Mercredi 24 octobre 2018 à 11h00 (Salle de confĂ©rence du bâtiment 17)
I will describe observations of magnetic disturbances in active region AR12673 between 1 and 3 September 2017 seen as a disruption of the moat flow several hours before the onset of strong flux emergence near the main sunspot. The moat flow is commonly known as a radially oriented strong outflow of photospheric plasma surrounding sunspots, which ends abruptly and shapes an annular pattern around the penumbra. Using highly accurate methods of tracking this photospheric flow applied to SDO/HMI data, we are able to describe the evolution of the moat surrounding the main sunspot of AR 12673. We find that several hours before the emergence of strong magnetic flux near the main sunspot, the moat boundaries are broken at these very same locations. This behavior is observed both on 1 and 3 September. There is no such behavior observed in the absence of flux emergence. These observational results pose the question of how often they occur in other active regions and whether the disruption of the moat flow might be, like in this case, an indication of impending enhanced magnetic activity or simply a coincidental event. I will also describe ongoing work using my new photospheric tracking framework adapted specifically for HMI and that enables systematic analyses of the moat flows as well as the relationship between supergranular flows and the magnetic evolution in emerging active regions.
Mardi 23 octobre 2018 à 11h00 (Salle de confĂ©rence du bâtiment 17)
Xudong SUN (University of Hawaii, USA)
The solar active region photospheric magnetic field evolves rapidly during major eruptive events, suggesting appreciable feedback from the corona. Using high-cadence vector magnetogram sequence, multi-wavelength coronal imaging, and numerical simulation, I will show how the observed photospheric "magnetic imprints” are highly structured in space and time, and how it can in principle be used to estimate the impulse of the Lorentz force that accelerates the coronal mass ejection (CME) plasma. In an archetypical event, the Lorentz force correlates well with the CME acceleration, but the total force impulse surprisingly exceeds the CME momentum by almost two orders of magnitude. Such a trend holds for the majority of the major eruptions from our survey sample. I propose a "gentle photospheric upwelling" scenario, where most of the Lorentz force is trapped in the lower atmosphere layer, counter-balanced by gravity of the upwelled mass. This unexpected effect dominates the momentum processes, but is negligible for the energy budget. I will discuss our follow-up project, how the upcoming high-sensitivity observations and new-generation numerical models may help elucidate the problem.
Jeudi 18 octobre 2018 à 16h00 (Salle de confĂ©rence du bâtiment 17)
Lucas Grosset (LESIA)
One of the main observational challenges for investigating the central regions of active galactic nuclei (AGN) at short wavelengths, using high angular resolution, and high contrast observations, is to directly detect the circumnuclear optically thick material hiding the central core emission when viewed edge-on. I will present my work on the dust structures of these inner regions and the main results I obtained during my four years in the LESIA. But I will focus particularly on one of the tool I used for this purpose, the simulation code MontAGN, computing the high angular resolution maps of dusty structures, including polarisation, through Monte-Carlo radiative transfer methods in the near infrared. Even at early development stages, simulated maps through MontAGN allowed me to constrain the geometry and density of the structures surrounding the AGN, in particular the dusty torus and the ionisation cone. Furthermore, after 2 years of development, the code is now almost ready for its first release and will be soon available after few upgrades.
Lundi 24 septembre 2018 à 11h00 (Salle de rĂ©union du bâtiment 14)
Atul MOHAN (Tata Institute of Fundamental Research, Pune, India)
The talk will focus on the study of weak energy release events in the Corona using the powerful new technique of snapshot spectroscopic imaging with Murchison Widefield Array (MWA). MWA observes the Sun in a frequency range of 80- 240 MHz at a spectro-temporal resolution of 40 kHz and 0.5 s. We imaged two 4 min long and 15 MHz wide band datasets at 0.5s and 16kHz resolution. This was a period of very low solar activity as per the metrics of high energy observations. This lead to nearly 46,000 images which were converted to brightness temperature maps. I will brief the techniques we employ to extract useful physical information from such large volume of data spread across 4 axes, namely 2 angular sky coordinates, frequency and time. I will focus on one of the events we studied and the interesting physical inferences.
Jeudi 20 septembre 2018 à 16h00 (Salle de confĂ©rence du bâtiment 17)
Santiago GarcĂa-Burillo (OAN)
We have used ALMA to image with 0.03-0.04"(2-3pc) spatial resolution the emission of a set of molecular lines, including CO(2-1), CO(3-2) and HCO+(4-3), and their underlying continuum emission in the circumnuclear disk of NGC1068, covering a region that extends from r=200pc down to the central 7-10pc-diameter torus of this prototypical Seyfert 2 galaxy, which was first detected in the 6-5 line of CO by GarcĂa-Burillo et al 2016. These new observations, by using three lines spanning three orders of magnitude in densities, reveal the many ’faces’ of the molecular torus in NGC1068. The torus shows a stratified layered structure spanning a radial range that goes from r=2-3pc to r=10pc. The kinematics of molecular gas in the torus are characterized by strong non-circular motions and enhanced turbulence. Furthermore, the CO(2-1) line emission has allowed us to image the outflowing molecular gas component emerging from the torus. A far-reaching question, to be answered by ongoing ALMA surveys of nearby AGNs (NUGA, GATOS), is whether similarly perturbed and turbulent tori are expected to be found ubiquitously in Seyfert galaxies, and if their properties should change as a function of parameters like the AGN luminosity, the degree of obscuration, or the Eddington ratio.
