Evolution of a Langmuir wave in a weakly inhomogeneous plasma in a longitudinal electric field

The spatial evolution of a Langmuir wave excited by external sources in a weakly inhomogeneous electron plasma in a longitudinal electrostatic field is considered. It is shown that, in a longitudinal electrostatic field, a Langmuir wave can only be amplified in an inhomogeneous plasma provided that the current of trapped electrons exceeds that of untrapped electrons. In this case, as the wave propagates through the inhomogeneous region where its phase velocity increases, some untrapped electrons become trapped in the wave potential wells. As a result, the current of trapped electrons increases and the wave is amplified. Moreover, in the regions where the bulk electrons are localized, the minima of the wave are amplified to a greater extent than its maxima.     

Decay instability of a laser pulse propagating in a transverse direction in a magnetized plasma

The decay instability of a lser pulse propagating across an external magnetic field in a subscritical plasma is investigated analytically and numerically. It is shown that, when the relaxation of the pulse is taken into account, the hydrodynamic growth rate of the decay instability is slower than that obtained earlier in the constant-amplitude pump wave approximation. The results of numerical simulations by a particle-in-cell method demonstrate that an increase in the amplitude of the parametrically excited waves is accompanied by a decrease in their group velocity; in this case, up to 85% of the laser energy is converted into the energy of the plasma particles. It is found that, under resonance conditions, the magnetic field acts to increase the energy of the accelerated ions that escape from the plasma slab through its front boundary.     

Three-body collisions in a plasma in a strong laser field

Electron-ion collisions in a high-density plasma in strong electromagnetic fields are considered. The applicability condition for the approximate model of pair collisions in strong fields are determined. It is shown that this condition is identical to the condition for the plasma to be transparent. Investigations were carried out by the test particle method generalized to the case of several scattering centers. An accurate calculation of short-range collisions is provided by a “jump” method that is based on the exact solution to the problem of the motion of a particle in a Coulomb potential. This method can also be applied in other approaches to simulating a collisional plasma (such as particle-in-cell and molecular dynamics methods).     

Ion acceleration in a dipole vortex in a laser plasma corona

Particle-in-cell simulations show that the inhomogeneity scale of the plasma produced in the interaction of high-power laser radiation with gas targets is of fundamental importance for ion acceleration. In a plasma slab with sharp boundaries, the quasistatic magnetic field and the associated electron vortex structure produced by fast electron beams both expand along the slab boundary in a direction perpendicular to the plasma density gradient, forming an extended region with a quasistatic electric field, in which the ions are accelerated. In a plasma with a smooth density distribution, the dipole magnetic field can propagate toward the lower plasma density in the propagation direction of the laser pulse. In this case, the electron density in an electric current filament at the axis of the magnetic dipole decreases to values at which the charge quasineutrality condition fails to hold. In electric fields generated by this process, the ions are accelerated to energies substantially higher than those characteristic of plasma configurations with sharp boundaries.     

Chaos in a seasonally perturbed SIR model: avian influenza in a seabird colony as a paradigm

Seasonality is a complex force in nature that affects multiple processes in wild animal populations. In particular, seasonal variations in demographic processes may considerably affect the persistence of a pathogen in these populations. Furthermore, it has been long observed in computer simulations that under seasonal perturbations, a host–pathogen system can exhibit complex dynamics, including the transition to chaos, as the magnitude of the seasonal perturbation increases. In this paper, we develop a seasonally perturbed Susceptible-Infected-Recovered model of avian influenza in a seabird colony. Numerical simulations of the model give rise to chaotic recurrent epidemics for parameters that reflect the ecology of avian influenza in a seabird population, thereby providing a case study for chaos in a host– pathogen system. We give a computer-assisted exposition of the existence of chaos in the model using methods that are based on the concept of topological hyperbolicity. Our approach elucidates the geometry of the chaos in the phase space of the model, thereby offering a mechanism for the persistence of the infection. Finally, the methods described in this paper may be immediately extended to other infections and hosts, including humans.     

Forging a biodiversity ethic in a multicultural context

New Zealand belongs to the Pacific region, a part of the world where human impacts have been both very recent and extreme in their effect. The New Zealand natural environment is rich in endemic taxa, but these are poorly equipped to cope with the effects of invasion by humans and exotic animals and plants. Polynesian immigrants brought to New Zealand a distinctive world view which gave rise to both tribal traditions and living traditions of the Maori. The resultant environmental ethic emphasises guardianship and stewardship, establishment of the right to use a resource, kinship obligations, and a balance between pairs of opposites. Nineteenth-century European colonists were ambivalent in their view of the environment, although a world view which emphasises dominion has tended to dominate. Two recent developments which are important factors in development of a multicultural biodiversity ethic are the enactment of the Resource Management Act 1991 and legal recognition of the principles of the Treaty of Waitangi. The intersection of these developments provides an opportunity to develop a new approach to environmental ethics especially in conceptualising significance, consultative processes, and developing a holistic and ecocentric use of resources.     

Superdiffusion in a Model for Diffusion in a Molecularly Crowded Environment

We present a model for diffusion in a molecularly crowded environment. The model consists of random barriers in a percolation network. Random walks in the presence of slowly moving barriers show normal diffusion for long times but anomalous diffusion at intermediate times. The effective exponents for square distance vs time usually are below one at these intermediate times, but they can also be larger than one for high barrier concentrations. Thus, we observe sub- and superdiffusion in a crowded environment.     

Prostatectomy in a county hospital; a review of 677 cases in a six-year period

Transurethral procedures were used in 620 of 677 cases in which prostatectomy was done (principally by residents supervised by a urologist) at a county hospital in a six-year period. Open operations were used in the other 57 cases. Results were classified as "excellent" in 46.8 per cent of the transurethral cases and as "good" in 36.3 per cent.     

LCA of a tomato crop in a multi-tunnel greenhouse in Almeria

Purpose

Protected crops have expanded significantly in the Mediterranean area over the last few decades as a successful means to provide abundant and high-quality produce. Although resources are generally used efficiently, greenhouse areas cause major environmental impacts. The aim of this work was to study, from an environmental point of view, the improvement capacity of greenhouse areas in the Mediterranean region and to assess several alternative agricultural practices to decrease their contribution to the environmental impacts in this system.

Materials and methods

The methodology used was life cycle assessment (LCA) based on a tomato crop grown in a multi-tunnel greenhouse in Almeria, on the southeast coast of Spain. The functional unit chosen was 1 ton of loose classic tomatoes. Five midpoint impact categories and one energy flow indicator were selected for their relevance. The agricultural practice alternatives evaluated were reduction of volume of substrate and fertilizers, extension of substrate and greenhouse life span and increase in renewable energy for electricity production.

Results and discussion

The results indicated that the main contributors to impact categories in the tomato production were structure, auxiliary equipment and fertilizers. Structure accounted for between 30 and 48?% of the contributions, depending on the impact category. The principal burdens in the auxiliary equipment stage were substrate and consumption of electricity. Fertilizers environmental impacts were due to emissions during their manufacture and application to the crop. In a best-case option, taking into account the best alternatives, contributions to the impact categories were reduced by between 17 and 30?%. The LCA methodology proved to be a useful tool to evaluate the environmental damage of this agricultural activity. The importance of including farm infrastructure in the assessment was demonstrated as it was a major contributor. The risk of eutrophication could be reduced by adjustment of the fertilizers?Cwater balance and implementation of a closed-loop irrigation system. Future technological improvements should be developed to increase yields and thereby directly reduce the environmental burdens per unit produce.

Conclusions

The present study served to assess the environmental impacts of a tomato crop in a multi-tunnel greenhouse on the coast of Almeria. The assessment was used to evaluate alternatives for improvement of cleaner production in greenhouse areas. Further research should focus on assuring the feasibility of the suggested options.     

Trophic interactions in a shallow lake following a reduction in nutrient loading: a long-term study

After the diversion of a nutrient-rich inflow, the eutrophic lake, Alderfen Broad, initially showed reduced total phosphorus concentrations and phytoplankton populations, clear water and the establishment of submerged macrophytes. Internal P loading then increased, perhaps stimulated by the senescence of submerged macrophytes and exacerbated by the lack of flushing. Cyanophytes appeared in the summer of two years. As a consequence of poor recruitment of roach (Rutilus rutilus (L.)), the chief zooplanktivore, and a summerkill of the fish population, populations of large-bodied Cladocera (Daphnia hyalina/ longispina and ultimately D. magna) developed. In the long-term, these may have limited the further development of phytoplankton populations and clear water and submerged macrophytes returned. During this latter period, internal P release has remained high (> 380 µg l^-1), thereby indicating the scope for biomanipulation even in eutrophic conditions. However, isolation of the lake has led to a decrease in water level (which through increased temperatures and lowered dissolved oxygen levels was probably responsible for the fish deaths) and further concentration of internal P load. Sediment is now being removed to reestablish greater water depth.     

Testing a Primary and a Secondary Endpoint in a Group Sequential Design

Summary We consider a clinical trial with a primary and a secondary endpoint where the secondary endpoint is tested only if the primary endpoint is significant. The trial uses a group sequential procedure with two stages. The familywise error rate (FWER) of falsely concluding significance on either endpoint is to be controlled at a nominal level α. The type I error rate for the primary endpoint is controlled by choosing any α‐level stopping boundary, e.g., the standard O'Brien–Fleming or the Pocock boundary. Given any particular α‐level boundary for the primary endpoint, we study the problem of determining the boundary for the secondary endpoint to control the FWER. We study this FWER analytically and numerically and find that it is maximized when the correlation coefficient ρ between the two endpoints equals 1. For the four combinations consisting of O'Brien–Fleming and Pocock boundaries for the primary and secondary endpoints, the critical constants required to control the FWER are computed for different values of ρ. An ad hoc boundary is proposed for the secondary endpoint to address a practical concern that may be at issue in some applications. Numerical studies indicate that the O'Brien–Fleming boundary for the primary endpoint and the Pocock boundary for the secondary endpoint generally gives the best primary as well as secondary power performance. The Pocock boundary may be replaced by the ad hoc boundary for the secondary endpoint with a very little loss of secondary power if the practical concern is at issue. A clinical trial example is given to illustrate the methods.     

Effect of a change in step direction from a forward to a lateral target in response to a sensory perturbation

The aim of the current study was to investigate the response of healthy older and young adults to a change in step direction from a forward to a lateral target in response to a sensory perturbation. Nine healthy older (75.1 ± 6.7 years; age range, 65–81 years) and nine young adults (27 ± 3.6 years; age range, 23–31 years) participated in the study. The sensory perturbation was a visual cue presented at random times while subjects stepped over an obstacle from quiet stance. For both young and elderly subjects there was an abrupt change in the slope of both shear ground reaction forces (GRFs) of the stance limb following the perturbation. The slope and peak of the change in GRFs was greater for the young subjects and the onset significantly earlier (205 ms compared to 271 ms). Changes in the GRFs were accompanied by an increase in bilateral gluteus medius and stance limb soleus activity. A late visual cue resulted in a delayed response for elderly subjects. These data show that a stepping response to a sensory perturbation was both delayed and of less magnitude for older adults which has implications for fall risk.     

Multiple mutations in a specific gene in a small population

Hereditary diseases have been reported with relatively high frequency in some small populations. Founder effect and genetic drift, associated or not with selective advantage of heterozygotes in case of recessive diseases, are the main explanations. Therefore, if we consider one population and one particular disease, only one deleterious allele should be observed.Determination of mutations has shown that in most cases the situation is more complex; more than one mutation is found among the patients. This finding can be explained by a multiple founder effect, with genetic drift and new mutations.     

Chaos and synchrony in a model of a hypercolumn in visual cortex

Neurons in cortical slices emit spikes or bursts of spikes regularly in response to a suprathreshold current injection. This behavior is in marked contrast to the behavior of cortical neurons in vivo, whose response to electrical or sensory input displays a strong degree of irregularity. Correlation measurements show a significant degree of synchrony in the temporal fluctuations of neuronal activities in cortex. We explore the hypothesis that these phenomena are the result of the synchronized chaos generated by the deterministic dynamics of local cortical networks. A model of a hypercolumn in the visual cortex is studied. It consists of two populations of neurons, one inhibitory and one excitatory. The dynamics of the neurons is based on a Hodgkin-Huxley type model of excitable voltage-clamped cells with several cellular and synaptic conductances. A slow potassium current is included in the dynamics of the excitatory population to reproduce the observed adaptation of the spike trains emitted by these neurons. The pattern of connectivity has a spatial structure which is correlated with the internal organization of hypercolumns in orientation columns. Numerical simulations of the model show that in an appropriate parameter range, the network settles in a synchronous chaotic state, characterized by a strong temporal variability of the neural activity which is correlated across the hypercolumn. Strong inhibitory feedback is essential for the stabilization of this state. These results show that the cooperative dynamics of large neuronal networks are capable of generating variability and synchrony similar to those observed in cortex. Auto-correlation and cross-correlation functions of neuronal spike trains are computed, and their temporal and spatial features are analyzed. In other parameter regimes, the network exhibits two additional states: synchronized oscillations and an asynchronous state. We use our model to study cortical mechanisms for orientation selectivity. It is shown that in a suitable parameter regime, when the input is not oriented, the network has a continuum of states, each representing an inhomogeneous population activity which is peaked at one of the orientation columns. As a result, when a weakly oriented input stimulates the network, it yields a sharp orientation tuning. The properties of the network in this regime, including the appearance of virtual rotations and broad stimulus-dependent cross-correlations, are investigated. The results agree with the predictions of the mean field theory which was previously derived for a simplified model of stochastic, two-state neurons. The relation between the results of the model and experiments in visual cortex are discussed.     

Fixation probability for a beneficial allele and a mutant strategy in a linear game under weak selection in a finite island model

The effect of population structure on the probability of fixation of a newly introduced mutant under weak selection is studied using a coalescent approach. Wright's island model in a framework of a finite number of demes is assumed and two selection regimes are considered: a beneficial allele model and a linear game among offspring. A first-order approximation of the fixation probability for a single mutant with respect to the intensity of selection is deduced. The approximation requires the calculation of expected coalescence times, under neutrality, for lineages starting from two or three sampled individuals. The results are obtained in a general setting without assumptions on the number of demes, the deme size or the migration rate, which allows for simultaneous coalescence or migration events in the genealogy of the sampled individuals. Comparisons are made with limit cases as the deme size or the number of demes goes to infinity or the migration rate goes to zero for which a diffusion approximation approach is possible. Conditions for selection to favor a mutant strategy replacing a resident strategy in the context of a linear game in a finite island population are addressed.     

Virulence evolution in a virus obeys a trade-off

The evolution of virulence was studied in a virus subjected to alternating episodes of vertical and horizontal transmission. Bacteriophage f1 was used as the parasite because it establishes a debilitating but non-fatal infection that can be transmitted vertically (from a host to its progeny) as well as horizontally (infection of new hosts). Horizontal transmission was required of all phage at specific intervals, but was prevented otherwise. Each episode of horizontal transmission was followed by an interval of obligate vertical transmission, followed by an interval of obligate horizontal transmission etc. The duration of vertical transmission was eight times longer per episode in one treatment than in the other, thus varying the relative intensity of selection against virulence while maintaining selection for some level of virus production. Viral lines with the higher enforced rate of infectious transmission evolved higher virulence and higher rates of virus production. These results support the trade-off model for the evolution of virulence.     

Dynamics of a mutualism in a multi-species context

Despite recent findings that mutualistic interactions between two species may be greatly affected by species external to the mutualism, the implications of such multi-species interactions for the population dynamics of the mutualists are virtually unexplored. In this paper, we ask how the mutualism between the shoot-base boring weevil Apion onopordi and the rust fungus Puccinia punctiformis is influenced by the dynamics of their shared host plant Cirsium arvense, and vice versa. In particular, we hypothesized that the distribution of the weevil's egg load between healthy and rust-infected thistles may regulate the abundance of the mutualists and their host plant. In contrast to our expectations we found that the dynamics of the mutualists are largely determined by the dynamics of their host. This is, to our knowledge, the first demonstration that the dynamics of a mutualism are driven by a third, non-mutualistic species.     

Hypoluteoidism in a bitch

Hypoluteoidism is characterized by insufficient secretion of progesterone by the corpora lutea during pregnancy. The resulting failure to maintain progesterone concentration above a critical level presumably could lead to fetal resorbtion as well as frank abortion. This report concerns a 2.5-year-old Bernese Mountain dog with a history of two previous pregnancies ending in abortion around Day 50 of pregnancy. The bitch was initially presented 2 days after mating. Physical and gynecological examination revealed no abnormalities. The infectious causes of abortion in the bitch, Brucella canis and herpesvirus, were excluded using serology. On Day 26 after mating, ultrasonography confirmed a pregnancy with at least four living fetuses. During the remaining part of the pregnancy repeated transabdominal ultrasonography and plasma progesterone measurements, using a RIA, were performed. On Day 42, ultrasonography revealed living fetuses but plasma progesterone concentration had decreased to 8.3 nmol/L, which is just above the threshold necessary to maintain a vital pregnancy. Oral treatment with 0.1mg medroxyprogesterone acetate (MPA) per kg body weight, once daily, was started and continued until Day 58 in order to prevent abortion due to progesterone deficiency. The endogenous plasma progesterone concentration decreased further, but pregnancy was maintained. On Day 59 a cesarean section was performed because of dystocia and four living and one dead pup were delivered. One puppy had severe facial malformations and was euthanised. The premature decrease in plasma progesterone concentration while ultrasonography demonstrated that the fetuses were still alive, and the maintenance of pregnancy during administration of MPA, strongly support the diagnosis of hypoluteoidism.     

The phosphorus-chlorophyll a relationship in periphytic communities in a controlled ecosystem

The correlation between total dissolved phosphorus input concentrations and periphytic chlorophyll a concentrations was examined after a four-week colonization period on artificial substrates in large, continuous-flow microcosms. A significant correlation was established; the data produced a linear regression and substantiated a recently proposed curvilinear regression analysis. The curvilinear equation can be used in future microcosm studies as a predictive aid for regulating phosphorus input concentrations.     

Dynamics of a vortex filament in a stratified medium

The behavior of a vortex filament in a perfectly conducting stratified medium is analyzed. It is shown that the equation describing oscillations of a straight filament is linear, but becomes substantially non-linear with increasing inclination angle. Effects related to the finite radius of the vortex core are considered, and dispersion relations for linear oscillations of a vortex column are derived.