176 research outputs found
Cordillera blanca : glaciares en la historia
La plus vaste couverture glaciaire de l'entre-tropiques n'apparaît comme un objet d'études que très tard, à la fin du 19ème siècle et, surtout grâce aux expéditions austro-allemandes, à partir des années 1930-1940. Le développement de l'alpinisme et un grand nombre de catastrophes meurtrières associées à la dynamique de ces glaciers (rupture de lacs de barrage morainique, avalanches) ont attiré l'attention sur eux et suscité des recherches glaciologiques. En 1980, ils comptent parmi les mieux surveillés des glaciers tropicaux, d'abord grâce à un programme d'étude développé par les Péruviens eux-mêmes. A un moment où le réchauffement global fait craindre un recul très important de la glaciation dans la zone tropicale, les glaciers de la Cordillère Blanche deviennent d'excellents indicateurs de l'évolution climatique en cours et la ressource en eau qui leur est associée constitue à la fois un sujet d'étude et un enjeu économique de haute importance. (Résumé d'auteur
Testing Lorentz symmetry with Lunar Laser Ranging
Lorentz symmetry violations can be parametrized by an effective field theory
framework that contains both general relativity and the standard model of
particle physics called the standard-model extension (SME). We present new
constraints on pure gravity SME coefficients obtained by analyzing lunar laser
ranging (LLR) observations. We use a new numerical lunar ephemeris computed in
the SME framework and we perform a LLR data analysis using a set of 20721
normal points covering the period of August, 1969 to December, 2013. We
emphasize that linear combination of SME coefficients to which LLR data are
sensitive and not the same as those fitted in previous postfit residuals
analysis using LLR observations and based on theoretical grounds. We found no
evidence for Lorentz violation at the level of for ,
for and , for
and
and for
. We improve previous constraints on SME
coefficient by a factor up to 5 and 800 compared to postfit residuals analysis
of respectively binary pulsars and LLR observations
Influence of the Tungurahua eruption on the ice core records of Chimborazo, Ecuador
International audienceThe comparison of two shallow ice cores recovered in 1999 and 2000 from the same place on the Chimborazo summit glacier revealed the influence of the coincident Tungurahua volcanic eruption on their stable isotope and chemical records. The surface snow melting and water percolation induced from the ash deposition caused a preferential elution and re-localization of certain ionic species, while the stable isotope records were not affected. Additionally, the comparison of the ionic amount and some selected ion ratios preserved along the ice core column reports under which processes the chemical species are introduced in the snow pack, as snow flake condensation nuclei, by atmospheric scavenging or by dry deposition. This preliminary study is essential for the interpretation of the deep Chimborazo ice core, or for other sites where surrounding volcanic activity influences the glaciochemical records
Mesures météorologiques, hydrologiques et glaciologiques sur le glacier du Zongo : année hydrologique 2001-2002
Mesures météorologiques, hydrologiques et glaciologiques sur le glacier du Zongo : année hydrologique 2000-2001
Kisspeptin Restores Pulsatile LH Secretion in Patients with Neurokinin B Signaling Deficiencies:Physiological, Pathophysiological and Therapeutic Implications
Pulsatile gonadotropin-releasing hormone (GnRH) is crucial to normal reproductive function and abnormalities in pulse frequency give rise to reproductive dysfunction. Kisspeptin and neurokinin B (NKB), neuropeptides secreted by the same neuronal population in the ventral hypothalamus, have emerged recently as critical central regulators of GnRH and thus gonadotropin secretion. Patients with mutations resulting in loss of signaling by either of these neuroendocrine peptides fail to advance through puberty but the mechanisms mediating this remain unresolved. We report here that continuous kisspeptin infusion restores gonadotropin pulsatility in patients with loss-of-function mutations in NKB (TAC3) or its receptor (TAC3R), indicating that kisspeptin on its own is sufficient to stimulate pulsatile GnRH secretion. Moreover, our findings suggest that NKB action is proximal to kisspeptin in the reproductive neuroendocrine cascade regulating GnRH secretion, and may act as an autocrine modulator of kisspeptin secretion. The ability of continuous kisspeptin infusion to induce pulsatile gonadotropin secretion further indicates that GnRH neurons are able to set up pulsatile secretion in the absence of pulsatile exogenous kisspeptin.Publisher PDFPeer reviewe
Rock Glaciers as Water Stores in the Bolivian Andes: An Assessment of Their Hydrological Importance
Optical Method for Determination of Carrier Density in Modulation Doped Quantum Wells
An optical method is suggested to determine the concentration of
two-dimensional electrons in modulation-doped quantum wells at low and moderate
electron densities between 10^{9} and 2x10^{11} cm^{-2}. The method is based on
an analysis of magneto-reflectivity spectra of charged excitons (trions). The
circular polarization degree and the oscillator strength of the charged
excitons contain information about the density and spin polarization of
two-dimensional electron gas. The method is applied to CdTe/(Cd,Mg)Te and
ZnSe/Zn,Mg)(S,Se) heterostructures
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