101 research outputs found
Observations of tidal melt and vertical strain at the FilchnerâRonne Ice Shelf, Antarctica
The FilchnerâRonne Ice Shelf experiences strong tidal forcing known to displace portions of the ice shelf by several meters over a tidal cycle. These large periodic displacements may cause significant variation of the ice shelf vertical strain. Further, tidal currents in the ice shelf cavity may be responsible for basal melt variations. We deployed autonomous phaseâsensitive radioâecho sounders at 17 locations across the ice shelf and measured basal motion and internal vertical ice motion at sufficiently short intervals to allow the resolution of all significant tidal constituents. Basal melt estimates with this surfaceâbased technique rely on accurate estimation of vertical strain changes in the ice shelf. We present a method that can separate the vertical strain changes from the total thickness changes at tidal time scales, yielding a tidal basal melt estimate. The method was used to identify vertical strain and basal melt variations at the predominant semiâdiurnal M2 tidal constituent. At most sites the tidal vertical strain was depthâindependent. Tidal deformation at four sites was controlled by local effects causing elastic bending. Significant tidal melt was observed to occur at six locations and upper bounds on the tidal melt amplitude were derived for the remaining sites. Finally, we show that observations of basal melt spectra, specifically at tidal frequencies and their multiples, can provide constraints on the hydrographic conditions near the ice base, such as the nonâtidal background ocean flow
The nature of ice intermittently accreted at the base of Ronne Ice Shelf, Antarctica, assessed using phaseâsensitive radar
In-situ phase-sensitive radar measurements from the Ronne Ice Shelf (RIS) reveal evidence of intermittent basal accretion periods at several sites that are melting in the long-term mean. Periods when ice is accreted at the ice-shelf base coincide with a decrease in the amplitude of the basal return of up to 4 dB. To quantify basal accretion we constrain simultaneously the dielectric constant, electrical conductivity, and thickness of the accreted ice. We do this by exploring the sensitivity of the received basal echo strength and phase to different transmit frequencies using the radar data in combination with a simple model. Along the western RIS we detect episodic basal accretion events leading to ice accumulation at a rate equivalent to 1-3 mm of meteoric ice per day. The inferred accumulation rates and electromagnetic properties of the accreted ice imply that these events are caused primarily by the deposition of frazil ice crystals. Our findings offer the possibility of monitoring and studying the evolution of boundaries between ice-shelf basal melting and accretion regimes using remote observations, collected from the ice-shelf surface
Strong tidal variations in ice flow observed across the entire Ronne Ice Shelf and adjoining ice streams
We present a compilation of GPS time series, including those for previously unpublished sites, showing that flow across the entire Ronne Ice Shelf and its adjoining ice streams is strongly affected by ocean tides. Previous observations have shown strong horizontal diurnal and semidiurnal motion of the ice shelf, and surface flow speeds of Rutford Ice Stream (RIS) are known to vary with a fortnightly (Msf) periodicity. Our new data set shows that the Msf flow modulation, first observed on RIS, is also found on Evans, Talutis, Institute, and Foundation ice streams, i.e. on all ice streams for which data are available. The amplitude of the Msf signal increases downstream of grounding lines, reaching up to 20âŻ% of mean flow speeds where ice streams feed into the main ice shelf. Upstream of ice stream grounding lines, decay length scales are relatively uniform for all ice streams but the speed at which the Msf signal propagates upstream shows more variation. Observations and modelling of tidal variations in ice flow can help constrain crucial parameters that determine the rate and extent of potential ice mass loss from Antarctica. Given that the Msf modulation in ice flow is readily observed across the entire region at distances of up to 80âŻkm upstream of grounding lines, but is not completely reproduced in any existing numerical model, this new data set suggests a pressing need to identify the missing processes responsible for its generation and propagation. The new GPS data set is publicly available through the UK Polar Data Centre at http://doi.org/10.5285/4fe11286-0e53-4a03-854c-a79a44d1e35
Using Multiple Sources of Knowledge to Investigate Northern Environmental Change: Regional Ecological Impacts of a Storm Surge in the Outer Mackenzie Delta, N.W.T.
Field data, remote sensing, and Inuvialuit knowledge were synthesized to document regional ecological change in the outer Mackenzie Delta and to explore the timing, causes, and implications of this phenomenon. In September 1999, a large magnitude storm surge inundated low-lying areas of the outer Mackenzie Delta. The storm was among the most intense on record and resulted in the highest water levels ever measured at the delta front. Synthesis of scientific and Inuvialuit knowledge indicates that flooding during the 1999 storm surge increased soil salinity and caused widespread vegetation death. Vegetation cover was significantly reduced in areas affected by the surge and was inversely related to soil salinity. Change detection analysis, using remotely sensed imagery bracketing the 1999 storm event, indicates severe impacts on at least 13 200 ha of terrestrial vegetation in the outer delta. Inuvialuit knowledge identifying the 1999 surge as anomalous is corroborated by geochemical profiles of permafrost and by a recently published paleo-environmental study, which indicates that storm surge impacts of this magnitude have not previously occurred during the last millennium. Almost a decade after the 1999 storm surge event, ecological recovery has been minimal. This broad-scale vegetation change is likely to have significant implications for wildlife and must be considered in regional ecosystem planning and in the assessment and monitoring of the cumulative impacts of development. Our investigations show that Inuvialuit were aware of the 1999 storm surge and the environmental impacts several years before the scientific and regulatory communities recognized their significance. This study highlights the need for multidisciplinary and locally informed approaches to identifying and understanding Arctic environmental change.La synthĂšse des donnĂ©es dâexploitation et de tĂ©lĂ©dĂ©tection de mĂȘme que des connaissances des Inuvialuit a Ă©tĂ© effectuĂ©e afin de rĂ©pertorier les changements Ă©cologiques enregistrĂ©s dans la rĂ©gion extĂ©rieure du delta du Mackenzie et dâexplorer la temporisation, les causes et les incidences de ce phĂ©nomĂšne. En septembre 1999, une onde de tempĂȘte de grande magnitude a inondĂ© les zones de faible Ă©lĂ©vation de lâextĂ©rieur du delta du Mackenzie. Il sâagit de la tempĂȘte la plus intense Ă nâavoir jamais Ă©tĂ© enregistrĂ©e, ce qui sâest traduit par les niveaux dâeau les plus Ă©levĂ©s Ă nâavoir jamais Ă©tĂ© mesurĂ©s Ă la hauteur du delta. La synthĂšse des donnĂ©es scientifiques et des connaissances des Inuvialuit nous montre que lâinondation de 1999 a eu pour effet dâaugmenter la salinitĂ© du sol et a entraĂźnĂ© la mort de la vĂ©gĂ©tation Ă grande Ă©chelle. La couverture vĂ©gĂ©tale a Ă©tĂ© rĂ©duite considĂ©rablement dans les zones visĂ©es par lâonde et Ă©tait inversement reliĂ©e Ă la salinitĂ© du sol. Lâanalyse des dĂ©tections de changement effectuĂ©e au moyen de lâimagerie tĂ©lĂ©dĂ©tectĂ©e dans le cas de la tempĂȘte de 1999 laisse entrevoir de fortes incidences sur au moins 13 200 hectares de vĂ©gĂ©tation terrestre dans lâextĂ©rieur du delta. Les connaissances des Inuvialuit, qui affirment que lâonde de 1999 Ă©tait anormale, sont corroborĂ©es par les profils gĂ©ochimiques du pergĂ©lisol ainsi que par une Ă©tude palĂ©oenvironnementale qui indique que des incidences de cette ampleur dĂ©coulant dâune onde de tempĂȘte ne se sont pas produites Ă un autre moment donnĂ© du dernier millĂ©naire. PrĂšs dâune dĂ©cennie aprĂšs lâonde de tempĂȘte de 1999, le rĂ©tablissement Ă©cologique Ă©tait minime. Ce changement de vĂ©gĂ©tation Ă grande Ă©chelle aura vraisemblablement dâimportantes incidences sur la faune et doit entrer en considĂ©ration dans la planification de lâĂ©cosystĂšme rĂ©gional ainsi que dans lâĂ©valuation et la surveillance des incidences cumulatives des travaux dâamĂ©nagement et de mise en valeur. Nos enquĂȘtes nous ont permis de constater que les Inuvialuit Ă©taient conscients des incidences environnementales de lâonde de tempĂȘte de 1999 plusieurs annĂ©es avant que les scientifiques et le personnel sâoccupant de la rĂ©glementation ne reconnaissent leur importance. Cette Ă©tude fait ressortir la nĂ©cessitĂ© dâavoir des mĂ©thodes multidisciplinaires et de faire appel aux gens de la rĂ©gion pour dĂ©terminer et comprendre les changements environnementaux dans lâArctique
Velocity-selective sublevel resonance of atoms with an array of current-carrying wires
Resonance transitions between the Zeeman sublevels of optically-polarized Rb
atoms traveling through a spatially periodic magnetic field are investigated in
a radio-frequency (rf) range of sub-MHz. The atomic motion induces the
resonance when the Zeeman splitting is equal to the frequency at which the
moving atoms feel the magnetic field oscillating. Additional temporal
oscillation of the spatially periodic field splits a motion-induced resonance
peak into two by an amount of this oscillation frequency. At higher oscillation
frequencies, it is more suitable to consider that the resonance is mainly
driven by the temporal field oscillation, with its velocity-dependence or
Doppler shift caused by the atomic motion through the periodic field. A
theoretical description of motion-induced resonance is also given, with
emphasis on the translational energy change associated with the internal
transition.Comment: 7 pages, 3 figures, final versio
A ground-based radar for measuring vertical strain rates and time-varying basal melt rates in ice sheets and shelves
The ApRES (autonomous phase-sensitive radio-echo sounder) instrument is a robust, lightweight and relatively inexpensive radar that has been designed to allow long-term, unattended monitoring of ice-shelf and ice-sheet thinning. We describe the instrument and demonstrate its capabilities and limitations by presenting results from three trial campaigns conducted in different Antarctic settings. Two campaigns were ice sheet-based â Pine Island Glacier and Dome C â and one was conducted on the Ross Ice Shelf. The ice-shelf site demonstrates the ability of the instrument to collect a time series of basal melt rates; the two grounded ice applications show the potential to recover profiles of vertical strain rate and also demonstrate some of the limitations of the present system
A new bathymetry for the southeastern Filchner-Ronne Ice Shelf: implications for modern oceanographic processes and glacial history
The FilchnerâRonne Ice Shelf, the ocean cavity beneath it and the Weddell Sea that bounds it, form an important part of the global climate system by modulating ice discharge from the Antarctic Ice Sheet and producing cold dense water masses that feed the global thermohaline circulation. A prerequisite for modeling the ice sheet and oceanographic processes within the cavity is an accurate knowledge of the subâiceâsheet bedrock elevation, but beneath the ice shelf where airborne radar cannot penetrate, bathymetric data are sparse. This paper presents new seismic point measurements of cavity geometry from a particularly poorly sampled region south of Berkner Island that connects the Filchner and Ronne ice shelves. An updated bathymetric grid formed by combining the new data with existing datasets reveals several new features. In particular, a sill running between Berkner Island and the mainland could alter ocean circulation within the cavity and change our understanding of paleoâiceâstream flow in the region. Also revealed are deep troughs near the grounding lines of Foundation and Support Force ice streams, which provide access for seawater with melting potential. Running an ocean tidal model with the new bathymetry reveals large differences in tidal current velocities, both within the new gridded region and further afield, potentially affecting subâiceâshelf melt rates
Updated Nucleosynthesis Constraints on Unstable Relic Particles
We revisit the upper limits on the abundance of unstable massive relic
particles provided by the success of Big-Bang Nucleosynthesis calculations. We
use the cosmic microwave background data to constrain the baryon-to-photon
ratio, and incorporate an extensively updated compilation of cross sections
into a new calculation of the network of reactions induced by electromagnetic
showers that create and destroy the light elements deuterium, he3, he4, li6 and
li7. We derive analytic approximations that complement and check the full
numerical calculations. Considerations of the abundances of he4 and li6 exclude
exceptional regions of parameter space that would otherwise have been permitted
by deuterium alone. We illustrate our results by applying them to massive
gravitinos. If they weigh ~100 GeV, their primordial abundance should have been
below about 10^{-13} of the total entropy. This would imply an upper limit on
the reheating temperature of a few times 10^7 GeV, which could be a potential
difficulty for some models of inflation. We discuss possible ways of evading
this problem.Comment: 40 pages LaTeX, 18 eps figure
Measurement of the - and -Dependence of the Asymmetry on the Nucleon
We report results for the virtual photon asymmetry on the nucleon from
new Jefferson Lab measurements. The experiment, which used the CEBAF Large
Acceptance Spectrometer and longitudinally polarized proton (NH) and
deuteron (ND) targets, collected data with a longitudinally
polarized electron beam at energies between 1.6 GeV and 5.7 GeV. In the present
paper, we concentrate on our results for and the related ratio
in the resonance and the deep inelastic regions for our lowest
and highest beam energies, covering a range in momentum transfer from
0.05 to 5.0 GeV and in final-state invariant mass up to about 3 GeV.
Our data show detailed structure in the resonance region, which leads to a
strong --dependence of for below 2 GeV. At higher , a
smooth approach to the scaling limit, established by earlier experiments, can
be seen, but is not strictly --independent. We add
significantly to the world data set at high , up to . Our data
exceed the SU(6)-symmetric quark model expectation for both the proton and the
deuteron while being consistent with a negative -quark polarization up to
our highest . This data setshould improve next-to-leading order (NLO) pQCD
fits of the parton polarization distributions.Comment: 7 pages LaTeX, 5 figure
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