1,721,008 research outputs found
Extreme Localized Geoelectric Fluctuations in Global Magnetospheric Simulations
No full textDriven by particle ejections from the Sun, magnetic storms create global geomagnetic field fluctuations which in turn produce geomagnetically induced currents (GICs) in ground-based systems. The space weather community has recently identified localized, short-lived high-magnitude geoelectric field enhancements that create large GICs over small spatial scales. Our goal is to determine whether the existing global magnetospheric modeling codes produce these localized geoelectric field enhancements. We examine the outputs of the SWMF, OpenGGCM, and LFM simulations of the 2003 Halloween storms. We then use the simulations to determine what physical processes are driving the local field enhancements. Preliminary results reveal that only the SWMF code reproduces the localizations. This difference is potentially due to its ionospheric conductance model, use of an adaptive grid, or lack of modeled dissipation. The physical driver of these enhancements is currently unclear
Fast Magnetic Reconnection Mediated by Plasmoid Instability
No full textMagnetic reconnection is generally believed to be the underlying mech- anism that powers explosive events in plasmas such as flares and substorms in the solar corona and sawtooth crashes in fusion plasmas. Firstly, this paper provides a fundamental description of the magnetic reconnection process by deriving the Sweet-Parker model. The original time-independent Sweet-Parker model assumes steady-state reconnection and neglects compressbility and plasma beta effects. In this paper, these effects will also be included and discussed using numerically sim- ulated data. Next, this paper discusses the nonlinear regime of plasmoid instability and shows that as a result of this instability, the system can realize a fast nonlinear reconnection rate
Magnetic Field Generation and Stability in High-Energy Density Plasmas
No full textThe generation of magnetic fields in plasmas, either astrophysical or in laser experiments, is not well understood. One proposed mechanism is the Biermann battery
effect, which goes with n p and is derived from a general form of Ohm's
law. This effect is derived from fluid mechanics approximations of plasmas, and has
not been determined from first principles kinematics. In this thesis, we perform a
particle-in-cell simulation of plasma particles and computationally examine this effect. We find that this effect does contribute to magnetic field. We further find that
the scalar pressure approximation does not fully cover the magnetic field generation
after early simulation times, and by replacing the pressure with the pressure tensor
achieve significantly better agreement. This thesis provides one of the first analyses
of this effect from kinematics
Shear dynamo, turbulence, and the magnetorotational instability
No full textThe formation, evolution, and detailed structure of accretion disks remain poorly understood, with wide implications across a variety of astrophysical disciplines. While the most pressing question -- what causes the high angular momentum fluxes that are necessary to explain observations? -- is nicely answered by the idea that the disk is turbulent, a more complete grasp of the fundamental processes is necessary to capture the wide variety of behaviors observed in the night sky. This thesis studies the turbulence in ionized accretion disks from a theoretical standpoint, in particular focusing on the generation of magnetic fields in these processes, known as dynamo. Such fields are expected to be enormously important, both by enabling the magnetorotational instability (which evolves into virulent turbulence), and through large-scale structure formation, which may transport angular momentum in different ways and be fundamental for the formation of jets.
The central result of this thesis is the suggestion of a new large-scale dynamo mechanism in shear flows -- the ``magnetic shear-current effect'' -- which relies on a positive feedback from small-scale magnetic fields. As well as being a very promising candidate for driving field generation in the central regions of accretion disks, this effect is interesting because small-scale magnetic fields have historically been considered to have a negative effect on the large-scale dynamo, damping growth and leading to dire predictions for final saturation amplitudes. Given that small-scale fields are ubiquitous in plasma turbulence above moderate Reynolds numbers, the finding that they could instead have a \emph{positive} effect in some situations is interesting from a theoretical and practical standpoint. The effect is studied using direct numerical simulation, analytic techniques, and novel statistical simulation methods.
In addition to the dynamo, much attention is given to the linear physics of disks and its relevance to turbulence. This is studied using nonmodal stability theory, which both provides a highly intuitive connection between global domains and the commonly studied shearing box, and suggests that transient linear growth can often be more important than spectral instability. These realizations motivate the use of the quasi-linear models that are applied extensively throughout the turbulence and dynamo studies later in the thesis
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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