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Reffet E.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation | du Pont S.C.,CNRS Complex Systems and Materials Laboratory | Hersen P.,CNRS Complex Systems and Materials Laboratory | Douady S.,CNRS Complex Systems and Materials Laboratory
Geology | Year: 2010

The shape of dunes depends on the history of wind regimes and sand availability. In deserts exposed to winds from two different directions but with comparable magnitude, dunes are found to be linear ridges, which are either perpendicular or parallel to the mean wind direction, depending on the angle between the two wind directions. These dunes, respectively observed for small and large angles between winds, are called transverse and longitudinal dunes. In both cases, their large width (hundreds of meters) and evolution time scale (years) strongly limit the investigation of their dynamics and thus our understanding of such structures. Here we show that, under water, similar structures can be obtained but at much smaller space and time scales. Performing controlled experiments together with numerical simulations, we highlight the physical mechanisms at play in the formation and long-term evolution of these structures. We show in particular that, while longitudinal dunes are stable and extend in time, transverse dunes are unstable. They evolve into wavy ridges and eventually break into barchans if the sand supply is too low. This fundamental difference is understood through the study of single sand piles and bars exposed to two winds. In the case of a large angle between winds, a sand pile grows a finger pointing in the average wind direction and transforms into a longitudinal dune. Such an elongation does not occur for a small angle where a sand pile evolves into a barchan. These results explain the morphological differences between straight and long longitudinal dunes and sinuous transverse dunes, while giving keys to infer the wind history or pattern state of development from the observation of dune shapes in the field. © 2010 Geological Society of America.

Aerts C.,Catholic University of Leuven | Aerts C.,Radboud University Nijmegen | Aerts C.,Hasselt University | Molenberghs G.,Hasselt University | And 3 more authors.
Astrophysical Journal | Year: 2014

We have composed a sample of 68 massive stars in our galaxy whose projected rotational velocity, effective temperature, and gravity are available from high-precision spectroscopic measurements. The additional seven observed variables considered here are their surface nitrogen abundance, rotational frequency, magnetic field strength, and the amplitude and frequency of their dominant acoustic and gravity modes of oscillation. A multiple linear regression to estimate the nitrogen abundance combined with principal component analysis, after addressing the incomplete and truncated nature of the data, reveals that the effective temperature and the frequency of the dominant acoustic oscillation mode are the only two significant predictors for the nitrogen abundance, while the projected rotational velocity and the rotational frequency have no predictive power. The dominant gravity mode and the magnetic field strength are correlated with the effective temperature but have no predictive power for the nitrogen abundance. Our findings are completely based on observations and their proper statistical treatment and call for a new strategy in evaluating the outcome of stellar evolution computations. © 2014. The American Astronomical Society. All rights reserved.

Demoulin P.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation
AIP Conference Proceedings | Year: 2010

Interplanetary Coronal Mass Ejections (ICMEs) are formed of plasma and magnetic field launched from the Sun into the Solar Wind (SW). These coherent magnetic structures, frequently formed by a flux rope, interact strongly with the SW. This interaction is reviewed by comparing the results obtained from in situ observations and numerical simulations. Like fast ships in the ocean, fast ICMEs drive an extended shock in front. ICMEs expand in all directions while traveling away from the Sun, a sheath of SW plasma and magnetic field accumulates in front of the ICME, which partially reconnects with the ICME magnetic field. Furthermore, not only do ICMEs have a profound impact on the heliosphere, but the type of SW encountered by an ICME has an important impact on its evolution (e.g. increase of mass, global deceleration, lost of magnetic flux and helicity, distortion of the configuration). © 2010 American Institute of Physics.

Samadi R.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation
Lecture Notes in Physics | Year: 2011

For more than ten years, solar-like oscillations have been detected and frequencies measured for a growing number of stars with various characteristics (e.g. different evolutionary stages, effective temperatures, gravities, metal abundances...). Excitation of such oscillations is attributed to turbulent convection and takes place in the uppermost part of the convective envelope. Since the pioneering work of Goldreich and Keeley (APJ, 211:934, 1977; 212:243, 1977) more sophisticated theoretical models of stochastic excitation were developed, which differ from each other both by the way turbulent convection is modeled and by the assumed sources of excitation. We review here these different models and their underlying approximations and assumptions. We emphasize how the computed mode excitation rates crucially depend on the way turbulent convection is described but also on the stratification and the metal abundance of the upper layers of the star. In turn we will show how the seismic measurements collected so far allow us to infer properties of turbulent convection in stars. © 2011 Springer-Verlag Berlin Heidelberg.

Lebreton Y.,CNRS Galaxies, Stars, Physics and Instrumentation Laboratory | Lebreton Y.,Rennes Institute of Physics | Goupil M.J.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation
Astronomy and Astrophysics | Year: 2012

Aims. We aim at characterizing the inward transition from convective to radiative energy transport at the base of the convective envelope of the solar-like oscillator HD 52265 recently observed by the CoRoT satellite. Methods. We investigated the origin of one specific feature found in the HD 52265 frequency spectrum. We modelled the star to derive the internal structure and the oscillation frequencies that best match the observations and used a seismic indicator sensitive to the properties of the base of the envelope convection zone. Results. The seismic indicators clearly reveal that to best represent the observed properties of HD 52265, models must include penetrative convection below the outer convective envelope. The penetrative distance is estimated to be ∼0.95H P, which corresponds to an extent over a distance representing 6.0 per cents of the total stellar radius, significantly larger than what is found for the Sun. The inner boundary of the extra-mixing region is found at 0.800 ± 0.004 R where R = 1.3 R ⊙ is the stellar radius. Conclusions. These results contribute to the tachocline characterization in stars other than the Sun. © 2012 ESO.

Gosain S.,U.S. National Solar Observatory | Demoulin P.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation | Lopez Fuentes M.,CONICET
Astrophysical Journal | Year: 2014

We present a study of two regular sunspots that exhibit nearly uniform twist from the photosphere to the corona. We derive the twist parameter in the corona and in the chromosphere by minimizing the difference between the extrapolated linear force-free field model field lines and the observed intensity structures in the extreme-ultraviolet images of the Sun. The chromospheric structures appear more twisted than the coronal structures by a factor of two. Further, we derive the vertical component of electric current density, jz, using vector magnetograms from the Hinode Solar Optical Telescope (SOT). The spatial distribution of jzhas a zebra pattern of strong positive and negative values owing to the penumbral fibril structure resolved by Hinode/SOT. This zebra pattern is due to the derivative of the horizontal magnetic field across the thin fibrils; therefore, it is strong and masks weaker currents that might be present, for example, as a result of the twist of the sunspot. We decompose jzinto the contribution due to the derivatives along and across the direction of the horizontal field, which follows the fibril orientation closely. The map of the tangential component has more distributed currents that are coherent with the chromospheric and coronal twisted structures. Moreover, it allows us to map and identify the direct and return currents in the sunspots. Finally, this decomposition of jzis general and can be applied to any vector magnetogram in order to better identify the weaker large-scale currents that are associated with coronal twisted/sheared structures. © 2014. The American Astronomical Society. All rights reserved..

Janvier M.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation | Demoulin P.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation | Dasso S.,University of Buenos Aires | Dasso S.,CONICET
Astronomy and Astrophysics | Year: 2013

Context. Coronal mass ejections (CMEs) are routinely tracked with imagers in the interplanetary space, while magnetic clouds (MCs) properties are measured locally by spacecraft. However, both imager and in situ data do not provide any direct estimation of the general flux rope properties. Aims. The main aim of this study is to constrain the global shape of the flux rope axis from local measurements and to compare the results from in-situ data with imager observations. Methods. We performed a statistical analysis of the set of MCs observed by WIND spacecraft over 15 years in the vicinity of Earth. We analyzed the correlation between different MC parameters and studied the statistical distributions of the angles defining the local axis orientation. With the hypothesis of having a sample of MCs with a uniform distribution of spacecraft crossing along their axis, we show that a mean axis shape can be derived from the distribution of the axis orientation. As a complement, while heliospheric imagers do not typically observe MCs but only their sheath region, we analyze one event where the flux rope axis can be estimated from the STEREO imagers. Results. From the analysis of a set of theoretical models, we show that the distribution of the local axis orientation is strongly affected by the overall axis shape. Next, we derive the mean axis shape from the integration of the observed orientation distribution. This shape is robust because it is mostly determined from the overall shape of the distribution. Moreover, we find no dependence on the flux rope inclination on the ecliptic. Finally, the derived shape is fully consistent with the one derived from heliospheric imager observations of the June 2008 event. Conclusions. We have derived a mean shape of MC axis that only depends on one free parameter, the angular separation of the legs (as viewed from the Sun). This mean shape can be used in various contexts, such as studies of high-energy particles or space weather forecasts. © ESO, 2013.

Demoulin P.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation | Dasso S.,University of Buenos Aires | Dasso S.,CONICET | Janvier M.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation
Astronomy and Astrophysics | Year: 2013

Context. Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections (ICMEs). One property of MCs is the presence of a magnetic flux rope. Is the difference between ICMEs with and without MCs intrinsic or rather due to an observational bias? Aims. As the spacecraft has no relationship with the MC trajectory, the frequency distribution of MCs versus the spacecraft distance to the MCs' axis is expected to be approximately flat. However, Lepping & Wu (2010, Ann. Geophys., 28, 1539) confirmed that it is a strongly decreasing function of the estimated impact parameter. Is a flux rope more frequently undetected for larger impact parameter? Methods. In order to answer the questions above, we explore the parameter space of flux rope models, especially the aspect ratio, boundary shape, and current distribution. The proposed models are analyzed as MCs by fitting a circular linear force-free field to the magnetic field computed along simulated crossings. Results. We find that the distribution of the twist within the flux rope and the non-detection due to too low field rotation angle or magnitude only weakly affect the expected frequency distribution of MCs versus impact parameter. However, the estimated impact parameter is increasingly biased to lower values as the flux rope cross section is more elongated orthogonally to the crossing trajectory. The observed distribution of MCs is a natural consequence of a flux rope cross section flattened on average by a factor 2 to 3 depending on the magnetic twist profile. However, the faster MCs at 1 AU, with V > 550 km s-1, present an almost uniform distribution of MCs vs. impact parameter, which is consistent with round-shaped flux ropes, in contrast with the slower ones. Conclusions. We conclude that the sampling of MCs at various distances from the axis does not significantly affect their detection. The large part of ICMEs without MCs could be due to a too strict criteria for MCs or to the fact that these ICMEs are encountered outside their flux rope or near the leg region, or they do not contain a flux rope. © 2013 ESO.

Reese D.R.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation
Astronomische Nachrichten | Year: 2010

Spurred by the spectroscopic and interferometric observations of rapidly rotating stars and the highly accurate pulsation data coming from asteroseismology space missions, theoreticians have spent many years developing models for such stars and studying their oscillations. This has led to the discovery of new phenomena and new types of pulsation modes. In what follows, a review is given of the different methods used to model the effects of rotation on stellar pulsations, as well as a description of these effects on inertial, gravito-inertial, r and acoustic modes. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Klein K.-L.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation | Trottet G.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation | Klassen A.,University of Kiel
Solar Physics | Year: 2010

Flares and coronal mass ejections (CMEs) contribute to the acceleration and propagation of solar energetic particles (SEP) detected in the interplanetary space, but the exact roles of these phenomena are yet to be understood. We examine two types of energetic particle tracers related with 15 CME-less flares that emit bright soft X-ray bursts (GOES X class): radio emission of flare-accelerated electrons and in situ measurements of energetic electrons and protons near 1 AU. The CME-less flares are found to be vigorous accelerators of microwave-emitting electrons, which remain confined in low coronal structures. This is shown by unusually steep low-frequency microwave spectra and by lack of radio emission from the middle and high corona, including dm - m wave type IV continua and metre-to-hectometre type III bursts. The confinement of the particles accelerated in CME-less flares agrees with the magnetic field configuration of these events inferred by others. Two events produced isolated metric type II bursts revealing coronal shock waves. None of the seven flares in the western hemisphere was followed by enhanced particle fluxes in the GOES detectors, but one, which was accompanied by a type II burst, caused a weak SEP event detected at SoHO and ACE. Three of the CME-less flares were followed within some hours by SEP-associated flares from the same active region. These SEP-producing events were clearly distinct from the CME-less ones by their association with fast and broad CMEs, dm - m wave radio emission, and intense DH type III bursts. We conclude that radio emission at decimetre and longer waves is a reliable indication that flare-accelerated particles have access to the high corona and interplanetary space. The absence of such emission can be used as a signal that no SEP event is to be expected despite the occurrence of a strong soft X-ray burst. © Springer Science+Business Media B.V. 2010.

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