154 research outputs found
Effects of electron inertia in collisionless magnetic reconnection
We present a study of collisionless magnetic reconnection within the
framework of full two-fluid MHD for a completely ionized hydrogen plasma,
retaining the effects of the Hall current, electron pressure and electron
inertia. We performed 2.5D simulations using a pseudo-spectral code with no
dissipative effects. We check that the ideal invariants of the problem are
conserved down to round-off errors. Our results show that the change in the
topology of the magnetic field lines is exclusively due to the presence of
electron inertia. The computed reconnection rates remain a fair fraction of the
Alfv\'en velocity, which therefore qualifies as fast reconnection
Energy spectrum, dissipation and spatial structures in reduced Hall magnetohydrodynamic
We analyze the effect of the Hall term in the magnetohydrodynamic turbulence
under a strong externally supported magnetic field, seeing how this changes the
energy cascade, the characteristic scales of the flow and the dynamics of
global magnitudes, with particular interest in the dissipation.
Numerical simulations of freely evolving three-dimensional reduced
magnetohydrodynamics (RHMHD) are performed, for different values of the Hall
parameter (the ratio of the ion skin depth to the macroscopic scale of the
turbulence) controlling the impact of the Hall term. The Hall effect modifies
the transfer of energy across scales, slowing down the transfer of energy from
the large scales up to the Hall scale (ion skin depth) and carrying faster the
energy from the Hall scale to smaller scales. The final outcome is an effective
shift of the dissipation scale to larger scales but also a development of
smaller scales. Current sheets (fundamental structures for energy dissipation)
are affected in two ways by increasing the Hall effect, with a widening but at
the same time generating an internal structure within them. In the case where
the Hall term is sufficiently intense, the current sheet is fully delocalized.
The effect appears to reduce impulsive effects in the flow, making it less
intermittent.Comment: 17 pages, 10 figure
Intermittency in Hall-magnetohydrodynamics with a strong guide field
We present a detailed study of intermittency in the velocity and magnetic
field fluctuations of compressible Hall-magnetohydrodynamic turbulence with an
external guide field. To solve the equations numerically, a reduced model valid
when a strong guide field is present is used. Different values for the ion skin
depth are considered in the simulations. The resulting data is analyzed
computing field increments in several directions perpendicular to the guide
field, and building structure functions and probability density functions. In
the magnetohydrodynamic limit we recover the usual results with the magnetic
field being more intermittent than the velocity field. In the presence of the
Hall effect, field fluctuations at scales smaller than the ion skin depth show
a substantial decrease in the level of intermittency, with close to monofractal
scaling.Comment: 10 pages, 8 figure
Slow and fast magneto-optical response of magnetite nanoparticles suspension
DC magnetic field applied to Fe3O4 nanoparticle suspension affects its light scattering. Time dependent variations in the light intensity transmitted through a suspension are observed after the magnetic field is switched-on. Two types of variations can be distinguished. Fast response takes less than millisecond while slow variations occur at the time interval from seconds to hundreds of minutes. Possible mechanisms of these variations are discussed
Energy spectrum of turbulent fluctuations in boundary driven reduced magnetohydrodynamics
The nonlinear dynamics of a bundle of magnetic flux ropes driven by
stationary fluid motions at their endpoints is studied, by performing numerical
simulations of the magnetohydrodynamic (MHD) equations. The development of MHD
turbulence is shown, where the system reaches a state that is characterized by
the ratio between the Alfven time (the time for incompressible MHD waves to
travel along the field lines) and the convective time scale of the driving
motions. This ratio of time scales determines the energy spectra and the
relaxation toward different regimes ranging from weak to strong turbulence. A
connection is made with phenomenological theories for the energy spectra in MHD
turbulence.Comment: Published in Physics of Plasma
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
Turbulence in the Solar Atmosphere: Manifestations and Diagnostics via Solar Image Processing
Intermittent magnetohydrodynamical turbulence is most likely at work in the
magnetized solar atmosphere. As a result, an array of scaling and multi-scaling
image-processing techniques can be used to measure the expected
self-organization of solar magnetic fields. While these techniques advance our
understanding of the physical system at work, it is unclear whether they can be
used to predict solar eruptions, thus obtaining a practical significance for
space weather. We address part of this problem by focusing on solar active
regions and by investigating the usefulness of scaling and multi-scaling
image-processing techniques in solar flare prediction. Since solar flares
exhibit spatial and temporal intermittency, we suggest that they are the
products of instabilities subject to a critical threshold in a turbulent
magnetic configuration. The identification of this threshold in scaling and
multi-scaling spectra would then contribute meaningfully to the prediction of
solar flares. We find that the fractal dimension of solar magnetic fields and
their multi-fractal spectrum of generalized correlation dimensions do not have
significant predictive ability. The respective multi-fractal structure
functions and their inertial-range scaling exponents, however, probably provide
some statistical distinguishing features between flaring and non-flaring active
regions. More importantly, the temporal evolution of the above scaling
exponents in flaring active regions probably shows a distinct behavior starting
a few hours prior to a flare and therefore this temporal behavior may be
practically useful in flare prediction. The results of this study need to be
validated by more comprehensive works over a large number of solar active
regions.Comment: 26 pages, 7 figure
Адсорбционная активность сырья водно-болотных растений Западной Сибири
In order to find effective sorbents of herbal origin, we’ve studied adsorption properties of aerial dry raw materials of 31 marsh-water, medical perspective, plant species. It was found that 8 of those 31 exercises adsorption activity (Shpagnum species).С целью поиска эффективных сорбентов растительного происхождения изучены адсорбционные свойства сухого сырья 31 вида широко распространенных в Западной Сибири водно-болотных растений. Для 8 из них установлены выраженные адсорбционные свойства (мхи рода сфагнум), что позволяет отнести их к числу перспективных видов для получения эффективных сорбентов
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
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