3,438 research outputs found
A new function of hydrocarbons in insect communication : maternal care and offspring signalling in the European earwig
International audienc
Hydrodynamic stress on fractal aggregates of spheres
We calculate the average hydrodynamic stress on fractal aggregates of spheres using Stokesian dynamics. We find that for fractal aggregates of force-free particles, the stress does not grow as the cube of the radius of gyration, but rather as the number of particles in the aggregate. This behavior is only found for random aggregates of force-free particles held together by hydrodynamic lubrication forces. The stress on aggregates of particles rigidly connected by interparticle forces grows as the radius of gyration cubed. We explain this behavior by examining the transmission of the tension along connecting lines in an aggregate and use the concept of a persistance length in order to characterize this stress transmission within an aggregate
Mesoscale dynamics on the Sun's surface from HINODE observations
Aims: The interactions of velocity scales on the Sun's surface, from
granulation to supergranulation are still not understood, nor are their
interaction with magnetic fields. We thus aim at giving a better description of
dynamics in the mesoscale range which lies between the two scales mentioned
above. Method: We analyse a 48h high-resolution time sequence of the quiet Sun
photosphere at the disk center obtained with the Solar Optical Telescope
onboard Hinode. The observations, which have a field of view of 100
\arcsec 100 \arcsec, typically contain four supergranules. We monitor
in detail the motion and evolution of granules as well as those of the radial
magnetic field. Results: This analysis allows us to better characterize Trees
of Fragmenting Granules issued from repeated fragmentation of granules,
especially their lifetime statistics. Using floating corks advected by measured
velocity fields, we show their crucial role in the advection of the magnetic
field and in the build up of the network. Finally, thanks to the long duration
of the time series, we estimate that the turbulent diffusion coefficient
induced by horizontal motion is approximately . Conclusions: These results demonstrate that the long living
families contribute to the formation of the magnetic network and suggest that
supergranulation could be an emergent length scale building up as small
magnetic elements are advected and concentrated by TFG flows. Our estimate for
the magnetic diffusion associated with this horizontal motion might provide a
useful input for mean-field dynamo models.Comment: to appear in A&A - 8 pages, 13 figures (degraded quality) - Full
resolution version available @
http://www.ast.obs-mip.fr/users/rincon/hinode_roudier_aa09.pd
Graphene Ripples as a Realization of a Two-Dimensional Ising Model: A Scanning Tunneling Microscope Study
Ripples in pristine freestanding graphene naturally orient themselves in an
array that is alternately curved-up and curved-down; maintaining an average
height of zero. Using scanning tunneling microscopy (STM) to apply a local
force, the graphene sheet will reversibly rise and fall in height until the
height reaches 60-70 percent of its maximum at which point a sudden, permanent
jump occurs. We successfully model the ripples as a spin-half Ising magnetic
system, where the height of the graphene is the spin. The permanent jump in
height, controlled by the tunneling current, is found to be equivalent to an
antiferromagnetic-to-ferromagnetic phase transition. The thermal load
underneath the STM tip alters the local tension and is identified as the
responsible mechanism for the phase transition. Four universal critical
exponents are measured from our STM data, and the model provides insight into
the statistical role of graphenes unusual negative thermal expansion
coefficient.Comment: 12 pages, 5 figures, 1 tabl
Comparison of Magnetic Flux Distribution between a Coronal Hole and a Quiet Region
Employing Big Bear Solar Observatory (BBSO) deep magnetograms and H
images in a quiet region and a coronal hole, observed on September 14 and 16,
2004, respectively, we have explored the magnetic flux emergence, disappearance
and distribution in the two regions. The following results are obtained: (1)
The evolution of magnetic flux in the quiet region is much faster than that in
the coronal hole, as the flux appeared in the form of ephemeral regions in the
quiet region is 4.3 times as large as that in the coronal hole, and the flux
disappeared in the form of flux cancellation, 2.9 times as fast as in the
coronal hole. (2) More magnetic elements with opposite polarities in the quiet
region are connected by arch filaments, estimating from magnetograms and
H images. (3) We measured the magnetic flux of about 1000 magnetic
elements in each observing region. The flux distribution of network and
intranetwork (IN) elements is similar in both polarities in the quiet region.
For network fields in the coronal hole, the number of negative elements is much
more than that of positive elements. However for the IN fields, the number of
positive elements is much more than that of negative elements. (4) In the
coronal hole, the fraction of negative flux change obviously with different
threshold flux density. 73% of the magnetic fields with flux density larger
than 2 Gauss is negative polarity, and 95% of the magnetic fields is negative,
if we only measure the fields with their flux density larger than 20 Gauss. Our
results display that in a coronal hole, stronger fields is occupied by one
predominant polarity; however the majority of weaker fields, occupied by the
other polarity
Interpretation of Light-Quenching Factor Measurements
We observe that the pattern of the quenching factors for scintillation light
from various ions, recently studied in in connection with dark matter
detectors, can be understood as a saturation phenomenon in which the light
output is simply proportional to track length, independent of the ion and its
energy. This observation is in accord with the high dE/dx limit of Birks' law.
It suggests a simple model for the intrinsic resolution of light detectors for
low energy ions, which we briefly discuss.Comment: Seven pages, seven figures, some with colo
Dynamic reconfiguration of human brain networks during learning
Human learning is a complex phenomenon requiring flexibility to adapt
existing brain function and precision in selecting new neurophysiological
activities to drive desired behavior. These two attributes -- flexibility and
selection -- must operate over multiple temporal scales as performance of a
skill changes from being slow and challenging to being fast and automatic. Such
selective adaptability is naturally provided by modular structure, which plays
a critical role in evolution, development, and optimal network function. Using
functional connectivity measurements of brain activity acquired from initial
training through mastery of a simple motor skill, we explore the role of
modularity in human learning by identifying dynamic changes of modular
organization spanning multiple temporal scales. Our results indicate that
flexibility, which we measure by the allegiance of nodes to modules, in one
experimental session predicts the relative amount of learning in a future
session. We also develop a general statistical framework for the identification
of modular architectures in evolving systems, which is broadly applicable to
disciplines where network adaptability is crucial to the understanding of
system performance.Comment: Main Text: 19 pages, 4 figures Supplementary Materials: 34 pages, 4
figures, 3 table
III.5 Stellar classification in CoRoT faint stars fields
This book is dedicated to all the people interested in the CoRoT mission and the beautiful data that were delivered during its six year duration. Either amateurs, professional, young or senior researchers, they will find treasures not only at the time of this publication but also in the future twenty or thirty years. It presents the data in their final version, explains how they have been obtained, how to handle them, describes the tools necessary to understand them, and where to find them. It also highlights the most striking first results obtained up to now. CoRoT has opened several unexpected directions of research and certainly new ones still to be discovered
Energy Conservation Constraints on Multiplicity Correlations in QCD Jets
We compute analytically the effects of energy conservation on the
self-similar structure of parton correlations in QCD jets. The calculations are
performed both in the constant and running coupling cases. It is shown that the
corrections are phenomenologically sizeable. On a theoretical ground, energy
conservation constraints preserve the scaling properties of correlations in QCD
jets beyond the leading log approximation.Comment: 11 pages, latex, 5 figures, .tar.gz version avaliable on
ftp://www.inln.unice.fr
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