Scattering line polarization in rotating, optically thick disks

To interpret observations of astrophysical disks it is essential to understand the formation process of the emitted light. If the disk is optically thick, scattering dominated and permeated by a Keplerian velocity field, Non-Local Thermodynamic Equilibrium radiative transfer modeling must be done to compute the emergent spectrum from a given disk model. We investigate Non-local thermodynamic equilibrium polarized line formation in different simple disk models and aim to demonstrate the importance of both radiative transfer effects and scattering as well as the effects of velocity fields. We self-consistently solve the coupled equations of radiative transfer and statistical equilibrium for a two level atom model by means of Jacobi iteration. We compute scattering polarization, that is Q/I and U/I line profiles. The degree of scattering polarization is significantly influenced by the inclination of the disk with respect to observer, but also by the optical thickness of the disk and the presence of rotation. Stokes U shows double-lobed profiles with amplitude which increases with the disk rotation. Our results suggest that the line profiles, especially the polarized ones, emerging from gaseous disks differ significantly from the profiles predicted by simple approximations. The profiles are diverse in shape, but typically symmetric in Stokes Q and antisymmetric in Stokes U. A clear indicator of disk rotation is the presence of Stokes U, which might prove to be a useful diagnostic tool. We also demonstrate that, for moderate rotational velocities, an approximate treatment can be used, where non-local thermodynamic equilibrium radiative transfer is done in the velocity field-free approximation and Doppler shift is applied in the process of spatial integration over the whole emitting surface.Comment: 16 pages; 12 figures; Accepted with revision for A&A. This is the version after first round of referee's suggestion

Inference of magnetic fields in inhomogeneous prominences

Most of the quantitative information about the magnetic field vector in solar prominences comes from the analysis of the Hanle effect acting on lines formed by scattering. As these lines can be of non-negligible optical thickness, it is of interest to study the line formation process further. We investigate the multidimensional effects on the interpretation of spectropolarimetric observations, particularly on the inference of the magnetic field vector. We do this by analyzing the differences between multidimensional models, which involve fully self-consistent radiative transfer computations in the presence of spatial inhomogeneities and velocity fields, and those which rely on simple one-dimensional geometry. We study the formation of a prototype line in ad hoc inhomogeneous, isothermal 2D prominence models. We solve the NLTE polarized line formation problem in the presence of a large-scale oriented magnetic field. The resulting polarized line profiles are then interpreted (i.e. inverted) assuming a simple 1D slab model. We find that differences between input and the inferred magnetic field vector are non-negligible. Namely, we almost universally find that the inferred field is weaker and more horizontal than the input field. Spatial inhomogeneities and radiative transfer have a strong effect on scattering line polarization in the optically thick lines. In real-life situations, ignoring these effects could lead to a serious misinterpretation of spectropolarimetric observations of chromospheric objects such as prominences.Comment: 11 pages, 9 figure

Methodes asymptotiques en transfert de rayonnement : applications a des problemes de formation de raie et de transport de rayonnement continu dans les etoiles

SIGLECNRS T 56795 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Transfert de rayonnement polarise et non-polarise avec redistribution en frequence : methodes asymptotiques et applications

CNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc

Do the quiet sun magnetic fields vary with the solar cycle?

International audienceAbstract The quiet Sun observed in polarized light exhibits a rich and complex magnetic structuring which is still not fully resolved nor understood. The present work is intended to contribute to the debate about the origin of the quiet sun magnetic fields, in relation or not to the global solar dynamo. We present analysis of center-to-limb polarization measurements obtained with the SOT/SP spectropolarimeter onboard the Hinode satellite outside active regions, in 2007 and 2013, i.e. at a minimum and a maximum of the solar cycle, respectively. We compare the spatial fluctuation Fourier spectra of unsigned circular and linear polarization images after corrections for polarization bias and focus variations between the two data sets. The decay of active regions is clearly a source of magnetic fields in the quiet Sun. It leads to a global increase of the polarization fluctuation power spectrum in 2013 in the network. In the internetwork, we observe no variation of the polarization fluctuation power at mesogranular and granular scales, whereas it increases at sub-granular scales. We interpret these results in the following way. At the mesogranular and granular scales very efficient mechanisms of magnetic field removal are operating in the internetwork, that leads to a dissipation or a concentration of magnetic fields on smaller scales. So the cycle-invariant magnetic signal that we detect at mesogranular and granular scales must be continuously created by a dynamo mechanism which is independent of the solar cycle

Étude à haute résolution angulaire de la granulation photosphérique solaire par une technique interspectrale

NICE-BU Sciences (060882101) / SudocSudocFranceF

Mesures du diamètre solaire depuis la Terre en haute précision en support de la mission Picard

L argument de la mesure du diamètre solaire est introduit dans le cadre plus ample de la variabilité solaire, et, en conséquence, de l influence du Soleil sur le climat de la Terre. Il est possible de mesure le diamètre solaire avec une précision suffisamment bonne pour des études de changements climatiques et de variations d irradiance en utilisant des données anciennes d éclipses totales. Cela permettrait l extension de la connaissance de la luminosité solaire en arrière de trois siècles, grâce à la connaissance du paramètre W=dlogL/dLogR. La méthode des éclipses et des grains de Baily est largement traitée, et un avancement significatif pare rapport aux derniers 40 ans a été obtenu avec la reconstruction de la fonction d assombrissement du limb solaire à partir de la lumière des grains de Baily , et la recherche du point d inflexion. Le cas de la dernière éclipse annulaire (15 janvier 2010) a été étudié en détail, tandis que l atlas des grains de Baily a été publié à partir de toutes les observations faites dans le monde par les membres du IOTA, aussi avec l analyse du diamètre solaire pendant l éclipse de 2006. Le travail de thèse a été développé pendant la transition entre l atlas photographique du limb lunaire publié par Watts en 1963, et corrigé dans les années suivantes par milliers d observations, et la carte réalisée avec le laser-altimètre de la mission lunaire japonaise Kaguya, publiée en novembre 2009. L autre méthode pour une mesure précise du diamètre solaire en alternative de la mission Picard / Picard-sol est le drift-scan utilisé soit par les astrolabes solaires, soit par les télescopes plus grands. Les observatoires de Locarno et Paris ont commencé des observations du soleil avec cette méthode avec des résultats encourageants. Pour la première fois, le mouvement de l image du Soleil entier a été détectée sur des fréquences de 1/100 Hz. Cela peut commencer à expliquer les résultats des campagnes d observation de Greenwich et Rome de 1850 à 1955. Cette activité sous-Hertz de la turbulence atmosphérique est un autre avancement significatif dans ce domaine de recherche. Le télescope géant à trou sténopeïque de la ligne méridienne de Santa Maria degli Angeli à Rome permet l introduction didactique de presque tous les arguments d astrométrie classique traités dans cette thèse. Cet essai didactique complète la thèse.The theme of the measurement of the solar diameter is introduced in the wider framework of solar variability, and, consequently of the influence of the Sun upon the Earth s climate. It is possible to measure the solar diameter with an accuracy enough good for studies on climate changes and irradiation variations using ancient data on total eclipses. This would permit to extend the knowledge of the solar luminosity back to three centuries, through the knowledge of the parameter W=dlogL/dLogR. The method of eclipses and Baily beads is widely discussed, and a significant improvement with respect to the last 40 years, has been obtained by reconstructing the Limb Darkening Function from the Baily s bead light curve, and the search of its inflexion point. The case of the last annular eclipse (Jan 15, 2010) has been studied in more detail, while the atlas of Baily s beads has bee published with all the observations made worldwide by IOTA members, along with an analysis of the solar diameter during the eclipse of 2006. The work of this thesis has been developed during the transition between the photographic atlas of the lunar limb published by Watts in 1963 and corrected in the following decades by thousands observations, and the laser-altimeter map made by the Kaguya lunar probe and published in November 2009. The other method for the accurate measurement of the solar diameter alternative to the Picard/Picard-sol mission is the drift-scan method user either by the solar astrolabes either larger telescope. The observatories of Locarno and Paris have started an observational program of the Sun with this method with encouraging results. For the first time an image motion of the whole Sun has been detected over frequencies of 1/100 Hz. This may start explain the puzzling results of the observational campaigns made in Greenwich and Rome from 1850 to 1955. This sub-Hertz activity of the atmospheric seeing is another fundamental achievement in this research field. A giant pinhole telescope as the meridian line of Santa Maria degli Angeli in Rome, permits to introduce almost all the arguments of classical astrometry presented in this thesis. In this consists the didactic outreach as a complement of this thesis.NICE-BU Sciences (060882101) / SudocSudocFranceItalyFRI

Polarized Radiation Transfer in 2D Geometry

International audienc