1,720,973 research outputs found
Modelling stellar coronal magnetic fields
Get PDFAbstractOur understanding of the structure and dynamics of stellar coronae has changed dramatically with the availability of surface maps of both star spots and also magnetic field vectors. Magnetic field extrapolations from these surface maps reveal surprising coronal structures for stars whose masses and hence internal structures and dynamo modes may be very different from that of the Sun. Crucial factors are the fraction of open magnetic flux (which determines the spin-down rate for the star as it ages) and the location and plasma density of closed-field regions, which determine the X-ray and radio emission properties. There has been recent progress in modelling stellar coronae, in particular the relative contributions of the field detected in the bright surface regions and the field that may be hidden in the dark star spots. For the Sun, the relationship between the field in the spots and the large scale field is well studied over the solar cycle. It appears, however, that other stars can show a very different relationship.</jats:p
MHD numerical simulations of Perseus A: formation of filaments and magnetic loops
No full textThe Perseus Cluster (A426) is the brightest cluster of galaxies observed in X-rays in the sky and its giant central galaxy (NGG1275) hosts the extended double radio source 3C84. There is a spectacular B.-alph-a nebulosity surrounding NGC1275 with loops and filaments that. are probably magnetized and extend over 100 kpc. The continuous blowing of bubbles leading to the propagation of shock fronts is also evident and more recently, outflow and infall velocities of several 1000 km/s have been detected associated to the surrounding filaments. We here present preliminary results of 2.5D MHD simulations of the Perseus cluster central region assuming that the production of the outflow structures and loops that arise from the surface of NGC1275 are due to turbulent injection triggered by recent star formation and SNe activity in the galaxy. This is in turn, probably induced by a, continuous gas infall from the satellite galaxies around NGC1275. Our simulations which include both, the turbulent gas outflow and gas infall from the surroundings, have revealed a, continuous formation of the observed features, like the filaments, the gigantic magnetic loops and weak shock fronts that propagate into the ICM medium with the observed velocities of 1000-5000 km/s. After 10 Myr, a, nearly steady state is established between the outflow material emerging from the central galaxy and the inflow gas from the surrounds. The outflow activity seems to retard the cannibalism action of the central galaxy over the surrounding galaxies. This result may have important implications over the evolution of the whole cluster as it seems to indicate that the SF and SNe production that are induced by the cannibalism may help to decelerate the later due to turbulence and outflow production. These results also offer important clues to the hot halo formation in the center of the cluster and in the suppression of cooling flows.</p
Evolution of magnetic fields in the IGM: kinetic MHD turbulence
No full textIn this work, we present 3D MHD simulations of non-helical, forced turbulence, with an anisotropic thermal pressure with respect to the orientation of the local magnetic field. Such anisotropy arises when the plasma, is weakly collisional, i.e., when the Larmor frequency is much greater than the ion-ion collision frequency. In this Kinetic MHD regime (KMHD), there are instabilities that give rise to fast; growing magnetic fluctuations in the smallest scales. The plasma that; fills the intergalactic and intracluster media has small density (n similar to 10(-3) cm(-3)), hence the effects of these instabilities could be important in the turbulent amplification of the magnetic fields there. In order to study the KMHD) turbulence, we have performed 3D numerical simulations employing a godunov-MHD) code (e.g., Kowal, Lazarian & Beresnyak 2007; Falceta-Goncalves, Lazarian & Kowal 2008). The power spectrum of the velocity and magnetic fields were calculated for two cases: when there is a pre-existing mean magnetic field, and when there is only an initial weak magnetic field.</p
Magnetic coronae of active main-sequence stars
No full textThe coronal structure of main sequence stars continues to puzzle us. While the solar corona is relatively well understood, it has become clear that even stars of the same mass as the Sun can display very non-solar coronal behaviour, particularly if they are rapid rotators or in a binary system. At masses greater than and also less than that of the Sun, the non-solar internal structure appears to affect both the geometry and dynamics of the stellar corona and the nature of the X-ray and radio emission. In this talk I will describe some recent advances in our understanding of the structure of the coronae of some of the most active (and interesting) main sequence stars.</p
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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