51,274 research outputs found
JPEG2000 Image Compression on Solar EUV Images
For future solar missions as well as ground-based telescopes, efficient ways
to return and process data have become increasingly important. Solar Orbiter,
e.g., which is the next ESA/NASA mission to explore the Sun and the
heliosphere, is a deep-space mission, which implies a limited telemetry rate
that makes efficient onboard data compression a necessity to achieve the
mission science goals. Missions like the Solar Dynamics Observatory (SDO) and
future ground-based telescopes such as the Daniel K. Inouye Solar Telescope, on
the other hand, face the challenge of making petabyte-sized solar data archives
accessible to the solar community. New image compression standards address
these challenges by implementing efficient and flexible compression algorithms
that can be tailored to user requirements. We analyse solar images from the
Atmospheric Imaging Assembly (AIA) instrument onboard SDO to study the effect
of lossy JPEG2000 (from the Joint Photographic Experts Group 2000) image
compression at different bit rates. To assess the quality of compressed images,
we use the mean structural similarity (MSSIM) index as well as the widely used
peak signal-to-noise ratio (PSNR) as metrics and compare the two in the context
of solar EUV images. In addition, we perform tests to validate the scientific
use of the lossily compressed images by analysing examples of an on-disk and
off-limb coronal-loop oscillation time-series observed by AIA/SDO.Comment: 25 pages, published in Solar Physic
The correlation energy functional within the GW-RPA approximation: exact forms, approximate forms and challenges
In principle, the Luttinger-Ward Green's function formalism allows one to
compute simultaneously the total energy and the quasiparticle band structure of
a many-body electronic system from first principles. We present approximate and
exact expressions for the correlation energy within the GW-RPA approximation
that are more amenable to computation and allow for developing efficient
approximations to the self-energy operator and correlation energy. The exact
form is a sum over differences between plasmon and interband energies. The
approximate forms are based on summing over screened interband transitions. We
also demonstrate that blind extremization of such functionals leads to
unphysical results: imposing physical constraints on the allowed solutions
(Green's functions) is necessary. Finally, we present some relevant numerical
results for atomic systems.Comment: 3 figures and 3 tables, under review at Physical Review
Orbital Kondo effect in Cobalt-Benzene sandwich molecules
We study a Co-benzene sandwich molecule bridging the tips of a Cu nanocontact
as a realistic model of correlated molecular transport. To this end we employ a
recently developed method for calculating the correlated electronic structure
and transport properties of nanoscopic conductors. When the molecule is
slightly compressed by the tips of the nanocontact the dynamic correlations
originating from the strongly interacting Co 3d shell give rise to an orbital
Kondo effect while the usual spin Kondo effect is suppressed due to Hund's rule
coupling. This non-trivial Kondo effect produces a sharp and
temperature-dependent Abrikosov-Suhl resonance in the spectral function at the
Fermi level and a corresponding Fano line shape in the low bias conductance
Predicting the frequencies of diverse exo-planetary systems
Extrasolar planetary systems range from hot Jupiters out to icy comet belts
more distant than Pluto. We explain this diversity in a model where the mass of
solids in the primordial circumstellar disk dictates the outcome. The star
retains measures of the initial heavy-element (metal) abundance that can be
used to map solid masses onto outcomes, and the frequencies of all classes are
correctly predicted. The differing dependences on metallicity for forming
massive planets and low-mass cometary bodies are also explained. By
extrapolation, around two-thirds of stars have enough solids to form Earth-like
planets, and a high rate is supported by the first detections of low-mass
exo-planets.Comment: 5 pages, 2 figures; accepted by MNRA
Traveling waves in rotating Rayleigh-Bénard convection: Analysis of modes and mean flow
Numerical simulations of the Boussinesq equations with rotation for realistic no-slip boundary conditions and a finite annular domain are presented. These simulations reproduce traveling waves observed experimentally. Traveling waves are studied near threshhold by using the complex Ginzburg-Landau equation (CGLE): a mode analysis enables the CGLE coefficients to be determined. The CGLE coefficients are compared with previous experimental and theoretical results. Mean flows are also computed and found to be more significant as the Prandtl number decreases (from sigma=6.4 to sigma=1). In addition, the mean flow around the outer radius of the annulus appears to be correlated with the mean flow around the inner radius
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