1,658 research outputs found
Validation of agent-based land use model by Markovian model : application to forest-agriculture transitions in Madagascar
The determination of transition rules that farmers adopt to manage crop-fallow after forest clearing, is essential for deciding a sustainable strategy for forest conservation. The effect of the type of farms with respect to these transition rules in forest border may mitigate incentive measures planned by forest conservation policy. Agent-base modeling (ABM) of land use is a relevant approach to manage the dynamics of heterogeneous mosaic landscapes such as the border of the Malagasy Eastern rainforest. Transition rules between six land uses (forest, fallow, crop, grass, plantation and paddy ?eld) are formalized at a plot level. A historical database containing transitions between the ?rst four land use states was used to calibrate transition models for the ecological and farmer land use dynamics. Three land-use models have been built: (1) a Markov chain (stochastic), (2) a timed automaton (deterministic), (3) and an agent-based model, which introduces the farmers. The land use ABM allows to test scenarios of deforestation with both varying initial population and farm spatial organization, size or strategy. The land use ABM is ?rst calibrated via a timed automaton, ?tting time delay parameters, the duration of each land use state (fallow, crop, grass), and the number of cropping cycles since the ?rst forest clearing. It is then validated with the help of a Markovian model, comparing two transition matrices with ?2 metrics. The two transition matrices were respectively created with historical data of plot land use, and with simulated data produced by the land use ABM. We ?nish with a general discussion on the validation of such a complex system with a simple mathematical model. (Résumé d'auteur
Superlattice Patterns in Surface Waves
We report novel superlattice wave patterns at the interface of a fluid layer
driven vertically. These patterns are described most naturally in terms of two
interacting hexagonal sublattices. Two frequency forcing at very large aspect
ratio is utilized in this work. A superlattice pattern ("superlattice-I")
consisting of two hexagonal lattices oriented at a relative angle of 22^o is
obtained with a 6:7 ratio of forcing frequencies. Several theoretical
approaches that may be useful in understanding this pattern have been proposed.
In another example, the waves are fully described by two superimposed hexagonal
lattices with a wavelength ratio of sqrt(3), oriented at a relative angle of
30^o. The time dependence of this "superlattice-II" wave pattern is unusual.
The instantaneous patterns reveal a time-periodic stripe modulation that breaks
the 6-fold symmetry at any instant, but the stripes are absent in the time
average. The instantaneous patterns are not simply amplitude modulations of the
primary standing wave. A transition from the superlattice-II state to a 12-fold
quasi-crystalline pattern is observed by changing the relative phase of the two
forcing frequencies. Phase diagrams of the observed patterns (including
superlattices, quasicrystalline patterns, ordinary hexagons, and squares) are
obtained as a function of the amplitudes and relative phases of the driving
accelerations.Comment: 15 pages, 14 figures (gif), to appear in Physica
Influence of the Coulomb Interaction on the Chemical Equilibrium of Nuclear Systems at Break-Up
The importance of a Coulomb correction to the formalism proposed by Albergo
et al. for determining the temperatures of nuclear systems at break-up and the
ensities of free nucleon gases is discussed. While the proposed correction has
no effect on the temperatures extracted based on double isotope ratios, it
becomes non-negligible when such temperatures or densities of free nucleon
gases are extracted based on multiplicities of heavier fragments of different
atomic numbers
Scale-free static and dynamical correlations in melts of monodisperse and Flory-distributed homopolymers: A review of recent bond-fluctuation model studies
It has been assumed until very recently that all long-range correlations are
screened in three-dimensional melts of linear homopolymers on distances beyond
the correlation length characterizing the decay of the density
fluctuations. Summarizing simulation results obtained by means of a variant of
the bond-fluctuation model with finite monomer excluded volume interactions and
topology violating local and global Monte Carlo moves, we show that due to an
interplay of the chain connectivity and the incompressibility constraint, both
static and dynamical correlations arise on distances . These
correlations are scale-free and, surprisingly, do not depend explicitly on the
compressibility of the solution. Both monodisperse and (essentially)
Flory-distributed equilibrium polymers are considered.Comment: 60 pages, 49 figure
Do quasi-regular structures really exist in the solar photosphere? I. Observational evidence
Two series of solar-granulation images -- the La Palma series of 5 June 1993
and the SOHO MDI series of 17--18 January 1997 -- are analysed both
qualitatively and quantitatively. New evidence is presented for the existence
of long-lived, quasi-regular structures (first reported by Getling and Brandt
(2002)), which no longer appear unusual in images averaged over 1--2-h time
intervals. Such structures appear as families of light and dark concentric
rings or families of light and dark parallel strips (``ridges'' and
``trenches'' in the brightness distributions). In some cases, rings are
combined with radial ``spokes'' and can thus form ``web'' patterns. The
characteristic width of a ridge or trench is somewhat larger than the typical
size of granules. Running-average movies constructed from the series of images
are used to seek such structures. An algorithm is developed to obtain, for
automatically selected centres, the radial distributions of the azimuthally
averaged intensity, which highlight the concentric-ring patterns. We also
present a time-averaged granulation image processed with a software package
intended for the detection of geological structures in aerospace images. A
technique of running-average-based correlations between the brightness
variations at various points of the granular field is developed and indications
are found for a dynamical link between the emergence and sinking of hot and
cool parcels of the solar plasma. In particular, such a correlation analysis
confirms our suggestion that granules -- overheated blobs -- may repeatedly
emerge on the solar surface. Based on our study, the critical remarks by Rast
(2002) on the original paper by Getling and Brandt (2002) can be dismissed.Comment: 21 page, 8 figures; accepted by "Solar Physics
Dissipation in ferrofluids: Mesoscopic versus hydrodynamic theory
Part of the field dependent dissipation in ferrofluids occurs due to the
rotational motion of the ferromagnetic grains relative to the viscous flow of
the carrier fluid. The classical theoretical description due to Shliomis uses a
mesoscopic treatment of the particle motion to derive a relaxation equation for
the non-equilibrium part of the magnetization. Complementary, the hydrodynamic
approach of Liu involves only macroscopic quantities and results in dissipative
Maxwell equations for the magnetic fields in the ferrofluid. Different stress
tensors and constitutive equations lead to deviating theoretical predictions in
those situations, where the magnetic relaxation processes cannot be considered
instantaneous on the hydrodynamic time scale. We quantify these differences for
two situations of experimental relevance namely a resting fluid in an
oscillating oblique field and the damping of parametrically excited surface
waves. The possibilities of an experimental differentiation between the two
theoretical approaches is discussed.Comment: 14 pages, 2 figures, to appear in PR
Hazardous near Earth asteroid mitigation campaign planning based on uncertain information on fundamental asteroid characteristics
Given a limited warning time, an asteroid impact mitigation campaign would hinge on uncertainty-based information consisting of remote observational data of the identified Earth-threatening object, general knowledge of near-Earth asteroids (NEAs), and engineering judgment. Due to these ambiguities, the campaign credibility could be profoundly compromised. It is therefore imperative to comprehensively evaluate the inherent uncertainty in deflection and plan the campaign accordingly to ensure successful mitigation. This research demonstrates dual-deflection mitigation campaigns consisting of primary (instantaneous/quasi-instantaneous) and secondary (slow-push) deflection missions, where both deflection efficiency and campaign credibility are taken into account. The results of the dual-deflection campaign analysis show that there are trade-offs between the competing aspects: the launch cost, mission duration, deflection distance, and the confidence in successful deflection. The design approach is found to be useful for multi-deflection campaign planning, allowing us to select the best possible combination of missions from a catalogue of campaign options, without compromising the campaign credibility
Angular Dependent Soft X-ray Absorption Spectroscopy of V2O5 and V6O13
We have measured the V 2p3/2 and O 1s x-ray absorption spectra of single
crystal V2O5 and V6O13 and compared to linear combinations of atomic orbitals
(LCAO) density of states (DOS) calculations. The spectra change dramatically
with incident angle. The use of polarized light and a single crystal limits the
number of transitions possible, revealing spectral features that cannot be
resolved on polycrystals (angle-integrated). The measured O 1s and V 2p3/2
spectra agree with the projected unoccupied O 2p and V 3d DOS respectively,
indicating that atomic and solid state effects, including V-O hybridization,
must be included to adequately describe the spectra.Comment: PostScript, 9 pages, 3 figure
Argon annealing of the oxygen-isotope exchanged manganite La_{0.8}Ca_{0.2}MnO_{3+y}
We have resolved a controversial issue concerning the oxygen-isotope shift of
the ferromagnetic transition temperature T_{C} in the manganite
La_{0.8}Ca_{0.2}MnO_{3+y}. We show that the giant oxygen-isotope shift of T_C
observed in the normal oxygen-isotope exchanged samples is indeed intrinsic,
while a much smaller shift observed in the argon annealed samples is an
artifact. The argon annealing causes the 18O sample to partially exchange back
to the 16O isotope due to a small 16O contamination in the Ar gas. Such a
contamination is commonly caused by the oxygen outgas that is trapped in the
tubes, connectors and valves. The present results thus umambiguously
demonstrate that the observed large oxygen isotope effect is an intrinsic
property of manganites, and places an important constraint on the basic physics
of these materials.Comment: 4 pages, 3 figures, submitted to PR
Isotope effect in impure high T_c superconductors
The influence of various kinds of impurities on the isotope shift exponent
\alpha of high temperature superconductors has been studied. In these materials
the dopant impurities, like Sr in La_{2-x}Sr_xCuO_4, play different role and
usually occupy different sites than impurities like Zn, Fe, Ni {\it etc}
intentionally introduced into the system to study its superconducting
properties.
In the paper the in-plane and out-of-plane impurities present in layered
superconductors have been considered. They differently affect the
superconducting transition temperature T_c. The relative change of isotope
shift coefficient, however, is an universal function of T_c/T_{c0} (T_{c0}
reffers to impurity free system) {\it i.e.} for angle independent scattering
rate and density of states function it does not depend whether the change of
T_c is due to in- or out-of-plane impurities. The role of the anisotropic
impurity scattering in changing oxygen isotope coefficient of superconductors
with various symmetries of the order parameter is elucidated. The comparison of
the calculated and experimental dependence of \alpha/\alpha_0, where \alpha_0
is the clean system isotope shift coefficient, on T_c/T_{c0} is presented for a
number of cases studied.
The changes of \alpha calculated within stripe model of superconductivity in
copper oxides resonably well describe the data on
La_{1.8}Sr_{0.2}Cu_{1-x}(Fe,Ni)_xO_4, without any fitting parameters.Comment: 8 pages, 6 figures, Phys. Rev. B67 (2003) accepte
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