2,231 research outputs found
Near-Limb Zeeman and Hanle Diagnostics
"Weak" magnetic-field diagnostics in faint objects near the bright solar disk
are discussed in terms of the level of non-object signatures, in particular, of
the stray light in telescopes. Calculated dependencies of the stray light
caused by diffraction at the 0.5-, 1.6-, and 4-meter entrance aperture are
presented. The requirements for micro-roughness of refractive and reflective
primary optics are compared. Several methods for reducing the stray light (the
Lyot coronagraphic technique, multiple stages of apodizing in the focal and
exit pupil planes, apodizing in the entrance aperture plane with a special
mask), and reducing the random and systematic errors are noted. An acceptable
level of stray light in telescopes is estimated for the V-profile recording
with a signal-to-noise ratio greater than three. Prospects for the limb
chromosphere magnetic measurements are indicated.Comment: 11 pages, 3 figure
Diffuse continuum transfer in H II regions
We compare the accuracy of various methods for determining the transfer of
the diffuse Lyman continuum in HII regions, by comparing them with a
high-resolution discrete-ordinate integration. We use these results to suggest
how, in multidimensional dynamical simulations, the diffuse field may be
treated with acceptable accuracy without requiring detailed transport
solutions. The angular distribution of the diffuse field derived from the
numerical integration provides insight into the likely effects of the diffuse
field for various material distributions.Comment: 12 pages, 7 figures, to be published in MNRA
Hydrodynamic equations for self-propelled particles: microscopic derivation and stability analysis
Considering a gas of self-propelled particles with binary interactions, we
derive the hydrodynamic equations governing the density and velocity fields
from the microscopic dynamics, in the framework of the associated Boltzmann
equation. Explicit expressions for the transport coefficients are given, as a
function of the microscopic parameters of the model. We show that the
homogeneous state with zero hydrodynamic velocity is unstable above a critical
density (which depends on the microscopic parameters), signaling the onset of a
collective motion. Comparison with numerical simulations on a standard model of
self-propelled particles shows that the phase diagram we obtain is robust, in
the sense that it depends only slightly on the precise definition of the model.
While the homogeneous flow is found to be stable far from the transition line,
it becomes unstable with respect to finite-wavelength perturbations close to
the transition, implying a non trivial spatio-temporal structure for the
resulting flow. We find solitary wave solutions of the hydrodynamic equations,
quite similar to the stripes reported in direct numerical simulations of
self-propelled particles.Comment: 33 pages, 11 figures, submitted to J. Phys.
Boltzmann and hydrodynamic description for self-propelled particles
We study analytically the emergence of spontaneous collective motion within
large bidimensional groups of self-propelled particles with noisy local
interactions, a schematic model for assemblies of biological organisms. As a
central result, we derive from the individual dynamics the hydrodynamic
equations for the density and velocity fields, thus giving a microscopic
foundation to the phenomenological equations used in previous approaches. A
homogeneous spontaneous motion emerges below a transition line in the
noise-density plane. Yet, this state is shown to be unstable against spatial
perturbations, suggesting that more complicated structures should eventually
appear.Comment: 4 pages, 3 figures, final versio
Remittances and the Dutch disease in Sub-Saharan Africa. A Dynamic Panel Approach
This paper investigates the effect of remittance inflows on the real exchange rate in Sub-Saharan Africa (SSA) using annual data from 1980 to 2008 for 34 SSA countries, generalised method of moments by Arellano and Bover (1995) andfeasible generalised least squares by Parks (1967) and Kmenta (1971, 1986). We find that when cross-sectional dependence and individual effects are controlled for, remittances to SSA as a whole appreciate the underlying real exchange rate ofrecipient countries. However the Dutch-disease effect is not experienced via the loss of export competitiveness, because the exchange rate appreciation is mitigated by monetary policy positioning and overdependence on imports due to low levels of domestic production in these countries. We also find reverse causality between remittances and the real exchange rate.Dutch disease, remittances, real exchange rate, Sub-Saharan Africa
Image-Derived Input Function Derived from a Supervised Clustering Algorithm: Methodology and Validation in a Clinical Protocol Using [11C](R)-Rolipram
Image-derived input function (IDIF) obtained by manually drawing carotid arteries (manual-IDIF) can be reliably used in [11C](R)-rolipram positron emission tomography (PET) scans. However, manual-IDIF is time consuming and subject to inter- and intra-operator variability. To overcome this limitation, we developed a fully automated technique for deriving IDIF with a supervised clustering algorithm (SVCA). To validate this technique, 25 healthy controls and 26 patients with moderate to severe major depressive disorder (MDD) underwent T1-weighted brain magnetic resonance imaging (MRI) and a 90-minute [11C](R)-rolipram PET scan. For each subject, metabolite-corrected input function was measured from the radial artery. SVCA templates were obtained from 10 additional healthy subjects who underwent the same MRI and PET procedures. Cluster-IDIF was obtained as follows: 1) template mask images were created for carotid and surrounding tissue; 2) parametric image of weights for blood were created using SVCA; 3) mask images to the individual PET image were inversely normalized; 4) carotid and surrounding tissue time activity curves (TACs) were obtained from weighted and unweighted averages of each voxel activity in each mask, respectively; 5) partial volume effects and radiometabolites were corrected using individual arterial data at four points. Logan-distribution volume (VT/fP) values obtained by cluster-IDIF were similar to reference results obtained using arterial data, as well as those obtained using manual-IDIF; 39 of 51 subjects had a VT/fP error of 10%. With automatic voxel selection, cluster-IDIF curves were less noisy than manual-IDIF and free of operator-related variability. Cluster-IDIF showed widespread decrease of about 20% [11C](R)-rolipram binding in the MDD group. Taken together, the results suggest that cluster-IDIF is a good alternative to full arterial input function for estimating Logan-VT/fP in [11C](R)-rolipram PET clinical scans. This technique enables fully automated extraction of IDIF and can be applied to other radiotracers with similar kinetics.ope
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