Observatoire de Paris Institut national de recherche scientifique français Univerité Pierre et Marie Curie Université Paris Diderot - Paris 7


  • Mardi 7 juin 2011 à 11h00 (Salle de confĂ©rence du bĂąt. 17)

    Test sur le ciel avec un prototype d’hypertĂ©lescope nommĂ© Carlina

    Hervé Le Coroller (Observatoire de Haute-Provence)

    Mise en cohĂ©rence des mirroirs primaires Ă  l’aide d’un laser supercontinuum

    Dans le cadre des Ă©tudes sur les technologies qui pourraient succĂ©der au VLTI nous construisons Ă  l’Observatoire de Haute‐Provence un dĂ©monstrateur d’un nouveau type d’interfĂ©romĂštre. Carlina ressemble Ă  un tĂ©lescope « classique » diluĂ© et pourrait aussi ĂȘtre un successeur aux ELTs. Il est configurĂ© comme le radio tĂ©lescope d’Arecibo avec un miroir primaire sphĂ©rique mais diluĂ© (constituĂ© de petits miroirs espacĂ©s) et fonctionnant dans le visible. Au dessus de ce rĂ©seau de miroir, une nacelle focale suspendue sous un ballon d’hĂ©lium ou sous des cĂąbles tendus entre deux montagnes rĂ©cupĂšre l’image Ă  haute rĂ©solution angulaire des Ă©toiles. L’absence de ligne Ă  retards, la simplicitĂ© du train optique et la mĂ©trologie interne de mise en cohĂ©rence des miroirs primaires de Carlina devraient confĂ©rer Ă  cet interfĂ©romĂštre une grande sensibilitĂ© (mv>15 pour un carlina fonctionnant avec 100 miroirs de 25 cm) et une trĂšs forte capacitĂ© d’imagerie (couverture UV riche). Nous testons actuellement l’ensemble du train optique de ce tĂ©lescope diluĂ©. Dans cet exposĂ©, nous dĂ©crirons en dĂ©tail l’asservissement et la mĂ©trologie pour rendre co‐ sphĂ©riques (mise en cohĂ©rence) avec une prĂ©cision de l’ordre du micron, les miroirs primaires. Nous concluons que Carlina, comme le LBT appartient Ă  une nouvelle famille d’instruments, les tĂ©lescopes diluĂ©s. Un tĂ©lescope diluĂ© d’une centaine de mĂštres de base, ouvrira de nouveaux champs de recherche en imageant les rĂ©gions centrales des AGNs, les microlentilles gravitationnelles, des Jupiters chauds, etc. De par ces caractĂ©ristiques techniques, un tel instrument sera complĂ©mentaire des ELTs et des interfĂ©romĂštres kilomĂ©triques.


  • Tuesday 10 May 2011 à 11h00 (Salle de confĂ©rence du bĂąt. 17)

    Causes and Consequences of nanodust in the interplanetary medium

    Ingrid Mann (Belgian Institute for Space Aeronomy BIRA/IASB, Brussels, Belgium; currently at LESIA )

    Fluxes of nanometric dust have been recently discovered near Earth’s orbit with electric field measurements onboard the STEREO space mission. The new measurements can be explained with nanodust that is accelerated in the solar wind after being formed during collisions of larger dust particles in the inner solar system. The momentum flux of the nanodust is small compared to that carried by the solar wind and the total cross-sectional area in a given volume in space of the nanodust population is smaller than that of the larger constituents of the interplanetary dust cloud. Hence the contribution of nanodust to the currently known dust interactions with the solar wind is small. The total mass flux observed in the nanodust is a small fraction of the mass that is destroyed by mutual collisions inside 1 AU, but the values are possibly beyond the fluxes that we calculate with dust collision and fragmentation models. The models have large uncertainties and it is quite possible that the nanodust does not form by direct fragmentation of the larger dust, but from a molten or processed phase of the dust material. Measuring the mass distribution of the nanodust in the interplanetary medium will give new insights in the fragmentation process and would allow re-considering the fragmentation models that are also applied in studies of dust evolution in the interstellar medium.


  • Tuesday 26 April 2011 à 11h00 (Salle de confĂ©rence du bĂąt. 17)

    In situ observations of magnetic reconnection in near-Earth space

    Alessandro RetinĂČ (LPP, École Polytechnique, Palaiseau)

    Magnetic reconnection is a universal process occurring at current sheets in astrophysical plasmas, where small-scale changes in the topology of the magnetic field lead to large-scale transport of plasma, acceleration of plasma jets, plasma heating and non-thermal particle acceleration. Reconnection is observed in the solar corona, in the solar wind, in planetary magnetospheres and is considered to play an important role in many other distant objects. Despite of many remote and in situ observations of reconnection, however, a number of key issues are still open; among them the microphysics, the mechanisms of non-thermal particle acceleration and the relationship with turbulence. Solving these issues from an experimental point of view requires in-situ observations of particle distributions functions and electromagnetic fields in reconnection regions, that are only available in the solar system through spacecraft measurements. Here we present some examples of in-situ observations, focusing on the near-Earth space.


  • Wednesday 6 April 2011 à 11h00 (Salle de confĂ©rence du bĂąt. 17)

    Progress on Chinese Spectral Radioheliograph

    Yihua Yan (National Astronomical Observatories, Chinese Academy of Sciences, Beijing)

    Imaging spectroscopy over cm-dm wave range is important for addressing fundamental processes in the solar eruptive phenomena. The Chinese Spectral Radioheliograph (CSRH) in 0.4-15 GHz range with high time, space and frequency resolutions is being constructed, which will open new observational windows on solar flares and CMEs. The CSRH array is located in a radio quiet region in Inner Mongolia of China. The array of CSRH in 0.4-2.0 GHz with 40 4.5m antennas has been mounted by the end of 2010. The array of CSRH in 2-15 GHz with 60 2m antennas will be established by 2013. The progress about the CSRH project is introduced. Some related programs on Chinese solar-terrestrial physics may also be introduced.


  • Thursday 31 March 2011 à 11h00 (Salle de confĂ©rence du ** bĂąt. 16 **)

    Wave modes and polarizations in planetary magnetospheres: Can propagation account for observed properties?

    Robert Mutel (University of Iowa)

    Radio emission from planetary magnetospheres manifests a rich variety of temporal, spectral, and polarization structure. In principle, analysis of these wave characteristics can be used to infer physical properties of the plasma, both at the site of generation and along the ray path to the observer. For radiation generated near the electron cyclotron frequency, the cyclotron maser instability (CMI), driven by parallel electron beams in converging magnetic fields, is well-accepted as the dominant generation mechanism. However, there are significant unresolved questions concerning the generation and propagation of individual wave modes and their polarization. In particular, both linear and nonlinear mode coupling and conversion at density boundaries has been invoked to explain the observed modes and polarizations. Recent spacecraft observations of terrestrial AKR (Cluster) and Saturnian SKR (Cassini) within their respective auroral acceleration regions have allowed new insights that may simplify this picture. Based on these observations, a simpler paradigm is emerging, in which L, Z, and X modes are generated at the CMI source region, and that observed intensity and polarization characteristics depend almost entirely on propagation effects between the source region and the spacecraft. We illustrate this scheme with ray-tracing results applied to several sample observations taken from Cluster (WBD) and Cassini (RPWS).


  • Mardi 29 mars 2011 à 11h00 (Salle de confĂ©rence du bĂąt. 17)

    Emissions X et phĂ©nomĂšnes d’Ă©changes de charge dans le systĂšme solaire et au-delĂ 

    Dimitra Koutroumpa (Goddard Space Flight Center, Greenbelt)

    L’Ă©change de charge est un processus physique de base qui implique le transfert d’Ă©lectrons liĂ©s entre des particules en collision. C’est un phĂ©nomĂšne qui joue un rĂŽle fondamental Ă  l’interface hĂ©liosphĂ©rique entre le vent solaire et le milieu interstellaire. Les Ă©changes de charge sont Ă©galement responsables pour l’Ă©mission des rayons X mous des comĂštes. Cette Ă©mission a Ă©tĂ© expliquĂ©e comme la dĂ©sexcitation radiative des Ă©tats excitĂ©s d’ions lourds du vent solaire qui sont peuplĂ©s lors des collisions avec des neutres comĂ©taires. Ailleurs dans le systĂšme solaire, les exosphĂšres des planĂštes comme la Terre et Mars, ainsi que les neutres interstellaires qui se propagent dans l’hĂ©liosphĂšre constituent des cibles neutres pour ce phĂ©nomĂšne. Les Ă©missions X post-Ă©changes de charge du vent solaire peuvent ĂȘtre Ă  la fois un signal d’intĂ©rĂȘt dans les Ă©tudes du systĂšme solaire, ou un bruit d’avant plan pour les Ă©tudes des plasmas chauds de la galaxie et au-delĂ . Les Ă©missions X par Ă©changes de charge dans la gĂ©ocouronne et l’hĂ©liosphĂšre entourent les observatoires en orbite terrestre et rajoutent un fond variable Ă  toute observation X dans le domaine d’Ă©nergies infĂ©rieures Ă  1.5 keV. Je vais prĂ©senter un rĂ©sumĂ© de nos connaissances actuelles sur ces phĂ©nomĂšnes, en insistant sur les Ă©missions X des planĂštes et de l’hĂ©liosphĂšre et la contribution de celle-lĂ  au fond diffus X interstellaire. Enfin, je vais briĂšvement discuter le rĂŽle de ce mĂ©canisme d’Ă©mission dans d’autres cas astrophysiques au-delĂ  du systĂšme solaire.


  • Mardi 22 mars 2011 à 11h00 (Salle de confĂ©rence du bĂąt. 17)

    The impact of slow solutions in the winds of massive stars

    Michel Curé (Université de Valparaiso, Chili)

    The standard theory that describe the wind of massive stars is the radiation driven wind model from Castor et al. (CAK 1975). When the influence of the stellar rotation is included, besides the standard solution of the CAK wind there also exists a new wind solution that we called the slow-omega solution (Curé 2004). We have already successfully described the two component wind of B[e] supergiants (Curé et al. 2005). Furthermore we apply this two component model for a classical Be stars with an oblate structure to describe the winds of these objects, explaining the observed contrast in density between equatorial and polar densities.
    Another slow solution exists when de line force parameter delta is larger than 0.25, we call this the slow-delta solution, which has a slower terminal velocity and lower mass loss rate than the fast solution.
    We have successfully applied this new slow-delta solution to explain the winds of A-Supergiants and we believe this solution can solve the weak wind problem. Finally we discuss the impact of this new slow-delta solution in the WML relationship.


  • Tuesday 15 February 2011 à 11h00 (Salle de confĂ©rence du bĂąt. 17)

    Helioseismic requirements for a better representation of overshoot at the base of the convective envelope in the Sun and solar-type stars

    Mario Monteiro (Director Centro de Astrofisica, Universidade do Porto, Portugal)

    The stratification near the base of the Sun’s convective envelope is governed by processes of convective overshooting and element diffusion, and the region is widely believed to play a key role in the solar dynamo. The stratification in that region gives rise to a characteristic signal in the frequencies of solar p modes, which has been used to determine the depth of the solar convection zone and to investigate the extent of convective overshoot.
    Previous helioseismic investigations have shown that the Sun’s spherically symmetric stratification in this region is smoother than that in a standard solar model without overshooting, and have ruled out simple models incorporating overshooting, which extend the region of adiabatic stratification and have a more-or-less abrupt transition to subadiabatic stratification at the edge of the overshoot region.
    In this talk we discuss the constraints from Helioseismology, reported in http://xxx.lanl.gov/abs/1102.0235, through a detailed comparison with the Sun of physically motivated models which have a smooth transition in stratification bridging the region from the lower convection zone to the radiative interior beneath. We find that such a model is in better agreement with the helioseismic data than a standard solar model.
    The seismic method developed to produce this analysis is discussed as well as the implications that such a result may have for the interpretation of seismic data of other solar-type stars.


  • Mardi 8 fĂ©vrier 2011 à 11h00 (Salle de confĂ©rence du bĂąt. 17)

    Voyage dans la région source des émissions radio aurorales de Saturne

    Laurent Lamy (LESIA)

    Durant les 4 derniĂšres dĂ©cennies, des observations Ă  distance ont permis d’identifier des Ă©missions radio intenses non-thermiques originaires des rĂ©gions aurorales des planĂštes magnĂ©tisĂ©es du systĂšme solaire (la Terre et les planĂštes gĂ©antes). Cependant, seule la rĂ©gion source de l’émission radio aurorale terrestre (AKR pour Auroral Kilometric Radiation) a pu ĂȘtre Ă©tudiĂ©e in situ grĂące Ă  de nombreuses observations spatiales (ISIS 1, Viking, Fast, Freja). Ces mesures ont permis de caractĂ©riser en dĂ©tail les conditions et le mĂ©canisme d’émission de ce rayonnement radio, gĂ©nĂ©rĂ© au dessus de l’atmosphĂšre par rĂ©sonance cyclotron avec des e- auroraux accĂ©lĂ©rĂ©s ( keV) circulant le long de lignes de champ magnĂ©tiques de haute latitude. Fin 2008, la mission Cassini traversait pour la premiĂšre fois la rĂ©gion aurorale oĂč le rayonnement kilomĂ©trique Ă©quivalent de Saturne (SKR pour Saturn Kilometric Radiation, dĂ©couvert par Voyager en 1980) prend naissance. En tirant partie des observations in situ simultanĂ©es fournies par divers instruments Ă  bord (radio, magnĂ©tomĂštre, particules), j’ai pu caractĂ©riser le plasma auroral et les propriĂ©tĂ©s de l’onde dans la rĂ©gion source, permettant de tester son mĂ©canisme d’émission (supposĂ© similaire Ă  celui de l’AKR) et contraindre sa source d’énergie libre. Par ailleurs, les antennes radio dĂ©tectant Ă  la fois des Ă©missions locales et distantes, j’ai pu dĂ©crire et quantifier l’évolution des propriĂ©tĂ©s de l’onde au cours de leur propagation dans un plasma magnĂ©tisĂ© (polarisation, diagramme d’émission apparent).


  • Thursday 3 February 2011 à 11h00 (Salle de confĂ©rence du bĂąt. 17)

    Statistical Properties of the Magnetic Field in Plage and Quiet Sun

    Alan Title (LMSAL, Palo Alto, CA, USA)

    As the magnetic features on the solar are seen at higher and higher spatial and temporal resolution it becomes increasingly more difficult to recognize the polarity pairs in the flux emergence processes. Modern feature recognition processes identify more fragments than bipoles and even when “groups” are identified a significant fraction of the flux is neglected in the the analysis. Here I discuss a method that generates distributions on a pixel by pixel basis. It is discovered that the distributions have power laws indices of -2.2 to -3.5 in QS and -1 to -1.3 in plage. The measurements of the total and mean flux show there is have nearly perfect polarity balance in the inner network regions independent of radial position on the solar disk and independent of the flux imbalance of the surrounding network fields. If time allows some new results from the AIA mission will be described