Spatial Drift Research Articles

Solution of Fokker-Planck equation for a broad class of drift and diffusion coefficients

A Langevin equation with variable drift and diffusion coefficients separable in time and space and its corresponding Fokker-Planck equation in the Stratonovich approach are considered. From this Fokker-Planck equation a class of exact solutions with the same spatial drift and diffusion coefficients is obtained. Furthermore, some details of this system are analyzed by using the spatial diffusion coefficient D(x) = sqrt[D]|x|(-θ/2).

Genetic population structure, fitness variation and the importance of population history in remnant populations of the endangered plant Silene chlorantha (Willd.) Ehrh. (Caryophyllaceae)

Habitat fragmentation can lead to a decline of genetic diversity, a potential risk for the survival of natural populations. Fragmented populations can become highly differentiated due to reduced gene flow and genetic drift. A decline in number of individuals can result in lower reproductive fitness due to inbreeding effects. We investigated genetic variation within and between 11 populations of the rare and endangered plant Silene chlorantha in northeastern Germany to support conservation strategies. Genetic diversity was evaluated using AFLP techniques and the results were correlated to fitness traits. Fitness evaluation in nature and in a common garden approach was conducted. Our analysis revealed population differentiation was high and within population genetic diversity was intermediate. A clear population structure was supported by a Bayesian approach, AMOVA and neighbour-joining analysis. No correlation between genetic and geographic distance was found. Our results indicate that patterns of population differentiation were mainly caused by temporal and/or spatial isolation and genetic drift. The fitness evaluation revealed that pollinator limitation and habitat quality seem, at present, to be more important to reproductive fitness than genetic diversity by itself. Populations of S. chlorantha with low genetic diversity have the potential to increase in individual number if habitat conditions improve. This was detected in a single large population in the investigation area, which was formerly affected by bottleneck effects.

Controlled AFM manipulation of small nanoparticles and assembly of hybridnanostructures

We demonstrate controlled manipulation of semiconductor and metallic nanoparticles(NPs) with 5–15 nm diameters and assemble these NPs into hybrid structures. Themanipulation is accomplished under ambient environment using a commercial atomic forcemicroscope (AFM). There are particular difficulties associated with manipulating NPs thissmall. In addition to spatial drift, the shape of an asymmetric AFM tip has to be takeninto account in order to understand the intended and actual manipulation results.Furthermore, small NPs often attach to the tip via electrostatic interaction and modify theeffective tip shape. We suggest a method for detaching the NPs by performing apseudo-manipulation step. Finally, we show by example the ability to assemble these smallNPs into prototypical hybrid nanostructures with well-defined composition and geometry.

Spatial dimensions increase the effect of cultural drift

The transition from hunter-gathering to agriculture (Neolithic) spread gradually across Europe from the Southeast. A reduction in cultural diversity of crop farming practices has been previously observed by comparing pre-LBK Neolithic sites in Greece and the Balkans (dated about 8500 yr BP) to LBK Neolithic sites in Central Europe (dated about 7000 yr BP). The decrease in crop diversity is statistically significant even when considering only the species less likely to have been subject to smaller productivity due to climatic factors (reductions in growing season, temperature, daylight, etc.). This reduction in cultural diversity has not been explained previously. In this paper we show that spatial drift, which occurred on the front of the advancing wave of pioneer settlements, can explain the observed loss of diversity during the LBK range expansion. Our results suggest that spatial dimensions can have a relevant effect also in other case studies in which cultural drift is important.

Seasonal and fine-scale spatial drift patterns in a tufadepositing barrage hydrosystem

We examined seasonal patterns of drifting macroinvertebrates, and transported moss and (in)organic particulate matter at a fine spatial scale within the tufa-depositing barrage hydrosystem of Plitvice Lakes, Croatia. Samples were collected at six sites along a stream reach during autumn, winter, spring and summer. The sampling sites differed in flow velocity, depending on their position within the study reach and distance from tufa barriers and associated waterfalls. Seasonality was pronounced for most physico-chemical measurements. The highest concentrations of transported (in)organic particulate matter, being related to increased amounts of moss in transport, were observed in autumn. Drift densities were highest at tufa barriers that had higher flow velocities. Drift abundance partially followed the seasonal pattern in transported (in)organic matter. Autumn peaks in drift of Oligochaeta, Coleoptera as well as some Diptera and Trichoptera were related to increased moss and particulate matter quantities, while observed increases in drifting Nematoda, Cladocera, Copepoda, Ephemeroptera, Simuliidae and Chironomidae in spring and summer were probably due to distribution shifts of larvae or to higher seston levels originating from the upstream lake. Although we found that faster flowing stream sites generally had higher drift densities, there typically was no significant correlation between flow velocity and the amount of drift. Therefore, our expectation that fine-scale spatial drift patterns primarily depend on flow differences between lotic habitats was not supported for this system. The positive correlation between transported moss and other measured drift parameters, both at the seasonal and fine spatial scale, suggests that drift patterns were influenced by trapping/refuge efficiency of moss-covered tufa substrate and respective seasonal changes in instream vegetation (moss) as linked to hydrochemistry and water temperature.

Single-molecule binding experiments on long time scales

We describe an approach for performing single-molecule binding experiments on time scales from hours to days, allowing for the observation of slower kinetics than have been previously investigated by single-molecule techniques. Total internal reflection fluorescence microscopy is used to image the binding of labeled ligand to molecules specifically coupled to the surface of an optically transparent flow cell. Long-duration experiments are enabled by ensuring sufficient positional, chemical, thermal, and image stability. Principal components of this experimental stability include illumination timing, solution replacement, and chemical treatment of solution to reduce photodamage and photobleaching; and autofocusing to correct for spatial drift.

Optimization of EFTEM image acquisition by using elastically filtered images for drift correction

Because of its high spatial resolution, energy-filtering transmission electron microscopy (EFTEM) has become widely used for the analysis of the chemical composition of nanostructures. To obtain the best spatial resolution, the precise correction of instrumental influences and the optimization of the data acquisition procedure are very important. In this publication, we discuss a modified image acquisition procedure that optimizes the acquisition process of the EFTEM images, especially for long exposure times and measurements that are affected by large spatial drift. To alleviate the blurring of the image caused by the spatial drift, we propose to take several EFTEM images with a shorter exposure time (sub-images) and merge these sub-images afterwards. To correct for the drift between these sub-images, elastically filtered images are acquired between two subsequent sub-images. These elastically filtered images are highly suitable for spatial drift correction based on the cross-correlation method. The use of the drift information between two elastically filtered images permits to merge the drift-corrected sub-images automatically and with high accuracy, resulting in sharper edges and an improved signal intensity in the final EFTEM image. Artefacts that are caused by prominent noise-peaks in the dark reference image have been suppressed by calculating the dark reference image from three images. Furthermore, using the information given by the elastically filtered images, it is possible to drift-correct a set of EFTEM images already during the acquisition. This simplifies the post-processing for elemental mapping and offers the possibility for active drift correction using the image shift function of the microscope, leading to an increased field of view.

Requantization transcoding for H.264/AVC video coding

In this paper, efficient solutions for requantization transcoding in H.264/AVC are presented. By requantizing residual coefficients in the bitstream, different error components can appear in the transcoded video stream. Firstly, a requantization error is present due to successive quantization in encoder and transcoder. In addition to the requantization error, the loss of information caused by coarser quantization will propagate due to dependencies in the bitstream. Because of the use of intra prediction and motion-compensated prediction in H.264/AVC, both spatial and temporal drift propagation arise in transcoded H.264/AVC video streams. The spatial drift in intra-predicted blocks results from mismatches in the surrounding prediction pixels as a consequence of requantization. In this paper, both spatial and temporal drift components are analyzed. As is shown, spatial drift has a determining impact on the visual quality of transcoded video streams in H.264/AVC. In particular, this type of drift results in serious distortion and disturbing artifacts in the transcoded video stream. In order to avoid the spatially propagating distortion, we introduce transcoding architectures based on spatial compensation techniques. By combining the individual temporal and spatial compensation approaches and applying different techniques based on the picture and/or macroblock type, overall architectures are obtained that provide a trade-off between computational complexity and rate-distortion performance. The complexity of the presented architectures is significantly reduced when compared to cascaded decoder–encoder solutions, which are typically used for H.264/AVC transcoding. The reduction in complexity is particularly large for the solution which uses spatial compensation only. When compared to traditional solutions without spatial compensation, both visual and objective quality results are highly improved.

Improved terahertz two-color plasma sources pumped by high intensity laser beam

We investigate the intensity dependent spatial drift of two-color plasma based terahertz (THz) sources. A simple scheme that uses an off-axis parabolic mirror is presented to overcome this shifting. In addition, the THz energy and electric field measurements are related via the real time images of the THz spot size.

Random partnerships in spatial game theory

We study the effects of random partnerships, introduced to interaction or replacement networks, on the evolution of cooperation in spatial game theory on a square lattice. For the spatial prisoner dilemma game, we show that, compared with the case without random partnership, cooperation can be enhanced regardless of whether a random partnership is introduced to an interaction or replacement network. Specifically, the enhancement of cooperation is strongest in the limit of zero randomness. We show explicitly that the cooperator frequency is a decreasing function of randomness, and the cooperation eventually vanishes once the randomness is strong enough. For the spatial snow drift game, we find that the enhancement of cooperation occurs only when a random partnership is introduced to an interaction network.

Impact of Balloon Drift Errors in Radiosonde Data on Climate Statistics

Abstract Radiosonde data are a valuable resource in the detection of climate change in the upper atmosphere. Long time series of stratospheric temperature data, carefully screened and corrected to remove errors, are available for this purpose. Normal reporting practice usually ascribes a fixed time and position (the station location) to all data reported in the ascent. In reality, the ascent may take around 90 min to complete and the spatial drift of the radiosonde may exceed 200 km. This note examines the magnitude of the errors associated with this practice using simulated radiosonde data generated from the ECMWF reanalysis archive. The results suggest that the temperature errors, while generally small in the troposphere, are locally significant in the stratosphere, particularly in the jet stream areas. However, the impact of the drift errors on global climate statistics is very small. Errors in the wind and humidity data are also examined.

EFTEM spectrum imaging at high-energy resolution

This paper deals with the application of high-energy resolution EFTEM image series and the corrections needed for reliable data interpretation. The detail of spectral information gained from an image series is largely determined by the intrinsic energy resolution. In this work we show that energy resolution values of as low as 0.8 eV in spectra extracted from EFTEM image series can be obtained with a small energy-selecting slit. At this resolution level aberrations of the energy filter, in particular the non-isochromaticity, can no longer be neglected. We show that the four most prominent factors for EFTEM image series data correction—spatial drift, non-isochromaticity, energy drift and image distortion—must not be treated independently but have to be corrected in unison. We present an efficient algorithm for this correction, and demonstrate the applied correction for the case of a GaN/AlN multilayer sample.

Separation and Conversion Dynamics of Four Nuclear Spin Isomers of Ethylene

Molecules with three or more nuclei of nonzero spin exist as discrete spin isomers whose interconversion in the gas phase is generally considered improbable. We have studied the interconversion process in ethylene by creating a sample depleted in the B2u nuclear spin isomer. The separation was achieved through spatial drift of this isomer induced by resonant absorption of narrow-band infrared light. Evolution of the depleted sample revealed conversion between B2u and B3u isomers at a rate linearly proportional to pressure, with a rate constant of 5.5 (+/-0.8) x 10(-4) s(-1) torr(-1). However, almost no change was observed in the Ag isomer populations. The results suggest a spin conversion mechanism in C2H4 via quantum relaxation within the same inversion symmetry.

High‐resolution FMRI at 1.5T using balanced SSFP

The resolution in conventional BOLD FMRI is considerably lower than can be achieved with other MRI methods, and is insufficient for many important applications. One major difficulty in robustly improving spatial resolution is the poor image quality in BOLD FMRI, which suffers from distortions, blurring, and signal dropout. This work considers the potential for increased resolution with a new FMRI method based on balanced SSFP. This method establishes a blood oxygenation sensitive steady-state (BOSS) signal, in which the frequency sensitivity of balanced SSFP is used to detect the frequency shift of deoxyhemoglobin. BOSS FMRI is highly SNR efficient and does not suffer from image distortions or signal dropout, making this method an excellent candidate for high-resolution FMRI. This study presents the first demonstration of high-resolution BOSS FMRI, using an efficient 3D stack-of-segmented EPI readout and combined acquisition at multiple center frequencies. BOSS FMRI is shown to enable high-resolution FMRI data (1 x 1 x 2 mm(3)) in both visual and motor systems using standard hardware at 1.5 T. Currently, the major limitation of BOSS FMRI is its sensitivity to temporal and spatial field drift.

Spatial Models in Marketing

Marketing science models typically assume that responses of one entity (firm or consumer) are unrelated to responses of other entities. In contrast, models constructed using tools from spatial statistics allow for cross-sectional and longitudinal correlations among responses to be explicitly modeled by locating entities on some type of map. By generalizing the notion of a map to include demographic and psychometric representations, spatial models can capture a variety of effects (spatial lags, spatial autocorrelation, and spatial drift) that impact firm or consumer decision behavior. Marketing science applications of spatial models and important research opportunities are discussed.

High gene flow between populations of Macrotermes michaelseni (Isoptera, Termitidae)

Termite alates are thought to be poor active flyers, and this should lead to considerable genetic differentiation on small spatial scales. However, using four microsatellite loci for the termite Macrotermes michaelseni we found low values of genetic differentiation (F ST ) across a spatial scale of even more than 50 km. Genetic differentiation between populations increased with spatial distance up to 50 km. Furthermore, up to this distance, the scatter around the linear regression of genetic differentiation versus spatial distance increased with spatial distance. This suggests that across such spatial distances gene flow and genetic drift are of about equal importance, and near equilibrium. Using a regional F ST as well as the distance between populations with non-significant F ST -values (up to 25 km), gene flow is sufficiently high so that populations may be regarded as panmictic on spatial scales of 25 to 50 km. The apparent contradiction between dispersal distances observed in the field and estimates of gene flow from genetic markers may be due to the masses of swarming alates. Assuming a leptokurtic distribution of dispersal distances, atleast some alates are expected to travel considerable distances, most likely by passive drift.

Ionospheric irregularities in the low‐latitude valley region observed with the Equatorial Atmosphere Radar

The geomagnetically low‐latitude valley region between the upper E region and the lower F1 region is studied with the Equatorial Atmosphere Radar (EAR) in Indonesia. Three‐meter‐scale field‐aligned irregularity echoes have been frequently observed in the valley region in association with the equatorial spread F (ESF) in the period from sunset to midnight. The valley region echoes usually appear at above 150 km and propagate downward with time. Rapid beam scanning of the EAR revealed that spatial structure, temporal variation, and drift velocity of the valley region echoes resemble those of ESF, which indicates that the dynamoelectric field in the equatorial F region controls the low‐latitude valley region irregularities. Perturbed electric fields associated with ESF map down to the low‐latitude valley region and can produce the perturbed plasma density structures as “images” of ESF structures. Image structure is effectively formed at altitudes below 200 km and is a source of the valley region irregularities observed with the EAR. It is suggested that intermediate layers should supply a plasma density gradient for excitation of 3‐m‐scale irregularities in the valley region through gradient drift instability.

Efficiency evaluation of MEV spatial sampling strategies: a scenario analysis

The minimum estimation variance (MEV) spatial sampling strategy is compared with some further strategies based on systematic designs and the sample mean. Since in environmental surveys data are usually collected repeatedly in time at sites whose selection is based on practical circumstances only, it seems worth measuring the efficiency of spatial sampling strategies through the expectation of design mean square errors under several superpopulation models assumed about the fixed population generating process. Relating the study to specific superpopulations, MEV efficiencies can be calculated and a quantitative evaluation is made possible by the use of scenario analyses for several sample sizes and different models of spatial drift and correlations. The MEV strategy revealed itself to be the relatively more efficient one under realistic conditions of nonstationary spatial drifts and bounded sample sizes.

Automated spatial drift correction for EFTEM image series

Energy filtering transmission electron microscopy (EFTEM) is a widely used technique in many areas of scientific research. Image contrast in energy-filtered images arises from specific scattering events such as the ionization of atoms. By combining a set of two or more images, relative sample thickness maps or elemental distribution maps can be easily created. It is also possible to acquire a whole series of energy-filtered images to do more complex data analysis. However, whenever several images are combined to extract certain information, problems are introduced due to sample drift between the exposures. In order to obtain artifact-free information, this spatial drift has to be taken care of. Manual alignment by overlaying and shifting the images to find the best overlap is usually very accurate but extremely time consuming for larger data sets. When large amounts of images are recorded in an EFTEM series, manual correction is no longer a reasonable option. Hence, automatic routines have been developed that are mostly based on the cross-correlation algorithm. Existing routines, however, sometimes fail and again make time consuming manual adjustments necessary. In this paper we describe a new approach to the drift correction problem by incorporating a statistical treatment of the data and we present our statistically determined spatial drift (SDSD) correction program. We show its improved performance by applying it to a typical EFTEM series data block.

SMART--a program to measure SEM resolution and imaging performance.

It is important to be able to measure the parameters, such as spatial resolution, astigmastism, signal-to-noise ratio, and drift and instability, that characterize the performance of a scanning electron microscope. These quantities can be determined most reliably by a Fourier analysis of digital micrographs from the instrument, recorded under conditions of interest. A program designed to implement all of the necessary steps in an automated manner has been developed as a 'macro' for the popular, and freely available, NIH Image and SCION Image programs.