Loops Of Flux Research Articles

Fractional-order-induced symmetric multi-scroll chaotic attractors and double bubble bifurcations in a memristive coupled Hopfield neural networks

The multi-state memristor is a type of memristor capable of memorizing multiple conductance states. This special feature is successfully used to mimic the behavior of neural synapses and simulate the influences of electromagnetic induction and radiation in artificial neural networks such as Hopfield neural networks. This paper introduces and examines a novel fractional-order Hopfield neural network model consisting of two non-identical sub-Hopfield neural networks coupled by a multi-stable flux controlled memristor (MCFHNN). The pinched hysteresis loops of the multi-stable flux controlled memristor are analyzed via numerical simulations. The equilibrium points of MCFHNN model and their stability are investigated. The introduced MCFHNN model is solved by using Adomian Decomposition Method (ADM). By using some examination methods such as spectral entropy complexity, bifurcation diagrams, phase portraits, maximum Lyapunov exponent and basins of attraction, we analyze the dynamic behaviors of MCFHNN model associated with coupling strength, fractional-order and initial states. Numerical results demonstrate that the proposed MCFHNN model is capable to develop abundant and complicated dynamics including, chaos, multiple transient transition behaviors, double bubble bifurcations, initial-boosted behavior and symmetric multi-scroll chaotic attractors. It is found that the complexity results are in agreement with those obtained from the bifurcation diagrams. This proves that the complexity can well reflect the dynamic behaviors of MCFHNN model. In order to support theoretical and numerical results, the designed MCFHNN model is implemented with the help of an Arduino-Due microcontroller board based on the Atmel SAM3X8E ARM Cortex-M3 processor. The results are in very good agreement with those obtained from computational simulations.

Tree Water Use Patterns as Influenced by Phenology in a Dry Forest of Southern Ecuador.

Tropical dry forests are composed of tree species with different drought coping strategies and encompass heterogeneous site conditions. Actual water use will be controlled by soil moisture availability. In a premontane dry forest of southern Ecuador, tree water use patterns of four tree species of different phenologies were studied along an elevational gradient, in which soil moisture availability increases with altitude. Main interest was the influence of variation in soil moisture, vapor pressure deficit, species (representing phenology), elevation, and tree diameter on water use. Special emphasis was put on the stem succulent, deciduous Ceiba trichistandra, as high water use rates and drought coping involving stem succulence was to be expected. Tree water use rates increased linearly with diameter across species at high soil water content. However, when soil moisture declined, sap flux densities of the species responded differently. The stem succulent, deciduous Ceiba and other deciduous tree species reduced sap flux sensitively, whereas sap flux densities of the evergreen (broad leaved) Capparis scabrida were increasing. This was also reflected in diurnal hysteresis loops of sap flux vs. vapor pressure deficit (VPD) of the air. Under dry soil conditions, Ceiba and other deciduous tree species had much smaller areas in the hysteresis loop, whereas the area of Capparis was largely enhanced compared to wet conditions. The evergreen Capparis potentially had access to deeper soil water resources as water use patterns suggest that top soil drought was tolerated. The deciduous species followed a drought avoidance strategy by being leafless in the dry season. The stem succulent deciduous Ceiba flushed leaves at the end of the dry season before the rainy season began and also re-flushed early in the dry season after a rain event; however, water use rates at this occasion remained low. Ceiba was also ready for fast and strong response in water use when conditions were most favorable during the wet season. The study thus indicates a strong influence of species’ drought coping strategy on water use patterns in tropical dry forests.

Dual Lattice Simulation of the Abelian Gauge-Higgs Model at Finite Density: An Exploratory Proof of Concept Study

The U(1) gauge-Higgs model with two flavors of opposite charge and a chemical potential is mapped exactly to a dual representation where matter fields correspond to loops of flux and the gauge fields are represented by surfaces. The complex action problem of the conventional formulation at finite chemical potential μ is overcome in the dual representation, and the partition sum has only real and nonzero contributions. We simulate the model in the dual representation using a generalized worm algorithm, explore the phase diagram, and study condensation phenomena at finite μ.

Gauge and matter fields as surfaces and loops: An exploratory lattice study of theZ3gauge-Higgs model

We discuss a representation of the Z(3) Gauge-Higgs lattice field theory at finite density in terms of dual variables, i.e., loops of flux and surfaces. In the dual representation the complex action problem of the conventional formulation is resolved and Monte Carlo simulations at arbitrary chemical potential become possible. A suitable algorithm based on plaquette occupation numbers and link-fluxes is introduced and we analyze the model at zero temperature and finite density both in the weak and strong coupling phases. We show that at zero temperature the model has different first order phase transitions as a function of the chemical potential both for the weak and strong coupling phases. The exploratory study demonstrates that alternative degrees of freedom may successfully be used for Monte Carlo simulations in several systems with gauge and matter fields.

Vector Control for Connectionless Two-Motor Synchronous System

According to two three-phase asynchronous motors system running by connectionless-mode, and considering the system to be required to run in strict synchronous way under different load, the method of field-oriented vector control is used. Thus, the control loops of rotor flux, torque and rotor speed are constructed to make two-motor system run synchronously by torque distribution. The simulation results illustrate its effectiveness and good dynamic and static performance.

Modelling magnetic flux emergence in the solar convection zone

[Abridged] Bipolar magnetic regions are formed when loops of magnetic flux emerge at the solar photosphere. Our aim is to investigate the flux emergence process in a simulation of granular convection. In particular we aim to determine the circumstances under which magnetic buoyancy enhances the flux emergence rate (which is otherwise driven solely by the convective upflows). We use three-dimensional numerical simulations, solving the equations of compressible magnetohydrodynamics in a horizontally-periodic Cartesian domain. A horizontal magnetic flux tube is inserted into fully developed hydrodynamic convection. We systematically vary the initial field strength, the tube thickness, the initial entropy distribution along the tube axis and the magnetic Reynolds number. Focusing upon the low magnetic Prandtl number regime (Pm<1) at moderate magnetic Reynolds number, we find that the flux tube is always susceptible to convective disruption to some extent. However, stronger flux tubes tend to maintain their structure more effectively than weaker ones. Magnetic buoyancy does enhance the flux emergence rates in the strongest initial field cases, and this enhancement becomes more pronounced when we increase the width of the flux tube. This is also the case at higher magnetic Reynolds numbers, although the flux emergence rates are generally lower in these less dissipative simulations because the convective disruption of the flux tube is much more effective in these cases. These simulations seem to be relatively insensitive to the precise choice of initial conditions: for a given flow, the evolution of the flux tube is determined primarily by the initial magnetic field distribution and the magnetic Reynolds number.

A simple but high-performance IM speed drive

The paper deals with an induction motor (IM) speed drive having high performance over a large speed range in the constant torque region. The drive is built up around an outer loop of speed equipped with an incremental encoder and two inner loops of torque and flux implemented in a direct torque control (DTC) scheme. The scheme takes advantage of the available speed to estimate the motor torque and the stator flux magnitude by means of the IM current model. This allows the full exploitation of the drive capabilities even in the low speed range. Furthermore, with a suitable representation of the IM current model, the drive can be operated down up to zero speed. The paper closes by analytically investigating the detuned behaviour of the scheme due to a rotor time constant mismatch. Experimental results substantiate the low-speed capabilities of the scheme and the findings on its detuned behaviour.

Helioseismically Determined Near‐Surface Flows Underlying a Quiescent Filament

The extended filaments seen in Hα images of the solar disk, and the corresponding prominences when viewed at the solar limb, are one of the great hallmarks of solar magnetism. Such arches of magnetic field and the coronal plasma structures they support are both beautiful and enigmatic. Many models of filament formation and maintenance invoke the existence of surface plasma flows, which are used to drive the magnetic reconnection needed to form twisted loops of flux held down by a coronal arcade. These flows are typically composed of a converging flow, which brings flux elements of opposite polarity together, combined with a tangential shear that stresses the coronal arcade. In this paper we present helioseismic measurements of near-surface flows underlying a single quiescent filament lying within a decayed active region. Newly devised high-resolution ring analyses (HRRA) with both 2° and 4° spatial resolution were applied to Doppler imaging data provided by the Michelson Doppler Imager (MDI) instrument on the SOHO spacecraft. A long-lived filament appearing in 2002 May and April was studied. We find that the filament channel is a region of vigorous subphotospheric convection. The largest observed scales of such convection span the region of weak magnetic field separating the active region's two polarities. Thus, the magnetic neutral line that forms the spine of the filament channel tends to lie along the centers of large convection cells. In temporal and spatial averages of the flow field, we do not find a systematic converging flow. However, we do detect a significant shearing flow parallel to the neutral line. This shear takes the form of two oppositely directed jets, one to either side of the neutral line and within 20 Mm of the line. The jets produce a net shear in the flow speed of 30 m s-1 occurring over a distance of 20 Mm.

Dynamic Performance Comparison of IRFO and SFO-SM Controlled Drives in Field-Weakening Region Using Variable-Saturation Regulators

This paper proposes to use variable-saturation regulators for Induction Motor (IM) drives operating in field-weakening region, and presents a comparative dynamic-performance analysis between the traditional Indirect Rotor Field Oriented (IRFO) control scheme and a new Stator Flux Oriented – Sliding Mode (SFO-SM) control scheme. The traditional IRFO control scheme has the d-axis current component, torque, rotor flux, and speed loops with four PI-type controllers. The SFO-SM control system is a Direct Torque Control – Space Vector Modulation (DTC-SVM) scheme with closed loops of torque and stator flux without current PI-type controllers. Both control schemes use regulators with variable-saturation in such a way that maximum dc-bus voltage utilization is reached and overcurrent problems are prevented. The experiments are carried out using dSPACE digital controllers and comparative results show that the SFO-SM controlled IM drive and IRFO one are characterized by similar dynamic responses in spite of structural simplicity of the SFO-SM control scheme.

A heavenly example of scale-free networks and self-organized criticality

The sun provides an explosive, heavenly example of self-organized criticality. Sudden bursts of intense radiation emanate from rapid rearrangements of the magnetic field network in the corona. Avalanches are triggered by loops of flux that reconnect or snap into lower-energy configurations when they are overly stressed. Our recent analysis of observational data reveal that the loops (links) and footpoints (nodes), where they attach on the photosphere, embody a scale-free network. The statistics of the avalanches and of the network structure are unified through a simple dynamical model where the avalanches and network co-generate each other into a complex, critical state. This particular example points toward a general dynamical mechanism for self-generation of complex networks.

A heavenly example of scale-free networks and self-organized criticality

The sun provides an explosive, heavenly example of self-organized criticality. Sudden bursts of intense radiation emanate from rapid rearrangements of the magnetic field network in the corona. Avalanches are triggered by loops of flux that reconnect or snap into lower-energy configurations when they are overly stressed. Our recent analysis of observational data reveal that the loops (links) and footpoints (nodes), where they attach on the photosphere, embody a scale-free network. The statistics of the avalanches and of the network structure are unified through a simple dynamical model where the avalanches and network co-generate each other into a complex, critical state. This particular example points toward a general dynamical mechanism for self-generation of complex networks.

String models of glueballs and the spectrum ofSU(N)gauge theories in2+1dimensions

The spectrum of glueballs in $2+1$ dimensions is calculated within an extended class of Isgur-Paton flux tube models and compared to lattice calculations of the low-lying $\mathrm{SU}(N&gt;~2)$ glueball mass spectrum. Our modifications of the model include a string curvature term and a new way of dealing with the short-distance cutoff. We find that the generic model is remarkably successful at reproducing the positive charge conjugation, $C=+,$ sector of the spectrum. The only large (and robust) discrepancy involves the ${0}^{\ensuremath{-}+}$ state, raising the interesting possibility that the lattice spin identification is mistaken and that this state is in fact ${4}^{\ensuremath{-}+}.$ Additionally, the Isgur-Paton model does not incorporate any mechanism for splitting $C=\ensuremath{-}$ from $C=+$ (in contrast with the case in $3+1$ dimensions), while the ``observed'' spectrum does show a substantial splitting. We explore several modifications of the model in an attempt to incorporate this physics in a natural way. At the qualitative level we find that this constrains our choice to the picture in which the $C=\ifmmode\pm\else\textpm\fi{}$ splitting is driven by mixing with new states built on closed loops of adjoint flux. However, a detailed numerical comparison suggests that a model incorporating an additional direct mixing between loops of opposite orientation is likely to work better, and that, in any case, a nonzero curvature term will be required. We also point out that a characteristic of any string model of glueballs is that the $\mathrm{SU}(\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty})$ mass spectrum will consist of multiple towers of states that are scaled up copies of each other. To test this will require a lattice mass spectrum that extends to somewhat larger masses than currently available.

MHD turbulence in radiating intense Z-pinches

A model for the physical processes that could occur in the turbulent interior of an imploding, radiating, intense Z-pinch is outlined. In this model the nonlinear evolution of the m = 0 pinch instability leads to the formation of tangled loops of magnetic flux that are jostled about by turbulent components of the velocity in the interior of the pinch. The pressure generated by the turbulent components of the velocity and magnetic field resist the implosive pressure from the pinch current and thus radiative collapse is prevented. A scaling law for the final collapse radius is obtained. Under certain conditions a “cold turbulent pinch” is predicted in which the radiated power is strongly decreased.

Closed-flux solutions to the quantum constraints for plane gravity waves

The metric for plane gravitational waves is quantized within the Hamiltonian framework, using a Dirac constraint quantization and the self-dual field variables proposed by Ashtekar. The $z$ axis (direction of travel of the waves) is taken to be the entire real line rather than the torus [manifold coordinatized by $(z,t)$ is $R\ifmmode\times\else\texttimes\fi{}R$ rather than ${S}_{1}\ifmmode\times\else\texttimes\fi{}R$]. Solutions to the constraints proposed in a previous paper involve open-ended flux lines running along the entire $z$ axis, rather than closed loops of flux; consequently, these solutions are annihilated by the Gauss constraint at interior points of the $z$ axis, but not at the two boundary points. The solutions studied in the present paper are based on closed flux loops and satisfy the Gauss constraint for all $z$.

Radiation transfer of gyrosynchrotron emission in stellar environments

Non-thermal radio emission has been detected from several kinds of active stars. Polarization and intensity measurements of the quiescent (i.e. non-flaring) emission indicate gyrosynchrotron emission. A three-dimensional magnetic field model for the stellar field is presented and the gyrosynchrotron intensity and polarization emerging from such a model is calculated and compared with observations. Model spectra agree well with observations. Model polarization results indicate that the emission region is more complex than the simple model and may indicate the presence of multipole components in the field, small loops of flux near the stellar surface and significant energy loss in the radiating electron distribution over time.

Small-Scale Horizontal Magnetic Fields in the Solar Photosphere

view Abstract Citations (156) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Small-Scale Horizontal Magnetic Fields in the Solar Photosphere Lites, B. W. ; Leka, K. D. ; Skumanich, A. ; Martinez Pillet, V. ; Shimizu, T. Abstract We present recent observations of quiet regions near the center of the solar disk using the Advanced Stokes Polarimeter. These observations reveal a component of the solar magnetic field heretofore unobserved: isolated, small-scale (typically 1"-2" or smaller), predominantly horizontal magnetic flux structures in the solar photosphere. These features occur in isolation of the well-known, nearly vertical flux concentrations usually seen in the photospheric "network." Hence we ascribe this horizontal flux to the photospheric "internetwork." They reveal themselves by the distinct signature of the Stokes Q and U polarization profiles, which are symmetric about the line center. The polarization signals are weak, with peak amplitudes typically ∼0.1%-0.2% of the continuum intensity in the resolved spectral profiles, but they are well above the noise level of these observations (≈0.05%). Such magnetic fields are weak (significantly less than 1000 G) and largely horizontal owing to the absence, or near absence, of accompanying Stokes V polarization when observed at the center of the solar disk. These horizontal field elements are often associated with blueshifted Stokes line profiles, and they often occur between regions of opposite polarity (but weak) Stokes V profiles. The horizontal elements are short-lived, typically lasting ∼5 minutes. Our observations suggest that we are viewing the emergence of small, concentrated loops of flux, carried upward either by granular convection or magnetic buoyancy. Even though these entities show weak field strengths, they also seem to be fairly common, implying that they could carry the order of 1024 Mx of magnetic flux to the surface on a daily basis. However, further observational study is needed to identify the specific nature of this phenomenon. Publication: The Astrophysical Journal Pub Date: April 1996 DOI: 10.1086/177028 Bibcode: 1996ApJ...460.1019L Keywords: POLARIZATION; SUN: MAGNETIC FIELDS; SUN: PHOTOSPHERE full text sources ADS |

Three-dimensional force-free looplike magnetohydrodynamic equilibria

Computations of three-dimensional force-free magnetohydrodynamic (MHD) equilibria, del x B = lambdaB with lambda = lambda(sub 0), a constant are presented. These equilibria are determined by boundary conditions on a surface corresponding to the solar photosphere. The specific boundary conditions used correspond to looplike magnetic fields in the corona. It is found that as lambda(sub 0) is increased, the loops of flux become kinked, and for sufficiently large lambda(sub 0), develop knots. The relationship between the kinking and knotting properties of these equilibria and the presence of a kink instability and related loss of equilibrium is explored. Clearly, magnetic reconnection must be involved for an unknotted loop equilibrium to become knotted, and speculations are made about the creation of a closed hyperbolic field line (X-line) about which this reconnection creating knotted field lines is centered.

Bistate t-expansion study of U(1) lattice gauge theory in 2+1 dimensions.

The compact formulation of U(1) Hamiltonian lattice gauge theory in 2 + 1 dimensions is studied using the $t$ expansion. The ground-state energy, average plaquette, specific heat, photon mass gap, and the ratio of the two lowest masses are investigated. Two contraction techniques are applied: a unistate scheme which uses only the strong-coupling vacuum for the trial state, and a bistate scheme which allows the introduction of variational parameters and arbitrarily large loops of electric flux in one of the trial states. The mass ratio obtained from the bistate contraction scheme exhibits precocious scaling. No evidence of a stable scalar glueball is found.

Abelianization of non-Abelian lattice gauge theories.

(2+1)-dimensional pure SU(2) lattice gauge theory on a square lattice is shown to be exactly equivalent to an Abelian gauge theory on a Kagom\'e lattice. The new dynamical variables create or annihilate a unit of an additive, color-invariant electric flux. They provide a complete (and not overcomplete) basis for physical states in the form of nonoriented "allowed" closed loops of unit flux. This also establishes 't Hooft's conjecture that a gauge theory of the Abelian subgroup is relevant for the confinement mechanism.

Subharmonic Shapiro steps in Josephson-junction arrays.

Subharmonic Shapiro step structure has been observed in the current-voltage characteristics of 300\ifmmode\times\else\texttimes\fi{}300 Nb-Au-Nb superconducting arrays. This subharmonic structure does not appear in the current-voltage characteristics of individual junctions, but appears to reflect a unique dynamical mode of the coupled system. A qualitative model is proposed that attributes the structure to the dynamical response of quantized loops of flux that are created by the antisymmetric self-magnetic field of the array bias current. Application of an external asymmetric magnetic field modifies the structure, which lends strong support to our model.