1,721,131 research outputs found
Electrostatic coherent structures generation by local heating in a collisionless plasma
No full textWe propose a new mechanism for the generation of electrostatic coherent structures and Langmuir fluctuations in a stratified plasma. The model is base on open boundary 1D Vlasov simulations where heated electrons are injected by applying a temporal continuous periodic modification of the width of the electron distribution function at one boundary of the simulation box. To our knowledge, that is the first time that a "localized heating" mechanism is studied as a driver for the development of bi(multi)polar electric field with their associated electron density holes and vortices in the phase space. After the development of many electrostatic coherent structures, the injected fast particles are free to move along their ballistic limit, while electrons moving at nearly, or less than, the thermal speed are diffused in phase space by the local fields. (c) 2007 Elsevier B.V. All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The detection of photospheric impacts from chromospheric impulsive events
No full textMultiwavelength solar observations were carried out at the THEMIS telescope in order to investigate the propagation of the plasma jets during and after a flare occurrence. The data obtained in the Hα, Na I D2, and Fe I 557.6 nm lines show that the perturbations detected in the higher layers due to a B-class flare do penetrate down to the photosphere. The observational evidence of such perturbations is crucial for the identification of the source of the solar oscillations
Low energy Langmuir cavitons: the asymptotic limit of weak turbulence
No full textLangmuir turbulence is an archetype of wave turbulence in plasma physics. By means of 1D-1V Vlasov-Poisson simulations, we show that coherent structures, called Langmuir cavitons, are generated by the long-time evolution of Langmuir weak-turbulence, thus illustrating the breakdown of a weak-turbulence regime. These structures correspond to an equilibrium between the pressure forces and the ponderomotive force resulting from high-frequency Langmuir oscillations. Langmuir cavitons are typical features of strong Langmuir turbulence expected to be generated at high energy and to saturate when Langmuir energy is of the order of the plasma thermal energy. Despite this wide-spread belief, here we observe that cavitons, emerging from weak Langmuir turbulence evolution, saturate at much lower energies. We show that these Langmuir coherent structures are characterized by a much larger length scale with respect to the Debye length. This gives evidence that “large” and “shallow” stable cavitons should be seen in space plasma observations. The transition toward strong turbulence is shown to be a consequence of an initial weak turbulent inverse cascade. Finally, the effective equation of state for ion acoustic oscillations is tested numerically from the kinetic model
Vlasov-Poisson simulations of electrostatic parametric instability for localized Langmuir wave packets in the solar wind
No full textRecent observation of large-amplitude Langmuir waveforms during a type III event in the solar wind has been interpreted as the signature of the electrostatic decay of beam-driven Langmuir waves. This mechanism is thought to be a first step to explain the generation of type III radio emission. The threshold for this parametric instability in the typical solar wind condition has been investigated through 1D-1V Vlasov-Poisson simulations. We show that the amplitude of the observed Langmuir beatlike waveforms is of the order of the effective threshold computed from the simulations. The expected levels of associated ion acoustic density fluctuations have also been computed for comparison with observations
Price of anarchy and price of stability in multi-agent project scheduling
No full textWe consider a project scheduling environment in which the activities are partitioned among a set of agents. The owner of each activity can decide its length, which is linearly related to its cost within a minimum (crash) and a maximum (normal) length. For each day the project makespan is reduced with respect to its normal value, a reward is offered to the agents, and each agent receives a given ratio of the reward. As in classical game theory, we assume that the agents’ parameters are common knowledge. We study the Nash equilibria of the corresponding non-cooperative game as a desired state where no agent is motivated to change his/her decision. Regarding project makespan as an overall measure of efficiency, here we consider the worst and the best Nash equilibria (i.e., for which makespan is maximum and, respectively, minimum among Nash equilibria). We show that the problem of finding the worst Nash equilibrium is NP-hard (finding the best Nash equilibrium is already known to be strongly NP-hard), and propose an ILP formulation for its computation. We then investigate the values of the price of anarchy and the price of stability in a large sample of realistic size problems and get useful insights for the project owner
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