25,540 research outputs found
Multi-beam Energy Moments of Multibeam Particle Velocity Distributions
High resolution electron and ion velocity distributions, f(v), which consist
of N effectively disjoint beams, have been measured by NASA's Magnetospheric
Multi-Scale Mission (MMS) observatories and in reconnection simulations.
Commonly used standard velocity moments generally assume a single
mean-flow-velocity for the entire distribution, which can lead to
counterintuitive results for a multibeam f(v). An example is the (false)
standard thermal energy moment of a pair of equal and opposite cold particle
beams, which is nonzero even though each beam has zero thermal energy. By
contrast, a multibeam moment of two or more beams has no false thermal energy.
A multibeam moment is obtained by taking a standard moment of each beam and
then summing over beams. In this paper we will generalize these notions,
explore their consequences and apply them to an f(v) which is sum of
tri-Maxwellians. Both standard and multibeam energy moments have coherent and
incoherent forms. Examples of incoherent moments are the thermal energy
density, the pressure and the thermal energy flux (enthalpy flux plus heat
flux). Corresponding coherent moments are the bulk kinetic energy density, the
RAM pressure and the bulk kinetic energy flux. The false part of an incoherent
moment is defined as the difference between the standard incoherent moment and
the corresponding multibeam moment. The sum of a pair of corresponding coherent
and incoherent moments will be called the undecomposed moment. Undecomposed
moments are independent of whether the sum is standard or multibeam and
therefore have advantages when studying moments of measured f(v).Comment: 27 single-spaced pages. Three Figure
Volunteer studies replacing animal experiments in brain research - Report and recommendations of a Volunteers in Research and Testing workshop
Study of the de Almeida-Thouless line using power-law diluted one-dimensional Ising spin glasses
We test for the existence of a spin-glass phase transition, the de
Almeida-Thouless line, in an externally-applied (random) magnetic field by
performing Monte Carlo simulations on a power-law diluted one-dimensional Ising
spin glass for very large system sizes. We find that an Almeida-Thouless line
only occurs in the mean field regime, which corresponds, for a short-range spin
glass, to dimension d larger than 6.Comment: 4 pages, 2 figures, 1 tabl
An explanation of the Newman-Janis Algorithm
After the original discovery of the Kerr metric, Newman and Janis showed that
this solution could be ``derived'' by making an elementary complex
transformation to the Schwarzschild solution. The same method was then used to
obtain a new stationary axisymmetric solution to Einstein's field equations now
known as the Kerr-newman metric, representing a rotating massive charged black
hole. However no clear reason has ever been given as to why the Newman-Janis
algorithm works, many physicist considering it to be an ad hoc procedure or
``fluke'' and not worthy of further investigation. Contrary to this belief this
paper shows why the Newman-Janis algorithm is successful in obtaining the
Kerr-Newman metric by removing some of the ambiguities present in the original
derivation. Finally we show that the only perfect fluid generated by the
Newman-Janis algorithm is the (vacuum) Kerr metric and that the only Petrov
typed D solution to the Einstein-Maxwell equations is the Kerr-Newman metric.Comment: 14 pages, no figures, submitted to Class. Quantum Gra
Dynamical Exchanges in Facilitated Models of Supercooled liquids
We investigate statistics of dynamical exchange events in coarse--grained
models of supercooled liquids in spatial dimensions , 2, and 3. The
models, based upon the concept of dynamical facilitation, capture generic
features of statistics of exchange times and persistence times. Here,
distributions for both times are related, and calculated for cases of strong
and fragile glass formers over a range of temperatures. Exchange time
distributions are shown to be particularly sensitive to the model parameters
and dimensions, and exhibit more structured and richer behavior than
persistence time distributions. Mean exchange times are shown to be Arrhenius,
regardless of models and spatial dimensions. Specifically, , with being the excitation concentration. Different dynamical
exchange processes are identified and characterized from the underlying
trajectories. We discuss experimental possibilities to test some of our
theoretical findings.Comment: 11 pages, 14 figures, minor corrections made, paper published in
Journal of Chemical Physic
Limit curve theorems in Lorentzian geometry
The subject of limit curve theorems in Lorentzian geometry is reviewed. A
general limit curve theorem is formulated which includes the case of converging
curves with endpoints and the case in which the limit points assigned since the
beginning are one, two or at most denumerable. Some applications are
considered. It is proved that in chronological spacetimes, strong causality is
either everywhere verified or everywhere violated on maximizing lightlike
segments with open domain. As a consequence, if in a chronological spacetime
two distinct lightlike lines intersect each other then strong causality holds
at their points. Finally, it is proved that two distinct components of the
chronology violating set have disjoint closures or there is a lightlike line
passing through each point of the intersection of the corresponding boundaries.Comment: 25 pages, 1 figure. v2: Misprints fixed, matches published versio
Color Dynamics in External Fields
We investigate the vacuum dynamics of U(1), SU(2), and SU(3) lattice gauge
theories in presence of external (chromo)magnetic fields, both in (3+1) and
(2+1) dimensions. We find that the critical coupling for the phase transition
in compact U(1) gauge theory is independent of the strength of an external
magnetic field. On the other hand we find that, both in (3+1) and (2+1)
dimensions, the deconfinement temperature for SU(2) and SU(3) gauge systems in
a constant abelian chromomagnetic field decreases when the strength of the
applied field increases. We conclude that the dependence of the deconfinement
temperature on the strength of an external constant chromomagnetic field is a
peculiar feature of non abelian gauge theories and could be useful to get
insight into color confinement.Comment: 26 pages, 14 figure
The Universal Cut Function and Type II Metrics
In analogy with classical electromagnetic theory, where one determines the
total charge and both electric and magnetic multipole moments of a source from
certain surface integrals of the asymptotic (or far) fields, it has been known
for many years - from the work of Hermann Bondi - that energy and momentum of
gravitational sources could be determined by similar integrals of the
asymptotic Weyl tensor. Recently we observed that there were certain overlooked
structures, {defined at future null infinity,} that allowed one to determine
(or define) further properties of both electromagnetic and gravitating sources.
These structures, families of {complex} `slices' or `cuts' of Penrose's null
infinity, are referred to as Universal Cut Functions, (UCF). In particular, one
can define from these structures a (complex) center of mass (and center of
charge) and its equations of motion - with rather surprising consequences. It
appears as if these asymptotic structures contain in their imaginary part, a
well defined total spin-angular momentum of the source. We apply these ideas to
the type II algebraically special metrics, both twisting and twist-free.Comment: 32 page
Signatures of Secondary Collisionless Magnetic Reconnection Driven by Kink Instability of a Flux Rope
The kinetic features of secondary magnetic reconnection in a single flux rope
undergoing internal kink instability are studied by means of three-dimensional
Particle-in-Cell simulations. Several signatures of secondary magnetic
reconnection are identified in the plane perpendicular to the flux rope: a
quadrupolar electron and ion density structure and a bipolar Hall magnetic
field develop in proximity of the reconnection region. The most intense
electric fields form perpendicularly to the local magnetic field, and a
reconnection electric field is identified in the plane perpendicular to the
flux rope. An electron current develops along the reconnection line in the
opposite direction of the electron current supporting the flux rope magnetic
field structure. Along the reconnection line, several bipolar structures of the
electric field parallel to the magnetic field occur making the magnetic
reconnection region turbulent. The reported signatures of secondary magnetic
reconnection can help to localize magnetic reconnection events in space,
astrophysical and fusion plasmas
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