1,720,981 research outputs found
Effects of bubble coalescence, permeability, and degassing process on the transition from explosive to effusive eruption of rhyolitic magma
No full textThis thesis consists of two parts that address bubble coalescence and the degassing process inside the magma conduit during volcanic eruptions. The first part focuses on analog fluid experiments and resultant scalings for bubble coalescence, due to either gravitational and capillary forces, with application to magmatic systems. I find that film drainage is due to capillary forces at dimensionless Bond numbers, Bo 0.25. The film drainage time scale is given by t ~ C ln(α) τ, and is orders of magnitude faster than previously assumed for magmatic systems. Here C ~ 10 is an empirical constant, α is the ratio of initial film thickness to film thickness at the time of rupture, and τ is the characteristic capillary or buoyancy time scale at values of Bo 0.25, respectively. These scalings could be used to estimate the time for bubble coalescence under static conditions, such as in a magma chamber or post-fragmentation in pyroclasts.
The second part of the thesis focuses on pyroclast permeability, a consequence of bubble coalescence during volcanic eruptions. I analyze porosity and permeability of rhyolitic pyroclasts from four different Plinian eruptions. One of these is the A.D. 1912 eruption of Novarupta volcano, Alaska, which is comprised of five different episodes ranging from explosive to effusive activity. For this eruption, I find that the degree of interconnectivity, measured as the ratio of connected to total porosity, decreases with phenocryst content and with increasing eruption intensity. Through numerical modeling of diffusive bubble growth during eruptive magma ascent, I conclude that magma permeability is unlikely a sufficient condition for the transition from explosive to effusive activity. Instead, it is likely that a decrease in magma ascent leads to the transition from explosive to effusive activity during the waning stages of the eruption. Subsequently, I compare and contrast the porosity and permeability data from all four Plinian eruptions investigated: the A.D. 181 Taupo eruption, New Zealand; the A.D. 1060 Glass Mountain eruption, California; the A.D. 1314 Kaharoa eruption; and the A.D. 1912 Novarupta eruption, Alaska. I find that the Kaharoa samples have the lowest values of porosity and permeability of these four Plinian eruptions. Porosity and permeability, as well as pyroclastic textures of Kaharoa samples closely resemble those of the effusive Episode V of the 1912 Novarupta eruption. I hypothesize that the Kaharoa eruption might have undergone a high degree of open-system degassing
Controls on magma supply from depth at Kīlauea Volcano, Hawai'i
No full textMagma supply rate from depth to the summit at Kīlauea volcano, Hawai'i is not constant through time. Even during the current Pu'u 'Ō'ō-Kupaianaha eruption (1983-present), a surge in supply occurred from 2003-2007. This surge was accompanied by summit inflation, increased CO2 emissions, and finally culminated in a dike intrusion and surface eruption. Recent work has suggested that a surge in supply is a result of an increase in dissolved volatiles in the melt zone. Alternatively, the surge could come from a mechanical change in the melt zone, perhaps as an increase in permeability or melt production. I develop a numerical model of two-phase flow between the melt zone and summit at Kīlauea to explore the role of volatile composition, conduit geometry, and pressure in the melt zone on supply rate. Results suggest that volatiles have a limited role in increasing supply, a dike-shaped conduit is likely, and pressure increases in the melt zone drive changes in supply. Additionally, a dike-shaped conduit yields a decoupling of conditions at the surface from those at depth, which impacts how conditions in the mantle below Kīlauea are inferred from surface deformation and summit gas emissions
High Temperature Plasma Dynamics in Solar Flares
No full textSpatially resolved spectroscopic observations show wing enhancements and broadening in extreme ultraviolet emission from particular hot iron lines. Several physical processes ranging from plasma turbulence, magnetic perturbations to non-Gaussian ion populations, and non-thermal physics have been proposed to play a role in their formation.
In this thesis I investigate in detail the role of plasma dynamics in spectral line shapes by studying the wing enhancements of Fe XXIII and XXIV observed during solar flares, using a field-aligned hydrodynamic model.
First I examine how plasma dynamics in a single, monolithic flaring loop contributes to the formation of line asymmetry. This is done by running 35 simulations that use the observed values and their uncertainties for the driving electron beam parameters. Next I study the effect of flaring loop length on spectral line shape and broadening by running simulations with different loop lengths and the same beam parameters.
The presence of sub-resolution structures, confirmed by increasingly high-resolution observations, and observational difficulties in isolating a monolithic loop from nearby loops, necessitate an investigation into the effect of superposed dynamics on some line asymmetries. Hence, I design multiloop models that are representative of three possible configurations of loops. Here I study how the resultant Fe XXIII spectral line profiles differ in each case and examine the differences between these multiloop models and the single loop model. I also briefly explore the role of a constant time delay in heating successive sub-loops of a multiloop configuration on the spectral line shape.
The results show that the single loop model can successfully reproduces line asymmetries, and the loop length plays an important role in explaining some of the key observations such as the positive correlation between Doppler shifts and line widths, and broad but symmetric hot Fe lines. For a multiloop model with sub-loops of the same length, an imposed heating time delay is an important factor that significantly alters the line profile shape from the single loop case. Whereas, multiloop models with sub-loops of varied lengths predict significantly different line profiles, such as asymmetric lines for longer durations and with large blue-shifts, without the necessity of introducing time delays
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
- …
