Sauf exception, les séminaires ont lieu sur le site de Meudon, dans la salle de conférence du bâtiment 17.

Prochains séminaires

  • Mardi 16 avril 2019 à 14h00 (Salle de réunion du bâtiment 14)

    Space Weather experiment at Oukaimeden Observatory in Morocco

    Aziza Bounhir (LPHEA, Université Cadi Ayyad, Maroc)

    Oukaimeden Observatory (31.2 N ; 8 W ; magnetic latitude 22.7 N ; altitude 3700 m), in the Atlas mountain in Morocco hosts an experiment dedicated to space weather and especially the study of the thermosphere/ionosphere coupling. The equipment consists of a Fabry-Perrot Interferometer (FPI), a wide-angle camera and a GPS station. Thermospheric winds and temperature as well as ionospheric structures that develop at an altitude of 250 km are produced along with the total electronic content of the ionosphere.

    Space weather is a new research field in our laboratory LPHEA (Laboratoire de Physique des Hautes Energies Astronomie et Astrophysique) at Cadi Ayyad university of Marrakech, since approximately 2014, when we dedicated a school to this topic. In 2010 an ISWI delegation came to Morocco in order to implement in the African continent experiments dedicated to space weather as it is not covered with in situ data. As a result, a collaboration between Cadi Ayyad university and the university of Illinois took place through the implementation of the camera and the FPI that belong to a network in the American sector.

    The subject matter of this talk is the presentation of the results obtained, that consist mainly on the thermospheric dynamics in quiet and disturbed conditions above Oukaimeden Obervatory. We have established the climatologies of the thermospheric winds and temperature, their seasonal behavior and their sensitivity to the solar cycle. The effect of geomagnetic storms on FPI data are also presented and a classification of the geomagnetic storms observed. The camera data and TEC measurements are also used to explore a certain aspect of the thermophere/ionophere coupling.

  • Jeudi 25 avril 2019 à 15h00 (Salle de réunion du bâtiment 14)

    Computation, visualization, and applications of relative magnetic helicity in the Sun

    Kostas Moraitis (LESIA)

    This talk summarizes three recent works regarding magnetic helicity, which is a measure of the geometrical complexity of a 3D magnetic field and a conserved quantity of ideal MHD. The first work is about an accurate method to compute relative magnetic helicity, the appropriate helicity for most natural plasmas, in spherical geometry, and especially in finite volumes. The second work deals with a method to visualize relative magnetic helicity through a density proxy called field line helicity. In the third work, after studying the evolution of relative magnetic helicity in a solar active region, we discuss the possibility of indicating AR eruptivity with helicity-related quantities.

Séminaires passés

  • Mardi 6 novembre 2018 à 14h00 (Salle de conférence du bâtiment 17)

    Dynamique du milieu interstellaire au Centre galactique : des nouveaux objects type G2 autour du trou noir supermassif

    Anna Ciurlo (UCLA)

    Au centre de la Voie lactée, l’environnement du trou noir supermassif (SMBH) présente un mélange complexe de jeunes étoiles, de gaz et de poussière. Le parsec central du Centre galactique a été observé à haut résolution avec le Keck pendant plus de 20 ans, le but principal étant de surveiller les étoiles en orbite autour du SMBH. Cependant, les caractéristiques du gaz situé au centre peuvent être examinées de près en utilisant cette base de données unique.

    En particulier, les observations effectuées avec le spectromètre à intégrale de champ OSIRIS au Keck nous permettent d’examiner les propriétés dynamiques du gaz et d’identifier un certain nombre d’objets aux propriétés inhabituelles. Certains d’entre eux ont de très hautes vitesses radiales (200-800 km/s) et changent de vitesse, positon ou de forme au cours du temps. Parmi ces objets on observe plusieurs nouveaux objets de « type G » (Gillessen et al. 2012) ou objets stellaires poussiéreux (dusty stellar objects DSO, Eckart et al. 2013).

    Nous présentons une étude de la morphologie et de la dynamique orbitale de ces objets compacts. Nous discutons des hypothèses pour leur formation et leur évolution. Ces objets pourraient jouer un rôle dans l’accrétion sur le SMBH. Initialement supposés être des nuages ​​de gaz compacts et poussiéreux, les DSO sont probablement des objets stellaires entourés d’enveloppes gonflées de poussière et de gaz. Diverses hypothèses ont été proposées mais aucun consensus général n’a encore été atteint sur leur nature. Jusqu’à présent, deux de ces objets ont été identifiés et étudiés en détail : G2 et G1. Nous décrivons ici les observations de trois autres DSO, y compris deux signalés pour la première fois, et utilisons cette population DSO pour contraindre leur nature intrinsèque.

  • Mardi 6 novembre 2018 à 14h00 (Salle de réunion du bâtiment 14)

    Impact of an axisymmetric magnetic field on gravity waves in rapidly rotating stars

    Vincent Prat (AIM, CEA)

    Internal gravity waves provide us with a unique way to probe the stellar interior of intermediate-mass and massive stars. Those waves are also able to transport angular momentum. However, the propagation, the frequencies of waves and the transport they induce may be strongly affected by rotation and the presence of a magnetic field. For slow rotators, rotation can be taken into account using perturbative methods. For rapid rotators, which is the case of many early-type pulsators such as gamma Doradus, delta Scuti, SPB and Be stars, such methods fail, and the eigenvalue problem is fully 2D and computationally expensive. The traditional approximation of rotation, which neglects the horizontal component of the rotation vector, allows for efficient computation of modes. Another approach is to consider only small-wavelength waves and model them as propagating rays. Using these two complementary methods, I will discuss the impact of an axisymmetric magnetic field on gravito-inertial waves and the implications for the detection of deep magnetic fields.

  • Mercredi 24 octobre 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Precursors of Magnetic Flux Emergence in the Moat Flows of Active Region AR12673

    I will describe observations of magnetic disturbances in active region AR12673 between 1 and 3 September 2017 seen as a disruption of the moat flow several hours before the onset of strong flux emergence near the main sunspot. The moat flow is commonly known as a radially oriented strong outflow of photospheric plasma surrounding sunspots, which ends abruptly and shapes an annular pattern around the penumbra. Using highly accurate methods of tracking this photospheric flow applied to SDO/HMI data, we are able to describe the evolution of the moat surrounding the main sunspot of AR 12673. We find that several hours before the emergence of strong magnetic flux near the main sunspot, the moat boundaries are broken at these very same locations. This behavior is observed both on 1 and 3 September. There is no such behavior observed in the absence of flux emergence. These observational results pose the question of how often they occur in other active regions and whether the disruption of the moat flow might be, like in this case, an indication of impending enhanced magnetic activity or simply a coincidental event. I will also describe ongoing work using my new photospheric tracking framework adapted specifically for HMI and that enables systematic analyses of the moat flows as well as the relationship between supergranular flows and the magnetic evolution in emerging active regions.

  • Mardi 23 octobre 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Magnetic Imprints and Momentum Processes in Major Solar Eruptions

    Xudong SUN (University of Hawaii, USA)

    The solar active region photospheric magnetic field evolves rapidly during major eruptive events, suggesting appreciable feedback from the corona. Using high-cadence vector magnetogram sequence, multi-wavelength coronal imaging, and numerical simulation, I will show how the observed photospheric "magnetic imprints” are highly structured in space and time, and how it can in principle be used to estimate the impulse of the Lorentz force that accelerates the coronal mass ejection (CME) plasma. In an archetypical event, the Lorentz force correlates well with the CME acceleration, but the total force impulse surprisingly exceeds the CME momentum by almost two orders of magnitude. Such a trend holds for the majority of the major eruptions from our survey sample. I propose a "gentle photospheric upwelling" scenario, where most of the Lorentz force is trapped in the lower atmosphere layer, counter-balanced by gravity of the upwelled mass. This unexpected effect dominates the momentum processes, but is negligible for the energy budget. I will discuss our follow-up project, how the upcoming high-sensitivity observations and new-generation numerical models may help elucidate the problem.

  • Jeudi 18 octobre 2018 à 16h00 (Salle de conférence du bâtiment 17)

    A Monte-Carlo radiative transfer code to explore the polarisation signatures of dusty disks and torii and its application to torii in active galactic nuclei

    Lucas Grosset (LESIA)

    One of the main observational challenges for investigating the central regions of active galactic nuclei (AGN) at short wavelengths, using high angular resolution, and high contrast observations, is to directly detect the circumnuclear optically thick material hiding the central core emission when viewed edge-on. I will present my work on the dust structures of these inner regions and the main results I obtained during my four years in the LESIA. But I will focus particularly on one of the tool I used for this purpose, the simulation code MontAGN, computing the high angular resolution maps of dusty structures, including polarisation, through Monte-Carlo radiative transfer methods in the near infrared. Even at early development stages, simulated maps through MontAGN allowed me to constrain the geometry and density of the structures surrounding the AGN, in particular the dusty torus and the ionisation cone. Furthermore, after 2 years of development, the code is now almost ready for its first release and will be soon available after few upgrades.

  • Lundi 24 septembre 2018 à 11h00 (Salle de réunion du bâtiment 14)

    Recent Results from Snapshot Spectroscopic Imaging studies of the Sun at m-wavelengths

    Atul MOHAN (Tata Institute of Fundamental Research, Pune, India)

    The talk will focus on the study of weak energy release events in the Corona using the powerful new technique of snapshot spectroscopic imaging with Murchison Widefield Array (MWA). MWA observes the Sun in a frequency range of 80- 240 MHz at a spectro-temporal resolution of 40 kHz and 0.5 s. We imaged two 4 min long and 15 MHz wide band datasets at 0.5s and 16kHz resolution. This was a period of very low solar activity as per the metrics of high energy observations. This lead to nearly 46,000 images which were converted to brightness temperature maps. I will brief the techniques we employ to extract useful physical information from such large volume of data spread across 4 axes, namely 2 angular sky coordinates, frequency and time. I will focus on one of the events we studied and the interesting physical inferences.

  • Jeudi 20 septembre 2018 à 16h00 (Salle de conférence du bâtiment 17)

    The many ’faces’ of the molecular torus of NGC1068

    Santiago García-Burillo (OAN)

    We have used ALMA to image with 0.03-0.04"(2-3pc) spatial resolution the emission of a set of molecular lines, including CO(2-1), CO(3-2) and HCO+(4-3), and their underlying continuum emission in the circumnuclear disk of NGC1068, covering a region that extends from r=200pc down to the central 7-10pc-diameter torus of this prototypical Seyfert 2 galaxy, which was first detected in the 6-5 line of CO by García-Burillo et al 2016. These new observations, by using three lines spanning three orders of magnitude in densities, reveal the many ’faces’ of the molecular torus in NGC1068. The torus shows a stratified layered structure spanning a radial range that goes from r=2-3pc to r=10pc. The kinematics of molecular gas in the torus are characterized by strong non-circular motions and enhanced turbulence. Furthermore, the CO(2-1) line emission has allowed us to image the outflowing molecular gas component emerging from the torus. A far-reaching question, to be answered by ongoing ALMA surveys of nearby AGNs (NUGA, GATOS), is whether similarly perturbed and turbulent tori are expected to be found ubiquitously in Seyfert galaxies, and if their properties should change as a function of parameters like the AGN luminosity, the degree of obscuration, or the Eddington ratio.

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