1,720,966 research outputs found
Inverse problems and numerical approximation for high-energy solar imaging
No full textThe Spectrometer/Telescope for Imaging X-rays (STIX) is the instrument onboard Solar Orbiter mission designed to observe solar flares and, in particular, to collect hard X-ray radiation emitted during these phenomena, with the ultimate goal of understanding the physics behind them. STIX adopts an indirect imaging technique: it is based on a double layer of grids to modulate the incoming radiation. The photon counts measured by its detectors are related to the so-called visibilities, i.e., a finite set of Fourier frequencies of the emitted X-ray radiation flux, that is the quantity of interest for image reconstruction. Due to the limited number of visibilities available (only 30 in the case of STIX), the reconstruction of X-ray images is an ill-posed inverse problem which must be treated properly in order to obtain reasonable reconstructions. In this thesis, after a detailed explanation of the design and hardware of the STIX instrument, the mathematical formulation of the data formation process is derived and studied. This allows explaining how the recorded data are mathematically related to a particular sampling of the Fourier transform of the radiation flux emitted during solar flares. Two different image reconstruction techniques are developed and investigated: a parametric method based on global optimization strategies, and a multi-scale formulation of CLEAN, a well-known algorithm, widely used in both solar physics and radio astronomy. Both methods are validated on STIX data as well as on data collected by RHESSI, a NASA satellite and the predecessor of STIX. In the last part of the thesis, we propose two methods to obtain regularization in direction of contiguous energy levels, to correlate the related visibilities, and thus the maps from them reconstructed. The first approach is based on the construction of visibility spectra as a function of energy and their inversion through Tikhonov regularization. Two consecutive ill-posed problem are solved: the first inversion allows obtaining information on visibilities in the electron space, where the physics of interest resides. By solving a second inverse ill-posed problem, it is possible now to obtain, from electron visibilities, electron maps whose pixel values are proportional to the number of accelerated electrons. Thus, from the electron flux images, important physical information on the evolution of solar flares can be obtained. Finally, the second approach is based on advanced interpolation techniques, focusing on the use of Variably Scaled Kernel, and on the projected Landweber method to perform extrapolation. We present theoretical analysis detailing the error bounds that highlight the dependence on the scale function for VSKs, and the convergence results of the Landweber scheme for specific definitions of the scale function. In this framework, the images are reconstructed by an interpolation approach in which the reconstructions of contiguous energy channels are used as regularization priors. Finally, this thesis is supplemented with an appendix describing the work done to take into account all the aspects that make STIX a real and not an ideal instrument
Multi-scale CLEAN for Fourier-based hard x-ray solar imaging
No full textMulti-scale deconvolution is an ill-posed inverse problem in imaging, with applications ranging from microscopy, through medical imaging, to astronomical remote sensing. In the case of high-energy space telescopes, multi-scale deconvolution algorithms need to account for the peculiar property of native measurements, which are sparse samples of the Fourier transform of the incoming radiation. The present paper proposes a multi-scale version of CLEAN, which is the most popular iterative deconvolution method in Fourier-based astronomical imaging. Using synthetic data generated according to a simulated but realistic source configuration, we show that this multi-scale version of CLEAN performs better than the original one in terms of accuracy, photometry, and regularization. Further, the application to a data set measured by the NASA Reuven Ramaty High Energy Solar Spectroscopic Imager shows the ability of multi-scale CLEAN to reconstruct rather complex flaring topographies
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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