1,803 research outputs found
The Prescriptive Jurisdictional Reach of U.S. Antitrust Law: Judge Learned Hand\u27s Requirement of a Substantive Anticompetitive Effect
The Prescriptive Jurisdictional Reach of U.S. Antitrust Law: Judge Learned Hand\u27s Requirement of a Substantive Anticompetitive Effect
Near-UV OH Prompt Emission in the Innermost Coma of 103P/Hartley 2
The Deep Impact spacecraft fly-by of comet 103P/Hartley 2 occurred on 2010
November 4, one week after perihelion with a closest approach (CA) distance of
about 700 km. We used narrowband images obtained by the Medium Resolution
Imager (MRI) onboard the spacecraft to study the gas and dust in the innermost
coma. We derived an overall dust reddening of 15\%/100 nm between 345 and 749
nm and identified a blue enhancement in the dust coma in the sunward direction
within 5 km from the nucleus, which we interpret as a localized enrichment in
water ice. OH column density maps show an anti-sunward enhancement throughout
the encounter except for the highest resolution images, acquired at CA, where a
radial jet becomes visible in the innermost coma, extending up to 12 km from
the nucleus. The OH distribution in the inner coma is very different from that
expected for a fragment species. Instead, it correlates well with the water
vapor map derived by the HRI-IR instrument onboard Deep Impact
\citep{AHearn2011}. Radial profiles of the OH column density and derived water
production rates show an excess of OH emission during CA that cannot be
explained with pure fluorescence. We attribute this excess to a prompt emission
process where photodissociation of HO directly produces excited
OH*() radicals. Our observations provide the first direct
imaging of Near-UV prompt emission of OH. We therefore suggest the use of a
dedicated filter centered at 318.8 nm to directly trace the water in the coma
of comets.Comment: 21 page
Fault Slip and Exhumation History of the Willard Thrust Sheet, Sevier FoldâThrust Belt, Utah: Relations to Wedge Propagation, Hinterland Uplift, and Foreland Basin Sedimentation
Zircon (UâTh)/He (ZHe) and zircon fission track thermochronometric data for 47 samples spanning the areally extensive Willard thrust sheet within the western part of the Sevier foldâthrust belt record enhanced cooling and exhumation during major thrust slip spanning approximately 125â90 Ma. ZHe and zircon fission track ageâpaleodepth patterns along structural transects and ageâdistance relations along stratigraphicâparallel traverses, combined with thermoâkinematic modeling, constrain the fault slip history, with estimated slip rates of ~1 km/Myr from 125 to 105 Ma, increasing to ~3 km/Myr from 105 to 92 Ma, and then decreasing as major slip was transferred onto eastern thrusts. Exhumation was concentrated during motion up thrust ramps with estimated erosion rates of ~0.1 to 0.3 km/Myr. Local cooling ages of approximately 160â150 Ma may record a period of regional erosion, or alternatively an early phase of limited... (see full abstract in article)
A Distribution of Large Particles in the Coma of Comet 103P/Hartley 2
The coma of comet 103P/Hartley 2 has a significant population of large
particles observed as point sources in images taken by the Deep Impact
spacecraft. We measure their spatial and flux distributions, and attempt to
constrain their composition. The flux distribution of these particles implies a
very steep size distribution with power-law slopes ranging from -6.6 to -4.7.
The radii of the particles extend up to 20 cm, and perhaps up to 2 m, but their
exact sizes depend on their unknown light scattering properties. We consider
two cases: bright icy material, and dark dusty material. The icy case better
describes the particles if water sublimation from the particles causes a
significant rocket force, which we propose as the best method to account for
the observed spatial distribution. Solar radiation is a plausible alternative,
but only if the particles are very low density aggregates. If we treat the
particles as mini-nuclei, we estimate they account for <16-80% of the comet's
total water production rate (within 20.6 km). Dark dusty particles, however,
are not favored based on mass arguments. The water production rate from bright
icy particles is constrained with an upper limit of 0.1 to 0.5% of the total
water production rate of the comet. If indeed icy with a high albedo, these
particles do not appear to account for the comet's large water production rate.
production rate.
Erratum: We have corrected the radii and masses of the large particles of
comet 103P/Hartley 2 and present revised conclusions in the attached erratum.Comment: Original article: 46 pages, 17 figures, 5 tables, published in
Icarus. Erratum: 5 pages, 1 table, accepted for publication in Icaru
Two-dimensional Spectral Analysis of Mesospheric Airglow Image Data
A technique to analyze short-period (\u3c1 hour) gravity wave structure in all-sky images of the airglow emissions is described. The technique involves spatial calibration, star removal, geographic projection, regridding, and flat fielding of the data prior to the determination of the horizontal wave parameters (wavelength, velocity, and period), by use of standard two-dimensional Fourier analysis techniques. The method was developed to exploit the information that is now available with wide-field solid state imaging systems. This technique permits interactive and quantitative investigations of large, complex data sets. Such studies are important for investigating gravity wave characteristics, their interaction with the airglow emissions, and their geographic and seasonal variability. We study one event of this type here and present possible evidence of a nonlinear waveâwave interaction in the upper atmosphere
Water Ice and Dust in the Innermost Coma of Comet 103P/Hartley 2
On November 4th, 2010, the Deep Impact eXtended Investigation (DIXI)
successfully encountered comet 103P/Hartley 2, when it was at a heliocentric
distance of 1.06 AU. Spatially resolved near-IR spectra of comet Hartley 2 were
acquired in the 1.05-4.83 micron wavelength range using the HRI-IR
spectrometer. We present spectral maps of the inner ~10 kilometers of the coma
collected 7 minutes and 23 minutes after closest approach. The extracted
reflectance spectra include well-defined absorption bands near 1.5, 2.0, and
3.0 micron consistent in position, bandwidth, and shape with the presence of
water ice grains. Using Hapke's radiative transfer model, we characterize the
type of mixing (areal vs. intimate), relative abundance, grain size, and
spatial distribution of water ice and refractories. Our modeling suggests that
the dust, which dominates the innermost coma of Hartley 2 and is at a
temperature of 300K, is thermally and physically decoupled from the
fine-grained water ice particles, which are on the order of 1 micron in size.
The strong correlation between the water ice, dust, and CO2 spatial
distribution supports the concept that CO2 gas drags the water ice and dust
grains from the nucleus. Once in the coma, the water ice begins subliming while
the dust is in a constant outflow. The derived water ice scale-length is
compatible with the lifetimes expected for 1-micron pure water ice grains at 1
AU, if velocities are near 0.5 m/s. Such velocities, about three order of
magnitudes lower than the expansion velocities expected for isolated 1-micron
water ice particles [Hanner, 1981; Whipple, 1951], suggest that the observed
water ice grains are likely aggregates.Comment: 51 pages, 12 figures, accepted for publication in Icaru
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