2,333 research outputs found
Effects of different field expansions on omnidirectional proton counting rate data
Field expansion effects on omnidirectional proton counting rate dat
Plasma clouds in the magnetosphere
Injection of hot plasma clouds into magnetosphere during magnetospheric substorm
Long term variations in high-energy geomagnetically trapped particles
Long term variations in high energy geomagnetically trapped particles from satellite detector dat
Stirring Unmagnetized Plasma
A new concept for spinning unmagnetized plasma is demonstrated
experimentally. Plasma is confined by an axisymmetric multi-cusp magnetic field
and biased cathodes are used to drive currents and impart a torque in the
magnetized edge. Measurements show that flow viscously couples momentum from
the magnetized edge (where the plasma viscosity is small) into the unmagnetized
core (where the viscosity is large) and that the core rotates as a solid body.
To be effective, collisional viscosity must overcome the ion-neutral drag due
to charge exchange collisions
Emergent singular solutions of non-local density-magnetization equations in one dimension
We investigate the emergence of singular solutions in a non-local model for a
magnetic system. We study a modified Gilbert-type equation for the
magnetization vector and find that the evolution depends strongly on the length
scales of the non-local effects. We pass to a coupled density-magnetization
model and perform a linear stability analysis, noting the effect of the length
scales of non-locality on the system's stability properties. We carry out
numerical simulations of the coupled system and find that singular solutions
emerge from smooth initial data. The singular solutions represent a collection
of interacting particles (clumpons). By restricting ourselves to the
two-clumpon case, we are reduced to a two-dimensional dynamical system that is
readily analyzed, and thus we classify the different clumpon interactions
possible.Comment: 19 pages, 13 figures. Submitted to Phys. Rev.
Fourth Order Algorithms for Solving the Multivariable Langevin Equation and the Kramers Equation
We develop a fourth order simulation algorithm for solving the stochastic
Langevin equation. The method consists of identifying solvable operators in the
Fokker-Planck equation, factorizing the evolution operator for small time steps
to fourth order and implementing the factorization process numerically. A key
contribution of this work is to show how certain double commutators in the
factorization process can be simulated in practice. The method is general,
applicable to the multivariable case, and systematic, with known procedures for
doing fourth order factorizations. The fourth order convergence of the
resulting algorithm allowed very large time steps to be used. In simulating the
Brownian dynamics of 121 Yukawa particles in two dimensions, the converged
result of a first order algorithm can be obtained by using time steps 50 times
as large. To further demostrate the versatility of our method, we derive two
new classes of fourth order algorithms for solving the simpler Kramers equation
without requiring the derivative of the force. The convergence of many fourth
order algorithms for solving this equation are compared.Comment: 19 pages, 2 figure
Structure of solar coronal loops: from miniature to large-scale
We will use new data from the High-resolution Coronal Imager (Hi-C) with
unprecedented spatial resolution of the solar corona to investigate the
structure of coronal loops down to 0.2 arcsec. During a rocket flight Hi-C
provided images of the solar corona in a wavelength band around 193 A that is
dominated by emission from Fe XII showing plasma at temperatures around 1.5 MK.
We analyze part of the Hi-C field-of-view to study the smallest coronal loops
observed so far and search for the a possible sub-structuring of larger loops.
We find tiny 1.5 MK loop-like structures that we interpret as miniature coronal
loops. These have length of the coronal segment above the chromosphere of only
about 1 Mm and a thickness of less than 200 km. They could be interpreted as
the coronal signature of small flux tubes breaking through the photosphere with
a footpoint distance corresponding to the diameter of a cell of granulation. We
find loops that are longer than 50 Mm to have a diameter of about 2 arcsec or
1.5 Mm, consistent with previous observations. However, Hi-C really resolves
these loops with some 20 pixels across the loop. Even at this greatly improved
spatial resolution the large loops seem to have no visible sub-structure.
Instead they show a smooth variation in cross-section. The fact that the large
coronal loops do not show a sub-structure at the spatial scale of 0.1 arcsec
per pixel implies that either the densities and temperatures are smoothly
varying across these loops or poses an upper limit on the diameter of strands
the loops might be composed of. We estimate that strands that compose the 2
arcsec thick loop would have to be thinner than 15 km. The miniature loops we
find for the first time pose a challenge to be properly understood in terms of
modeling.Comment: Accepted for publication in A&A (Jun 19, 2013), 11 pages, 10 figure
Detecting chaos in particle accelerators through the frequency map analysis method
The motion of beams in particle accelerators is dominated by a plethora of
non-linear effects which can enhance chaotic motion and limit their
performance. The application of advanced non-linear dynamics methods for
detecting and correcting these effects and thereby increasing the region of
beam stability plays an essential role during the accelerator design phase but
also their operation. After describing the nature of non-linear effects and
their impact on performance parameters of different particle accelerator
categories, the theory of non-linear particle motion is outlined. The recent
developments on the methods employed for the analysis of chaotic beam motion
are detailed. In particular, the ability of the frequency map analysis method
to detect chaotic motion and guide the correction of non-linear effects is
demonstrated in particle tracking simulations but also experimental data.Comment: Submitted for publication in Chaos, Focus Issue: Chaos Detection
Methods and Predictabilit
Multiscale analysis of materials with anisotropic microstructure as micropolar continua
Multiscale procedures are often adopted for the continuum modeling of materials composed of a specific micro-structure. Generally, in mechanics of materials only two-scales are linked. In this work the original (fine) micro-scale description, thought as a composite material made of matrix and fibers/particles/crystals which can interact among them, and a scale-dependent continuum (coarse) macro-scale are linked via an energy equivalence criterion. In particular the multiscale strategy is proposed for deriving the constitutive relations of anisotropic composites with periodic microstructure and allows us to reduce the typically high computational cost of fully microscopic numerical analyses. At the microscopic level the material is described as a lattice system while at the macroscopic level the continuum is a micropolar continuum, whose material particles are endowed with orientation besides position. The derived constitutive relations account for shape, texture and orientation of inclusions as well as internal scale parameters, which account for size effects even in the elastic regime in the presence of geometrical and/or load singularities. Applications of this procedure concern polycrystals, wherein an important descriptor of the underlying microstructure gives the orientation of the crystal lattice of each grain, fiber reinforced composites, as well as masonry-like materials. In order to investigate the effects of micropolar constants in the presence of material non central symmetries, some numerical finite element simulations, with elements specifically formulated for micropolar media, are presented. The performed simulations, which extend several parametric analyses earlier performed [1], involve two-dimensional media, in the linear framework, subjected to compression loads distributed in a small portion of the medium
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