Central Dipole Research Articles

Features of calcium and strontium distribution on the surface of CP star HD49976

Modelling the magnetic field structure in HD49976, we obtained a map of its magnetic field distribution on the surface and its main parameters. Simulating the distribution of Ca and Sr by the phase dependences of the Ca II 3934 A and Sr II 4215 A equivalent line widths revealed that their abundances are most likely increased between the magnetic poles. An insufficiently clear result is due to the structure of the magnetic field of HD49976, which proved to be complex, not corresponding to the central dipole. It is modelled by two dipoles, the stronger of them is in the center of the star, and the other, weaker, is at distance of 0.3R* from the center. The synthetic spectrum method yielded abundances of 28 elements in the moments of magnetic field extrema. The carbon and sulfur abundances proved to be close to the solar values, silicon showed a deficit of 1.0 dex, lithium, calcium, iron and the iron peak elements, strontium, rare earths are in excess, especially significant in the case of rare earths. High-resolution spectropolarimetric observations closely covering the entire rotation period are required to investigate the distribution of chemical elements in complex multi-dipole magnetic configurations, applying the Magnetic Doppler Imaging method.

Impact of varying the DC bias stripline connection angle on terahertz coplanar stripline dipole antenna characteristics

We investigated the dependence of the input impedance and radiation characteristics on the connection angle between the DC bias stripline and the center dipole of a terahertz coplanar stripline dipole antenna in a semi-infinite substrate. Three connection angles, namely 0° (typical horizontal connection), 45° (diagonal connection), and 90° (vertical connection), were examined and an overall performance comparison was provided. The antenna with a sufficiently long bias stripline exhibited a traveling-wave behavior with stable impedance variation, regardless of the connection angle between the DC bias stripline and the center dipole. The antenna with diagonal and vertical bias connections performed much better than the typical H-shaped electrode with a horizontal bias connection in terms of radiation characteristics exhibiting flat gain and low frequency cut-off of the radiation efficiency. In addition, the antenna with a diagonal bias connection exhibited a better radiation pattern and more efficient use of a given substrate area than that with a vertical bias connection. The result indicates that in terms of the overall aspect of the terahertz stripline dipole antenna designs on a high-dielectric-constant substrate, the antenna with a diagonal bias connection is the best configuration.

Electric potentials in magnetic dipole fields normal and oblique to a surface in plasma: Understanding the solar wind interaction with lunar magnetic anomalies

We experimentally investigated the solar wind interaction with moderate‐strength lunar magnetic anomalies in which the electrons are magnetized but the ions remain unmagnetized. Previously, we studied the plasma sheaths above an insulating surface in a magnetic dipole field oriented parallel to the surface. In this paper, when the dipole field is oriented normal to the surface, the surface potential largely rises, and a potential bump forms in the sheath in the magnetic cusp region due to a significant magnetic mirror reflection of the electrons. It is also found that the electrons are shielded from the central dipole wings and diverted into the side of the wings. When the dipole field obliquely intersects the surface, an asymmetric potential distribution develops. Our experimental results indicate that lunar surface charging can be greatly modified in the magnetic anomaly regions, creating extreme local electrical environments.

Interaction between wedge disclination dipoles and core–shell nanowires with interface effects

Abstract In order to understand the influence of disclinations on mechanical properties of nanostructured composite materials, the interaction between a wedge disclination dipole and core–shell nanowires with interface effects is investigated. By using the complex variable method, the analytical solutions of complex potentials are derived for the interaction of a wedge disclination dipole and a core–shell nanowire considering interface effects. The stress fields, the elastic strain energy and the force acting on the wedge disclination dipole center are obtained. The influence of the feature of disclination dipole, the nanoscale coating thickness, the uniform eigenstrain and the material elastic dissimilarity on the equilibrium location and the stability of the dipole is studied when the size of the core–shell nanowire is reduced to nanometer scale. The results show that the impact of interface stresses on the equilibrium and the motion of the dipole near the core–shell nanowire is significant, namely, interface stresses have important effect on the strengthening and toughening of the nanostructured composite materials. The material properties of coating layer are dominant factors in the influence of the equilibrium location and the stability of disclination dipole for serious mismatch (i.e. very hard or very soft) and/or thick coating layer.

Magnetic field measurement of the star HD 35298

Based on the spectropolarimetric data obtained at the 6-m telescope, a study of the magnetic field and physical parameters of the magnetic He-weak star HD 35298 was performed. A comparison of the results of magnetic field measurements by various methods has been carried out. The star’s magnetic field varies in the range from −3 to +3 kG. The field geometry is explained in terms of the oblique rotator model. The obtained magnetic field variation curve can be described by a central dipole with the dipole axis inclined to the axis of rotation by β = 60°, and the magnetic field strength at the pole of B p = 11.5 kG. The data on the variability of spectral lines of some metals are presented, allowing to make an assumption that the stellar surface is heavily spotted.

Mesogen polarity effects on biaxial nematics. Centrally located dipoles

We investigate the phase organisation of thermotropic biaxial Gay-Berne (GB) mesogens yielding a biaxial nematic (Nb) phase upon endowing them with a central point dipole. We study the effects of changing the strength and orientation of the dipole on the phase behaviour, and in particular we examine, using molecular dynamics (MD) simulations, the possibility of improving the stability of the Nb phase. After mapping the boundaries of the Nb phase, we find that the strength of the embedded dipole is the parameter with the strongest influence on the mesogenic properties, while its orientation plays a minor role. For these central dipole systems, we find that the Nb phase organisation is stable only for mesogens with relatively weak dipole moments, while it disappears if electrostatic interactions become comparable in magnitude with dispersion interactions.

Self-assembly of spherical colloidal particles with off-centered magnetic dipoles

Fluids of spherical colloids possessing an off-centered embedded magnetic dipole were investigated by using Monte Carlo simulations. Systems of colloids with different strengths and directions of the embedded dipole moment confined in a 2D space without and with an external magnetic field applied were considered. The fluids were characterized by radial distribution functions, angular distribution functions, cluster data, and energetic data. In the absence of an external field, the colloids form dimers and trimers at sufficiently large magnetic moment without the tendency of forming chains of colloids as appearing in systems with particles possessing a central magnetic dipole. In the presence of an external field, chains of colloids aligned in a zigzag fashion were formed for a field parallel to the plane of the particles, whereas the colloidal ordering was suppressed in the presence of a field perpendicular to that plane. The findings agree surprisingly well with the recent experimental observations on fluids containing spherical polymer colloids with embedded single-domain magnetic hematite cubes (S. Sacanna, L. Rossi, and D. J. Pine, J. Am. Chem. Soc., 2012, 134, 6112).

EFFECTS OF ANTENNA DESIGN PARAMETERS ON THE CHARACTERISTICS OF A TERAHERTZ COPLANAR STRIPLINE DIPOLE ANTENNA

This paper presents the antenna design parameter dependency on the impedance and radiation characteristics of a terahertz coplanar stripline dipole antenna. The antenna response is numerically investigated by applying a semi-inflnite substrate and by generating a constant voltage source to drive a signal on the antenna. In this way, we can analyze the antenna characteristics without the photoconductive material response and the substrate lens geometrical efiects. Further, we explain the mechanism underlying the preferable uses of several millimeter length DC bias striplines in a typical THz coplanar stripline dipole antenna design. The antenna, consisting of a center dipole connected to long bias striplines, has a traveling wave characteristic supporting an attenuated current, rather than a resonant characteristic supporting a standing wave of current. The traveling wave behavior produces stable antenna input impedances and minimal changes in the antenna radiation patterns. We also found that the length of the center dipole has a prominent efiect on the antenna gain response.

Coupling of diamond nanocrystals to a high-Qwhispering-gallery microresonator

We theoretically investigate a cavity quantum electrodynamics system in which a high-$Q$ whispering-gallery-mode (WGM) microresonator interacts with multiple diamond nanocrystals each containing a single nitrogen-vacancy (NV) center. With a tapered fiber waveguide coupling the WGMs, the transmission spectrum of the system exhibits a twofold-splitting phenomenon, where the first fold of splitting originates from the nanocrystals' Rayleigh scattering and the second fold stems from the NV center dipole coupling with the WGMs. The interacting system can serve as a platform to generate quantum entanglement between two NV centers embedded in distant diamond nanocrystals. Our further study reveals that the Rayleigh scattering of the nanocrystals plays a positive role in generating entanglement in some cases. The analytical results show that the relatively strong scattering leads to a larger concurrence for the fixed dipole-cavity detunings. This investigation may hold potential for solid-state quantum information processing applications.

Specificities of magnetic field structures in chemically peculiar stars

We continue our program of the study of large-scale structures of magnetic fields in chemically peculiar stars. In this paper we analyze eight stars, out of which three stars have the structure of a central dipole, three—the structure of a dipole shifted along the axis, and two—of a dipole shifted across the axis. High-precision measurements (with σ = 50 and 80 G) are available for two stars (HD62140 and HD71866, respectively). The model phase dependences agree with the measurements within the errors. This result shows that the hypothesis about the dipole structure of the magnetic fields of CP stars is well founded.

Peculiarities of the magnetic field distribution over the surface of CP-stars. I. HD37017, 37479, 75049, 125823, 200775, V 380 Ori

The structures of the magnetic fields of 6 stars, of which 4 are Main Sequence stars and 2 are Herbig Ae/Be stars, are studied using published data. It turns out that HD125823, HD200775, and V380 Ori have a central magnetic dipole field structure, HD75049 has a dipole displaced along the axis, and HD37017 and 37479 have a dipole displaced perpendicular to the axis. It was unexpectedly found that the young Ae/Be Herbig stars HD200775 and V380 Ori have simple central dipole field structures (to within the limits of error of the measurements) and the expected disturbance of the field structures caused by disk accretion was not found.

Magnet Design and Measurement Results of the Solenoids and Bunch Compressor Bending Magnets of the SwissFEL Test Facility

The SwissFEL Injector Test Facility at the Paul Scherrer Institute (PAI/Switzerland) started its operation in August 2010. This Facility represents the first stage of the 6 GeV acceleration complex foreseen for the SwissFEL X-Ray coherent light source planned at the PSI. The 250 MeV Injector consists of an S-Band RF photo injector followed by four S-Band acceleration structures of 4 m length each. For fine focusing corrections each accelerating structure is surrounded by four solenoids (WFS) of 0.8 m length and a maximum field of 0.1 T. Each solenoid consists of two coils in one yoke tube. To decrease the bunch length of the electron beam a magnetic bunch compressor chicane with four dipoles (AFBC2) providing a maximum magnetic field of 0.4 T for a length of 0.25 m was build. The central dipoles of the compressor chicane are sited on a movable girder to allow extensive compression studies with dipole angles between 0° and 5°. The preservation of a high brightness and a low emittance for the femto-second electron pulse impose severe tolerances in the field quality and in the magnet positioning. In this paper we review the design of two magnet types and discuss the results of field quality and magnetic axis measurements in terms of the specifications.

Infrared luminescence from silicon nanostructures heavily doped with boron

Intense highly polarized radiation from silicon nanostructures heavily doped with boron to 5 × 1021 cm−3 is studied as a function of temperature, forward current, and an additional lateral electric field. The features of the radiation intensity and degree of polarization suggest that an important role in the formation of the luminescence spectra is played by the ordered system of B+-B− dipoles, formed as a result of the reconstruction of shallow boron acceptors as centers with negative correlation energy. The results obtained are interpreted within a proposed model based on two-electron adiabatic potentials, according to which radiation results from donor-acceptor recombination via boron dipole center states, involving shallow phosphorus donors.

Asymmetry of geomagnetic polarity: Equatorial dipole, Pangaea, and the Earth’s core

The paper presents the results of analyzing the set of dual-polarity paleomagnetic results the Global Paleomagnetic Database (GPMDB). The dataset was expanded by the results from the Paleomagnetic Data Catalogue for the USSR and with new data published after 2005. Some results were rejected to avoid the influence of overprints of ancient and recent magnetization. Overall, 59 dual-polarity results for the lithospheric plates of Baltica, Laurentia, and Siberia with their immediate framing were selected for the analysis in the interval of ages 207–359 Ma. The new data confirmed the model of the paleomagnetic field, according to which the field contains a long-lived component corresponding to the equatorial dipole which is responsible for the non-antipodal paleomagnetic directions in the zones of normal and reverse polarity in sedimentary and volcanic rock sequences. Retaining its value of 5–8% of the central axial dipole, the equatorial dipole changed its polarity a few times during the interval 359–207 Ma. The northern poles of the dipole formed two antipodal groups on the Earth’s surface, which lie within or near the subduction zones on the periphery of the Pangaea Supercontinent. Such localization of the equatorial dipole is suggested to be related to the ascending branches of the mantle convection and to the topography of both boundaries of the outer Earth’s core.

General magnetic field on the weakly‐active yellow giant Pollux and on the old dwarf star 61 Cyg A

AbstractPollux is yellow giant with known regular surface magnetic field about 1 Gauss. An analysis of the periodic behavior of the magnetic field of Pollux was performed. For this purpose, we used literature data together with spectropolarimetric observations, which were carried out during 10 nights in 2010 at the Crimean Astrophysical Observatory, and spectropolarimetric observations, which were obtained at Bohyunsan Optical Astronomy Observatory during 4 nights in 2007. The radial velocity measurements of Pollux show variations with a period of 592.9 days due to the planetary companion. The rotational period of the Pollux equals to 491.5 days. Improved dipole parameters are presented. The centered dipole gives the angle between spin axis and line of sight, i ∼ 45°, and the angle between the spin and dipole axes, β ∼ 135°. We suppose that a few points that lie outside of the dipole magnetic field curve by more than three errors are the result of the emergence of magnetic unipolar spots on the stellar surface. We discuss the possibility of these events in comparison with solar‐like star 61 Cyg A and the Sun. Numerical simulations of the magnetic geometry lead us to the conclusion that the spots may have an angular radius of ∼1.5° with the magnetic intensity ∼3000 G. Such active regions can be formed at the same latitudes as on the Sun near the 40° from the equator (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Biaxial Nematic Phase in Model Bent-Core Systems

We determine the bifurcation phase diagrams with isotropic (I), uniaxial (N(U)) and biaxial (N(B)) nematic phases for model bent-core mesogens using Onsager-type theory. The molecules comprise two or three Gay-Berne interacting ellipsoids of uniaxial and biaxial shape and a transverse central dipole. The Landau point is found to turn into an I-N(B) line for the three-center model with a large dipole moment. For the biaxial ellipsoids, a line of Landau points is observed even in the absence of the dipoles.

The Interaction between a Screw Dipole in Matrix and an Elliptical Inhomogeneity Containing a Confocal Crack

The interaction between a screw dipole inside the matrix and an elastic elliptical inhomogeneity containing a confocal crack under longitudinal shear is investigated. By using the complex method of elasticity, singularity analysis of stress function and Riemann boundary problem, the complex functions of matrix and inhomogeneity are obtained in the series form. And the singularity stress field of matrix, the image force and image torque acting on the center of screw dipole and the stress intensity factor of crack tip are deduced. Then the influence of the dip angle of screw dipole, the size of inhomogeneity and the misfit of materials on image force, torque of couple and the stress intensity are analyzed. The results show that the image force varies periodically as and there is two critical value of to make image force getting its maximum and minimum; the decreasing of relative shear modulus and the increasing of inhomogeneity may lead to the increasing of image force; the variation of will change the direction of screw dipole on stress intensity factor.

Magnetic field structures in chemically peculiar stars

We report the results of magnetic field modelling of around 50 CP stars, performed using the “magnetic charges” technique. The modelling shows that the sample reveals four main types of magnetic configurations: 1) a central dipole, 2) a dipole, shifted along the axis, 3) a dipole, shifted across the axis, and 4) complex structures. The vast majority of stars has the field structure of a dipole, shifted from the center of the star. This shift can have any direction, both along and across the axis. A small percentage of stars possess field structures, formed by two or more dipoles.

MHD simulations of jet formation - protostellar jets & applications to AGN jets

AbstractJet formation MHD simulations are presented considering a variety of model setups. The first approach investigates the interrelation between the disk magnetisation profile and jet collimation. Our results suggest (and quantify) that outflows launched from a very concentrated region at the inner disk tend to be weakly collimated. In the second approach, jet formation is investigated from a magnetic field configuration consisting of a stellar dipole superposed by a strong disk field. We find that the central dipole considerably de-collimates the disk wind. In addition, reconnection flares are launched in the interaction region of disk and stellar magnetic field, subsequently changing the outflow mass flux by factors of two. The time interval between flare ejection is about 1000 Keplerian periods - surprisingly similar to the observed time lag between jet knots. The third approach considers radiative pressure effects on jet collimation - an environment which is interesting mainly for outflows from massive young stars (but also for relativistic jets). Finally we present relativistic MHD simulations of jet formation from accretion disks extenting the previous non-relativistic approaches.

Elastic behaviour of a wedge disclination dipole near a sharp crack emanating from a semi-elliptical blunt crack

The interaction between a wedge disclination dipole and a crack emanating from a semi-elliptic hole is investigated. Utilising the complex variable method, the closed form solutions are derived for complex potentials and stress fields. The stress intensity factor at the tip of the crack and the image force acting on the disclination dipole center are also calculated. The influence of the morphology of the blunt crack and the position of the disclination dipole on the shielding effect to the crack and the image force is examined in detail. The results indicate that the shielding or anti-shielding effect to the stress intensity factor increases when the wedge disclination dipole approaches the tip of the crack. The effects of the morphology of the blunt crack on the stress intensity factor of the crack and the image force are very significant.