Thesis (Ph.D.)--Boston University / The formation, evolution and eruption of solar active regions is a main theme in solar physics. Ultimately the goal is predicting when, where and how an eruption will occur, which will greatly aid space weather forecasting. Special kinds of S-shaped active regions (sigmoids) facilitate this line of research, since they provide conditions that are easier to disentangle and have a high probability for erupting as flares and/or coronal mass ejections (CME). Several theories have been proposed for the formation, evolution, and eruption of solar active regions. Testing these against detailed models of sigmoidal regions can provide insight into the dominant mechanisms and conditions required for eruption. This thesis explores the behavior of solar sigmoids via both observational and magnetic modeling studies. Data from the most modern space-based solar observatories are utilized in addition to state-of-the-art three-dimensional data-driven magnetic field modeling to gain insight into the physical processes controlling the evolution and eruption of solar sigmoids. We use X-ray observations and the magnetic field models to introduce the reader to the underlying magnetic and plasma structure defining these regions. By means of a large comprehensive observational study we investigate the formation and evolution mechanism. Specifically, we show that flux cancellation is a major mechanism for building the underlying magnetic structure associated with sigmoids, namely magnetic flux ropes. We make use of topological analysis to describe the complicated magnetic field structure of the sigmoids. We show that when data-driven models are used in sync with MHD simulations and observations we can arrive at a consistent picture of the scenario for CME onset, namely the positive feedback between reconnection at a generalized X-line and the torus instability. In addition we show that topological analysis is of great use in analyzing the post-eruption flare- and CME-associated observational features. Such analysis is used to extend the standard 2D flare/CME models to 3D and to find potentially large implications of topology to understanding 3D reconnection and the seed populations of energetic particles in CMEs.

The International Workshop on "Measuring Techniques in Liquid Metal Flows" (MTLM Workshop) was organised in frame of the Dresden "Innovationskolleg Magnetofluiddynamik". The subject of the MTLM Workshop was limited to methods to determine physical flow quantities such as velocity, pressure, void fraction, inclusion properties, crystallisation fronts etc. The present proceedings contain abstracts and viewgraphs of the oral presentations. During the last decades numerical simulations have become an important tool in industry and research to study the structure of flows and the properties of heat and mass transfer. However, in case of liquid metal flows there exists a significant problem to validate the codes with experimental data due to the lack of available measuring techniques. Due to the material properties (opaque, hot, chemical aggressive) the measurement of flow quantities is much more delicate in liquid metals compared to ordinary water flows. The generalisation of results obtained by means of water models to real liquid metal flows has often to be considered as difficult due to the problems to meet the actual values of non-dimensional flow parameters (Re, Pr, Gr, Ha, etc.). Moreover, a strong need has to be noted to make measuring techniques available to monitor and to control flow processes in real industrial facilities.

Induction heating is a common industrial process used for the reheating of billets before extrusion or forging. In this work the influence of the coil and work piece geometry, the effect of the electrical properties of the work piece, and the coil current and frequency, on the magnetic flux density and resulting work piece heating rates were studied. A combination of 1D analytical solutions, 2D axial symmetric finite element modelling and precise measurements has been used. Dozens of heating and magnetic field experiments have been conducted, with steadily increasing sophistication and measurement accuracy. The development of the experimental techniques will be described in the ‘cover’ and related to the later results published in the supplements. Experimental results are compared to predictions obtained from analytical and numerical models. The published measurements obtained for the billet heating experiments consisted of: billet electrical conductivity with &lt;0.5% error, applied currents with &lt;1% error, magnetic flux densities with 1-2% error, calorifically determined heating rates with &lt;2% error and electrical reactive power with &lt;~2% error. 2 D axial symmetric finite element models were obtained, which describe the measured results with less than a 2% difference (i.e. an ‘error’ of the same magnitude as the measurement uncertainty). Heating and reactive power results predicted by the FEM model are in excellent agreement with analytical solutions from 50 Hz to 500 kHz (differences from &lt;1% to 6%). A modified 1D short coil correction factor is presented which accounts for the interaction of the coil and work piece geometry, electrical properties and operating frequency, on the average magnetic flux density of the coil/work piece air-gap and the resulting heating rate. Using this factor, the average magnetic flux density in the air-gap can be estimated analytically within 2-3% and the heating rates of billets of known electrical properties can be estimated, with typical errors on the order of 5%. / <p>QC 20130618</p>

The Hanle effect has been proposed as a new diagnostic of circumstellar magnetic fields for early-type stars, for which it is sensitive to field strengths in the 1-300 G range. In this paper we compute the polarized P Cygni line profiles that result from the Hanle effect. For modeling the polarization, we employ a variant of the "last scattering approximation." For cases in which the Sobolev optical depths are greater than unity, the emergent line intensity is assumed to be unpolarized, while for smaller optical depths, the Stokes source functions for the Hanle effect with optically thin line scattering are used. For a typical P Cygni line, the polarized emission forms in the outer wind, because the Sobolev optical depth is large at the inner wind. For low surface field strengths, weak P Cygni lines are needed to measure the circumstellar field. For high values of the surface fields, both the Zeeman and Hanle diagnostics can be used, with the Zeeman effect probing the photospheric magnetic fields and the Hanle effect measuring the magnetic field in the wind flow. Polarized line profiles are calculated for a self-consistent structure of the flow and the magnetic geometry based on the WCFields model, which is applicable to slowly rotating stellar winds with magnetic fields drawn out by the gas flow. For surface fields of a few hundred gauss, we find that the Hanle effect can produce line polarizations in the range of a few tenths of a percent up to about 2%.

In these pages we will go through the topic of astrophysical magnetic fields, focusing on galactic fields, their observation and the theories that have been developed for a proper understanding of the these physical phenomena.We review the main work in the study of galactic magnetic fields, often seeing how it is important to deal with problems of general validity in order to be able to point out the right elements needed for a correct interpretation of specific situations. We also aim to summarize some of the conflicts that arise using different theoretical approaches to be proficient in future choices of our research guidelines.This thesis consists in an introductory text and three papers dealing with some specific topics that are introduced in the first three chapters.In the first chapter we will talk about the state of the art of the observations of galactic fields. We review current techniques and observations.In the second chapter we describe the current theories that best describe the generation of magnetic fields. We also mention here two of the three works presented in this licentiate thesis. We will then deal with the possibility to have a proper measure of the $\alpha$ effect in numerical simulationsof dynamo action.Then we consider a particular aspect of magnetic helicity, that is, its connection with the topology of the magnetic field in a given system.In the third chapter we focus on theories related to galactic fields and their validity.We also present our work on the generation of vorticity in the interstellar medium as well as a study ofturbulent diffusivity in a system presenting spherical expansion waves.

The measurement of vector magnetic fields on the sun is one of the most important diagnostic tools for characterizing solar activity. The ubiquitous solar wind is guided into interplanetary space by open magnetic field lines in the upper solar atmosphere. Highly-energetic solar flares and Coronal Mass Ejections (CMEs) are triggered in lower layers of the solar atmosphere by the driving forces at the visible ``surface'' of the sun, the photosphere. The driving forces there tangle and interweave the vector magnetic fields, ultimately leading to an unstable field topology with large excess magnetic energy, and this excess energy is suddenly and violently released by magnetic reconnection, emitting intense broadband radiation that spans the electromagnetic spectrum, accelerating billions of metric tons of plasma away from the sun, and finally relaxing the magnetic field to lower-energy states. These eruptive flaring events can have severe impacts on the near-Earth environment and the human technology that inhabits it. This dissertation presents a novel inversion method for inferring the properties of the vector magnetic field from telescopic measurements of the polarization states (Stokes vector) of the light received from the sun, in an effort to develop a method that is fast, accurate, and reliable. One of the long-term goals of this work is to develop such a method that is capable of rapidly-producing characterizations of the magnetic field from time-sequential data, such that near real-time projections of the complexity and flare-productivity of solar active regions can be made. This will be a boon to the field of solar flare forecasting, and should help mitigate the harmful effects of space weather on mankind's space-based endeavors. To this end, I have developed an inversion method based on genetic algorithms (GA) that have the potential for achieving such high-speed analysis.

The primary goal of this research was to investigate the effects associated with the application of magnetic fields to single electrodes.

No description available.

<p> The de Haas-van Alphen (dHvA) effect has been studied in single crystals of calcium using the modulation method in magnetic fields up to 55 kOe. Four distinct orbits were observed with dHvA frequency minima at the [110] or [100] directions. The results do not support calculations predicting a disconnected first-band Fermi surface for calcium. The dHvA data of crystalline calcium is consistent with the topology of the two-OPW model in which the first band is connected and pockets of electrons are about the point L of the second zone.</p> / Thesis / Master of Science (MSc)

<p> Field modulation techniques were used to observe the de Haas-van Alphen effect in magnetic fields up to 5.5 tesla and at temperatures below 1.1°K. A data acquisition system recorded on magnetic tape the large amount of data necessary for computer fourier analysis of the oscillations. All of the orbits predicted by Keeton and Loucks' model of the Fermi surface of mercury were at least tentatively identified. The data on the β, τ and α orbits were in essential agreement with previous work. The γ and X-face orbits were also investigated in some detail, while tentative identification was made of the μ and T-face orbits. When the data permitted, the areas were fitted to ellipsoids or hyperboloids of revolution by a least squares calculation.</p> <p> A search was made for modifications to the de Haas-van Alphen theory due to phonons. Accurate torque de Haas-van Alphen amplitude measurements were taken as a function of temperature and magnetic field. The analysis of the results revealed no systematic dependence of either the cyclotron effective mass or the Dingle temperature on temperature from 1.25°K to 4.2°K or on magnetic field from 1.5 tesla to 2.3 tesla. Thus no effects due to phonons were observed.</p> <p> A method of observing the open orbits in metallic single crystals was developed and used to observe the open orbits in mercury. The method utilized the eddy currents induced in the sample by the rotation of a magnetic field. This provided a signal which was dependent on the conductivity in the plane perpendicular to the open orbit. The torque amplitude, which indicated the number of open orbit carriers, was used to detect the angular range of the bands of open orbits in mercury. The method was experimentally simple since no special sample geometry was necessary and no electrical connections to the sample were needed.</p> / Thesis / Doctor of Philosophy (PhD)