1,721,060 research outputs found
Introduction of thermal activation in forward modeling of hysteresis loops for single-domain magnetic particles and implications for the interpretation of the Day diagram
Full text linkSynthetic hysteresis loops were generated by numerically solving the classical Stoner- Wohlfarth model and a thermally activated Stoner-Wohlfarth model for a set of randomly oriented magnetic grains. Although computationally intensive this method allows forward modeling of hysteresis loops of single-domain (SD) and viscous grains. In the classic Stoner-Wohlfarth model the shape of the modeled loops can be modified by changing the distribution of the anisotropy energy but all the loops will all have similar hysteresis parameters Msr/Ms and Hcr/Hc corresponding to that of a theoretical assemblage of SD particles. The thermally activated Stoner-Wohlfarth model, which allows the magnetic moment of each grain to switch between two energy minima according to Boltzmann statistics, extends the SD model toward superparamagnetic (SP) grains and introduces a volume dependency. Numerical simulation using the thermally activated model shows that the shapes of SD loops are modified by the effect of the thermal energy if the particles are sufficiently small. The major effect of the thermal disturbance is observed in highly viscous particles (smaller than approximately 0.03 mm in diameter, for magnetite) where it strongly reduces the coercivity and to a lesser extent the remanent magnetization. The effect on the hysteresis parameters is a large increase in Hcr/Hc and a decrease in Msr/Ms, by factors that vary with anisotropy distribution, grain volume and measurement time. For certain grain sizes, these result in hysteresis parameters that are similar to those attributed to pseudosingle-domain (PSD) grains
Meteoric smoke fallout revealed by superparamagnetism in Greenland ice
Full text linkMeteoric material reaching Earth contains an appreciable percentage of iron, much of which can be oxidized into nanometric-size particles produced by ablation and subsequent condensation in the atmosphere. New measurements of isothermal remanent magnetization (IRM) show that magnetic particles of extraterrestrial origin can be distinguished from terrigenous particles based on their smaller superparamagnetic (SP) size as inferred from magnetic relaxation and by the poor correlation of the SP fraction with dust contents. The magnetic relaxation data suggest that extraterrestrial magnetic particles are in the size range of about 7 – 17 nm, which is compatible with the expected size of condensed particles. The concentration of extraterrestrial material in Greenland ice was estimated from the magnetic relaxation data. Assuming an iron content typical of average chondritic composition, the results correspond to a particles concentration of 0.78 ± 0.22 ppb for Greenland, good agreement with results based on iridium concentrations in NGRIP ice samples
Meteoric smoke concentration in the Vostok ice core estimated from superparamagnetic relaxation and some consequences for estimates of Earth accretion rate
Full text linkWe measured the magnetization of glacial and interglacial ice from the Vostok core to estimate the meteoric smoke concentration in Antarctic ice. We have found that, within the uncertainty of the method, the smoke concentration in ice in Antarctica is equivalent to that previously measured in Greenland ice. The virtually identical smoke concentrations despite the different ice accumulation rates in Greenland and Antarctica suggest that wet deposition is the main deposition mechanism for such ultra-small particles. Given the typical scavenging ratios for atmospheric aerosols, this would imply that previous estimates of accretion rate based on dry deposition are likely to be appreciably overestimated
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
Isothermal remanent magnetization of Greenland ice: Preliminary results
Full text linkThe magnetic mineral content of wind“transported dust should reflect atmospheric transport dynamics and conditions in its source areas, and could thus be used as an environmental proxy. To test the feasibility of determining the magnetic mineral content in polar ice, isothermal remanent magnetization (IRM) was measured on a small suite of Greenland ice samples of Holocene (interglacial) and Last Glacial Maximum (LGM) age. Although the extremely low dust concentrations limit weak field (susceptibility) measurements, all samples contained an easily measurable concentration of magnetic minerals that can be estimated using IRM intensity provided that special precautions are used. IRM experiments at liquid nitrogen temperatures indicate ice magnetic properties which are consistent with that expected from varying concentrations of magnetite or maghemite. Interestingly, the Holocene ice samples tend to have higher magnetic concentrations, despite having much lower total polar dust contents, than the few LGM ice samples tested thus far
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
Magnetostratigraphy and Biostratigraphy of the Carnian/Norian Boundary Interval from the Pizzo Mondello Section (Sicily)
Full text linkWe present new magnetostratigraphic and biostratigraphic data from an Upper Triassic limestone section named Pizzo Mondello. This section is 141m-thick and crops out in the Sicani Mountains of western Sicily (Italy). The Pizzo Mondello section is one of the Tethyan best in virtue of its high quality of exposure, accessibility, stratigraphic continuity and good magnetostratigraphic data. We recognize a sequence of six magnetozones, from M1 to M6, each subdivided into a lower predominantly normal and an upper predominantly reversed magnetozone. This sequence of magnetozones spans the Carnian/Norian boundary according to conodont biostratigraphy. The Carnian/Norian boundary should fall in the upper part of magnetozone M3 which should correspond to chron E14 in the Newark reference sequence of polarity reversals. The comparison of data from the Newark basin, Pizzo Mondello and the Late Triassic Bolcektasi Tepe section from the literature seems to suggest the existence of a disconformity in the Tethyan marine domain at around the Carnian/Norian boundary which we tentatively relate to intraPangea tectonic mobility. A consistent correlation of Newark/Pizzo Mondello with published data from the Norian Tethyan marine Kavur Tepe and Scheiblkogel sections is still elusive, suggesting that the validity of the original polarity interpretation of the Kavur Tepe section could usefully be critically reviewed
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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