Large-scale Nonuniformities Research Articles

Modeling the Wavelength Dependence of Pixel Response Nonuniformity of a CCD Sensor

Precision measurements in astronomy require stringent control of systematics such as those arising from imperfect correction of sensor effects. In this work, we develop a parametric method to model the wavelength dependence of pixel response nonuniformity (PRNU) for a laser-annealed backside-illuminated charge-coupled device. The model accurately reproduces the PRNU patterns of flat-field images taken at nine wavelengths from 290 to 950 nm, leaving the rms residuals no more than 0.2% in most cases. By removing the large-scale nonuniformity in the flat fields, the rms residuals are further reduced. This model fitting approach gives more accurate predictions of the PRNU than cubic-spline interpolation does with fewer free parameters. It can be applied to make PRNU corrections for individual objects according to their spectral energy distribution to reduce the photometry errors caused by the wavelength-dependent PRNU, if sub-percent level precision is required.

Constraining the interiors of asteroids through close encounters

ABSTRACT Knowledge of the interior density distribution of an asteroid can reveal its composition and constrain its evolutionary history. However, most asteroid observational techniques are not sensitive to interior properties. We investigate the interior constraints accessible through monitoring variations in angular velocity during a close encounter. We derive the equations of motion for a rigid asteroid’s orientation and angular velocity to arbitrary order and use them to generate synthetic angular velocity data for a representative asteroid on a close Earth encounter. We develop a toolkit AIME (Asteroid Interior Mapping from Encounters) which reconstructs asteroid density distribution from these data, and we perform injection-retrieval tests on these synthetic data to assess AIME’s accuracy and precision. We also perform a sensitivity analysis to asteroid parameters (e.g. asteroid shape and orbital elements), observational setup (e.g. measurement precision and cadence), and the mapping models used. We find that high precision in rotational period estimates (≲0.27 s) is necessary for each cadence, and that low perigees (≲ 18 Earth radii) are necessary to resolve large-scale density non-uniformities with uncertainties of $\sim 0.1{{\ \rm per\ cent}}$ of the local density under some models.

Efficiency of Mixture Separation in Distillation Columns with Structured Packings under Conditions of Dynamically Controlled Irrigation

The paper presents the results of an experimental study of separation efficiency and distribution of flow parameters over the cross-section of the distillation columns. The experiments were carried out on the large-scale research setup “Large Freon Column” with the diameter of 0.6 and 0.9 m. Experimental data were obtained at uniform and non-uniform irrigation of the Mellapak 350.Y and Sulzer 500X structured packings by the controlled liquid distributor. The mixture of refrigerants R114 and R21 was used as the working liquid at the pressure of 3 bar in the column. The liquid distributor with independently controlled valves for each drip point was used for controlled packing irrigation. Local temperature distributions of the liquid phase and counter-current vapor flow in the cross-section and over the packing height were recorded using a sequence of fast-response temperature sensors. Periodic irrigation of the structured packing can affect large-scale non-uniformity of mixture composition and flow parameter distribution over the cross-section and height of the transfer unit. Significantly non-uniform packing irrigation within a separate semiperiod leads to rearrangement of the structure of large-scale non-uniformity inside the packing.The new method for dynamically controlled irrigation of the packing is suggested. This method is aimed at destruction of stationary large-scale maldistribution over the column cross-section and achievement of higher separation efficiency of a completely wetted packing.

Propagation of a global coronal wave and its interaction with large-scale coronal magnetic structures

Context. Global coronal waves associated with solar eruptions (the so-called EIT waves) often encounter coronal holes and solar active regions and interact with these magnetic structures. This interaction leads to a number of observed effects such as wave reflection and transmission. Aims. We consider the propagation of a large-scale coronal shock wave and its interaction with large-scale non-uniformities of the background magnetic field and plasma parameters. Methods. Using the Lare2d code, we performed 2.5-dimensional simulations of the interaction of a large-scale single-pulse fast-mode magnetohydrodynamic shock wave of weak-to-moderate intensity with the region of enhanced Alfvén speed as well as with that of reduced Alfvén speed. We analysed simple models of non-uniformity and the surrounding plasma to understand the basic effects in wave propagation. Results. We found the reflected waves of plasma compression and rarefaction, transmitted waves that propagate behind or ahead of the main part of the wave, depending on properties of the plasma non-uniformity, and secondary wave fronts. The obtained results are important to the correct interpretation of the global coronal wave propagation in the solar corona, understanding of theoretical aspects of the interaction of large-scale coronal shock waves with large-scale coronal magnetic structures, and diagnostics of coronal plasma parameters.

Features of liquid mixtures separation in large-scale distillation columns with structured packing. New ideas and approaches

Negative vapor stratification along the height of distillation column caused by different density of vapor mixture components and higher temperature at the column bottom, leads to formation of large-scale maldistribution of temperature and mixture composition over the column cross-section even at uniform irrigation of the structured packing. Experimental results concerning the dynamic effect of packing irrigation on separation efficiency of the two-component mixture of R-21 and R-114 are presented in this paper. The structured packing Zulser 350Y was installed in the distillation column with the diameter of 0.9 m. Experiments were carried out on the 10- and 19-layer packing with an overall height of 2.1 and 4 m, respectively. The liquid distributor with independently controlled 126 valves for each irrigation point, developed by the authors, was used for packing irrigation. The experiments showed that the periodic impact of the irrigation system on the large-scale non-uniformity of mixture composition, formed in the packing, could significantly affect the distribution of flow parameters over the cross-section and height of the mass transfer unit. Essentially nonuniform periodic irrigation of the packing can improve the separation efficiency of the column within 20%, if the switching periods are comparable with the times of formation of large-scale non-uniformity.

Three-dimensional modeling of direct-drive cryogenic implosions on OMEGA

The effects of large-scale (with Legendre modes ≲10) laser-imposed nonuniformities in direct-drive cryogenic implosions on the OMEGA Laser System are investigated using three-dimensional hydrodynamic simulations performed using the newly developed code ASTER. Sources of these nonuniformities include an illumination pattern produced by 60 OMEGA laser beams, capsule offsets (∼10–20 μm), and imperfect pointing, power balance, and timing of the beams (with typical σrms∼10 μm, 10%, and 5 ps, respectively). Two implosion designs using 26-kJ triple-picket laser pulses were studied: a nominal design, in which an 874-μm-diameter capsule is illuminated by about the same-diameter beams, and a more hydrodynamically efficient “R75” design using a 900-μm-diameter capsule and beams of 75% of this diameter. Simulations show that nonuniformities caused by capsule offsets and beam imbalance have the largest effect on implosion performance. These nonuniformities lead to significant distortions of implosion cores, resulting in an increased residual kinetic energy and incomplete stagnation. The shape of distorted cores can be well characterized using neutron images but is less represented by 4–8 keV x-ray images. Simulated neutron spectra from perturbed implosions show large directional variations because of bulk motion effects and up to an ∼2 keV variation of the hot-spot temperature inferred from these spectra. The R75 design suffers more from illumination nonuniformities. Simulations show an advantage of this design over the nominal design when the target offset and beam power imbalance σrms are reduced to less than 5 μm and 5%, respectively.

Heel effect adaptive flat field correction of digital x-ray detectors

Anode heel effect renders large-scale background nonuniformities in digital radiographs. Conventional offset/gain calibration is performed at mono source-to-image distance (SID), and disregards the SID-dependent characteristic of heel effect. It results in a residual nonuniform background in the corrected radiographs when the SID settings for calibration and correction differ. In this work, the authors develop a robust and efficient computational method for digital x-ray detector gain correction adapted to SID-variant heel effect, without resorting to physical filters, phantoms, complicated heel effect models, or multiple-SID calibration and interpolation. The authors present the Duo-SID projection correction method. In our approach, conventional offset/gain calibrations are performed only twice, at the minimum and maximum SIDs of the system in typical clinical use. A fast iterative separation algorithm is devised to extract the detector gain and basis heel patterns from the min/max SID calibrations. The resultant detector gain is independent of SID, while the basis heel patterns are parameterized by the min- and max-SID. The heel pattern at any SID is obtained from the min-SID basis heel pattern via projection imaging principles. The system gain desired at a specific acquisition SID is then constructed using the projected heel pattern and detector gain map. The method was evaluated for flat field and anatomical phantom image corrections. It demonstrated promising improvements over interpolation and conventional gain calibration/correction methods, lowering their correction errors by approximately 70% and 80%, respectively. The separation algorithm was able to extract the detector gain and heel patterns with less than 2% error, and the Duo-SID corrected images showed perceptually appealing uniform background across the detector. The Duo-SID correction method has substantially improved on conventional offset/gain corrections for digital x-ray imaging in an SID-variant environment. The technique is relatively simple, and can be easily incorporated into multiple-point gain calibration/correction techniques. It offers a potentially valuable tool for preprocessing digital x-ray images to boost image quality of mammography, chest and cardiac radiography, as well as automated computer aided diagnostic radiology.

Direct evidence of gas-induced laser beam smoothing in the interaction with thin foils

The process of laser beam homogenization in a gas medium placed in front of a thin metallic foil has been studied. Experiments were performed using the Prague Asterix Laser System iodine laser [Jungwirth et al., Phys. Plasmas 8, 2495 (2001)] working at 0.44μm wavelength and irradiance of about 1015W∕cm2. Homogenization was detected both by directly analyzing the transmitted laser beam and by studying the shock breakout on the foil rear side. Results show that the gas ionization by the laser pulse induces a strong refraction and produces an effective smoothing of large-scale intensity nonuniformities.

Gas-induced smoothing of laser beams studied by interaction with thin foils

We studied the process of laser beam homogenization in a gas medium placed in front of a thin metallic foil. Experiments were performed using the Prague PALS iodine laser working at 0.44 µm wavelength and irradiances of about 1015 W cm−2. Homogenization was detected both by directly analysing the transmitted laser beam and by studying the shock breakout on the foil rear side. Results show that the gas ionization by the laser pulse induces a strong refraction and produces an effective smoothing of large-scale intensity non-uniformities.

Influence of Well-Width Fluctuations on the Electronic Structure of GaN/AlxGa1-xN Multiquantum Wells with Graded Interfaces

Experimental and computation results based on chemical composition assessment of metal-organic chemical vapour deposition grown undoped GaN/AlxGa1-xN multiquantum well structures in the low composition limit of x = 0.07 and wide wells demonstrate composition fluctuations in the barrier layers which lead to large-scale nonuniformities and inequivalence of the different wells. As a consequence the experimental photoluminescence spectra at low temperature show a double peak structure indicative of well-width fluctuations by one lattice parameter (2 monolayers).

Mathematical Modeling of the Formation of Type IIId Solar Decameter Radio Bursts with Echo Components

Using numerical simulation, we investigate the possibility that echo components of type IIId solar decameter bursts can be produced through refraction of radio emission in a coronal plasma with large-scale regular electron density inhomogeneities. It is shown that the observed time profiles of radio burst intensity with three or four maxima can be related to the presence in the middle corona of streamers and a localized regular nonuniformity whose electron density exceeds background electron density by a factor of more than 3 – 4. The nonuniformity scale in this case is comparable with the optical diameter of the solar disk. The observed radio burst echo components with delays longer than 3 s are explained by the production of additional radio emission propagation modes within a “transverse" refraction waveguide arising between the localized electron density nonuniformity and deeper layers in the corona. As these additional modes are reflected from the streamers, they can reach the Earth. Calculations of the time profile of radio burst intensity take into consideration the influence of scattering by turbulent coronal inhomogeneities and of collisional absorption. Comparison of the modeling results with the observational data shows that the calculated values of some profile parameters differ from the observed values. One of the possible reasons is that the method used does not take into account the diffraction leakage of radio emission through the large-scale nonuniformity.

Extended defects in GaN films grown at high growth rate

A description of the present knowledge of the layer microstructure typical forthick GaN films grown at high growth rates is presented. The GaN layers aregrown by hydride vapour phase epitaxy on sapphire and different buffers areused. The variety of extended defects present in such highly mismatchedsystem are summarized, with the emphasis on their impact on the crystalquality. The defects are reviewed in two main categories according to themicrostructural development during growth: large-scale nonuniformities andmicrostructural crystallographic defects. The first category comprisesthree-dimensional structural features developed mainly in the interface region,while the second are typical extended defects, i.e., dislocations with differentBurgers vectors, nanopipes, inversion domain boundaries and stacking faults.The quality of the layers was improved vastly as a consequence of ourunderstanding of the correlation of growth parameters and microstructure.

Mixing in fully chaotic flows

Passive scalar mixing in fully chaotic flows is usually explained in terms of Lyapunov exponents, i.e., rates of particle pair separations. We present a unified review of this approach (which encapsulates also other nonchaotic flows) and investigate its limitations. During the final stage of mixing, when the scalar variance decays exponentially, Lyapunov exponents can fail to describe the mixing process. The failure occurs when another mixing mechanism, first introduced by Fereday et al. [Phys. Rev. E 65, 035301 (2002)], leads to a slower decay than the mechanism based on Lyapunov exponents. Here we show that this mechanism is governed by the large-scale nonuniformities of the flow which are different from the small scale stretching properties of the flow that are captured by the Lyapunov exponents. However, during the initial stage of mixing, i.e., the stage when most of the scalar variance decays, Lyapunov exponents describe well the mixing process. We develop our theory for the incompressible and diffusive baker map, a simple example of a chaotic flow. Nevertheless, our results should be applicable to all chaotic flows.

Intramural virtual electrodes during defibrillation shocks in left ventricular wall assessed by optical mapping of membrane potential.

It is believed that defibrillation is due to shock-induced changes of transmembrane potential (DeltaV(m)) in the bulk of ventricular myocardium (so-called virtual electrodes), but experimental proof of this hypothesis is absent. Here, intramural shock-induced DeltaV(m) were measured for the first time in isolated preparations of left ventricle (LV) by an optical mapping technique. LV preparations were excised from porcine hearts (n=9) and perfused through a coronary artery. Rectangular shocks (duration 10 ms, field strength E approximately 2 to 50 V/cm) were applied across the wall during the action potential plateau by 2 large electrodes. Shock-induced DeltaV(m) were measured on the transmural wall surface with a 16x16 photodiode array (resolution 1.2 mm/diode). Whereas weak shocks (E approximately 2 V/cm) induced negligible DeltaV(m) in the wall middle, stronger shocks produced intramural DeltaV(m) of 2 types. (1) Shocks with E>4 V/cm produced both positive and negative intramural DeltaV(m) that changed their sign on changing shock polarity, possibly reflecting large-scale nonuniformities in the tissue structure; the DeltaV(m) patterns were asymmetrical, with DeltaV-(m)>DeltaV+(m). (2) Shocks with E>34 V/cm produced predominantly negative DeltaV(m) across the whole transmural surface, independent of the shock polarity. These relatively uniform polarizations could be a result of microscopic discontinuities in tissue structure. Strong defibrillation shocks induce DeltaV(m) in the intramural layers of LV. During action potential plateau, intramural DeltaV(m) are typically asymmetrical (DeltaV-(m)>DeltaV+(m)) and become globally negative during very strong shocks.

Thick GaN Films Grown on Sapphire: Defects in Highly Mismatched Systems

In this paper, we review the present knowledge of the defects in thick GaN films grown by hydride vapor phase epitaxy (HVPE) on sapphire substrates. We summarize the defects present in such highly mismatched system into three main categories: large-scale nonuniformities, microstructure crystallographic defects, and point defects. We begin by describing the layer structure typical for thick films grown at very high growth rates, and concentrate on large-scale three-dimensional defects and their impact on the crystal quality. Then we briefly review the current understanding of the variety of extended structural defects with the emphasis on their role in the morphology and relaxation mechanisms in thick heteroepitaxial layers. The discussion is completed by reviewing the point defects in HVPE nitride materials and their influence on the optical properties of GaN films.

A Moving Cold Front in the Intergalactic Medium of A3667

We present results from a Chandra observation of the central region of the galaxy cluster A3667 with emphasis on the prominent sharp X-ray brightness edge spanning 0.5 Mpc near the cluster core. Our temperature map shows large-scale nonuniformities characteristic of the ongoing merger, in agreement with earlier ASCA results. The brightness edge turns out to be a boundary of a large cool gas cloud moving through the hot ambient gas, very similar to the fronts discovered by Chandra in A2142. The higher quality of the A3667 data allows the direct determination of the cloud velocity. At the leading edge of the cloud, the gas density abruptly increases by a factor of 3.9 ± 0.8, while the temperature decreases by a factor of 1.9 ± 0.2 (from 7.7 to 4.1 keV). The ratio of the gas pressures inside and outside the front shows that the cloud moves through the ambient gas at near-sonic velocity, M = 1 ± 0.2 or v = 1400 ± 300 km s-1. In front of the cloud, we observe the compression of the ambient gas with an amplitude expected for such a velocity. A smaller surface brightness discontinuity is observed further ahead, ~350 kpc in front of the cloud. We suggest that it corresponds to a weak bow shock, implying that the cloud velocity may be slightly supersonic. Given all the evidence, the cold front appears to delineate the remnant of a cool subcluster that recently has merged with A3667. The cold front is remarkably sharp. The upper limit on its width, 35 or 5 kpc, is several times smaller than the Coulomb mean free path. This is a direct observation of suppression of the transport processes in the intergalactic medium, most likely by magnetic fields.

Photometry and polarimetry of the classical Herbig Ae star VV Ser

We present the results of our long-term photometric and polarimetric observations of the classical Herbig Ae star VV Ser, performed at the Crimean Astrophysical Observatory as part of a program of photometric and polarimetric monitoring of UX Ori stars. We recorded an unusually deep minimum of VV Ser (ΔV≈3m), with a turn of the color tracks in the V-(U-B) and V-(B-V) diagrams (“the blueing effect”) observed for the first time for this star. The increase of the linear polarization during the minimum brightness was consistent with expectations for variable circumstellar extinction models, and the maximum polarization in the B band reached a record value for UX Ori stars in the deepest part of the minimum (12.8±1.4%). Our results cannot be explained by models with an axially symmetrical circumstellar dust disk consisting of silicate grains. They point to the existence of a large-scale nonuniformity in the azimuthal dust distribution near VV Ser attributable to the presence of a second component or protoplanet.

On Stochastic Motion of Vortex Loop

Stochastic behavior of a single vortex loop in HeII moving in its self-induced velocity field under the influence of an additional external random force has been discussed. The model under consideration is connected to the so-called low-temperature superfluid turbulence. The character of a stirring force has been determined by random actions of medium which are thermal excitations, of large-scale nonuniformities of flow and of chaotic motion of other loops. In the paper we have studied in details the first case which of course leads to the thermal equilibrium state.

Direct observation of large-scale nonuniformities in hydride vapor-phase epitaxy-grown gallium nitride by cathodoluminescence

Hydride vapor phase epitaxial GaN films grown on sapphire without a buffer are found to contain large-scale regions with high electron concentration located close to the interface. These regions are composed of individual columns forming an irregular but quasicontinuous layer, while the rest of the film has a much lower carrier concentration. The highly doped regions are easily visualized using cathodoluminescence. The coexistence of regions with low and high electron concentration allows us to explain the concurrent evidence of high film quality in photoluminescence, Raman spectroscopy and x-ray diffraction, and a high electron concentration measured in transport studies.

Space-time structure of the magnetic field of a laser plasma and methods for its enhancement outside the plasma

A considerable enhancement (more than three orders of magnitude) of the magnetic field outside a laser plasma under a large-scale nonuniformity of the experimental configuration has been observed. Some fundamental peculiarities, which indicate the active role of the target in magnetic field generation, have been established. For example, the target topology, the ratio of its dimensions, the conductivity in contact with a plasma ``ionization aureole'' were found to be important factors, that can determine the structure of magnetic field of the laser plasma.