1,721,008 research outputs found
A test electron model for the study of three dimensional magnetic reconnection effects
No full textUnderstanding the mechanisms that enable the conversion of the explosive release of magnetic energy into the electron energization that is experimentally observed in space and laboratory plasmas represents a long-standing question in the study of magnetic reconnection.
We present a test particle model able to follow the dynamics of the electron guiding centers in the presence of the electromagnetic fields characterizing a non-steady-state, three-dimensional magnetohydrodynamic (3D-MHD) description of magnetic reconnection. The resulting electron equations, based on a relativistic Hamiltonian formulation of the particle dynamics, have been implemented in a code that evolves the spatial position and the velocity of the electrons according to the fields obtained by the numerical solutions of a two-fluid reconnection model. By the calculation of the electron distribution function in the phase space, the electron kinetic moments can be obtained with a δf technique particularly suitable during the early stages of the reconnection process, which can be replaced by a full-f approach in the nonlinear reconnection phase. Thanks to its capability to reproduce the electron response in a quite realistic reconnection configuration, this numerical tool represents a new instrument for the investigation of the nonlinear electron dynamics in particularly complex reconnection regions, such as those characterized by magnetic field line stochasticity
Stability of a weakly collisional plasma with runaway electrons
Full text linkWe investigate the problem of the tearing stability of a post-disruption weakly
collisional plasma where the current is completely carried by runaway electrons. We adopt here
a two fluid model which takes into account also ion sound Larmor radius and electron inertia
effects in the description of the reconnection process. In the past, it has been demonstrated in
[Helander et al. Phys. Plasmas 14, 12, (2007)] that in the purely resistive regime the presence
of runaway electrons in plasma has a significant effect on the saturated magnetic island width.
In particular, runaway electrons generated during disruption can cause an increase of 50% in
the saturated magnetic island width with respect to the case with no runaway electrons. These
results were obtained adopting a periodic equilibrium magnetic field that limited the analysis to
small size saturated magnetic islands. Here we present our results to overcome this limitation
adopting a non-periodic Harris’ type equilibrium magnetic field. Preliminary results on the
effects of the ion sound Larmor radius effects will also be presented
Coexistence of Plasmoid and Kelvin-Helmholtz Instabilities in Collisionless Plasma Turbulence
Full text linkThe plasmoid formation in collisionless plasmas, where magnetic reconnection within turbulence may take place driven by the electron inertia, is analyzed. We find a complex situation in which, due to the presence of strong velocity shears, the typical plasmoid formation, observed to influence the energy cascade in the magnetohydrodynamic context, has to coexist with the Kelvin-Helmholtz (KH) instability. We find that the current density layers may undergo the plasmoid or the KH instability depending on the local values of the magnetic and velocity fields. The competition among these instabilities affects not only the evolution of the current sheets, that may generate plasmoid chains or KH-driven vortices, but also the energy cascade, that is different for the magnetic and kinetic spectra
Gyrofluid analysis of electron β e effects on collisionless reconnection
Full text linkThe linear and nonlinear evolutions of the tearing instability in a collisionless plasma with a strong guide field are analysed on the basis of a two-field Hamiltonian gyrofluid model. The model is valid for a low ion temperature and a finite. The finite effect implies a magnetic perturbation along the guide field direction, and electron finite Larmor radius effects. A Hamiltonian derivation of the model is presented. A new dispersion relation of the tearing instability is derived for the case and tested against numerical simulations. For the equilibrium electron temperature is seen to enhance the linear growth rate, whereas we observe a stabilizing role when electron finite Larmor radius effects become more relevant. In the nonlinear phase, stall phases and faster than exponential phases are observed, similarly to what occurs in the presence of ion finite Larmor radius effects. Energy transfers are analysed and the conservation laws associated with the Casimir invariants of the model are also discussed. Numerical simulations seem to indicate that finite effects do not produce qualitative modifications in the structures of the Lagrangian invariants associated with Casimirs of the model
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
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