• Vendredi 21 juin 2019 à 14h00 (Salle de conférence du bâtiment 17)

    Deciphering the Early Phase of Solar Eruptions

    Xin Cheng (School of Astronomy and Space Science, Nanjing University& Max Planck Institute for Solar System Research)

    Coronal mass ejections (CMEs) and solar flares are the large-scale and most energetic eruptive phenomena in the solar system and able to release a large quantity of high-speed plasma and magnetic flux into the solar wind, probably seriously affect the safety of human high-tech activities in outer space. Nevertheless, the key questions of how and when CMEs/flares are initiated are still puzzled. Very importantly, the AIA data show that the pre-eruptive configuration of CMEs/flares may appear as a coherent plasma channel structure with a temperature up to 10 million degree, even pre-existing prior to the eruption. This thus allows us to decipher the physics during the initiation and early phase of solar eruptions. In this talk, I will first present observational features of magnetic reconnection during the slow rise phase that plays an important role in creating and heating the pre-existing flux rope. For the second part, I will talk some common behaviours of the early evolution of solar eruptions, which probably can be used to distinguish different CME/flare models.

  • Vendredi 21 juin 2019 à 14h00 (Salle 204 du bâtiment 18)

    The entourage of the Galactic Black Hole : a jet, a chimney, and a rope

    Mark Morris (UCLA Galactic Center group, USA)

    Although its mass is rather modest by the standards of supermassive black holes found in the centers of galaxies, the black hole at the center of our Milky Way Galaxy, weighing in at 4 million solar masses, manifests itself in a variety of ways in terms of its effect on its environment. In this talk, I will describe the strengthening X-ray evidence for the presence of a jet on the scale of a parsec as well as X-ray evidence for exhaust vents on scales of hundreds of parsecs that are candidates for carrying energetic particles out to the gigantic gamma-ray emitting Fermi Bubble straddling the Galactic plane. Closer to the black hole, I’ll show recent observations with the SOFIA Observatory of the highly ordered magnetic field in the central few parsecs, and with the VLA of a magnetic filament — a synchrotron-illuminated flux rope — that appears to emanate from the black hole.

  • Mardi 18 juin 2019 à 11h00 (Salle de conférence du bâtiment 16)

    Origins of density fluctuations
 in the solar corona explored with time-dependent MHD simulations

    Léa Griton (IRAP)

    Quasi-periodic density fluctuations appear ubiquitous in the regions where the solar wind forms and accelerates. The origin of these fluctuations is still debated and could result from a number of physical processes including rising MHD waves, periodic impulsive heating or continual magnetic reconnection. The recent analysis of deep field imaging campaigns carried out with the STEREO COR-2 instrument highlight the omnipresence of density fluctuations with periodicities around 40 minutes in both fast and slow solar winds. We use the time-dependent model of the solar wind MULTI-VP already tested against observations for a steady-state corona, to explore the mechanisms at play in the low corona that could produce such density fluctuations higher up in the atmosphere. We first test the idea that impulsive and periodic heating near the transition region could lead to density fluctuations higher up in the atmosphere. We investigate how such density fluctuations can be transmitted out in the solar wind beyond the sonic point. This study provides a modeling framework for the future analysis that can be carry out with the remote-sensing and in situ data that will be acquired by the Parker Solar Probe and the Solar Orbiter over the next decade.

  • Mercredi 5 juin 2019 à 14h00 (Salle de conférence du bâtiment 17)

    Horizontal turbulent transport in stars : progresses and challenges

    Stéphane Mathis (LDE3, CEA Saclay)

    In his seminal work on the transport of angular momentum and chemicals in stellar radiation zones (Zahn 1992), Jean-Paul Zahn proposed a coherent and complete formalism based on the hypothesis of a strong anisotropic turbulent transport stronger in the horizontal direction because of the stable stratification of these regions. This allowed him to derive equations to describe the evolution of rotating stars in a formalism that assumes the so-called « shellular » rotation, where the rotation mostly varies along the radial direction. This coherent theory, often called rotational mixing, has been broadly and successfully implemented in stellar evolution codes and applied to many types of stars.

    However, very few prescriptions (at least three) have been proposed in the literature for the turbulent transport in the horizontal direction. Moreover, while based on the hypothesis of an anisotropy of turbulent transport induced by stable stratification in rotating radiation zones, none of them have an explicit dependence on the buoyancy frequency neither rotation. In addition, the resulting predictions are unable to reproduce the weak differential rotation observed in the Sun and stars thanks to helio- and asteroseismology.

    In this context, understanding properties of turbulent transport in rotating stratified fluids is at the forefront of fundamental fluid dynamics research using theory, numerical simulations and laboratory experiments. In this seminar, based on last advances in the field, I will show how new prescriptions for the anisotropy of transport and related horizontal eddy diffusion coefficient that have an explicit dependence on stratification, rotation, and thermal diffusion can be derived.

    One of these new prescriptions has been implemented in a stellar evolution code and applications to solar-type stars’ evolution from their PMS to advanced stages of evolution have been computed. Obtained results show a potentially stronger but self-regulated turbulent transport that should be taken into account but cannot reproduce the observed rotation profiles.

    These results will be discussed in the general context of on-going progresses for the theory, the modelling, and numerical simulations of hydrodynamic and magneto-hydrodynamic mechanisms that transport angular momentum in stellar interiors and of the confrontation of their predictions to seismic constrains on their rotation.

  • Lundi 6 mai 2019 à 11h00 (Salle de réunion du bâtiment 16)

    The Faraday effect in Saturn Kilometric Radiation observed by Cassini/RPWS

    Ulrich Taubenschuss (Department of Space Physics, IAP, Czech Academy of Sciences, Prague, Czech Republic)

    Unstable particle distributions in the auroral regions of Saturn’s magnetosphere produce a powerful planetary radio emission known as Saturn Kilometric Radiation (SKR). SKR has been continuously monitored by the High Frequency Receiver (built at LESIA) of the RPWS instrument onboard the Cassini spacecraft from 2003 to 2017. While SKR is known to be circularly polarized when observed near the equator, it turned out to be elliptically polarized when observed from high latitudes and fully linearly polarized within its source region. Linear/elliptical wave polarization provide the conditions for the well known “Faraday rotation” effect to occur, which is a rotation of the axis of linear polarization as a function of frequency and the plasma conditions encountered along the ray path. We will present the basic theory of Faraday rotation and comment on the possibility to derive information about the wave propagation medium. Examples of Faraday rotation from Cassini/RPWS will be shown from a preliminary search through the dataset of the first 6 years of the Cassini mission.

  • 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.

  • Jeudi 18 avril 2019 à 16h00 (Salle de conférence du bâtiment 17)

    Possible Challenges to the Standard Model of Cosmology

    Behnam Javanmardi (LESIA)

    The Lambda-Cold-Dark-Matter (ΛCDM) model has been understood to be successful in explaining many cosmological observations, therefore widely accepted as the standard model of cosmology. Nevertheless, it is faced with some interesting challenges with yet no solutions. In my talk, after giving a brief and simple review of the current status of ΛCDM, I will present some of the challenges to this model and my research related to them. In particular, I will talk about an unexpected observed correlation between bulge mass of disk galaxies and the number of their dwarf satellites, and also the significant tension between local and cosmic measurements of the current expansion rate of the Universe.

  • 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.

  • Mercredi 20 mars 2019 à 14h00 (Salle de conférence du bâtiment 17)

    Fourier-based WaveFront Sensing at LAM : a quick overview

    Olivier Fauvarque (LAM)

    At the condition to use adequate focal masks, optical Fourier filtering turns out to convert phase fluctuations into intensity variations with a great efficiency. If two wave front sensors in particular - the Pyramid and the Zernike – have, during the last decades, shown very promising performance making them serious candidates for the next AO systems of the Extremely Large Telescopes, a general and theoretical scheme to finely define their behavior was missing.

    This seminar intends to present the latest theoretical results that have been done by the LAM AO team and its partners in this emerging instrumental field of research. In particular, the problem of Optical Gain tracking will be adressed through the powerful Kernel mathematical framework. We will then see how to choose proper phase reconstructor depending on the sensors’ optical characteristics. We will also introduce a new kind of optical device allowing to improve the linearity of Fourier-based WFSs without any loss of sensitivity. Finally, a presentation of the LOOPS test bench (designed to generate and test a large diversity of Fourier-based WFSensors) will serve as conclusion.

  • Mercredi 20 mars 2019 à 11h00 (Salle de conférence du bâtiment 17)

    Recovering thermodynamics from spectral profiles observed by IRIS using machine and deep learning techniques

    Alberto Sainz Dalda (Lockheed-Martin Solar and Astrophysics Laboratory / BAERI, USA)

    We present three novel methods to recover the physical information from spectral profiles suitable to be inverted from an iterative solution of the radiative transfer equation. We combine the meaningful results provided by these traditional methods with machine and deep learning techniques to obtain similar-quality results in a easy-to-use, faster way. We have applied these new methods to Mg II h&k lines observed by IRIS. As a result, we are able to obtain the thermodynamics in the chromosphere and high photosphere in a few CPU-minutes, speeding up the process in a factor of 105-106. The open-source code developed to this aim will allow the community to use IRIS observations to open a new window to a host of solar phenomena.

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