Dissertations / Theses: 'The origin of magnetic field'

1

Schwarte, Judith. "Modelling the earth's magnetic field of magnetospheric origin from CHAMP data." [S.l. : s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=971057001.

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Xu, Hao. "The AGN origin of cluster magnetic fields." Diss., [La Jolla] : University of California, San Diego, 2009. http://wwwlib.umi.com/cr/ucsd/fullcit?p3356297.

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Thesis (Ph. D.)--University of California, San Diego, 2009.
Title from first page of PDF file (viewed July 7, 2009). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 132-139).

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Barnes, D. J. "Origin and evolution of large-scale magnetic fields." Thesis, University College London (University of London), 2015. http://discovery.ucl.ac.uk/1466179/.

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Magnetic elds are ubiquitous at all scales in the Universe and have been observed in galaxies and clusters of galaxies via observations of di use radio emission and Faraday Rotation Measures. Despite the observations, the origin and impact of the magnetic elds in these systems is poorly understood. In this thesis we develop a state of the art cosmological Smoothed Particle Magnetohydrodynamics code, GCMHD+, to enable the study of the magnetic elds of the largest bound structures in the Universe. Using a wide range of idealized test problems, we justify our choice of free parameters and demonstrate the performance of the code relative to analytical solutions and the results produced by a grid based MHD scheme. We then used the code to investigate the evolution of a seed magnetic eld due to the formation of structure. By varying the numerical scheme, we demonstrate that the growth of magnetic elds in galaxy clusters are very sensitive to the growth of numerical divergence of the magnetic eld. We nd that amplitude and topology of the cluster magnetic eld are insensitive to the mass or formation history of the cluster. Using high resolution simulations, we show that a primordial seed magnetic eld is capable of reproducing a wide range of observations of large-scale magnetic elds in galaxy clusters. Additionally, we examine the impact of the formation of spiral structure in a disc galaxy on the galactic magnetic eld. We nd that the numerical scheme can become unstable unless the divergence cleaning scheme is limited. We nd that the rotation of the galaxy produces a disc orientated magnetic eld with a spiral structure and large-scale eld reversals. The formation of spiral arms ampli es the ambient G magnetic eld to 20 G, in agreement with the observations of spiral galaxies. We conclude that additional physics is required to produce a more realistic galactic magnetic eld.

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Wilmot-Smith, Antonia. "The origin and dynamic interaction of solar magnetic fields." Thesis, St Andrews, 2008. http://hdl.handle.net/10023/417.

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Beck, Alexander Maximilian. "On the origin and growth of cosmic magnetic fields." Diss., Ludwig-Maximilians-Universität München, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:19-164179.

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Schwarte, Judith [Verfasser]. "Modelling the earth's magnetic field of magnetospheric origin from CHAMP data / Geoforschungszentrum Potsdam. Von Judith Schwarte." Potsdam : Geoforschungszentrum, 2004. http://d-nb.info/971057001/34.

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7

Pariev, Vladimir Ivanovich. "Magnetic fields: Their origin and manifestation in accretion disks around supermassive black holes." Diss., The University of Arizona, 2001. http://hdl.handle.net/10150/279820.

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The magnetic field dynamo in the inner part of accretion disks around supermassive black holes in AGNs may be an important mechanism for the generation of magnetic fields in galaxies and in extragalactic space. We consider dynamo with the necessary helicity generation produced by star-disk collisions. Gas heated by a star passing through the disk is buoyant and form rising and expanding plume of plasma. Due to Coriolis forces the flow produced by plumes have coherent helicity. This helicity is the source of alpha effect in the alpha-O dynamo in differentially rotating accretion disk. We apply the mean field dynamo theory to the ensemble of plumes produced by star-disk collisions. We demonstrate the existence of the dynamo and evaluate the growth rate of magnetic field. The estimate of the nonlinear saturated state of the dynamo gives the magnetic field exceeding equipartition with the thermal energy in the accretion disk. Thus, star-disk collision dynamo can be important in generating dynamically significant magnetic fields, which could alter the disk structure and be the source of the energy flow in extragalactic jets. We present results of numerical simulations of the kinematic dynamo produced by star-disk collisions. It was found that for about one star-disk collision per period of rotation of the inner edge of an accretion disk, the typical value of the threshold magnetic Reynolds number is of the order of 100. The generated mean magnetic field has predominantly even parity. We also present theoretical consideration of magnetic dynamo in New Mexico dynamo experiment, which is currently under construction. The experiment utilizes Couette flow and driven jets of liquid sodium to simulate astrophysical alpha-O dynamos in the laboratory. We perform numerical simulations with ideally conducting boundary and evaluate the changes, which vacuum boundary conditions introduce in our numerical results. We also develop the theory of the MHD Ekman boundary layer in differentially rotating conducting fluid. The Ekman layer is formed at the end plates in the experiment. We show that the Ekman layer does not influence the generation of the large scale magnetic field in the experiment.

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Aoyama, Tadashi. "A study on the origin of small-scale field-aligned currents as observed in topside ionosphere at middle and low latitudes." 京都大学 (Kyoto University), 2017. http://hdl.handle.net/2433/225408.

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9

Beck, Alexander Maximilian [Verfasser], and Harald [Akademischer Betreuer] Lesch. "On the origin and growth of cosmic magnetic fields / Alexander Maximilian Beck. Betreuer: Harald Lesch." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2013. http://d-nb.info/1046503154/34.

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10

Bertin, Alexandre. "Geometrical frustration and quantum origin of spin dynamics." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GRENY014/document.

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Cette thèse se concentre sur l'étude de composés magnétiques géométriquement frustrés où les ions magnétiques se situent aux sommets d'un réseau de tétraèdres partageant leurs sommets: les composés pyrochlores. Deux familles de formule chimique R2M2O7, où R est un lanthanide et M= Ti, Sn, sont particulièrement intéressantes puisqu'elles présentent une grande variété d'états magnétiques exotiques. Premièrement, nous avons étudié le champ cristallin agissant au site de la terre rare dans l'approximation de Stevens où uniquement le terme fondamental est considéré. Un jeu unique de paramètres a été déterminé pour chaque famille considérée grâce à une analyse globale incluant des spectres de neutrons inélastiques de plusieurs composés. Ensuite, nous avons caractérisé avec un large éventail de techniques les propriétés physiques à basse température de Nd2Sn2O7. En dessous de la température de transition Tc=0.91 K, ce composé possède un ordre magnétique à longue portée dans la configuration de spins dite "all-in-all-out". Une persistance de la dynamique de spins a été révélée dans la phase ordonnée, attribuée à des excitations unidimensionnelles de spins. Une dynamique de spins anormalement lente est également reportée dans la phase paramagnétique. Enfin, nous avons apporté quelques informations sur les deux états fondamentaux proposés pour le composé très étudié Tb2Ti2O7: premièrement, l'apparition d'une transition Jahn-Teller à basse température est suggérée mais l'absence d'élargissement des pics de Bragg réfute la présence d'une transition structurale. Enfin ce composé pourrait être un exemple d'une glace de spin quantique mais l'existence d'un plateau d'aimantation n'est pas évident jusqu'à T=20 mK
This Phd thesis focuses on the study of magnetically frustrated compounds where magnetic ions lie at the vertices of a corner-sharing tetrahedra network: the pyrochlore compounds. The two series of chemical formula R2M2O7, where R is a lanthanide and M=Ti, Sn, are of peculiar interest since they display a large variety of exotic magnetic ground states. First, we have studied the crystal-electric-field acting at the rare earth within the Stevens approximation where only the ground state multiplet is considered. A single set of parameters for each families of interest has been determined through a global analysis including several inelastic neutron scattering spectra of various compounds. Then, we have characterised with a large panel of techniques the low temperature physical properties of Nd2Sn2O7. This compound enters a long-range magnetic order at transition temperature Tc=0.91 K with an ``all-in-all-out'' spin configuration. A persistence of spin dynamics has been found in the ordered phase, ascribed to one-dimensional spin loops excitations. Anomalously slow paramagnetic spin fluctuations are also reported. Finally, we have brought information on the two proposed ground states of the widely studied compound Tb2Ti2O7: first, a Jahn-Teller transition is claimed to occur at low temperatures but no broadening of the Bragg peaks is seen down to T=4 K precluding premises of a structural transition. Secondly, this compound could be a realisation of a quantum spin-ice but no definitive evidence of a magnetisation plateau is found down to T=20 mK

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11

Darah, Alhashmi A. "Magnetic field generation within an ambient magnetic field." Thesis, University of Newcastle Upon Tyne, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.505835.

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Potter, D. K. "Magnetic remanences of gyromagnetic origin in rocks and other magnetic materials." Thesis, University of Newcastle Upon Tyne, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.372568.

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13

Satter, Md Abdus. "A theory for dilute magnetic alloys : the origin of magnetic anisotropy." Thesis, University of Warwick, 1989. http://wrap.warwick.ac.uk/101517/.

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In this thesis, a formalism for studying the anisotropic interaction between two substitutional magnetic impurities and the magnetic anisotropic effect in a dilute noble metal-transition metal magnetic alloy has been developed from relativistic scattering theory. The theoretical development and the computational techniques of this formalism are based on relativistic spin-polarised scattering theory and relativistic band structure frameworks. For studying the magnetic anisotropic effect a convenient ‘working’ frame of reference with its axes oriented along the fee crystal axes is set up. This formalism is applied to study the situation for two Fe impurities in both paramagnetic Au (heavy) and Cu hosts. For AuFe dilute alloy, the two impurity site interaction as a function of separation is not oscillatory and the anisotropic effect is found to be less than the two site interaction itself only by an order of magnitude. Apart from the anisotropic coupling of the two impurity spins to the separation vector, for the first time, another weak anisotropic coupling to the crystal axes is also contained in the two site interaction. These anisotropic effects are the results of the relativistic spin-orbit interaction which are incorporated into the formalism.

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14

Petropoulos, Labros Spiridon. "Magnetic field issues in magnetic resonance imaging." Case Western Reserve University School of Graduate Studies / OhioLINK, 1993. http://rave.ohiolink.edu/etdc/view?acc_num=case1060710667.

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15

Brett, Belinda Jane. "The galactic magnetic field." Thesis, University of Plymouth, 1986. http://hdl.handle.net/10026.1/1696.

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Ostroumov, Ivan Victorovich. "Investigation of magnetic field." Thesis, Polit. Challenges of science today: XIV International Scientific and Practical Conference of Young Researchers and Students, April 2–3, 2014 : theses. – К., 2014. – 34p, 2014. http://er.nau.edu.ua/handle/NAU/26583.

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Earth's magnetic field – is a field around our planet, where there are magnetic forces. Magnetic field is characterized by intensity vector. Intensity vector “T” is the sum of the vectors strengths of several fields: The main magnetic field (dipole uniform) is formed by electric currents in the earth's core, the magnetization of the upper layers of the earth creates a so-called continental field, ferromagnetic ore unevenly located in the earth's surface create local anomalous, magnetic fields that distort the main field of the Earth. Typically, the vector T is estimated from its projections, on the NED coordinate system (Mx, My, Mz components). Also important two angles: declination and inclination, which indicated position of intensity vector in space, horizontal H and vertical Z components indicate intensity of T. Declination is positive for an eastward deviation of the field relative to true north. It can be estimated by comparing the magnetic north/south heading on a compass with the direction of a celestial pole. The inclination is given by an angle that can assume values between -90° (up) to 90° (down). In the northern hemisphere, the field points downwards. It is straight down at the North Magnetic Pole and rotates upwards as the latitude decreases until it is horizontal (0°) at the magnetic equator.

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17

Gunther, Janelle. "Defects in liquid crystal polymers : their origins and behavior in magnetic and flow fields." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/43466.

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18

Zhang, Qi. "Magnetic Rendering: Magnetic Field Control for Haptic Interaction." Thesis, Université d'Ottawa / University of Ottawa, 2015. http://hdl.handle.net/10393/32613.

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As a solution to mid-air haptic actuation with strong and continuous tactile force, Magnetic Rendering is presented as an intuitive haptic display method applying an electromagnet array to produce a magnetic field in mid-air where the force field can be felt as magnetic repulsive force exerted on the hand through the attached magnet discs. The magnetic field is generated by a specifically designed electromagnet array driven by direct current. By attaching small magnet discs on the hand, the tactile sensation can be perceived by the user. This method can provide a strong tactile force on multiple points covering user’s hand and avoid cumbersome attachments with wires, thus it is suitable for a co-located visual and haptic display. In my work, the detailed design of the electromagnet array for haptic rendering purposes is introduced, which is modelled and tested using Finite Element Method simulations. The model is characterized mathematically, and three methods for controlling the magnetic field are applied accordingly: direct control, system identification and adaptive control. The performance of the simulated model is evaluated in terms of magnetic field distribution, force strength, operation distance and force stiffness. The control algorithms are implemented and tested on a 3-by-3 and a 15-by-15 model, respectively. Simulations are performed on a 15-by-15 model to generate a haptic human face, which results in a smooth force field and accurate force exertion on the control points.

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19

Beck, Paul Beck Paul. "Magnetic field assisted biomaterials processing /." [S.l.] : [s.n.], 2009. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=18292.

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20

Glover, Paul Martin. "High field magnetic resonance imaging." Thesis, University of Nottingham, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335575.

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21

Bingaman, Adam Neal. "Tilt-Compensated Magnetic Field Sensor." Thesis, Virginia Tech, 2010. http://hdl.handle.net/10919/33295.

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Motion and tilt have long hindered the accuracy, reliability, and response of magnetic detection systems. Perturbations in the magnetic field reading resulting from motion cause degradation of the output signal, compromising the performance and reliability of the magnetometer system.

The purpose of this document is to describe the development, construction, and testing of a tilt-stabilized three-axis magnetic field sensor. The sensor is implemented as a three-axis general-purpose magnetic field sensor, with the additional capability of being implemented as a compass. Design and construction of system hardware is discussed, along with software development and implementation.

Finite impulse response filters are designed and implemented in hardware to filter the acquired magnetic signals. Various designs of median filters are simulated and tested for smoothing inclination signal irregularities and noise. Trigonometric conversions necessary for tilt-compensation are calculated in software using traditional methods, as well as the Coordinate Rotation Digital Computer (CORDIC) algorithm. Both calculation methods are compared for execution time and efficiency.

Successful incorporation of all design aspects leads to detection and output of stable earth magnetic fields, sinusoidal signals, and aperiodic signatures while the magnetometer system is subject to significant tilt motion. Optimized system execution time leads to a maximum detectable signal bandwidth of 410 Hz. Integration of azimuth angle calculation is incorporated and is successfully tested with minimal error, allowing the system to be used as a compass.

Results of the compensated system tests are compared to non-compensated results to display system performance, including tilt-compensation effectiveness, noise attenuation, and operational speed.
Master of Science

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22

Goodluck, Olufemi W. (Olufemi Waheed). "Magnetic separation of strongly magnetic particles using alternating field." Thesis, McGill University, 1986. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=65920.

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23

Miao, Jingqi. "The theory of magnetic polarons and magnetic field effect in diluted magnetic semiconductors." Thesis, University of Hull, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.264955.

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24

Arpinar, Volkan Emre. "Analysis Of Magnetic Resonance Imaging In Inhomogenous Main Magnetic Field." Phd thesis, METU, 2009. http://etd.lib.metu.edu.tr/upload/12611008/index.pdf.

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In this study, analysis of Magnetic Resonance Imaging (MRI) in inhomogeneous main magnetic field is conducted. A numerical model based on Bloch equation is implemented for MRI, to understand effect of inhomogeneous magnetic field to Magnetic Resonance (MR) signal. Using the model, relations between inhomogeneity levels in main magnetic field with energy, decay time, bandwidth of the FID signal is investigated. Also relation between the magnetic field inhomogeneity and field of view is determined. To simulate measurement noise in the FID signal under inhomogeneous main magnetic field, noise model for MRI with homogeneous main field is altered. Following the numerical model development an image reconstruction algorithm for inhomogeneous main magnetic field is developed to remove undesirable effect of field inhomogeneity in image reconstruction. To evaluate capability of the reconstruction algorithm, the algorithm is tested for several input parameters which results in different noise levels in the FID signal. Then reconstruction errors are analysed to gain information about feasibility of MRI in inhomogeneous main magnetic field.

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25

Luck, Stanley David. "The role of magnetic field gradients in nuclear magnetic resonance." Thesis, University of British Columbia, 1986. http://hdl.handle.net/2429/27541.

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A high resolution NMR probe was modified with gradient coils (31 mm diameter) for the measurement of translational diffusion and for microscopic imaging, and a larger set of gradient coils (15 cm diameter) was constructed for surface coil diffusion measurements. The magnitudes of the gradients produced by these coils were determined from the linewidths and lineshapes of gradient spectra. In diffusion experiments using the pulsed gradient method of Stejskal and Tanner, induced eddy currents and slow variation of the magnetic field at the sample interfered with measurements at short echo times. For these experiments the known diffusion coefficient of water was used to determine the effective gradient in each experiment. The diffusion coefficient of acrylonitrile was measured from the decay of single, double and triple quantum echoes using a modified pulsed field gradient spin echo pulse sequence. In the second part of this thesis, three examples of living systems were studied. The first involved the application of pulsed gradient spin echo measurements to characterize the motion of water and lipid, in-vivo, in human forearm. Spin echo spectra from human forearm gave a water signal that was attributed to extracellular water because of relatively long spin-spin relaxation time (0.8 s) compared to that of intracellular water (20-30 ms). Comparison of the diffusivity of water, from experiments at two different echo times suggest that the major part of the motion of water, in-vivo, was due to directionally randomized bulk flow rather than molecular diffusion. The second application involved the chemical shift resolved mapping of the proton distribution, in one-dimension, along the anteroposterior direction, of pupae of the Douglas-fir cone moth Barbara colfaxiana. Proton distribution maps showed that the distribution of the aqueous fluid depended upon the vertical orientation, head pointing upward or downward, of the pupae. Finally, two dimensional images of mature caps of the marine alga Acetabularia mediterranea were obtained using the normal spin echo sequence as well as with T₁, T₂ and diffusion contrasting. D₂O—contrasting was obtained by briefly submerging the caps in D₂O. All of these images showed features resembling the radial structure of the caps. The resolution was estimated by comparison with microscopic views of the caps and was found to be 0.1 mm, determined as the smallest distinguishable feature in the image.
Science, Faculty of
Chemistry, Department of
Graduate

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26

Lukawska, Anna Beata. "THERMAL PROPERTIES OF MAGNETIC NANOPARTICLES IN EXTERNAL AC MAGNETIC FIELD." Wright State University / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=wright1401441820.

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27

Watterson, P. A. "Force-free magnetic field evolution in the reversed-field pinch." Thesis, University of Cambridge, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.355048.

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28

Carter, John P. "Magnetic field generation in shock waves." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1994. http://edocs.nps.edu/npspubs/scholarly/theses/1994/June/94Jun_Carter.pdf.

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29

Virtanen, I. (. Ilpo). "Asymmetry of the heliospheric magnetic field." Doctoral thesis, University of Oulu, 2013. http://urn.fi/urn:isbn:9789526202563.

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Abstract This thesis studies the structure and evolution of the large scale heliospheric magnetic field. The work covers the space age, the period when satellite measurements revolutionized our knowledge about space. Now, this period is known to be the declining phase of the grand modern maximum of solar activity. The thesis addresses how the hemispherical asymmetry of solar activity is seen in the photospheric magnetic field and how it appears in the corona and in the heliosphere until the termination shock. According to geomagnetic and heliospheric observations, the heliospheric current sheet has been southward shifted around the solar minima since 1930s. Using Ulysses probe observations, we derive an accurate estimate of 2° for the southward shift of the heliospheric current sheet during two very different solar minimum in the mid 1990s and 2000s. The overall structure of the heliospheric magnetic field has changed significantly now when the grand modern maximum has come to an end. During the present low solar activity the polar fields are weaker and the heliospheric current sheet covered a wide latitudinal range during the previous minimum. When the heliospheric current sheet is wide the asymmetry is less visible at the Earth’s orbit. We extend our study to the outer heliosphere using measurements made by Voyager and Pioneer probes and show that the hemispherical asymmetry in the coronal hole evolution, and the related southward shift of the heliospheric current sheet, are seen until the termination shock. In order to understand the origin of the hemispherical asymmetry, we complete a multipole analysis of the solar magnetic field since 1976. We find that the minimum time southward shift of the heliospheric current sheet is due to the quadrupole component of the coronal magnetic field. The quadrupole term exists because the generation and transport of the magnetic flux in the Sun tends to proceed differently in the northern and southern hemispheres. During this and the following decade the Sun is most likely going to be less active than it has been since 1920s. Therefore it is probable that the hemispherical asymmetry of the heliospheric magnetic field will be less visible in the ecliptic plane in the near future. Now, when the Sun seems to be at the maximum of cycle 24, we are looking forward to see how the polar fields and the heliospheric magnetic field are formed when approaching the following solar minimum. It is possible that, as the activity rises again after the present and future low cycles, the hemispherical asymmetry will be opposite to that of the 20th century and the minimum time heliospheric current sheet would be northward shifted.

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Brada, Matej. "Graphene electronic devices in magnetic field." Thesis, Loughborough University, 2016. https://dspace.lboro.ac.uk/2134/21800.

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This thesis discusses the two dimensional allotrope of carbon known as graphene in presence of magnetic field, with special focus on edge states. The structure of graphene is described in detail and from the structure, two models are formed. The Dirac equation is a good description of graphene for large samples, far away from edges, where the boundaries can be ignored. However, it causes problems with most types of edge and hard wall approximation has to be implemented. The Dirac equation is described in detail and used to obtain an energy spectrum, wavefunction and density of states for graphene edge in a strong magnetic field. For comparison, a Bohr-Sommerfield approximation was used to find the dispersion relation and compare it to the results obtained numerically from the Dirac equation. The second model, better fitting for nano-scale systems, is the tight binding model. This model was utilized to find Energy spectrum for graphene flakes in magnetic field, which resembles Hofstadter's butterfly spectrum. The spectrum was analyzed and periodic oscillations of magnetisation dependent on magnetic field (known as the de Haas-van Alphen effect) were described. The oscillation of magnetisation depends on the shape of the dot, even though the main properties remain the same: at low magnetic field, periodic oscillations due to Aharonov-Bohm effect, turning into more chaotic oscillations depending on the boundary conditions of the given quantum dot.

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Pointer, Graham Richard. "The magnetic field of AB Doradûs." Thesis, University of St Andrews, 2001. http://hdl.handle.net/10023/12940.

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Observations of AB Doradus, a nearby, rapidly-rotating K0 dwarf are analysed, and the surface magnetic field is shown to be approximated by a potential field. Evolving the surface magnetic field according to diffusion and the observed differential rotation still yields good correlation between the calculated and observed radial field after 30 days, contradictory to the results of Barnes et al. (1998), leading to the conclusion that there is an additional cause for the evolution of the magnetic field. The chromospheric magnetic field is modelled as a potential field with a source surface. Using the stability criteria g.B = 0 and B.V(g.B) < 0, places where prominences can be stable are investigated. For agreement with the results of Donati et al. (2000)- that prominences form preferentially near the equatorial plane and at and beyond corotation- it is necessary to add a quasidipolar field of maximum strength ~20G.

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32

Olander, Terese. "The magnetic field of ε Eri." Thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-201519.

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The magnetic eld of the star, ε Eri, is investigated. It is a nearbyK2V star. Previous studies have shown indications of a magnetic eld,but no measurements have been done with polarization. The Zeemaneect causes the lines in the spectrum to be split. The light also becomespolarized. By looking at the circular polarization, we can get informationabout the longitudinal magnetic eld.We study the polarization proles in the high-resolution spectra of εEri, to increase the S/N ratio, the LSD method is used. By combiningmany lines in the spectra, the shape of the averaged line can be derived.Using this approach, we obtain Stokes I, V, and the null spectra. Theyare then compared. From these, the mean longitudinal magnetic eld iscalculated. Data from two separate observing runs are compared: Onefrom 2010 and the other from 2011. Based on this analysis we nd a largereduction of magnetic eld strength of ε Eri in just one year.

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33

Leichle, Thierry C. "A micromachined resonant magnetic field sensor." Thesis, Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/13833.

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34

Robinson, Alexander Joseph. "Magnetic field effects on biological systems." Thesis, University of Oxford, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.543006.

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35

Pulecio, Javier F. "Field-Coupled Nano-Magnetic Logic Systems." Scholar Commons, 2010. http://scholarcommons.usf.edu/etd/3608.

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The following dissertation addresses the study of nano-magnetic devices configured to produce logic machines through magnetostatic coupling interactions. The ability for single domain magnets to reliably couple through magnetostatic interactions is essential to the proper functionality of Magnetic Cellular Automata (MCA) devices (p. 36). It was significant to explore how fabrication defects affected the coupling reliability of MCA architectures. Both ferromagnetic and anti-ferromagnetic coupling architectures were found to be robust to common fabrication defects. Experiments also verified the functionality of the previously reported MCA majority gate [1] and a novel implementation of a ferromagnetic MCA majority gate is reported. From these results, the study of clocking Magnetic Cellular Automata (MCA) interconnect architectures was investigated (p. 54). The wire architectures were saturated under distinct directions of an external magnetic field. The experimental results suggested ferromagnetic coupled wires were able to mitigate magnetic frustrations better than anti-ferromagnetic coupled wires. Simulations were also implemented supporting the experimental results. Ferromagnetic wires were found to operate more reliably and will likely be the primary interconnects for MCA. The first design and implementation of a coplanar cross wire system for MCA was constructed which consisted of orthogonal ferromagnetic coupled wires (p. 68). Simulations were implemented of a simple crossing wire junction to analyze micro-magnetic dynamics, data propagation, and associated energy states. Furthermore, two systems were physically realized; the first system consisted of two coplanar crossing wires and the second was a more complex system consisting of over 120 nano-magnetic cells. By demonstrating the combination of all the possible logic states of the first system and the low ground state achieved by the second system, the data suggested coplanar cross wire systems would indeed be a viable architecture in MCA technology. Finally, ongoing research of an unconventional method for image processing using nano-magnetic field-based computation is presented (p. 79). In magnetic field-based computing (MFC), nano-disks were mapped to low level segments of an image, and the magnetostatic coupling of magnetic dipole moments was directly related to the saliency of a low level segment for grouping. A proof of concept model for two MFC systems was implemented. Details such as the importance of fabricating circular nano-magnetic cells to mitigate shape anisotropy, experimental coupling analysis via Magnetic Force Microscopy, and current results from a complex MFC system is outlined.

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36

Herreros, Quentin. "Very low field magnetic resonance imaging." Phd thesis, Université René Descartes - Paris V, 2013. http://tel.archives-ouvertes.fr/tel-01064053.

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The aim of this thesis is to perform Magnetic Resonance Imaging at very low field (from 1 mT to 10 mT). A new kind of sensor called "mixed sensor" has been used to achieve a good detectivity at low frequencies. Combining a superconducting loop and a giant magnetoresistance, those detectors have a competitive equivalent field noise compared to existing devices (Tuned coils, SQUIDs and Atomic Magnetometers). They have been combined with flux transformers to increase the coupling between the sample and the sensor. A complete study has been performed to adapt it to mixed sensors and then maximize the gain. This set has been incorporated in an existing small MRI device to test its robustness in real conditions. In parallel, several MRI sequences (GE, SE, FLASH, EPI, ...) have been integrated and adapted to very low field requirements. They have been used to perform in-vivo three dimensional imaging and relaxometry studies on well known products to test their reliability. Finally, a larger setup adapted for full-head imaging has been designed and built to perform images on a larger working volume.

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Rodgers, Christopher T. "Magnetic field effects in chemical systems." Thesis, University of Oxford, 2007. http://ora.ox.ac.uk/objects/uuid:f5878b88-c5ba-4cbd-83af-857431aef66e.

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Magnetic fields influence the rate and/or yield of chemical reactions that proceed via spin correlated radical pair intermediates. The field of spin chemistry centres around the study of such magnetic field effects (MFEs). This thesis is particularly concerned with the effects of the weak magnetic fields B₀ ~ 1mT relevant in the ongoing debates on the mechanism by which animals sense the geomagnetic field and on the putative health effects of environmental electromagnetic fields. Relatively few previous studies have dealt with such weak magnetic fields. This thesis presents several new theoretical tools and applies them to interpret experimental measurements. Chapter 1 surveys the development and theory of spin chemistry. Chapter 2 introduces the use of Tikhonov and Maximum Entropy Regularisation methods as a new means of analysing MARY field effect data. These are applied to recover details of the diffusive motion of reacting pyrene and N,N-dimethylaniline radicals. Chapter 3 gives a fresh derivation and appraisal of an approximate, semiclassical approach to MFEs. Monte Carlo calculations allow the elucidation of several "rules of thumb" for interpreting MFE data. Chapter 4 discusses recent optically-detected zero-field EPR measurements, adapting the gamma-COMPUTE algorithm from solid state NMR for their interpretation. Chapter 5 explores the role of RF polarisation in producing MFEs. The breakdown in weak fields of the familiar rotating frame approximation is analysed. Chapter 6 reviews current knowledge and landmark experiments in the area of animal magnetoreception. The origins of the sensitivity of European robins Erithacus rubecula to the Earth’s magnetic field are given particular attention. In Chapter 7, Schulten and Ritz’s hypothesis that avian magnetoreception is founded on a radical pair mechanism (RPM) reaction is appraised through calculations in model systems. Chapter 8 introduces quantitative methods of analysing anisotropic magnetic field effects using spherical harmonics. Chapter 9 considers recent observations that European robins may sometimes be disoriented by minuscule RF fields. These are shown to be consistent with magnetoreception via a radical pair with no (effective) magnetic nuclei in one of the radicals.

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Fenton, Neil David. "Optical and magnetic ligand-field studies." Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.317773.

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Sulaiman, Ali Haidar. "The near-Saturn magnetic field environment." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/44209.

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Shock waves exist throughout the universe and are fundamental to understanding the nature of collisionless plasmas. The complex coupling between charged particles and electromagnetic fields in plasmas give rise to a whole host of mechanisms for dissipation and heating across shock waves, particularly at high Mach numbers. While ongoing studies have investigated these process extensively both theoretically and via simulations, their observations remain few and far between. This thesis presents a study of very high Mach number shocks in a parameter space that has been poorly explored and identifies reformation using in situ magnetic field observations from the Cassini spacecraft at Saturn's bow shock. This gives an insight into quasi-perpendicular shocks across two orders of magnitude in Alfvén Mach number (MA) and spanning Earth-like to Astrophysical-like regimes. The work here shows evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted timescale of ~0.3 τc, where τc is the ion gyroperiod. The relationship between these reformation signatures, magnetic overshoot and variability are also presented. The final part of this thesis characterises the region downstream of Saturn's bow shock, the magnetosheath. The results show a comprehensive overview of the configuration of the magnetic field in a non-axisymmetric magnetosheath. This non-axisymmetry is revealed to have an impact in the rotation of the magnetic field and is significant enough to influence the magnetic shear at the magnetopause.

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Aigner, Simon. "Magnetic field microscopy using ultracold atoms." [S.l. : s.n.], 2007. http://nbn-resolving.de/urn:nbn:de:bsz:16-opus-79425.

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Takeo, Hiroshi. "Copper electrodeposition in a magnetic field." PDXScholar, 1985. https://pdxscholar.library.pdx.edu/open_access_etds/3550.

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The effect of a magnetic field on copper electrodeposition was investigated. Copper was electrodeposited onto square copper cathodes 1 sq cm in area from an aqueous solution (0.5 M CuSO4, 0.5 M H2SO. A glass cell was placed between the pole pieces of an electromagnet, and the magnetic fields applied were in the range from 0 to 12.5 kG. The current density was in the range from 80 mA/sq cm to 880 mA/sq cm. In each of the experiments, cell current, cell voltage, and cell temperature were monitored with a microcomputer. The weight change, deposit surface and cross section morphology, and the hardness were also found. Anodes used in the experiments were studied to see the effect of various conditions on the surface finish. Copper was also electrodeposited onto copper grids in order to study how the uniformity of the deposit is affected by an applied magnetic field.

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Tucker, Philip John Yorke. "Magnetic instability, magnetoconvection and magnetic field generation in a plane layer." Thesis, University of Exeter, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263204.

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Ng, Bryan. "Understanding magnetic field reversal mechanisms in mesoscopic magnetic multilayer ring structures." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45395.

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Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.
Includes bibliographical references.
Patterned pseudo spin-valve rings show great promise for device applications due to their non-volatility and variety of stable magnetic states. However, the magnetic reversal of these elements under an applied field is complex due to the magnetostatic coupling between the two ferromagnetic layers. Elliptical rings are electrically probed using highly symmetric Wheatstone bridges in conjunction with traditional four-point electrical measurements and micromagnetic simulations. New insight into domain wall nucleation and propagation events are elucidated. The resulting behavior is found to yield large signals at very low fields, making these devices ideal for device applications in data storage and computer logic. 360° domain walls are found to be extremely stable until fields as high as 10000e, but are positionally uncontrollable in elliptical rings. Rhombic rings were investigated as a geometry that can nucleate, propagate and pin domain walls more easily. Measurements and simulations confirm that the same reversal mechanisms exist and domain walls are more systematically positioned. The control over 3600 domain walls is valuable since reversals can occur without nucleation by decoupling the wall into a reverse domain. As a result, rhombic rings are useful as devices that can perform device functions at extremely low fields.
by Bryan Ng.
M.Eng.

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Rinkevich, A. B., M. I. Samoylovich, and A. F. Belyanin. "Effective Conductivity and Magnetic Permeability of Nanostructured Materials in Magnetic Field." Thesis, Sumy State University, 2013. http://essuir.sumdu.edu.ua/handle/123456789/35340.

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The problem of homogenization the nanostructured materials placed in DC magnetic field has been discussed. The experimental data are obtained using metallic superlattices, metal-dielectric thin films and 3D-nanostructured materials. All these materials contain ferro- or ferrimagnetic component. The trans-mission and reflection coefficients were measured on the waves of millimeter waveband. It has been shown that the experimental frequency spectra of the coefficients in zero magnetic field can be described by the effective conductivity and dielectric permittivity. The spectra of ferromagnetic resonance, however, cannot be calculated correctly with the averaged magnetization. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35340

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Sipilä, Pekka [Verfasser]. "Real-Time Magnetic Field Monitoring in Magnetic Resonance Imaging / Pekka Sipilä." Aachen : Shaker, 2011. http://d-nb.info/1069050512/34.

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Ignace, R., and K. G. Gayley. "Circumstellar Magnetic Field Diagnostics from Line Polarization." Universität Potsdam, 2007. http://opus.kobv.de/ubp/volltexte/2008/1806/.

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Maguire, Steven. "Magnetic field control of silver nanoparticle formation." Thesis, University of Ottawa (Canada), 2006. http://hdl.handle.net/10393/27390.

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Silver nanoparticles can be readily generated in micellar environments by ketyl radicals formed from the photoreduction of benzophenone in the presence of a suitable hydrogen donor. The yield of these ketyl radicals can be increased by extending the lifetime of the triplet radical pair through Zeeman splitting of the triplet sublevels in an externally applied magnetic field. This provides control over the rate of photogeneration of nanoparticles under very mild conditions. The rate of photogeneration can be monitored by the distinctive surface plasmon resonance absorption around 420 nm. In this work, micelles of sodium dodecyl sulphate (SDS) were employed, and 1,4-cyclohexadiene (1,4-CHD), an excellent hydrogen donor, was used to promote the generation of ketyl radicals. When benzophenone and a silver salt are added to this system and it is irradiated in the presence of a magnetic field, the rate of appearance of the plasmon band is enhanced. In addition to serving as a hydrogen donor, 1,4-CHD also has a stabilising influence on the nanoparticles, adsorbing onto the surface and preventing aggregation. 1,4-CHD added to a solution of nanoparticles synthesised without the diene present will even break up existing aggregates.

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Unger, Philip Peter. "NMR-based microprobes for magnetic field measurements." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/NQ62675.pdf.

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Uno, Hideharu. "New constraints on Mercury's internal magnetic field." Thesis, University of British Columbia, 2009. http://hdl.handle.net/2429/7716.

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Three-component vector magnetic field observations from MESSENGER’s first two flybys of Mercury have confirmed the presence of an internal field, along with external fields related to magnetospheric current systems. We use techniques in inverse theory to investigate structure in Mercury’s inter nal magnetic field permitted by the Mariner 10 and MESSENGER flyby data, and structures recoverable while spacecraft is in orbit. We remove external fields predicted by a parameterized magnetospheric model from the flyby observations. We estimate noise contributions from long-wavelength uncertainties in the external field and from un-modeled short-wavelength features. Internal field models are parameterized to spherical harmonic degree and order 8, with regularization constraints applied to the power spectrum. The field is predominantly dipolar but additional latitudinal and longitudinal structure is required to fit the data. Enhanced radial magnetic field in the region of the Mariner 10 and MESSENGER flybys latitudes is seen. Contributions to the internal field predicted by Aharonson and others for a long-wavelength crustal field are present (namely, the g⁰₁, g⁰₃, and g²₃ spherical harmonic coefficients), but our hypothesis testing has shown that the field is dominated by the g⁰₁ term rather than the proposed g⁰₃ term. Further hypothesis testing has determined that the dipole tilt of Mercury is in the range of 30 to 13° with 13° as a strict upper bound. Observations from the upcoming MESSENGER flyby will provide additional, and critical, low-latitude coverage. Analyses of flyby data have shown that much of the recovery internal field depends successfully characterizing the external field signatures that are present in the data. Currently, the limited ninnber of ob servations prevents us from constructing a reliable external field model, but these external field models will improve as data becomes more abundant. We also investigate recovery of three simulated core fields using synthetic data during MESSENGER’s orbital phase, under the assumption that long wavelength external fields can be modeled and removed. The results show excellent recovery of the dipole field and of field structure at mid-northern to high latitudes out to degree and order 10, providing encouraging results for future identification and characterization of core fields.

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Zhu, Weiwei. "The cooling of high-magnetic-field pulsars." Thesis, McGill University, 2011. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=104725.

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Prior to ~20 years ago, only two kinds of pulsars were known: RPPs and accretion-powered pulsars. The rapid advance of X-ray astronomy in the past few decades has led to the discovery of magnetic-powered pulsars, namely "magnetars". Magnetars were first identified with the SGRs which exhibit sporadic soft gamma-ray bursts. More recently, another group of pulsars, the AXPs, characterised by their bright persistent X-ray emission that is more powerful than their spin-down luminosity, were also recognized as members of the magnetar family. Both SGRs and AXPs have very high (10¹⁴-10¹⁵ G) magnetic fields as inferred from their spin-down. Studying AXP behaviour might help us understand the physics of magnetars and their connections with normal pulsars. In Chapter 3, I present our work on the X-ray afterglow of the AXP 1E 2259+586. It is the first AXP to exhibit a SGR-like outburst. It went through a major outburst in 2002. We studied the X-ray afterglow of this outburst, using ten XMM observations taken before and after the outburst. We found that the AXP's flux decayed following a power-law of index -0.69±0.03, remarkably similar what was found from the afterglow of some SGR outbursts. We also found a strong correlation between spectral hardness and X-ray flux, as seen in other AXPs. In Chapter 4 I present our work on searching for X-ray variability from the glitching AXP 1E 1841-045. This is one of the most frequent glitchers among AXPs. Magnetar theories and observations suggest that there could be a connection between magnetar glitches and their X-ray variability. However, we found no evidence of glitch-related X-ray variability from archival X-ray data of 1E 1841-045 taken between 1993 and 2006. Our finding supports the existence of radiatively silent glitches in AXPs. Interestingly, there is also a group of RPPs that have spin-down magnetic fields close to those of the magnetars (~10¹³G). These high-magnetic-field RPPs may share some observational properties with the magnetars. In Chapter 5, I present the first X-ray detection of the high-magnetic-field RPP B1916+14. We found that the pulsar's emission is likely thermal, with a surface temperature in the range of 0.08-0.23 keV. We did not detect pulsations in the data, and set a 1σ upper limit on the pulsed fraction in the 0.1-2 keV band of ~0.7. The origin of the thermal emission is not well constrained. We cannot rule out initial cooling or return current heating for this pulsar. To look for evidence of magnetic-field-decay heating, a deeper observation is needed. In Chapter 6, I present our work on CHANDRA X-ray observations of the high-magnetic-field RPP J1718-3718. We detected X-ray pulsations at the pulsar's period with 52%±13% pulsed fraction in the 0.8-2 keV band. We found, from a merged spectrum of multiple observations, a blackbody temperature of 0.19±0.02 keV, slightly higher than predicted by standard cooling models. However, the best-fit neutron star atmosphere model is consistent with standard cooling. We also found that the pulsar's bolometric luminosity represents 0.3 of its spin-down power, assuming a distance of 4.5kpc. Finally, we compared the blackbody temperatures measured for the high-magnetic-field pulsars with those from low-magnetic field rotation-powered pulsars of the same age, and found evidence of the former being on average hotter than the latter, as predicted by magneto-thermal evolution models that attempt to unify high-magnetic-field RPPs with magnetars.
Dans le chapitre 3, je présente mon travail sur la lueur résiduelle de AXP 1E2259+586. Il s'agit du premier pulsar anormal à partir duquel des pulsations similaires à celles des SGR ont été détectées. Ce pulsar a eu un sursaut énergétique majeur en 2002. Nous avons étudié la lueur résiduelle de ce sursaut au travers 10 observations faites avec le télescope XMM prises avant et après le sursaut. Nous avons trouvé que le flux du pulsar a diminué en suivant une fonction de puissance dont l'indice, −0.69±0.03, est remarquablement similaire à celui trouvé dans le rayonnement résiduel des sursauts des SGR. Nous avons aussi trouvé une corrélation entrela dureté du spectre et le flux en rayons-X, une corrélation qui est aussi observée dans d'autres AXP. Dans le chapitre 4, je présente mon travail sur la recherche de variations dans la luminosité-X de AXP 1E 1841−045. Ce pulsar est parmi les AXP qui présentent le plus fréquemment des sauts de fréquences, ou glitchs. Plusieurs théories des magnétars suggèrent une connection entre les glitchs et la variation de la luminosité-X. Malgré celà, nous n'avons pas trouvé de preuve de l'éxistence de ces variations dans la luminosité-X de 1E 1841−045 dans des observations prises entre 1993 et 2006 avec des télescopes variés. Ceci démontre l'existence de glitchs silencieux. Il est intéressant de noter qu'il existe un groupe de pulsars normaux, dont la source de luminosité est leur énergie rotationelle, qui ont un champ magnétique élevé et proche de celui des magnétars ( 1013 G). Certaines caractéristiques de ces pulsars à champ magnétique élevé sont similaires à celles des magnétars. Dans le chapitre 5, je présente la première détection en rayons-X du pulsar à champs magnétique élevé B1916+14. Nous avons trouvé que le spectre d'émission de ce pulsar est probablement thermique, avec une température de surface entre 0.08–0.23 keV. Nous n'avons pas détecté de pulsations régulières dans les données, avec une limite supérieure (1)de 0.7 sur la fraction pulsée entre 0.1 et 2 keV. Il est aussi difficile de déterminer si le spectre d'émission thermique observé est dû à un refroidissement initial ou à un courant qui réchauffe la surface du pulsar en ce moment. C'est pourquoi plus d'observations sont requises afin de prouver que le réchauffement de la surface est dû à une diminution du champ magnétique. Dans le chapitre 6, je présente mon travail effectué sur des observations faites avec le télescope Chandra du pulsar a champ magnétique élevé RPP J1718−3718. Nous avons détecté des pulsations régulières en rayons-X à un interval égal à celui de la fréquence rotationelle de ce pulsar et avec une fraction pulsée de 52%±13% entre 0.8 et 2 keV. Nous avons trouvé, en étudiant le spectre combiné de plusieurs observations,une température de corps noir de 0.19±0.02 keV. C'est une température un peu plus élevée que celle prédite par les modèles standards de refroidissement. Par contre, les modèles numériques d'atmosphère des étoiles à neutrons est en accord avec les modèles standards de refroidissement. Nous avons aussi trouvé que la luminosité bolométrique représente 0.3 de la puissance due à la perte d'énergie rotationelle, si l'on suppose une distance de 4.5 kpc. Finalement, nous avons comparé les températures de corps noirs des pulsars normaux à champ magnétique élevé avec ceux de pulsars normaux du même âge ayant un champ magnétique faible, et nous avons trouvé que les premiers avaient des températures plus élevées, comme le prédisent les modèles magnéto-thermiques ayant comme but d'unifier les pulsars normaux à champs magnétiques élevés et les magnétars.

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Dissertations / Theses on the topic 'The origin of magnetic field'

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