26,103 research outputs found
Quasi-static granular flow of ice mélange
We use Landsat 8 imagery to generate ice mélange velocity fields at Greenland’s three most
productive outlet glaciers: Jakobshavn Isbræ, 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 Isbræ 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
An infrared proper motion study of the Orion bullets
We report the first IR proper motion measurements of the Herbig-Haro objects
in the Orion Molecular Cloud--One using a four-year time baseline. The [Fe II]
emitting bullets are moving of order 0.08 arcsec per year, or at about 170
\kms. The direction of motion is similar to that inferred from their
morphology. The proper motions of \h2 emitting wakes behind the [Fe II]
bullets, and of newly found \h2 bullets, are also measured. \h2 bullets have
smaller proper motion than [Fe II] bullets, while \h2 wakes with leading [Fe
II] bullets appear to move at similar speeds to their associated bullets. A few
instances of variability in the emission can be attributed to dense, stationary
clumps in the ambient cloud being overrun, setting up a reverse--oriented
bullet. Differential motion between [Fe II] bullets and their trailing \h2
wakes is not observed, suggesting that these are not separating, and also that
they have reached a steady--state configuration over at least 100 years. The
most distant bullets have, on average, larger proper motions, but are not
consistent with free expansion. Nevertheless an impulsive, or short--lived
( 1,000 years) duration for their origin seems likely.Comment: 9 pages, 8 figure
Quasi-static granular flow of ice mélange
We use Landsat 8 imagery to generate ice mélange velocity fields at Greenland’s three most
productive outlet glaciers: Jakobshavn Isbræ, 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 Isbræ 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
The southern dust pillars of the Carina Nebula
We present preliminary results from a detailed study towards four previously
detected bright mid-infrared sources in the southern part of the Carina Nebula:
G287.73--0.92, G287.84--0.82, G287.93--0.99 and G288.07--0.80. All of these
sources are located at the heads of giant dust pillars that point towards the
nearby massive star cluster, Trumpler 16. It is unclear if these pillars are
the prime sites for a new generation of triggered star formation or if instead
they are the only remaining parts of the nebula where ongoing star fromation
can take place.Comment: 2 pages, to appear in the proceedings of "Hot Star Workshop III: The
Earliest Phases of Massive Star Birth" (ed. P.A. Crowther
Mapping warm molecular hydrogen with Spitzer's Infrared Array Camera (IRAC)
Photometric maps, obtained with Spitzer's Infrared Array Camera (IRAC), can
provide a valuable probe of warm molecular hydrogen within the interstellar
medium. IRAC maps of the supernova remnant IC443, extracted from the Spitzer
archive, are strikingly similar to spectral line maps of the H2 pure rotational
transitions that we obtained with the Infrared Spectrograph (IRS) instrument on
Spitzer. IRS spectroscopy indicates that IRAC Bands 3 and 4 are indeed
dominated by the H2 v=0-0 S(5) and S(7) transitions, respectively. Modeling of
the H2 excitation suggests that Bands 1 and 2 are dominated by H2 v=1-0 O(5)
and v=0-0 S(9). Large maps of the H2 emission in IC433, obtained with IRAC,
show band ratios that are inconsistent with the presence of gas at a single
temperature. The relative strengths of IRAC Bands 2, 3, and 4 are consistent
with pure H2 emission from shocked material with a power-law distribution of
gas temperatures. CO vibrational emissions do not contribute significantly to
the observed Band 2 intensity. Assuming that the column density of H2 at
temperatures T to T+dT is proportional to T raised to the power -b for
temperatures up to 4000 K, we obtained a typical estimate of 4.5 for b. The
power-law index, b, shows variations over the range 3 to 6 within the set of
different sight-lines probed by the maps, with the majority of sight-lines
showing b in the range 4 to 5. The observed power-law index is consistent with
the predictions of simple models for paraboloidal bow shocks.Comment: 27 pages, including 11 figures. Accepted for publication in Ap
Irrigation and drainage performance assessment: practical guidelines
Irrigation management / Drainage / Performance evaluation / Performance indexes / Evapotranspiration / Precipitation / Water balance / Participatory rural appraisal / Databases / Simulation
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