Dipolar Structures Research Articles

Sub- and super-luminar propagation of structures satisfying Poynting-like theorem for incompressible generalized hydrodynamic fluid model depicting strongly coupled dusty plasma medium

The strongly coupled dusty plasma has often been modelled by the Generalized Hydrodynamic (GHD) model used for representing visco-elastic fluid systems. The incompressible limit of the model which supports transverse shear wave mode is studied in detail. In particular, dipole structures are observed to emit transverse shear waves in both the limits of sub- and super-luminar propagation, where the structures move slower and faster than the phase velocity of the shear waves, respectively. In the sub-luminar limit the dipole gets engulfed within the shear waves emitted by itself, which then backreacts on it and ultimately the identity of the structure is lost. However, in the super-luminar limit the emission appears like a wake from the tail region of the dipole. The dipole, however, keeps propagating forward with little damping but minimal distortion in its form. A Poynting-like conservation law with radiative, convective, and dissipative terms being responsible for the evolution of W, which is similar to “enstrophy” like quantity in normal hydrodynamic fluid systems, has also been constructed for the incompressible GHD equations. The conservation law is shown to be satisfied in all the cases of evolution and collision amidst the nonlinear structures to a great accuracy. It is shown that monopole structures which do not move at all but merely radiate shear waves, the radiative term, and dissipative losses solely contribute to the evolution of W. The dipolar structures, on the other hand, propagate in the medium and hence convection also plays an important role in the evolution of W.

Parsimony, Exhaustivity and Balanced Detection in Neocortex.

The layout of sensory brain areas is thought to subtend perception. The principles shaping these architectures and their role in information processing are still poorly understood. We investigate mathematically and computationally the representation of orientation and spatial frequency in cat primary visual cortex. We prove that two natural principles, local exhaustivity and parsimony of representation, would constrain the orientation and spatial frequency maps to display a very specific pinwheel-dipole singularity. This is particularly interesting since recent experimental evidences show a dipolar structures of the spatial frequency map co-localized with pinwheels in cat. These structures have important properties on information processing capabilities. In particular, we show using a computational model of visual information processing that this architecture allows a trade-off in the local detection of orientation and spatial frequency, but this property occurs for spatial frequency selectivity sharper than reported in the literature. We validated this sharpening on high-resolution optical imaging experimental data. These results shed new light on the principles at play in the emergence of functional architecture of cortical maps, as well as their potential role in processing information.

Convection in deformed bodies: The effect of equatorial ellipticity on convective behavior

Tidal interactions between bodies such as hot jupiter planets and their host stars are likely to result in non-spherical geometries. These elliptical instabilities may have interesting effects on interior fluid convective patterns, which in turn could influence the nature of the magnetic dynamo within these planets. Simulations of thermal convection in the 2D rotating equatorial plane are conducted to determine to first order the effect of equatorial eccentricity on convection for varying density contrasts with differing convective vigor and rotation rates. This survey is conducted in two dimensions in order to simulate a broad range of eccentricities and to maximize the parameter space explored. The location of the three regimes documented in previous work (Evonuk and Samuel, 2012), dipolar flow, transitional flow, and differential flow, are found to be offset by the introduction of equatorial eccentricity to the system. The introduction of equatorial eccentricity changes the fluid behavior such that bodies with high amounts of deformation are likely to have weaker differential flows shifting their behavior towards transitional and dipolar flow structures. A scaling law based on the convective Rossby number, density contrast, and the eccentricity of the equatorial plane can therefore provide a way to estimate which regime a given body lies in.

Bipolar solar magnetic fields

Aims. Understanding magnetic fields in the solar corona is closely related to the complex nature of the often nonlinear differential equations describing such structures. Based on the ansatz of force-free fields, a class of solutions is derived and discussed that allows for axisymmetric bipolar magnetic fields. Methods. Allowed dipolar solutions for self-similar axisymmetric force-free magnetic fields use the formalism of a Grad-Shafranov equation involving the vector potential. For separable solutions involving poloidal fields decaying radially as r −n , there are no dipolar field structures for the decay index n 1. Results. In the domain n < 1 dipolar field structures are possible in restricted ranges of the angular coordinate θ depending on the value for n. Outside of the restricted domains there are no dipolar solutions, but there can be multipole solutions. The limiting case of the parameter n → 0 has been discussed previously, so that now the full regime 0 n < ∞ is covered.

THEMIS two-point measurements of the cross-tail current density: A thick bifurcated current sheet in the near-Earth plasma sheet.

The basic properties of the near‐Earth current sheet from 8 RE to 12 RE were determined based on Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations from 2007 to 2013. Ampere's law was used to estimate the current density when the locations of two spacecraft were suitable for the calculation. A total of 3838 current density observations were obtained to study the vertical profile. For typical solar wind conditions, the current density near (off) the central plane of the current sheet ranged from 1 to 2 nA/m2 (1 to 8 nA/m2). All the high current densities appeared off the central plane of the current sheet, indicating the formation of a bifurcated current sheet structure when the current density increased above 2 nA/m2. The median profile also showed a bifurcated structure, in which the half thickness was about 3 RE. The distance between the peak of the current density and the central plane of the current sheet was 0.5 to 1 RE. High current densities above 4 nA/m2 were observed in some cases that occurred preferentially during substorms, but they also occurred in quiet times. In contrast to the commonly accepted picture, these high current densities can form without a high solar wind dynamic pressure. In addition, these high current densities can appear in two magnetic configurations: tail‐like and dipolar structures. At least two mechanisms, magnetic flux depletion and new current system formation during the expansion phase, other than plasma sheet compression are responsible for the formation of the bifurcated current sheets.

Effect of an arbitrary spin orientation on the quadrupolar structure of an extended body in a Schwarzschild spacetime

The influence of an arbitrary spin orientation on the quadrupolar structure of an extended body moving in a Schwarzschild spacetime is investigated. The body dynamics is described by the Mathisson-Papapetrou-Dixon model, without any restriction on the motion or simplifying assumption on the associated spin vector and quadrupole tensor, generalizing previous works. The equations of motion are solved analytically in the limit of small values of the characteristic length scales associated with the spin and quadrupole variables with respect to the characteristic length of the background curvature. The solution provides all corrections to the circular geodesic on the equatorial plane taken as the reference trajectory due to both the dipolar and quadrupolar structures of the body as well as the conditions which the nonvanishing components of the quadrupole tensor must fulfill in order for the problem to be self-consistent.

Swelling enhanced remanent magnetization of hydrogels cross-linked with magnetic nanoparticles.

Hydrogels that are pH-sensitive and partially cross-linked by cobalt ferrite nanoparticles exhibit remarkable remanent magnetization behavior. The magnetic fields measured outside our thin disks of ferrogel are weak, but in the steady state, the field dependence on the magnetic content of the gels and the measurement geometry is as expected from theory. In contrast, the time-dependent behavior is surprisingly complicated. During swelling, the remanent field first rapidly increases and then slowly decreases. We ascribe the swelling-induced field enhancement to a change in the average orientation of magnetic dipolar structures, while the subsequent field drop is due to the decreasing concentration of nanoparticles. During shrinking, the field exhibits a much weaker time dependence that does not mirror the values found during swelling. These observations provide original new evidence for the markedly different spatial profiles of the pH during swelling and shrinking of hydrogels.

Three-dimensional Mapping of Static Magnetic Fields over a Semi-anechoic Chamber

The geomagnetic field is a kind of natural potential field in the Earth. A three-year research program for exploration of this field has been conducted in the Lombok Island-Indonesia, where extreme geomagnetic anomalies with two very strong dipolar structures exist. The research aims to construct a system to collect and concentrate geomagnetic fields, in order to possibly use the concentrated fields for geomagnetic power plants or to integrate the system with a field pick-up unit scheme by means of wireless power transfer. The designed geomagnetic concentrator system has been tested in a self-arranged semi-anechoic chamber with a pair of Helmholtz coils, induced with DC currents to simulate the regional ambient static geomagnetic fields. Several tests have proven the performance of the system in onedimensional space. This paper presents the results of detailed three-dimensional measurements of static magnetic fields in the semi-anechoic chamber. Static magnetic fields over the entire chamber are drawn in their magnitudes and directions, by interpolating data obtained in regular grids of 50cm × 50cm. In specific areas, where the Helmholtz coil is placed, extra grids of 25cm × 25cm are inserted to sharpen the fields’ depictions. Results show that by inducing 1 A current on each of coils will produce magnetic fields, concentrated over the surrounding area of Helmholtz coil. The intensities of magnetic fields over this area are about 15,00045,000 nT, which can be used to model the geomagnetic fields of Lombok Island. Using the results of 3D field mapping, it will be possible to get the optimum placement of the geomagnetic concentrator system when it is tested on the chamber.

Revealing the most disturbing tendency error of Zebiak–Cane model associated with El Niño predictions by nonlinear forcing singular vector approach

The nonlinear forcing singular vector (NFSV) approach is used to identify the most disturbing tendency error of the Zebiak–Cane model associated with El Nino predictions, which is most potential for yielding aggressively large prediction errors of El Nino events. The results show that only one NFSV exists for each of the predictions for the predetermined model El Nino events. These NFSVs cause the largest prediction error for the corresponding El Nino event in perfect initial condition scenario. It is found that the NFSVs often present large-scale zonal dipolar structures and are insensitive to the intensities of El Nino events, but are dependent on the prediction periods. In particular, the NFSVs associated with the predictions crossing through the growth phase of El Nino tend to exhibit a zonal dipolar pattern with positive anomalies in the equatorial central-western Pacific and negative anomalies in the equatorial eastern Pacific (denoted as “NFSV1”). Meanwhile, those associated with the predictions through the decaying phase of El Nino are inclined to present another zonal dipolar pattern (denoted as “NFSV2”), which is almost opposite to the NFSV1. Similarly, the linear forcing singular vectors (FSVs), which are computed based on the tangent linear model, can also be classified into two types “FSV1” and “FSV2”. We find that both FSV1 and NFSV1 often cause negative prediction errors for Nino-3 SSTA of the El Nino events, while the FSV2 and NFSV2 usually yield positive prediction errors. However, due to the effect of nonlinearities, the NFSVs usually have the western pole of the zonal dipolar pattern much farther west, and covering much broader region. The nonlinearities have a suppression effect on the growth of the prediction errors caused by the FSVs and the particular structure of the NFSVs tends to reduce such suppression effect of nonlinearities, finally making the NFSV-type tendency error yield much large prediction error for Nino-3 SSTA of El Nino events. The NFSVs, compared to the FSVs, are more applicable in describing the most disturbing tendency error of the Zebiak–Cane model since they consider the effect of nonlinearities. The NFSV-type tendency errors may provide information concerning the sensitive areas where the model errors are much more likely to yield large prediction errors for El Nino events. If the simulation skills of the states in the sensitive areas can be improved, the ENSO forecast skill may in turn be greatly increased.

The natural oscillation of two types of ENSO events based on analyses of CMIP5 model control runs

The eastern- and central-Pacific El Nio-Southern Oscillation (EP- and CP-ENSO) have been found to be dominant in the tropical Pacific Ocean, and are characterized by interannual and decadal oscillation, respectively. In the present study, we defined the EP- and CP-ENSO modes by singular value decomposition (SVD) between SST and sea level pressure (SLP) anomalous fields. We evaluated the natural features of these two types of ENSO modes as simulated by the pre-industrial control runs of 20 models involved in phase five of the Coupled Model Intercomparison Project (CMIP5). The results suggested that all the models show good skill in simulating the SST and SLP anomaly dipolar structures for the EP-ENSO mode, but only 12 exhibit good performance in simulating the tripolar CP-ENSO modes. Wavelet analysis suggested that the ensemble principal components in these 12 models exhibit an interannual and multi-decadal oscillation related to the EP- and CP-ENSO, respectively. Since there are no changes in external forcing in the pre-industrial control runs, such a result implies that the decadal oscillation of CP-ENSO is possibly a result of natural climate variability rather than external forcing.

Elastic octopoles and colloidal structures in nematic liquid crystals.

We propose a simple theoretical model which explains the formation of dipolar two- (2D) and three-dimensional (3D) colloidal structures in nematic liquid crystals. The colloidal particles are treated as effective hard spheres interacting via their elastic dipole, quadrupole, and octopole moments. It is shown that the octopole moment plays an important role in the formation of 2D and 3D nematic colloidal crystals. We generalize this assumption to the case of an external electric field and theoretically explain a giant electrostriction effect in 3D crystals observed recently.

Dipolar and V-shaped structures incorporating methylenepyran and diazine fragments

Thirteen novel dipolar and V-shaped chromophores with pyranylidene electron-donating part, diazine electron-withdrawing part and various π-linkers were synthesized. The extent of intramolecular charge transfer, structure-property relationships and optical properties were further investigated by UV/Vis absorption, electrochemistry, and DFT calculations.

Nematic-field-driven positioning of particles in liquid crystal droplets.

Common nematic oils, such as 5CB, experience planar anchoring at aqueous interfaces. When these oils are emulsified, this anchoring preference and the resulting topological constraints lead to the formation of droplets that exhibit one or two point defects within the nematic phase. Here, we explore the interactions of adsorbed particles at the aqueous interface through a combination of experiments and coarse-grained modeling, and demonstrate that surface-active particles, driven by elastic forces in the droplet, readily localize to these defect regions in a programmable manner. When droplets include two nanoparticles, these preferentially segregate to the two poles, thereby forming highly regular dipolar structures that could serve for hierarchical assembly of functional structures. Addition of sufficient concentrations of surfactant changes the interior morphology of the droplet, but pins defects to the interface, resulting in aggregation of the two particles.

Vortex formation in nonlinearly coupled modes in a magnetized quantum plasma

Linear and nonlinear properties of coupled modes in a magnetized quantum plasma in the presence of electron Fermi pressure are studied in a nonuniform magnetoplasma composed of electrons, ions, and extremely massive and negatively charged immobile dust grains. Stationary solutions of the nonlinear equations that govern the dynamics of coupled modes are presented. It is found that electrostatic dipolar vortex structure can form in such a plasma. The dipolar structures in dense plasmas are observed to be formed on a much shorter scalelength by comparison with their classical counterparts. It is found that the increasing Fermi temperature shortens the scalelength over which the nonlinear coherent structures are formed. The relevance of the present investigation with regard to the dense astrophysical plasmas is also pointed out.

Motion of Axisymmetric Magnetic Eddies with Swirl

We consider the motion of axisymmetric magnetic eddies with swirl in ideal magnetohydrodynamic (MHD) flow. The magnetic field is assumed to be toroidal, while the velocity field has both toroidal and poloidal components. The contour-dynamics formu- lation by Hattori and Moffatt (2006) for the case without swirl is extended to include swirl velocity so that the cross helicity does not vanish in general. The strength of the vortex sheets that appear on the contours varies with time under the influence of the centrifugal force due to swirl and the magnetic tension due to the Lorentz force. Numerical simulation using the contour-dynamics formulation shows that there exist counter-propagating dipolar structures whose radius is given by a balance between the centrifu- gal force and the magnetic tension. These structures are well described by the steady solutions obtained by perturbation expansion. The effects of vorticity inside the eddy on the motion of eddies are also investigated.

Conjugated push-pull salts derived from linear benzobisthiazole: preparation and optical properties

AbstractA series of novel monomethylated salts derived from linear benzobisthiazole was prepared. The push-pull attributes of these new compounds are represented by a quaternised azolium cycle as the acceptor part at one end of the structure and the dialkylamino- or diarylamino-substituted benzene ring as the donor part at the opposite end. Both moieties are connected by a conjugated linker consisting of one or two double bonds. Such dipolar structures are promising candidates for non-linear optical materials. The quantum-chemical indices describing linear and non-linear optical properties were obtained from semi-empirical calculations. The relationships between the chemical structure and non-linear optical properties of the cations studied were obtained. Effective conjugation was confirmed by measuring the optical properties in the UV-VIS region.

Linear and nonlinear dynamics of current-driven waves in dusty plasmas

The linear and nonlinear dynamics of a recently proposed plasma mode of dusty plasma is studied using kappa distribution for electrons. This electrostatic wave can propagate in the plasma due to the sheared flow of electrons and ions parallel to the external magnetic field in the presence of stationary dust. The coupling of this wave with the usual drift wave and ion acoustic wave is investigated. D'Angelo's mode is also modified in the presence of superthermal electrons. In the nonlinear regime, the wave can give rise to dipolar vortex structures if the shear in flow is weaker and tripolar vortices if the flow has steeper gradient. The results have been applied to Saturn's magnetosphere corresponding to negatively charged dust grains. But the theoretical model is applicable for positively charged dust as well. This work will be useful for future observations and studies of dusty environments of planets and comets.

Electrostatic vortex structures in a rotating magnetized electron-positron plasma with stationary ions

Nonlinear electrostatic vortex structures in the rotating magnetized electron-positron-ion plasmas are investigated. The electrons and positrons are considered to be dynamic while ions are taken to be stationary to neutralize only plasma background. In linear limit, the dispersion relation for electrostatic wave in a rotating electron-positron-ion plasma is discussed for both local and nonlocal cases. It is also found that conditions for the existence of dipolar vortex structures are modified in the presence of stationary ions in a magnetized rotating electron-positron plasma.

Strain-vorticity induced secondary motion in shallow flows

Deviations from two-dimensionality of a shallow flow that is dominated by bottom friction are quantified in terms of the spatial distribution of strain and vorticity as described by the Okubo-Weiss function. This result is based on a Poisson equation for the pressure in a quasi-horizontal (primary) flow. It is shown that the Okubo-Weiss function specifies vertical pressure gradients, which for their part drive vertical (secondary) motion. An asymptotic expansion of these gradients based on the smallness of the vertical to horizontal scale ratio demonstrates that the sign and magnitude of secondary circulation inside the fluid layer is dictated by the signs and magnitude of the Okubo-Weiss function. As a consequence of this, secondary motion as well as nonzero horizontal divergence do also depend on the strength, i.e., the Reynolds number of the primary flow. The theory is exemplified by two generic vortical structures (monopolar and dipolar structures). Most importantly, the theory can be applied to more complicated turbulent shallow flows in order to assess the degree of two-dimensionality using measurements of the free-surface flow only.

Elastic interactions between colloidal microspheres and elongated convex and concave nanoprisms in nematic liquid crystals

We study mutual alignment and interactions between colloidal particles of dissimilar shapes and dimensions when dispersed in a nematic host fluid. Convex pentagonal and concave starfruit-shaped nanoprisms and microspheres induce dipolar or quadrupolar director structures. The ensuing elastic pair interactions between microspheres and nanoprisms are highly dependent on the nanoparticle shape being omnidirectionally attractive for convex prisms but strongly anisotropic for concave prisms. Elastic deformations due to spherical particles cause well-defined alignment of complex-shaped nanoparticles at distances much larger than the microsphere size. We characterize distance and angular dependencies of elastic pair interaction forces, torques, and binding energies. The studied elasticity-mediated self-assembly of metal and dielectric nanoparticles with dissimilar shapes and sizes opens new possibilities for self-assembly based fabrication of structured mesoscopic composites with predesigned properties.