6,380 research outputs found
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
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.GrĂące Ă leur importante couverture de glace et au riche Ă©talage de comportement des glaciers, les monts St. Elias continuent de servir de laboratoire naturel enviable pour la recherche glaciologique. Des Ă©tudes rĂ©centes ont Ă©tĂ© motivĂ©es, en partie, par la magnitude et la vitesse des changements observĂ©s dans les glaciers de lâendroit, qui sont riches en glace au point que la perte de glace a une incidence mesurable sur le niveau gĂ©nĂ©ral de la mer. La dĂ©tection et lâattribution de ces changements de mĂȘme que les recherches Ă lâĂ©gard des processus des glaciers ont servi de thĂšme central Ă des projets qui ont Ă©tĂ© mis en oeuvre au cours de la derniĂšre dĂ©cennie Ă la station de recherche du lac Kluane. Les objectifs scientifiques de ces projets consistent (1) Ă quantifier les changements rĂ©cents relativement Ă lâaire et au volume du glacier Kaskawulsh, puis Ă les mettre dans une perspective historique, (2) Ă faire enquĂȘte sur la variabilitĂ© gĂ©nĂ©rale de la rĂ©action du glacier vis-Ă -vis du climat et de lâinfluence modulatrice de la dynamique de la glace, et (3) Ă caractĂ©riser le contrĂŽle hydromĂ©canique par rapport au glissement du glacier. Une vaste gamme de mĂ©thodes est employĂ©e pour parvenir Ă ces fins, allant des mesures manuelles sur le terrain Ă la tĂ©lĂ©dĂ©tection spatiale. JusquâĂ maintenant, les observations indiquent que les glaciers ne sont pas en Ă©quilibre et que dâimportants changements se produisent quant Ă lâaire, au volume et Ă la dynamique du glacier. Des modĂšles informatiques sont utilisĂ©s pour gĂ©nĂ©raliser ces rĂ©sultats ainsi que pour cerner les processus les plus critiques Ă notre comprĂ©hension du systĂšme couplĂ© glacier-climat
Modeling Sediment Transport in Ice-Walled Subglacial Channels and Its Implications for Esker Formation and Proglacial Sediment Yields
Sediment yields from glacierized basins are used to quantify erosion rates on seasonal to decadal timescales as well as conditions at the glacier bed, and eskers hold valuable information about past subglacial hydraulic conditions in their spatial organization, geometry, and sedimentary structures. Ultimately, eskers are a record of past glacioâfluvial sediment transport, but there is currently no physical model for this process. We develop a 1âD model of morphodynamics in semicircular bedrockâfloored subglacial channels. We adapt a sediment conservation law developed for mixed alluvialâbedrock conditions to subglacial channels. Channel evolution is a function of melt opening by viscous heat dissipation from flowing water and creep closure of the overlying ice, to which we add the closure or enlargement due to sediment deposition or removal, respectively. We apply the model to an idealized landâterminating glacier and find that temporary sediment accumulation in the vicinity of the terminus, or the formation of an incipient esker, is inherent to the dynamics of the channelized water flow. The alluviation of the bed combined with the pressurized channel flow produces unexpected patterns of sediment evacuation: We show that the direction of hysteresis between sediment and water discharge is not necessarily linked to a supplyâ or transportâlimited system, as has been hypothesized for proglacial sediment yields. We also find that the deposition of an incipient esker is a function of a compromise between water discharge and sediment supply, but perhaps more importantly, iceâsurface slope and the temporal pattern of water delivery to the bed
Fiber-Cavity-Based Optomechanical Device
We describe an optomechanical device consisting of a fiber-based optical
cavity containing a silicon nitiride membrane. In comparison with typical
free-space cavities, the fiber-cavity's small mode size (10 {\mu}m waist, 80
{\mu}m length) allows the use of smaller, lighter membranes and increases the
cavity-membrane linear coupling to 3 GHz/nm and quadratic coupling to 20
GHz/nm^2. This device is also intrinsically fiber-coupled and uses glass
ferrules for passive alignment. These improvements will greatly simplify the
use of optomechanical systems, particularly in cryogenic settings. At room
temperature, we expect these devices to be able to detect the shot noise of
radiation pressure.Comment: 4 pages, 3 figures; the following article has been submitted to
Applied Physics Letter
as parameter of Minkowski metric in effective theory
With the proper choice of the dimensionality of the metric components, the
action for all fields becomes dimensionless. Such quantities as the vacuum
speed of light c, the Planck constant \hbar, the electric charge e, the
particle mass m, the Newton constant G never enter equations written in the
covariant form, i.e., via the metric g^{\mu\nu}. The speed of light c and the
Planck constant are parameters of a particular two-parametric family of
solutions of general relativity equations describing the flat isotropic
Minkowski vacuum in effective theory emerging at low energy:
g^{\mu\nu}=diag(-\hbar^2, (\hbar c)^2, (\hbar c)^2, (\hbar c)^2). They
parametrize the equilibrium quantum vacuum state. The physical quantities which
enter the covariant equations are dimensionless quantities and dimensionful
quantities of dimension of rest energy M or its power. Dimensionless quantities
include the running coupling `constants' \alpha_i; topological and geometric
quantum numbers (angular momentum quantum number j, weak charge, electric
charge q, hypercharge, baryonic and leptonic charges, number of atoms N, etc).
Dimensionful parameters include the rest energies of particles M_n (or/and mass
matrices); the gravitational coupling K with dimension of M^2; cosmological
constant with dimension M^4; etc. In effective theory, the interval s has the
dimension of 1/M; it characterizes the dynamics of particles in the quantum
vacuum rather than geometry of space-time. We discuss the effective action, and
the measured physical quantities resulting from the action, including
parameters which enter the Josepson effect, quantum Hall effect, etc.Comment: 18 pages, no figures, extended version of the paper accepted in JETP
Letter
Neutrino Pair Bremsstrahlung in Neutron Star Crusts: a Reappraisal
We demonstrate that band-structure effects suppress bremsstrahlung of
neutrino pairs by electrons in the crusts of neutron stars at temperatures of
the order of and below. Taking this into account,
together with the fact that recent work indicates that the masses of neutron
star crusts are considerably smaller than previously estimated, we find
neutrino pair bremsstrahlung to be much less important for the thermal
evolution of neutron stars than earlier calculations suggested.Comment: 11 plain LaTeX pages, 3 figures available on request, NORDITA-93/72
A/S/
Bring a plate: facilitating experimentation in the Welcome Dinner Project
Drawing on in-depth empirical research, we explore a project called The Welcome Dinner (WDP). The WDP aims to bring together ânewly arrivedâ people and âestablished Australiansâ to meet and âshare storiesâ over a potluck meal in âthe comfort of their own homeâ. The purpose is to create meaningful connections, new friendships and social solidarities. In this paper, we focus on the micro-contexts of the dinners and the minute activities and techniques that facilitators use in hosting. Our aim is not to analyse the effects of the project but rather the design and meaning of the activities. As a form of âdesigned everyday multiculturalismâ, focused on welcoming new arrivals to Australia, it takes effort, skill and labour to manage the contact between different cultural groups over organised meals. Thus, facilitators take over the hosting of the lunches and dinners to run activities, which are imagined to lubricate social dynamics and relations, and produce convivial commensal affects and behaviours. Drawing on theories of training activities as embodied and cognitive experimentations, which enable new knowledge practices and social relations, we analyse field notes and interviews about the facilitation, structure and activities at the WDP home dinners
Contribution of the massive photon decay channel to neutrino cooling of neutron stars
We consider massive photon decay reactions via intermediate states of
electron-electron-holes and proton-proton-holes into neutrino-antineutrino
pairs in the course of neutron star cooling. These reactions may become
operative in hot neutron stars in the region of proton pairing where the photon
due to the Higgs-Meissner effect acquires an effective mass that
is small compared to the corresponding plasma frequency. The contribution of
these reactions to neutrino emissivity is calculated; it varies with the
temperature and the photon mass as
for . Estimates show that these processes appear as extra
efficient cooling channels of neutron stars at temperatures K.Comment: accepted to publication in Zh. Eksp. Teor. Fiz. (JETP
Nuclear Effects on Bremsstrahlung Neutrino Rates of Astrophysical Interest
We calculate in this work the rates for the neutrino pair production by
nucleon-nucleon bremsstrahlung taking into account the full contribution from a
nuclear one-pion-exchange potential. It is shown that if the temperatures are
low enough (), the integration over the nuclear part can be done
for the general case, ranging from the completely degenerate (D) to the
non-degenerate (ND) regime. We find that the inclusion of the full nuclear
contribution enhances the neutrino pair production by and
bremsstrahlung by a factor of about two in both the D and ND limits when
compared with previous calculations. This result may be relevant for the
physical conditions of interest in the semitransparent regions near the
neutrinosphere in type II supernovae, cooling of neutron stars and other
astrophysical situations.Comment: 11 pages, no figures, LaTex file. submitted to PR
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