1,720,964 research outputs found
Earth-based spectroscopy of Jupiter’s Galilean moons
No full textThis thesis uses spatially resolved spectral observations, primarily from the ground based Very Large Telescope (VLT), to model and understand the surface composition of the Galilean moons. Previous studies have proposed various compositional hypotheses, so I developed a Markov Chain Monte Carlo (MCMC) spectral inversion routine which allows detailed investigation of the different compositional mixtures which are consistent with an observed spectrum. I use this MCMC routine to quantify uncertainties on fitted abundances from infrared spectral observations of the icy moons Europa and Ganymede, and quantitatively demonstrate how the hydrated salt mixture on these moons cannot be fully constrained using near-infrared spectroscopy alone.
I produced compositional mapping datasets by analysing observations from the VLT/SPHERE, VLT/MUSE and Galileo/NIMS instruments to study the abundances and spatial distributions of ices, volatiles, and contaminants on the surfaces of the Galilean moons. On Europa, I mapped the contrasts between the high acid abundance (∼ 60%) near the trailing apex and high water ice abundances (∼ 50%) at high latitudes, and constrained the possible spatial distributions of hydrated salts. Modelling results for Ganymede were dominated by water ice in younger areas (abundances up to 50%) and a spectrally flat darkening agent in older terrain (up to 60% abundance). The water ice grain size on Ganymede was found to have strong latitudinal and longitudinal contrasts, driven by global scale trends in temperature and plasma bombardment. Mapping of the 590nm SO2 absorption band on Io identified stronger absorptions at mid latitudes and in the Pele ejecta blanket. The 577.3nm O2 band on Ganymede was found to be spatially constrained to the closed magnetic field line region, with the strongest absorptions on the trailing hemisphere. Comparison of the datasets is used to demonstrate how ground-based telescopes can carry out compositional mapping which was previously only possible using spacecraft. These datasets fill spatial and spectral gaps in observations of the moons and will help to prepare and guide the next decade of exploration of these worlds by the JUICE and Europa Clipper spacecraft.
I led a successful observing campaign of Ganymede with VLT/SPHERE in 2021, and work from this thesis is published in King et al. (2022) and King and Fletcher (2022).</p
JWST NIRCam and NIRSpec reduction and plotting code
No full textPython code to reduce JWST NIRCam and NIRSpec rateints files, producing the callings files used in the data analysis. The reduction makes use of calwebb along with other steps as described in Nichols et al. (2024), and produce Figures A1-A3.</p
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
Global Modeling of Ganymede's Surface Composition: Near‐IR Mapping From VLT/SPHERE
No full textWe present maps of Ganymede's surface composition with almost complete longitude coverage, acquired using high spatial resolution near-infrared (0.95–1.65 μm) observations from the ground-based VLT/SPHERE instrument. Observed reflectance spectra were modeled using a Markov Chain Monte Carlo method to estimate abundances and associated uncertainties of water ices, acids, salts and a spectrally flat darkening agent. Results confirm Ganymede's surface is dominated by water ice in young bright terrain (impact craters, sulci), and low-albedo spectrally flat material in older dark terrain (e.g., Galileo Regio). Ice grain size has strong latitudinal and longitudinal gradients, with larger grains at the equator and on the trailing hemisphere. These trends are consistent with the effects of the latitudinal thermal gradient and global variations in radiation driven sputtering. Sulfuric acid has a low abundance and appears potentially spatially correlated with plasma bombardment, where Ganymede's poles are exposed to the external Jovian magnetic field. Best-estimate abundances suggest a mixture of salts could be present, although their low abundances, spectral degeneracies and associated uncertainties mean individual salt species cannot be detected with confidence. If present, sodium magnesium sulfate and magnesium chlorate appear tentatively correlated with exogenic plasma bombardment, while magnesium chloride and sulfate appear tentatively correlated with younger terrain, implying a possible endogenic origin. MCMC modeling was also performed on Galileo/NIMS data, showing comparable distributions. The high spatial resolution of SPHERE allows the precise mapping of small scale (<150 km) surface features, which could be used along with higher spectral resolution observations to jointly confirm the presence and distribution of potential species.</p
Global Modeling of Ganymede's Surface Composition: Near‐IR Mapping From VLT/SPHERE
No full textWe present maps of Ganymede's surface composition with almost complete longitude coverage, acquired using high spatial resolution near-infrared (0.95–1.65 μm) observations from the ground-based VLT/SPHERE instrument. Observed reflectance spectra were modeled using a Markov Chain Monte Carlo method to estimate abundances and associated uncertainties of water ices, acids, salts and a spectrally flat darkening agent. Results confirm Ganymede's surface is dominated by water ice in young bright terrain (impact craters, sulci), and low-albedo spectrally flat material in older dark terrain (e.g., Galileo Regio). Ice grain size has strong latitudinal and longitudinal gradients, with larger grains at the equator and on the trailing hemisphere. These trends are consistent with the effects of the latitudinal thermal gradient and global variations in radiation driven sputtering. Sulfuric acid has a low abundance and appears potentially spatially correlated with plasma bombardment, where Ganymede's poles are exposed to the external Jovian magnetic field. Best-estimate abundances suggest a mixture of salts could be present, although their low abundances, spectral degeneracies and associated uncertainties mean individual salt species cannot be detected with confidence. If present, sodium magnesium sulfate and magnesium chlorate appear tentatively correlated with exogenic plasma bombardment, while magnesium chloride and sulfate appear tentatively correlated with younger terrain, implying a possible endogenic origin. MCMC modeling was also performed on Galileo/NIMS data, showing comparable distributions. The high spatial resolution of SPHERE allows the precise mapping of small scale (<150 km) surface features, which could be used along with higher spectral resolution observations to jointly confirm the presence and distribution of potential species.</p
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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