Tesis sobre el tema "Magnetic inhomogeneities"
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Pasquato, Ester. "Effects of stellar surface inhomogeneities on astrometric accuracy". Doctoral thesis, Universite Libre de Bruxelles, 2011. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/209872.
Texto completoIn the case of a red supergiant star, the average photocentre shift is about 0.1 AU. Such a photocentric noise translates in a 10% inaccuracy on the parallax (independently of the distance), which becomes larger than the statistical error on the parallax derived from the data reduction for stars that are up to about 4 kpc away. For the most nearby stars, we derive an inaccuracy on the parallax that can be 10 times its statistical error. Finally we estimate that up to about 4000 stars among red supergiants and bright giants may have astrometric parameters that are inaccurate at levels bigger than expected because of the surface brightness asymmetries. In the determination of this number, a crucial role is played by the Gaia observable magnitude range. The fact that Gaia will not observe stars brighter than 5.6 in the Gaia G band means that the closest stars will not be observed. Yet, the impact of the surface brightness asymmetries is proportional to their angular size, meaning that the stars whose astrometric accuracy would be most affected are not observed.
Various non-Gaussian spot models (as applicable in the case of magnetic spots) have been implemented and analytical predictions for the effects of such magnetic spots are computed for the most representative classes of magnetic stars.
Another effect of the presence of surface brightness asymmetries is their impact on Gaia data processing flow. The quality of the fit of the data is evaluated with the F2 parameter that is a transformation of χ2 such that it has a unit normal distribution when the model is adequate and it is independent of the number of measurements. If the goodness-of-fit F2 of the single-star solution is not good enough (F2>3), a chain of solution of growing complexity is tried until a satisfactory one (with F2<3) is obtained. If no good solution is found, a so-called stochastic solution is computed where a "cosmic" error is added to the data in order to obtain a single-star solution with F2=0. We show that the photocentre noise induces an increase in the goodness-of-fit parameter, causing this chain of solutions to be entered. Depending on the characteristics of the photocentre noise, a variable fraction of the stars in our simulations end up with a non-single-star solution. Yet, we show that these (orbital) solutions are not acceptable because non-significant or non-physical. Finally, an important fraction of stars is assigned a stochastic solution with a cosmic noise matching well the photocentric noise.
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Les asymétries de brillance de surface sont une caractéristique commune des étoiles. Parmi d'autres effets, elles provoquent une différence entre la projection du centre de masse et le photocentre. L'évolution de ces structures de surface rend cette différence variable avec le temps. Dans certains cas, le déplacement du photocentre peut être une fraction non négligeable du rayon de l'étoile R et, si R>1 UA, de la parallaxe. Nous examinons l'impact des asymétries de brillance de surface sur la solution astrométrique de Gaia et sur le processus de traitement des données. En particulier nous dérivons des expressions analytiques pour le changement des paramètres astrométriques déerivées pour une étoile simple, par rapport aux paramètres pour une étoile uniformément lumineuse, en fonction des caractéristiques des asymétries de brillance de surface. Ces prévisions sont confirmées par les résultats de simulations du traitement des données astrométriques de Gaia, auxquelles des mouvements du photocentre causés par des asymétries de brillance de surface ont été ajoutés en utilisant un modèle gaussien markovien.
Dans le cas d'une étoile super-géante rouge, le décalage moyen du photocentre est d'environ 0.1 UA. Un bruit photocentrique de cette amplitude se traduit dans une imprécision de 10% sur la parallaxe (indépendamment de la distance), qui peut devenir plus grande que l'erreur statistique sur la parallaxe déerivée par la réduction des données, pour les étoiles plus proches d'environ 4 kpc. Pour les étoiles les plus proches, nous évaluons une imprécision sur la parallaxe qui peut être 10 fois leur erreur statistique. Finalement, nous estimons que jusqu'à environ 4000 étoiles parmi les super-géantes rouges et géantes brillantes peuvent avoir des paramètres astrométriques inexactes à des niveaux plus grands que prévu en raison des asymétries de brillance de surface. Dans la détermination de ce nombre, la gamme de magnitudes observables par Gaia joue un rôle crucial. Le fait que Gaia n'observera pas les étoiles plus brillantes que 5.6 mag (en bande Gaia) signifie que les étoiles les plus proches ne seront pas observées. Pourtant, l'impact des asymétries de brillance de surface est proportionnel à leur taille angulaire, ce qui signifie que les étoiles dont la précision astrométrique seraient la plus affecté ne seront pas observées.
Différents modèles de taches ont été réalisés et des prédictions analytiques pour les effets de ces taches magnétiques sont calculés pour les classes les plus représentatives des étoiles magnétiques.
Un autre effet de la présence des asymétries de brillance de surface est leur impact sur le traitement des données de Gaia. La qualité de l'ajustement des données est évaluée avec le paramètre F2 qui est une transformation de χ2 telle qu'il ait une distribution normale lorsque le modèle est adéquat. Si la qualité de l'ajustement F2 de la solution étoile-simple n'est pas acceptable (F2>3), une chaîne de solutions de complexité croissante est essayée jusqu'à ce qu'une solution satisfaisante (avec F2<3) soit obtenue. Si aucune solution satisfaisante n'est trouvée, une solution dite stochastique est calculée où une erreur "cosmique" est ajoutée aux données afin d'obtenir une solution étoile-simple avec F2=0. Nous montrons que le bruit du photocentre induit une augmentation de F2, ce qui provoque l'activation de cette chaîne de solutions. Selon les caractéristiques du bruit du photocentre, une solution étoile-non-simple est obtenue pour une fraction variable des étoiles dans nos simulations. Nous montrons que ces solutions (orbitales) ainsi obtenues ne sont pas acceptables car non significatives ou non-physiques. Enfin, une fraction importante d'étoiles se voient attribuer une solution stochastique avec un bruit cosmique correspondant au bruit photocentrique.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished
Assländer, Jakob [Verfasser]. "Static Field Inhomogeneities in Magnetic Resonance Encephalography: Effects and Mitigation / Jakob Assländer". München : Verlag Dr. Hut, 2014. http://d-nb.info/1063221269/34.
Texto completoBaumann, Christoph. "Magnetic and structural inhomogeneities in single layered manganites La1-xSr1+xMnO4 : hyperfine field investigations /". Berlin : Mensch-und-Buch-Verl, 2005. http://bvbr.bib-bvb.de:8991/F?func=service&doc_library=BVB01&doc_number=014182273&line_number=0001&func_code=DB_RECORDS&service_type=MEDIA.
Texto completoAssländer, Jakob [Verfasser] y Jürgen [Akademischer Betreuer] Hennig. "Static field inhomogeneities in magnetic resonance encephalography : : effects and mitigation = Statische Magnetfeldinhomogenitäten in der Magnetresonanzencephalographie : Effekte und Mitigation". Freiburg : Universität, 2014. http://d-nb.info/1123481458/34.
Texto completoKörzdörfer, Gregor [Verfasser], Bernhard [Akademischer Betreuer] Hensel y Bernhard [Gutachter] Hensel. "Analysis and Mitigation of the Effect of Magnetic Field Inhomogeneities and Undersampling Artifacts on Magnetic Resonance Fingerprinting / Gregor Körzdörfer ; Gutachter: Bernhard Hensel ; Betreuer: Bernhard Hensel". Erlangen : Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 2020. http://d-nb.info/1211822125/34.
Texto completoReich, Jan Christoph [Verfasser] y G. [Akademischer Betreuer] Drexlin. "Magnetic Field Inhomogeneities and Their Influence on Transmission and Background at the KATRIN Main Spectrometer / Jan Christoph Reich. Betreuer: G. Drexlin". Karlsruhe : KIT-Bibliothek, 2013. http://d-nb.info/1031709037/34.
Texto completoBenitez, Mendieta Jessica. "An efficient and semiautomatic segmentation method for 3D surface reconstruction of the lumbar spine from Magnetic Resonance Imaging (MRI)". Thesis, Queensland University of Technology, 2016. https://eprints.qut.edu.au/101274/1/Jessica_Benitez%20Mendieta_Thesis.pdf.
Texto completoSplitthoff, Daniel Nicolas [Verfasser] y Jürgen [Akademischer Betreuer] Hennig. "SENSE shimming (SSH) : : fast detection of B0 field inhomogeneities in magnetic resonance imaging = SENSE Shimming (SSH) : schnelle Detektion von B0 Feldinhomogenitäten in der Magnet-Resonanz-Bildgebung". Freiburg : Universität, 2012. http://d-nb.info/1123467404/34.
Texto completoDe, Biasi Federico. "Matrix-Assisted NMR". Doctoral thesis, Università degli studi di Padova, 2019. http://hdl.handle.net/11577/3424861.
Texto completoLemarchand, Nadège. "Impacts of cosmic inhomogeneities on the CMB : primordial perturbations in two-field bouncing cosmologies and cosmic magnetism in late-time structures Secondary CMB anisotropies from magnetized haloes I. Power spectra of the Faraday rotation angle and conversion rate". Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS510.
Texto completoThe Cosmic Microwave Background (CMB) is a key cosmological probe, that sets tight constraints on the CDM model of the Universe. Released 380000 years after the big bang, it exhibits tiny anisotropies in temperature and polarisation which trace the cosmic inhomogeneities at different epochs of the Universe. On the one hand, primary anisotropies give access to inflation, during which the primordial perturbations are generated. On the other hand, secondary anisotropies trace inhomogeneities in the recent Universe, which have evolved into large scale structures through gravity, starting from the primordial ones. Hence CMB anisotropies are a powerful probe of both the origin of inhomogeneities in the very early Universe, and their evolved state in the late-time Universe. This thesis deals with two aspects of inhomogeneities by first considering their production in an extension of the inflationary scenario, and second by predicting the impact of magnetic fields in large scale structures on the secondary CMB polarised anisotropies.Despite its successes, inflation does not solve the initial big bang singularity issue, where gravity might need to be quantised. In Loop Quantum Cosmology (LQC), this singularity is replaced by a quantum bounce. Single field LQC with quadratic potential has already been studied and predicts an inflation phase following the bounce. Then, primordial inhomogeneities are not only produced during inflation, but also during the bounce and the contraction preceding it. Here, I considered a multifield extension of LQC with two fields: a massive one as being the inflaton, and a massless one used as an internal clock. I first studied the background evolution of the Universe both analytically and numerically. I showed that far in the contraction, the massive field dominates the energy budget. I have also checked that inflation remains likely to happen, despite the presence of the massless field. Secondly, I investigated how perturbations are produced. Unlike the one-field case, they are now described by an isocurvature component in addition to the standard adiabatic one, the former being characteristic of multifield models, for which Planck has put upper limits. In the remote past of the contraction, these two kinds of perturbations are highly coupled. I showed how to set their initial conditions by using appropriate variables mixing both kinds of perturbations, making the coupling subdominant. These perturbations remain to be propagated through the bounce down to the end of inflation to get their primordial (cross)spectra, to be subsequently compared to observational constraints.Since its released, the CMB traveled through large scale structures before reaching us. This leads to secondary anisotropies by its interaction with these structures, like e.g. gravitational deflection or the SZ effect in clusters. Magnetic fields have been observed in galaxies and larger structures. Since these structures are also filled with free electrons, this should lead to the Faraday Rotation (FR) effect which rotates the primordial linear polarisation, turning E into B modes, and to the Faraday Conversion (FC) effect which converts linear into circular polarisation. I revisited these sources of secondary anisotropies by computing the angular power spectra of the FR angle and the FC rate by large-scale structures. I used the halo model paying special attention to the impact of magnetic field projections. I found angular power spectra peaking at multipoles 104. Assuming a mass-independent magnetic field, the angular power spectra scale with the amplitude of matter perturbations as 83. This scaling is however degenerated with the one of the magnetic field with halos’ mass. I finally detail how to compute the full angular power spectra of polarised anisotropies, starting from the FR and FC power spectra. I also show how to reconstruct the FR and FC fields from the CMB adapting the estimators developed for lensing reconstruction
Chakraborty, Akash. "Effets des inhomogénéités nanométriques sur les propriétés magnétiques de systèmes magnétiques dilués". Phd thesis, Université de Grenoble, 2012. http://tel.archives-ouvertes.fr/tel-00947328.
Texto completoKudoda, Ayman Mohamed ELhadi Mohamed. "Modelling the Inhomogeneities of the extragalactic background light". Thesis, 2016. http://hdl.handle.net/10539/19295.
Texto completoThis work investigates the impact of the extragalactic background light fluctuations on very high energy !-ray spectra from distant blazars. We calculate the extragalactic background light spectral energy distribution using a model that extends those proposed by Razzaque et al. (2009ApJ.697.483R) and Finke et al. (2010ApJ.712.238F). We introduce a model for fluctuations in the extragalactic background light based on fluctuations in the star formation rate density, since these two fluctuations can reasonably be expected to be correlated. Fluctuations in the star formation rate are estimated from the semi-analytical galaxy catalogue of Guo et al. (2013MNRAS.428.1351G), we use his model to derive the resulting opacities for !-rays from distant sources. We determine the mean, lower and upper limits for the scatter of the star formation rate density, which then allow us to compute corresponding limits on the extragalactic background light spectrum. We then calculate the impact of these fluctuations limits on the !-ray optical depth. This appears to be the first detailed analytical model that aims to account for the impact of extragalactic background light fluctuations on the !-ray opacity. The model predicts relatively high variations ( 15%) on the opacity in the energy range less than 100 GeV for nearby sources. The impact is found to be smaller (⇠ 5%) for very high energy !-rays from distant sources.
Chen, Xinhe. "Domain-wall pinning by magnetic inhomogeneities and temperature dependence of coercivity". 1990. http://hdl.handle.net/1993/16996.
Texto completoLavrador, Rui Filipe David. "Correction of image distortions in magnetic resonance imaging: intensity inhomogeneities correction and evaluation". Master's thesis, 2010. http://hdl.handle.net/10316/14173.
Texto completoRuppert, Kai. "High-resolution 3D MR imaging using novel k-space trajectories and improved contrast based on sub-voxel magnetic field inhomogeneities /". 1999. http://wwwlib.umi.com/dissertations/fullcit/9930105.
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