LTD
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
56.17%
The diffusion of astrophysical magnetic fields in conducting fluids in the presence of turbulence depends on whether magnetic fields can change their topology via reconnection in highly conducting media. Recent progress in understanding fast magnetic reconnection in the presence of turbulence reassures that the magnetic field behavior in computer simulations and turbulent astrophysical environments is similar, as far as magnetic reconnection is concerned. This makes it meaningful to perform MHD simulations of turbulent flows in order to understand the diffusion of magnetic field in astrophysical environments. Our studies of magnetic field diffusion in turbulent medium reveal interesting new phenomena. First of all, our three-dimensional MHD simulations initiated with anti-correlating magnetic field and gaseous density exhibit at later times a de-correlation of the magnetic field and density, which corresponds well to the observations of the interstellar media. While earlier studies stressed the role of either ambipolar diffusion or time-dependent turbulent fluctuations for de-correlating magnetic field and density, we get the effect of permanent de-correlation with one fluid code, i.e., without invoking ambipolar diffusion. In addition...

## THE ROLE OF DIFFUSIVITY QUENCHING IN FLUX-TRANSPORT DYNAMO MODELS

GUERRERO, Gustavo; DIKPATI, Mausumi; PINO, Elisabete M. de Gouveia Dal
Fonte: IOP PUBLISHING LTD Publicador: IOP PUBLISHING LTD
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
46.04%
In the nonlinear phase of a dynamo process, the back-reaction of the magnetic field upon the turbulent motion results in a decrease of the turbulence level and therefore in a suppression of both the magnetic field amplification (the alpha-quenching effect) and the turbulent magnetic diffusivity (the eta-quenching effect). While the former has been widely explored, the effects of eta-quenching in the magnetic field evolution have rarely been considered. In this work, we investigate the role of the suppression of diffusivity in a flux-transport solar dynamo model that also includes a nonlinear alpha-quenching term. Our results indicate that, although for alpha-quenching the dependence of the magnetic field amplification with the quenching factor is nearly linear, the magnetic field response to eta-quenching is nonlinear and spatially nonuniform. We have found that the magnetic field can be locally amplified in this case, forming long-lived structures whose maximum amplitude can be up to similar to 2.5 times larger at the tachocline and up to similar to 2 times larger at the center of the convection zone than in models without quenching. However, this amplification leads to unobservable effects and to a worse distribution of the magnetic field in the butterfly diagram. Since the dynamo cycle period increases when the efficiency of the quenching increases...

## Numerical studies of diffusion and amplification of magnetic fields in turbulent astrophysical plasmas; Estudos numéricos de difusão e amplificação de campos magnéticos em plasmas astrofísicos turbulentos

Lima, Reinaldo Santos de
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Relevância na Pesquisa
66.25%
In this thesis we investigated two major issues in astrophysical flows: the transport of magnetic fields in highly conducting fluids in the presence of turbulence, and the turbulence evolution and turbulent dynamo amplification of magnetic fields in collisionless plasmas. The first topic was explored in the context of star-formation, where two intriguing problems are highly debated: the requirement of magnetic flux diffusion during the gravitational collapse of molecular clouds in order to explain the observed magnetic field intensities in protostars (the so called "magnetic flux problem") and the formation of rotationally sustained protostellar discs in the presence of the magnetic fields which tend to remove all the angular momentum (the so called "magnetic braking catastrophe"). Both problems challenge the ideal MHD description, usually expected to be a good approximation in these environments. The ambipolar diffusion, which is the mechanism commonly invoked to solve these problems, has been lately questioned both by observations and numerical simulation results. We have here investigated a new paradigm, an alternative diffusive mechanism based on fast magnetic reconnection induced by turbulence, termed turbulent reconnection diffusion (TRD). We tested the TRD through fully 3D MHD numerical simulations...

## The angular deviation of ultra high energy cosmic rays in intergalactic magnetic fields

Clay, R.; Cook, S.; Dawson, B.; Smith, A.; Lampard, R.
Fonte: ELSEVIER SCIENCE BV Publicador: ELSEVIER SCIENCE BV
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.82%
The angular deviation associated with the propagation of ultra high energy cosmic ray protons through turbulent intergalactic magnetic fields is examined including the effect of interactions with the cosmic microwave background. It is found that the deviation of the particles is consistent with diffusion ideas but the magnitude of the deviation is appreciably less than suggested by simple models for a given mean field strength and observational energy.; R. W. Clay, S. Cook, B. R. Dawson, A. G. K. Smith and R. Lampard; Copyright © 1998 Published by Elsevier Science B.V.

## Cosmic ray propagation in turbulent magnetic fields.

Shibamura, Kenji
Relevância na Pesquisa
65.87%
In this research, the diffusion of high energy cosmic rays (protons) in turbulent magnetic fields was investigated using a Monte Carlo technique.; Thesis (M.Sc.) -- University of Adelaide, School of Chemistry and Physics, 2005; Title page, abstract and table of contents only. The complete thesis in print form is available from the University of Adelaide Library.

## Instabilities in a Crystal Growth Melt Subjected to Alternating Magnetic Fields

Davis, Kenny
ENG
Relevância na Pesquisa
55.96%
In confined bulk crystal growth techniques such as the traveling heater method, base materials in an ampoule are melted and resolidified as a single crystal. During this process, flow control is desired so that the resulting alloy semiconductors are uniform in composition and have minimal defects. Such control allows for tuned lattice parameters and bandgap energy, properties necessary to produce custom materials for specific electro-optical applications. For ternary alloys, bulk crystal growth methods suffer from slow diffusion rates between elements, severely limiting growth rates and reducing uniformity. Exposing the electrically conducting melt to an external alternating magnetic field can accelerate the mixing. A rotating magnetic field (RMF) can be used to stir the melt in the azimuthal direction, which reduces temperature variations and controls the shape at the solidification front. A traveling magnetic field (TMF) imposes large body forces in the radial and axial directions, which helps reduce the settling of denser components and return them to the growth front. In either case, mixing is desired, but turbulence is not. At large magnetic Taylor numbers the flow becomes unstable to first laminar and then turbulent transitions. It is imperative that crystal growers know when these transitions will occur and how the flow physics is affected. Here...

## Ferrofluid surface and volume flows in uniform rotating magnetic fields

Elborai, Shihab M. (Shihab Mahmoud), 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 260 p.
ENG
Relevância na Pesquisa
46.11%
Ferrofluid surface and volume effects in uniform dc and rotating magnetic fields are studied. Theory and corroborating measurements are presented for meniscus shapes and resulting surface driven flows, spin-up flows, and Hele-Shaw cell flows and instabilities. To characterize the water-based and oil-based ferrofluids used in experiments, measurements were made of the magnetization curve, surface tension, viscosity, density, and the speed of sound. Extensive measurements of the height and shape of ferrofluid menisci in applied uniform dc magnetic fields show that the height of the meniscus increases for vertical applied magnetic fields, whereas horizontal magnetic fields decrease meniscus height. An approximate energy minimization analysis agrees with the observed trends in ferrofluid meniscus height. The effects of ferrofluid meniscus curvature on spin-up flow were modeled under simplified assumptions. Analytical solutions were derived for two dimensional low Reynolds number flows and extended results were obtained numerically using COMSOL's Multiphysics finite element software package (FEMLAB) to solve for three dimensional recirculating flows at higher Reynolds numbers.; (cont.) Familiar magnetostatic energy expressions in linear magnetic media were extended to non-linear magnetization relations. These energy expressions were applied to study the effects of linear and non-linear magnetization on flows and instabilities in Hele-Shaw cells with simultaneously applied in-plane rotating and dc axial magnetic fields. Ultrasound velocimetry of the spin-up flow in the bulk region of water-based ferrofluids conclusively demonstrates the co-rotation of the bulk of the ferrofluid with the applied rotating magnetic field with and without a free surface. Careful ultrasound investigation of flow profiles at different heights in uncovered ferrofluid cylinders showed flow direction reversal between the counter-rotating top free surface and the co-rotating bulk region of the ferrofluid. A framework for a numerical solution of the coupled governing equations of conservation of linear and angular momentum in magnetic spin-up flows that considers all the terms in the first Shliomis magnetization relaxation equation was formulated and solved. Previous solutions in the literature which decouple the magnetic and fluid mechanical dynamics by neglecting the linear and spin velocities in the magnetization relaxation equations result in no spin-up flow in uniform magnetic fields in the absence of spin diffusion effects.; (cont.) Contrary to the commonly held view in the literature...

## The current-free electric double layer in a coronal magnetic funnel

Boswell, Roderick; Marsch, Eckart; Charles, Christine
Fonte: IOP Publishing Publicador: IOP Publishing
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.77%
Current-free double layers (CFDLs) have been recently discovered in a number of laboratory devices, when a low collisional plasma is forced to expand from a high magnetic field source region to a low magnetic field diffusion region. This experimental setu

## On the Physics of Primordial Magnetic Fields

Battaner, E.; Lesch, H.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.1%
There are at present more then 30 theories about the origin of cosmic magnetic fields at galactic and intergalactic scales. Most of them rely on concepts of elementary particle physics, like phase transitions in the early Universe, string theory and processes during the inflationary epoch. Here we present some more astrophysical arguments to provide some guidance through this large number and variety of models. Especially the fact that the evolution of magnetic fields depends on the spatial coherence scale of the fields leds to some interesting conclusions, which may rule out the majority of the theoretical scenarios. In principle one has to distinguish between the large-scale and small-scale magnetic fields. Large scale fields are defined as those as becoming sub-horizon at that redshift at which the mass energy density becomes equal to the photon energy density, which we name as equality. Small scale fields which are sub-horizon even before equality, i.e. with scales lower than (present) few Mpc cannot survive the radiation era and cannot reach recombination, because of the effects of magnetic diffusion and photon diffusion. Therefore mechanisms based on phase transitions become unlike, as they provide magnetic fields on scales smaller than the horizon. Thus...

## Multi-scale Radio-IR Correlations in M31 and M33: The Role of Magnetic Fields and Star Formation

Tabatabaei, F. S.; Berkhuijsen, E. M.; Frick, P.; Beck, R.; Schinnerer, E.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.04%
Interstellar magnetic fields and the propagation of cosmic ray electrons have an important impact on the radio-infrared (IR) correlation in galaxies. This becomes evident when studying different spatial scales within galaxies. We investigate the correlation between the infrared (IR) and free-free/synchrotron radio continuum emission at 20 cm from the two local group galaxies M31 and M33 on spatial scales between 0.4 and 10 kpc. The multi-scale radio-IR correlations have been carried out using a wavelet analysis. The free-free and IR emission are correlated on all scales, but on some scales the synchrotron emission is only marginally correlated with the IR emission. The synchrotron-IR correlation is stronger in M33 than in M31 on small scales (<1 kpc), but it is weaker than in M31 on larger scales. Taking the smallest scale on which the synchrotron-IR correlation exists as the propagation length of cosmic ray electrons, we show that the difference on small scales can be explained by the smaller propagation length in M33 than in M31. On large scales, the difference is due to the thick disk/halo in M33, which is absent in M31. A comparison of our data with data on NGC6946, the LMC and M51 suggests that the propagation length is determined by the ratio of ordered-to-turbulent magnetic field strength...

## The kink-type instability of toroidal stellar magnetic fields with thermal diffusion

Ruediger, G.; Kitchatinov, L. L.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.07%
The stability of toroidal magnetic fields in rotating radiative stellar zones is studied for realistic values of both the Prandtl numbers. The two considered models for the magnetic geometry represent fields with odd and even symmetry with respect to the equator. In the linear theory in Boussinesq approximation the resulting complex eigenfrequency (including growth rate and drift rate) are calculated for a given radial wavenumber of a nonaxisymmetric perturbation with m=1. The ratio of the Alfven frequency, \Omega_A, to the rate of the basic rotation, \Omega, controls the eigenfrequency of the solution. For strong fields with \Omega_A > \Omega the solutions do not feel the thermal diffusion. The growth rate runs with \Omega_A and the drift rate is close to -\Omega so that the magnetic pattern will rest in the laboratory system. For weaker fields with \Omega_A < \Omega the growth rate strongly depends on the thermal conductivity. For fields with dipolar parity and for typical values of the heat conductivity the resulting very small growth rates are almost identical with those for vanishing gravity. For fields with dipolar symmetry the differential rotation of any stellar radiative zone (like the solar tachocline) is shown as basically stabilizing the instability independent of the sign of the shear. Finally...

## Transport phenomena in stochastic magnetic mirrors

Malyshkin, Leonid; Kulsrud, Russell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.05%
Parallel thermal conduction along stochastic magnetic field lines may be reduced because the heat conducting electrons become trapped and detrapped between regions of strong magnetic field (magnetic mirrors). The problem reduces to a simple but realistic model for diffusion of mono-energetic electrons based on the fact that when there is a reduction of diffusion, it is controlled by a subset of the mirrors, the principle mirrors. The diffusion reduction can be considered as equivalent to an enhancement of the pitch angle scattering rate. Therefore, in deriving the collision integral, we modify the pitch angle scattering term. We take into account the full perturbed electron-electron collision integral, as well as the electron-proton collision term. Finally, we obtain the four plasma transport coefficients and the effective thermal conductivity. We express them as reductions from the classical values. We present these reductions as functions of the ratio of the magnetic field decorrelation length to the electron mean free path at the thermal speed $V_T=\sqrt{2kT/m_e}$. We briefly discuss an application of our results to clusters of galaxies. Key words: magnetic fields: conduction --- magnetic fields: diffusion --- methods: analytical --- plasmas; Comment: 25 pages...

## Transport of Cosmic Rays in Chaotic Magnetic Fields

Casse, Fabien; Lemoine, Martin; Pelletier, Guy
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.14%
The transport of charged particles in disorganised magnetic fields is an important issue which concerns the propagation of cosmic rays of all energies in a variety of astrophysical environments, such as the interplanetary, interstellar and even extra-galactic media, as well as the efficiency of Fermi acceleration processes. We have performed detailed numerical experiments using Monte-Carlo simulations of particle propagation in stochastic magnetic fields in order to measure the parallel and transverse spatial diffusion coefficients and the pitch angle scattering time as a function of rigidity and strength of the turbulent magnetic component. We confirm the extrapolation to high turbulence levels of the scaling predicted by the quasi-linear approximation for the scattering frequency and parallel diffusion coefficient at low rigidity. We show that the widely used Bohm diffusion coefficient does not provide a satisfactory approximation to diffusion even in the extreme case where the mean field vanishes. We find that diffusion also takes place for particles with Larmor radii larger than the coherence length of the turbulence. We argue that transverse diffusion is much more effective than predicted by the quasi-linear approximation, and appears compatible with chaotic magnetic diffusion of the field lines. We provide numerical estimates of the Kolmogorov length and magnetic line diffusion coefficient as a function of the level of turbulence. Finally we comment on applications of our results to astrophysical turbulence and the acceleration of high energy cosmic rays in supernovae remnants...

## Luminosity and cooling of highly magnetised white dwarfs: Suppression of luminosity by strong magnetic fields

Bhattacharya, Mukul; Mukhopadhyay, Banibrata; Mukerjee, Subroto
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.05%
We investigate the luminosity and cooling of highly magnetised white dwarfs. We consider white dwarfs with electron-degenerate core and nondegenerate surface layers where cooling occurs by diffusion of photons. We find the temperature and density profiles in the surface layers or envelope of white dwarfs for radially constant and varying magnetic fields by solving the magnetostatic equilibrium and photon diffusion equations in a Newtonian framework. We also obtain the properties of white dwarfs at the core-envelope interface, when the core is assumed to be practically isothermal due to large thermal conductivity. With the increase in magnetic field, the interface temperature and density are found to be increasing. While the interface radius also increases with the increase in magnetic field when the field is hypothesised to be constant throughout the star, the interface radius decreases for varying fields. However, for white dwarfs having fixed interface radius or interface temperature, we find that the luminosity significantly decreases, falling in the range ~ 10^{-6}-10^{-13} solar luminosity, with the increase in magnetic field strength at the interface and hence envelope, in the corresponding range ~ 10^9-10^{11} G, in particular for the varying magnetic fields which are expected to be more realistic. This is remarkable as it argues for magnetised white dwarfs to be dimmer and be practically hidden in the H-R diagram. We also find the cooling rates corresponding to these luminosities. Interestingly...

## Magnetic fields of neutron stars

Reisenegger, Andreas
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.08%
Neutron stars contain the strongest magnetic fields known in the Universe. In this paper, I discuss briefly how these magnetic fields are inferred from observations, as well as the evidence for their time-evolution. I show how these extremely strong fields are actually weak in terms of their effects on the stellar structure, as is also the case for magnetic stars on the upper main sequence and magnetic white dwarfs, which have similar total magnetic fluxes. I propose a scenario in which a stable hydromagnetic equilibrium (containing a poloidal and a toroidal field component) is established soon after the birth of the neutron star, aided by the strong compositional stratification of neutron star matter, and this state is slowly eroded by non-ideal magnetohydrodynamic processes such as beta decays and ambipolar diffusion in the core of the star and Hall drift and breaking of the solid in its crust. Over sufficiently long time scales, the fluid in the neutron star core will behave as if it were barotropic, because, depending on temperature and magnetic field strength, beta decays will keep adjusting the composition to the chemical equilibrium state, or ambipolar diffusion will decouple the charged component from the neutrons. Therefore...

## On the resilience of helical magnetic fields to turbulent diffusion and the astrophysical implications

Blackman, Eric G.; Subramanian, Kandaswamy
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.18%
The extent to which large scale magnetic fields are susceptible to turbulent diffusion is important for interpreting the need for in situ large scale dynamos in astrophysics and for observationally inferring field strengths compared to kinetic energy. By solving coupled equations for magnetic energy and magnetic helicity in a system initiated with isotropic turbulence and an arbitrarily helical large scale field, we quantify the decay rate of the latter for a bounded or periodic system. The energy associated with the non-helical magnetic field rapidly decays by turbulent diffusion, but the decay rate of the helical component depends on whether the ratio of its magnetic energy to the turbulent kinetic energy exceeds a critical value given by M_{1,c} =(k_1/k_2)^2, where k_1 and k_2 are the wave numbers of the large and forcing scales. Turbulently diffusing helical fields to small scales while conserving magnetic helicity requires a rapid increase in total magnetic energy. As such, only when the helical fields are sub-critical can they so diffuse. When super-critical, the large scale helical field decays slowly, at a rate determined by microphysical dissipation even when macroscopic turbulence is present. Amplification of small scale magnetic helicity abates the turbulent diffusion. Two implications are that: (1) Standard arguments supporting the need for in situ large scale dynamos based on the otherwise rapid turbulent diffusion of large scale fields require re-thinking since only the non-helical field is so diffused in a closed system. Boundary terms could however provide potential pathways for rapid change of the large scale helical field. (2) Since M_{1...

## Stability of toroidal magnetic fields in stellar interiors

Ibañez-Mejia, Juan C.; Braithwaite, Jonathan
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.11%
We present 3D MHD simulations of purely toroidal and mixed poloidal-toroidal magnetic field configurations to study the behavior of the Tayler instability. For the first time the simultaneous action of rotation and magnetic diffusion are taken into account and the effects of a poloidal field on the dynamic evolution of unstable toroidal magnetic fields is included. In the absence of diffusion, fast rotation (rotation rate compared to Alfv\'en frequency) is able to suppress the instability when the rotation and magnetic axes are aligned and when the radial field strength gradient p < 1.5. When diffusion is included, this system turns unstable for diffusion dominated and marginally diffusive dominated regions. If the magnetic and rotation axes are perpendicular to each other the stabilizing effect induced by the Coriolis force is scale dependent and decreases with increasing wavenumber. In toroidal fields with radial field gradients bigger than p > 1.5, rapid rotation does not suppress the instability but instead introduces a damping factor to the growth rate in agreement with the analytic predictions. For the mixed poloidal-toroidal fields we find an unstable axisymmetric mode, not predicted analytically, right at the stability threshold for the non-axisymmetric modes; it has been argued that an axisymmetric mode is necessary for the closure of the Tayler-Spruit dynamo loop.; Comment: 12 pages...

## Cosmic Ray Parallel and Perpendicular Transport in Turbulent Magnetic Fields

Xu, Siyao; Yan, Huirong
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.06%
A correct description of cosmic-ray (CR) diffusion in turbulent plasma is essential for many astrophysical and heliospheric problems. This paper aims to present physical diffusion behavior of CRs in actual turbulent magnetic fields, model of which has been numerically tested. We perform test particle simulations in compressible magnetohydrodynamic turbulence. We obtain scattering and spatial diffusion coefficients by tracing particle trajectories. We find no resonance gap for pitch-angle scattering at 90$^\circ$. Our result confirms the dominance of mirror interaction with compressible modes for most pitch angles as revealed by the nonlinear theory. For cross-field transport, our results are consistent with normal diffusion predicted earlier for large scales. The diffusion behavior strongly depends on the Alfvenic Mach number and particle's parallel mean free path. We, for the first time, numerically derive the dependence of M_A^4 for perpendicular diffusion coefficient with respect to the mean magnetic field. We conclude that CR diffusion coefficients are anisotropic in sub-Alfvenic turbulence and spatially correlated to the local turbulence properties. On scales smaller than the injection scale, we find that CRs are superdiffusive. We emphasize the importance of our results in a wide range of astrophysical processes...

## Effects of strain, electric, and magnetic fields on lateral electron spin transport in semiconductor epilayers

Hruska, M.; Kos, S.; Crooker, S. A.; Saxena, A.; Smith, D. L.
Tipo: Artigo de Revista Científica