LESIA - Observatoire de Paris

Presentation of the new journal "Nature Astronomy"

Agnès Fave — 2016-06-30T08:41:26Z

Nature Astronomy is a new member of the Nature family set for launch in January next year. It will be the main Nature journal dedicated to the field and therefore aims to publish the most significant advances (of observational, theoretical, computational, methodological or technological nature) in astronomy, astrophysics and planetary science. As recently hired associate editor for Nature Astronomy, I will give a (relatively brief) presentation of the journal, its purpose and structure, how it fits within the current landscape of available journals. I will also include basic information on "how to publish in Nature". The main purpose of the seminar is to get questions (of any kind !) and to give an "inside view" of the editorial process within Nature. Nature Astronomy is in its early stages and the editorial team (including myself) has just started working, so any kind of input from its target audience is more than welcome and can shape the scope of the journal.

Utilizing the Wavefront Sensor for Direct Imaging of Exoplanets from the Ground

Agnès Fave — 2016-06-24T12:00:18Z

A new generation of telescopes with mirror diameters of 20 m or more, called extremely large telescopes (ELTs) has the potential to provide unprecedented imaging and spectroscopy of exo-planetary systems, if the difficulties in achieving the extremely high dynamic range required to differentiate the planetary signal from the star can be overcome to a sufficient degree. I will argue that millisecond imaging in the science camera could potentially lead to a breakthrough in the field. My approach combines the wavefront sensor data stream with simultaneous science camera exposures to simultaneously determine the planetary image and the non-common path aberrations of the optical system. I emphasize that this approach is not a form of differential imaging, such as ADI or SDI.

History of Research on Solar Energetic Particle Events : The Evolving Paradigm

2016-06-15T12:47:17Z

I review the evolution of research on the acceleration of solar energetic particles (SEPs) – beginning with Forbush's observation of two ground level events in 1942, through the first suggestion of the two-phase (flare and shock) picture of Wild, Smerd, and Weiss (1963), on up to the present day. During my own time working in the field, I have seen SEP studies go from a pre-paradigmatic state to one with a well-defined conceptual framework. Highlights include the linkage of CMEs and shocks to SEP events by Kahler, Cane and others, the seminal paper by Reames, Lin, and von Rosenvinge (1985) that linked 3He-rich events with small electron events, and the recognition of the importance of seed particles and shock geometry by Tylka et al. (2005). The notion that large high-energy (> 25 MeV/nuc) “gradual” SEP events originate primarily at CME-driven shocks was challenged by the observation of large SEP events by ACE in 1997-1998 that had “impulsive” event characteristics and has been questioned again more recently in studies by Dierckxsens et al. (2015), Trottet et al. (2015), and Grechnev et al. (2015). Of these, I focus on the Grechnev et al. study that considered >100 MeV protons. The presence of two particle acceleration mechanisms at the Sun (flare and shock) and the oscillation in emphasis between these mechanisms remain a spur for progress in this field.

A new high dynamic low noise InfraRed camera designed and developed at LPA

Agnès Fave — 2016-05-06T09:10:09Z

Attention : changement de date !

La plupart des signaux étudiés issus des réponses optiques des nano-structures, sont très faibles. Ils doivent être détectés via une instrumentation très bas bruit. Depuis un peu moins de deux décennies les détecteurs photo-sensibles de qualité scientifique (CCD, CMOS etc …) ont fait leur apparition. Les systèmes monochromateurs à photodiodes à avalanche ont, alors, été progressivement remplacés sur nos expérimentations par des caméras CCD (plus récemment CMOS et CMOS hybridés) couplées à des spectromètres. Dans le même temps les longueurs d'ondes ont évoluées en fonctions des réponses des objets étudiés et c'est vers les basses énergies que se situent de nouveaux domaines d'études. Ainsi, nous avons développé, au LPA, une caméra Infrarouge InGaAs, bas bruit, refroidie, architecturée autour d'un détecteur surprenant de chez NIT (New Imaging Technologies). Je vous parlerai, lors de ce séminaire, de cette caméra, sa conception, ses caractéristiques, la cohabitation originale entre des réponses (natives) linéaires et logarithmiques sur une même image permettant une dynamique 'quasi infinie', de ses premiers résultats expérimentaux, des applications en vues et enfin des futurs développements très prometteurs.

Aqueous alteration on Early Mars / Premiers environnements aqueux sur Mars

2016-05-06T08:58:28Z

I will present a review of new advances in our understansing of the states of liquid water during the early era (>3.5 Gya) of Mars, made possible in recent years thanks to numerous landed and orbital missions, and comparison with geochemical models and terrestrial analog sites. Mars remains slow to yield its secrets : the most essential questions regarding its past habitability and geological evolution are still open, and much debated. Several prospects to tackle these issues will be (...)

- Saison 2015-2016

Magnetohydrodynamics modeling of the onset and early dynamics of eruptive flares

Agnès Fave — 2016-05-06T08:41:10Z

- Saison 2015-2016

Exocomètes dans le disque de gaz circumstellaire de Beta Pictoris

Agnès Fave — 2016-04-20T07:49:11Z

Le jeune système planétaire autour de Beta Pictoris contient de nombreux petits corps actifs. Les observations spectroscopiques de Beta Pictoris révèlent en effet un taux élevé de transits de petits corps en évaporation, autrement dit des exo-comètes (Kiefer et al. 2014). Je présenterai l'analyse du millier de spectres de Beta Pictoris collectés avec HARPS entre 2003 et 2011, qui nous a fourni un échantillon d'environ 6000 signatures d'absorption variables identifiées à des exocomètes en transit devant Beta Pictoris. Nous avons mis en évidence que ces comètes appartiennent à deux populations distinctes avec des propriétés physiques marquées. Une première famille, produisant des raies d'absorption peu profondes, a été attribué à des comètes épuisées, pris au piège dans une résonance de moyen-mouvement avec une planète massive. La deuxième famille produisant des raies d'absorption profondes est possiblement liée à la fragmentation récente d'un ou plusieurs corps parents.

First Results from The New Horizons Encounter at Pluto. Pluto/Charon Thermal Emission

Agnès Fave — 2016-04-13T14:40:46Z

The instruments on board the New Horizons spacecraft, measured key characteristics of Pluto and Charon during the July 14, 2015, flyby. The data collected has beentransmitted to Earth since the encounter a process thatwill continue for the rest of 2016. High resolution images have been obtained along with spatially resolved spectroscopy in the infrared and ultraviolet revealing a world of extraordinary character. Additionally, during flyby the Radio Science Experiment (REX), in the NH X-band radio transceiver, recorded powerful uplink transmissions from Earth stations, as well a broadband radiometric power from the surface of Pluto and Charon. The REX recording of the uplinks produced a precise measurement of the surface pressure, the temperature structure of the lower atmosphere, and the surface radius of Pluto. In addition, REX measured thermal emission to a precision of 0.1K, from Pluto at 4.2-cm wavelength during two linear scans across the disk at close range when both the dayside and the night side were visible. A bonus in the equatorial thermal scan was the detection of a bistatic reflection of a specially designed uplink from near Pluto's specular point. This work was supported by NASA's New Horizons project.

Inversion of spectropolarimetric data for vector magnetic field maps, taking into account unresolved magnetic structures

Agnès Fave — 2016-01-21T09:46:53Z

This method has been developed at LESIA and is based on the UNNOFIT inversion code (Landolfi, Landi Degl'Innocenti, Arena, 1984, Sol. Phys., 93, 269). This code applies the Levenberg-Marquardt algorithm to make coincide the observed Stokes profiles with the theoretical ones stemmed from the Unno-Rachkovsky integration of the transfer equation through a Milne-Eddington atmosphere model. The work developed at LESIA has consisted in taking into account unresolved magnetic structures, by introducing an additional parameter to determine, a lower than unity magnetic filling factor. We will show that, in sunspot center, the results coincide with those obtained without this feature, because in sunspot umbra the magnetic filling factor is unity. But the farther from the sunspot center, the more different is the retrieved field inclination, and other observation results confirm the UNNOFIT ones. HMI onboard SDO records spectropolarimetric data of the full disk every 12 mn, and the data are available for downloading. It is then possible to select an active region of interest, and to prepare UNNOFIT field vectors of it. Examples of such work done on request by some LESIA solar team members, will be cited. This seminar is devoted to advertise about this possibility available at LESIA. The online HMI/SDO field vectors are obtained without assuming any magnetic filling factor. The farther from the sunspot center, the more different they are from the UNNOFIT results.

Lifecycle of a large-scale polar coronal pseudostreamer/cavity system

Agnès Fave — 2016-01-21T09:43:43Z

We report on an exceptional large-scale coronal pseudostreamer/cavity system in the southern polar region of the solar corona, which was visible for approximately a year starting in February 2014. It is unusual to see such a large closed-field structure embedded within the open polar coronal hole. We investigate this structure to document its formation, evolution and eventually its shrinking process, using data from both the PROBA2/SWAP and SDO/AIA UV imagers. In particular, we used EUV tomography to find the overall shape and internal structure of the pseudostreamer, and to determine its 3D temperature and density structure using then DEM analysis. We found that the cavity temperature is extremely stable with time and is essentially at similar or slightly hotter temperature than the surrounding pseudostreamer. Two regimes in cavity thermal properties was observed : the first 5 months corresponds to the lower density depletion associated with the highly multi-thermal plasma, while the shrinking period exhibits the exact opposite behavior. As the thermodynamic properties are strongly correlated with the magnetic structure, these results provide constraints on both the trigger of CMEs and the processes that maintain cavities stability for such a long lifetime.