1,721,125 research outputs found
Etude de l’exosphère de Mercure par le spectromètre PHEBUS de la mission BepiColombo
No full textMercure est la planète tellurique la moins explorée du système solaire, trois missions seulement l’ont survolée. Mariner 10 a effectué trois survols de la planète en 1974 et 1975, découvrant une atmosphère très mince, similaire à celle de la Lune, contenant de l’hélium (He) et de l’hydrogène (H). Le sodium (Na), le potassium (K) et le calcium (Ca) ont été détectés plus tard à partir d’observations depuis la Terre. La mission MESSENGER, lancée en 2004, a non seulement survolé Mercure à trois reprises mais a également orbité la planète durant quatre ans. Elle a détecté deux nouvelles espèces, le magnésium (Mg) et le manganèse (Mn). En parallèle, deux autres espèces ont été découvertes grâce à des observations depuis la Terre : le fer (Fe) et l’aluminium (Al). BepiColombo est la troisième mission à destination de Mercure. Lancée en 2018, la mission conjointe ESA JAXA doit effectuer six survols de Mercure avant de s’insérer en orbite fin 2025. Trois survols de Mercure ont déjà été réalisés lors desquels PHEBUS (Probing the Hermean Exosphere By Ultraviolet Spectroscopy) a pu observer l’exosphère de Mercure. L’instrument est composé de deux détecteurs UV (EUV et FUV) et de deux canaux visibles (c404 et c422). Le canal c404 est dédié à la ligne d’émission du K à 404,7 nm tandis que le canal c422 est dédié à la ligne d’émission du Ca à 422,8 nm. La géométrie d’observation des deux premiers survols est similaire, la sonde approchant la planète côté nuit, traversant son ombre avant de se diriger côté jour. La sonde s’est approchée à ∼200 km de la surface au point le plus proche côté nuit. PHEBUS pointait vers le nord, dans une direction opposée au soleil. Le taux de comptage enregistré par les deux canaux visibles illustre la géométrie d’observation, notamment le passage dans l’ombre de Mercure. Le maximum est atteint peu après la sortie d’éclipse. Le taux de comptage diminue ensuite à mesure que la sonde s’éloigne de Mercure. Lors des survols, les signaux des deux canaux sont pollués par des pics sporadiques dont l’origine reste incertaine. Malgré ces pics, le canal c422 a clairement détecté du Ca. Le signal c422 montre non seulement une concentration de Ca à l’aube mais également une couronne de Ca très étendue du côté matin qui n’a pas été rapportée par MESSENGER. La hauteur d’échelle déduite de nos profils (2500-2800 km) est en accord avec la valeur rapportée par MESSENGER à un angle d’anomalie vraie similaire. En utilisant le modèle de Chamberlain, j’ai déterminé une température à l’exobase très élevée (>50 000K), ce qui est en accord avec les résultats de MESSENGER. La grande hauteur d’échelle et la température à l’exobase élevée impliquent un processus de libération très énergétique. Les données semblent également indiquer une asymétrie jour/nuit qui pourrait signifier que la source de Ca est principalement du côté jour ou être la conséquence d’un décalage de la source de Ca par rapport à la région de l’aube. La prochaine étape consiste à utiliser un modèle exosphérique 3D prenant en compte la photoionisation et la distribution non uniforme du Ca. Enfin, le pointage de PHEBUS lors de ces deux premiers survols n’a pas permis d’observer la potentielle composante froide du Ca puisque les altitudes 50,000 K), in agreement with MESSENGER results. Both the large scale height and temperature at the exobase imply a very energetic release process. I also report a day/night asymmetry in the Ca exosphere that could indicate that the source of Ca is predominantly on the dayside or be the consequence of a shift of the main source of Ca away from the dawn region. The next step consists in using a 3D exospheric model that will include the effects of photoionization and non uniform distribution of Ca. Finally, PHEBUS pointing during the flybys did not allow to observe the potential cold component of Ca as the altitudes <200 km were not explored.The c404 channel detected species at low altitudes on the morning side during the flybys, potentially K or Mn. This K emission line was never detected by MESSENGER. As for Mn, MESSENGER has not observed it continuously, its detection was confined to certain seasons and to certain regions of the exosphere. These detection conditions are very different from those of BepiColombo. It is therefore difficult to identify with certainty the detected species. Further observations are necessary. Note that the scale height deduced from the c404 profile is ∼135 km and that the Chamberlain model applied to the c404 profile seems to indicate a temperature at the exobase <3000 K
Update on modeling and data analysis of heliospheric solar wind charge exchange X-ray emission
No full textInternational audienceAbout 15 years ago, charge exchange (CX) X-ray emission was discovered in comet observations, and was identified as the radiative decay of excited states of highly-charge solar wind ions populated in collisions with neutral cometary material. This non-thermal X-ray emission mechanism is now generally acknowledged in planetary environments (e.g. Mars, Earth), as well as interstellar atoms sweeping through the heliosphere. In this paper I present the most recent improvements made in simulations of the heliospheric CX X-ray emission. The model results are compared to X-ray data from Suzaku, XMM-Newton and Chandra spanning over a 10-year period, and some conclusions are drawn on the heliospheric contribution to the diffuse soft X-ray background. The solar system CX X-ray sources can serve as prototypes in terms of modeling and diagnostics to more distant astrophysical objects where CX emission signatures are being discovere
Thermal Emission or Charge Exchange from the North Polar Spur?
No full textXMM-Newton Proposal ID 06516302We propose a coupled observation of the North Polar Spur (NPS) and a nearby off-spur region in order to determine whether the soft X-ray spectrum of the NPS is solely produced by hot plasma thermal emission, or whether some fraction of the emission is produced by charge exchange (CX) collisions between neutral particles and ions within the hot gas envelope of the Loop I/NPS superbubble. Our goal is to confirm and measure the shift in energy of the He-like O VII and Ne IX triplet centroids measured at low significance in previous observations of the NPS with XMM-Newton and Suzaku. These shifts are predicted by the CX emission models but not in thermal emission models. We request the new ON/OFF observations of the NPS in order to avoid large spatial variations of the cosmi
Models of Heliospheric solar wind charge exchange X-ray emission
No full textInternational audienceThe first models of the solar wind charge exchange (SWCX) X-ray production in the heliosphere were developed shortly after the discovery of SWCX emission at the end of 1990s. Since then, continuous monitoring of the global solar wind evolution through the solar cycle has allowed better constraints on its interaction with the interstellar neutrals. We have a fairly accurate description of the interstellar neutral density distributions in interplanetary space. However, the solar wind heavy ion fluxes, and especially their short term variability and propagation through interplanetary space, have remained relatively elusive due to the sparseness or lack of in situ data, especially towards high ecliptic latitudes. In this talk, I will present a summary the heliospheric SWCX modeling efforts, and an overview of the global solar cycle variability of heliospheric SWCX emission, while commenting on the difficulties of modeling the real-time variability of the heliospheric X-ray signal
Etude des émissions gazeuses UV/X et des phénomènes d'échange de charge dans l'Héliosphère : application à des systèmes astrophysiques analogues.
No full textCharge-exchange is a very efficient phenomenon whenever an ionized gas interacts with a neutral gas. My thesis included a thorough analysis of the occurence of this phenomenon and its effects in the heliosphere and other astrophysical systems.At first, I used interstellar (IS) H distributions in order to model the backscattered Lyman-α emission observed with the SOHO/SWANinstrument and finally constrain the IS hydrogen parameters. This study allowed to demonstrate the deflection of the IS hydrogen flow with respect to the IS helium flow. This deflection is due to the coupling of IS hydrogen atoms to IS protons in the heliospheric interface and the creation of a secondary population of hydrogen, which is heated, decelerated and deviated with respect to primary IS atoms. The deflection of this secondary population with respect to the primary flow is the direct proof that the structure of the heliosphere is not axisymetric, but tilted under the pressure of an inclined local IS magnetic field.In a second part, I modeled the EUV and soft X-ray emissions in the heliosphere due to charge-exchange between heavy solar wind ions and IS hydrogen and helium. I have built the first time-dependent model of heliospheric X-ray emission in response to solar wind composition and flux temporal variations, taking into account the solar rotation and the solar wind radial propagation. I applied the model to X-ray data obtained with XMM, Chandra and Suzaku telescopes. I have demonstrated that in the 0.3-1 keV range the heliospheric X-ray emission is responsible for all the diffuse emission previously attributed to hot gas in the so-called Local Interstellar Bubble (100 parsecs wide). A preliminary analysis of the lower energy emission (0.1-0.3 keV) during the last months of my thesis shows that the heliospheric contribution may very well explain most of the supposed Local Bubble emission at these energy ranges too.Finally, I have used a hybrid model of theMartian plasma environment to model the charge-exchange induced soft X-ray emission from the Martian exosphere. I have used input exosphere parameters from Lyman-α measurements of Mars Express/SPICAM instrument and compared my results with recent Xray observations of Mars with XMM and Chandra satellites.L'échange de charge est un processus de couplage très efficace lorsqu'un gaz ionisé interagit avec un gaz neutre. Le but de ma thèse était d'étudier l'occurence de ce phénomène et ses effets dans l'héliosphère et d'autres systèmes astrophysiques.J'ai en premier lieu déterminé les paramètres du flot d'hydrogène atomique interstellaire autour du soleil et mis en évidence sa déflection à l'entrée dans l'héliosphère. Cette d´eflection, due à la création d'une population d'atomes secondaires lors des échanges de charge atomes H-protons, est la conséquence de la distorsion de l'héliosphère sous l'effet du champ magn´etique interstellaire. L'étude était basée sur l'analyse et la modélisation des données de l'instrument SWAN sur SOHO, qui mesure l'émission de rétro-diffusion résonnante du rayonnement solaire à Lyman-α (121.6nm) par les atomes d'hydrogène dans l'espace interplanétaire et sa ré-absorption par une cellule montée devant les photomètres.Dans une deuxième phase, j'ai étudié les émissions EUV/X (0.1-1 keV) dues aux échanges de charge entre les ions lourds du vent solaire et les atomes d'hydrogène et d'hélium. J'ai développé un modèle statique et une variante dynamique pour calculer les émissions X dans l'héliosphère en tenant compte des variations de l'activité solaire à grande (cycle de 11 ans) et courte (sursauts de vent solaire) échelle. J'ai appliqué mon modèle à une série d'observations X avec les télescopes XMM, Chandra et Suzaku. L'étude a montré que l'héliosphère est responsable de toute l'´emission entre 0.3 et 1 keV attribuée auparavant à la Bulle Interstellaire Locale, une structure de 100 parsecs environ, supposée remplie de gaz très chaud. Les résultats préliminaires dans le domaine 0.1-0.3 keV montrent également que la contribution héliosphérique est du même ordre que celle attribuée à la Bulle Locale.Finalement, en utilisant des modèles existants de l'interface entre le vent solaire et l'exosphère de Mars, j'ai calculé les émissions X dues aux échanges de charge entre les ions lourds du vent solaire et les atomes planétaires, montré comment elles révèlent les structures magnétiques autour de la planète, et les ai comparées aux observations récentes Chandra/XMM
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Properties of the Local Diffuse X-ray Background Below 1/4 keV
No full textThe observed Diffuse X-ray Background (DXB) is made of different components that are difficult to separate because of similar observational properties. At very low energy, two local components are predominant, emission from the Local Hot Bubble (LHB), a 1M K plasma surrounding the solar system 100-300 pc in all directions, and Solar Wind Charge eXchange (SWCX), produced when solar wind ions interact with neutral gas in the interplanetary medium. The “Diffuse X-ray emission from the Local galaxy” (DXL) mission consists of a series of sounding rocket launches to study and separate the SWCX and LHB x-ray emissions. DXL-2 launched in December 2015 and used four large area proportional counters carrying three unique filters (carbon, boron, and beryllium-based). It scanned through the Helium Focusing Cone (HFC) in two perpendicular directions to look for the spatial signature of SWCX on top of the LHB emission. We find that the SWCX contribution to the diffuse x-ray background for the ¼ keV band in the galactic plane to be 30±6% and an average over the whole sky to be 30±6% the rest of the emission is dominated by LHB. Similarly, for ¾ keV we find it to be 21±9% in-plane and 14±6% for the whole sky. We used the observation along the ecliptic longitude to measure the pointing direction of the Sun’s velocity vector, which we quantify to be λ= 73.5±3.6°. We extrapolate our result to identify SWCX and its contribution to ROSAT All Sky Survey (RASS) maps and to derive physical parameters of the LHB. Our results will stand as a strong probe to correct for the embedded SWCX offset for any of the past or future observations on diffuse x-ray emission.</p
Solar Wind Charge Exchange X-ray emission in the Heliosphere, and other implications on the local diffuse X-ray background
No full textInternational audienceI will present an overview of our current knowledge of the solar wind charge exchange (SWCX) X-ray emission from the Heliosphere and its contribution to the local diffuse background. I will detail the comparison of self-consistent calculations of the heliospheric SWCX component to observations made with ROSAT (in the 1/4 keV band)) and XMM-Newton, Chandra and Suzaku (in the 3/4 keV band), as well as the possible effect on the presence/temperature of the Local Bubble hot gas. Finally, I will expose the need in atomic data input from theoretical calculations and laboratory measurements, as well as the implications of the charge-exchange mechanism in different aspects of diffuse X-ray emission
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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