1,740 research outputs found
Extreme nonlinear electrodynamics in metamaterials with very small linear dielectric permittivity
We consider a sub-wavelength periodic layered medium whose slabs are filled
by arbitrary linear metamaterials and standard nonlinear Kerr media and we show
that the homogenized medium behaves as a Kerr medium whose parameters can
assume values not available in standard materials. Exploiting such a parameter
availability, we focus on the situation where the linear relative dielectric
permittivity is very small thus allowing the observation of the extreme
nonlinear regime where the nonlinear polarization is comparable with or even
greater than the linear part of the overall dielectric response. The behavior
of the electromagnetic field in the extreme nonlinear regime is very peculiar
and characterized by novel features as, for example, the transverse power flow
reversing. In order to probe the novel regime, we consider a class of fields
(transverse magnetic nonlinear guided waves) admitting full analytical
description and we show that these waves are allowed to propagate even in media
with since the nonlinear polarization produces a
positive overall effective permittivity. The considered nonlinear waves
exhibit, in addition to the mentioned features, a number of interesting
properties like hyper-focusing induced by the phase difference between the
field components.Comment: 12 pages, 7 figure
Complete controllability of quantum systems
Sufficient conditions for complete controllability of -level quantum
systems subject to a single control pulse that addresses multiple allowed
transitions concurrently are established. The results are applied in particular
to Morse and harmonic-oscillator systems, as well as some systems with
degenerate energy levels. Morse and harmonic oscillators serve as models for
molecular bonds, and the standard control approach of using a sequence of
frequency-selective pulses to address a single transition at a time is either
not applicable or only of limited utility for such systems.Comment: 8 pages, expanded and revised versio
LAND COVER CHARACTERIZATION USING MULTITEMPORAL RED, NEAR-IR, AND THERMAL-IR DATA FROM NOAA/AVHRR
A simple land cover classification scheme is proposed based on energy absorption and exchange properties of various land cover types, observable from remote sensing. Seasonal trajectories of the Normalized Difference Vegetation Index (NDVI) and surface temperature (Ts), routinely available from NOAA/AVHRR (National Oceanic and Atmospheric Administration/Advanced Very High Resolution Radiometer), are used to characterize different land cover types into four groups: water limited (shrub, grass), energy limited (wetlands, boreal forests, snow, ice, and water), atmospherically coupled (aerodynamically rough canopies, forests), and atmospherically decoupled (aerodynamically smooth canopies, crops). Further separation is achieved using growing-season average NDVI for shrub and grass, seasonal NDVI amplitude for deciduous vs. evergreen, and near-infrared (NIR) reflectance for broadleaf vs. needleleaf vegetation.
The methodology using threshold-based rules is completely remote sensing based; classification rules are simple and easily modifiable. A first test of this logic over the continental United States, when compared with existing maps, showed that the methodology adequately captures the spatial distribution of various land cover types. The logic is also useful for monitoring seasonal dynamics of land cover, evapotranspiration, and disturbances due to fire, floods, insects/disease, and other anthropogenic processes. Future improvements needed to deal with mixed landscapes and global implementation details are discussed
Estimation of Regional Surface Resistance to Evapotranspiration from NDVI and Thermal-IR AVHRR Data
Infrared surface temperatures from satellite sensors have been used to infer evaporation and soil moisture distribution over large areas. However, surface energy partitioning to latent versus sensible heat changes with surface vegetation cover and water availability. We tested a hypothesis that the relationship between surface temperature and canopy density is sensitive to seasonal changes in canopy resistance of conifer forests. Surface temperature (Ts) and canopy density were computed for a 20 × 25 km forested region in Montana, from the NOAA/AVHRR for 8 days during the summer of 1985. A forest ecosystem model, FOREST-BGC, simulated canopy resistance (Rc) for the same period.
For all eight days. surface temperatures had high association with canopy density, measured as Normalized Difference Vegetation Index (NDVI) (R2 = 0.73 − 0.91), implying that latent heat exchange is the major cause of spatial variations in surface radiant temperatures. The slope of Ts and NDVI, σ, was sensitive to changes in canopy resistance on two contrasting days of canopy activity. The trajectory of σ followed seasonal changes in canopy resistance simulated by the model. The relationship found between σ and Rc (R2 = 0.92), was nonlinear, expected because Rc values beyond 20 s cm−1 do not influence energy partitioning significantly. The slope of Ts and NDVI, σ, could provide a useful parameterization of surface resistance in regional evapotranspiration research
Imaging the Near Field
In an earlier paper we introduced the concept of the perfect lens which
focuses both near and far electromagnetic fields, hence attaining perfect
resolution. Here we consider refinements of the original prescription designed
to overcome the limitations of imperfect materials. In particular we show that
a multi-layer stack of positive and negative refractive media is less sensitive
to imperfections. It has the novel property of behaving like a fibre-optic
bundle but one that acts on the near field, not just the radiative component.
The effects of retardation are included and minimized by making the slabs
thinner. Absorption then dominates image resolution in the near-field. The
deleterious effects of absorption in the metal are reduced for thinner layers.Comment: RevTeX, (9 pages, 8 figures
Sensitivity of Moderate Resolution Imaging Spectroradiometer (MODIS) Terrestrial Primary Production to the Accuracy of Meteorological Reanalyses
The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA\u27s satellites, Terra and Aqua, dramatically improves our ability to accurately and continuously monitor the terrestrial biosphere. MODIS information is used to estimate global terrestrial primary production weekly and annually in near-real time at a 1-km resolution. MODIS terrestrial primary production requires daily gridded assimilation meteorological data as inputs, and the accuracy of the existing meteorological reanalysis data sets show marked differences both spatially and temporally. This study compares surface meteorological data sets from three well-documented global reanalyses, NASA Data Assimilation Office (DAO), European Centre for Medium-Range Weather Forecasts (ECMWF) (ERA-40) and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis 1, with observed weather station data and other gridded data interpolated from the observations, to evaluate the sensitivity of MODIS global terrestrial gross and net primary production (GPP and NPP) to the uncertainties of meteorological inputs both in the United States and the global vegetated areas. NCEP tends to overestimate surface solar radiation, and underestimate both temperature and vapor pressure deficit (VPD). ECMWF has the highest accuracy but its radiation is lower in tropical regions, and the accuracy of DAO lies between NCEP and ECMWF. Biases in temperature are mainly responsible for large VPD biases in reanalyses. MODIS NPP contains more uncertainties than GPP. Global total MODIS GPP and NPP driven by DAO, ECMWF, and NCEP show notable differences (\u3e20 Pg C/yr) with the highest estimates from NCEP and the lowest from ECMWF. Again, the DAO results lie somewhere between NCEP and ECMWF estimates. Spatially, the larger discrepancies among reanalyses and their derived MODIS GPP and NPP occur in the tropics. These results reveal that the biases in meteorological reanalyses can introduce substantial error into GPP and NPP estimations, and emphasize the need to minimize these biases to improve the quality of MODIS GPP and NPP products
Developing Satellite-derived Estimates of Surface Moisture Status
Recent research has shown that the combination of spectral vegetation indices with thermal infrared observations may provide an effective method for parameterizing surface processes at large spatial scales. In this paper, we explore the remotely sensed surface temperature (Ts)/normalized difference vegetation index (NDVI) relationship regarding a) influence of biome type on the slope of Ts/NDVI, b) automating the definition of the relationship so that the surface moisture status can he compared with Ts/NDVI at continental scales. The analysis was carded out using 1) NOAA Advanced Very High Resolution Radiometer (AVHRR) data over a 300-km × 300-km area in western Montana under various land-use practices (grass, crops, and forests), 2) Earth Resources Observations Systems Data Center continental United States biweekly composite AVHRR data.
A strong negative relationship was observed between NDVI and Ts over all biome types. The similarity of the Ts/NDVI relationships over different biomes indicated that fraction of vegetation cover has strong influence on the spatial variability of Ts. A substantial change in the Ts/NDVI relationship was observed over forests between wet and dry days. In comparison, no change was observed over irrigated crops.
Results from the automated approach agreed well with those using manual selection. At continental scales, the slope of Ts/NDVI is strongly correlated to crop-moisture index values indicating that Ts/NDVI relation is sensitive to surface moisture conditions. Upon further development, this relationship may be useful for parameterizing surface moisture conditions in climate models, decomposition studies, and fire weather monitoring
The effect of gain saturation in a gain compensated perfect lens
The transmission of evanescent waves in a gain-compensated perfect lens is
discussed. In particular, the impact of gain saturation is included in the
analysis, and a method for calculating the fields of such nonlinear systems is
developed. Gain compensation clearly improves the resolution; however, a number
of nonideal effects arise as a result of gain saturation. The resolution
associated with the lens is strongly dependent on the saturation constant of
the active medium.Comment: to appear in J. Opt. Soc. Am.
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