Lundi 23 janvier 2017 à 16h00 (Salle de confĂ©rence du bâtiment 17)
Anne-Lise Maire (MPIA, Heidelberg, Germany)
The SAO 206462 (HD 135344B) disk is one of the few known transitional disks showing asymmetric features in both scattered light and thermal emission. Giant planets in formation have been suggested for accounting for the disk morphology. We obtained new high-contrast and high-resolution near-infrared images of the target with the VLT planet finder instrument SPHERE. The spiral arms and the outer rim of the inner cavity are revealed at high sensitivities without the need for image post-processing. We do not detect any close-in companion candidates. As for the derivation of the detection limits on putative giant planets embedded in the disk, we show that the knowledge of the disk aspect ratio and viscosity is critical for the estimation of the attenuation of a planet signal by the protoplanetary dust because of the gaps that these putative planets can open. Given the assumptions on these parameters, the mass limits can vary from 2-5 to 4-7 Jupiter masses at separations beyond the disk spiral arms. Our deep detection limits provide new constraints on a few recent predictions of massive planets in the outer disk based on the spiral density wave theory.
Jeudi 3 novembre 2016 à 11h00 (Salle de confĂ©rence du bâtiment 17)
Milan Maksimovic (LESIA)
L’instrument Radio & Plasmas Waves (RPW) de la mission Solar Orbiter de l’ESA a la capacité de mesurer à la fois les ondes électriques depuis le continu jusqu’à 16 MHz et les ondes magnétiques depuis quelques Hertz jusqu’à 500 kHz. Ce faisant, RPW permettra d’observer des phénomènes physiques à la fois in-situ dans le vent solaire entourant la sonde et par télédétection pour des ondes radio générées près du Soleil. Après avoir introduit brièvement les objectifs scientifiques de la mission je détaillerai un peu plus ceux spécifiques à l’instrument. Je décrierai ensuite comment ces objectifs scientifiques on été implémentés ainsi que l’implication technique du LESIA.
Jeudi 27 octobre 2016 à 11h00 (Salle de confĂ©rence du bâtiment 17)
Anne Schreiner (University of Cologne)
Analytical dissipation models for solar wind turbulence are usually derived as a function of the perpendicular wavenumber, however, magnetic field fluctuations in the solar wind are mostly measured for angles between the mean magnetic field and the solar wind velocity less than 90°. Particularly for small field-to-flow angles, dissipation processes at electron scales influence the spectral scaling of the sub-ion range due to sampling effects, which occur in one-dimensional measurements where several different wavevectors contribute to the spectral energy density at a certain spacecraft frequency. To help better understand the physical mechanism of the dissipation process and its influence on the sub-ion range, we develop a three-dimensional dissipation model under the assumption of critically balanced turbulence and damping via wave-particle interactions of kinetic AlfvĂ©n waves obtained from linear Vlasov theory. From the three-dimensional energy distribution in k-space we calculate based on a forward modeling approach by von Papen & Saur (2015) reduced one-dimensional power spectra, which we compare to a set of spectral energy densities obtained from one- dimensional Cluster measurements by Alexandrova et al. (2012).
Vendredi 21 octobre 2016 à 11h00 (Salle de confĂ©rence du bâtiment 17)
Adolfo F. Viñas (NASA Goddard Space Flight Center, Geospace Physics Laboratory, Greenbelt MD, USA)
Kinetic-scale plasma wave packet resolved with the Fast Plasma Investigation on MMS The Fast Plasma Investigation on the Magnetospheric Multiscale mission provides two orders of magnitude higher time resolution than previous magnetospheric missions. Fast measurements from a constellation of four closely spaced observatories enable unprecedented investigations of kinetic-scale plasma physics. We present observations taken in the first year of MMS operation of structures with spatial scales below the thermal ion gyroradius. We analyze a kinetic Alfven wave packet in a reconnection jet near Earth’s dayside magnetopause. Multi-spacecraft particle and fields measurements are used to demonstrate that this packet travels obliquely to the magnetic field away from the magnetic reconnection site, and that energy is being transferred from the wave into the plasma. Further examination of the electron velocity distribution in the wave results in the discovery of a population trapped by the magnetic mirror force. This population accounts for 50% of the density variation in the wave packet and may lead to the generation of secondary instabilities. Implications for the future THOR mission will be discussed.
Mercredi 12 octobre 2016 à 11h00 (Salle de rĂ©union du bâtiment 14)
Kostas Allisandrakis (University of Ioannina, Grèce)
Jeudi 6 octobre 2016 à 14h00 (Salle de confĂ©rence du bâtiment 17)
John D Landstreet (Western Univ., London, Ontario, Canada)
We have observational detections of magnetic fields in at least some stars of the major stages of stellar evolution, from the pre-main sequence to the commonest stellar final state, the white dwarfs. However, the evolution of a field as a star evolves from one stage to another is still very poorly understood. The white dwarfs are particularly puzzling. Naively, their fairly rare megaGauss magnetic fields could be the descendents, by flux conservation, of the fairly rare kiloGauss magnetic fields of upper main sequence stars. But how these main sequence fields survive the giant and AGB stages of stellar evolution, or how “new" fields are produced in some young white dwarfs (perhaps by binary merger events) is still far from clear. Theoretical advances depend on a good foundation of observations to test and refine models. S. Bagnulo, G. Valyavin and I have been working to provide a systematic observational description of the occurrence and field geometry of magnetic white dwarfs, especially close to the weak field limit of the very broad observed field strength distribution. In the course of this work, which uses data from ESO FORS, WHT ISIS, and the Russian 6-m telescope, we have developed a powerful method of using ESPaDOnS on the CFHT to detect and ultimately model very weak white dwarf fields. This talk will survey some of our interesting first results.
Vendredi 30 septembre 2016 à 11h00 (Salle de rĂ©union du bâtiment 14)
Kostas Moraitis (LESIA)
Mardi 27 septembre 2016 à 11h00 (Salle de rĂ©union du bâtiment 14)
Eoin Carley (LESIA)
Electron acceleration in the solar corona is often associated with flares and the eruption of twisted magnetic structures known as flux ropes. However, the locations and mechanisms of such particle acceleration during the flare and eruption are still subject to much investigation. Observing the exact sites of particle acceleration can help conrm how the are and eruption are initiated and evolve. Here we use the Atmospheric Imaging Assembly to analyse a flare and erupting flux rope on 2014-April-18, while observations from the Nan-cay Radio Astronomy Facility allows us to diagnose the sites of electron acceleration during the eruption. Our analysis shows evidence for a pre-formed flux rope which slowly rises and becomes destabilised at the time of a C-class flare, plasma jet and the escape of >75 keV electrons from rope center into the corona. As the eruption proceeds, continued acceleration of electrons with energies of -5 keV occurs above the flux rope for a period over 5 minutes. At are peak, one site of electron acceleration is located close to the are site while another is driven by the erupting flux rope into the corona at speeds of up to 400 km/s. Energetic electrons then fill the erupting volume, eventually allowing the flux rope legs to be clearly imaged from radio sources at 150-445MHz. Following the analysis of Joshi et al. (2015), we conclude that the sites of energetic electrons are consistent with flux rope eruption via a tether-cutting or flux cancellation scenario inside a magnetic fan-spine structure. In total, our radio observations allow us to better understand the evolution of a flux rope eruption and its associated electron acceleration sites, from eruption initiation to propagation into the corona.
