2,101 research outputs found
Simulating spatial variability of cereal yields from historical yield maps and satellite imagery
[Abstract]: The management of spatial variability of crop yields relies on the availability of affordable and accurate spatial data. Yield maps are a direct measure of the crop yields, however, costs and difficulties in collection and processing to generate yield maps results in poor availability of such data in Australia. In this study, we used historical mid-season normalised difference vegetation index (NDVI), generated from Landsat imagery over 4 years. Using linear regression model, the NDVI was compared to the actual yield map from a 257 ha paddock. The difference between actual and predicted yield showed that 77% and 93% of the paddock area had an error of <20% and <30%, respectively. The linear model obtained in the paddock was used to simulate crop yield for an adjoining paddock of 162 ha. On an average of 4 years, the difference between actual and simulated yield showed that 87% of the paddock had an error of <20%. However, this error varied from season to season. Paddock area with <20% error increased exponentially with decreasing in-crop rainfall between anthesis and crop maturity. Furthermore, the error in simulating crop yield also varied with the soil constraints. Paddock zones with high concentrations of subsoil chloride and surface soil exchangeable sodium percentage generally had higher percent of error in simulating crop yields. Satellite imagery consistently over-predicted cereal yields in areas with subsoil constraints, possibly due to chloride-induced water stress during grain filling. The simulated yield mapping methodology offers an opportunity to identify within-field spatial variability using satellite imagery as a surrogate measure of biomass. However, the ability to successfully simulate crop yields at farm scale or regional scale requires wider evaluation across different soil types and climatic conditions
Magnification relations in gravitational lensing via multidimensional residue integrals
We investigate the so-called magnification relations of gravitational lensing
models. We show that multidimensional residue integrals provide a simple
explanation for the existence of these relations, and an effective method of
computation. We illustrate the method with several examples, thereby deriving
new magnification relations for galaxy lens models and microlensing (point mass
lensing).Comment: 16 pages, uses revtex4, submitted to Journal of Mathematical Physic
Electrical expression of spin accumulation in ferromagnet/semiconductor structures
We treat the spin injection and extraction via a ferromagnetic
metal/semiconductor Schottky barrier as a quantum scattering problem. This
enables the theory to explain a number of phenomena involving spin-dependent
current through the Schottky barrier, especially the counter-intuitive spin
polarization direction in the semiconductor due to current extraction seen in
recent experiments. A possible explanation of this phenomenon involves taking
into account the spin-dependent inelastic scattering via the bound states in
the interface region. The quantum-mechanical treatment of spin transport
through the interface is coupled with the semiclassical description of
transport in the adjoining media, in which we take into account the in-plane
spin diffusion along the interface in the planar geometry used in experiments.
The theory forms the basis of the calculation of spin-dependent current flow in
multi-terminal systems, consisting of a semiconductor channel with many
ferromagnetic contacts attached, in which the spin accumulation created by spin
injection/extraction can be efficiently sensed by electrical means. A
three-terminal system can be used as a magnetic memory cell with the bit of
information encoded in the magnetization of one of the contacts. Using five
terminals we construct a reprogrammable logic gate, in which the logic inputs
and the functionality are encoded in magnetizations of the four terminals,
while the current out of the fifth one gives a result of the operation.Comment: A review to appear in Mod. Phys. Lett.
Bringing closure to microlensing mass measurement
Interferometers offer multiple methods for studying microlensing events and
determining the properties of the lenses. We investigate the study of
microlensing events with optical interferometers, focusing on narrow-angle
astrometry, visibility, and closure phase. After introducing the basics of
microlensing and interferometry, we derive expressions for the signals in each
of these three channels. For various forecasts of the instrumental performance,
we discuss which method provides the best means of measuring the lens angular
Einstein radius theta_E, a prerequisite for determining the lens mass. If the
upcoming generation of large-aperture, AO-corrected long baseline
interferometers (e.g. VLTI, Keck, OHANA) perform as well as expected, theta_E
may be determined with signal-to-noise greater than 10 for all bright events.
We estimate that roughly a dozen events per year will be sufficiciently bright
and have long enough durations to allow the measurement of the lens mass and
distance from the ground. We also consider the prospects for a VLTI survey of
all bright lensing events using a Fisher matrix analysis, and find that even
without individual masses, interesting constraints may be placed on the bulge
mass function, although large numbers of events would be required.Comment: 23 pages, aastex, submitted to Ap
The imprints of primordial non-gaussianities on large-scale structure: scale dependent bias and abundance of virialized objects
We study the effect of primordial nongaussianity on large-scale structure,
focusing upon the most massive virialized objects. Using analytic arguments and
N-body simulations, we calculate the mass function and clustering of dark
matter halos across a range of redshifts and levels of nongaussianity. We
propose a simple fitting function for the mass function valid across the entire
range of our simulations. We find pronounced effects of nongaussianity on the
clustering of dark matter halos, leading to strongly scale-dependent bias. This
suggests that the large-scale clustering of rare objects may provide a
sensitive probe of primordial nongaussianity. We very roughly estimate that
upcoming surveys can constrain nongaussianity at the level |fNL| <~ 10,
competitive with forecasted constraints from the microwave background.Comment: 16 pages, color figures, revtex4. v2: added references and an
equation. submitted to PRD. v3: simplified derivation, additional reference
A New Survey for Giant Arcs
We report on the first results of an imaging survey to detect strong
gravitational lensing targeting the richest clusters selected from the
photometric data of the Sloan Digital Sky Survey (SDSS) with follow-up deep
imaging observations from the Wisconsin Indiana Yale NOAO (WIYN) 3.5m telescope
and the University of Hawaii 88-inch telescope (UH88). The clusters are
selected from an area of 8000 deg^2 using the Red Cluster Sequence technique
and span the redshift range 0.1 < z < 0.6, corresponding to a comoving
cosmological volume of ~ 2 Gpc^3. Our imaging survey thus targets a volume more
than an order of magnitude larger than any previous search. A total of 240
clusters were imaged of which 141 had sub-arcsecond image quality. Our survey
has uncovered16 new lensing clusters with definite giant arcs, an additional 12
systems for which the lensing interpretation is very likely, and 9 possible
lenses which contain shorter arclets or candidate arcs which are less certain
and will require further observations to confirm their lensing origin. The
number of new cluster lenses detected in this survey is likely > 30. Among
these new systems are several of the most dramatic examples of strong
gravitational lensing ever discovered with multiple bright arcs at large
angular separation. These will likely become 'poster-child' gravitational
lenses similar to Abell 1689 and CL0024+1654. The new lenses discovered in this
survey will enable future sysetmatic studies of the statistics of strong
lensing and its implications for cosmology and our structure formation
paradigm.Comment: 19 pages, 7 pages of Figures, submitted to AJ. Fixed Typo
β-Adrenergic Receptor Stimulation Induces Endoplasmic Reticulum Stress in Adult Cardiac Myocytes: Role in Apoptosis
Accumulation of misfolded proteins and alterations in calcium homeostasis induces endoplasmic reticulum (ER) stress, leading to apoptosis. In this study, we tested the hypothesis that β-AR stimulation induces ER stress, and induction of ER stress plays a pro-apoptotic role in cardiac myocytes. Using thapsigargin and brefeldin A, we demonstrate that ER stress induces apoptosis in adult rat ventricular myocytes (ARVMs). β-AR-stimulation (isoproterenol; 3h) significantly increased expression of ER stress proteins, such as GRP-78, Gadd-153, and Gadd-34, while activating caspase-12 in ARVMs. In most parts, these effects were mimicked by thapsigargin. β-AR stimulation for 15 min increased PERK and eIF-2α phosphorylation. PERK phosphorylation remained higher, while eIF-2α phosphorylation declined thereafter, reaching to ∼50% below basal levels at 3 h after β-AR stimulation. This decline in eIF-2a phosphorylation was prevented by β1-AR, not by β2-AR antagonist. Forskolin, adenylyl cyclase activator, simulated the effects of ISO on eIF-2α phosphorylation. Salubrinal (SAL), an ER stress inhibitor, maintained eIF-2α phosphorylation and inhibited β-ARstimulated apoptosis. Furthermore, inhibition of caspase-12 using z-ATAD inhibited β-AR-stimulated and thapsigargininduced apoptosis. In vivo, β-AR stimulation induced ER stress in the mouse heart as evidenced by increased expression of GRP-78 and Gadd-153, activation of caspase-12, and dephosphorylation of eIF-2α. SAL maintained phosphorylation of eIF-2α, inhibited activation of caspase-12, and decreased β-AR-stimulated apoptosis in the heart. Thus, β-AR stimulation induces ER stress in cardiac myocytes and in the heart, and induction of ER stress plays a pro-apoptotic role
Self-Lensing Models of the LMC
All of the proposed explanations for the microlensing events observed towards
the LMC have difficulties. One of these proposed explanations, LMC
self-lensing, which invokes ordinary LMC stars as the long sought-after lenses,
has recently gained considerable popularity as a possible solution to the
microlensing conundrum. In this paper, we carefully examine the set of LMC
self-lensing models. In particular, we review the pertinent observations made
of the LMC, and show how these observations place limits on such self-lensing
models. We find that, given current observational constraints, no purely LMC
disk models are capable of producing optical depths as large as that reported
in the MACHO collaboration 2-year analysis. Besides pure disk, we also consider
alternate geometries, and present a framework which encompasses the previous
studies of LMC self-lensing. We discuss which model parameters need to be
pushed in order for such models to succeed. For example, like previous workers,
we find that an LMC halo geometry may be able to explain the observed events.
However, since all known LMC tracer stellar populations exhibit disk-like
kinematics, such models will have difficulty being reconciled with
observations. For SMC self-lensing, we find predicted optical depths differing
from previous results, but more than sufficient to explain all observed SMC
microlensing. In contrast, for the LMC we find a self-lensing optical depth
contribution between 0.47e-8 and 7.84e-8, with 2.44e-8 being the value for the
set of LMC parameters most consistent with current observations.Comment: 20 pages, Latex, 14 figures, submitted to Ap
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