2,660 research outputs found
Tracer Measurements in Growing Sea Ice Support Convective Gravity Drainage Parameterizations
Gravity drainage is the dominant process redistributing solutes in growing sea ice. Modeling gravity drainage is therefore necessary to predict physical and biogeochemical variables in sea ice. We evaluate seven gravity drainage parameterizations, spanning the range of approaches in the literature, using tracer measurements in a sea ice growth experiment. Artificial sea ice is grown to around 17 cm thickness in a new experimental facility, the Roland von Glasow air‐sea‐ice chamber. We use NaCl (present in the water initially) and rhodamine (injected into the water after 10 cm of sea ice growth) as independent tracers of brine dynamics. We measure vertical profiles of bulk salinity in situ, as well as bulk salinity and rhodamine in discrete samples taken at the end of the experiment. Convective parameterizations that diagnose gravity drainage using Rayleigh numbers outperform a simpler convective parameterization and diffusive parameterizations when compared to observations. This study is the first to numerically model solutes decoupled from salinity using convective gravity drainage parameterizations. Our results show that (1) convective, Rayleigh number‐based parameterizations are our most accurate and precise tool for predicting sea ice bulk salinity; and (2) these parameterizations can be generalized to brine dynamics parameterizations, and hence can predict the dynamics of any solute in growing sea ic
Far-Ultraviolet Dust Albedo Measurements in the Upper Scorpius Cloud Using the SPINR Sounding Rocket Experiment
The Spectrograph for Photometric Imaging with Numeric Reconstruction (SPINR)
sounding rocket experiment was launched on 2000 August 4 to record
far-ultraviolet (912-1450 A) spectral and spatial information for the giant
reflection nebula in the Upper Scorpius region. The data were divided into
three arbitrary bandpasses (912-1029 A, 1030-1200 A, and 1235-1450 A) for which
stellar and nebular flux levels were derived. These flux measurements were used
to constrain a radiative transfer model and to determine the dust albedo for
the Upper Scorpius region. The resulting albedos were 0.28+/-0.07 for the
912-1029 A bandpass, 0.33+/-0.07 for the 1030-1200 A bandpass, and 0.77+/-0.13
for the 1235-1450 A bandpass
Towards the model driven organization
Todays modern organization is faced with a need for rapid response to changes from external business pressures by updating systems and operational procedures. The effect of such continuous evolution eventually leads to sub- optimal configurations of its underlying systems. The management of continuous business change is compromised by uncertainty due to the inadequacy of existing mechanisms for responding to multiple change drivers thus leading to signifi- cant organizational costs. This represents a major opportunity for seeking greater efficiencies. To date, there has been little or no attempt to apply model driven principles or approaches to addressing these issues. We present a new vision of a Model Driven Organisation (MDO) that has the potential to increase produc- tivity by promoting integration of business processes and collaborations across the organisation whilst supporting safe and convenient adaptations that enable rapid response to change whilst maintaining integrity of the systems within the organisation. The approach proposed is based on the use of modelling languages and simulation technologies that support abstractions for understanding business goals through to specification of IT systems and ultimately to deployed systems. The paper motivates the problem and proposes a definition of the MDO. We val- idate the proposal through an illustrative case and conclude with a review of the state of the art leading to a roadmap of research and emergent grand challenges towards achieving the MDO vision
Quantum calculations of Coulomb reorientation for sub-barrier fusion
Classical mechanics and Time Dependent Hartree-Fock (TDHF) calculations of
heavy ions collisions are performed to study the rotation of a deformed nucleus
in the Coulomb field of its partner. This reorientation is shown to be
independent on charges and relative energy of the partners. It only depends
upon the deformations and inertias. TDHF calculations predict an increase by
30% of the induced rotation due to quantum effects while the nuclear
contribution seems negligible. This reorientation modifies strongly the fusion
cross-section around the barrier for light deformed nuclei on heavy collision
partners. For such nuclei a hindrance of the sub-barrier fusion is predicted.Comment: accepted for publication in Physical Review Lette
Radio Observations of HD 80606 Near Planetary Periastron
This paper reports Very Large Array observations at 325 and 1425 MHz (90cm
and 20cm) during and near the periastron passage of HD 80606b on 2007 November
20. We obtain flux density limits (3-sigma) of 1.7 mJy and 48 microJy at 325
and 1425 MHz, respectively, equivalent to planetary luminosity limits of 2.3 x
10^{24} erg/s and 2.7 x 10^{23} erg/s. These are well above the Jovian value
(at 40 MHz) of 2 x 10^{18} erg/s. The motivation for these observations was
that the planetary magnetospheric emission is driven by a stellar
wind-planetary magnetosphere interaction so that the planetary luminosity would
be elevated. Near periastron, HD 80606b might be as much as 3000 times more
luminous than Jupiter. Recent transit observations of HD 80606b provide
stringent constraints on the planetary mass and radius, and, because of the
planet's highly eccentric orbit, its rotation period is likely to be
"pseudo-synchronized" to its orbital period, allowing a robust estimate of the
former. We are able to make robust estimates of the emission frequency of the
planetary magnetospheric emission and find it to be around 60--90 MHz. We
compare HD 80606b to other high-eccentricity systems and assess the detection
possibilities for both near-term and more distant future systems. Of the known
high eccentricity planets, only HD 80606b is likely to be detectable, as HD
20782B b and HD 4113b are both likely to have weaker magnetic field strengths.
Both the forthcoming "EVLA low band" system and the Low Frequency Array may be
able to improve upon our limits for HD 80606b, and do so at a more optimum
frequency. If the low-frequency component of the Square Kilometre Array
(SKA-lo) and a future lunar radio array are able to approach their thermal
noise limits, they should be able to detect an HD 80606b-like planet, unless
the planet's luminosity increases by substantially less than a factor of 3000.Comment: 9 pages; accepted for publication in A
Methane Mitigation:Methods to Reduce Emissions, on the Path to the Paris Agreement
The atmospheric methane burden is increasing rapidly, contrary to pathways compatible with the goals of the 2015 United Nations Framework Convention on Climate Change Paris Agreement. Urgent action is required to bring methane back to a pathway more in line with the Paris goals. Emission reduction from “tractable” (easier to mitigate) anthropogenic sources such as the fossil fuel industries and landfills is being much facilitated by technical advances in the past decade, which have radically improved our ability to locate, identify, quantify, and reduce emissions. Measures to reduce emissions from “intractable” (harder to mitigate) anthropogenic sources such as agriculture and biomass burning have received less attention and are also becoming more feasible, including removal from elevated-methane ambient air near to sources. The wider effort to use microbiological and dietary intervention to reduce emissions from cattle (and humans) is not addressed in detail in this essentially geophysical review. Though they cannot replace the need to reach “net-zero” emissions of CO2, significant reductions in the methane burden will ease the timescales needed to reach required CO2 reduction targets for any particular future temperature limit. There is no single magic bullet, but implementation of a wide array of mitigation and emission reduction strategies could substantially cut the global methane burden, at a cost that is relatively low compared to the parallel and necessary measures to reduce CO2, and thereby reduce the atmospheric methane burden back toward pathways consistent with the goals of the Paris Agreement
Non-local thermodynamic equilibrium effects determine the upper atmospheric temperature structure of the ultra-hot Jupiter KELT-9b
Several results indicate that the atmospheric temperature of the ultra-hot
Jupiter KELT-9b in the main line formation region is a few thousand degrees
higher than predicted by self-consistent models. We test whether non-local
thermodynamic equilibrium (NLTE) effects are responsible for the presumably
higher temperature. We employ the Cloudy NLTE radiative transfer code to
self-consistently compute the upper atmospheric temperature-pressure (TP)
profile of KELT-9b, assuming solar metallicity. The Cloudy NLTE TP profile is
2000 K hotter than that obtained with previous models assuming local
thermodynamic equilibrium (LTE). In particular, in the 1-10 bar range
the temperature increases from 4000 K to 8500 K, remaining
roughly constant at lower pressures. We find that the high temperature in the
upper atmosphere of KELT-9b is driven principally by NLTE effects modifying the
Fe and Mg level populations, which strongly influence the atmospheric thermal
balance. We employ Cloudy to compute LTE and NLTE synthetic transmission
spectra on the basis of the TP profiles computed in LTE and NLTE, respectively,
finding that the NLTE model generally produces stronger absorption lines than
the LTE model (up to 30%), which is largest in the ultraviolet. We compare the
NLTE synthetic transmission spectrum with the observed H and H
line profiles obtaining an excellent match, thus supporting our results. The
NLTE synthetic transmission spectrum can be used to guide future observations
aiming at detecting features in the planet's transmission spectrum. Metals,
such as Mg and Fe, and NLTE effects shape the upper atmospheric temperature
structure of KELT-9b and thus affect the mass-loss rates derived from it.
Finally, our results call for checking whether this is the case also of cooler
planets.Comment: Accepted for publication on A&A. The abstract has been shortened to
fit the available spac
Lifetime measurements in Co and Co
Lifetimes of the and states in Co and the
state in Co were measured using the recoil distance Doppler
shift and the differential decay curve methods. The nuclei were populated by
multi-nucleon transfer reactions in inverse kinematics. Gamma rays were
measured with the EXOGAM Ge array and the recoiling fragments were fully
identified using the large-acceptance VAMOS spectrometer. The E2 transition
probabilities from the and states to the ground
state could be extracted in Co as well as an upper limit for the
(E2) value in Co. The experimental
results were compared to large-scale shell-model calculations in the and
model spaces, allowing to draw conclusions on the single-particle
or collective nature of the various states.Comment: 8 pages, 8 figures, 1 table, accepted for publication in Physical
Review
Low Temperature Opacities
Previous computations of low temperature Rosseland and Planck mean opacities
from Alexander & Ferguson (1994) are updated and expanded. The new computations
include a more complete equation of state with more grain species and updated
optical constants. Grains are now explicitly included in thermal equilibrium in
the equation of state calculation, which allows for a much wider range of grain
compositions to be accurately included than was previously the case. The
inclusion of high temperature condensates such as AlO and CaTiO
significantly affects the total opacity over a narrow range of temperatures
before the appearance of the first silicate grains.
The new opacity tables are tabulated for temperatures ranging from 30000 K to
500 K with gas densities from 10 g cm to 10 g cm.
Comparisons with previous Rosseland mean opacity calculations are discussed. At
high temperatures, the agreement with OPAL and Opacity Project is quite good.
Comparisons at lower temperatures are more divergent as a result of differences
in molecular and grain physics included in different calculations. The
computation of Planck mean opacities performed with the opacity sampling method
are shown to require a very large number of opacity sampling wavelength points;
previously published results obtained with fewer wavelength points are shown to
be significantly in error. Methods for requesting or obtaining the new tables
are provided.Comment: 39 pages with 12 figures. To be published in ApJ, April 200
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