4,689 research outputs found
Dissipative Dynamics of Collisionless Nonlinear Alfven Wave Trains
The nonlinear dynamics of collisionless Alfven trains, including resonant
particle effects is studied using the kinetic nonlinear Schroedinger (KNLS)
equation model. Numerical solutions of the KNLS reveal the dynamics of Alfven
waves to be sensitive to the sense of polarization as well as the angle of
propagation with respect to the ambient magnetic field. The combined effects of
both wave nonlinearity and Landau damping result in the evolutionary formation
of stationaryOA S- and arc-polarized directional and rotational
discontinuities. These waveforms are freqently observed in the interplanetary
plasma.Comment: REVTeX, 6 pages (including 5 figures). This and other papers may be
found at http://sdphpd.ucsd.edu/~medvedev/papers.htm
Effective gravity from a quantum gauge theory in Euclidean space-time
We consider a gauge theory in an Euclidean -dimensional
space-time, which is known to be renormalizable to all orders in perturbation
theory for . Then, with the help of a space-time representation of
the gauge group, the gauge theory is mapped into a curved space-time with
linear connection. Further, in that mapping the gauge field plays the role of
the linear connection of the curved space-time and an effective metric tensor
arises naturally from the mapping. The obtained action, being quadratic in the
Riemann-Christoffel tensor, at a first sight, spoils a gravity interpretation
of the model. Thus, we provide a sketch of a mechanism that breaks the
color invariance and generates the Einstein-Hilbert term, as well as a
cosmological constant term, allowing an interpretation of the model as a
modified gravity in the Palatini formalism. In that sense, gravity can be
visualized as an effective classical theory, originated from a well defined
quantum gauge theory. We also show that, in the four dimensional case, two
possibilities for particular solutions of the field equations are the de Sitter
and Anti de Sitter space-times.Comment: 20 pages; Final version accepted for publication in Class.Quant.Gra
Comparison of Reconstruction Algorithms for Brain Stroke Microwave Imaging
The aim of this paper is to describe and compare the performances of three image reconstruction algorithms that can be used for brain stroke microwave imaging. The algorithms belong to the class of non-linear iterative algorithms and are capable of providing a quantitative map of the imaged scenario. The first algorithm is the Contrast Source Inversion (CSI) method, which uses the Finite Element Method (FEM) to discretize the domain of interest. The second one is the Subspace-Based Optimization Method (SOM) that has some properties in common with the CSI method, and it also uses FEM to discretize the domain. The last one is the Distorted Born Iterative Method with the inverse solver Two-step Iterative Shrinkage/Thresholding (DBIM-TwIST), which exploits the forward Finite Difference Time Domain (FDTD) solver. The reconstruction examples are created with 3-D synthetic data modelling realistic brain tissues with the presence of a blood region, representing the stroke area in the brain, whereas the inversion step is carried out using a 2-D model
Twisted topological structures related to M-branes
Studying the M-branes leads us naturally to new structures that we call
Membrane-, Membrane^c-, String^K(Z,3)- and Fivebrane^K(Z,4)-structures, which
we show can also have twisted counterparts. We study some of their basic
properties, highlight analogies with structures associated with lower levels of
the Whitehead tower of the orthogonal group, and demonstrate the relations to
M-branes.Comment: 17 pages, title changed on referee's request, minor changes to
improve presentation, typos correcte
- …