Electric field generation by the electron beam filamentation instability: Filament size effects

The filamentation instability (FI) of counter-propagating beams of electrons is modelled with a particle-in-cell simulation in one spatial dimension and with a high statistical plasma representation. The simulation direction is orthogonal to the beam velocity vector. Both electron beams have initially equal densities, temperatures and moduli of their nonrelativistic mean velocities. The FI is electromagnetic in this case. A previous study of a small filament demonstrated, that the magnetic pressure gradient force (MPGF) results in a nonlinearly driven electrostatic field. The probably small contribution of the thermal pressure gradient to the force balance implied, that the electrostatic field performed undamped oscillations around a background electric field. Here we consider larger filaments, which reach a stronger electrostatic potential when they saturate. The electron heating is enhanced and electrostatic electron phase space holes form. The competition of several smaller filaments, which grow simultaneously with the large filament, also perturbs the balance between the electrostatic and magnetic fields. The oscillations are damped but the final electric field amplitude is still determined by the MPGF.Comment: 14 pages, 10 plots, accepted for publication in Physica Script

PIC Simulations of the Temperature Anisotropy-Driven Weibel Instability: Analyzing the perpendicular mode

An instability driven by the thermal anisotropy of a single electron species is investigated in a 2D particle-in-cell (PIC) simulation. This instability is the one considered by Weibel and it differs from the beam driven filamentation instability. A comparison of the simulation results with analytic theory provides similar exponential growth rates of the magnetic field during the linear growth phase of the instability. We observe in accordance with previous works the growth of electric fields during the saturation phase of the instability. Some components of this electric field are not accounted for by the linearized theory. A single-fluid-based theory is used to determine the source of this nonlinear electric field. It is demonstrated that the magnetic stress tensor, which vanishes in a 1D geometry, is more important in this 2-dimensional model used here. The electric field grows to an amplitude, which yields a force on the electrons that is comparable to the magnetic one. The peak energy density of each magnetic field component in the simulation plane agrees with previous estimates. Eddy currents develop, which let the amplitude of the third magnetic field component grow, which is not observed in a 1D simulation.Comment: accepted by Plasma Physics and Controlled Fusio

Spatial self-structuring accelerates adaptive speciation in sexual populations

Questions: How does spatial self-structuring influence the waiting time until adaptive speciation in a population with sexual reproduction? Which mechanisms underlie this effect? Model: Using a spatially explicit individual-based multi-locus model of adaptive speciation, we investigate the evolution of a sexually reproducing population, with different levels of spatial self-structuring induced by different distances of natal dispersal. We analyze how waiting times until speciation are affected by the mobility of individuals, the number of loci determining the phenotype under disruptive selection, and the mating costs for individuals preferring rare phenotypes. Conclusions: Spatial self-structuring facilitates the evolution of assortative mating and accelerates adaptive speciation. We identify three mechanisms that are responsible for this effect: (i) spatial self-structuring promotes the evolution of assortativity by providing assortative mating "for free," as individuals find phenotypically similar mates within their spatial clusters; (ii) it helps assortatively mating individuals with rare phenotypes to find mating partners even when the selected phenotype is determined by a large number of loci, so that strict assortativity is difficult; and (iii) it renders speciation less sensitive to costs of assortative mating, especially for individuals preferring rare phenotypes

The filamentation instability driven by warm electron beams: Statistics and electric field generation

The filamentation instability of counterpropagating symmetric beams of electrons is examined with 1D and 2D particle-in-cell (PIC) simulations, which are oriented orthogonally to the beam velocity vector. The beams are uniform, warm and their relative speed is mildly relativistic. The dynamics of the filaments is examined in 2D and it is confirmed that their characteristic size increases linearly in time. Currents orthogonal to the beam velocity vector are driven through the magnetic and electric fields in the simulation plane. The fields are tied to the filament boundaries and the scale size of the flow-aligned and the perpendicular currents are thus equal. It is confirmed that the electrostatic and the magnetic forces are equally important, when the filamentation instability saturates in 1D. Their balance is apparently the saturation mechanism of the filamentation instability for our initial conditions. The electric force is relatively weaker but not negligible in the 2D simulation, where the electron temperature is set higher to reduce the computational cost. The magnetic pressure gradient is the principal source of the electrostatic field, when and after the instability saturates in the 1D simulation and in the 2D simulation.Comment: 10 pages, 6 figures, accepted by the Plasma Physics and Controlled Fusion (Special Issue EPS 2009

Implications of Habitat Choice for Protected Polymorphysms

In this paper we reexamine how heterogeneous heterogeneous environments can enable protected polymorphisms. Building on the classical models by Levene and Dempster of dispersal and selection in two habitats, we systematically investigate how the maintenance of polymorphisms is affected by (i) local versus global density regulation and (ii) constant versus variable output from habitats to the next generation. We show that, for populations capable of habitat choice, a third independent and fundamental class of models needs to be considered. It is characterized by local density regulation (like Levene's model) and variable habitat output (like Dempster's model). Our results indicate that the conditions determining whether a system allows for protected polymorphisms qualitatively differ in the presence and absence of matching habitat choice (which occurs when individuals prefer the habitat to which they are best adapted). Without such habitat choice, the salient distinction is not between local or global density regulation but rather between constant or variable habitat output. With matching habitat choice this situation is reversed. Analysis of the third class of models introduced here suggests that the joint evolution of matching habitat choice and local- adaptation polymorphism is easier than was previously understood

Does Density-Dependent Individual Growth Simplify Dynamics in Age-Structured Populations? A General Model Applied to Perch, "Perca fluviatilis"

Availability of resources is a limiting factor for many populations. Diminished resource availability due to intraspecific competition is expected to decrease the annual growth increments of individuals. We study an age- structured population model for individuals with indeterminate growth and annual reproduction; parameters of the model are chosen to characterize a population with life history similar to the Eurasian perch. Different variants of this model are analyzed, all of which have a potential for exhibiting non-equilibrium population fluctuations. We demonstrate that by incorporating density-dependent individual growth into these models changes the dynamics of these populations by damping or even eradicating fluctuations in abundance and biomass. This finding offers an explanation for the observed stable dynamics of unperturbed perch populations. Further, density-dependent individual growth may also be a significant factor for contributing to the conspicuous empirical rarity of non-equilibrium population dynamics in general

Ecosystem Vulnerability to Species Loss

Species losses have always occurred as a natural phenomenon, but the pace at which species are going extinct has recently accelerated dramatically as a result of human activities. The disappearance of a species can have far-reaching and often unexpected consequences for other species, since changes can propagate throughout ecosystems. Hence, the following questions arise: - How does the collapse of one ecosystem compartment (species or functional groups) influence the remaining ecosystem compartments? - How is an ecosystem.s structure related to its vulnerability to compartment collapses

Ecological flow analysis of network collapse II: Indicators of ecosystem level vulnerability

Using donor-controlled, bottom-up equations to describe network collapse we systematically investigate the impact each species has on the survival or extinction of other species. Short of extinction, one can determine the integrated losses experienced by the ecosystem. These losses are aggregated into system level indicators, such as entropy, average gain/loss, average time to extinction, etc. The methodology is applied to 18 ecological flow networks available in the literature. We calculate the correlations between various indicators and determine high positive correlation between: number of nodes & maximal trophic level; connectedness & average entropy losses; number of nodes & average number of extinct nodes; and, maximum trophic level & evenness of links. A high negative correlation was found between: number of nodes & connectedness; connectedness & maximal trophic level; maximum tropic level & average entropy loss; and, connectedness & evenness of flows. Lastly, a low correlation was found between: average number of extinct compartments & evenness of flows; number of nodes & evenness of stocks; and, evenness of flows & evenness of stocks

Ecological flow analysis of network collapse I: New methodology to investigate network collapse dynamics

This research builds on standard ecological network analysis techniques in order to investigate the impact of removing species (nodes) on the remaining of the network species. The flow network is expressed as a system of dynamical equations such that the removal of one node has time-forward impacts on the remaining nodes. The approach allows one to determine the gain or loss experienced by each other compartment in the model and the time for such impact to occur. The general methodology is demonstrated on the Cone Spring Ecosystem. These results indicate that collapse of certain species exert more control on the overall network organization. We also investigate model sensitivity to determine discount rate robustness and discuss further research