490 research outputs found
Fusion approach for remotely sensed mapping of agriculture (FARMA):A scalable open source method for land cover monitoring using data fusion
The increasing availability of very-high resolution (VHR; <2 m) imagery has the potential to enable agricultural monitoring at increased resolution and cadence, particularly when used in combination with widely available moderate-resolution imagery. However, scaling limitations exist at the regional level due to big data volumes and processing constraints. Here, we demonstrate the Fusion Approach for Remotely Sensed Mapping of Agriculture (FARMA), using a suite of open source software capable of efficiently characterizing time-series field-scale statistics across large geographical areas at VHR resolution. We provide distinct implementation examples in Vietnam and Senegal to demonstrate the approach using WorldView VHR optical, Sentinel-1 Synthetic Aperture Radar, and Sentinel-2 and Sentinel-3 optical imagery. This distributed software is open source and entirely scalable, enabling large area mapping even with modest computing power. FARMA provides the ability to extract and monitor sub-hectare fields with multisensor raster signals, which previously could only be achieved at scale with large computational resources. Implementing FARMA could enhance predictive yield models by delineating boundaries and tracking productivity of smallholder fields, enabling more precise food security observations in low and lower-middle income countries.</p
Thermal Decay of the Cosmological Constant into Black Holes
We show that the cosmological constant may be reduced by thermal production
of membranes by the cosmological horizon, analogous to a particle ``going over
the top of the potential barrier", rather than tunneling through it. The
membranes are endowed with charge associated with the gauge invariance of an
antisymmetric gauge potential. In this new process, the membrane collapses into
a black hole, thus the net effect is to produce black holes out of the vacuum
energy associated with the cosmological constant. We study here the
corresponding Euclidean configurations ("thermalons"), and calculate the
probability for the process in the leading semiclassical approximation.Comment: 14 pages, 6 figures. Minor correction
Effect of membrane character and solution chemistry on microfiltration performance
To help understand and predict the role of natural organic matter (NOM) in the fouling of low-pressure membranes, experiments were carried out with an apparatus that incorporates automatic backwashing and long filtration runs. Three hollow fibre membranes of varying character were included in the study, and the filtration of two different surface waters was compared. The hydrophilic membrane had greater flux recovery after backwashing than the hydrophobic membranes, but the efficiency of backwashing decreased at extended filtration times. NOM concentration of these waters (7.9 and 9.1 mg/L) had little effect on the flux of the membranes at extended filtration times, as backwashing of the membrane restored the flux to similar values regardless of the NOM concentration. The solution pH also had little effect at extended filtration times. The backwashing efficiency of the hydrophilic membrane was dramatically different for the two waters, and the presence of colloid NOM alone could not explain these differences. It is proposed that colloidal NOM forms a filter cake on the surface of the membranes and that small molecular weight organics that have an adsorption peak at 220 nm but not 254 nm were responsible for “gluing” the colloids to the membrane surface. Alum coagulation improved membrane performance in all instances, and this was suggested to be because coagulation reduced the concentration of “glue” that holds the organic colloids to the membrane surface
Threshold configurations in the presence of Lorentz violating dispersion relations
A general characterization of lower and upper threshold configurations for
two particle reactions is determined under the assumptions that the single
particle dispersion relations E(p) are rotationally invariant and monotonic in
p, and that energy and momentum are conserved and additive for multiple
particles. It is found that at a threshold the final particle momenta are
always parallel and the initial momenta are always anti-parallel. The
occurrence of new phenomena not occurring in a Lorentz invariant setting, such
as upper thresholds and asymmetric pair production thresholds, is explained,
and an illustrative example is given.Comment: 5 pages, 3 figure
Classical Solutions in a Lorentz-violating Maxwell-Chern-Simons Electrodynamics
We take as starting point the planar model arising from the dimensional
reduction of the Maxwell Electrodynamics with the (Lorentz-violating)
Carroll-Field-Jackiw term. We then write and study the extended Maxwell
equations and the corresponding wave equations for the potentials. The solution
to these equations show some interesting deviations from the usual MCS
Electrodynamics, with background-dependent correction terms. In the case of a
time-like background, the correction terms dominate over the MCS sector in the
region far from the origin, and establish the behaviour of a massless
Electrodynamics (in the electric sector). In the space-like case, the solutions
indicate the clear manifestation of spatial anisotropy, which is consistent
with the existence of a privileged direction is space.Comment: latex, 8 page
A no-go for no-go theorems prohibiting cosmic acceleration in extra dimensional models
A four-dimensional effective theory that arises as the low-energy limit of
some extra-dimensional model is constrained by the higher dimensional Einstein
equations. Steinhardt & Wesley use this to show that accelerated expansion in
our four large dimensions can only be transient in a large class of
Kaluza-Klein models that satisfy the (higher dimensional) null energy condition
[1]. We point out that these no-go theorems are based on a rather ad-hoc
assumption on the metric, without which no strong statements can be made.Comment: 20 page
A realisation of Lorentz algebra in Lorentz violating theory
A Lorentz non-invariant higher derivative effective action in flat spacetime,
characterised by a constant vector, can be made invariant under infinitesimal
Lorentz transformations by restricting the allowed field configurations. These
restricted fields are defined as functions of the background vector in such a
way that background dependance of the dynamics of the physical system is no
longer manifest. We show here that they also provide a field basis for the
realisation of Lorentz algebra and allow the construction of a Poincar\'e
invariant symplectic two form on the covariant phase space of the theory.Comment: text body edited, reference adde
Propagators and WKB-exactness in the plane wave limit of AdSxS
Green functions for the scalar, spinor and vector fields in a plane wave
geometry arising as a Penrose limit of are obtained. The
Schwinger-DeWitt technique directly gives the results in the plane wave
background, which turns out to be WKB-exact. Therefore the structural
similarity with flat space results is unveiled. In addition, based on the local
character of the Penrose limit, it is claimed that for getting the correct
propagators in the limit one can rely on the first terms of the direct geodesic
contribution in the Schwinger-DeWitt expansion of the original propagators .
This is explicitly shown for the Einstein Static Universe, which has the same
Penrose limit as with equal radii, and for a number of other
illustrative cases.Comment: 18 pages, late
Spontaneous Breaking of Lorentz Invariance
We describe how a stable effective theory in which particles of the same
fermion number attract may spontaneously break Lorentz invariance by giving
non-zero fermion number density to the vacuum (and therefore dynamically
generating a chemical potential term). This mecanism yields a finite vacuum
expectation value could relate to work on signals of Lorentz violation
in electrodynamics.Comment: revtex4, 11 pages, 5 figures; v2:references added; v3:more references
added, typos fixed, some points in sect. IV clarified; v4:even more
references added, discussion in sect. V extended; v5:replaced to match
published version (minor corrections of form
Effects of sea temperature and stratification changes on seabird breeding success
As apex predators in marine ecosystems, seabirds may primarily experience climate change impacts indirectly, via changes to their food webs. Observed seabird population declines have been linked to climate-driven oceanographic and food web changes. However, relationships have often been derived from relatively few colonies and consider only sea surface temperature (SST), so important drivers, and spatial variation in drivers, could remain undetected. Further, explicit climate change projections have rarely been made, so longer-term risks remain unclear. Here, we use tracking data to estimate foraging areas for eleven black-legged kittiwake (Rissa tridactyla) colonies in the UK and Ireland, thus reducing reliance on single colonies and allowing calculation of colony-specific oceanographic conditions. We use mixed models to consider how SST, the potential energy anomaly (indicating density stratification strength) and the timing of seasonal stratification influence kittiwake productivity. Across all colonies, higher breeding success was associated with weaker stratification before breeding and lower SSTs during the breeding season. Eight colonies with sufficient data were modelled individually: higher productivity was associated with later stratification at three colonies, weaker stratification at two, and lower SSTs at one, whilst two colonies showed no significant relationships. Hence, key drivers of productivity varied among colonies. Climate change projections, made using fitted models, indicated that breeding success could decline by 21 – 43% between 1961-90 and 2070-99. Climate change therefore poses a longer-term threat to kittiwakes, but as this will be mediated via availability of key prey species, other marine apex predators could also face similar threats
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