Ice Age Epochs and the Sun's Path Through the Galaxy

We present a calculation of the Sun's motion through the Milky Way Galaxy over the last 500 million years. The integration is based upon estimates of the Sun's current position and speed from measurements with Hipparcos and upon a realistic model for the Galactic gravitational potential. We estimate the times of the Sun's past spiral arm crossings for a range in assumed values of the spiral pattern angular speed. We find that for a difference between the mean solar and pattern speed of Omega_Sun - Omega_p = 11.9 +/- 0.7 km/s/kpc the Sun has traversed four spiral arms at times that appear to correspond well with long duration cold periods on Earth. This supports the idea that extended exposure to the higher cosmic ray flux associated with spiral arms can lead to increased cloud cover and long ice age epochs on Earth.Comment: 14 pages, 3 figures, accepted for publication in Ap

Cascade Failure in a Phase Model of Power Grids

We propose a phase model to study cascade failure in power grids composed of generators and loads. If the power demand is below a critical value, the model system of power grids maintains the standard frequency by feedback control. On the other hand, if the power demand exceeds the critical value, an electric failure occurs via step out (loss of synchronization) or voltage collapse. The two failures are incorporated as two removal rules of generator nodes and load nodes. We perform direct numerical simulation of the phase model on a scale-free network and compare the results with a mean-field approximation.Comment: 7 pages, 2 figure

Nitrogênio mineral no solo e índice de clorofila na folha como indicadores da necessidade de nitrogênio para o milho.

O objetivo deste trabalho foi o de avaliar parâmetros de solo (N mineral) e planta (índice de clorofila) como indicadores da necessidade de adubação nitrogenada para o milho, em um estádio fenológico que permita a correção de deficiência deste nutriente, sem prejuízo à produtividade. Os parâmetros de solo e planta, avaliados no estádio V7-folhas, apresentaram-se com potencial de serem utilizados como indicadores da necessidade de adubação nitrogenada de cobertura para o milho. Em solo sob plantio direto, com teores de N mineral (N-NH + N-N03) da ordem de 20 kg/ha na camada de 0 - 0,10 m e de 30 kg/ha na camada de 0,10 - 0,30 m, associado aos valores dos índices de clorofila na folha, na faixa de 50 a 52, foram indicativos da não necessidade de aplicação de adubação nitrogenada de cobertura, com potencial de produtividade de grãos da ordem de 9 a 10 t/ha

Long-Tailed Trapping Times and Levy Flights in a Self-Organized Critical Granular System

We present a continuous time random walk model for the scale-invariant transport found in a self-organized critical rice pile [Christensen et al., Phys. Rev. Lett. 77, 107 (1996)]. From our analytical results it is shown that the dynamics of the experiment can be explained in terms of L\'evy flights for the grains and a long-tailed distribution of trapping times. Scaling relations for the exponents of these distributions are obtained. The predicted microscopic behavior is confirmed by means of a cellular automaton model.Comment: 4 pages, RevTex, includes 3 PostScript figures, submitted to Phys. Rev. Let

Elastic String in a Random Medium

We consider a one dimensional elastic string as a set of massless beads interacting through springs characterized by anisotropic elastic constants. The string, driven by an external force, moves in a medium with quenched disorder. We present evidence that the consideration of longitudinal fluctuations leads to nonlinear behavior in the equation of motion which is {\it kinematically} generated by the motion of the string. The strength of the nonlinear effects depends on the anisotropy of the medium and the distance from the depinning transition. On the other hand the consideration of restricted solid on solid conditions imposed to the growth of the string leads to a nonlinear term in the equation of motion with a {\it diverging} coefficient at the depinning transition.Comment: 9 pages, REVTEX, figures available upon request from [email protected]

Dynamics of driven interfaces near isotropic percolation transition

We consider the dynamics and kinetic roughening of interfaces embedded in uniformly random media near percolation treshold. In particular, we study simple discrete ``forest fire'' lattice models through Monte Carlo simulations in two and three spatial dimensions. An interface generated in the models is found to display complex behavior. Away from the percolation transition, the interface is self-affine with asymptotic dynamics consistent with the Kardar-Parisi-Zhang universality class. However, in the vicinity of the percolation transition, there is a different behavior at earlier times. By scaling arguments we show that the global scaling exponents associated with the kinetic roughening of the interface can be obtained from the properties of the underlying percolation cluster. Our numerical results are in good agreement with theory. However, we demonstrate that at the depinning transition, the interface as defined in the models is no longer self-affine. Finally, we compare these results to those obtained from a more realistic reaction-diffusion model of slow combustion.Comment: 7 pages, 9 figures, to appear in Phys. Rev. E (1998

Universality Classes for Interface Growth with Quenched Disorder

We present numerical evidence that there are two distinct universality classes characterizing driven interface roughening in the presence of quenched disorder. The evidence is based on the behavior of $\lambda$, the coefficient of the nonlinear term in the growth equation. Specifically, for three of the models studied, $\lambda \rightarrow \infty$ at the depinning transition, while for the two other models, $\lambda \rightarrow 0$.Comment: 11 pages and 3 figures (upon request), REVTeX 3.0, (submitted to PRL

Gauge Theories with Lorentz-Symmetry Violation by Symplectic Projector Method

The violation of Lorentz symmetry is studied from the point of view of a canonical formulation. We make the usual analysis on the constraints structure of the Carroll-Field-Jackiw model. In this context we derive the equations of motion for the physical variables and check out the dispersion relations obtained from them. Therefore, by the analysis using Symplectic Projector Method (SPM), we can check the results about this type of Lorentz breaking with those in the recent literature: in this sense we can confirm that the configuration of $v^{\mu}$ space-like is stable, and the $v^{\mu}$ time-like carry tachionic modes.Comment: 7 pages and no figure

Compact QED3_3 - a simple example of a variational calculation in a gauge theory

We apply a simple mean field like variational calculation to compact QED in 2+1 dimensions. Our variational ansatz explicitly preserves compact gauge invariance of the theory. We reproduce in this framework all the known results, including dynamical mass generation, Polyakov scaling and the nonzero string tension. It is hoped that this simple example can be a useful reference point for applying similar approximation techniques to nonabelian gauge theories.Comment: 18 pages, OUTP- 94-23 P, TPI-MINN-94/37-

Anisotropic Interface Depinning - Numerical Results

We study numerically a stochastic differential equation describing an interface driven along the hard direction of an anisotropic random medium. The interface is subject to a homogeneous driving force, random pinning forces and the surface tension. In addition, a nonlinear term due to the anisotropy of the medium is included. The critical exponents characterizing the depinning transition are determined numerically for a one-dimensional interface. The results are the same, within errors, as those of the ``Directed Percolation Depinning'' (DPD) model. We therefore expect that the critical exponents of the stochastic differential equation are exactly given by the exponents obtained by a mapping of the DPD model to directed percolation. We find that a moving interface near the depinning transition is not self-affine and shows a behavior similar to the DPD model.Comment: 9 pages, 13 figures, REVTe