47 research outputs found
Random walk of magnetic field lines for different values of the energy-range spectral index
An analytical nonlinear description of field-line wandering in partially
statistically magnetic systems was proposed recently [A. Shalchi, I. Kourakis,
Astronomy and Astrophysics, 470, 405 (2007)]. In this article we investigate
the influence of the wave-spectrum in the energy-range onto field line random
walk by applying this formulation. It is demonstrated that in all considered
cases we clearly obtain a superdiffusive behaviour of the field-lines. If the
energy-range spectral index exceeds unity a free-streaming behaviour of the
field-lines can be found for all relevant length-scales of turbulence. Since
the superdiffusive results obtained for the slab model are exact, it seems that
superdiffusion is the normal behavior of field line wandering.Comment: Submitted to Physics of Plasmas; 13 pages, no figure
Pattern Formation in Interface Depinning and Other Models: Erratically Moving Spatial Structures
We study erratically moving spatial structures that are found in a driven
interface in a random medium at the depinning threshold. We introduce a
bond-disordered variant of the Sneppen model and study the effect of extremal
dynamics on the morphology of the interface. We find evidence for the formation
of a structure which moves along with the growth site. The time average of the
structure, which is defined with respect to the active spot of growth, defines
an activity-centered pattern. Extensive Monte Carlo simulations show that the
pattern has a tail which decays slowly, as a power law. To understand this sort
of pattern formation, we write down an approximate integral equation involving
the local interface dynamics and long-ranged jumps of the growth spot. We
clarify the nature of the approximation by considering a model for which the
integral equation is exactly derivable from an extended master equation.
Improvements to the equation are considered by adding a second coupled equation
which provides a self-consistent description. The pattern, which defines a
one-point correlation function, is shown to have a strong effect on ordinary
space-fixed two-point correlation functions. Finally we present evidence that
this sort of pattern formation is not confined to the interface problem, but is
generic to situations in which the activity at succesive time steps is
correlated, as for instance in several other extremal models. We present
numerical results for activity-centered patterns in the Bak-Sneppen model of
evolution and the Zaitsev model of low-temperature creep.Comment: RevTeX, 18 pages, 19 eps-figures, To appear in Phys. Rev.
Metabolism of ascites tumor cells. Iii. Effect of 2-deoxyglucose phosphorylation on phosphorus metabolism.
Metabolism of ascites tumor cells. IV. Enzymatic reactions involved in adenosinetriphosphate degradation induced by 2-deozyglucose.
Reaction-diffusion fronts under stochastic advection
We study front propagation in stirred media using a simplified modelization of the turbulent flow. Computer simulations reveal the existence of the two limiting propagation modes observed in recent experiments with liquid phase isothermal reactions. These two modes respectively correspond to a wrinkled although sharp propagating interface and to a broadened one. Specific laws relative to the enhancement of the front velocity in each regime are confirmed by our simulations