8,150 research outputs found
Mapeamento de áreas de campo limpo úmido no Distrito Federal a partir de fusão de imagens multiespectrais.
Generic differential geometry of hyperbolic plane curves
We study hyperbolic invariants of hyperbolic plane curves as applications of the singularity theory of smooth function
Elastic Convection in Vibrated Viscoplastic Fluids
We observe a new type of behavior in a shear thinning yield stress fluid:
freestanding convection rolls driven by vertical oscillation. The convection
occurs without the constraint of container boundaries yet the diameter of the
rolls is spontaneously selected for a wide range of parameters. The transition
to the convecting state occurs without hysteresis when the amplitude of the
plate acceleration exceeds a critical value. We find that a non-dimensional
stress, the stress due to the inertia of the fluid normalized by the yield
stress, governs the onset of the convective motion.Comment: 4 pages, 6 figure
Modeling tree cover changes in a pasture-dominated landscape by adopting silvopastoral practices in a dry forest region in Central Brazil.
Sustained Magnetorotational Turbulence in Local Simulations of Stratified Disks with Zero Net Magnetic Flux
We examine the effects of density stratification on magnetohydrodynamic
turbulence driven by the magnetorotational instability in local simulations
that adopt the shearing box approximation. Our primary result is that, even in
the absence of explicit dissipation, the addition of vertical gravity leads to
convergence in the turbulent energy densities and stresses as the resolution
increases, contrary to results for zero net flux, unstratified boxes. The ratio
of total stress to midplane pressure has a mean of ~0.01, although there can be
significant fluctuations on long (>~50 orbit) timescales. We find that the time
averaged stresses are largely insensitive to both the radial or vertical aspect
ratio of our simulation domain. For simulations with explicit dissipation, we
find that stratification extends the range of Reynolds and magnetic Prandtl
numbers for which turbulence is sustained. Confirming the results of previous
studies, we find oscillations in the large scale toroidal field with periods of
~10 orbits and describe the dynamo process that underlies these cycles.Comment: 13 pages, 18 figures, submitted to Ap
Monitoramento e detecção de desmatamento no bioma Cerrado matogrossense utilizando imagens de multisensores.
In the last decades, Brazil has become a global agricultural power and the Cerrado biome (Brazilian savanna) has been playing an important role in the Brazilian agriculture growth. To better analyze the biome human disturbance dynamics, it is necessary to develop and adopt effective methods of assessment and monitoring of land use and land cover changes. The goal is to provide adequate land cover classifications and implement an operational monitoring system in the Cerrado biome, since there is only a few attempts to control the degradation of this biome. This monitoring system can be accomplished using MODIS images, as this sensor has great potential for studies about the seasonal dynamics of Cerrado vegetation phytophysiognomies. Due to this new dynamics, the main objective of this work was to apply the PRODES and DETER like methodologies to detect and map deforestation in the Cerrado biome of Mato Grosso State, Brazil, using Landsat and MODIS data. The proposed methodology was able to detect correctly 65% of all MODIS detected polygons; this represented 74% of estimated area of deforestation. Also, it showed suitability to identify new deforested areas in both shrubland and forestland areas with a tendency to misclassify smaller polygons (< 50 ha) of deforestation
The Effect of the Hall Term on the Nonlinear Evolution of the Magnetorotational Instability: I. Local Axisymmetric Simulations
The effect of the Hall term on the evolution of the magnetorotational
instability (MRI) in weakly ionized accretion disks is investigated using local
axisymmetric simulations. First, we show that the Hall term has important
effects on the MRI when the temperature and density in the disk is below a few
thousand K and between 10^13 and 10^18 cm^{-3} respectively. Such conditions
can occur in the quiescent phase of dwarf nova disks, or in the inner part
(inside 10 - 100 AU) of protoplanetary disks. When the Hall term is important,
the properties of the MRI are dependent on the direction of the magnetic field
with respect to the angular velocity vector \Omega. If the disk is threaded by
a uniform vertical field oriented in the same sense as \Omega, the axisymmetric
evolution of the MRI is an exponentially growing two-channel flow without
saturation. When the field is oppositely directed to \Omega, however, small
scale fluctuations prevent the nonlinear growth of the channel flow and the MRI
evolves into MHD turbulence. These results are anticipated from the
characteristics of the linear dispersion relation. In axisymmetry on a field
with zero-net flux, the evolution of the MRI is independent of the size of the
Hall term relative to the inductive term. The evolution in this case is
determined mostly by the effect of ohmic dissipation.Comment: 31 pages, 3 tables, 12 figures, accepted for publication in ApJ,
postscript version also available from
http://www.astro.umd.edu/~sano/publications
Superconductivity of the One-Dimensional d-p Model with p-p transfer
Using the numerical diagonalization method, we investigate the
one-dimensional - model, simulating a Cu-O linear chain with strong
Coulomb repulsions. Paying attention to the effect of the transfer energy
between the nearest neighbor oxygen-sites, we calculate the critical
exponent of correlation functions based on the Luttinger liquid
relations and the ground state energy as a function of an external
flux . We find that the transfer increases the charge
susceptibility and the exponent in cooperation with the repulsion
at Cu-site. We also show that anomalous flux quantization occurs for
. The superconducting region is presented on a phase diagram of
vs. plane.Comment: 4 pages, RevTex + 5 PS figures include
Coronae as Consequence of Large Scale Magnetic Fields in Turbulent Accretion Disks
Non-thermal X-ray emission in compact accretion engines can be interpreted to
result from magnetic dissipation in an optically thin magnetized corona above
an optically thick accretion disk. If coronal magnetic field originates in the
disk and the disk is turbulent, then only magnetic structures large enough for
their turbulent shredding time to exceed their buoyant rise time survive the
journey to the corona. We use this concept and a physical model to constrain
the minimum fraction of magnetic energy above the critical scale for buoyancy
as a function of the observed coronal to bolometric emission. Our results
suggest that a significant fraction of the magnetic energy in accretion disks
resides in large scale fields, which in turn provides circumstantial evidence
for significant non-local transport phenomena and the need for large scale
magnetic field generation. For the example of Seyfert AGN, for which of order
30 per cent of the bolometric flux is in the X-ray band, we find that more than
20 per cent of the magnetic energy must be of large enough scale to rise and
dissipate in the corona.Comment: submitted to ApJL, 2 fig
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