Oscillatory subglacial drainage in the absence of surface melt

The presence of strong diurnal cycling in basal water pressure records obtained during the melt season is well established for many glaciers. The behaviour of the drainage system outside the melt season is less well understood. Here we present borehole observations from a surge-type valley glacier in the St Elias Mountains, Yukon Territory, Canada. Our data indicate the onset of strongly correlated multi-day oscillations in water pressure in multiple boreholes straddling a main drainage axis, starting several weeks after the disappearance of a dominant diurnal mode in August 2011 and persisting until at least January 2012, when multiple data loggers suffered power failure. Jökulhlaups provide a template for understanding spontaneous water pressure oscillations not driven by external supply variability. Using a subglacial drainage model, we show that water pressure oscillations can also be driven on a much smaller scale by the interaction between conduit growth and distributed water storage in smaller water pockets, basal crevasses and moulins, and that oscillations can be triggered when water supply drops below a critical value. We suggest this in combination with a steady background supply of water from ground water or englacial drainage as a possible explanation for the observed wintertime pressure oscillations

Normal mode splitting and mechanical effects of an optical lattice in a ring cavity

A novel regime of atom-cavity physics is explored, arising when large atom samples dispersively interact with high-finesse optical cavities. A stable far detuned optical lattice of several million rubidium atoms is formed inside an optical ring resonator by coupling equal amounts of laser light to each propagation direction of a longitudinal cavity mode. An adjacent longitudinal mode, detunedby about 3 GHz, is used to perform probe transmission spectroscopy of the system. The atom-cavity coupling for the lattice beams and the probe is dispersive and dissipation results only from the finite photon-storage time. The observation of two well-resolved normal modes demonstrates the regime of strong cooperative coupling. The details of the normal mode spectrum reveal mechanical effects associated with the retroaction of the probe upon the optical lattice.Comment: 4 pages, 3 figure

Contemporary Glacier Processes and Global Change: Recent Observations from Kaskawulsh Glacier and the Donjek Range, St. Elias Mountains

With an extensive ice cover and rich display of glacier behaviour, the St. Elias Mountains continue to be an enviable natural laboratory for glaciological research. Recent work has been motivated in part by the magnitude and pace of observed glacier change in this area, which is so ice-rich that ice loss has a measurable impact on global sea level. Both detection and attribution of these changes, as well as investigations into fundamental glacier processes, have been central themes in projects initiated within the last decade and based at the Kluane Lake Research Station. The scientific objectives of these projects are (1) to quantify recent area and volume changes of Kaskawulsh Glacier and place them in historical perspective, (2) to investigate the regional variability of glacier response to climate and the modulating influence of ice dynamics, and (3) to characterize the hydromechanical controls on glacier sliding. A wide range of methods is being used, from ground-based manual measurements to space-based remote sensing. The observations to date show glaciers out of equilibrium, with significant ongoing changes to glacier area, volume, and dynamics. Computer models are being used to generalize these results, and to identify the processes most critical to our understanding of the coupled glacier-climate system.Grâce à leur importante couverture de glace et au riche étalage de comportement des glaciers, les monts St. Elias continuent de servir de laboratoire naturel enviable pour la recherche glaciologique. Des études récentes ont été motivées, en partie, par la magnitude et la vitesse des changements observés dans les glaciers de l’endroit, qui sont riches en glace au point que la perte de glace a une incidence mesurable sur le niveau général de la mer. La détection et l’attribution de ces changements de même que les recherches à l’égard des processus des glaciers ont servi de thème central à des projets qui ont été mis en oeuvre au cours de la dernière décennie à la station de recherche du lac Kluane. Les objectifs scientifiques de ces projets consistent (1) à quantifier les changements récents relativement à l’aire et au volume du glacier Kaskawulsh, puis à les mettre dans une perspective historique, (2) à faire enquête sur la variabilité générale de la réaction du glacier vis-à-vis du climat et de l’influence modulatrice de la dynamique de la glace, et (3) à caractériser le contrôle hydromécanique par rapport au glissement du glacier. Une vaste gamme de méthodes est employée pour parvenir à ces fins, allant des mesures manuelles sur le terrain à la télédétection spatiale. Jusqu’à maintenant, les observations indiquent que les glaciers ne sont pas en équilibre et que d’importants changements se produisent quant à l’aire, au volume et à la dynamique du glacier. Des modèles informatiques sont utilisés pour généraliser ces résultats ainsi que pour cerner les processus les plus critiques à notre compréhension du système couplé glacier-climat

Some genus 3 curves with many points

Using an explicit family of plane quartic curves, we prove the existence of a genus 3 curve over any finite field of characteristic 3 whose number of rational points stays within a fixed distance from the Hasse-Weil-Serre upper bound. We also provide an intrinsic characterization of so-called Legendre elliptic curves