1,835 research outputs found
The INSU and DMN network of ST radars
Due to their capabilities of measuring wind profiles with good time and height resolution, Stratosphere-Troposphere (ST) are well adapted to carry out atmospheric research. In France, a Very High Frequency (VHF) and an Ultrahigh Frequency (UHF) ST radar are working for research purposes. The INSU (Institut National des Sciences de l'Univers) and the DMN (Direction de la Meteorologie Nationale) networks are discussed
Magic composite pulses
I describe composite pulses during which the average dipolar interactions
within a spin ensemble are controlled while realizing a global rotation. The
construction method used is based on the average Hamiltonian theory and rely on
the geometrical properties of the spin-spin dipolar interaction only. I present
several such composite pulses robust against standard experimental defects in
NRM: static or radio-frequency field miscalibration, fields inhomogeneities.
Numerical simulations show that the magic sandwich pulse sequence, a pulse
sequence that reverse the average dipolar field while applied, is plagued by
defects originating from its short initial and final \pi/2 radio-frequency
pulses. Using the magic composite pulses instead of \pi/2 pulses improves the
magic sandwich effect. A numerical test using a classical description of NMR
allows to check the validity of the magic composite pulses and estimate their
efficiency.Comment: 22 pages, 6 figure
A calcareous nannofossil and organic geochemical study of marine palaeoenvironmental changes across the Sinemurian/Pliensbachian (early Jurassic, ~191Ma) in Portugal
The Sinemurian/Pliensbachian boundary (~ 191 Ma) is acknowledged as one of the most important steps in the radiation of planktonic organisms, especially primary producers such as dinoflagellates and coccolithophores. To date, there is no detailed study documenting changes in planktonic assemblages related to palaeoceanographic changes across this boundary. The aim of this study is to characterize the palaeoenvironmental changes occurring across the Sinemurian/Pliensbachian boundary at the São Pedro de Moel section (Lusitanian Basin, Portugal) using micropalaeontology and organic geochemistry approaches. Combined calcareous nannofossil assemblage and lipid biomarker data document for a decrease in primary productivity in relation to a major sea-level rise occurring above the boundary. The Lusitanian Basin was particularly restricted during the late Sinemurian with a relatively low sea level, a configuration that led to the recurrent development of black shales. After a sharp sea-level fall, the basin became progressively deeper and more open during the earliest Pliensbachian, subsequently to a major transgression. This sea-level increase seems to have been a global feature and could have been related to the opening of the Hispanic Corridor that connected the Tethys and palaeo-Pacific oceans. The palaeoceanographic and palaeoclimatic changes induced by this opening may have played a role in the diversification of coccolithophores with the first occurrence or colonization of Tethyan waters by placolith-type coccoliths
Model compilation: An approach to automated model derivation
An approach is introduced to automated model derivation for knowledge based systems. The approach, model compilation, involves procedurally generating the set of domain models used by a knowledge based system. With an implemented example, how this approach can be used to derive models of different precision and abstraction is illustrated, and models are tailored to different tasks, from a given set of base domain models. In particular, two implemented model compilers are described, each of which takes as input a base model that describes the structure and behavior of a simple electromechanical device, the Reaction Wheel Assembly of NASA's Hubble Space Telescope. The compilers transform this relatively general base model into simple task specific models for troubleshooting and redesign, respectively, by applying a sequence of model transformations. Each transformation in this sequence produces an increasingly more specialized model. The compilation approach lessens the burden of updating and maintaining consistency among models by enabling their automatic regeneration
Discriminative Parameter Estimation for Random Walks Segmentation
The Random Walks (RW) algorithm is one of the most e - cient and easy-to-use
probabilistic segmentation methods. By combining contrast terms with prior
terms, it provides accurate segmentations of medical images in a fully
automated manner. However, one of the main drawbacks of using the RW algorithm
is that its parameters have to be hand-tuned. we propose a novel discriminative
learning framework that estimates the parameters using a training dataset. The
main challenge we face is that the training samples are not fully supervised.
Speci cally, they provide a hard segmentation of the images, instead of a
proba- bilistic segmentation. We overcome this challenge by treating the opti-
mal probabilistic segmentation that is compatible with the given hard
segmentation as a latent variable. This allows us to employ the latent support
vector machine formulation for parameter estimation. We show that our approach
signi cantly outperforms the baseline methods on a challenging dataset
consisting of real clinical 3D MRI volumes of skeletal muscles.Comment: Medical Image Computing and Computer Assisted Interventaion (2013
The CoRoT target HD 49933: 2- Comparison of theoretical mode amplitudes with observations
From the seismic data obtained by CoRoT for the star HD 49933 it is possible,
as for the Sun, to constrain models of the excitation of acoustic modes by
turbulent convection. We compare a stochastic excitation model described in
Paper I (arXiv:0910.4027) with the asteroseismology data for HD 49933, a star
that is rather metal poor and significantly hotter than the Sun. Using the mode
linewidths measured by CoRoT for HD 49933 and the theoretical mode excitation
rates computed in Paper I, we derive the expected surface velocity amplitudes
of the acoustic modes detected in HD 49933. Using a calibrated quasi-adiabatic
approximation relating the mode amplitudes in intensity to those in velocity,
we derive the expected values of the mode amplitude in intensity. Our amplitude
calculations are within 1-sigma error bars of the mode surface velocity
spectrum derived with the HARPS spectrograph. The same is found with the mode
amplitudes in intensity derived for HD 49933 from the CoRoT data. On the other
hand, at high frequency, our calculations significantly depart from the CoRoT
and HARPS measurements. We show that assuming a solar metal abundance rather
than the actual metal abundance of the star would result in a larger
discrepancy with the seismic data. Furthermore, calculations that assume the
``new'' solar chemical mixture are in better agreement with the seismic data
than those that assume the ``old'' solar chemical mixture. These results
validate, in the case of a star significantly hotter than the Sun and Alpha Cen
A, the main assumptions in the model of stochastic excitation. However, the
discrepancies seen at high frequency highlight some deficiencies of the
modelling, whose origin remains to be understood.Comment: 8 pages, 3 figures (B-W and color), accepted for publication in
Astronomy & Astrophysics. Corrected typo in Eq. (4). Updated references.
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