1,721,014 research outputs found
Solar wind spectral analysis in heliosheath from Voyager 2 data
No full textThe solar wind is a supersonic flow of magnetized plasma. It is time-dependent on all scales and expands with distance. The flow has fluctuations on a broad range of scales and frequencies. This fluctuations are not just convected outward but show energy cascades among the different scales. The solar wind turbulence peculiar phenomenology has been comprehensively reviewed by Tu and Marsch [11] and Bruno and Carbone [2]. As the distance from the sun increases, the available data on plasma and magnetic field become increasingly scarce. At distance of the order of 1 astronomic unit (AU), several measurement have been performed by various crafts, but, nowadays, only the Voyager spacecrafts can measure data in the heliosheath, the outermost layer in heliosphere where the solar wind is slowed by the pressure of the interstellar gas, and only the Voyager 2 craft can measure both plasma and magnetic fields (Voyager 1 can measure only the magnetic field, and Pioneer 10 and 11 has ceased communications). Taken together, the Voyager 1 and 2 probes have collected over 11 year of data in the heliosheath. The Voyager plasma experiment observes plasma currents in the energy/charge range 10 – 5950 eV /q using four modulated-grid Faraday cup detectors [1]. The observed currents are fit to convected isotropic proton Maxwellian distributions to derive the parameters (velocity, density, and temperature) used in this work. Magnetic field and plasma data are taken the COHO web site and MIT Space Plasma Group repository. Several studies have been done in order to extend the existing models to make them consistent with the energetic particles and magnetic fields measured in the heliosheath, but so far an exhaustive explanation has not yet been obtained. In particular, the differences between the energetic particle intensity variations seen by the two crafts are unexplained. The electron intensity measured by Voyager 2 varies steeper by a factor of 10 in a single year, while the same quantity from Voyager 1 changes gradually over time.[7] A possible explanation can be the presence of bubble of turbulence that travels in the heliosheat. Therefore, a characterization of turbulence and its intermittency is necessary to explain this phenomenology. The aim of this work is to provide the first spectral analysis of heliosheath solar wind, trying to characterize the plasma turbulence in that region by estimating the spectral slopes. A first result is represented in figure 2, where it can be seen that the low frequency spectral slope is lower when the electron intensity is low. In order to compute spectra, signal reconstruction techniques are mandatory: at distance over 80 AU, available data are very spotty. For the plasma velocity, there are 97% of missings due to unsteadiness in the signals, see 1, the most important of which are: tracking gaps due to the V2 location and due to limited deep space network availability; interference from other instruments; possible errors in the measurement chain (from the Faraday cups up to the data acquisition system and the signal shipping to Earth); the temporal sequence of the nuclear propulsion used to control the Voyager trajectory and to assist in several critical repositionings of the craft. For data recovery, we mainly use two different methods. The first method used is based on the correlation computation [8] that allows to reconstruct correlations and use it to compute spectra. Better results can be achieved implementing the maximum likelihood reconstruction by Rybicki and Press [10] based on a minimum-variance recovery with a stochastic component. The second methods comes from the Compress Sensing, a recent technique widely used in telecommunications, that provides the reconstruction of the signal from a sparse dataset [3], by using sparse Fourier matrices [9]. The methods used have been previously tested on 1979 data and on synthetic fluid turbulent fields. Results were in good agreement with the literature, and allows to compute largest spectra of solar wind at 5 AU, with frequencies ranging from 10-7 to 10-2 Hz [4, 5, 6]
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
Cross and magnetic helicity in the outer heliosphere from Voyager 2 observations
Full text linkPlasma velocity and magnetic field measurements from the Voyager 2 mission are used to study solar wind turbulence in the slow solar wind at two different heliocentric distances, 5 and 29 astronomical units, sufficiently far apart to provide information on the radial evolution of this turbulence. The magnetic helicity and the cross-helicity, which express the correlation between the plasma velocity and the magnetic field, are used to characterize the turbulence. Wave number spectra are computed by means of the Taylor hypothesis applied to time resolved single point Voyager 2 measurements. The overall picture we get is complex and difficult to interpret. A substantial decrease of the cross-helicity at smaller scales (over 1-3 h of observation) with increasing heliocentric distance is observed. At 5 AU the only peak in the probability density of the normalized residual energy is negative, near −0.5. At 29 AU the probability density becomes doubly peaked, with a negative peak at −0.5 and a smaller peak at a positive values of about 0.7. A decrease of the cross-helicity for increasing heliocentric distance is observed, together with a reduction of the unbalance towards the magnetic energy of the fluctuations. For the smaller scales, at 29 AU the normalized polarization is small and positive on average (about 0.1), but it is zero at 5 AU. For the larger scales, the polarization is low and positive at 5 AU (average around 0.1) while it is negative (around −0.15) at 29 AU
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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