Mercredi 5 juin 2019 à 14h00 (Salle de conférence du bâtiment 17)
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)
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)
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)
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)
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)
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)
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 10^{5}-10^{6}. 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.
Mardi 19 mars 2019 à 11h00 (Salle de conférence du bâtiment 17)
Deborah Baker (University College London, Mullard Space Science Laboratory, UK)
Elemental abundance variations are tracers of physical processes throughout the Universe, with the cosmic reference standard being the solar elemental composition. Knowledge of the Sun’s elemental composition underpins our understanding of the flow of mass and energy from deep in the interior, through the outer atmosphere, into the heliosphere. As the Sun’s outer atmosphere originates from the photosphere, it is not trivial that the elemental abundances of the photosphere and corona are different. Recent Hinode/EIS and SDO/EVE results suggest that the observed distribution and evolution of elemental composition are closely linked with the magnetic activity and heating processes in the Sun’s outer atmosphere. I will review the key results and show how variation in elemental composition may be used as a tracer of physical processes on the Sun.
Lundi 4 mars 2019 à 14h00 (Amphithéâtre Evry Schatzman, bâtiment 18)
Ingo P. Waldmann (Deputy Director UCL Centre of Space Exoplanet Data, Dept. of Physics & Astronomy, University College London)
The field of exoplanetary spectroscopy is as fast moving as it is new. Analysing currently available observations of exoplanetary atmospheres often invoke large and correlated parameter spaces that can be difficult to map or constrain. This is true for both : the data analysis of observations as well as the theoretical modelling of their atmospheres. Issues of low signal-to-noise data and large, non-linear parameter spaces are nothing new and commonly found in many fields of engineering and the physical sciences. Recent years have seen vast improvements in statistical data analysis and machine learning that have revolutionised fields as diverse as telecommunication, pattern recognition, medical physics and cosmology. In many aspects, data mining and non-linearity challenges encountered in other data intensive fields are directly transferable to the field of extrasolar planets as well as planetary sciences. In this seminar, I will discuss our new deep learning framework, ExoGAN (Tzingales & Waldmann, 2018, AJ), designed to address some of these atmospheric modelling challenges using generative adversarial networks. I will then proceed to discuss our new hyper-spectral image classification code, PlanetNET (Waldmann & Griffith, in press, Nat. Astr.), able to automatically and accurately map Saturn’s clouds using Cassini/VIMS data. As we firmly move into the era of ‘big data’ for both planetary (e.g. Juno) and exoplanetary sciences (e.g. JWST, Ariel), intelligent algorithms will play an important part in facilitating the analysis of these rich data sets in the future.
Mardi 26 février 2019 à 11h00 (Salle de réunion du bâtiment 14)
Joten Okamoto (NAOJ, Japan)
Sunspots are concentrations of magnetic fields on the solar surface. Then, where is the strongest field in each sunspot ? It is generally located in an umbra, but sometimes stronger fields are found outside umbrae, such as a penumbra and a light bridge. The formation mechanism of such strong fields outside umbrae is still puzzling. Now we have numerous high-quality datasets taken with the Hinode/Spectro-Polarimeter over 10 years, which motivate us to address this question via a statistical analysis of strongest fields in sunspots. Hence, we complied a ranking list of active regions by their largest field strengths and investigated conditions for appearance or formation of strong magnetic fields. In this seminar, we will introduce a sunspot with a field strength of 6250 G as a case study, and then discuss the key features to produce strong fields in a statistical sample.