626 research outputs found
Magnetohydrodynamic evolution of magnetic skeletons
The heating of the solar corona is likely to be due to reconnection of the
highly complex magnetic field that threads throughout its volume. We have run a
numerical experiment of an elementary interaction between the magnetic field of
two photospheric sources in an overlying field that represents a fundamental
building block of the coronal heating process. The key to explaining where, how
and how much energy is released during such an interaction is to calculate the
resulting evolution of the magnetic skeleton. A skeleton is essentially the web
of magnetic flux surfaces (called separatrix surfaces) that separate the
coronal volume into topologically distinct parts. For the first time the
skeleton of the magnetic field in a 3D numerical MHD experiment is calculated
and carefully analysed, as are the ways in which it bifurcates into different
topologies. A change in topology normally changes the number of magnetic
reconnection sites.
In our experiment, the magnetic field evolves through a total of six distinct
topologies. Initially, no magnetic flux joins the two sources. Then a new type
of bifurcation, called a global double-separator bifurcation, takes place: this
bifurcation is likely to be one of the main ways in which new separators are
created in the corona (separators are field lines at which 3D reconnection
takes place). This is the first of five bifurcations in which the skeleton
becomes progressively more complex before simplifying. Surprisingly, for such a
simple initial state, at the peak of complexity there are five separators and
eight flux domains present.Comment: 18 pages, 5 figure
Pion-Kaon Scattering near the Threshold in Chiral SU(2) Perturbation Theory
In the context of chiral SU(2) perturbation theory, pion-kaon scattering is
analysed near the threshold to fourth chiral order. The scattering amplitude is
calculated both in the relativistic framework and by using an approach similar
to heavy baryon chiral perturbation theory. Both methods lead to equivalent
results. We obtain relations between threshold parameters, valid to fourth
chiral order, where all those combinations of low-energy constants which are
not associated with chiral-symmetry breaking terms drop out. The remaining
low-energy constants can be estimated using chiral SU(3) symmetry.
Unfortunately, the experimental information is not precise enough to test our
low-energy theorems.Comment: 38 pages, 5 figures, PhD Thesis, references adde
Density functional method for nonequilibrium electron transport
We describe an ab initio method for calculating the electronic structure,
electronic transport, and forces acting on the atoms, for atomic scale systems
connected to semi-infinite electrodes and with an applied voltage bias. Our
method is based on the density functional theory (DFT) as implemented in the
well tested Siesta approach (which uses non-local norm-conserving
pseudopotentials to describe the effect of the core electrons, and linear
combination of finite-range numerical atomic orbitals to describe the valence
states). We fully deal with the atomistic structure of the whole system,
treating both the contact and the electrodes on the same footing. The effect of
the finite bias (including selfconsistency and the solution of the
electrostatic problem) is taken into account using nonequilibrium Green's
functions. We relate the nonequilibrium Green's function expressions to the
more transparent scheme involving the scattering states. As an illustration,
the method is applied to three systems where we are able to compare our results
to earlier ab initio DFT calculations or experiments, and we point out
differences between this method and existing schemes. The systems considered
are: (1) single atom carbon wires connected to aluminum electrodes with
extended or finite cross section, (2) single atom gold wires, and finally (3)
large carbon nanotube systems with point defects.Comment: 18 pages, 23 figure
Svestka's Research: Then and Now
Zdenek Svestka's research work influenced many fields of solar physics,
especially in the area of flare research. In this article I take five of the
areas that particularly interested him and assess them in a "then and now"
style. His insights in each case were quite sound, although of course in the
modern era we have learned things that he could not readily have envisioned.
His own views about his research life have been published recently in this
journal, to which he contributed so much, and his memoir contains much
additional scientific and personal information (Svestka, 2010).Comment: Invited review for "Solar and Stellar Flares," a conference in honour
of Prof. Zden\v{e}k \v{S}vestka, Prague, June 23-27, 2014. This is a
contribution to a Topical Issue in Solar Physics, based on the presentations
at this meeting (Editors Lyndsay Fletcher and Petr Heinzel
A Connection between Twistors and Superstring Sigma Models on Coset Superspaces
We consider superstring sigma models that are based on coset superspaces G/H
in which H arises as the fixed point set of an order-4 automorphism of G. We
show by means of twistor theory that the corresponding first-order system,
consisting of the Maurer-Cartan equations and the equations of motion, arises
from a dimensional reduction of some generalised self-dual Yang-Mills equations
in eight dimensions. Such a relationship might help shed light on the explicit
construction of solutions to the superstring equations including their hidden
symmetry structures and thus on the properties of their gauge theory duals.Comment: v3: 16 pages, typos fixed and minor clarifications adde
Search for Global Dipole Enhancements in the HiRes-I Monocular Data above 10^{18.5} eV
Several proposed source models for Ultra-High Energy Cosmic Rays (UHECRs)
consist of dipole distributions oriented towards major astrophysical landmarks
such as the galactic center, M87, or Centaurus A. We use a comparison between
real data and simulated data to show that the HiRes-I monocular data for
energies above 10^{18.5} eV is, in fact, consistent with an isotropic source
model. We then explore methods to quantify our sensitivity to dipole source
models oriented towards the Galactic Center, M87, and Centaurus A.Comment: 17 pages, 31 figure
Energetics, forces, and quantized conductance in jellium modeled metallic nanowires
Energetics and quantized conductance in jellium modeled nanowires are
investigated using the local density functional based shell correction method,
extending our previous study of uniform in shape wires [C. Yannouleas and U.
Landman, J. Phys. Chem. B 101, 5780 (1997)] to wires containing a variable
shaped constricted region. The energetics of the wire (sodium) as a function of
the length of the volume conserving, adiabatically shaped constriction leads to
formation of self selecting magic wire configurations. The variations in the
energy result in oscillations in the force required to elongate the wire and
are directly correlated with the stepwise variations of the conductance of the
nanowire in units of 2e^2/h. The oscillatory patterns in the energetics and
forces, and the correlated stepwise variation in the conductance are shown,
numerically and through a semiclassical analysis, to be dominated by the
quantized spectrum of the transverse states at the narrowmost part of the
constriction in the wire.Comment: Latex/Revtex, 11 pages with 5 Postscript figure
Global phylogenetic analysis of Escherichia coli and plasmids carrying the mcr-1 gene indicates bacterial diversity but plasmid restriction
To understand the dynamics behind the worldwide spread of the mcr-1 gene, we determined the population structure of Escherichia coli and of mobile genetic elements (MGEs) carrying the mcr-1 gene. After a systematic review of the literature we included 65 E. coli whole genome sequences (WGS), adding 6 recently sequenced travel related isolates, and 312 MLST profiles. We included 219 MGEs described in 7 Enterobacteriaceae species isolated from human, animal and environmental samples. Despite a high overall diversity, 2 lineages were observed in the E. coli population that may function as reservoirs of the mcr-1 gene, the largest of which was linked to ST10, a sequence type known for its ubiquity in human faecal samples and in food samples. No genotypic clustering by geographical origin or isolation source was observed. Amongst a total of 13 plasmid incompatibility types, the IncI2, IncX4 and IncHI2 plasmids accounted for more than 90% of MGEs carrying the mcr-1 gene. We observed significant geographical clustering with regional spread of IncHI2 plasmids in Europe and IncI2 in Asia. These findings point towards promiscuous spread of the mcr-1 gene by efficient horizontal gene transfer dominated by a limited number of plasmid incompatibility types
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