5,192 research outputs found
Higher Order Modulation Equations for a Boussinesq Equation
In order to investigate corrections to the common KdV approximation to long
waves, we derive modulation equations for the evolution of long wavelength
initial data for a Boussinesq equation. The equations governing the corrections
to the KdV approximation are explicitly solvable and we prove estimates showing
that they do indeed give a significantly better approximation than the KdV
equation alone. We also present the results of numerical experiments which show
that the error estimates we derive are essentially optimal
Nanoflare Evidence from Analysis of the X-Ray Variability of an Active Region Observed with Hinode/XRT
The heating of the solar corona is one of the big questions in astrophysics.
Rapid pulses called nanoflares are among the best candidate mechanisms. The
analysis of the time variability of coronal X-ray emission is potentially a
very useful tool to detect impulsive events. We analyze the small-scale
variability of a solar active region in a high cadence Hinode/XRT observation.
The dataset allows us to detect very small deviations of emission fluctuations
from the distribution expected for a constant rate. We discuss the deviations
in the light of the pulsed-heating scenario.Comment: 6 pages, 4 figure
Decoupled and unidirectional asymptotic models for the propagation of internal waves
We study the relevance of various scalar equations, such as inviscid
Burgers', Korteweg-de Vries (KdV), extended KdV, and higher order equations (of
Camassa-Holm type), as asymptotic models for the propagation of internal waves
in a two-fluid system. These scalar evolution equations may be justified with
two approaches. The first method consists in approximating the flow with two
decoupled, counterpropagating waves, each one satisfying such an equation. One
also recovers homologous equations when focusing on a given direction of
propagation, and seeking unidirectional approximate solutions. This second
justification is more restrictive as for the admissible initial data, but
yields greater accuracy. Additionally, we present several new coupled
asymptotic models: a Green-Naghdi type model, its simplified version in the
so-called Camassa-Holm regime, and a weakly decoupled model. All of the models
are rigorously justified in the sense of consistency
All Coronal Loops are the Same: Evidence to the Contrary
The 1998 April 20 spectral line data from the Coronal Diagnostics
Spectrometer (CDS) on the {\it Solar and Heliospheric Observatory} (\SOHO)
shows a coronal loop on the solar limb. Our original analysis of these data
showed that the plasma was multi-thermal, both along the length of the loop and
along the line of sight. However, more recent results by other authors indicate
that background subtraction might change these conclusions, so we consider the
effect of background subtraction on our analysis. We show Emission Measure (EM)
Loci plots of three representative pixels: loop apex, upper leg, and lower leg.
Comparisons of the original and background-subtracted intensities show that the
EM Loci are more tightly clustered after background subtraction, but that the
plasma is still not well represented by an isothermal model. Our results taken
together with those of other authors indicate that a variety of temperature
structures may be present within loops.Comment: Accepted for publication in ApJ Letter
Orbital Ordering in ferromagnetic Lu2V2O7
We have observed the orbital ordering in the ferromagnetic Mott-insulator
Lu2V2O7 by the polarized neutron diffraction technique. The orbital ordering
pattern determined from the observed magnetic form factors can be explained in
terms of a linear combination of wave functions |yz>, |zx> and |xy>; |0> =
(1/3)^(1/2) |xy> + (1/3)^(1/2)|yz> + (1/3)^(1/2) |zx> which is proportional to
|(x + y + z)^2 - r^2>; where each orbital is extended toward the center-of-mass
of the V tetrahedron. We discuss the stability of the ferromagnetic Lu2V2O7,
using a Hubbard Hamiltonian with these three orbitals.Comment: 17pages. to be published in J. Phys. Soc. Jpn. 74 (2005
Superlubricity mechanism of diamond-like carbon with glycerol. Coupling of experimental and simulation studies
We report a unique tribological system that produces superlubricity under boundary lubrication conditions with extremely little wear. This system is a thin coating of hydrogen-free amorphous Diamond-Like-Carbon (denoted as ta-C) at 353 K in a ta-C/ta-C friction pair lubricated with pure glycerol. To understand the mechanism of friction vanishing we performed ToF-SIMS experiments using deuterated glycerol and 13C glycerol. This was complemented by first-principles-based computer simulations using the ReaxFF reactive force field to create an atomistic model of ta-C. These simulations show that DLC with the experimental density of 3.24 g/cc leads to an atomistic structure consisting of a 3D percolating network of tetrahedral (sp3) carbons accounting for 71.5% of the total, in excellent agreement with the 70% deduced from our Auger spectroscopy and XANES experiments. The simulations show that the remaining carbons (with sp2 and sp1 character) attach in short chains of length 1 to 7. In sliding simulations including glycerol molecules, the surface atoms react readily to form a very smooth carbon surface containing OH-terminated groups. This agrees with our SIMS experiments. The simulations find that the OH atoms are mostly bound to surface sp1 atoms leading to very flexible elastic response to sliding. Both simulations and experiments suggest that the origin of the superlubricity arises from the formation of this OH-terminated surface
Fermi Surface of Alpha-Uranium at Ambient Pressure
We have performed de Haas-van Alphen measurements of the Fermi surface of
alpha-uranium single crystals at ambient pressure within the alpha-3 charge
density wave (CDW) state from 0.020 K - 10 K and magnetic fields to 35 T using
torque magnetometry. The angular dependence of the resulting frequencies is
described. Effective masses were measured and the Dingle temperature was
determined to be 0.74 K +/- 0.04 K. The observation of quantum oscillations
within the alpha-3 CDW state gives new insight into the effect of the charge
density waves on the Fermi surface. In addition we observed no signature of
superconductivity in either transport or magnetization down to 0.020 K
indicating the possibility of a pressure-induced quantum critical point that
separates the superconducting dome from the normal CDW phase.Comment: 11 pages, 4 figures, 3 table
Temperature and Emission-Measure Profiles Along Long-Lived Solar Coronal Loops Observed with TRACE
We report an initial study of temperature and emission measure distributions
along four steady loops observed with the Transition Region and Coronal
Explorer (TRACE) at the limb of the Sun. The temperature diagnostic is the
filter ratio of the extreme-ultraviolet 171-angstrom and 195-angstrom
passbands. The emission measure diagnostic is the count rate in the
171-angstrom passband. We find essentially no temperature variation along the
loops. We compare the observed loop structure with theoretical isothermal and
nonisothermal static loop structure.Comment: 10 pages, 3 postscript figures (LaTeX, uses aaspp4.sty). Accepted by
ApJ Letter
Damage spreading in two dimensional geometrically frustrated lattices: the triangular and kagome anistropic Heisenberg model
The technique of damage spreading is used to study the phase diagram of the
easy axis anisotropic Heisenberg antiferromagnet on two geometrically
frustrated lattices. The triangular and kagome systems are built up from
triangular units that either share edges or corners respectively. The
triangular lattice undergoes two sequential Kosterlitz-Thouless transitions
while the kagome lattice undergoes a glassy transition. In both cases, the
phase boundaries obtained using damage spreading are in good agreement with
those obtained from equilibrium Monte Carlo simulations.Comment: 7 pages, 4 figure
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