73 research outputs found
Thermophysical properties of warm dense hydrogen
We study the thermophysical properties of warm dense hydrogen using quantum
molecular dynamics simulations. New results are presented for the pair
distribution functions, the equation of state, the Hugoniot curve, and the
reflectivity. We compare with available experimental data and predictions of
the chemical picture. Especially, we discuss the nonmetal-to-metal transition
which occurs at about 40 GPa in the dense fluid
Quantum molecular dynamics simulations for the nonmetal-to-metal transition in fluid helium
We have performed quantum molecular dynamics simulations for dense helium to
study the nonmetal-to-metal transition at high pressures. We present new
results for the equation of state and the Hugoniot curve in the warm dense
matter region. The optical conductivity is calculated via the Kubo-Greenwood
formula from which the dc conductivity is derived. The nonmetal-to-metal
transition is identified at about 1 g/ccm. We compare with experimental results
as well as with other theoretical approaches, especially with predictions of
chemical models.Comment: 4 pages, 5 figure
Ab-initio-Simulationen fĂŒr die Zustandsgleichung und TransportgröĂen von dichtem Wasserstoff
In dieser Arbeit werden die Resultate aus Ab-initio-Simulationen fĂŒr warmen dichten Wasserstoff vorgestellt. Diese kombinieren klassische Molekulardynamik
mit Dichtefunktionaltheorie. Es werden neben der thermischen und der kalorischen Zustandsgleichung auch die elektrische und WĂ€rmeleitfĂ€higkeit sowie das Reflexionsvermögen, der Absorptionskoeffizient und die Thermokraft prĂ€sentiert. Es wurde ein Nichtmetall-Metall-Ăbergang bei hohen DrĂŒcken beobachtet. Vergleiche mit vorhandenen Experimenten bestĂ€tigten die ZuverlĂ€ssigkeit der
vorgestellten Resultate.The results obtained from ab-initio simulations for warm dense hydrogen are presented in this work. The simulations combine classical molecular dynamics and density functional theory. Besides the thermal and the caloric equation of
state, also the electrical and thermal conductivity as well as the reflectivity, the absorption coefficient and the thermopower are presented. A nonmetal-to-metal transition at high pressures was observed. Comparison with
experiments showed best agreement and confirmed the reliability of the presented results
Electronic transport coefficients from ab initio simulations and application to dense liquid hydrogen
Using Kubo's linear response theory, we derive expressions for the
frequency-dependent electrical conductivity (Kubo-Greenwood formula),
thermopower, and thermal conductivity in a strongly correlated electron system.
These are evaluated within ab initio molecular dynamics simulations in order to
study the thermoelectric transport coefficients in dense liquid hydrogen,
especially near the nonmetal-to-metal transition region. We also observe
significant deviations from the widely used Wiedemann-Franz law which is
strictly valid only for degenerate systems and give an estimate for its valid
scope of application towards lower densities
Ab initio Equation of State data for hydrogen, helium, and water and the internal structure of Jupiter
The equation of state of hydrogen, helium, and water effects interior
structure models of giant planets significantly. We present a new equation of
state data table, LM-REOS, generated by large scale quantum molecular dynamics
simulations for hydrogen, helium, and water in the warm dense matter regime,
i.e.for megabar pressures and temperatures of several thousand Kelvin, and by
advanced chemical methods in the complementary regions. The influence of
LM-REOS on the structure of Jupiter is investigated and compared with
state-of-the-art results within a standard three-layer model consistent with
astrophysical observations of Jupiter. Our new Jupiter models predict an
important impact of mixing effects of helium in hydrogen with respect to an
altered compressibility and immiscibility.Comment: to appear in ApJ in August 2008, 11 figure
Cryptic diets of forage fish: jellyfish consumption observed in the Celtic Sea and Western English Channel
To establish if fishesâ consumption of jellyfish changes through the year, we conducted a molecular gutâcontent assessment on opportunistically sampled species from the Celtic Sea in October and compared these with samples previously collected in February and March from the Irish Sea. Mackerel Scomber scombrus were found to feed on hydrozoan jellyfish relatively frequently in autumn, with rare consumption also detected in sardine Sardina pilchardus and sprat Sprattus sprattus. By October, moon jellyfish Aurelia aurita appeared to have escaped predation, potentially through somatic growth and the development of stinging tentacles. This is in contrast with sampling in February and March where A. aurita ephyrae were heavily preyed upon. No significant change in predation rate was observed in S. sprattus, but jellyfish predation by S. scombrus feeding in autumn was significantly higher than that seen during winter. This increase in consumption appears to be driven by the consumption of different, smaller jellyfish species than were targeted during the winter
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
The role of the cerebellum in adaptation: ALE metaâanalyses on sensory feedback error
It is widely accepted that unexpected sensory consequences of selfâaction engage the cerebellum. However, we currently lack consensus on where in the cerebellum, we find fineâgrained differentiation to unexpected sensory feedback. This may result from methodological diversity in taskâbased human neuroimaging studies that experimentally alter the quality of selfâgenerated sensory feedback. We gathered existing studies that manipulated sensory feedback using a variety of methodological approaches and performed activation likelihood estimation (ALE) metaâanalyses. Only half of these studies reported cerebellar activation with considerable variation in spatial location. Consequently, ALE analyses did not reveal significantly increased likelihood of activation in the cerebellum despite the broad scientific consensus of the cerebellum's involvement. In light of the high degree of methodological variability in published studies, we tested for statistical dependence between methodological factors that varied across the published studies. Experiments that elicited an adaptive response to continuously altered sensory feedback more frequently reported activation in the cerebellum than those experiments that did not induce adaptation. These findings may explain the surprisingly low rate of significant cerebellar activation across brain imaging studies investigating unexpected sensory feedback. Furthermore, limitations of functional magnetic resonance imaging to probe the cerebellum could play a role as climbing fiber activity associated with feedback error processing may not be captured by it. We provide methodological recommendations that may guide future studies
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