From Small-Scale Dynamo to Isotropic MHD Turbulence

We consider the problem of incompressible, forced, nonhelical, homogeneous, isotropic MHD turbulence with no mean magnetic field. This problem is essentially different from the case with externally imposed uniform mean field. There is no scale-by-scale equipartition between magnetic and kinetic energies as would be the case for the Alfven-wave turbulence. The isotropic MHD turbulence is the end state of the turbulent dynamo which generates folded fields with small-scale direction reversals. We propose that the statistics seen in numerical simulations of isotropic MHD turbulence could be explained as a superposition of these folded fields and Alfven-like waves that propagate along the folds.Comment: kluwer latex, 7 pages, 7 figures; Proceedings of the International Workshop "Magnetic Fields and Star Formation: Theory vs. Observations", Madrid, 21-25 April 2003 -- published version (but the e-print is free of numerous typos introduced by the publisher

Seasonal Changes in Feeding Success, Activity Patterns, and Weights of Nonbreeding Sanderlings (Calidris alba)

Seasonal changes in Sanderling (Calidris alba) feeding success, time budgets, and weights were followed throughout the nonbreeding season at Bodega Bay, California. Sanderlings spent more time roosting in fall than in winter, and in fall adults spent more time roosting than juveniles. Sanderling prey capture rates were high in fall and spring and declined through winter. Sanderling weights paralleled seasonal changes in feeding success and activity patterns: birds were heaviest in fall and spring and lightest in winter. These results reflect lowered food availability in winter and imply that birds may have difficulty balancing their energy budgets during part of the nonbreeding season

Habitat-Specific Impacts of Multiple Consumers on Plant Population Dynamics

Multiple consumers often attack seeds, seedlings, and adult plants, but their population-level consequences remain uncertain. We examined how insect and small mammal consumers influenced the demography and abundance of the perennial shrub, bush lupine (Lupinus arboreus). In grassland and dune habitats we established replicate experimental lupine populations in 81-m2 plots that were either protected from, or exposed to, herbivorous voles and granivorous mice (via fencing) and/or root feeding insects (via insecticide treatment). Populations were initiated with transplanted seedlings in 1999 and 2000. We followed the demography of these cohorts, subsequent generations, and the seed bank for 5.5 years. Voles and insects killed many seedlings in dune (1999 only) and grassland (1999 and 2000) habitats. After 2000, insects and voles had minimal effects on seedling or adult survival. Seed predation by granivorous mice, however, greatly depressed seedling recruitment, resulting in lower adult lupine abundance in control plots vs. those protected from rodents. In grasslands, initial effects of voles and insects on seedling survival produced large differences among treatments in adult plant density and the cumulative number of seeds produced throughout the experiment. Differences among grassland populations in seed rain, however, had little influence on the magnitude of seedling recruitment into this habitat. Instead, recruitment out of a preexisting seed bank compensated for the lack of seed production in populations exposed to consumers. Shading by dense adults in plots protected from consumers limited seedling establishment within these populations. Although differences among populations in cumulative seed rain did not influence adult establishment, populations protected from consumers accumulated substantially larger seed banks than controls. These results illustrate how density dependence, habitat-specific seed dynamics, and particular demographic impacts of consumers interact to shape plant population responses to consumers

Consumers Limit the Abundance and Dynamics of a Perennial Shrub with a Seed Bank

For nearly 30 years, ecologists have argued that predators of seeds and seedlings seldom have population-level effects on plants with persistent seed banks and density-dependent seedling survival. We parameterized stage-based population models that incorporated density dependence and seed dormancy with data from a 5.5-year experiment that quantified how granivorous mice and herbivorous voles influence bush lupine (Lupinus arboreus) demography. We asked how seed dormancy and density-dependent seedling survival mediate the impacts of these consumers in dune and grassland habitats. In dune habitat, mice reduced analytical lambda (the intrinsic rate of population growth) by 39%, the equilibrium number of above-ground plants by 90%, and the seed bank by 98%; voles had minimal effects. In adjacent grasslands, mice had minimal effects, but seedling herbivory by voles reduced analytical lambda by 15% and reduced both the equilibrium number of aboveground plants and dormant seeds by 63%. A bootstrap analysis demonstrated that these consumer effects were robust to parameter uncertainty. Our results demonstrate that the quantitative strengths of seed dormancy and density-dependent seedling survival-not their mere existence-critically mediate consumer effects. This study suggests that plant population dynamics and distribution may be more strongly influenced by consumers of seeds and seedlings than is currently recognized

Thermal conduction and particle transport in strong MHD turbulence, with application to galaxy-cluster plasmas

We investigate field-line separation in strong MHD turbulence analytically and with direct numerical simulations. We find that in the static-magnetic-field approximation the thermal conductivity in galaxy clusters is reduced by a factor of about 5-10 relative to the Spitzer thermal conductivity of a non-magnetized plasma. We also estimate how the thermal conductivity would be affected by efficient turbulent resistivity.Comment: Major revision: higher resolution simulations lead to significantly different conclusions. 26 pages, 10 figure

Acceleration of energetic particles by large-scale compressible magnetohydrodynamic turbulence

Fast particles diffusing along magnetic field lines in a turbulent plasma can diffuse through and then return to the same eddy many times before the eddy is randomized in the turbulent flow. This leads to an enhancement of particle acceleration by large-scale compressible turbulence relative to previous estimates in which isotropic particle diffusion is assumed.Comment: 13 pages, 3 figures, accepted for publication in Ap

Speciation in Golden-Plovers, Pluvialis dominica and P. fulva: Evidence from the Breeding Grounds

Two forms of golden-plover have long been considered subspecies, Pluvialisdominica dominica and P. d. fulva. Prior studies have shown differences between forms in breeding distributions, wintering distributions, plumage, morphology, molt, and maturation schedules. We report clear and consistent differences in breeding vocalizations and nesting habitat, and strict assortative mating in areas of sympatry in western Alaska. These results indicate a greater degree of differentiation between the forms than was previously appreciated. They are appropriately treated as separate species and should be referred to under the names Pluvialis dominica, for the American Golden-Plover, and Pluvialis fulva, for the Pacific Golden-Plover

Going to Extremes: Why Do Sanderlings Migrate to the Neotropics?

Sanderlings (Calidris alba) breed within a small latitudinal range in the arctic while spreading in winter virtually throughout temperate and tropical marine beaches of the world. This paper examines spatial variation in Sanderlings nonbreeding density across the New World, documents annual cycle differences between populations wintering in California and those wintering in Peru and Chile, and then explores demographic and ecological factors underlying Sanderlings migration to different wintering grounds. Densities during the nonbreeding season are higher on the Pacific coast than on the Atlantic at all censused latitudes in the New World, and reach a peak in southwestern Peru and northwestern Chile adjacent to the Humboldt Current. Populations wintering in California spend a larger fraction of the year on the wintering site than do those wintering in Peru and Chile. Adults replace primaries during prebasic molt in both regions, as do first-winter Sanderlings in Peru and Chile. First-winter birds in California do not molt primaries. Comparisons of weight and time-activity budgets near the northern and southern ends of the winter distributions along the Pacific coast of the western hemisphere indicate that resource conditions are more favorable for Sanderlings in the south

Structure of Small-Scale Magnetic Fields in the Kinematic Dynamo Theory

In the interstellar medium and protogalactic plasmas, the magnetic Prandtl number is very large, and the kinematic dynamo therefore produces a broad spectrum of growing magnetic fluctuations at small (subviscous) scales. The condition for the onset of nonlinear effects depends on the structure of the field lines. We study the statistical correlations that are set up in the field pattern and show that the magnetic-field lines possess a folding structure, where most of the scale decrease is due to the field variation across itself, while the scale of the field variation along itself stays approximately constant. Specifically, we find that, though both the magnetic energy and the mean square curvature of the field lines grow exponentially, the field strength and the field-line curvature are anticorrelated, i.e. the curved field is relatively weak, while the growing field is relatively flat. The detailed analysis of the statistics of the curvature shows that it possesses a stationary limiting distribution with the bulk located at the values of curvature comparable to the characteristic wave number of the velocity field and a power-like tail extending to large values of curvature where it is cut off by the resistive regularization. The growth of the curvature occurs in a small fraction of the total volume of the system, is due to the intermittent nature of the curvature distribution, and is limited only by the resistive cut-off. The implication of the folding effect is that the advent of the Lorentz back reaction occurs when the magnetic energy approaches that of the smallest turbulent eddies