1,213 research outputs found
Investigation of magnetopause reconnection models using two colocated, low‐altitude satellites: A unifying reconnection geometry
Ion precipitation data from two co-orbiting Defense Meteorological Satellite Program satellites (F6 and F8) are used to investigate magnetopause reconnection models. We examine differential fluxes between 30 eV and 30 keV, from a Southern Hemisphere, prenoon pass during the morning of January 10, 1990. Data from the first satellite to pass through the region (F6) show two distinct ion energy dispersions •-1 ø of latitude apart, between 76 ø and 79 ø magnetic latitude. The electron data exhibit similar features at around the same region but with no or little energy dispersion, consistent with their high velocities. We suggest that the two energy dispersions can be explained by two separate injections resulting from two bursts of magnetopause reconnection. Data from the second satellite (F8), which moved through the same region I rain later, reveal the same energy-dispersed structures, only further poleward and with less overall flux. This temporal evolution is consistent with two recently reconnected flux tubes releasing their plasma as they move antisunward away from dayside merging sites. However, an observed overlap between the two ion energy dispersions suggests a more complex reconnection geometry than usual models can accommodate. We propose a generalized reconnection scenario that unifies the Bursty Single X-Line and the Multiple X-Line Reconnection models. A simple time-of-flight particle precipitation model is constructed to reproduce the ion dispersions and their overlap. The modeling results suggest that for time-dependent reconnection the dispersion overlap is observed clearly at low altitudes only for a short period compared with the evolution timescale of the ion precipitation
Computation of the Vortex Free Energy in SU(2) Gauge Theory
We present the first measurement of the vortex free-energy order parameter at
weak coupling for SU(2) in simulations employing multihistogram methods. The
result shows that the excitation probability for a sufficiently thick vortex in
the vacuum tends to unity. This is rigorously known to provide a necessary and
sufficient condition for maintaining confinement at weak coupling in SU(N)
gauge theories.Comment: 7 pages, LaTeX with 3 eps figures, minor changes, replacement of Fig.
Electroviscous effects of simple electrolytes under shear
On the basis of a hydrodynamical model analogous to that in critical fluids,
we investigate the influences of shear flow upon the electrostatic contribution
to the viscosity of binary electrolyte solutions in the Debye-H\"{u}ckel
approximation. Within the linear-response theory, we reproduce the classical
limiting law that the excess viscosity is proportional to the square root of
the concentration of the electrolyte. We also extend this result for finite
shear. An analytic expression of the anisotropic structure factor of the charge
density under shear is obtained, and its deformation at large shear rates is
discussed. A non-Newtonian effect caused by deformations of the ionic
atmosphere is also elucidated for . This finding
concludes that the maximum shear stress that the ionic atmosphere can support
is proportional to , where , and
are, respectively, the shear rate, the Debye screening
length and the Debye relaxation time with being the relative diffusivity at
the infinite dilution limit of the electrolyte.Comment: 13pages, 2figure
On the equation of state of a dense columnar liquid crystal
An accurate description of a columnar liquid crystal of hard disks at high
packing fractions is presented using an improved free-volume theory. It is
shown that the orientational entropy of the disks in the one-dimensional fluid
direction leads to a different high-density scaling pressure compared to the
prediction from traditional cell theory. Excellent quantitative agreement is
found with recent Monte-Carlo simulation results for various thermodynamic and
structural properties of the columnar state.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Some comments on developments in exact solutions in statistical mechanics since 1944
Lars Onsager and Bruria Kaufman calculated the partition function of the
Ising model exactly in 1944 and 1949. Since then there have been many
developments in the exact solution of similar, but usually more complicated,
models. Here I shall mention a few, and show how some of the latest work seems
to be returning once again to the properties observed by Onsager and Kaufman.Comment: 28 pages, 5 figures, section on six-vertex model revise
Fluctuations in the Irreversible Decay of Turbulent Energy
A fluctuation law of the energy in freely-decaying, homogeneous and isotropic
turbulence is derived within standard closure hypotheses for 3D incompressible
flow. In particular, a fluctuation-dissipation relation is derived which
relates the strength of a stochastic backscatter term in the energy decay
equation to the mean of the energy dissipation rate. The theory is based on the
so-called ``effective action'' of the energy history and illustrates a
Rayleigh-Ritz method recently developed to evaluate the effective action
approximately within probability density-function (PDF) closures. These
effective actions generalize the Onsager-Machlup action of nonequilibrium
statistical mechanics to turbulent flow. They yield detailed, concrete
predictions for fluctuations, such as multi-time correlation functions of
arbitrary order, which cannot be obtained by direct PDF methods. They also
characterize the mean histories by a variational principle.Comment: 26 pages, Latex Version 2.09, plus seceq.sty, a stylefile for
sequential numbering of equations by section. This version includes new
discussion of the physical interpretation of the formal Rayleigh-Ritz
approximation. The title is also change
Nonequilibrium steady state thermodynamics and fluctuations for stochastic systems
We use the work done on and the heat removed from a system to maintain it in
a nonequilibrium steady state for a thermodynamic-like description of such a
system as well as of its fluctuations. Based on a generalized Onsager-Machlup
theory for nonequilibrium steady states we indicate two ambiguities, not
present in an equilibrium state, in defining such work and heat: one due to a
non-uniqueness of time-reversal procedures and another due to multiple
possibilities to separate heat into work and an energy difference in
nonequilibrium steady states. As a consequence, for such systems, the work and
heat satisfy multiple versions of the first and second laws of thermodynamics
as well as of their fluctuation theorems. Unique laws and relations appear only
to be obtainable for concretely defined systems, using physical arguments to
choose the relevant physical quantities. This is illustrated on a number of
systems, including a Brownian particle in an electric field, a driven torsion
pendulum, electric circuits and an energy transfer driven by a temperature
difference.Comment: 39 pages, 3 figur
In-Plane Magnetic Field Induced Anisotropy of 2D Fermi Contours and the Field Dependent Cyclotron Mass
The electronic structure of a 2D gas subjected to a tilted magnetic field,
with a strong component parallel to the GaAs/AlGaAs interface and a weak
component oriented perpendicularly, is studied theoretically. It is shown that
the parallel field component modifies the originally circular shape of a Fermi
contour while the perpendicular component drive an electron by the Lorentz
force along a Fermi line with a cyclotron frequency given by its shape. The
corresponding cyclotron effective mass is calculated self-consistently for
several concentrations of 2D carriers as a function of the in-plane magnetic
field. The possibility to detect its field-induced deviations from the zero
field value experimentally is discussed.Comment: written in LaTeX, 9 pages, 4 figures (6 pages) in 1 PS file
(compressed and uuencoded) available on request from [email protected],
SM-JU-93-
Electronic Structure of Three-Dimensional Superlattices Subject to Tilted Magnetic Fields
Full quantum-mechanical description of electrons moving in 3D structures with
unidirectional periodic modulation subject to tilted magnetic fields requires
an extensive numerical calculation. To understand magneto-oscillations in such
systems it is in many cases sufficient to use the quasi-classical approach, in
which the zero-magnetic-field Fermi surface is considered as a
magnetic-field-independent rigid body in k-space and periods of oscillations
are related to extremal cross-sections of the Fermi surface cut by planes
perpendicular to the magnetic-field direction. We point out cases where the
quasi-classical treatment fails and propose a simple tight-binding
fully-quantum-mechanical model of the superlattice electronic structure.Comment: 8 pages, 7 figures, RevTex, submitted to Phys. Rev.
Nematic-Isotropic Spinodal Decomposition Kinetics of Rod-like Viruses
We investigate spinodal decomposition kinetics of an initially nematic
dispersion of rod-like viruses (fd virus). Quench experiments are performed
from a flow-stabilized homogeneous nematic state at high shear rate into the
two-phase isotropic-nematic coexistence region at zero shear rate. We present
experimental evidence that spinodal decomposition is driven by orientational
diffusion, in accordance with a very recent theory.Comment: 17 pages, 6 figures, accepted in Phys. Rev.
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