Characteristic Boundaries Research Articles

Quantitative Brewster angle microscopy measurements of chiral symmetry breaking and chiral boundaries in fatty acid Langmuir monolayers

Brewster angle microscopy (BAM) measurements of the low temperature region of the phase diagram of eicosanoic acid monolayers at an acidic subphase are performed. The existence of a new chiral I phase intervening between the L2 and L 2 ' phase recently discovered by Durbin et al. [M.K. Durbin, A. Malik, A.G. Richter, R. Ghastkadvi, T. Gog, P. Dutta, J. Chem. Phys. 106, 8216 (1997)] using grazing incidence X-ray diffraction (GIXD) is confirmed. Chiral symmetry breaking within the monolayers creates characteristic boundaries separating domains of opposite handedness. These disclination lines are associated with a jump in the tilt azimuth of the director, while the underlying hexatic orientation is continuous across the boundary. The disclination lines are observed with the Brewster angle microscope and analyzed as a function of surface pressure. The jump in tilt azimuth is determined. Agreement with an extended Landau theory proposed by Durbin is achieved.

Cooperative DNA-binding by Bicoid provides a mechanism for threshold-dependent gene activation in the Drosophila embryo.

The Bicoid morphogen directs pattern formation along the anterior-posterior (A-P) axis of the Drosophila embryo. Bicoid is distributed in a concentration gradient that decreases exponentially from the anterior pole, however, it transcribes target genes such as hunchback in a step-function-like pattern; the expression domain is uniform and has a sharply defined posterior boundary. A 'gradient-affinity' model proposed to explain Bicoid action states that (i) cooperative gene activation by Bicoid generates the sharp on/off switch for target gene transcription and (ii) target genes with different affinities for Bicoid are expressed at different positions along the A-P axis. Using an in vivo yeast assay and in vitro methods, we show that Bicoid binds DNA with pairwise cooperativity; Bicoid bound to a strong site helps Bicoid bind to a weak site. These results support the first aspect of the model, providing a mechanism by which Bicoid generates sharp boundaries of gene expression. However, contrary to the second aspect of the model, we find no significant difference between the affinity of Bicoid for the anterior gene hunchback and the posterior gene knirps. We propose, instead, that the arrangement of Bicoids bound to the target gene presents a unique signature to the transcription machinery that, in combination with overall affinity, regulates the extent of gene transcription along the A-P axis.

Structure of grain boundaries: Correlation to supercurrent transport in textured Bi2Sr2Can−1CunOx bulk material

Two kinds of characteristic grain boundaries were observed in textured Bi2Sr2Can−1CunOx (n = 2 and 3) bulk material: one (P-type) is nearly parallel to the (001) plane, and the other (N-type) is approximately normal to the (001) plane. Low-angle tilt N-type boundaries are composed of arrays of dislocations. However, for a small c-axis misorientation, the regions between the dislocation cores are still well connected, providing “pathways” for supercurrents crossing the boundary plane. The P-type boundaries exhibit compositionally and structurally modulated faceting. Although we see local regions of the low Tc (2201) phase at low-angle tilt (<10°) P-type boundaries, there are also “pathways” crossing the boundary plane made up of the high Tc (2212) and (2223) phases. The characteristics of such low-angle tilt grain boundary structures can therefore be modeled to provide general insight into the correlation between high critical current densities and low-texture breadths. On the other hand, a weak link could be formed at high-angle (> 10°) boundaries where there are the low Tc (2201) or insulating phases.

Characteristic boundary transitions in energetic particle data (60– ≥ 260 keV) recorded at comets P/Grigg-Skjellerup and P/Halley by the EPONA instrument on Giotto

An overview of published magnetic field, low energy electron and low to intermediate energy ion data recorded on board Giotto during an encounter with P/Grigg-Skjellerup (G-S) on 10 July, 1992 is presented and these measurements compared with particle data secured by the onboard energetic particles experiment EPONA ( E = 60 to ≥260 keV). It is shown that, in general, the same cometary boundaries (Bow Wave, Bow Shock, Mystery Boundary Transitions (MBTs)) were sensed in energetic particles as by the other experiments. Elevated fluxes of accelerated particles, interpreted to be ions of species M = 16–18, were identified in the G-S cometosheath. It is inferred that heavy cometary ions of species M = 28–33 were present in the innermost region of the comet (as viewed in the sunward direction). Energetic particles recorded by EPONA on Giotto at P/Halley are considered relative to other particles and fields measurements obtained onboard. Evidence based on pitch angle data indicates that, inbound, the MBTs at G-S and at Halley, were spatially associated with changes from trapped to “flowing” distributions of water group ions, suggesting that mirroring of these particles was taking place during encounter in the magnetic field piled up around each comet. Downstream of the MBTs, outbound, flowing distributions were, in each case, identified. Evidence of MBTs in energetic particle data are, herein, reported for the first time. The roles of various mechanisms in accelerating cometary ions in association with Bow Wave/Bow Shock boundaries at G-S and at Halley are inferred from comparisons between experimental and theoretical spectra pertinent to each comet. Similarities and differences between the signatures of characteristic boundaries recorded in each case are discussed in the light of basic differences between the comets themselves, and between the ambient interplanetary circumstances pertaining during each cometary encounter.

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The effect of strain induced to alloys prior to the semi-solid processing (SSP) on microstructures was investigated on a Al–7%Si–5%Mg alloy. The microstructural changes during heating and isothermal holding at a semi-solid state were examined on a strain-induced alloy. The strain was induced by cold-rolling with a reduction ratio of 10 to 30%. The effect of cold-rolling on producing spherical and grain-refined α phase increases with increasing the reduction ratio up to approximately 20% and almost saturates at higher reduction ratio than 20%. During heating to 580°C, the characteristic boundaries become visible on the inside of the dendrite-shaped α phase. Simultaneously, the eutectic silicon particles are partially melted and the liquid phase preferentially flows into the small capillaries of boundaries. With increasing temperature the boundaries become more distinctive and act to break down the dendrites into small grains. The boundaries are considered to be formed as a result of recrystallization. The isothermal holding at 580°C of semi-solid temperature region for up to 600 s has essentially no eflfect to increase the size of the α phase and to change morphologies.

Characteristic boundaries of the Hermean magnetosphere and energetic particles close to the planet

Characteristic Hermean magnetospheric boundaries identified in magnetic field and plasma data recorded on board the Mariner 10 spacecraft during fly-bys of Mercury in March 1974 and March 1975 are described. Energetic particle measurements contemporaneously made on board, and ambiguities inherent in their interpretation, are discussed and investigations to detect an atmosphere and ionosphere at Mercury described. Scientific questions concerning the magnetosphere that could be addressed using a future Mercury Orbiter are outlined and reasons advanced as to why it is important to include an Energetic Particle Detector in the payload of such a mission. The relative locations of a Bow Shock and Magnetopause identified at Mercury in magnetic and plasma electron data recorded during the two encounters considered, correlate well with the calculated positions of the same boundaries in the Earth's magnetosphere when equivalent interplanetary conditions are assumed, when aberration in the solar wind flow is taken into account and when observations made in the Hermean magnetosphere at a distance of xR M are compared with corresponding observations in the Terrestrial Magnetosphere at a distance of 8 XR E. The Bow Shock and Magnetopause signatures recorded at Mercury are consistent with the geometry which is expected for interaction between a planet centred magnetic dipole and the solar wind. Although the origin of the magnetic field at Mercury is not yet uniquely established, its dipole moment is estimated to be 5.1 ± 0.3 × 10 22 G cm 3 (or about 0.04–0.09% that of the Earth). The geometrically determined distance to the Magnetopause stagnation point of solar wind flow is 1.45 ± 0.15 R M. On average, this distance can, however, be expected to be somewhat larger, 1.8 ± 0.2 R M, due to solar wind variability and to the highly eccentric orbit of Mercury. Right-hand polarized whistler modes (≤ 10 Hz) upstream of the Bow Shock, a Polar Cusp region about twice the size of that pertaining at the Earth, a Current Sheet and an extended Magnetic Tail were each recorded at Mercury. The impulsive acceleration of particles in this Tail to energies of several hundred keV suggests similarities with terrestrial substorm events. It is a matter of debate if electrons alone, or electrons and protons, participate in the acceleration process. The thin atmosphere makes no significant contribution to the plasma of the Magnetosphere and there is no appreciable ionosphere. The restricted nature of the magnetic measurements made aboard Mariner 10, and the effect of technical problems suffered by the instruments recording plasma and energetic particle data in limiting the deductions that can presently be made using these records, require further “particles and fields” studies to be carried out in the Hermean Magnetosphere. A candidate mission to enable such investigations is the “Mercury Orbiter” (MO) currently under study by the European Space Agency. Since the source of the magnetic field at Mercury is presently in doubt, observations made over an extended period from this spacecraft, while flying in a low orbit, are required to determine the higher order multiple moments of the planetary magnetic field, and to deduce its topology within the Magnetosphere. Such data would also provide insights into the present nature of the interior of Mercury and elucidate its thermal history. It is recommended to include in the MO payload an Energetic Particle Detector to investigate transient intense bursts of relativistic electrons (possibly also protons) in the Magnetic Tail at energies of several hundred keV. This would allow the basic process of charged particle acceleration in a Magnetic Tail-Neutral Sheet configuration to be investigated. Interdisciplinary investigations of the link between magnetospheric and exospheric dynamics would provide an insight into mechanisms whereby, in the absence of a significant ionosphere, solar wind interaction leads to the development of a magnetospheric current system which connects with the planetary surface. Energetic particle measurements would also support Cruise Phase studies made on board the planned Mercury Orbiter. It is expected in this connection that outside, but close to, the Hermean Magnetosphere, the dynamics of the relatively unevolved solar wind flow could be studied from a unique vantage point using the proposed. Energetic Particle Detector.

Scanning tunneling microscopy of the phase transition between H/Si(100)-(2 x 1) and H/Si(100)-(3 x 1).

The transition from H/Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) to H/Si(100)-(3\ifmmode\times\else\texttimes\fi{}1) that occurs upon dosing atomic hydrogen at 400 K has been studied using scanning tunneling microscopy. It is characterized by a series of interactions between hydrogen and the strained silicon bonds via local adsorption and thermal decomposition of dihydride units. The transition starts from a paired H adsorption on a monohydride dimer on the 2\ifmmode\times\else\texttimes\fi{}1 surface, followed by the formation of local 3\ifmmode\times\else\texttimes\fi{}1 domains. The growth of 3\ifmmode\times\else\texttimes\fi{}1 domains is found anisotropic along the dimer row direction due to characteristic antiphase boundaries. A lateral dihydride-row diffusion has been observed, indicating that the local 3\ifmmode\times\else\texttimes\fi{}1 domains are correlated in their expansions, suggesting a plausible pathway for conversion of the spreading 3\ifmmode\times\else\texttimes\fi{}1 domains into a well-defined phase. Etching species have always been found on the surface, which are related to defects on the initially clean surface and breaking of back bonds during atomic hydrogen dosing. \textcopyright{} 1996 The American Physical Society.

Planned investigation of energetic particle populations (∼20–500 keV) in the close Martian environment

Energetic particle observations made by the Irish SLED instrument on the Phobos 2 spacecraft in 1989 have revealed the presence, within the overall energy range < 30 keV – > 3.2 MeV, of variously located energetic particle populations in the close Marian environment. The signatures of characteristic boundaries have also been recorded for the first time in energetic particles in the distant Martian magnetotail. The new SLED-II instrument on the Mars-94 Mission is designed to study in detail, with 4π measurement capability, these and other energetic particle phenomena at Mars, while operating, over an extended period, at low altitudes above the planet.

Cycling and skewing of exit measures for planar systems

We apply the exit conditioning approach to study two particular phenomena of the characteristic boundary exit problem: cycling of exit measures on periodic boundaries and skewing of exit measures at saddle points on the boundary. Sufficient conditions are given for each phenomenon to occur. Some results about the quasipotential function of Wentzell and Freidlin are established for characteristic boundaries in general.

First identification in energetic particles of characteristic plasma boundaries at Mars and an account of various energetic particle populations close to the planet

Signatures of characteristic boundaries, interpreted to be the bow shock and magnetopause with, between them, the magnetosheath, have been recorded for the first time in energetic particles (< 30 keV–> 3.2 MeV) in the downstream nightside Martian environment by the SLED instrument aboard Phobos 2. Also, energetic particles, interpreted to be oxygen ions, have been recorded by SLED at four distinct locations close to Mars. These include (a) anisotropic fluxes at the terminator shocks with energies of up to at least 72 keV; (b) anisotropic fluxes with energies of up to at least 225 keV inside the magnetopause at a height above the planet of approximately 900 km in the subsolar part of the magnetosphere; (c) fluxes with energies of up to at least 3.2 MeV in the flanks of the magnetosheath displaying quasiperiodic variations (period approximately 45 min) which are synchronous across the recorded energy spectrum and correlated in time with changes in the local magnetic field; (d) beams of oxygen ions with energies of up to at least 55 keV travelling out along open field lines in the magnetotail with, in some cases, a suggestion of confinement close to the neutral sheet. A preliminary discussion is provided concerning the energization of the various populations of particles identified.

Phase boundaries and domain boundaries in crystals

Domain boundaries, i.e. interfaces between different orientation variants of the same crystal species, and phase boundaries, i.e. interfaces between two different modifications of the same compound, exhibit rather similar features. This has been investigated by means of polarized light and X-ray topography for a series of structurally related sulphates which were grown as large single crystals from aqueous solution. The major results are as follows:(i) Domain interfaces frequently adopted only a few orientations which are low-energy boundaries of best structural fit. These preferred orientations may be parallel to low-indexed lattice planes or to non-crystallographic planes (W and Wt walls according to the classification of Sapriel,Phys. Rev. B12, 1975, 5128). Illustrations of such (transition-induced) domain boundaries in KLiSO4, NH4LiSO4, RbLiSO4, CsLiSO4,(NH4)3H(SO4)2 will be presented.(ii) For many first-order transitions the phase boundaries prefer planes of minimum strains, i.e. low energy, which again may be low-index lattice planes or non-crystallographic planes. These preferred orientations can be calculated from the strain tensor of the transition with the relative lattice-parameter changes as tensor components). If the transition isotherm deviates from the minimum strain orientation, characteristic zigzag boundaries with segments parallel to the (symmetrically equivalent) preferred planes may result. Zigzag phase boundaries have been observed in RbLiSO4 and {N(CH3 4}2ZnCl4.(iii) The shape and the density of transition-induced domains is influenced by the orientation of the phase boundaries and its velocity of motion. For the 708 K. transition of KLiSO4 and the 413 K transition of (NH4 3H(SO4 2 (in both cases loss of the trigonal axis), among the minimum-strain domain boundaries those normal to the phase boundary are preferred. In N(CH3)4 2ZnCl4, the domain density increases with the phase boundary velocity.

Boundary Local Time and Small Parameter Exit Problems with Characteristic Boundaries

The exit problem for an asymptotically small random perturbation of a stable dynamical system $x(t)$ in a region D is considered. The connection between the distribution of the position of first exit and the equilibrium density of the perturbed system subject to reflection from the boundary of D is developed. Earlier work treated the case in which $x(t)$ enters D nontangentially. Here the case in which $x(t)$ is everywhere tangent to the boundary is examined. The “small-noise” asymptotics of the boundary local time turn out to be of primary importance.

Numerical simulation of flows with locally characteristic boundaries

Numerical simulation of flows with locally characteristic boundaries

Asymptotic Analysis of a Singular Perturbation Problem

We study, in a rectangle $0 < x < a$ and $0 < y < b$, the Dirichlet problem for an elliptic differential equation of the form \[ - \epsilon \Delta u\epsilon + p\frac{{\partial u\epsilon }}{{\partial x}} + qu\epsilon = f(x,y)\] where $\epsilon $ is a small parameter $0 < \epsilon \ll 1$, $\Delta $ is the Laplacian operator, p is a positive number, q is a nonnegative number and all of the input data are smooth. We establish a constructive procedure for obtaining an asymptotic approximation of arbitrary order with respect to $\epsilon $ of this singular perturbation problem, and also give a proof of its uniform validity in the closed rectangle by the use of the maximum principle and exponential estimates of all boundary or corner layer functions. The corner singularities of parabolic boundary layer functions are removed by introducing elliptic boundary layer functions along the characteristic boundaries $y = 0$ and $y = b$. Both ordinary corner layer functions and elliptic corner layer functions are employed at the outflow corners $(a,0)$ and $(a,b)$. An application is made to settle a long-standing problem in the magnetohydrodynamic flow in a rectangular duct.

Diffusion Across Characteristic Boundaries with Critical Points

We consider the problems of the effect of small white noise perturbations on a deterministic dynamical system in the plane with (i) an asymptotically stable equilibrium point or limit cycle and (ii) an equilibrium point surrounded by closed trajectories. The mean exit time and the distribution of exit points for each problem is determined by solving singularly perturbed elliptic boundary value problems in domains with closed characteristic boundaries with critical points. Uniformly valid asymptotic solutions are constructed for each of the problems. For the asymptotically stable equilibrium point, the method of matched asymptotic expansions with the integral condition of Matkowsky and Schuss is employed. A method of averaging combined with boundary layer analysis is used for the problem of an equilibrium point surrounded by closed trajectories. The influence on the solutions, of the critical points on the boundary, is exhibited and explained. An application to the physical pendulum is given. Finally our r...

Delineation of seasonal changes of chlorophyll frontal boundaries in Mediterranean Coastal waters with Nimbus-7 Coastal Zone Color Scanner data

The seasonal changes in chlorophyll-like pigment distribution in the Northwestern Mediterranean Sea (Gulf of Lions) were monitored during the year 1979. Data were collected from the Coastal Zone Color Scanner (CZCS), carried on the Nimbus-7 satellite. A series of cloud-free images covering the whole year was selected and processed to evaluate chlorophyll-like pigment concentrations. The upwelling subsurface radiance of the ocean ( L ss) was extracted from the apparent upwelling signal reaching the satellite sensor ( L O) using an algorithm removing atmospheric effects. Chlorophyll-like pigment concentration ( C) was then derived from L ss in different wavelengths. Characteristic boundaries between water masses with different phytoplankton content were obtained. The results demonstrate that phytoplankton distribution is a good indicator of seasonal variations of oceanic fronts. Features such as coastal upwellings, cyclonic eddies, or plume of the Rhône river could be monitored.

Effects of annealing on deformation textures in galena

Experimental annealing of galena samples with known deformation histories shows that this mineral has the necessary properties to be a valuable source of information about low-grade deformational environments. An- nealed galena displays recovery and/or recrystallization features dependent upon the type of texture inherited from the tectonic event, which in turn is closely linked to deformation temperatures. In samples deformed at temperatures less than 200 ~ C in the laboratory, later annealing produced subgrains, mosaics of new grains, or rapid grain boundary migration as the annealing temperatures were varied from 200 ~ C to 700 ~ C. Kink bands maintained characteristic straight simple boundaries inherited from the deformation event. Samples deformed above 300 ~ C devel- oped syntectonically recrystallized textures. Kink bands had been converted to elongate grains with complex sutured grain boundaries during deformation, and mosaics of new grains were found in highly deformed regions. These textures were extremely stable through later annealing. Despite our changing annealing temperatures through 500 ~ C, we did not produce similar textures from both low and high temperature deformation runs. Examination of polished and etched galena from low-grade tectonic set- tings may well be worth the effort since galena textures appear to display features indicative of deformational evironments, even after being subjected to considerable post-tectonic thermal perturbations.

Analytical Methods for a Singular Perturbation Problem in a Sector

From the exact solution of an elliptic boundary value problem in a sector, asymptotic approximations with respect to a small parameter are derived. The asymptotic expansion is uniformly valid in the boundary layers. Also the phenomena for the case of almost characteristic boundaries are discussed.

An elliptic singular perturbation problem with almost characteristic boundaries

An elliptic singular perturbation problem with almost characteristic boundaries

EVOLUTION OF ROCK COMPOSITION AND GEOCHEMICAL PROCESSES IN THE SEDIMENTARY SHELL OF THE EARTH*

ABSTRACTAnalyses of the chemistry of the earth's sedimentary shell (and the crystalline crust) reveal evolutionary trends in sedimentary rock composition, depositional environments, and the intensity and relative importance of geochemical processes. Successive changes in the mode of crustal movement, the intensity of volcanic and intrusive activity, and in the structural plan of continents (reduction of the area of active geosynclines, increase of the area of platforms) caused a regular transformation of the petrographic composition of ancient erosion areas and corresponding changes in the composition of terrigenous material that at different stages was supplied to depositional areas (gradually lower amounts of greywacke, decrease of average Na, Mg, Fe and Al content, inverse development of K and Ca). Sediment‐composition and lithological proportion in the sedimentary shell changed in the course of time: the abundance of volcanogenic rocks decreased, the abundance of carbonates and evaporites increased, the types of prevailing carbonate accumulation changed (siderite→dolomite→calcite), the degree of compositional differentiation of terrigenous sediments increased. These changes were accompanied by regular evolutionary changes in the chemical composition of the most important sedimentary rock types: the lowering of average Na and Fegen contents, the increase of Corg, Ssulf and Spyr concentration, the inverse K development, the increase of the Ca/Mg ratio, the rise of the iron oxidation degree, the displacement of isotope ratios of oxygen and sulfur, etc.A definite trend in the development of the main types of sedimentary geochemical processes is established. The processes of clastogenesis and hydrolysis developed inversely: the importance of the first process decreased, but that of the second increased in the course of time. The intensity of oxidation and precipitation processes also increased through time; chemical precipitation was predominant during the early development stages, and biogenic precipitation predominated during the later stages. Characteristic boundaries in the evolution of sedimentary rock composition and geochemical processes are traced, which correspond to the boundaries A and Pt1, Pt1 and Pt3, Rf3 and V, Pz3 and Mz. Discontinuous variations in the concentrations and ratios of elements in sediments and in the intensity of some geochemical processes are confined to these boundaries. They were determined by changes in the composition of erosion areas, by the increasing importance of platform types of sedimentary processes, by large changes in the biosphere, by the evolution of the atmosphere and ocean composition, and by changes in the types of surface equilibria in the dynamic system: atmosphere‐ocean‐stratisphere.