Tuesday 5 February 2013 à 11h00 (Salle de conférence du bât. 17)
Yang Guo (School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China)
Magnetic field in the solar atmosphere plays a key role in various solar activities. However, it is still difficult to measure the coronal magnetic field. The force-free model is usually adopted to construct the 3D coronal magnetic structure. We test a nonlinear force-free field (NLFFF) optimization code in spherical geometry using an analytical solution from Low and Lou, which is well reconstructed when the boundary condition is properly managed. Analytical tests also show that NLFFF code in the spherical geometry performs better than that in the Cartesian one when the field of view of the bottom boundary is large. Additionally, we apply the NLFFF model to an active region observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO) both before and after an M8.7 flare. By comparing the extrapolated magnetic field lines with the extreme ultraviolet (EUV) observations by the Atmospheric Imaging Assembly on board SDO, we find that the NLFFF performs better than the PFSS not only for the core field of the flare productive region, but also for large EUV loops higher than 50 Mm.
Lundi 14 janvier 2013 à 11h00 (Salle de conférence du bât. 17)
Justin Kasper (Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA)
Plasma carrying a spectrum of counter-propagating field-aligned ion-cyclotron waves can strongly and preferentially heat ions through a stochastic Fermi mechanism. Such a process has been proposed to explain the extreme temperatures, temperature anisotropies, and speeds of ions in the solar corona and solar wind. We quantify how differential flow between ion species results in a Doppler shift in the wave spectrum that can prevent this strong heating. Two critical values of differential flow are derived for strong heating of the core and tail of a given ion distribution function. Our comparison of these predictions to observations from the Wind spacecraft reveals excellent agreement. Solar wind helium that meets the condition for strong core heating is nearly seven times hotter than hydrogen on average. Ion-cyclotron resonance contributes to heating in the solar wind, and there is a close link between heating, differential flow, and temperature anisotropy.
Friday 11 January 2013 à 11h00 (Salle de conférence du bât. 17)
W. Gekelman (Department of Physics and Astronomy, UCLA, California 90095, USA)
Magnetic flux ropes are due to helical currents and form a dense carpet of arches on the surface of the sun. Occasionally one tears loose as a coronal mass ejection and its rope structure is detected by satellites close to the earth. Current sheets in plasma can tear into filaments and these are nothing other than flux ropes. Ropes are not static, they exert mutual J × B forces causing them to twist about each other and merge. Kink instabilities cause them to violently smash into each other and reconnect at the point of contact. We report on experiments done in in the large plasma device the LAPD (L = 17m, dia = 60cm, 0.3 ≤ B0z ≤ 2.5kG, n ≅ 2 × 1012 cm −3 ) at UCLA on three dimensional flux ropes. Two or three magnetic flux ropes are generated from initially adjacent pulsed current channels or a series of ropes from the tearing of a current sheet. in a background magnetized plasma. The currents and magnetic fields form exotic shapes with no ignorable direction and no magnetic nulls. Volumetric space-time data show multiple reconnection sites with time-dependent locations. Mach probes measure three dimensional plasma flow which jet out from reconnection regions and spiral around the rope(s) magnetic field. The concept of a quasi-separatrix layer (QSL), a tool to understand and visualize 3D magnetic field line reconnection without null points is introduced. We will explain what it is and how it is derived from our data. Here the QSL is a narrow ribbon-like region(s) that twist between field lines. It (they) will be shown in detail from data sets acquired at up to 50,000 spatial locations. Within the QSL(s) field lines that start close to one another rapidly diverge as they pass through one or more reconnection sites. The magnetic helicity, which is a measure of the linkage of magnetic fields is evaluated from volumetric data in both cases and its rate of change is used to estimate the plasma resistivity. Heating and other co-existing waves (the ropes are Alfvén waves in disguise) will be presented. Flux ropes and QSLs show promise in understanding what 3D reconnection really is!
This work was done at the Basic Plasma Science Facility at UCLA and funded by NSF and DOE
Figure caption: The figure is a rendering of the experimental data. The blue and green tubes are magnetic field lines associated with the flux ropes. (Note several “green” field lines have rotated in space and are smashing into the “blue” ones). The orange field lines are plasma flow. The flow speed is as large as Mach 0.3. The blue surface is a Quasi-Seperatrix layer of value 1000. The flow goes through the QSL at one point and then spirals around the flux ropes. The two axis markers are two meters apart in the z direction (the machine axis). Data was acquired at 50,000 locations over a volume (δx=24 cm, δy = 24 cm, δz = 10m). Many other images and movies will be shown in the talk.
Friday 11 January 2013 à 11h00 (Salle de conférence du bât. 17)
W. Gekelman (Department of Physics and Astronomy, UCLA, California 90095, USA)
Magnetic flux ropes are due to helical currents and form a dense carpet of arches on the surface of the sun. Occasionally one tears loose as a coronal mass ejection and its rope structure is detected by satellites close to the earth. Current sheets in plasma can tear into filaments and these are nothing other than flux ropes. Ropes are not static, they exert mutual J × B forces causing them to twist about each other and merge. Kink instabilities cause them to violently smash into each other and reconnect at the point of contact. We report on experiments done in in the large plasma device the LAPD (L = 17m, dia = 60cm, 0.3 ≤ B0z ≤ 2.5kG, n ≅ 2 × 1012 cm −3 ) at UCLA on three dimensional flux ropes. Two or three magnetic flux ropes are generated from initially adjacent pulsed current channels or a series of ropes from the tearing of a current sheet. in a background magnetized plasma. The currents and magnetic fields form exotic shapes with no ignorable direction and no magnetic nulls. Volumetric space-time data show multiple reconnection sites with time-dependent locations. Mach probes measure three dimensional plasma flow which jet out from reconnection regions and spiral around the rope(s) magnetic field. The concept of a quasi-separatrix layer (QSL), a tool to understand and visualize 3D magnetic field line reconnection without null points is introduced. We will explain what it is and how it is derived from our data. Here the QSL is a narrow ribbon-like region(s) that twist between field lines. It (they) will be shown in detail from data sets acquired at up to 50,000 spatial locations. Within the QSL(s) field lines that start close to one another rapidly diverge as they pass through one or more reconnection sites. The magnetic helicity, which is a measure of the linkage of magnetic fields is evaluated from volumetric data in both cases and its rate of change is used to estimate the plasma resistivity. Heating and other co-existing waves (the ropes are Alfvén waves in disguise) will be presented. Flux ropes and QSLs show promise in understanding what 3D reconnection really is!
This work was done at the Basic Plasma Science Facility at UCLA and funded by NSF and DOE
Figure caption: The figure is a rendering of the experimental data. The blue and green tubes are magnetic field lines associated with the flux ropes. (Note several “green” field lines have rotated in space and are smashing into the “blue” ones). The orange field lines are plasma flow. The flow speed is as large as Mach 0.3. The blue surface is a Quasi-Seperatrix layer of value 1000. The flow goes through the QSL at one point and then spirals around the flux ropes. The two axis markers are two meters apart in the z direction (the machine axis). Data was acquired at 50,000 locations over a volume (δx=24 cm, δy = 24 cm, δz = 10m). Many other images and movies will be shown in the talk.
Jeudi 13 décembre 2012 à 15h00 (Salle de conférence du ** bât. 14 **)
Ji Wu, Ying Liu, Hao Liu
Le format du séminaire est inhabituel. Il consiste en 3 présentations de 20 minutes chacune :
Prof. Ji Wu (Director General, Natinal space scien Center, Beijing) : "Strategic Pioneer Program on Space Science"
Dr. Ying Liu (PI of the SPORT project) : " SPORT Scientific Objectives"
Dr. Hao Liu (Chief Engineer of the SPORT project) : "Technical design of SPORT"
Tuesday 11 December 2012 à 11h00 (Salle de conférence du bât. 17)
Gabby Provan (Physics and Astronomy, University of Leicester, United Kingdom)
Saturn’s magnetosphere chimes with oscillations at periods close to the planetary rotation period. The oscillatory period changes slowly over time [Galopeau and Lecacheux, 2000; Gurnett et al., 2005, Kurth et al., 2007, 2008], with slightly different periods being observed in the Northern and southern hemispheres [Gurnett et al., 2009a, Lamy, 2011]. Both periods are observed in the equatorial plane [Provan et al., 2011]. This talk aims to explore the periods, amplitudes and polarization of the magnetic field oscillations from 2004 and 2012. I will examine the relationship of these oscillations to the SKR, and show how these oscillations are related to the variations in the UV auroral power as observed by the Hubble spacecraft [Nichols et al., 2010a], the location of the UV auroral oval [Nichols et al., 2008,2010b, Provan et al., 2009], the position of the magnetopause and bow shock [Clarke et al., 2006, 2010a,b] and the vertical displacement and thickness of Saturn’s plasma sheet. Finally, I will examine how recent abrupt disruptions in the amplitudes of the magnetic oscillations may be related to the occurrence of Saturn’s Great White Spot.
Mardi 30 octobre 2012 à 14h00 (Salle de conférence du bât. 17)
Olivier Guyon (Subaru Telescope, Nat. Astr. Obs. of Japan & Steward Obs., Univ. of Arizona)
Thanks to developments in optics, large format detector arrays and computing, wide field imaging systems are becoming increasingly common in astronomy. I will illustrate how this offers new opportunities for detection and characterization of exoplanets. ASTROMETRY : Deep wide field imaging of fields around nearby stars can conceptually lead to a high precision absolute astrometric measurement, enabling detection of planets around nearby stars and measurement of their orbits and masses. At the micro-arcsecond level required for observing Earth-mass planets in habitable zones, image distortion effects are however orders of magnitude larger than the signal. I will show that these distortions can be calibrated and removed by applying small dots, covering approximately 1% of the pupil area, on the primary mirror. This allows full utilization of wide field image for astrometric measurements, offering sub-microarcsecond precision. GROUND-BASED EXOPLANET TRANSIT SURVEY : Commercial DSLR cameras offer the most cost-effective solution for obtaining high etendue (product of collecting area and field of view). They are however generally considered inappropriate for high precision photometry due to strong pixelization errors in the RGB color detector. I will show that this limitation can be overcome by data reduction, and describe our plan for a ground-based global network of low-cost DSLR-based units.
Mardi 23 octobre 2012 à 14h00 (Amphithéâtre Evry Schatzman (bât 18))
Chiara Ferrari (OCA)
Séminaire LOFAR-SKA, organisé par l’Action Spécifique SKA-LOFAR de l’INSU et l’axe "Radiotélescopes du XXIème siècle" de l’Observatoire de Paris.
L’interféromètre basse-fréquence LOFAR est maintenant opérationnel. Il s’agit d’un télescope capable d’aborder des sujets astronomiques très variés, allant de la planétologie à l’étude des galaxies et des amas de galaxies, jusqu’à l’époque de la ré-ionisation. Dans ce séminaire, je présenterai un des projets clés de LOFAR, le "Key Project Surveys". Je mettrai l’accent sur l’énorme potentiel de cet instrument pour développer des relevés profonds et étendus, avec un forte impact sur des domaines assez variés de l’astrophysique extra-galactique. Je présenterai enfin les premiers résultats scientifiques obtenus sur l’étude de l’émission radio des amas de galaxies.
Vendredi 7 septembre 2012 à 11h00 (Salle de conférence du bât. 17)
Sebastien Hess (LATMOS)
Les phénomènes auroraux sont observés à toutes les planètes possédant un champ magnétique interne. Ils sont observés sur une vaste gamme de longueurs d’ondes, du domaine X aux émissions radio basses fréquences. Chaque gamme de longueur d’onde apporte un jeu d’informations complémentaires sur l’interaction magnétosphérique qui génère les aurores. Les sources des émissions aurorales se situent en générale dans la haute atmosphère de la planète. Le domaine radio se démarque par la large gamme d’altitude dans laquelle sont situées les sources, depuis la limite collisionnelle de l’atmosphère jusqu’à plusieurs milliers de kilomètres, voir plus selon les planètes. La fréquence des sources étant liée à leur altitude, la radio permet d’obtenir une information en altitude, ce que ne permettent pas les autres longueurs d’onde, mais au détriment des informations en longitude-latitude. Au contraire des autres longueurs d’onde, on ne peut pas pour l’instant faire d’image en radio basses fréquences. Je montrerai qu’il est possible de modéliser les caractéristiques des sources suffisamment bien pour pouvoir retrouver leur position en longitude-latitude à partir de la morphologie des émissions dans le plan temps-fréquence et d’en déduire des informations d’importance pour la compréhension de la physique des phénomènes auroraux et des magnétosphères.
