5,643 research outputs found
Study and interpretation of the millimeter-wave spectrum of Venus
The effects of the Venus atmospheric constituents on its millimeter wavelength emission are investigated. Specifically, this research describes the methodology and the results of laboratory measurements which are used to calculate the opacity of some of the major absorbers in the Venus atmosphere. The pressure broadened absorption of gaseous SO2/CO2 and gaseous H2SO4/CO2 has been measured at millimeter wavelengths. We have also developed new formalisms for computing the absorptivities of these gases based on our laboratory work. The complex dielectric constant of liquid sulfuric acid has been measured and the expected opacity from the liquid sulfuric acid cloud layer found in the atmosphere of Venus has been evaluated. The partial pressure of gaseous H2SO4 has been measured which results in a more accurate estimate of the dissociation factor of H2SO4. A radiative transfer model has been developed in order to understand how each atmospheric constituent affects the millimeter wave emissions from Venus. Our results from the radiative transfer model are compared with recent observations of the micro-wave and millimeter wave emissions from Venus. Our main conclusion from this work is that gaseous H2SO4 is the most likely cause of the variation in the observed emission from Venus at 112 GHz
Understanding the variation in the millimeter-wave emission of Venus
Recent observations of the millimeter-wave emission from Venus at 112 GHz (2.6 mm) have shown significant variations in the continuum flux emission that may be attributed to the variability in the abundances of absorbing constituents in the Venus atmosphere. Such constituents include gaseous H2SO4, SO2, and liquid sulfuric acid (cloud condensates). Recently, Fahd and Steffes have shown that the effects of liquid H, SO4, and gaseous SO2 cannot completely account for this measured variability in the millimeter-wave emission of Venus. Thus, it is necessary to study the effect of gaseous H2SO4 on the millimeter-wave emission of Venus. This requires knowledge of the millimeter-wavelength (MMW) opacity of gaseous H2SO4, which unfortunately has never been determined for Venus-like conditions. We have measured the opacity of gaseous H2SO4 in a CO2 atmosphere at 550, 570, and 590 K, at 1 and 2 atm total pressure, and at a frequency of 94.1 GHz. Our results, in addition to previous centimeter-wavelength results are used to verify a modeling formalism for calculating the expected opacity of this gaseous mixture at other frequencies. This formalism is incorporated into a radiative transfer model to study the effect of gaseous H2SO4 on the MMW emission of Venus
The catalytic role of beta effect in barotropization processes
The vertical structure of freely evolving, continuously stratified,
quasi-geostrophic flow is investigated. We predict the final state
organization, and in particular its vertical structure, using statistical
mechanics and these predictions are tested against numerical simulations. The
key role played by conservation laws in each layer, including the fine-grained
enstrophy, is discussed. In general, the conservation laws, and in particular
that enstrophy is conserved layer-wise, prevent complete barotropization, i.e.,
the tendency to reach the gravest vertical mode. The peculiar role of the
-effect, i.e. of the existence of planetary vorticity gradients, is
discussed. In particular, it is shown that increasing increases the
tendency toward barotropization through turbulent stirring. The effectiveness
of barotropisation may be partly parameterized using the Rhines scale . As this parameter decreases (beta increases) then
barotropization can progress further, because the beta term provides enstrophy
to each layer
Positive semi-definite embedding for dimensionality reduction and out-of-sample extensions
In machine learning or statistics, it is often desirable to reduce the
dimensionality of a sample of data points in a high dimensional space
. This paper introduces a dimensionality reduction method where
the embedding coordinates are the eigenvectors of a positive semi-definite
kernel obtained as the solution of an infinite dimensional analogue of a
semi-definite program. This embedding is adaptive and non-linear. A main
feature of our approach is the existence of a non-linear out-of-sample
extension formula of the embedding coordinates, called a projected Nystr\"om
approximation. This extrapolation formula yields an extension of the kernel
matrix to a data-dependent Mercer kernel function. Our empirical results
indicate that this embedding method is more robust with respect to the
influence of outliers, compared with a spectral embedding method.Comment: 16 pages, 5 figures. Improved presentatio
A canonical Frobenius structure
We show that it makes sense to speak of THE Frobenius manifold attached to a
convenient and nondegenerate Laurent polynomialComment: 24 page
Can past gamma-ray bursts explain both INTEGRAL and ATIC/PAMELA/Fermi anomalies simultaneously?
Gamma-ray bursts (GRBs) have been invoked to explain both the 511 keV
emission from the galactic bulge and the high-energy positron excess inferred
from the ATIC, PAMELA, and Fermi data. While independent explanations can be
responsible for these phenomena, we explore the possibility of their common
GRB-related origin by modeling the GRB distribution and estimating the rates.
For an expected Milky Way long GRB rate, neither of the two signals is generic;
the local excess requires a 2% coincidence, while the signal from the galactic
center requires a 20% coincidence with respect to the timing of the latest GRB.
The simultaneous explanation requires a 0.4% coincidence. Considering the large
number of statistical "trials" created by multiple searches for new physics,
the coincidences of a few per cent cannot be dismissed as unlikely.
Alternatively, both phenomena can be explained by GRBs if the galactic rate is
higher than expected. We also show that a similar result is difficult to obtain
assuming a simplified short GRB distribution.Comment: 4 pages; version accepted for publicatio
Thermodynamics of the three-dimensional Hubbard model: Implications for cooling cold atomic gases in optical lattices
We present a comprehensive study of the thermodynamic properties of the
three-dimensional fermionic Hubbard model, with application to cold fermionic
atoms subject to an optical lattice and a trapping potential. Our study is
focused on the temperature range of current experimental interest. We employ
two theoretical methods - dynamical mean-field theory and high-temperature
series - and perform comparative benchmarks to delimitate their respective
range of validity. Special attention is devoted to understand the implications
that thermodynamic properties of this system have on cooling. Considering the
distribution function of local occupancies in the inhomogeneous lattice, we
show that, under adiabatic evolution, the variation of any observable (e.g.,
temperature) can be conveniently disentangled into two distinct contributions.
The first contribution is due to the redistribution of atoms in the trap during
the evolution, while the second one comes from the intrinsic change of the
observable. Finally, we provide a simplified picture of the cooling procedure
recently proposed in J.-S. Bernier et al., Phys. Rev. A 79, 061601 (2009) by
applying this method to an idealized model.Comment: 17 pages, 27 figures, version published in PR
Provenance for SPARQL queries
Determining trust of data available in the Semantic Web is fundamental for
applications and users, in particular for linked open data obtained from SPARQL
endpoints. There exist several proposals in the literature to annotate SPARQL
query results with values from abstract models, adapting the seminal works on
provenance for annotated relational databases. We provide an approach capable
of providing provenance information for a large and significant fragment of
SPARQL 1.1, including for the first time the major non-monotonic constructs
under multiset semantics. The approach is based on the translation of SPARQL
into relational queries over annotated relations with values of the most
general m-semiring, and in this way also refuting a claim in the literature
that the OPTIONAL construct of SPARQL cannot be captured appropriately with the
known abstract models.Comment: 22 pages, extended version of the ISWC 2012 paper including proof
Tunneling study of cavity grade Nb: possible magnetic scattering at the surface
Tunneling spectroscopy was performed on Nb pieces prepared by the same
processes used to etch and clean superconducting radio frequency (SRF)
cavities. Air exposed, electropolished Nb exhibited a surface superconducting
gap delta=1.55 meV, characteristic of clean, bulk Nb. However the tunneling
density of states (DOS) was broadened significantly. The Nb pieces treated with
the same mild baking used to improve the Q-slope in SRF cavities, reveal a
sharper DOS. Good fits to the DOS were obtained using Shiba theory, suggesting
that magnetic scattering of quasiparticles is the origin of the gapless surface
superconductivity and a heretofore unrecognized contributor to the Q-slope
problem of Nb SRF cavities.Comment: 3 pages, 3 figure
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