1,105 research outputs found
Quasi-static granular flow of ice mélange
We use Landsat 8 imagery to generate ice mĂ©lange velocity fields at Greenlandâs three most
productive outlet glaciers: Jakobshavn IsbrĂŠ, Helheim Glacier, and Kangerdlugssuaq Glacier. Winter velocity
fields are generally steady and highly uniform. Summer velocity fields, on the other hand, tend to be much
more variable and can be uniform, compressional, or extensional. We rarely observe compressional flow
at Jakobshavn IsbrĂŠ or extensional flow at Helheim Glacier, while both are observed at Kangerdlugssuaq
Glacier. Transverse velocity profiles from all three locations are suggestive of viscoplastic flow, in which
deformation occurs primarily in shear zones along the fjord walls. We analyze the transverse profiles in
the context of quasi-static flow using continuum rheologies for granular materials and find that the force
per unit width that ice mélange exerts on glacier termini increases exponentially with the ice mélange
length-to-width ratio and the effective coefficient of friction. Our estimates of ice mélange resistance are
consistent with other independent estimates and suggest that ice mélange may be capable of inhibiting
iceberg calving events, especially during winter. Moreover, our results provide geophysical-scale support for
constitutive relationships for granular materials and suggest a potential avenue for modeling ice mélange
dynamics with continuum models.From acknowledgments:
Funding for this project was provided by the U.S. National Science Foundation (DMR-1506446 and DMR-1506307).
Digital elevation models were provided by the Polar Geospatial Center under the U.S. National Science Foundation (OPP-1043681, OPP-1559691, and OPP-1542736)Ye
Quasi-static granular flow of ice mélange
We use Landsat 8 imagery to generate ice mĂ©lange velocity fields at Greenlandâs three most
productive outlet glaciers: Jakobshavn IsbrĂŠ, Helheim Glacier, and Kangerdlugssuaq Glacier. Winter velocity
fields are generally steady and highly uniform. Summer velocity fields, on the other hand, tend to be much
more variable and can be uniform, compressional, or extensional. We rarely observe compressional flow
at Jakobshavn IsbrĂŠ or extensional flow at Helheim Glacier, while both are observed at Kangerdlugssuaq
Glacier. Transverse velocity profiles from all three locations are suggestive of viscoplastic flow, in which
deformation occurs primarily in shear zones along the fjord walls. We analyze the transverse profiles in
the context of quasi-static flow using continuum rheologies for granular materials and find that the force
per unit width that ice mélange exerts on glacier termini increases exponentially with the ice mélange
length-to-width ratio and the effective coefficient of friction. Our estimates of ice mélange resistance are
consistent with other independent estimates and suggest that ice mélange may be capable of inhibiting
iceberg calving events, especially during winter. Moreover, our results provide geophysical-scale support for
constitutive relationships for granular materials and suggest a potential avenue for modeling ice mélange
dynamics with continuum models.From acknowledgments:
Funding for this project was provided by the U.S. National Science Foundation (DMR-1506446 and DMR-1506307).
Digital elevation models were provided by the Polar Geospatial Center under the U.S. National Science Foundation (OPP-1043681, OPP-1559691, and OPP-1542736)Ye
Quantifying flow and stress in ice mĂ©lange, the worldâs largest granular material.
Tidewater glacier fjords are often filled with a collection of calved icebergs, brash ice, and sea ice. For glaciers with high calving rates, this âm Ìelangeâ of ice can be jam-packed, so that the flow of ice fragments is mostly determined by granular interactions. In the jammed state, ice m Ìelange has been hypothesized to influence iceberg calving and capsize, dispersion and attenuation of ocean waves, injection of freshwater into fjords, and fjord circulation. However, detailed measurements of ice m Ìelange are lacking due to difficulties in instrumenting remote, ice-choked fjords. Here we characterize the flow and associated stress in icem Ìelange, using a combination of terrestrial radar data, laboratory experiments, and numerical simulations. We find that, during periods of terminus quiescence, ice m Ìelange experiences laminar flow over timescales of hours to days. The uniform flow fields are bounded by shear margins along fjord walls where force chains between granular icebergs terminate. In addition, the average force per unit width that is transmitted to the glacier terminus, which can exceed 107N/m, increases exponentially with them Ìelange length-to-width ratio. These âbuttressingâ forces are sufficiently high to inhibit the initiation of large-scale calving events, supporting the notion that ice m Ìelange can be viewed as a weak granular ice shelf that transmits stresses from fjord walls back to glacier termini.Ye
Seismic Tremor Reveals Spatial Organization and Temporal Changes of Subglacial Water System
©2019. American Geophysical Union. All Rights Reserved.Subglacial water ïŹow impacts glacier dynamics and shapes the subglacial environment. However, due to the challenges of observing glacier beds, the spatial organization of subglacial water systems and the time scales of conduit evolution and migration are largely unknown. To address these questions, we analyze 1.5â to 10âHz seismic tremor that we associate with subglacial water ïŹow,
that is, glaciohydraulic tremor, at Taku Glacier, Alaska, throughout the 2016 melt season. We use frequencyâdependent polarization analysis to estimate glaciohydraulic tremor propagation direction (related to the subglacial conduit location) and a degree day melt model to monitor variations in meltâwater
input. We suggest that conduit formation requires sustained water input and that multiconduit ïŹow paths can be distinguished from singleâconduit ïŹow paths. Theoretical analysis supports our seismic interpretations that subglacial discharge likely ïŹows through a singleâconduit in regions of steep hydraulic potential gradients but may be distributed among multiple conduits in regions with shallower potential gradients. Seismic tremor in regions with multiple conduits evolves through abrupt jumps between stable conïŹgurations that last 3â7 days, while tremor produced by singleâconduit ïŹow remains more stationary.
We also ïŹnd that polarized glaciohydraulic tremor wave types are potentially linked to the distance from source to station and that multiple peak frequencies propagate from a similar direction. Tremor appears undetectable at distances beyond 2â6 km from the source. This new understanding of the spatial organization and temporal development of subglacial conduits informs our understanding of dynamism within the subglacial hydrologic system.Raw seismic data described in this paper are available through the Incorporated Research Institutions for Seismology Data Management Center (http://ds.iris.edu/mda/ZQ? timewindow=2015â2016; Amundson et al., 2015). The raw weather data used in this paper can be found through the Arctic Data Center (https://doi.org/ 10.18739/A2H98ZC7V; Bartholomaus & Walter, 2018). Python code developed to carry out the analyses presented here is available at https://github.com/ voremargot/SeismicâTremorâRevealsâ SpatialâOrganizationâandâTemporalâ Changesâof SubglacialâWaterâSystem and https://github.com/ tbartholomaus/med_spec. This study was made possible with support from the University of Texas Institute for Geophysics and the University of Idaho. We thank Ginny Catania for the loan of weather stations. J. P. W.'s and J. M. A.'s contributions to this work were supported by the U.S. National Science Foundation (OPPâ1337548 and OPPâ 1303895). T. C. B. thanks Dylan Mikesell for an early conversation, which inspired the analysis presented here.Ye
Seismic Tremor Reveals Spatial Organization and Temporal Changes of Subglacial Water System
©2019. American Geophysical Union. All Rights Reserved.Subglacial water ïŹow impacts glacier dynamics and shapes the subglacial environment. However, due to the challenges of observing glacier beds, the spatial organization of subglacial water systems and the time scales of conduit evolution and migration are largely unknown. To address these questions, we analyze 1.5â to 10âHz seismic tremor that we associate with subglacial water ïŹow,
that is, glaciohydraulic tremor, at Taku Glacier, Alaska, throughout the 2016 melt season. We use frequencyâdependent polarization analysis to estimate glaciohydraulic tremor propagation direction (related to the subglacial conduit location) and a degree day melt model to monitor variations in meltâwater
input. We suggest that conduit formation requires sustained water input and that multiconduit ïŹow paths can be distinguished from singleâconduit ïŹow paths. Theoretical analysis supports our seismic interpretations that subglacial discharge likely ïŹows through a singleâconduit in regions of steep hydraulic potential gradients but may be distributed among multiple conduits in regions with shallower potential gradients. Seismic tremor in regions with multiple conduits evolves through abrupt jumps between stable conïŹgurations that last 3â7 days, while tremor produced by singleâconduit ïŹow remains more stationary.
We also ïŹnd that polarized glaciohydraulic tremor wave types are potentially linked to the distance from source to station and that multiple peak frequencies propagate from a similar direction. Tremor appears undetectable at distances beyond 2â6 km from the source. This new understanding of the spatial organization and temporal development of subglacial conduits informs our understanding of dynamism within the subglacial hydrologic system.Raw seismic data described in this paper are available through the Incorporated Research Institutions for Seismology Data Management Center (http://ds.iris.edu/mda/ZQ? timewindow=2015â2016; Amundson et al., 2015). The raw weather data used in this paper can be found through the Arctic Data Center (https://doi.org/ 10.18739/A2H98ZC7V; Bartholomaus & Walter, 2018). Python code developed to carry out the analyses presented here is available at https://github.com/ voremargot/SeismicâTremorâRevealsâ SpatialâOrganizationâandâTemporalâ Changesâof SubglacialâWaterâSystem and https://github.com/ tbartholomaus/med_spec. This study was made possible with support from the University of Texas Institute for Geophysics and the University of Idaho. We thank Ginny Catania for the loan of weather stations. J. P. W.'s and J. M. A.'s contributions to this work were supported by the U.S. National Science Foundation (OPPâ1337548 and OPPâ 1303895). T. C. B. thanks Dylan Mikesell for an early conversation, which inspired the analysis presented here.Ye
Probing neutrino mass with multilepton production at the Tevatron in the simplest R-parity violation model
We analyze the production of multileptons in the simplest supergravity model
with bilinear violation of R parity at the Fermilab Tevatron. Despite the small
R-parity violating couplings needed to generate the neutrino masses indicated
by current atmospheric neutrino data, the lightest supersymmetric particle is
unstable and can decay inside the detector. This leads to a phenomenology quite
distinct from that of the R-parity conserving scenario. We quantify by how much
the supersymmetric multilepton signals differ from the R-parity conserving
expectations, displaying our results in the plane. We
show that the presence of bilinear R-parity violating interactions enhances the
supersymmetric multilepton signals over most of the parameter space, specially
at moderate and large .Comment: 26 pages, 23 figures. Revised version with some results corrected and
references added. Conclusions remain the sam
Meltwater Intrusions Reveal Mechanisms for Rapid Submarine Melt at a Tidewater Glacier
Submarine melting has been implicated as a driver of glacier retreat and sea level rise, but to date melting has been difficult to observe and quantify. As a result, melt rates have been estimated from parameterizations that are largely unconstrained by observations, particularly at the near-vertical termini of tidewater glaciers. With standard coefficients, these melt parameterizations predict that ambient
melting (the melt away from subglacial discharge outlets) is negligible compared to discharge-driven melting for typical tidewater glaciers. Here, we present new data from LeConte Glacier, Alaska, that challenges this paradigm. Using autonomous kayaks, we observe ambient meltwater intrusions that are ubiquitous within 400 m of the terminus, and we provide the first characterization of their properties, structure, and distribution. Our results suggest that ambient melt rates are substantially higher (Ă100) than standard theory predicts and that ambient melting is a significant part of the total submarine melt flux. We explore modifications to the prevalent melt parameterization to provide a path forward for improved modeling of ocean-glacier interactions.This work was funded by NSF OPP Grants 1503910, 1504191, 1504288,
and 1504521 and National Geographic Grant CP4-171R-17. Additionally, this research was supported by the NOAA Climate and Global Change Postdoctoral Fellowship Program, administered by UCARâs Cooperative Programs for the Advancement of Earth System Science (CPAESS) under award #NA18NWS4620043B. These observations would not be possible without the skilled engineering team who developed the autonomous kayaksâincluding Jasmine Nahorniak, June Marion, Nick McComb, Anthony Grana, and Corwin Perrenâand also the Captain and crew of the M/V Amber Anne. We thank Donald Slater and an anonymous reviewer for valuable feedback that improved this manuscript. Data availability: All of the oceanographic data collected by ship and kayak have been archived with the National Centers for Environmental Information (Accession 0189574, https://accession.nodc.noaa.gov/ 0189574). The glacier data have been archived at the Arctic Data Center (https://doi.org/10.18739/A22G44).Ye
Testing Color Evaporation in Photon-Photon Production of J/Psi at CERN LEP II
The DELPHI Collaboration has recently reported the measurement of J/Psi
production in photon-photon collisions at LEP II. These newly available data
provide an additional proof of the importance of colored c bar{c} pairs for the
production of charmonium because these data can only be explained by
considering resolved photon processes. We show here that the inclusion of color
octet contributions to the J/Psi production in the framework of the color
evaporation model is able to reproduce this data. In particular, the
transverse-momentum distribution of the J/Psi mesons is well described by this
model.Comment: 10 pages, 5 Figures, Revtex
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