188 research outputs found
Effect of the electronic pressure on the energy and magnetic moment of charged test particles in turbulent electromagnetic fields
In this work we perform direct numerical simulations of three-dimensional
magnetohydrodynamics with a background magnetic field, representing solar wind
plasma, and introduce test particles to explore how a turbulent electromagnetic
environment affects them. Our focus is on the terms of the electric field
present in the generalized Omh's Law that are usually dismissed as unimportant.
These are the Hall and the electronic pressure (EP) terms, but we concentrate
primarily on the latter. We discover that the EP term generates an acceleration
of the particles, which represent protons, in the direction parallel to the
background magnetic field, in contrast to the known preferential perpendicular
energization. By studying the electric field itself, we are able to detect the
type of structures of the EP field that produce such parallel acceleration.
These are thin and elongated structures placed on top of a monotonic and
near-zero background. A statistical study to understand the real significance
of the electronic pressure term is also performed
Rapid directional alignment of velocity and magnetic field in magnetohydrodynamic turbulence
We show that local directional alignment of the velocity and magnetic field
fluctuations occurs rapidly in magnetohydrodynamics for a variety of
parameters. This is observed both in direct numerical simulations and in solar
wind data. The phenomenon is due to an alignment between the magnetic field and
either pressure gradients or shear-associated kinetic energy gradients. A
similar alignment, of velocity and vorticity, occurs in the Navier Stokes fluid
case. This may be the most rapid and robust relaxation process in turbulent
flows, and leads to a local weakening of the nonlinear terms in the small scale
vorticity and current structures where alignment takes place.Comment: 4 pages, 6 figure
Hall effect in a strong magnetic field: Direct comparisons of compressible magnetohydrodynamics and the reduced Hall magnetohydrodynamic equations
In this work we numerically test a model of Hall magnetohydrodynamics in the presence of a strong mean magnetic field: the reduced Hall magnetohydrodynamic model RHMHD derived by Gomez et al., Phys. Plasmas 15, 102303 2008 with the addition of weak compressible effects. The main advantage of this model lies in the reduction of computational cost. Nevertheless, up until now the degree of agreement with the original Hall MHD system and the range of validity in a regime of turbulence were not established. In this work direct numerical simulations of three-dimensional Hall MHD turbulence in the presence of a strong mean magnetic field are compared with simulations of the weak compressible RHMHD model. The results show that the degree of agreement is very high when the different assumptions of RHMHD, such as spectral anisotropy, are satisfied. Nevertheless, when the initial conditions are isotropic but the mean magnetic field is maintained strong, the results differ at the beginning but asymptotically reach a good agreement at relatively short times. We also found evidence that the compressibility still plays a role in the dynamics of these systems, and the weak compressible RHMHD model is able to capture these effects. In conclusion the weak compressible RHMHD model is a valid approximation of the Hall MHD turbulence in the relevant physical context. © 2010 American Institute of Physics.Fil: Martin, Luis Nicolas. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Dmitruk, Pablo Ariel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; ArgentinaFil: Gomez, Daniel Osvaldo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentin
Самоорганізовані метал-напівпровідникові мікро- та наноструктури Au-GaAs для застосування у плазмонній фотовольтаїці
Au-GaAs metal-semiconductor composite microstructures have been prepared by an anisotropic etching of n-GaAs (100) wafers doped with Te (1016 to 1017 cm-3) with subsequent photostimulated chemical deposition of noble metal (Au) on formed semiconductor quasigratings. The microrelief topology of GaAs surface is controlled by the anisotropic etching conditions. Au metal was deposited on the structured GaAs surface as randomly placed nanoparticles of various shape and size and/or nanowires on the top of the hills of formed semiconductor microstructure. As the number of Au nanoparticles increases, they tend to localize on the ledges of the GaAs microrelief forming a system of approximately parallel nanowires. Obtained periodic structures with submicron to microns periods without Au nanoparticles and with deposited nanoparticles have been studied by means of scanning electron microscopy, optical spectroscopy (photoluminescence spectroscopy at room temperature), and photoelectric measurements. The decrease of the relative intensity of main photoluminescence band for samples with Au nanostructures compared to ones without nanoparticles deposition and simultaniously changes of the shape of photocurrent spectra of Au-GaAs microstructures have been observed. Such correlation in behaviour of measured spectra make formed Au-GaA metal-semiconductor microstructures perspective for the application in plasmonic photovoltaics.
Pages of the article in the issue: 139 - 142
Language of the article: EnglishМетал-напівпровідникові композитні мікроструктури Au-GaAs були отримані шляхом анізотропного травлення з подальшим фотостимульованим хімічним осадженням благородного металу (Au) на сформовані напівпровідникові квазігратки. Золото наносилося на структуровану поверхню GaAs у вигляді хаотично розміщених наночастинок металу та/або нанодротів на вершинах пагорбів напівпровідникової мікроструктури. Сформовані структури вивчаються за допомогою скануючої електронної мікроскопії, оптичної спектроскопії та фотоелектричних вимірювань. Створені метал-напівпровідникові мікроструктури Au-GaAs є перспективними для застосування в плазмонній фотовольтаїці, що підтверджується поведінкою спектрів фотолюмінесценції та змінами форми спектрів фотоструму
Metal nanoparticle-enhanced photocurrent in GaAs photovoltaic structures with microtextured interfaces
Anisotropic turbulent model for solar coronal heating
Context : We present a self-consistent model of solar coronal heating,
originally developed by Heyvaert & Priest (1992), in which we include the
dynamical effect of the background magnetic field along a coronal structure by
using exact results from wave MHD turbulence (Galtier et al. 2000). Aims : We
evaluate the heating rate and the microturbulent velocity for comparison with
observations in the quiet corona, active regions and also coronal holes.
Methods :The coronal structures are assumed to be in a turbulent state
maintained by the slow erratic motions of the magnetic footpoints. A
description for the large-scale and the unresolved small-scale dynamics are
given separately. From the latter, we compute exactly (or numerically for
coronal holes) turbulent viscosites that are finally used in the former to
close self-consistently the system and derive the heating flux expression.
Results : We show that the heating rate and the turbulent velocity compare
favorably with coronal observations. Conclusions : Although the Alfven wave
turbulence regime is strongly anisotropic, and could reduce a priori the
heating efficiency, it provides an unexpected satisfactory model of coronal
heating for both magnetic loops and open magnetic field lines.Comment: 13 pages, 7 figure
Electron and proton heating by solar wind turbulence
Previous formulations of heating and transport associated with strong
magnetohydrodynamic (MHD) turbulence are generalized to incorporate separate
internal energy equations for electrons and protons. Electron heat conduction
is included. Energy is supplied by turbulent heating that affects both
electrons and protons, and is exchanged between them via collisions. Comparison
to available Ulysses data shows that a reasonable accounting for the data is
provided when (i) the energy exchange timescale is very long and (ii) the
deposition of heat due to turbulence is divided, with 60% going to proton
heating and 40% into electron heating. Heat conduction, determined here by an
empirical fit, plays a major role in describing the electron data
Influence of interparticle interaction on melting of gold nanoparticles in Au/polytetrafluoroethylene nanocomposites
Некоторые аспекты подготовки специалистов по радиотехнике в современных условиях
Обговорюються проблеми підготовки фахівців напряму радіотехніка в сучасних умовах. Аналізується наведений навчальний план, пропонуються шляхи покращення підготовки фахівцівThe problem of experts preparing in radio engineering specialists' in modem environment has been discussed. The education plan is analysed and the improvement ways of experts preparing is proposedОбсуждаются проблемы подготовки специалистов направления радиотехники в современных условиях. Анализируется содержание учебного плана, предлагаются пути улучшения подготовки специалисто
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