LESIA - Observatoire de Paris

  • Mardi 26 janvier 2016 à 11h00 (Salle de réunion du bâtiment 14)

    Lifecycle of a large-scale polar coronal pseudostreamer/cavity system

    Chloé Guennou (LESIA)

    We report on an exceptional large-scale coronal pseudostreamer/cavity system in the southern polar region of the solar corona, which was visible for approximately a year starting in February 2014. It is unusual to see such a large closed-field structure embedded within the open polar coronal hole. We investigate this structure to document its formation, evolution and eventually its shrinking process, using data from both the PROBA2/SWAP and SDO/AIA UV imagers. In particular, we used EUV tomography to find the overall shape and internal structure of the pseudostreamer, and to determine its 3D temperature and density structure using then DEM analysis. We found that the cavity temperature is extremely stable with time and is essentially at similar or slightly hotter temperature than the surrounding pseudostreamer. Two regimes in cavity thermal properties was observed : the first 5 months corresponds to the lower density depletion associated with the highly multi-thermal plasma, while the shrinking period exhibits the exact opposite behavior. As the thermodynamic properties are strongly correlated with the magnetic structure, these results provide constraints on both the trigger of CMEs and the processes that maintain cavities stability for such a long lifetime.

  • Mercredi 20 janvier 2016 à 10h30 (Salle de conférence du bâtiment 17)

    Asymptotic analysis of dipolar mixed modes in red giant stars

    Masao Takata (Department of Astronomy, University of Tokyo)

    Recent observations of oscillations in subgiant and red giant stars have given a great impact on our understanding of the structure and evolution of the stars. The most important property of the oscillations is the detection of dipolar mixed modes, which can probe the stellar structure both in the core and in the envelope. In order to understand those modes from a theoretical point of view, an asymptotic analysis is performed under the assumption that the oscillations are composed of short-wavelength waves both in the core and in the envelope. Compared to the conventional analysis in the theory of stellar oscillation, the present analysis has the following new aspects : (i) the perturbation to the gravitational potential is fully taken into account (namely, the Cowling approximation is not assumed) ; (ii) the strong interaction between the oscillations in the central gravity-wave (G) cavity and those in the envelope acoustic-wave (P) cavity is considered. The formulae that are derived in the analysis include the quantisation condition, which determines the eigenfrequencies of the modes, and the amplitude ratio between the G cavity and the P cavity. Based on these, the origin and implication of the empirical phase shift, which is usually expressed as ’epsilon’, in the frequency formula is particularly discussed. The results should be useful in interpreting the observed oscillation spectra.

  • Mercredi 16 décembre 2015 à 11h00 (Salle de conférence du bâtiment 17)

    Non-Maxwellian distributions in the solar corona : Diagnostics for a transient coronal loop

    Jaroslav Dudík (Academy of Sciences, République Tchèque)

    Observations of the solar wind show pervasive presence of the non-Maxwellian kappa-distributions of electron energies characterized by a power-law high-energy tail. Theory has shown that such distributions can arise in the solar corona during impulsive heating by magnetic reconnection and also wave-particle interactions. We explored the consequences of the presence of such kappa-distributions on the optically thin spectra of the solar corona emitted in the X-rays, UV, and visible wavelengths. In particular, several line combinations allow for diagnostics of the kappa-distributions. A diagnostic from the EUV line ratios observed by Hinode/EIS space-borne spectrometer is performed and it is shown that the electron distribution can be strongly non-Maxwellian.

  • Jeudi 10 décembre 2015 à 11h00 (Salle de conférence du bâtiment 17)

    Interferometric observations of microlensing events

    Clément Ranc (IAP)

    Gravitational microlensing is a powerful technique to detect extrasolar planets at galactic distances, and holds great promises in detecting populations of brown dwarfs companions to stars. The new generations of alert telescopes have strongly increased the number of detected microlensing events for the last few years and now exceed 2000 per year. Among these events, planet searches routinely yield over 20 detections. The physical properties of the lensing systems are inferred either from the detection of specific physical effects (source size, all kind of parallax measurements etc.), or from additional observations using high angular resolution imaging. Observing microlensing events with an interferometer is a new additional way to break the degeneracies, by resolving the multiple images that are created during an event. Until now, no observation has yet succeeded.

    After a description of the main features of gravitational microlensing, I will present a formalism that closely combines interferometric and microlensing observable quantities. From this formalism arise the resulting observable quantities that are naturally constrained through an interferometric observation. Then, I will present an up-to-date analysis of the expected number of targets in the light of new microlensing surveys.

    The recent improvements in the sensitivity of long baseline interferometers such as CHARA and VLTI open new perspectives regarding interferometric observations of microlensing events. The observational strategy requires an efficient alert system that shall involve the already existing photometric follow-up of the events. Interferometric microlensing observations carry great promises to characterize completely many more microlensing systems in a near future.

  • Mardi 1er décembre 2015 à 11h00 (Salle de conférence du bâtiment 17)

    Stellar Astrophysics with CASSINI

    Paul N. Stewart (University of Sydney)

    The multi-national, multi-billion-dollar, Cassini mission has resulted in amazing insights into the complex Saturn system, dramatically improving our understanding of the planet, and its moons and rings. One particularly successful method employs the observation of bright stars as the planet’s rings pass in front, allowing the study of the ring system. In this presentation I will demonstrate how such observations can also be used to investigate the stars themselves.

    The technique is shown to be effective for measuring the spatial and spectral structure of evolved stars, including identification of molecular layers in the stellar atmosphere. It enables the recovery of high-angular-resolution 2D images of the inner regions of complex stellar systems, achieving resolutions not possible with regular telescopes. These observations are demonstrated to help constrain models of the behaviour of Mira variable stars, and to change our understanding of the inner nebula around IRC+10216.

  • Jeudi 26 novembre 2015 à 10h45 (Amphithéâtre Evry Schatzman)

    Face à la raréfaction des ressources, quelle innovation pour demain ?

    Philippe Bihouix

    L’approche technologique peut-elle résoudre les problèmes environnementaux ? Les métaux sont à la base de notre civilisation industrielle, mais leur disponibilité, face à un accroissement continu du prélèvement des ressources pour soutenir notre modèle actuel de développement, sera sans doute un des défis majeurs du XXIème siècle.

    Les enjeux techniques de la croissance verte sont complexes : une innovation fondée sur les nouvelles technologies, souvent consommatrices de ressources rares et plus difficiles à recycler, nous conduit dans l’impasse. Une économie circulaire fondée sur le recyclage généralisé, peut-elle être une réponse  ? Quelles autres solutions imaginer ?

    Philippe Bihouix est ingénieur centralien. Il est coauteur de l’ouvrage Quel futur pour les métaux (EDP sciences, 2010), et auteur de L’âge des low tech (Seuil, 2014).

  • Mercredi 18 novembre 2015 à 14h00 (Salle de conférence du bâtiment 17)

    Observations planétaires et exoplanétaires par le JWST

    Pierre Drossart et Emmanuel Lellouch

    Retour de la conférence ESLAB/JWST du 12 au 16 octobre 2015.

  • Mardi 10 novembre 2015 à 11h00 (Salle de conférence du bâtiment 17)

    Small scale energy release on the Sun

    Costas Allisandrakis (University of Ioannina, Grèce)

  • Lundi 5 octobre 2015 à 11h00 (Salle de réunion du bâtiment 14)

    From decametric radio emission of shocks to the study of the role of microwave emission on particle acceleration

    Pietro Zucca (LESIA)

  • Mardi 29 septembre 2015 à 11h00 (Salle de réunion du bâtiment 14)

    Numerical Simulations of Dynamic Phenomena in the Solar Corona

    Tibor Török (Predictive Science Inc.)

    The numerical modeling of the solar corona and of its dynamical phenomena has experienced significant progress in recent years. Present magnetohydrodynamic (MHD) simulations can be run on large spherical grids, include a realistic description of the energy transfer (thermodynamics) in the corona, and are capable of incorporating photospheric measurements as boundary condition for the magnetic field. These capabilities allow us to model the large-scale magnetic field and plasma distribution of the corona, the structure and dynamics of active regions, streamers, and coronal holes, as well as transient eruptive phenomena such as jets and coronal mass ejections (CMEs) in an increasingly realistic manner. In this talk, I will present some of these simulations and briefly discuss the next steps that lie ahead.

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