1,390 research outputs found
On the construction of discrete approximations to linear differential expressions
Algorithm for generating discrete approximations in terms of ordinates for linear differential expression
Patents and Competition in the Automobile Industry
Methane (CH4) fluxes from world rivers are still poorly constrained, with measurements restricted mainly to temperate climates. Additional river flux measurements, including spatio-temporal studies, are important to refine extrapolations. Here we assess the spatio-temporal variability of CH4 fluxes from the Amazon and its main tributaries, the Negro, Solimoes, Madeira, Tapajos, Xingu, and Para Rivers, based on direct measurements using floating chambers. Sixteen of 34 sites were measured during low and high water seasons. Significant differences were observed within sites in the same river and among different rivers, types of rivers, and seasons. Ebullition contributed to more than 50% of total emissions for some rivers. Considering only river channels, our data indicate that large rivers in the Amazon Basin release between 0.40 and 0.58 Tg CH4 yr(-1). Thus, our estimates of CH4 flux from all tropical rivers and rivers globally were, respectively, 19-51% to 31-84% higher than previous estimates, with large rivers of the Amazon accounting for 22-28% of global river CH4 emissions.Funding Agencies|FAPESP [08/58089-9, 2011/06609-1, 2011/14502-2, 2012/17359-9]</p
Beware of simple methods for structure-based virtual screening: the critical importance of broader comparisons
We discuss how data unbiasing and simple methods such as protein-ligand Interaction FingerPrint (IFP) can overestimate virtual screening performance. We also show that IFP is strongly outperformed by target-specific machine-learning scoring functions, which were not considered in a recent report concluding that simple methods were better than machine-learning scoring functions at virtual screening
Global shallow water magnetohydrodynamic waves in the solar tachocline
We derive analytical solutions and dispersion relations of global magnetic
Poincar\'e (magneto-gravity) and magnetic Rossby waves in the approximation of
shallow water magnetohydrodynamics. The solutions are obtained in a rotating
spherical coordinate system for strongly and weakly stable stratification
separately in the presence of toroidal magnetic field. In both cases magnetic
Rossby waves split into fast and slow magnetic Rossby modes. In the case of
strongly stable stratification (valid in the radiative part of the tachocline)
all waves are slightly affected by the layer thickness and the toroidal
magnetic field, while in the case of weakly stable stratification (valid in the
upper overshoot layer of the tachocline) magnetic Poincar\'e and fast magnetic
Rossby waves are found to be concentrated near the solar equator, leading to
equatorially trapped waves. However, slow magnetic Rossby waves tend to
concentrate near the poles, leading to polar trapped waves. The frequencies of
all waves are smaller in the upper weakly stable stratification region than in
the lower strongly stable stratification one
Overdamped Alfven waves due to ion-neutral collisions in the solar chromosphere
Alfvenic waves are ubiquitous in the solar atmosphere and their dissipation
may play an important role in atmospheric heating. In the partially ionized
solar chromosphere, collisions between ions and neutrals are an efficient
dissipative mechanism for Alfven waves with frequencies near the ion-neutral
collision frequency. The collision frequency is proportional to the ion-neutral
collision cross section for momentum transfer. Here, we investigate Alfven wave
damping as a function of height in a simplified chromospheric model and compare
the results for two sets of collision cross sections, namely those of the
classic hard-sphere model and those based on recent quantum-mechanical
computations. We find important differences between the results for the two
sets of cross sections. There is a critical interval of wavelengths for which
impulsively excited Alfven waves are overdamped as a result of the strong
ion-neutral dissipation. The critical wavelengths are in the range from 1 km to
50 km for the hard-sphere cross sections, and from 1 m to 1 km for the
quantum-mechanical cross sections. Equivalently, for periodically driven Alfven
waves there is an optimal frequency for which the damping is most effective.
The optimal frequency varies from 1 Hz to 10^2 Hz for the hard-sphere cross
sections, and from 10^2 Hz to 10^4 Hz for the quantum-mechanical cross
sections. Future observations at sufficiently high spatial or temporal
resolution may show the importance of high-frequency Alfven waves for
chromospheric heating. For instance, the Atacama Large Millimeter/submillimeter
Array (ALMA) may be able to detect the critical wavelengths and optimal
frequencies and so to test the effective collision cross section in the
chromospheric plasma.Comment: Accepted for publication in A&
Energy losses of fast heavy-ion projectiles in dense hydrogen plasmas
It has been recently shown that the Bethe-Larkin formula for the energy
losses of fast heavy-ion projectiles in dense hydrogen plasmas is corrected by
the electron-ion correlations [Phys. Rev. Lett. \textbf{101}, 075002 (2008)].
We report numerical estimates of this correction based on the values of
obtained by numerical simulations in [Phys. Rev. E \textbf{61},
3470 (2000)]. We also extend this result to the case of projectiles with
dicluster charge distribution. We show that the experimental visibility of the
electron-ion correlation correction is enhanced in the case of dicluster
projectiles with randomly orientated charge centers. Although we consider here
the hydrogen plasmas to make the effect physically more clear, the
generalization to multispecies plasmas is straightforward.Comment: 5 pages, 1 figure. International Conference on Strongly Coupled
Coulomb Systems 2008, Camerino (Italy). To appear in J. Phys.
Automated data reduction workflows for astronomy
Data from complex modern astronomical instruments often consist of a large
number of different science and calibration files, and their reduction requires
a variety of software tools. The execution chain of the tools represents a
complex workflow that needs to be tuned and supervised, often by individual
researchers that are not necessarily experts for any specific instrument. The
efficiency of data reduction can be improved by using automatic workflows to
organise data and execute the sequence of data reduction steps. To realize such
efficiency gains, we designed a system that allows intuitive representation,
execution and modification of the data reduction workflow, and has facilities
for inspection and interaction with the data. The European Southern Observatory
(ESO) has developed Reflex, an environment to automate data reduction
workflows. Reflex is implemented as a package of customized components for the
Kepler workflow engine. Kepler provides the graphical user interface to create
an executable flowchart-like representation of the data reduction process. Key
features of Reflex are a rule-based data organiser, infrastructure to re-use
results, thorough book-keeping, data progeny tracking, interactive user
interfaces, and a novel concept to exploit information created during data
organisation for the workflow execution. Reflex includes novel concepts to
increase the efficiency of astronomical data processing. While Reflex is a
specific implementation of astronomical scientific workflows within the Kepler
workflow engine, the overall design choices and methods can also be applied to
other environments for running automated science workflows.Comment: 12 pages, 7 figure
Enumeration of left braces with additive group
We show that the number of isomorphism classes of left braces of order~
with additive group isomorphic to is
. This completes the classification of left braces of order~,
that turn out to fall into isomorphism classes.Comment: Ric. Mat., in press. 9 pages. arXiv admin note: substantial text
overlap with arXiv:2210.1282
Brachytherapy structural shielding calculations using Monte Carlo generated, monoenergetic data
To provide a method for calculating the transmission of any broad photon beam with a known energy spectrum in the range of 20 keV-1090 keV, through concrete and lead, based on the superposition of corresponding monoenergetic data obtained from Monte Carlo simulation
- …