1,720,970 research outputs found
Granular velocities of the Sun from speckle interferometry
No full textWe present observations of granular velocities and their relation to the granular intensity pattern from the disc center of the Sun. They were obtained in June 1997 with the Vacuum Tower Telescope at the Observatorio del Teide on Tenerife. High spatial resolution, 0." 4-0." 5 for the velocities, was achieved with speckle methods applied to two-dimensional narrow-band images in Na D-2 from the "Gottingen" Fabry-Perot interferometer. The velocity observations refer to a geometric height of 50-200 km (above tau(5) = 1). Velocity amplitudes of +/- 2.2 km s(-1) are seen. The high velocity regions are small-scale and the upflows coincide frequently with the bright borders of granules or with small-scale brightenings. A statistical analysis reveals only a rough consistency to the -5/3 or -17/3 law of log(power) vs. log(wavenumber) expected for isotropic turbulence (cf. Espagnet et al. 1993). We consider an agreement with such power laws as accidental since the intensity and velocity power spectra found here decrease smoothly from flat ones at low wavenumbers to steep ones at high wavenumbers. The coherence of velocity and intensity fluctuation stays above 0.5 up to horizontal wavenumbers k(h) approximate to 11 Mm(-1)(corresponds to 0." 8) and the phase difference between intensity and velocity stays stable down to structures of 0." 5. While the intensity pattern exhibits a clearly non-Gaussian distribution, the velocity distribution can be represented by a Gaussian with a "macroturbulent" velocity of 0.825 km s(-1
On the use of Bowen compensators for polarimetry
No full textAbstract. | For high precision polarimetry it is very desirable to observe with a telescope that is free of instrumental polarization. Yet it is often inevitable to introduce optical devices which generate crosstalk between the Stokes Q; U components and the V component due to phase retardations upon reflection at mirror surfaces. We show that Bowen compensators are well suited to compensate for these phase changes. They consist of 2 rotatable =8 phase retarder plates, and it can be shown that they act on polarized light either by a rotation of the axes of the polarization ellipse followed by a prescribed, desired phase change or by a phase change followed by rotation of the ellipse. We presen
Speckle spectro-polarimetry of solar magnetic structures
No full textWe present speckle observations of small-scale magnetic structures on the Sun. They were obtained with the "Göttingen"Fabry-Perot interferometer (FPI) in the Vacuum Tower Telescope at the Observatorio del Teide, Tenerife, from quiet and active regions close to disc center. A Stokes V polarimeter was added to the FPI to measure V profiles in the Fe I 6302.5 Åline. The setup allows image reconstruction with speckle methods. The achieved spatial resolution in the magnetograms is 0\farcs4-0\farcs5. We describe the observational technique and the data reduction. The results from small-scale magnetic flux elements in a very quiet region, in an active region with pores and abnormal granulation, and in a sunspot with its surroundings are discussed. In the quiet Sun, granular dynamics dominate the time evolution of the magnetic elements. Flux occurs in both bright intergranular points and in dark intergranular spaces. Likewise, with the present spatial resolution, no preference of magnetic flux in abnormal granulation in an active region can be found. Flux occurs in both bright (abnormal) small-scale granules and in the darker spaces in between them. The small sunspot studied has very little magnetic flux in its ambient quiet regions, especially no strong, conspicuous concentrations of returned flux, i.e. of flux with polarity opposite to that in the sunspot
Granular velocities of the Sun from speckle interferometry
No full textWe present observations of granular velocities and their relation to the granular intensity pattern from the disc center of the Sun. They were obtained in June 1997 with the Vacuum Tower Telescope at the Observatorio del Teide on Tenerife. High spatial resolution, 0." 4-0." 5 for the velocities, was achieved with speckle methods applied to two-dimensional narrow-band images in Na D-2 from the "Gottingen" Fabry-Perot interferometer. The velocity observations refer to a geometric height of 50-200 km (above tau(5) = 1). Velocity amplitudes of +/- 2.2 km s(-1) are seen. The high velocity regions are small-scale and the upflows coincide frequently with the bright borders of granules or with small-scale brightenings. A statistical analysis reveals only a rough consistency to the -5/3 or -17/3 law of log(power) vs. log(wavenumber) expected for isotropic turbulence (cf. Espagnet et al. 1993). We consider an agreement with such power laws as accidental since the intensity and velocity power spectra found here decrease smoothly from flat ones at low wavenumbers to steep ones at high wavenumbers. The coherence of velocity and intensity fluctuation stays above 0.5 up to horizontal wavenumbers k(h) approximate to 11 Mm(-1)(corresponds to 0." 8) and the phase difference between intensity and velocity stays stable down to structures of 0." 5. While the intensity pattern exhibits a clearly non-Gaussian distribution, the velocity distribution can be represented by a Gaussian with a "macroturbulent" velocity of 0.825 km s(-1
High resolution 2D-spectroscopy of granular dynamics
No full textSpectroscopic data with high spatial resolution are used to study the
granular dynamics of the Sun. The observations were obtained with the
"Göttingen"two-dimensional (2D) Fabry-Perot interferometer in the
Vacuum Tower Telescope at the Observatorio del Teide/Tenerife. Time
sequences of spectral scans across the non-magnetic
Fe
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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