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Magnetic Buoyancy Instabilities in Deep, Twisted Magnetic Layers
Get PDFIn observing the solar magnetic field, possibly the most prominent features visible on the surface are sunspots, which emerge at di!erent latitudes as the solar cycle progresses. Sunspot pairs are believed to be formed by concentrated bundles of mainly toroidal magnetic field (flux tubes) looping through the surface. These regions exhibit surprisingly ordered patterns of behavior such as the Hale’s Polarity Law, Joy’s Law, and the Solar Hemispherical Helicity Rule (SHHR). The latter states that emerging flux in the Northern hemisphere generally has left-handed current helicity, whereas the Southern hemisphere has right handed. While there is magnetic field at all scales on the sun, the origins of these active regions and the connection between large-scale dynamo generated fields and active region scales is a long standing and difficult question. The flux tubes that form sunspots most likely originate from magnetic buoyancy instabilities that occur in the solar tachocline and then subsequently rise through the convective zone. Although there are many theories of the origin of the SHHR, the helicity content in the emerging flux is often claimed to be a direct result of the helicity (or angulation) in the originating dynamo field.Magnetic buoyancy instabilities and their non-linear evolution have been studied and simulated by others in 2D and 3D, generally using magnetic slabs that possess infinite gradients at their interfaces, which guarantees that said instability occurs. In this work, we extend these ideas, allowing the initial conditions to have a gently varying interface between magnetic and non-magnetic layers with variable width (while still satisfying criteria to initiate the instability). We look for differences in the evolution of the instabilities in this new scenario. This setup allows us to then add a horizontal poloidal field component to the previous horizontal toroidal component, thereby creating a horizontal field that varies in direction over the depth of the magnetic interface. We study the instabilities of this setup and examine the emerging flux tubes for any resultant helicity in order to explore the relationship between helicity in emerging magnetic structures and that in the originating field.Overall, we found that when the magnetic interface was wider, the instability proceeded in a distinctly different fashion depending on the particular aspects of the originating layer. In our setup, the fluid generally went unstable lower down in the transition layer and created different geometries of the magnetic structures and secondary instabilities due, at least in part to, the stronger buoyancy forces deeper in the layer and to the necessity of deformation of overlaying field. The instabilities are rapid and mix efficiently, reaching a stable end-state much faster in the deep layer cases than the shallow layer version.Furthermore, when adding twist to the originating fields, we found that the resultant helicity in the emerging magnetic structures does indeed depend on the initial profile of the field. In general, a dominant outer helical layer of the flux tube is found, the chirality of which is directly dependent on the angle of poloidal and toroidal field at the point of maximum instability in the layer, even if the structure rose through field of the opposite angulation
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Estimating a Local Source Approximation for the Ultraviolet Background Radiation in Cosmological Settings Using Lambda Iteration
Get PDFAfter the epoch of reionization, Lyman continuum radiation permeate the universe. Immediately after reionization (redshifts ), the influence of the cosmic Ultraviolet (UV) background on the Intergalactic Medium (IGM) and the Circumgalactic Medium (CGM) is important to understand. We focus on building a 1D radiative transfer algorithm using the Accelerated Lambda Iteration scheme and the Lambda Iteration scheme to solve the time-independent radiative transfer equation. This approach captures the dynamics of radiation fields, and the convergence, accuracy, and mathematical limitations of these schemes are thoroughly tested. We then expand the algorithm to include a 4th order Runge-Kutta scheme to trace the fraction of Hydrogen atoms between the UV background and the center of a massive galaxy. In our calculations, the UV background dominates the ionization of the IGM and CGM near our example star-forming galaxy
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Magnetic Flux Tube Dynamics in the Deep Solar Interior
Get PDFThis doctoral work is motivated by magnetic field transport processes in the deep interior of the Sun and their eventual observational signatures at the solar surface. In particular, we are inspired by a combination of previous computational and observational work. First of all, based on observations at the solar surface of sunspots embedded in active regions, it is widely believed that large-scale, strong magnetic flux emerges from the Sun's deep interior in the form of arched, cylindrical tube-like structures often known simply as "magnetic flux tubes." The buoyant transport of these flux tubes from the solar interior towards the surface plays an essential role in the emergence of active regions, the formation of sunspots, and the overall solar dynamo. A range of two- and three-dimensional computational work in the past has focused on the formation and transport of magnetic flux tubes from the deep solar interior. Many such studies have assumed the existence of such magnetic structures and studied highly simplified models like the buoyant rise of an isolated flux tube in a quiescent, field-free environment. Here, motivated by their formation, (e.g. Cline et al., 2003a, Brummell et al., 2002), we systematically attempt to remove some of these restrictive assumptions and study the rise of a toroidal flux tube embedded in a large-scale poloidal background magnetic field, with and without the presence of turbulent convection.In this thesis, we have divided our work into two major parts. In the first part, we investigate the rise of a \emph{non-isolated} toroidal flux tube through a volume-filling large-scale magnetic field in a quiescent background state. We find that positive and negative twisted flux tubes show starkly different dynamics. In particular, for a given large-scale magnetic field, flux tubes of one sign of the twist are more likely to rise than the other. We have created a mathematical model based on the forces acting on the flux tube that can explain and even predict the observed asymmetric rise dynamics for a given flux tube twist and background field orientation. This reveals a filtering region in the parameter space that we refer to as Selective Rise Regime (SRR). To better understand the statistical relevance of the SRR, we also carry out Monte Carlo (MC) simulations of multiple flux tubes rising through the background field. This study further shows that the SRR, along with the MC simulations, plausibly explains the broad range of solar helicity observations that are collectively known as the Solar Hemispheric Helicity rules (SHHR). The SHHR declare primarily that there is a preferred helicity of the emerging flux in each hemisphere (negative for Northern; positive for Southern). Our model provides an explanation for this major bias. Furthermore, the SHHR is a weak rule, obeyed only 60-80 % of the time. Our models also provide good physical reasons for statistical violations of the rule, and further makes predictions about the cycle dependence of the rule that could be tested. In the second part, we examine a more realistic and therefore more complex version of the same thing. Specifically, we consider the buoyant rise of magnetic flux tubes from their formation in a deep radiative zone, through a turbulent overshooting convection zone that self-consistently arranges a volume-filling large-scale background field. Despite the presence of much more complicated dynamics, we still find the same preferential rise of flux tubes of a particular twist, thus establishing the robustness of SRR mechanism. The presence of convection does however add another layer of statistical fluctuations to the observed asymmetry in the flux tube dynamics, which we investigate by Monte Carlo-like suites of simulations to again confront variations in the SHHR
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Numerical explorations in MHD phenomena with connections to the solar, geo, and lunar dynamos
Get PDFMany celestial bodies possess magnetic fields generated somehow by dynamo action within the interior of the object. These magnetic fields can be dynamically significant. For example, sunspots in magnetically-active regions of the sun are related to the flares and coronal mass ejections which create the space weather that influences life on Earth. Furthermore, the earthly protection from such events is created by the Earth's interior geodynamo. Understanding such dynamics is in general a complex highly nonlinear problem and we frequently turn to numerical simulations. However, for many of these circumstances, even modern supercomputers lack the power to model the extreme parameter regimes of the true objects. For that reason, we often resort to simpler simulations as experimental laboratories to explore ideas that might be relevant. In this thesis, we adopt this approach and use direct numerical simulations to study some unusual ideas on aspects of MHD phenomena relating to the solar, geo, and lunar dynamos. The first of these novel studies examines the operation of essentially nonlinear dynamos (ENDs), where nonlinear effects are dominant from the start (as opposed to becoming important after a linear epoch). Here the model design is inspired by the solar tachocline where there is a strong toroidal shear, but this END study has additional relevance to the subcritical nature of the geodynamo. In a second study, we employ another novel theory, that of stoked nondynamos to provide an alternative scenario for some unexplained observations of the lunar magnetic field. Here, we examine a non-closed system that combines a lunar core dynamo with a surrounding nondynamo basal magma ocean as a plausible mechanism to explain unexpectedly high paleointensities found in lunar surface rocks. In the final study of this thesis, we return to the Sun to investigate the origin of the solar hemispheric helicity rules (SHHR) via three dimensional simulations of twisted magnetic flux tubes in the presence of rotating convection. This study examines the efficacy of a highly-simplified theoretical mechanism, the -effect, via fully resolved three dimensional MHD simulations
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
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Dynamics of weakly non-Boussinesq convection, convective overshooting and magnetic field confinement in a spherical shell
Get PDFThis doctoral work is motivated by the Sun and solar-type stars, which consist of an unstable convection zone (CZ) that lies on top of a stably stratified radiative zone (RZ). The dynamics occurring at the CZ-RZ interface are not well understood, and yet they are known to play a significant role in processes such as transport of chemical species, angular momentum and magnetic fields. To shed some new light on this complicated problem, we have compartmentalized this work into three main chapters. In the first part, in order to mimic stellar-like conditions, we study convection in a weakly non-Boussinesq gaseous spherical shell in the low-Prandtl number regime assuming a constant adiabatic temperature gradient and employing fixed flux at the inner boundary. We find the remarkable emergence of a subadiabatic layer within the domain for sufficiently turbulent flows enhanced by large variations in the superadiabaticity across the shell. However, convection remains vigorous everywhere across the shell thus indicating that it is a highly non-local process. In the second part, we further extend our study to include a stable region below the convective zone and we investigate the dynamics of overshooting/penetrative convection. We observe that the overshooting of the turbulent motions into the RZ depends on three different parameters: the relative stability of the stable zone, the transition width between the two, and the intensity of the turbulence. We find that, in the parameter regime studied, these overshooting motions manage to partially alter the thermal stratification, but not so efficiently as to create a fully mixed adiabatic region. We have built a model of these processes that could be useful for stellar evolution codes. In the third and final part, we also add a poloidal dipole magnetic field initially contained in the stable zone and study its interaction with the turbulent motions. Our numerical results are categorized into non-dynamo and dynamo cases. In the non-dynamo cases, the field diffuses outward, and its field lines open up and penetrate in the CZ. At the same time, a large fraction of its energy is removed due to the turbulent diffusion by the convective motions. In the dynamo cases, the field starts diffusing outward but its interaction with the turbulent motions leads to a small-scale essentially kinematic dynamo within the CZ and the overshoot region. In both of these cases, we find that the dipole field cannot remain confined in the RZ by the turbulent motions
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
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