Página 1 dos resultados de 729 itens digitais encontrados em 0.017 segundos

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

Fonte: IOP PUBLISHING LTD
Publicador: IOP PUBLISHING LTD

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

56.17%

#diffusion#ISM: magnetic fields#magnetohydrodynamics (MIID)#stars: formation#turbulence#STAR-FORMATION#MHD TURBULENCE#MAGNETOHYDRODYNAMIC TURBULENCE#INTERSTELLAR TURBULENCE#ASTROPHYSICAL IMPLICATIONS#HYDROMAGNETIC TURBULENCE

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...

Link permanente para citações:

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

Fonte: IOP PUBLISHING LTD
Publicador: IOP PUBLISHING LTD

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

46.04%

#MHD#Sun: magnetic fields#SOLAR DYNAMO#MAGNETIC-FIELD#DIFFERENTIAL ROTATION#MERIDIONAL FLOW#ALPHA#SIMULATIONS#CONVECTION#MECHANISM#SHEAR

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...

Link permanente para citações:

## 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

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

Publicado em 17/05/2013
EN

Relevância na Pesquisa

66.25%

#astrophysical plasmas#campos magnéticos#formação estelar#intracluster medium#magnetic fields#meio intra-aglomerado de galáxias#MHD#MHD#numerical simulations#plasmas astrofísicos#simulações numéricas

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...

Link permanente para citações:

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

Fonte: ELSEVIER SCIENCE BV
Publicador: ELSEVIER SCIENCE BV

Tipo: Artigo de Revista Científica

Publicado em //1998
EN

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.

Link permanente para citações:

## Cosmic ray propagation in turbulent magnetic fields.

Fonte: Universidade de Adelaide
Publicador: Universidade de Adelaide

Tipo: Tese de Doutorado

Publicado em //2005

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.

Link permanente para citações:

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

Fonte: Universidade Rice
Publicador: Universidade Rice

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...

Link permanente para citações:

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

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...

Link permanente para citações:

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

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

Link permanente para citações:

## On the Physics of Primordial Magnetic Fields

Fonte: Universidade Cornell
Publicador: Universidade Cornell

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...

Link permanente para citações:

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

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...

Link permanente para citações:

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

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...

Link permanente para citações:

## Transport phenomena in stochastic magnetic mirrors

Fonte: Universidade Cornell
Publicador: Universidade Cornell

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...

Link permanente para citações:

## Transport of Cosmic Rays in Chaotic Magnetic Fields

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 14/09/2001

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...

Link permanente para citações:

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/09/2015

Relevância na Pesquisa

46.05%

#Astrophysics - Solar and Stellar Astrophysics#Astrophysics - High Energy Astrophysical Phenomena#General Relativity and Quantum Cosmology

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...

Link permanente para citações:

## Magnetic fields of neutron stars

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 11/05/2013

Relevância na Pesquisa

46.08%

#Astrophysics - Solar and Stellar Astrophysics#Astrophysics - High Energy Astrophysical Phenomena#Physics - Plasma Physics

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...

Link permanente para citações:

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

46.18%

#Astrophysics - Cosmology and Nongalactic Astrophysics#Astrophysics - Astrophysics of Galaxies#Astrophysics - Solar and Stellar Astrophysics#Physics - Plasma Physics

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...

Link permanente para citações:

## Stability of toroidal magnetic fields in stellar interiors

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/03/2015

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...

Link permanente para citações:

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

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...

Link permanente para citações:

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 01/11/2005

Relevância na Pesquisa

46.16%

We construct a spin-drift-diffusion model to describe spin-polarized electron
transport in zincblende semiconductors in the presence of magnetic fields,
electric fields, and off-diagonal strain. We present predictions of the model
for geometries that correspond to optical spin injection from the absorption of
circularly polarized light, and for geometries that correspond to electrical
spin injection from ferromagnetic contacts. Starting with the Keldysh Green's
function description for a system driven out of equilibrium, we construct a
semiclassical kinetic theory of electron spin transport in strained
semiconductors in the presence of electric and magnetic fields. From this
kinetic theory we derive spin-drift-diffusion equations for the components of
the spin density matrix for the specific case of spatially uniform fields and
uniform electron density. We solve the spin-drift-diffusion equations
numerically and compare the resulting images with scanning Kerr microscopy data
of spin-polarized conduction electrons flowing laterally in bulk epilayers of
n-type GaAs. The spin-drift-diffusion model accurately describes the
experimental observations. We contrast the properties of electron spin
precession resulting from magnetic and strain fields. Spin-strain coupling
depends linearly on electron wave vector and spin-magnetic field coupling is
independent of electron wave vector. As a result...

Link permanente para citações:

## Turbulent Diffusion of Magnetic Fields in Weakly Ionized Gas

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/09/2002

Relevância na Pesquisa

46.08%

The diffusion of uni-directional magnetic fields by two dimensional turbulent
flows in a weakly ionized gas is studied. The fields here are orthogonal to the
plane of fluid motion. This simple model arises in the context of the decay of
the mean magnetic flux to mass ratio in the interstellar medium. When ions are
strongly coupled to neutrals, the transport of a large--scale magnetic field is
driven by both turbulent mixing and nonlinear, ambipolar drift. Using a
standard homogeneous and Gaussian statistical model for turbulence, we show
rigorously that a large-scale magnetic field can decay on at most turbulent
mixing time scales when the field and neutral flow are strongly coupled. There
is no enhancement of the decay rate by ambipolar diffusion. These results
extend the Zeldovich theorem to encompass the regime of two dimensional flows
and orthogonal magnetic fields, recently considered by Zweibel (2002). The
limitation of the strong coupling approximation and its implications are
discussed.

Link permanente para citações: