LESIA - Observatoire de Paris

  • Tuesday 29 June 2010 à 11h00 (Salle de conférence du bât. 17)

    Combining Observations and Simulations to Advance our Understanding of Solar Eruptions

    Noé Lugaz (Institute for Astronomy, University of Hawaii)

    As solar cycle 24 slowly begins, thanks to the always-expanding float of satellites observing the Sun and the heliosphere, immense progresses can be expected in the forecasting and understanding of space weather, in particular regarding the initiation and propagation of coronal mass ejections (CMEs). To make a full use of the new observation capabilities, numerical simulations are often required, in particular to separate instrumental effects from the observed physical phenomena. This is particularly true for line-of-sight observations, such as coronagraphic and heliospheric images, as well as for in-situ measurements for complex series of CMEs. In this talk, I will discuss recent progresses in determining CME physical properties from white-light images, both in the corona (LASCO) and in the heliosphere (SECCHI) with the help of numerical magneto-hydrodynamics models. I will also discuss how numerical simulations can be used to test and validate existing analysis methods used on real data, and to propose new ones. Finally, I will explore how magneto-hydrodynamics models can help explaining in situ measurements at 1 AU, from isolated and multiple CMEs.


  • Tuesday 22 June 2010 à 11h00 (Salle de conférence du bât. 17)

    Dynamics of CMEs and Evolution of CME Magnetic Fields in Interplanetary Space

    Valbona Kunkel (George Mason University and Naval Research Laboratory)

    Coronal mass ejections (CMEs) constitute an important class of solar wind (SW) structures, having practical implications for geomagnetic conditions. With the new SECCHI/STEREO observations, it is now possible to observe CME trajectories in interplanetary space. This allows direct comparison of CME models with CME data over a much wider region in space than has been possible. Among the existing CME models, the erupting flux rope model (FRM) of Chen (1996) is the best tested model. It has been extensively shown to be able to replicate observed CME dynamics extending to the SECCHI HI1 field of view ( 100 Rs) (Kunkel and Chen 2010). Of the physical CME parameters, the strength and evolution of CME magnetic fields have not been accessible to direct measurement. In a theory-data comparison paper, however, Krall et al. (2006) showed that the FRM solution that best fit the observed halo CME data also yielded a flux rope whose magnetic field at 1 AU was in reasonable agreement with that of a magnetic cloud detected by the ACE spacecraft. This suggests that by fitting observed CME trajectories, the FRM is able to correctly predict magnetic cloud fields at 1 AU. In this talk, I will discuss a systematic study of this possibility. The talk will show in detail how the magnetic field of a CME evolves through interplanetary space, emphasizing the quantitative relationship between the CME trajectory and the evolution of the CME magnetic field. The theory will be applied to CME dynamics observed by SECCHI coronagraphs and the magnetic fields of associated magnetic clouds observed by instruments at 1 AU. The discussion will focus on a physical understanding that can be used to interpret observational data and numerical results of simulation models of CMEs.


  • Friday 4 June 2010 à 11h00 (Salle de conférence du bât. 17)

    Observing the Solar Corona: A First Look at Data from SDO.

    Leon Golub (Smithsonian Astrophysical Observatory, Cambridge)

    The Solar Dynamics Observatory (SDO) includes a set of telescopes called the Atmospheric Imaging Assembly (AIA). The AIA provides full Sun images of the solar corona in seven different UV and three EUV passbands, at a spatial resolution comparable to that of our previous small Explorer satellite, the Transition Region and Coronal Explorer (TRACE). We will present a description of the scientific goals of the mission and early results from this experiment. In keeping with NASA’s policy, all of the images from SDO are freely available to the entire scientific community worldwide.


  • Vendredi 4 juin 2010 à 09h30 (Salle de conférence du bât. 17)

    Reconstruction of Quiet and Transient Solar Wind Structures in the Inner Heliosphere

    P. K. Manoharan (National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Udhagamandalam (Ooty), India )

    In this talk, I will discuss results of three-dimensional evolution of the solar wind density and speed in the inner heliosphere. The primary solar wind data sets used in these studies have been obtained from the interplanetary scintillation (IPS) measurements made at the Ooty Radio Telescope (India), which is capable of measuring scintillation of a large number of radio sources per day and solar wind estimates along different cuts of the heliosphere, allow to image the three-dimensional structure of the ambient solar wind and propagating transients, such as coronal mass ejections (CMEs) and/or co-rotating interaction regions (CIRs) in the inner heliosphere. Results indicate that the interaction between the CME (or the CIR) and the background solar wind determines the radial evolution of its speed and size. Ooty studies play a key role in quantifying the drag force imposed on disturbances by the solar wind interaction, which is essential in modeling the propagation characteristics of disturbances within 1-AU heliosphere. Solar wind structures obtained from the IPS observations have also been compared with in-situ measurements obtained from the near-Earth space missions. Such studies have a great importance in understanding the prediction of CME/CIR-associated space weather at near-Earth space.


  • Wednesday 2 June 2010 à 14h30 (Salle de conférence du bât. 17)

    The heliosphere as a laboratory for cosmic dust research

    Ingrid Mann (Institute for Space Aeronomie, Uccle, Belgium; School of Science and Engineering, Kinki University, Higashi-Osaka, Japan)

    Astrophysics and planetary science consider numerous dust phenomena, but most of them are derived from theoretical considerations. The dust of sizes of the order of several 10 nm and smaller, denoted as nano dust, evades most observations, since the cross sections for scattering and for thermal emission of a solid particle drop steeply in the size range that is maller than the considered wavelength. On the other, the nano dust, because of its large surface area relative to the small mass efficiently interacts with surrounding atoms, ions, molecules and radiation. Laboratory studies of nano dust are very limited, but show, for instance, that heating, sublimation, charging and alteration of the microstructure evolve differently for the nano dust than for larger particles. Space measurements allow utilizing the solar system as a laboratory. In-situ measurements show that the flux of interstellar dust that enters the solar system is modulated in the solar wind. Nano dust that forms within the solar system by fragmentation of larger objects is accelerated in the solar wind and crosses Earth orbit with speed of about 300 km/s. The recent measurements with STEREO near 1 AU and future in-situ measurements in the inner solar system provide an exciting new tool for studying cosmic dust and its interactions in a plasma.


  • Tuesday 11 May 2010 à 11h00 (Salle de conférence du bât. 17)

    The solar corona in a new light: initial results from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory

    Karel Schrijver (Stanford-Lockheed Institute for Space Research)

    The recently-launched Solar Dynamics Observatory is changing our view of solar activity, with its three state-of-the-art instruments for helioseismic and magnetic imaging (HMI), for observations of the solar corona (AIA), and the solar spectral irradiance (EVE). The Atmospheric Imaging Assembly observes the entire visible solar corona at the best resolution of its predecessors, but with seven EUV channels to provide full thermal coverage, and with a cadence of about 10 seconds. Its data rate exceeds those of SOHO’s EIT by a factor of 10,000 and of TRACE by 1,000. These enormous advances reveal details of the solar corona never seen before. In its first month of observations, AIA has already seen multiple filament eruptions, flares, active regions, and coronal holes. In this talk, I will review some of the first results from these early observations, which include post-eruption thermal evolution, global coronal waves and coronal-loop oscillations, coronal rain, and the intricate interconnections on scales from small ephemeral regions up to a solar radius.


  • Jeudi 29 avril 2010 à 11h00 (Salle de conférence du bât. 17)

    Properties and origin of transient horizontal magnetic fields

    Ryohko ISHIKAWA (National Astronomical Observatory of Japan/ University of Tokyo)

    My talk is a comprehensive summary of the properties of the enigmatic transient horizontal magnetic fields in the photosphere. It is found that the internetwork regions are covered with ubiquitous granular-sized horizontal magnetic fields (e.g., Lites et al. 2008). Using the Solar Optical Telescope on board Hinode, we study the temporal evolution of these horizontal fields. These horizontal fields are highly transient with lifetime ranging from 1 to 10 minutes and the occurrence rate is quite high. Thus, the term of “transient horizontal fields (THMFs)” is used for them. We carry out the full Stokes inversion with the SIRGAUS code (Bellot Rubio 2003) for a THMF, and identify an omega-shaped flux tube with magnetic field strength of 400 G rising through the line forming layer of the Fe I 630.2 nm lines. These horizontal magnetic fields are observed in the quiet Sun, a weak plage region, and the polar regions (Ito et al. 2010). Occurrence rate and magnetic field strength distribution are the same in these different regions in spite of considerable difference in the amount of vertical magnetic flux. We present statistical properties of transient horizontal magnetic fields such as lifetime, size, and the location of their appearance and disappearance with respect to granular structure. I will summarize their magneto-hydrodynamic properties, and discuss their origin as inferred from these comprehensive observations.


  • Jeudi 18 février 2010 à 11h00

    L’émergence de flux magnétique solaire et son influence sur le cycle de 22 ans

    Laurène Jouve (DAMTP Cambridge)

    Durant ce séminaire, je me propose de présenter l’étude numérique multidimensionnelle de différents aspects du magnétisme solaire, avec un intérêt particulier pour le lien entre son origine interne et ses manifestations en surface. Via des simulations MHD 3D en géométrie sphérique, nous cherchons à comprendre le mécanisme d’émergence de champs magnétiques toroïdaux de la base de la zone convective solaire jusqu’à la surface où ils forment des régions actives. Dans le cas de l’introduction d’un champ initialement faible, l’émergence de boucles Ω est favorisée. Pour ce cas particulier, les régions bipolaires apparaissant en haut de notre domaine de calcul tendent à posséder une orientation Est-Ouest, conformément à la loi de Joy. Les interactions avec les écoulements moyens seront analysées pour différentes intensités initiales de champ magnétique. Nous étudierons l’influence de l’émergence de flux magnétique sur le fonctionnement même de la dynamo solaire. En réintroduisant certains résultats de ces calculs 3D dans des modèles de dynamo champ moyen de type Babcock-Leighton, on remarque notamment que le temps de montée des tubes, dépendant fortement de l’intensité du champ à la base de la zone convective, introduit une modulation de l’amplitude du cycle. Cette activité solaire modulée ainsi que le diagramme papillon résultant sont ainsi plus compatibles avec les observations que le modèle standard de Babcock-Leighton. Nous présenterons également les premiers calculs d’interaction entre tubes de flux torsadés au sein de la zone convective, avec un intérêt particulier pour les régions actives résultantes. Enfin, qu’en est-il de l’influence de l’activité magnétique solaire sur notre environnement terrestre ? Je présenterai un premier effort d’application en physique solaire de techniques d’assimilation de données sophistiquées utilisées en météorologie potentiellement prometteuses pour la prédiction du cycle d’activité solaire.


  • Mardi 16 février 2010 à 11h00 (Salle de conférence du bât. 17)

    Aspects énergétiques des oscillations de type solaire ; du Soleil aux étoiles

    Kévin Belkacem (Université de Liège, Belgique)

    Les oscillations de type solaire sont observées depuis près de quarante ans dans la Soleil et depuis plus d’une dizaine d’années dans quelques étoiles proches. L’avènement de l’astérosismologie spatiale, avec CoRoT mais aussi Kepler, permet depuis peu d’observer une myriade d’étoiles pulsant sur de tels modes. Ces oscillations sont maintenant détectées aussi bien dans les étoiles de séquence principale, les géantes rouges, mais également depuis peu dans une étoile massive. Les couches supérieures des régions convectives sont le lieu de mouvements turbulents vigoureux. Ces mouvements turbulents génèrent des fluctuations de pressions dont une faible partie va, par un processus stochastique, exciter les modes propres de la cavité stellaire. L’amplitude de ces modes stochastiquement excités résulte d’un équilibre entre excitation et amortissement. La modélisation de ces deux mécanismes nous renseigne alors sur les propriétés dynamiques de la convection turbulente dans les étoiles. Je présenterai les fondements théoriques de l’excitation et l’amortissement des modes de type solaire, ainsi que des comparaisons aux observations héliosismiques. J’évoquerai ensuite comment il est possible de faire de l’astérosismologie à l’aide des amplitudes, et non plus seulement des fréquences, et je montrerai comment les derniers résultats de la mission CoRoT permettent et permettront de contraindre les processus dynamiques dans les étoiles.


  • Monday 15 February 2010 à 11h00 (Salle de conférence du bât. 17)

    Structure, Composition and Habitability of super-Earths

    Diana Valencia (Observatoire de la Cote d’Azur)

    Within the research field of exoplanets there is a new type of planets: super-Earths. These are planets voided of a massive atmosphere and thus resemble the Terrestrial Planets and Icy Satellites in the Solar System. The first generation of data comprises masses and radius and last year the first two transiting super-Earths were reported: CoRoT-7b and GJ1214b. I will discuss the composition of these two planets based on the data and internal structure models. While they are similar in mass, CoRoT-7b is small and thus may be terrestrial, while GJ1214b is larger and has a volatile-rich atmosphere. Despite a robust mass-radius relation for super-Earths, there is an intrinsic degeneracy in composition. I will present the limitations on what can be inferred from mass-radius measurements and future steps to disentangle valid compositions. In addition, a subset of the terrestrial super-Earths might have evolved to be habitable. I will present results on the thermal state and interior dynamics of these planets. Preliminary results show that a suitable mass range for planets to exhibit habitable conditions is 1-5 M_Earth.


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