205 research outputs found

    The collective gyration of a heavy ion cloud in a magnetized plasma

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    In both the ionospheric barium injection experiments CRIT 1 and CRIT 2, a long duration oscillation was seen with a frequency close to the gyro frequency of barium and a time duration of about one second. A model for the phenomena which was proposed for the CRIT 1 experiment is compared to the results from CRIT 2 which made a much more complete set of measurements. The model follows the motion of a low Beta ion cloud through a larger ambient plasma. The internal field of the model is close to antiparallel to the injection direction v sub i but slightly tilted towards the self polarization direction E sub p = -V sub i by B. As the ions move across the magnetic field, the space charge is continuously neutralized by magnetic field aligned electron currents from the ambient ionosphere, drawn by the divergence in the perpendicular electric field. These currents give a perturbation of the magnetic field related to the electric field perturbation by Delta E/Delta B approximately equal to V sub A. The model predictions agree quite well with the observed vector directions, field strengths, and decay times of the electric and magnetic fields in CRIT 2. The possibility to extend the model to the active region, where the ions are produces in this type of self-ionizing injection experiments, is discussed

    Where are global vegetation greening and browning trends significant?

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    Faire l’Union. La refondation des parties de droite après les élections de 2002

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    Un nouveau parti de droite, pour le moment dénommé UMP, a été mis en place entre des deux tours de l'élection présidentielle. Cet article se donne pour objectif d'analyser les enjeux de cette transformation. La genèse de l'UMP permet de saisir dans quelle mesure un parti poli-tique peut être conçu comme la connexion d'un ensemble de systèmes de coopération organisés autour de postes à conquérir. En effet, l'impératif d'unification de la droite française est lié au décalage persistant existant entre les systèmes de coopération législatif et présidentiel. Mais les questions soulevées par cette fusion partisane (l'UMP inclut le RPR, DL et une majeure partie de l'UDF) montrent que cette vision ne peut suffire et que les partis ne sont pas seulement des systèmes de coopération mais aussi des systèmes de production. Dès lors, leurs logiques d'organisation internes pèsent sur ce travail de transformation du social en politique, de la plu-ralité en homogénéité. Dans le cas de l'UMP, il s'agit d'abord de mettre en commun des modes de fonctionnement, en particulier de définir la place et la légitimité accordées respectivement aux adhérents et aux élus, mais aussi d'organiser l'expression de la pluralité idéologique par la mise en place de courants

    Climate change and equestrian empires in the Eastern Steppes: new insights from a high-resolution Lake Core in Central Mongolia

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    The repeated expansion of East Asian steppe cultures was a key driver of Eurasian history, forging new social, economic, and biological links across the continent. Climate has been suggested as important driver of these poorly understood cultural expansions, but paleo-climate records from the Mongolian Plateau often suffer from poor age control or ambiguous proxy interpretation. Here, we use a combination of geochemical analyses and comprehensive radiocarbon dating to establish the first robust and detailed record of paleo-hydrological conditions for Lake Telmen, Mongolia, covering the past ~4000 years. Our record shows that humid conditions coincided with solar minima, and hydrological modelling confirms the high sensitivity of the lake to paleo-climate changes. Careful comparisons with archaeological and historical records suggest that in the vast semi-arid grasslands of eastern Eurasia, solar minima led to reduced temperatures, less evaporation, and high biomass production, expanding the power base for pastoral economies and horse cavalry. Our findings suggest a crucial link between temperature dynamics in the Eastern Steppe and key social developments, such as the emergence of pastoral empires, and fuel concerns that global warming enhances water scarcity in the semi-arid regions of interior Eurasia.1. Introduction 2. Results 2.1 Sediment core chronology 2.2 Sedimentological and geochemical analyses 2.3 Isotope analyses, evaporation index (EI), and paleohydrology 3. Discussion 3.1 External forcing on the regional climate 3.2 Hydrological modelling 3.3 Climate impact on human history in Mongolia Method
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