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1

Magnetic flux noise in strongly anisotropic superconductors  

NASA Astrophysics Data System (ADS)

Magnetic noise due to thermally activated movements of flux vortices has been calculated taking into account fluctuations modes of nonrigid vortices. It has been shown that at low frequencies, below the crossover frequency, the noise spectrum of a layered superconductor is identical to that of a continuous material. Three regimes of spectral behavior, ln?, ?-1/2, and ?-3/2, have been predicted to be present in this frequency range. Characteristic frequencies separating different regimes depend on the geometry of the flux pickup loop. At high frequencies, above the crossover frequency, bending of vortices leads to a Lorentzian shape of noise spectra. The value of the crossover frquency is not influenced by the particularities of the flux-measuring arrangement and depends only on the material properties and applied magnetic field.

Ashkenazy, V. D.; Jung, G.; Shapiro, B. Ya.

1995-04-01

2

Magnetic flux noise in copper oxide superconductors  

SciTech Connect

Magnetic flux noise and flux creep in thin films and single crystals of YBa{sub 2}Cu{sub 3}O{sub 7-x}, Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+x}, Tl{sub 2}Ca{sub 2}Ba{sub 2}Cu{sub 3}O{sub x}, and TlCa{sub 2}Ba{sub 2}Cu{sub 3}O{sub x} are measured with a superconducting quantum interference device (SQUID). The noise power spectrum generally scales as 1/f (f is frequency) from 1 Hz to 1 kHz, increases with temperature, and decreases in higher-quality films. It is proportional to the magnetic field B in which the sample is cooled, at least in the range 0.1 mT < B < 3 mT. A model of thermally activated vortex motion is developed which explains the dependence of the noise on frequency, temperature, current, and applied magnetic field. The pinning potential is idealized as an ensemble of double wells, each with a different activation energy separating the two states. From the noise measurements, this model yields the distribution of pinning energies in the samples, the vortex hopping distance, the number density of mobile vortices, and the restoring force on a vortex at a typical pinning site. The distribution of pinning energies in YBa{sub 2}Cu{sub 3}O{sub 7-x} shows a broad peak below 0.1 eV. The small ambient magnetic field, and the detection of noise even in the absence of a driving force, insure that the measured pinning energies are characteristic of isolated vortices near thermal equilibrium. The observed vortex density in fields much less than 0.1 mT is too large to be explained by the ambient field, suggesting a mechanism intrinsic to the sample which produces trapped vortices.

Ferrari, M.J.

1991-11-01

3

Correlation between magnetic flux leakage and magnetic Barkhausen noise: Stress dependence in pipeline steel  

NASA Astrophysics Data System (ADS)

The tensile-stress dependence of angular-dependent magnetic Barkhausen noise (MBN) was investigated on the inner and outer surfaces of four sections of pipeline steel. Stresses up to 330 MPa or 70% of the yield strength of the steel were applied in either the circumferential or axial pipe direction. An effective MBN energy (MBNENERGY), defined as the time integral of the squared voltage MBN signal, was calculated. The variation of the ratio of the MBNENERGY in the pipe axis direction to that in the circumferential direction was correlated with the stress-dependent variation of the amplitude of radial magnetic flux leakage (MFLpp) signals measured on the outside of the pipe from simulated corrosion pits (13 mm diameter ball-milled pits) in sections of pipeline steel under axial magnetization that was applied from the outer surface of the pipe. The percentage variation of the MFLpp signal with stress was found to agree within uncertainty, with the MFLpp signal variation with stress from defects placed on the pipe wall far surface, at 1.1-1.2 T pipe wall flux density. The MBNENERGY, ratio is proposed as a measure of the relative anisotropy in line with the magnetizing field to that perpendicular to it. It is this relative anisotropy that defines the degree to which lines of flux may pass around a high reluctance defect by either remaining within the steel or being forced into the air.

Mandal, K.; Hauge, C.; Weyman, P.; Sijgers, B.; Atherton, D. L.; Krause, T. W.

1997-05-01

4

Flux controlled magnetic barkhausen noise measurements on grain oriented electrical steels  

NASA Astrophysics Data System (ADS)

This paper investigates magnetic Barkhausen noise (MBN) in grain-oriented electrical steel using flux-control and its relationship to power losses obtained from standard Epstein tests. The effects of flux, frequency, grain size and texture in five samples of similar thickness with known core loss values were examined. A comparison with a non-oriented sample indicated that the grain oriented Si-Fe has higher magnetic anisotropy and greater MBN signal variability with position, which is associated with its larger grain size. Angular MBN measurements demonstrated a major easy axis along the rolling direction (RD) that was attributed to its Goss texture. An inverse relation between MBN energy and core loss values was observed. The inverse relation was associated with a greater proportion of Barkhausen events with magnetization changes projected out of the plane of the sample. This generates microscopic eddy currents that do not add vectorily to the bulk eddy currents and thereby reduces excess power losses.

Samimi, Arash A.; Krause, Thomas W.; Clapham, Lynann

2012-05-01

5

Correlation between magnetic flux leakage and magnetic Barkhausen noise: Stress dependence in pipeline steel  

Microsoft Academic Search

The tensile-stress dependence of angular-dependent magnetic Barkhausen noise (MBN) was investigated on the inner and outer surfaces of four sections of pipeline steel. Stresses up to 330 MPa or 70% of the yield strength of the steel were applied in either the circumferential or axial pipe direction. An effective MBN energy (MBNENERGY), defined as the time integral of the squared

T. W. Krause; K. Mandal; C. Hauge; P. Weyman; B. Sijgers; D. L. Atherton

1997-01-01

6

Gamma-magnetic normalization - new effect to reduce flux-gate magnetometer noise level  

NASA Astrophysics Data System (ADS)

It is the author's opinion confirmed by numerous experiments, that the FGM noise level (NL) is determined not by Barkhausen jumps during the core remagnetization from positive to negative state as the majority of designers believe, but by non-repeatability of the magnetic domains transition from negative to positive states and back. This shows the way how to reduce the magnetic noise: to manufacture the magnetic material with a structure which will create conditions for magnetic domain walls to glide easily and uniformly when changing their orientation leading to minimal efforts at cyclic remagnetization. Ideally, such a material may be represented as a solid "liquid" with freely floating uniform magnetic domains without walls friction. To reduce the specific NL of the materials, several post-melting processing technologies were developed. A set of experiments made by many investigators has shown that the best results gives the magnetic materials annealing in vacuum or in any inert gas applying by this during all annealing time the alternative magnetic field, imitating core excitation field during FGM operation. If to accept the "solid liquid" model, this mechanism of NL decreasing has clear physical explanation: permanent re-magnetization of domains leads to the structural improvements favorable namely for the homogenization of transitions, rise of temperature gives necessary energy for the impurities liquidation. Probably, M. Acuna was the first who reported that during FGM operation in space its NL is decreasing with time and attributed this to the relaxation of mechanical stresses in the core material in weightlessness conditions [1]. We studied in details the conditions in which the core material is in space: weightlessness, vacuum and radiation. Mechanical stresses relaxation hypothesis was rejected because the internal forces in any solid body are much stronger as those to which a gravity force might have influence. Also the tests of FGM sensor in vacuum chamber both in operation and switched off conditions showed any influence of this parameter on NL. And radiation influence, the dose of which was selected approximately equal to yearly dose at near-Earth orbit (~10 krad), revealed interesting facts: the magnetic material itself and the FGM sensor in non-operation state showed any dependence on radiation, whereas this dose applied to the operating FGM sensor lead to the marked NL reducing. This new effect was named "gamma-magnetic normalization" and also can be explained by the accepted model: the mechanism is the same as above, but, because ?-quantum have much greater energy as thermal one, more "rigid" impurities are eliminated at ?-radiation action. The experimental results confirming this hypothesis are presented in the report. This work was partially supported by NASU Contract 1-71/11/1531. [1] Acuna M M, 2002. Space based magnetometers. Rev. Sci. Instr. 73(11) pp. 3717-36.

Korepanov, V.

2012-04-01

7

Magnetic-Flux Pump.  

National Technical Information Service (NTIS)

A magnetic flux pump is described for increasing the intensity of a magnetic field by transferring flux from one location to the magnetic field. The device includes a pair of communicating cavities formed in a block of superconducting material, and a pist...

A. F. Hildebrandt D. D. Elleman F. C. Whitmore

1966-01-01

8

Modeling Flux Noise in SQUIDs due to Hyperfine Interactions  

NASA Astrophysics Data System (ADS)

Recent experiments implicate spins on the surface of metals as the source of flux noise in superconducting quantum interference devices and indicate that these spins are able to relax without conserving total magnetization. We present a model of 1/f flux noise in which electron spins on the surface of metals can relax via hyperfine interactions. Our results indicate that flux noise would be significantly reduced in superconducting materials where the most abundant isotopes do not have nuclear moments, such as zinc and lead.

Wu, Jiansheng; Yu, Clare C.

2012-06-01

9

Theory of Flux-Flow Noise in Superconductors.  

National Technical Information Service (NTIS)

Voltage noise in magnetic flux-containing type-II superconductors whose linear dimensions are large by comparison with the field-dependent penetration depth lambda/sub B/ is attributable to the motion of singly-quantized flux vortices. The instantaneous v...

J. R. Clem

1978-01-01

10

Low-frequency Flux Noise in SQUIDs and Superconducting Qubits  

NASA Astrophysics Data System (ADS)

Superconducting qubits are a leading candidate for scalable quantum information processing. In order to realize the full potential of these qubits, it is necessary to develop a more complete understanding of the microscopic physics that governs dissipation and dephasing of the quantum state. In the case of the Josephson phase and flux qubits, the dominant dephasing mechanism is an apparent low-frequency magnetic flux noise with a 1/f spectrum. The origin of this excess noise is not understood. We report the results of SQUID measurements that explore the dependence of the excess low-frequency flux noise on SQUID inductance, geometry, materials, and temperature. We discuss contributions to the measured noise from temperature fluctuations, trapped vortices in the superconducting films, and surface magnetic states in the native oxides of the superconductors. We discuss implications of our measurements for qubit dephasing.

Sendelbach, Steven; Hover, David; Kittel, Achim; Mueck, Michael; McDermott, Robert

2008-03-01

11

The Mems Flux Concentrator: a Device for Improving Magnetic Sensors  

Microsoft Academic Search

The MEMS flux concentrator, a device for minimizing the effect of 1\\/f noise in magnetic sensors is described. In its static configuration, the MEMS flux concentrator's is similar to that usually employed with flux concentrators, i.e., soft magnetic materials are used to increase the field at the position of the magnetic sensor. The difference is that the MEMS flux concentrator

Alan Edelstein; Gregory Fischer

2003-01-01

12

Quantum wire network with magnetic flux  

NASA Astrophysics Data System (ADS)

The charge transport and the noise of a quantum wire network, made of three semi-infinite external leads attached to a ring crossed by a magnetic flux, are investigated. The system is driven away from equilibrium by connecting the external leads to heat reservoirs with different temperatures and/or chemical potentials. The properties of the exact scattering matrix of this configuration as a function of the momentum, the magnetic flux and the transmission along the ring are explored. We derive the conductance and the noise, describing in detail the role of the magnetic flux. In the case of weak coupling between the ring and the reservoirs, a resonant tunneling effect is observed. We also discover that a non-zero magnetic flux has a strong impact on the usual Johnson-Nyquist law for the pure thermal noise at small temperatures.

Caudrelier, Vincent; Mintchev, Mihail; Ragoucy, Eric

2013-10-01

13

Novel behavior of magnetic flux lines in type II superconductors  

Microsoft Academic Search

In this thesis we present several studies in the properties of magnetic flux lines in type II superconductors. We have carried out a model calculation of the flux noise produced by vortex avalanches in a Type-II superconductor, using a simple kinetic model proposed by Bassler and Paczuski. Over a broad range of frequencies, we find that the flux noise SFw

Gregory Allan Mohler

2001-01-01

14

Mean square flux noise in SQUIDs and qubits: numerical calculations  

NASA Astrophysics Data System (ADS)

The performance of SQUIDs and superconducting qubits based on magnetic flux is degraded by the presence of magnetic flux noise with a spectral density scaling approximately inversely with frequency. It is generally accepted that the noise arises from the random reversal of spins on the surface of the superconductors. We introduce a numerical method of calculating the mean square flux noise \\langle {\\Phi }_{{}}^{2}\\rangle from independently fluctuating spins on the surface of thin-film loops of arbitrary geometry. By reciprocity, \\langle {\\Phi }_{{}}^{2}\\rangle is proportional to , where B(r) is the magnetic field generated by a circulating current around the loop and r varies over the loop surface. By discretizing the loop nonuniformly, we efficiently and accurately compute the current distribution and resulting magnetic field, which may vary rapidly across the loop. We use this method to compute \\langle {\\Phi }_{{}}^{2}\\rangle in a number of scenarios in which we systematically vary physical parameters of the loop. We compare our simulations to an earlier analytic result predicting that \\langle {\\Phi }_{{}}^{2}\\rangle \\propto R/W in the limit where the loop radius R is much greater than the linewidth W. We further show that the previously neglected contribution of edge spins to \\langle {\\Phi }_{{}}^{2}\\rangle is significant—even dominant—in narrow-linewidth loops.

Anton, S. M.; Sognnaes, I. A. B.; Birenbaum, J. S.; O'Kelley, S. R.; Fourie, C. J.; Clarke, John

2013-07-01

15

Open magnetic flux and magnetic flux closure during sawtooth events  

Microsoft Academic Search

We use IMAGE-FUV observations of the polar aurora and measurements of the ionospheric convection from the SuperDARN radar network to study several sawtooth events previously reported in the literature. We estimate the amount of open magnetic flux in the Earth magnetosphere during a significant part of these sawtooth intervals as well as the magnetic flux opening and closure rates, that

B. Hubert; S. E. Milan; A. Grocott; S. W. H. Cowley; J. C. Gérard

2008-01-01

16

Flux noise in SQUIDs: Effects of deposited surface films  

NASA Astrophysics Data System (ADS)

Magnetic flux noise in SQUIDs and superconducting qubits with a spectral density S?(f) scaling as 1/(f/1 Hz)^? is understood to arise from the random reversal of spins localized at the surface of the superconducting film. We present experimental results showing the effects on S?(f) of Au, SiNx, NbN, and Al2O3 films deposited on the upper surface of Nb and NbN dc SQUID loops. For each measurement, we fabricated six identical SQUIDs on a single chip and then capped the surface of either half or all the SQUID loops. Certain capping layers, such as Au, had no discernible effect on S?(f) with regard to the magnitude, slope ?, and temperature dependence. On the other hand, some capping layers significantly reduced S?(1 Hz)---by a factor of about two in the case of SiNx. Furthermore, some layers significantly affected the value of ? and the temperature dependence of both S?(1 Hz) and ?. These results further establish the importance of the role of the surface of the SQUID loop on its flux noise. We discuss implications for microscopic models of flux noise in light of these measurements.

O'Kelley, S. R.; Anton, S. M.; Birenbaum, J. S.; Clarke, John; Hilton, G. C.; Cho, H.-M.; Irwin, K. D.; Nugroho, C. D.; Dove, A. F.; Olson, G. A.; Yoscovits, Z. R.; Orlyanchik, V.; van Harlingen, D. J.; Eckstein, J. N.

2013-03-01

17

Magnetic noise in structured hard magnets.  

PubMed

The anomalous Hall effect of wires patterned from (Co{90}Fe{10}/Pt){n} multilayers, with 10magnetization process in a small volume of maze domains. Time-independent 1/f noise appears in samples with a quality factor Q<1 at points on the hysteresis loop where the magnetization reverses continuously. The magnetic noise is associated with reversible excursions of segments of a domain wall approximately 100 nm long. Barkhausen jumps are observed close to either the switching field or the saturation field where the noise power spectrum varies as 1/f{1.7}, and its magnitude decays with time. PMID:20366735

Diao, Zhu; Nowak, E R; Feng, Gen; Coey, J M D

2010-01-28

18

Evolution of magnetic flux ropes associated with flux transfer events and interplanetary magnetic clouds  

Microsoft Academic Search

Spacecraft observations suggest that flux transfer events and interplanetary magnetic clouds may be associated with magnetic flux ropes which are magnetic flux tubes containing helical magnetic field lines. In the magnetic flux ropes, the azimuthal magnetic field is superposed on the axial field. The time evolution of a localized magnetic flux rope is studied. A two-dimensional compressible MHD simulation code

C. Q. Wei; L. C. Lee; S. Wang; S.-I. Akasofu

1991-01-01

19

Evolution of magnetic flux ropes associated with flux transfer events and interplanetary magnetic clouds  

Microsoft Academic Search

Spacecraft observations suggest that flux transfer events and interplanetary magnetic clouds may be associated with magnetic flux ropes which are magnetic flux tubes containing helical magnetic field lines. In the magnetic flux ropes, the azimuthal magnetic field (B{sub θ}) is superposed on the axial field (B{sub z}). In this paper the time evolution of a localized magnetic flux rope is

C. Q. Wei; L. C. Lee; S. Wang; S.-I. Akasofu

1991-01-01

20

Noise and Dephasing from Surface Magnetic States in Superconducting Circuits  

NASA Astrophysics Data System (ADS)

Superconducting qubits are a leading candidate for scalable quantum information processing. In order to realize the full potential of these qubits, it is necessary to develop a more complete understanding of the microscopic physics that governs dissipation and dephasing of the quantum state. In the case of the Josephson phase and flux qubits, the dominant dephasing mechanism is an apparent low-frequency magnetic flux noise with a 1/f spectrum and a magnitude of several ??0/Hz^ 1/2 at 1 Hz, where ?0= h/2e is the magnetic flux quantum. Recent qubit results are compatible with the excess low-frequency noise measured by researchers at Berkeley more that 20 years ago in a series of experiments on SQUIDs cooled to millikelvin temperatures. The origin of this excess noise was never understood. Here we describe studies of flux noise and temperature- dependent magnetization in SQUIDs cooled to millikelvin temperatures. We observe that the flux threading the SQUIDs increases as 1/T as temperature is lowered; moreover, the flux change is proportional to the density of trapped vortices. The data is compatible with the thermal polarization of unpaired surface spins in the trapped fields of the vortices. In the absence of trapped flux, we observe evidence of spin- glass freezing at low temperature. These results suggest a microscopic explanation for the universal 1/f flux noise in SQUIDs and superconducting qubits, and suggest that suitable surface treatments of the superconducting films will lower the density of magnetic states, leading to superconducting devices with lower noise and solid-state qubits with improved coherence times.

McDermott, Robert

2009-03-01

21

Noise and nonlinearities in digital magnetic recording systems  

NASA Astrophysics Data System (ADS)

Various types of noise and nonlinearities in digital magnetic recording systems are investigated in this dissertation. Measurement techniques and analyzing methods are developed to understand each phenomenon. The nonlinearities due to the replay process using MR sensors are studied in Chapter 4. The nonlinearities are determined by comparing the measured signal with that obtained from a linear analysis. A characterization method of transition noise is developed in Chapter 5. Approximating transition noise by several leading 'modes' allows the noise parameters to be determined experimentally. Chapter 6 covers the investigation of disk substrate texture induced noise. The noise mechanism and characteristics are systematically studied. An analytical noise correlation function that directly relates the noise with the fluctuations of the textured disk surface is also developed in this chapter. An error rate model including colored and nonstationary noise is developed to further understand the impact of noise on system performance in Chapter 7. Noise with different characteristics is shown to influence the system performance differently. In addition, the influence of texture noise is examined in term of each noise parameter based upon the noise model developed in Chapter 6. Finally, in Chapter 8, the effect of finite write field rise time on recording performance is studied. Recording performance predicted by a simplified analytical model is compared with the measurements. It is shown that a slow flux rise time causes a degraded field gradient during writing, which results in a broader written transition, a larger NLTS, and noisier transition boundaries.

Xing, Xinzhi

1998-11-01

22

MAGNETIC FLUX BALANCE IN THE HELIOSPHERE  

SciTech Connect

Understanding the evolution of magnetic flux in the heliosphere remains an unresolved issue. The current solar minimum between cycles 23 and 24 is anomalously long, which gives rare insight into the long-term evolution of heliospheric magnetic flux when the coronal mass ejection (CME) rate and the flux emergence rate from CMEs were very low. The precipitous drop of heliospheric magnetic flux to levels lower than have ever been observed directly shows that there may be a persistent loss of open magnetic flux through disconnection, the reconnection between opposite polarity heliospheric magnetic field lines relatively near the Sun (beneath the Alfven point). Here, we develop a model for the levels of magnetic flux in the inner heliosphere balancing new flux injected by CMEs, flux lost through disconnection, and closed flux lost through interchange reconnection near the Sun. This magnetic flux balance is a fundamental property that regulates the plasma and radiation environment of our solar system.

Schwadron, N. A. [Department of Astronomy, Boston University, Boston, MA 02215 (United States); Connick, D. E.; Smith, C. [Physics Department, Space Science Center, University of New Hampshire, Durham, NH (United States)

2010-10-20

23

Measurement of Integrated Low Frequency Flux Noise in Superconducting Flux/Phase Qubits  

NASA Astrophysics Data System (ADS)

We measured the integrated low frequency flux noise (~1 m?0) of an rf SQUID as a flux qubit by fitting the resonant peaks from photon assistant tunneling (PAT). The energy relaxation time Tl between the ground and first excited states in the same potential well, measured directly in time domain, is 3 ns. From these results we identified low frequency flux noise as the dominant source of decoherence. In addition, we found that the measured values of integrated flux noise in three qubits of various sizes differ more than an order of magnitude.

Mao, Bo; Qiu, Wei; Han, Siyuan

2008-11-01

24

The Mems Flux Concentrator: a Device for Improving Magnetic Sensors  

NASA Astrophysics Data System (ADS)

The MEMS flux concentrator, a device for minimizing the effect of 1/f noise in magnetic sensors is described. In its static configuration, the MEMS flux concentrator's is similar to that usually employed with flux concentrators, i.e., soft magnetic materials are used to increase the field at the position of the magnetic sensor. The difference is that the MEMS flux concentrator is driven by electrostatic fields to perform oscillatory motion. This oscillatory motion modulates the field at the position of the sensor at a frequency that is twice the drive frequency. The magnetic sensors will detect at the modulation frequency and will be unaffected by pickup at the drive frequency. The modulation of the magnetic field has the effect of shifting the operating frequency from the high noise region for low frequency magnetic sensor to a higher frequency region where the noise can be two orders of magnitude lower. Two designs are presented, one in which there is an oscillatory motion of the flux concentrators about a torsional suspension and another in which a comb drive is used to obtain 15-20 micron oscillatory in-plane motion. The later design has two advantages. First, using a comb drive makes it much easier to design a layout for a practical device that provides enough space on the chip between the flux concentrators for the magnetic sensor. Second, the motion of the flux concentrators in the design using the comb drive is easier to model because the force is independent of the displacement. By a suitable choice of parameters, one can adjust the oscillatory frequency to be above 10 kHz and the amplitude of the magnetic field modulation to be greater than 3. Progress in fabricating the device will be presented.

Edelstein, Alan; Fischer, Gregory

2003-03-01

25

Solar Photospheric Magnetic Flux Tubes: Observations  

NASA Astrophysics Data System (ADS)

The magnetic field on the solar surface (SOLAR MAGNETIC FIELD) is highly filamentary, with a considerable fraction being in the form of flux tubes (roughly speaking bundles of concentrated field lines). Small-scale flux tubes have diameters below approximately 400 km. They appear bright and are the basic magnetic features underlying the magnetic network and active region plages (SOLAR CHROMOSPHER...

Solanki, S.; Murdin, P.

2000-11-01

26

Flux emergence, flux imbalance, magnetic free energy and solar flares  

NASA Astrophysics Data System (ADS)

Emergence of complex magnetic flux in the solar active regions lead to several observational effects such as a change in sunspot area and flux embalance in photospheric magnetograms. The flux emergence also results in twisted magnetic field lines that add to free energy content. The magnetic field configuration of these active regions relax to near potential-field configuration after energy release through solar flares and coronal mass ejections. In this paper, we study the relation of flare productivity of active regions with their evolution of magnetic flux emergence, flux imbalance and free energy content. We use the sunspot area and number for flux emergence study as they contain most of the concentrated magnetic flux in the active region. The magnetic flux imbalance and the free energy are estimated using the HMI/SDO magnetograms and Virial theorem method. We find that the active regions that undergo large changes in sunspot area are most flare productive. The active regions become flary when the free energy content exceeds 50% of the total energy. Although, the flary active regions show magnetic flux imbalance, it is hard to predict flare activity based on this parameter alone.

Choudhary, Debi Prasad; Gosain, Sanjay; Gopalswamy, Nat; Manoharan, P. K.; Chandra, R.; Uddin, W.; Srivastava, A. K.; Yashiro, S.; Joshi, N. C.; Kayshap, P.; Dwivedi, V. C.; Mahalakshmi, K.; Elamathi, E.; Norris, Max; Awasthi, A. K.; Jain, R.

2013-10-01

27

One-sided fluxes -- A magnetic curiosity?  

Microsoft Academic Search

It is shown that a previously unknown class of magnetization patterns exists in planar structures which have the unique property that all the flux escapes from one surface with none leaving the other side. A simple case is a constant amplitude rotating vector magnetization where the sense of rotation dictates which surface has no flux. More complicated magnetization patterns are

John C. Mallinson

1973-01-01

28

Magnetic flux tube evolution in solar wind anisotropic magnetic turbulence  

Microsoft Academic Search

The magnetic turbulence in the solar wind causes a magnetic field line transport that is reflected in the propagation in space of charged particles. Assuming a small localized source, the distribution in space of energetic particles is determined, in part, by the shape of the magnetic flux tube. The spatial evolution of a magnetic flux tube is studied here by

G. Zimbardo; P. Pommois; P. Veltri

2004-01-01

29

Dynamical decoupling and noise spectroscopy with a superconducting flux qubit  

NASA Astrophysics Data System (ADS)

We demonstrate dynamical decoupling in a superconducting flux qubit with a long energy-relaxation time, T1= 12,s. Low-frequency noise acts to dephase the qubit, reducing its transverse coherence time T2. At the noise-optimal bias point we observe a free-induction decay time T2^* = 2.5,s and T1-limited spin-echo decay, T2E= 2,1. Biased away from this point, the increased sensitivity to flux noise leads to increased echo and free-induction decay rates. We moderate the dephasing effects of this noise by applying dynamical-decoupling sequences with up to 200 ?-pulses. Using the CPMG sequence, we achieve a more than 50-fold enhanced decay time over T2^*, and Gaussian pure-dephasing times T> 100,s. We use the filtering property of this pulse sequence to facilitate spectroscopy of the environmental noise and reconstruct its 1/f power spectral density, which we independently confirm by a Rabi-spectroscopy approach. We characterize the noise sources coupling to the energy-bias and tunnel-coupling terms of the Hamiltonian.

Bylander, Jonas; Gustavsson, Simon; Yan, Fei; Yoshihara, Fumiki; Harrabi, Khalil; Cory, David; Nakamura, Yasunobu; Tsai, Jaw-Shen; Oliver, William D.

2011-03-01

30

Magnetic flux amplification by Lenz lenses  

NASA Astrophysics Data System (ADS)

Tailoring magnetic flux distribution is highly desirable in a wide range of applications such as magnetic sensors and biomedicine. In this paper we study the manipulation of induced currents in passive devices in order to engineer the distribution of magnetic flux intensity in a given region. We propose two different approaches, one based on especially designed wire loops (Lenz law) and the other based on solid conductive pieces (eddy currents). The gain of such devices is mainly determined by geometry giving perspective of high amplification. We consistently modeled, simulated, and executed the proposed devices. Doubled magnetic flux intensity is demonstrated experimentally for a moderate aspect ratio.

Schoenmaker, J.; Pirota, K. R.; Teixeira, J. C.

2013-08-01

31

Analysis of Vibration and Acoustic Noise in Permanent Magnet Motors.  

NASA Astrophysics Data System (ADS)

The drive motor is a frequent source of vibration and acoustic noise in many precision spindle motors. One of the electromagnetic sources of vibration in permanent magnet motors is the torque ripple, consisting of the reluctance torque and electromagnetic torque fluctuation. This type of vibration is becoming more serious with the advent of new high-grade magnets with increased flux density. Acoustic noise of electromagnetic origin is difficult to predict and its exact mechanism is unclear. The mechanism of noise generation should be revealed to design a quieter motor which is the modern customer's demand. For motor operation at low speeds and loads, torque ripple due to the reluctance torque is often a source of vibration and control difficulty. The reluctance torque in a motor was calculated from the flux density by a finite element method and the Maxwell stress method. Effects of design parameters, such as stator slot width, permanent slot width, airgap length and magnetization direction, were investigated. Magnet pole shaping, by gradually decreasing the magnet thickness toward edges, yields a sinusoidal shape of the reluctance torque with reduced harmonics, thus reducing the vibration. This dissertation also presents two motor design techniques: stator tooth notching and rotor pole skewing with magnet pole shaping, and the effect of each method on the output torque. The analysis shows that the reluctance torque can be nearly eliminated by the suggested designs, with minimal sacrifice of the output torque. In permanent magnet DC motors, the most popular design type is the trapezoidal back electro-motive force (BEMF), for switched DC controllers. It is demonstrated that the output torque profile of one phase energized is qualitatively equivalent to the BEMF profile for motors with reduced reluctance torque. It implies that design of BEMF profile is possible by magnetic modeling of a motor, without expensive and time-consuming experiments for different designs. The effect of various design parameters on the output torque and torque ripple are discussed. Design parameters include winding patterns, magnetization direction, magnet arc length, number of segments in poles and magnet pole shaping. New designs of trapezoidal BEMF motors are proposed to reduce the electromagnetic torque ripple. Magnet stepping and magnet edge shaping with reduced arc length, significantly reduce torque ripple, with minimal sacrifice of the maximum output torque. Acoustic noise of electromagnetic origin is investigated using a magnetic frame which emulates a DC motor. The driving electromagnetic force is calculated using finite element analysis and the resulting vibration and acoustic noise is measured. Acoustic noise of purely electromagnetic origin was also tested with a DC brushless motor to confirm the results of the magnetic frame. The mechanism of noise generation in a DC motor is a quasi-static response of a stator not only at the fundamental frequency but also at higher harmonic frequencies of alternating switched DC, which is a current characteristic of a DC motor. Noise generation is significantly aggravated when some of those harmonics are close to the resonant frequencies of the stator. Therefore, acoustic noise is highly dependent upon the excitation current shape, as higher harmonics may match with resonant frequencies of the stator.

Hwang, Sangmoon

32

HELICAL LENGTHS OF MAGNETIC CLOUDS FROM THE MAGNETIC FLUX CONSERVATION  

SciTech Connect

We estimate axial lengths of helical parts in magnetic clouds (MCs) at 1 AU from the magnetic flux (magnetic helicity) conservation between solar active regions (ARs) and MCs with the event list of Leamon et al. Namely, considering poloidal magnetic flux (PHI{sub P}) conservation between MCs and ARs, we estimated L{sub h} in MCs, where L{sub h} is the axial length of an MC where poloidal magnetic flux and magnetic twist exist. It is found that L{sub h} is 0.01-1.25 AU in the MCs. If the cylinder flux rope picture is assumed, this result leads to a possible new picture of the cylinder model whose helical structure (namely, poloidal magnetic flux) localizes in a part of a MC.

Yamamoto, Tetsuya T. [Solar-Terrestrial Environment Laboratory, Nagoya University, Chikusa, Nagoya, 464-8601 (Japan); Kataoka, R. [Interactive Research Center of Science, Tokyo Institute of Technology, Meguro, 152-8550 (Japan); Inoue, S., E-mail: tyamamot@stelab.nagoya-u.ac.j [National Institute of Information and Communications Technology, Koganei, 184-8795 (Japan)

2010-02-10

33

Photospheric processes and magnetic flux tubes  

NASA Astrophysics Data System (ADS)

New high-resolution observations reveal that small-scale magnetic flux concentrations have a delicate substructure on a spatial scale of 0.1''. Their basic structure can be interpreted in terms of a magnetic flux sheet or tube that vertically extends through the ambient weak-field or field-free atmosphere with which it is in mechanical equilibrium. A more refined interpretation comes from new three-dimensional magnetohydrodynamic simulations that are capable of reproducing the corrugated shape of magnetic flux concentrations and their signature in the visible continuum. Faculae are another manifestation of small-scale magnetic flux concentrations. It is shown that the characteristic asymmetric shape of the contrast profile of faculae is an effect of radiative transfer across the rarefied atmosphere of the magnetic flux concentration. Also discussed are three-dimensional radiation magnetohydrodynamic simulations of the integral layers from the top of the convection zone to the mid-chromosphere. They show a highly dynamic chromospheric magnetic field, marked by rapidly moving filaments of stronger than average magnetic field that form in the compression zone downstream and along propagating shock fronts. The simulations confirm the picture of flux concentrations that strongly expand through the photosphere into a more homogeneous, space filling chromospheric field. Future directions in the simulation of small-scale magnetic fields are indicated with a few examples from recent reports. The second part of these lecture notes is devoted to a few basic properties of magnetic flux tubes that can be considered to be an abstraction of the more complicated flux concentrations known from observations and numerical simulations. By analytical means we will find that an electrical current flows in a sheet at the surface of a flux-tube for which location we also derive the mechanical equilibrium condition. The equations for constructing a magnetohydrostatic flux tube embedded in a gravitationally stratified atmosphere are derived. It is shown that the expansion of a flux tube with height sensibly depends on the difference in the thermal structure between the atmosphere of the flux tube and the surrounding atmosphere. Furthermore, we will find that radiative equilibrium produces a smaller temperature gradient within the flux tube compared to that in the surrounding atmosphere. The condition for interchange stability is derived and it is shown that small-scale magnetic flux concentrations are liable to the interchange instability.

Steiner, Oskar

2007-07-01

34

Magnetic flux lines as relativistic strings  

Microsoft Academic Search

It is suggested that the relativistic string of the dual resonance model of hadrons is a line of quantised magnetic flux. Accordingly, quarks have magnetic charge. Assuming quarks of magnetic charge +g,-g, baryons are composed of three quarks. States of one, two, four or five quarks will not normally occur.

L. J. Tassie

1973-01-01

35

Magnetic flux line in a disordered medium  

Microsoft Academic Search

We numerically study a single flux-line motion through a disordered medium, on the basis of the Langevin-type stochastic equation of motion. The flux line is driven by a transport current and also fluctuates by the disorder and the thermal noise. Performing large-scale simulations of the model for parameters associated with the YBCO sample, we examine some scaling exponents characterizing the

Yoshihisa Enomoto; Sadamichi Maekawa

1994-01-01

36

Magnetic flux, Wilson line, and orbifold  

Microsoft Academic Search

We study torus\\/orbifold models with magnetic flux and Wilson line backgrounds. The number of zero modes and their profiles depend on those backgrounds. That has interesting implications from the viewpoint of particle phenomenology.

Hiroyuki Abe; Kang-Sin Choi; Tatsuo Kobayashi; Hiroshi Ohki

2009-01-01

37

Plastic deformation effects on magnetic Barkhausen noise  

Microsoft Academic Search

Plastically deformed steel specimens were analyzed using surface magnetic Barkhausen noise (MBN). Three types of samples were investigated: mild steel specimens uniaxially deformed up to 40% strain, mild steel specimens deformed in bending to 0.2% longitudinal strain, cold-rolled nuclear reactor pressure vessel steel specimens from 0% to 60% reduction ratio. The angular dependence of an “MBNenergy” term was used to

C.-G. Stefanita; L. Clapham; D. L. Atherton

2000-01-01

38

Plastic deformation effects on magnetic Barkhausen noise  

Microsoft Academic Search

Plastically deformed steel specimens were analyzed using surface magnetic Barkhausen noise (MBN). Three types of samples were investigated: mild steel specimens uniaxially deformed up to 40% strain, mild steel specimens deformed in bending to 0.2% longitudinal strain, cold-rolled nuclear reactor pressure vessel steel specimens from 0% to 60% reduction ratio. The angular dependence of an ``MBNenergy'' term was used to

C.-G. Stefanita; L. Clapham; D. L. Atherton

2000-01-01

39

Vector Magnetic Field in Emerging Flux Regions  

NASA Astrophysics Data System (ADS)

A crucial phase in magnetic flux emergence is the rise of magnetic flux tubes through the solar photosphere, which represents a severe transition between the very different environments of the solar interior and corona. Multi-wavelength observations with Flare Genesis, TRACE, SoHO, and more recently with the vector magnetographs at THEMIS and Hida (DST) led to the following conclusions. The fragmented magnetic field in the emergence region - with dipped field lines or bald patches - is directly related with Ellerman bombs, arch filament systems, and overlying coronal loops. Measurements of vector magnetic fields have given evidence that undulating "serpentine" fields are present while magnetic flux tubes cross the photosphere. See the sketch below, and for more detail see Pariat et al. (2004, 2007); Watanabe et al. (2008):

Schmieder, B.; Pariat, E.

40

Equivalent Magnetic Noise Limit of Low-Cost GMI Magnetometer  

Microsoft Academic Search

We present a noise analysis of a giant magnetoimpedance (GMI) sensor using a peak detector at the optimal magnetic field working bias point of a sensor wire, by considering internal noise sources (intrinsic GMI device associated noise sources and conditioning electronic noise sources). An expression is obtained for the theoretical expected noise for known electronic design parameters and physical characteristics

Lehui Ding; SÉbastien Saez; Christophe Dolabdjian; Luiz G. C. Melo; Arthur Yelon; David Menard

2009-01-01

41

Magnetic flux lines as relativistic strings  

Microsoft Academic Search

It is suggested that the relativistic string of the dual resonance model ; of hadrons is a line of quantized magnetic flux. Accordingly, quarks have ; maaetic charge. Assuming quarks of magnetic charge +g, --2g, baryons are ; composed of three quarks. States of one, two, four or five quarks will not ; normally occur. (auth);

L Tassie

1973-01-01

42

Modeling Magnetic Flux Emergence in HD 106225  

NASA Astrophysics Data System (ADS)

Using a ``thin flux tube'' approximation proposed by Spruit (1981), and working with a numerical simulation previously used to model solar magnetic flux dynamics, we attempt to create magnetic flux tube emergence in HD 106225, a rapidly rotating subgiant star, which has been observed to have both x-ray emission and starspots. Initial attempts, placing the stellar dynamo at the base of the convection zone, as has been done in solar modeling, were unsuccessful. Fisher, McClymont, & Chou (1991) show that, if the pressure scale height at the base of the convection zone is a significant fraction of the radius at that point (Lambda /R > 0.3), the magnetic tension force increases faster than the buoyancy as the tube is stretched, and no rise occurs from a tube anchored at the base of the convection zone. However, Spruit & van Ballegooijen (1982a, b) prove that, for stars with sufficiently deep convection zones, it is possible to create a buoyantly stable flux tube in the convection zone itself. The stability of a given mode again depends on the value of Lambda /R at a given radius. If Lambda /R > 0.3, a flux tube is able to maintain null buoyancy in the convection zone proper. Using Spruit & van Ballegooijen's solutions, we are able to run numerical simulations that model stable flux tubes in the convection zone. We have also been able to create the rise of a magnetic flux perturbation by placing the base of the ring at a stable convective radius with the perturbation extending to a more buoyant radius. Initial attempts have been based on a simplified model, placing a flux tube at zero latitude with no rotation. We also attempt simulations which allow for flux rise at non-zero latitudes and include Coriolis force. Using these results, we examine the implications that such a magnetic field pattern has for the stellar dynamo.

Wills, M. J.; Deluca, E. E.; Guenther, D. B.

1996-05-01

43

Poloidal magnetic flux control experiments in Rotamak  

NASA Astrophysics Data System (ADS)

A new set of equilibrium coils has been installed in cylindrical chamber rotamak to allow for an active control of poloidal magnetic flux in 40-ms plasma discharges. The coils are powered by programmable current source with 1-3 ms response time. The coils allow controlling both the shape of the plasma and the boundary flux magnitude. Without the current in the coils, the boundary flux drops from vacuum value of 0.30 mWb to 0.05-0.10 mWb after the plasma current is generated. If the current is applied to the coils, the boundary flux can be maintained within 0.22-0.26 mWb range, thus keeping the separatrix away from chamber walls during plasma shot. The new system also includes passive flux conserving rings that help to eliminate fast variations of the boundary flux.

Petrov, Yuri; Yang, Xiaokang; Huang, Tian-Sen

2008-11-01

44

Color magnetic flux tubes in dense QCD  

SciTech Connect

QCD is expected to be in the color-flavor locking phase in high baryon density, which exhibits color superconductivity. The most fundamental topological objects in the color superconductor are non-Abelian vortices which are topologically stable color magnetic flux tubes. We present numerical solutions of the color magnetic flux tube for diverse choices of the coupling constants based on the Ginzburg-Landau Lagrangian. We also analytically study its asymptotic profiles and find that they are different from the case of usual superconductors. We propose the width of color magnetic fluxes and find that it is larger than naive expectation of the Compton wavelength of the massive gluon when the gluon mass is larger than the scalar mass.

Eto, Minoru [Theoretical Physics Laboratory, RIKEN, Saitama 351-0198 (Japan); Nitta, Muneto [Department of Physics, and Research and Education Center for Natural Sciences, Keio University, 4-1-1 Hiyoshi, Yokohama, Kanagawa 223-8521 (Japan)

2009-12-15

45

Optimization of multiply acquired magnetic flux density Bz using ICNE-Multiecho train in MREIT  

Microsoft Academic Search

The aim of magnetic resonance electrical impedance tomography (MREIT) is to visualize the electrical properties, conductivity or current density of an object by injection of current. Recently, the prolonged data acquisition time when using the injected current nonlinear encoding (ICNE) method has been advantageous for measurement of magnetic flux density data, Bz, for MREIT in the signal-to-noise ratio (SNR). However,

Hyun Soo Nam; Oh In Kwon

2010-01-01

46

Low-frequency noise in high-transition-temperature superconducting multilayer magnetometers in ambient magnetic fields  

SciTech Connect

Each magnetometer consisted of a dc superconducting quantum interference device (SQUID), with the YBa{sub 2}Cu{sub 3}O{sub 7-x} washer patterned into 4 {mu}m lines, coupled to a multiturn, multilayer flux transformer containing flux dams. The noise at 1 Hz did not increase when bare SQUIDs or magnetometers were cooled and operated in fields up to values well above the magnetic field of the earth. When the magnetic field was changed, the noise in a bare SQUID was constant up to a threshold field of 12 {mu}T. The addition of a flux transformer containing flux dams increased the magnetic field sensitivity by a factor of 43, while reducing the threshold field only moderately, to 5 {mu}T.

Cho, Hsiao-Mei; Andresen, Y. T.; Clarke, John; DiIorio, M. S.; Yang, K.-Y.; Yoshizumi, S.

2001-10-08

47

Trapped magnetic flux in superconducting niobium samples  

NASA Astrophysics Data System (ADS)

Trapped magnetic flux is known to be one cause of residual losses in bulk niobium superconducting radio frequency cavities. In the Meissner state an ambient magnetic field should be expelled from the material. Disturbances such as lattice defects or impurities have the ability to inhibit the expulsion of an external field during the superconducting transition so that the field is trapped. We have investigated the effect the treatment history of bulk niobium has on the trapped flux and which treatment leads to minimal flux trapping. For that purpose, we measured the fraction of trapped magnetic flux in niobium samples representing cavities with different typical treatment histories. The differences between single crystal and polycrystalline material as well as the influence of spatial temperature gradients and different cooling rates were investigated. In addition, the progression of the release of a trapped field during warm-up was studied. We found that heat treatment reduces trapped flux considerably and that single crystal samples trap less flux than polycrystalline niobium. As a consequence, the single crystal sample with 1200°C baking trapped the smallest amount of field which is about 42%. Moreover, the release of the trapped field during warm-up was observed to progress over a broad temperature range for the baked single crystal samples.

Aull, S.; Kugeler, O.; Knobloch, J.

2012-06-01

48

Topology of magnetic flux ropes and formation of fossil flux transfer events and boundary layer plasmas  

Microsoft Academic Search

At the dayside magnetopause, magnetic flux ropes can form as a result of multiple X line reconnection. In this process the presence of at least two parallel X lines leads to the formation of a magnetic flux rope with each additional X line giving another flux rope. For a finite extent of these X lines the magnetic flux of the

L. C. Lee; Z. W. Ma; Z. F. Fu; A. Otto

1993-01-01

49

Plastic deformation effects on magnetic Barkhausen noise  

NASA Astrophysics Data System (ADS)

Plastically deformed steel specimens were analyzed using surface magnetic Barkhausen noise (MBN). Three types of samples were investigated: mild steel specimens uniaxially deformed up to 40% strain, mild steel specimens deformed in bending to 0.2% longitudinal strain, cold-rolled nuclear reactor pressure vessel steel specimens from 0% to 60% reduction ratio. The angular dependence of an ``MBNenergy'' term was used to examine magnetic anisotropy changes with increasing deformation, with linear MBN scans used to monitor changes in magnetic properties along the specimen axis. Average MBNenergy values were observe to vary significantly with stress within the elastic region, yet only slightly in the plastic range of deformation. Changes in the shape of the pulse height distribution curves were evidence of plastic deformation effects on domain wall pinning sites. We conclude that a complex interplay between stress and strain mechanisms, residual stresses and crystallographic texture generates variations in MBN signal. .

Stefanita, C.-G.; Clapham, L.; Atherton, D. L.

2000-05-01

50

What causes the Heliospheric Magnetic Flux Excess?  

NASA Astrophysics Data System (ADS)

Ulysses results indicate that the total magnetic flux inside the heliosphere, ?, can be obtained from the radial field component measured at a single spacecraft multiplied by the square of the radial distance and averaged over a solar rotation, < BR r2>. This result is contrary to a large increase in ? with distance, called the Flux Excess published in the JGR by Owens et al. (2008) and by Lockwood et al. (2009a,b) who attribute it to variations in solar wind speed. Ulysses data and a mathematical simulation are used to show that the Flux Excess is caused by replacing BR with the modulus, |BR|. The modulus rectifies the large amplitude magnetic field variations normally present in measurements of BR and increases the mean of < |BR| r2> relative to < BR r2>. The variance of the magnetic fluctuations decreases less rapidly with distance than BR and produces a progressively larger error in < |BR| r2> resulting in the Flux Excess. The advisability of defining ? in terms of |BR|, of using < |BR| r2> beyond 1 AU and the applicability of the Lockwood et al. (2009) proposed correction to the Flux Excess are questioned.

Smith, E. J.

2011-12-01

51

Magnetic flux evolution in highly shaped plasmas  

NASA Astrophysics Data System (ADS)

The resistive evolution of magnetic flux in toroidal devices is studied. The formulation is applicable to general nonaxisymmetric (three-dimensional) toroidal configurations. In particular, it can treat highly shaped, high ?, three-dimensional stellarator configurations, as well as two-dimensional (axisymmetric) tokamak plasmas. The time evolution of the poloidal magnetic flux is posed in terms of the rotational transform, ?, and allows for a transparent inclusion of stellarator specific current-free contributions to ?. Strong diamagnetic and paramagnetic contributions to toroidal magnetic flux, as evident in spherical tokamaks and similar concepts, are calculated by direct iteration with an equilibrium solver. The nonlinear evolution equation is derived using a susceptance matrix formulation originally introduced by Grad and co-workers [Bateman, Nucl. Fusion 13, 227 (1973)]. Here, it is extended to general, nonstraight field line coordinate systems. The basic equations are described, explicit expressions for the susceptance matrix are given, and example applications using the stand-alone code, THRIFT (THRee-dimensional Inductive Flux evolution in Toroidal devices), are discussed.

Strand, P. I.; Houlberg, W. A.

2001-06-01

52

Concentrated winding axial flux permanent magnet motor with plastic bonded magnets and sintered segmented magnets  

Microsoft Academic Search

Direct drive axial flux permanent magnet (PM) motors are a cost effective and an energy saving choice for industrial use. Open slots make concentrated winding machines a favourable configuration with respect to manufacturing. However, open slots expose rotor surface magnets to large flux pulsations and the losses of sintered magnets may not be neglected. Plastic bonded magnets have very low

Hanne Jussila; Pia Salminen; Asko Parviainen; Janne Nerg; Juha Pyrhönen

2008-01-01

53

Magnetic merging in colliding flux tubes  

NASA Astrophysics Data System (ADS)

We develop an analytical theory of reconnection between colliding, twisted magnetic flux tubes. Our analysis is restricted to direct collisions between parallel tubes and is based on the collision dynamics worked out by Bogdan (1984). We show that there is a range of collision velocities for which neutral point reconnection of the Parker-Sweet type can occur, and a smaller range for which reconnection leads to coalescence. Mean velocities within the solar convection zone are probably significantly greater than the upper limit for coalescence. This suggests that the majority of flux tube collisions do not result in merging, unless the frictional coupling of the tubes to the background flow is extremely strong.

Zweibel, Ellen G.; Rhoads, James E.

1995-02-01

54

Stochastic flux freezing and magnetic dynamo  

SciTech Connect

Magnetic flux conservation in turbulent plasmas at high magnetic Reynolds numbers is argued neither to hold in the conventional sense nor to be entirely broken, but instead to be valid in a statistical sense associated to the ''spontaneous stochasticity'' of Lagrangian particle trajectories. The latter phenomenon is due to the explosive separation of particles undergoing turbulent Richardson diffusion, which leads to a breakdown of Laplacian determinism for classical dynamics. Empirical evidence is presented for spontaneous stochasticity, including numerical results. A Lagrangian path-integral approach is then exploited to establish stochastic flux freezing for resistive hydromagnetic equations and to argue, based on the properties of Richardson diffusion, that flux conservation must remain stochastic at infinite magnetic Reynolds number. An important application of these results is the kinematic, fluctuation dynamo in nonhelical, incompressible turbulence at magnetic Prandtl number (Pr{sub m}) equal to unity. Numerical results on the Lagrangian dynamo mechanisms by a stochastic particle method demonstrate a strong similarity between the Pr{sub m}=1 and 0 dynamos. Stochasticity of field-line motion is an essential ingredient of both. Finally, some consequences for nonlinear magnetohydrodynamic turbulence, dynamo, and reconnection are briefly considered.

Eyink, Gregory L. [Department of Applied Mathematics and Statistics and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218 (United States)

2011-05-15

55

Stochastic flux freezing and magnetic dynamo  

NASA Astrophysics Data System (ADS)

Magnetic flux conservation in turbulent plasmas at high magnetic Reynolds numbers is argued neither to hold in the conventional sense nor to be entirely broken, but instead to be valid in a statistical sense associated to the “spontaneous stochasticity” of Lagrangian particle trajectories. The latter phenomenon is due to the explosive separation of particles undergoing turbulent Richardson diffusion, which leads to a breakdown of Laplacian determinism for classical dynamics. Empirical evidence is presented for spontaneous stochasticity, including numerical results. A Lagrangian path-integral approach is then exploited to establish stochastic flux freezing for resistive hydromagnetic equations and to argue, based on the properties of Richardson diffusion, that flux conservation must remain stochastic at infinite magnetic Reynolds number. An important application of these results is the kinematic, fluctuation dynamo in nonhelical, incompressible turbulence at magnetic Prandtl number (Prm) equal to unity. Numerical results on the Lagrangian dynamo mechanisms by a stochastic particle method demonstrate a strong similarity between the Prm=1 and 0 dynamos. Stochasticity of field-line motion is an essential ingredient of both. Finally, some consequences for nonlinear magnetohydrodynamic turbulence, dynamo, and reconnection are briefly considered.

Eyink, Gregory L.

2011-05-01

56

Stochastic flux freezing and magnetic dynamo.  

PubMed

Magnetic flux conservation in turbulent plasmas at high magnetic Reynolds numbers is argued neither to hold in the conventional sense nor to be entirely broken, but instead to be valid in a statistical sense associated to the "spontaneous stochasticity" of Lagrangian particle trajectories. The latter phenomenon is due to the explosive separation of particles undergoing turbulent Richardson diffusion, which leads to a breakdown of Laplacian determinism for classical dynamics. Empirical evidence is presented for spontaneous stochasticity, including numerical results. A Lagrangian path-integral approach is then exploited to establish stochastic flux freezing for resistive hydromagnetic equations and to argue, based on the properties of Richardson diffusion, that flux conservation must remain stochastic at infinite magnetic Reynolds number. An important application of these results is the kinematic, fluctuation dynamo in nonhelical, incompressible turbulence at magnetic Prandtl number (Pr(m)) equal to unity. Numerical results on the Lagrangian dynamo mechanisms by a stochastic particle method demonstrate a strong similarity between the Pr(m)=1 and 0 dynamos. Stochasticity of field-line motion is an essential ingredient of both. Finally, some consequences for nonlinear magnetohydrodynamic turbulence, dynamo, and reconnection are briefly considered. PMID:21728673

Eyink, Gregory L

2011-05-27

57

Scattering from a magnetic flux line due to the Lorentz force of the return flux  

Microsoft Academic Search

The differential cross section for the scattering of an electron by an impenetrable, infinitely long magnetic flux line with return magnetic flux uniformly distributed on the surface of a cylinder of radius a is calculated. The scattering is due to the Lorentz force from the magnetic field of the return flux. In the limit as the radius of the return

Donald H. Kobe; J. Q. Liang

1988-01-01

58

Analytical calculation of magnetic flux lines in 3-D space  

Microsoft Academic Search

We propose a new analytical method for computation of magnetic flux lines in 3-D space. Using the results obtained from finite element analysis and the magnetic flux line equation in 3-D space, the sequence of line segments that construct the magnetic flux line is computed analytically. The proposed method reduces computational time by nearly five times for the same accuracy

Vlatko Cingoski; Manabu Ichinose; Kazufumi Kaneda; Hideo Yamashita

1994-01-01

59

Observations of Synchronous Magnetic Flux Emergence  

NASA Astrophysics Data System (ADS)

We report on observations of spatially-separated, nearly simultaneous small-scale magnetic flux emergence seen in SDO/HMI magnetograms. In the prototypical event, two pairs of bipoles emerge with a nearly identical north-south orientation, the beginning of the separate emergence events begins within 3 hours of each other, and the distance between the pair is approximately twice the bipolar separation distance. This suggests a common subsurface origin. We present a detailed analysis of this emergence event and the associated evolving coronal magnetic field topology, show other examples of similar events, and present a preliminary statistical analysis of the likelihood of such events occurring by chance.

Lamb, Derek; DeForest, C.

2013-07-01

60

Motion of magnetic flux lines in magnetohydrodynamics  

Microsoft Academic Search

A gauge-free description of magnetohydrodynamic flows of an ideal incompressible fluid, which takes into account the freezing-in\\u000a of the magnetic field and the presence of cross invariants containing the vorticity, is obtained. This description is an extension\\u000a of the canonical formalism well-known in ordinary hydrodynamics to the dynamics of frozen-in flux lines. Magnetohydrodynamics\\u000a is studied as the long-wavelength limit of

V. P. Ruban

1999-01-01

61

Magnetic flux tube models in superstring theory  

Microsoft Academic Search

Superstring models describing curved 4-dimensional magnetic flux tube backgrounds are exactly solvable in terms of free fields. We first consider the simplest model of this type (corresponding to a ‘Kaluza-Klein’ a = ?3 Melvin background). Its 2d action has a flat but topologically non-trivial 10-dimensional target space (there is a mixing of the angular coordinate of the 2-plane with an

J. G. Russo; A. A. Tseytlin

1996-01-01

62

Evaluation of front flux guide-type magnetic tunnel junction heads  

Microsoft Academic Search

Magnetic tunnel junction (MTJ) heads have been successfully fabricated using the free layer as flux guide to prevent electrical short during the definition of the air bearing surface (ABS). For a 6 Gbits\\/in2 design, an output as high as 5740 ?Vpp was achieved for 3 mA sense current, and the output waveform was stable and noise free. Noise analysis confirmed

K. Shimazawa; O. Redon; N. Kasahara; J. J. Sun; K. Sato; T. Kagami; S. Saruki; T. Umehara; Y. Fujita; S. Yarimizu; S. Araki; H. Morita; M. Matsuzaki

2000-01-01

63

Flux Rope Acceleration and Enhanced Magnetic Reconnection  

NASA Astrophysics Data System (ADS)

Recent flare and CME observations have shown that flux ropes are accelerated in the low corona in the form of coronal mass ejections or filament eruptions or soft X-ray plasma ejecta. The conclusion from these observations is that the most intense peak in the flare hard X-ray emissions and the maximum rate of increase in soft X-ray emission occur at the time of maximum acceleration of the flux rope's rising motion. To understand these observations we present results of our 2-1/2D MHD simulations of arcade field evolutions by employing a nonuniform anomalous resistivity. The simulation results relate the flux rope's accelerated rising motion with an enhanced magnetic reconnection rate and thus an enhanced reconnection electric field in the current sheet during the flare rise phase. The simulation results provide good quantitative agreement with observations of the acceleration of flux ropes (CMEs) for several CME-flare events. For the X-class flare events the peak reconnection electric field is ˜ O(103 V/m) or larger, enough to accelerate electrons to over 100 keV in a field-aligned distance of 0.1 km and produce impulsive hard X-ray emission observed during the flare rise phase, consistent with the estimated reconnection rate obtained from the magnetogram data and two-ribbon emissions. Comparisons of the flux rope height, velocity and acceleration between our simulation results and observed CME-flare events will be presented.

Ren, Y.; Cheng, C. Z.; Choe, G. S.; Qiu, J.; Moon, Y. J.

2003-12-01

64

Magnetic Field Noise from High Permeability Magnetic Shields for Precision Measurements  

Microsoft Academic Search

High permeability magnetic shields often generate magnetic field noise that can limit the sensitivity of precision measurements. We show that calculations based on the fluctuation-dissipation theorem allow evaluation of magnetic field noise, either by current or spin fluctuations, from high permeability metals and ferrites over a broad frequency range. For example, the noise spectrum of a mu-metal shield generally exhibits

S.-K. Lee; S. J. Smullin; T. W. Kornack; M. V. Romalis

2007-01-01

65

Harmonic Fluxes and Electromagnetic Forces of Concentric Winding Brushless Permanent Magnet Motor  

NASA Astrophysics Data System (ADS)

Brushless permanent magnet motors have been widely used in home applications and industrial fields. These days, high efficiency and low noise motors are demanded from the view point of environment. Electromagnetic noise and iron loss of the motor are produced by the harmonic fluxes and electromagnetic forces. However, order and space pattern of these have not been discussed in detail. In this paper, fluxes, electromagnetic forces and magneto-motive forces of brushless permanent magnet motors with concentric winding were analyzed analytically, experimentally and numerically. Time harmonic fluxes and time electromagnetic forces in the air gap were measured by search coils on the inner surface of the stator teeth and analyzed by FEM. Space pattern of time harmonic fluxes and time electromagnetic forces were worked out with experiments and FEM. Magneto motive forces due to concentric winding were analyzed with equations and checked by FEM.

Ishibashi, Fuminori; Takemasa, Ryo; Matsushita, Makoto; Nishizawa, Takashi; Noda, Shinichi

66

Exploring ISEE-3 magnetic cloud polarities with electron heat fluxes  

SciTech Connect

We have used solar wind electron heat fluxes to determine the magnetic polarities of the interplanetary magnetic fields (IMF) during the ISEE-3 observations in 1978{endash}1982. That period included 14 magnetic clouds (MCs) identified by Zhang and Burlaga. The MCs have been modeled as single magnetic flux ropes, and it is generally assumed that they are magnetically closed structures with each end of the flux rope connected to the Sun. The flux rope model is valid only if the magnetic polarity of each MC does not change during the passage of ISEE-3 through the MC. We test this model with the heat flux data, using the dominant heat flux in bidirectional electron heat fluxes to determine the MC polarities. The polarity changes within at least 2, and possibly 6, of the 14 MCs, meaning that those MCs can not fit the model of a single flux rope. {copyright} {ital 1999 American Institute of Physics.}

Kahler, S.W. [Air Force Research Laboratory, 29 Randolph Rd, Hanscom AFB, Massachusetts 01731 (United States); Crooker, N.U. [Center for Space Physics, Boston University, 725 Commonwealth Ave., Boston, Massachusetts 02215 (United States); Gosling, J.T. [Los Alamos National Laboratory, MS D 466, Los Alamos, New Mexico 87545 (United States)

1999-06-01

67

SEED BANKS FOR MAGNETIC FLUX COMPRESSION GENERATORS  

SciTech Connect

In recent years the Lawrence Livermore National Laboratory (LLNL) has been conducting experiments that require pulsed high currents to be delivered into inductive loads. The loads fall into two categories (1) pulsed high field magnets and (2) the input stage of Magnetic Flux Compression Generators (MFCG). Three capacitor banks of increasing energy storage and controls sophistication have been designed and constructed to drive these loads. One bank was developed for the magnet driving application (20kV {approx} 30kJ maximum stored energy.) Two banks where constructed as MFCG seed banks (12kV {approx} 43kJ and 26kV {approx} 450kJ). This paper will describe the design of each bank including switching, controls, circuit protection and safety.

Fulkerson, E S

2008-05-14

68

Signal extraction using ensemble empirical mode decomposition and sparsity in pipeline magnetic flux leakage nondestructive evaluation  

Microsoft Academic Search

The commonly used and cost effective corrosion inspection tools for the evaluation of pipelines utilize the magnetic flux leakage (MFL) technique. The MFL signal is usually contaminated by various noise sources. In this paper, we propose that the pipeline flaw MFL signal is extracted using the ensemble empirical mode decomposition (EEMD) and the sparsity. At first, we introduce the EEMD

Liang Chen; Xing Li; Xun-Bo Li; Zuo-Ying Huang

2009-01-01

69

Advanced signal processing of magnetic flux leakage data obtained from seamless gas pipeline  

Microsoft Academic Search

Natural gas is normally transported through a vast network of pipelines. A major segment of this network employs seamless pipes. The manufacturing processes associated with the production of seamless pipes contribute to a helical variation in the grain properties of the pipe. This introduces an artifact, known as the seamless pipe noise (SPN), in the data obtained from magnetic flux

Muhammad Afzal; Satish Udpa

2002-01-01

70

Vector Magnetic Property Measurement in Magnetic Steel Sheets under High Magnetic Flux Density Conditions  

NASA Astrophysics Data System (ADS)

In order to develop higher torque, lower weight and smaller sized magnetic actuators for driving electrical vehicles and robot arms, designing under high magnetic field and high exciting voltage is most effective. It is therefore very important to understand vector magnetic properties in electrical steel sheets under high magnetic flux density conditions. The vector magnetic properties mean the relationship between the magnetic flux density vector and the magnetic field strength vector and it is very difficult to measure the each component under high magnetic flux density conditions because of the magnetic saturation and the magnetic anisotropy. In this paper, accurate measurement techniques of the vector magnetic properties over 1.5T and measured vector magnetic properties are reported and discussed.

Maeda, Yoshitaka; Todaka, Takashi; Enokizono, Masato

71

Stability and equilibrium of emerged magnetic flux  

NASA Astrophysics Data System (ADS)

We analyse stability and equilibrium of a unipolar large-scale magnetic field pervading a plane horizontal subphotospheric layer with the possible implications for sunspots in mind. Eddy diffusivity is applied to account for the effects of the small-scale convective turbulence. Diffusivity quenching by magnetic field results in a secondary large-scale instability. A linear stability analysis is performed to define the marginal stability boundary in parametric space and the unstable mode structure. The nonlinear dynamics of the unstable modes are followed numerically. The original state of a uniform vertical magnetic field is transformed via the instability into the nonlinear dynamical equilibrium with a highly intermittant distribution of the magnetic field. Magnetic flux is concentrated in a relatively small area surrounded by an almost field-free region. The role of the fluid motion in the hydromagnetic equilibrium is emphasized. Although the relevance of the instability to the process of sunspot formation is rather questionable, the resulting equilibrium structures are similar to mature spots in their thermal and magnetic properties. Also, the simulated flow structure agrees with helioseismic tomography results.

Kitchatinov, L. L.; Mazur, M. V.

2000-02-01

72

Noise-cancelling quadrature magnetic position, speed and direction sensor  

DOEpatents

An array of three magnetic sensors in a single package is employed with a single bias magnet for sensing shaft position, speed and direction of a motor in a high magnetic noise environment. Two of the three magnetic sensors are situated in an anti-phase relationship (i.e., 180.degree. out-of-phase) with respect to the relationship between the other of the two sensors and magnetically salient target, and the third magnetic sensor is situated between the anti-phase sensors. The result is quadrature sensing with noise immunity for accurate relative position, speed and direction measurements.

Preston, Mark A. (Niskayuna, NY); King, Robert D. (Schenectady, NY)

1996-01-01

73

Improvement of the superconducting magnetic levitation system for the determination of the magnetic flux quantum  

Microsoft Academic Search

The authors describe an improvement of the preliminary superconducting magnetic levitation system in progress for the absolute determination of the magnetic flux quantum. This improvement includes the development of the flux-up method to determine the flux in terms of the Josephson voltage. The improvement is essential for the determination of the magnetic flux quantum as well as of the coil

T. Endo; Y. Sakamoto; F. Shiota; K. Nakayama; Y. Nezu; M. Kikuzawa; K. Hara

1989-01-01

74

Chara cterization of Wire Rope Defects from Magnetic Flux Leakage Signals  

Microsoft Academic Search

When using magnetic flux leakage to inspect wire rope, fwo fypes of signals are produced for analysis, nurn.l-y, Lo=calized Fault (LF) and Loss of Magnetic Cross-section Area (LMA)' However, these signals contain a lot of other noises which makes the defects difficult to perfectly analyze. Therefore, this paper rs written to characterize and analyze signals of the wire rope defects

C. Jomdecha; A. PrateePasen; W. Methong

2003-01-01

75

Flux Leakage Measurements for Defect Characterization Using a High Precision 3AXIAL Gmr Magnetic Sensor  

Microsoft Academic Search

High-precision magnetic field sensors are of increasing interest in non destructive testing (NDT). In particular GMR-sensors (giant magneto resistance) are qualified because of their high sensitivity, high signal-to-noise ratio and high spatial resolution. With a GMR-gradiometer and a 3D-GMR-magnetometer we performed magnetic flux leakage measurements of artificial cracks and cracks of a depth of <=50 mum still could be dissolved

M. Pelkner; M. Blome; V. Reimund; H.-M. Thomas; M. Kreutzbruck

2011-01-01

76

Magnetic flux leakage inspection of tailor-welded blanks  

Microsoft Academic Search

A feasibility study was conducted on the application of magnetic flux leakage (MFL) inspection to the evaluation of weld quality in automotive tailor-welded blanks (TWB). Using a permanent magnet configuration, magnetic flux was directed through the weld region of a TWB. A Hall effect sensor was coupled to the movement of a digital plotter and was, thereby, scanned around the

S. O'Connor; L. Clapham; P. Wild

2002-01-01

77

On-line noise monitoring at the Fast Flux Test Facility  

SciTech Connect

An automated noise surveillance and diagnostics system (ANSDS) is being demonstrated at the Fast Flux Test Facility (FFTF). Three low-level, in-vessel fission chambers (LLFMs), three ex-vessel compensated ion chambers (CICs), and two accelerometers on the mechanism of one advanced absorber (ADVAB) control rod were monitored with an automated noise surveillance and diagnostic system (ANSDS) in late 1983.

Mullens, J.A.; Thie, J.A.; Campbell, L.R.

1984-01-01

78

The Evolution of Open Magnetic Flux Driven by Photospheric Dynamics  

Microsoft Academic Search

The coronal magnetic field is of paramount importance in solar and heliospheric physics. Two profoundly different views of the coronal magnetic field have emerged. In quasi-steady models, the predominant source of open magnetic field is in coronal holes. In contrast, in the interchange model, the open magnetic flux is conserved, and the coronal magnetic field can only respond to the

Jon A. Linker; Roberto Lionello; Zoran Mikic; Viacheslav S. Titov; Spiro K. Antiochos; Spiro K

2011-01-01

79

Simulation of magnetic flux leakage (MFL) analysis using FEMM software  

Microsoft Academic Search

Magnetic flux leakage (MFL) is a method used in oil and gas piping systems which has potential in medical gas piping inspection, cleaning and gauging tasks without interrupting the pipeline transportation process. MFL is a magnetic method of nondestructive testing that applies a magnet to magnetize the steel. At areas where there is corrosion or missing metal, the magnetic field

Zulkarnay Zakaria; M. S. B. Mansor; A. H. Jahidin; M. S. Z. Azlan; Ruzairi Abdul Rahim

2010-01-01

80

Magnetic Flux Pileup and Magnetic Field Dipolarization During Substorm  

NASA Astrophysics Data System (ADS)

During the period from July to October in 2004, the orbit apogee of Double Star TC-1 of 13Re located in the magnetotail, which made it possible to survey some basic features of substorm processes in the magnetotail. In the present paper we make a case study of substorm occurring on Sep. 17, 2004. At about 0117 UT FUV WIC on board IMAGE observed an aurora breakup. Almost at the same time the LANL-01A and 90-085 at the geosynchronous altitude detected dispersionless injection of energetic electrons. About 2-3 minutes later a weak dipolarization was seen by NOAA geosynchronous satellite GOES 12. Interestingly, ten minutes before substorm onset, Cluster observed an earthward flow at (-15.00, 2.05, 3.50) Re(GSM). At 0116 UT (one min before onset) TC-1 saw a beginning of magnetic flux pile-up at its position (-10.26, -1.36, 1.01) REGSM, characterized by an increasing of Bz component with almost constant Bx and thermal pressure. At about 0126 UT (9 minutes after onset) TC-1 observed a local dipolarization of the magnetic field which was characterized by a rapid decrease of Bx component and a sharp jump in the thermal pressure, together with a continuous increase of Bz. In the literature, some authors treated the flux pile-up and dipolarization in the tail as a single process named `dipolarization'. However, as TC-1 measurements show in this event, flux pile-up and dipolarization are characterized by completely different signatures. The same feature holds for many other events. Detailed inspections of TC-1 data in this event show that the beginning of flux pile-up was about 10 minutes preceding the dipolarization. Besides, the latter lasted only for about two minutes, whereas the former kept for almost one hour. All these aforementioned differences imply that flux pile-up and dipolarization are two different dynamical processes. Nevertheless, further studies indicate that they are `cause-effect' related: The magnetic flux pileup may lead the field lines with `good' curvature to change to the `bad' ones, further compresses the magnetic field and yields disturbances. These can set up appropriate conditions for development of instabilities and then trigger substorm expansion onset. In summary, this case study seems consistent with the synthesis substorm model of near-Earth neutral line (NENL) and near-Earth current disruption (NECD).

Zhang, H.; Pu, Z.; Cao, X.; Fu, S.; Xiao, C.; Zong, Q.; Liu, Z.; Cao, J.; Korth, A.; Frazen, M.; Carr, C.; Reme, H.; Glassmeier, K.

2005-12-01

81

Fiber-optic Faraday-effect magnetic-field sensor based on flux concentrators.  

PubMed

The principles and performance of a fiber-optic Faraday-effect magnetic-field sensor designed around an yttrium-iron-garnet (YIG) sensing element and two flux concentrators are described. The system design exploits the technique of polarization-rotated reflection in which a single polarization-maintaining optical fiber links the sensor head to the optical source and detection system. In the sensing head, ferrite flux concentrators are magnetically coupled to the YIG sensing element to achieve maximum sensitivity. The system exhibits a noise equivalent field of 6 pT/?Hz and a 3-dB bandwidth of~10 MHz. PMID:21068992

Deeter, M N

1996-01-01

82

Unbalanced magnetic forces in rotational unsymmetrical transverse flux machine  

Microsoft Academic Search

The torque and unbalanced magnetic forces in a small flux concentrating permanent magnet transverse flux machine (FCPM-TFM) in segmented compact structure, which are the resultants of the tangential; axial and normal magnetic forces, respectively, are in general influenced by pole-teeth-winding configuration. The paper calculates the torque and unbalanced magnetic forces using finite element (FE-) method by solving Maxwell stress tensor

Salwa Baserrah; Keno Rixen; Bernd Orlik

2010-01-01

83

Thermal magnetic noise generated by an eddy current shielded room  

SciTech Connect

Using a SQUID-based second-order gradiometer, we have measured the thermal magnetic noise generated by a thick aluminum plate and an eddy current shielded room used for biomagnetic measurements. The measured noise agrees with the theoretical prediction for the thermal noise fields generated by infinite conducting slabs. The noise is substantial near the walls of the shielded room, but is predicted to be less than 0.02 fT/..sqrt..Hz at 1 Hz at the center of the room. This is too small to influence the biomagnetic measurements usually taken at this location.

Stroink, G.; MacAulay, C.

1986-04-01

84

Enhancement in magnetic field sensitivity and reduction in equivalent magnetic noise by magnetoelectric laminate stacks  

NASA Astrophysics Data System (ADS)

We have investigated the equivalent magnetic noise and magnetic field sensitivity for a magnetoelectric (ME) sensor unit of N numbers of ME laminates stacked together. Our results show with increasing N that the modeled and measured equivalent magnetic noises decreased by a factor of N and that the magnetic field sensitivities increased by N. For Metglas/Pb(Mg1/3,Nb2/3)O3-PbTiO3 laminates, the equivalent magnetic noise decreased and the magnetic field sensitivity increased by a factors of 2.1 and 2.3, respectively, for N = 4 relative to that for N = 1.

Li, Menghui; Gao, Junqi; Wang, Yaojin; Gray, David; Li, Jiefang; Viehland, D.

2012-05-01

85

Magnetic Flux Pileup and Magnetic Field Dipolarization during Substorm  

NASA Astrophysics Data System (ADS)

During the period from July to October in 2004 the orbit apogee of Double Star TC-1 of 13RE located in the magnetotail which made it possible to survey some basic features of substorm processes in the magnetotail In the present paper we make a case study of substorm occurring on Sep 17 2004 At about 0117 UT FUV WIC on board IMAGE observed an aurora breakup Almost at the same time the LANL-01A and 90-085 at the geosynchronous altitude detected dispersionless injection of energetic electrons About 2-3 minutes later a weak dipolarization was seen by NOAA geosynchronous satellite GOES 12 Interestingly ten minutes before substorm onset Cluster observed an earthward flow at -15 00 2 05 3 50 REGSM At 0116 UT one min before onset TC-1 saw a beginning of magnetic flux pile-up at its position -10 26 -1 36 1 01 REGSM characterized by an increasing of Bz component with almost constant Bx and thermal pressure At about 0126 UT 9 minutes after onset TC-1 observed a local dipolarization of the magnetic field which was characterized by a rapid decrease of Bx component and a sharp jump in the thermal pressure together with a continuous increase of Bz In the literature some authors treated the flux pile-up and dipolarization in the tail as a single process named odipolarization - However as TC-1 measurements show in this event flux pile-up and dipolarization are characterized by completely different signatures The same feature holds for many other events Detailed inspections of TC-1 data in this event show that the beginning of flux pile-up was 10 minutes

Zhanghui, H.; Pu, Z. Y.; Liu, Z. X.; Korth, A.; Frazen, M.; Friedel, R.; Zong, Q. G.; Reme, H.; Glassmeier, K. H.

86

Remanent state noise measurements on magnetic recording media  

NASA Astrophysics Data System (ADS)

Media noise has been investigated as a function of both the isothermal remanent magnetisation (IRM) and DC-demagnetisation (DCD) processes on a modified audio transport. Captured noise time series from a given remanent state has been analysed for spectral content using signal processing software. These noise spectra display changes in magnetic microstructure of the tested media with remanent state. Measurements have been made on a range of commercial and laboratory produced tapes. The paper concentrates on examples of commercial samples which have been chosen to indicate various noise characteristics. Conventional media usually show similar features, but changes in remanent state noise has been observed in tapes for new formats and in recent manufacture of conventional formats. This indicates changes in magnetic microstructure associated with new particles and dispersion techniques.

McCann, S. M.; Bissell, P. R.; Onions, T.; Mercer, T.

1998-03-01

87

Reconnection of Twisted Magnetic Flux Tubes  

NASA Astrophysics Data System (ADS)

We present 3D MHD simulations of the collision and reconnection of pairs of twisted, isolated magnetic flux tubes at various collision angles. Such reconnection is likely to be an important source of energy release in solar flares, and could play a role in coronal mass ejection dynamics. We show that the dynamics of the reconnection depends strongly on the collision angle between the tube axes and on the relative sign of twist of the tubes. The most energetic interaction is a slingshot interaction, analogous to the reconnection often seen in 2D simulations. But, depending on the configuration, the tubes can also bounce without reconnecting, merge into a single tube, or tunnel through each other. We will discuss these various interactions, the topological changes they bring about, and the magnetic energy released. In addition we will present an analytical model which explains some of the results, in particular the tunnel and slingshot interactions, in terms of a simple energy calculation based on helicity conservation. This work was supported by NASA and ONR grants, an ITP-NSF grant, and a grant of computer time from the DoD/HPC Program.

Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K.

2002-05-01

88

Numerical study of magnetic flux lines in random media  

Microsoft Academic Search

We study numerically the dynamical behavior of magnetic flux lines in type-II superconductors, on the basis of the stochastic equation of motion for interacting flux lines with anisotropic effective mass. We first discuss a computationally efficient modeling for this equation and then carry out three-dimensional simulations for the flux line dynamics in random media. We particularly examine the effects of

Yoshihisa Enomoto; Ken-Ichi Katsumi

1992-01-01

89

Suppression of Leakage Flux in Magnetic Air Circuit Breakers  

Microsoft Academic Search

The need for increased ratings of magnetic air breakers has required critical studies of and improvements in the various factors involved in the interrupting process in these devices. One important factor, the magnetic field, has been extensively explored by means of probe coils connected to integrating circuits in such a manner that instantaneous values of useful flux, leakage flux, and

A. P. Strom

1958-01-01

90

A mathematical model of the reusable linear magnetic flux compressor  

Microsoft Academic Search

The reusable linear magnetic flux compressor not only can offer fairly high energy density and work repeatedly, but also can be statically inert prior to firing. Therefore, it can be regarded as an ideal pulsed power supply for the electric gun. Based on theoretical studies of this magnetic flux compressor, a mathematical model is developed and described in this paper.

Ying Wang; Jun Li; Zanji Wang; Min Gao; Yanjie Cao

2001-01-01

91

Numerical 3D simulations of buoyant magnetic flux tubes  

NASA Astrophysics Data System (ADS)

We have examined instabilities of non-thin buoyant magnetic flux tubes ascending through a solar convection zone model using numerical 3D MHD experiments. The experiments show that the fate of the flux tubes is entirely dependent on the internal topology of the magnetic field lines in the flux tube; if the initial topology is too simple the tube is quickly disrupted by a Rayleigh-Taylor like instability. The disruption is prevented or delayed if the field has a component that makes the topology non-trivial. Even a weak random or twisting component, an order of magnitude weaker than the longitudinal magnetic field, is sufficient to let the tube ascend as a more or less coherent structure. These 3D results may resolve the apparent contradiction between the success of experiments using the thin flux tube approximation to study the buoyant rise of magnetic flux tubes, and the rapid break-up of flux tubes found in 2D experiments.

Dorch, S. B. F.; Nordlund, A.

1998-10-01

92

Magnetohydrodynamic simulations of the motion of magnetic flux tubes through a magnetized plasma  

Microsoft Academic Search

Magnetohydrodynamic simulations of the evolution of a flux tube accelerated through a stationary magnetized plasma are presented. As the flux tube moves through the external plasma, its shape becomes distorted and reconnection can take place between the flux tube and external fields. The coupling between the moving flux tube and the external plasma is generally efficient, with simulated flux tube

P. J. Cargill; J. Chen; D. S. Spicer; S. T. Zalesak

1996-01-01

93

Signal photon flux and background noise in a coupling electromagnetic detecting system for high-frequency gravitational waves  

SciTech Connect

A coupling system among Gaussian-type microwave photon flux, a static magnetic field, and fractal membranes (or other equivalent microwave lenses) can be used to detect high-frequency gravitational waves (HFGWs) in the microwave band. We study the signal photon flux, background photon flux, and the requisite minimal accumulation time of the signal in the coupling system. Unlike the pure inverse Gertsenshtein effect (G effect) caused by the HFGWs in the gigahertz band, the electromagnetic (EM) detecting scheme proposed by China and the U.S. HFGW groups is based on the composite effect of the synchroresonance effect and the inverse G effect. The key parameter in the scheme is the first-order perturbative photon flux (PPF) and not the second-order PPF; the distinguishable signal is the transverse first-order PPF and not the longitudinal PPF; the photon flux focused by the fractal membranes or other equivalent microwave lenses is not only the transverse first-order PPF but the total transverse photon flux, and these photon fluxes have different signal-to-noise ratios at the different receiving surfaces. Theoretical analysis and numerical estimation show that the requisite minimal accumulation time of the signal at the special receiving surfaces and in the background noise fluctuation would be {approx}10{sup 3}-10{sup 5} seconds for the typical laboratory condition and parameters of h{sub rms}{approx}10{sup -26}-10{sup -30}/{radical}(Hz) at 5 GHz with bandwidth {approx}1 Hz. In addition, we review the inverse G effect in the EM detection of the HFGWs, and it is shown that the EM detecting scheme based only on the pure inverse G effect in the laboratory condition would not be useful to detect HFGWs in the microwave band.

Li Fangyu; Yang Nan; Fang Zhenyun; Wen Hao [Department of Physics, Chongqing University, Chongqing 400044 (China); Baker, Robert M. L. Jr. [GRAWAVE registered LLC, 8123 Tuscany Avenue, Playa del Rey, California 90293 (United States); Stephenson, Gary V. [Seculine Consulting, P.O. Box 925, Redondo Beach, California 90277 (United States)

2009-09-15

94

Magnetic flux emergence and associated dynamic phenomena in the Sun.  

PubMed

We present a review of the process of magnetic flux emergence in the Sun. We focus on observations and numerical experiments that explore the dynamical rise of magnetic fields from the solar interior to the corona. We describe the response of the highly stratified solar atmosphere on flux emergence and, consequently, we present a comprehensive picture of the coupling between solar dynamic events and flux emergence. We discuss potential applications of this process in other astrophysical environments. PMID:22665894

Archontis, Vasilis

2012-07-13

95

Magnetic noises due to direct-current electric railcars  

Microsoft Academic Search

Biomagnetic signals are very weak compared with those of environmental magnetic fields due to trains and motor cars. In Japan, fluctuation and frequency dependence of the environmental magnetic noises at low frequencies in urban areas are considerably different between the daytime when direct-current (dc) electric railcars are running and the nighttime when there is no traffic. In Kagoshima city, a

K. Yamazaki; K. Kato; K. Kobayashi; A. Haga; Y. Kumagai; K. Fujiwara

96

Spatially variable Rician noise in magnetic resonance imaging.  

PubMed

Magnetic resonance images tend to be influenced by various random factors usually referred to as "noise". The principal sources of noise and related artefacts can be divided into two types: arising from hardware (acquisition coil arrays, gradient coils, field inhomogeneity); and arising from the subject (physiological noise including body motion, cardiac pulsation or respiratory motion). These factors negatively affect the resolution and reproducibility of the images. Therefore, a proper noise treatment is important for improving the performance of clinical and research investigations. Noise reduction becomes especially critical for the images with a low signal-to-noise ratio, such as those typically acquired in diffusion tensor imaging at high diffusion weightings. The standard methods of signal correction usually assume a uniform distribution of the standard deviation of the noise across the image and evaluate a single correction parameter for the whole image. We pursue a more advanced approach based on the assumption of an inhomogeneous distribution of noise in space and evaluate correction factors for each voxel individually. The Rician nature of the underlying noise is considered for low and high signal-to-noise ratios. The approach developed here has been examined using numerical simulations and in vivo brain diffusion tensor imaging experiments. The efficacy and usefulness of this approach is demonstrated here and the resultant effective tool is described. PMID:22209560

Maximov, Ivan I; Farrher, Ezequiel; Grinberg, Farida; Shah, N Jon

2011-12-10

97

Magnetic Flux Ropes in the Martian Atmosphere: Global Characteristics  

NASA Astrophysics Data System (ADS)

We report observations of magnetic fields amplitude, which consist of a series of individual spikes in the Martian atmosphere. A minimum variance analysis shows that these spikes form twisted cylindrical filaments. These small diameter magnetic filaments are commonly called magnetic flux ropes. We examine the global characteristics of magnetic flux ropes, which are observed on 5% of the elliptical orbits of Mars Global Surveyor. Flux ropes are more often observed in Venus' atmosphere (70% of the orbits). In this paper we report some of the global characteristics of the flux ropes identified in the Martian atmosphere. No flux ropes are observed in the southern hemisphere of Mars. Most of them occur at high solar zenith angles, close to the terminator plane, and at high latitude with altitudes below 400 km. The orientation of the flux ropes appears random while in the case of Venus the orientation is more horizontal near the terminator for altitudes greater than 200 km. We have identified fewer flux ropes for SZA between 40 to 60 deg and for SZA lower than 20 deg, like in the case of Venus (Elphic and Russell, 1983b). Statistically, Mars' ionosphere with SZA range between 40circ to 60circ is less magnetized than near the subsolar point. As the Martian ionosphere is quite often magnetized by the magnetic components of the crustal field, this crustal magnetic field seems to inhibit the flux ropes formation in the southern hemisphere. However, some orbits without crustal magnetic field, called magnetic cavities, were observed without flux ropes. So the flux ropes formation process seems to be uppressed by another factor, like the solar wind dynamic pressure for Venus (Krymskii and Breus, 1988).

Vignes, D.; Acuña, M. H.; Connerney, J. E. P.; Crider, D. H.; Rème, H.; Mazelle, C.

2004-03-01

98

Protostellar Accretion Flows Destabilized by Magnetic Flux Redistribution  

NASA Astrophysics Data System (ADS)

Magnetic flux redistribution lies at the heart of the problem of star formation in dense cores of molecular clouds that are magnetized to a realistic level. If all of the magnetic flux of a typical core were to be dragged into the central star, the stellar field strength would be orders of magnitude higher than the observed values. This well-known magnetic flux problem can in principle be resolved through non-ideal MHD effects. Two-dimensional (axisymmetric) calculations have shown that ambipolar diffusion, in particular, can transport magnetic flux outward relative to matter, allowing material to enter the central object without dragging the field lines along. We show through simulations that such axisymmetric protostellar accretion flows are unstable in three dimensions to magnetic interchange instability in the azimuthal direction. The instability is driven by the magnetic flux redistributed from the matter that enters the central object. It typically starts to develop during the transition from the prestellar phase of star formation to the protostellar mass accretion phase. In the latter phase, the magnetic flux is transported outward mainly through advection by strongly magnetized low-density regions that expand against the collapsing inflow. The tussle between the gravity-driven infall and magnetically driven expansion leads to a highly filamentary inner accretion flow that is more disordered than previously envisioned. The efficient outward transport of magnetic flux by advection lowers the field strength at small radii, making the magnetic braking less efficient and the formation of rotationally supported disks easier in principle. However, we find no evidence for such disks in any of our rotating collapse simulations. We conclude that the inner protostellar accretion flow is shaped to a large extent by the flux redistribution-driven magnetic interchange instability. How disks form in such an environment is unclear.

Krasnopolsky, Ruben; Li, Zhi-Yun; Shang, Hsien; Zhao, Bo

2012-09-01

99

Magnetic field analysis of a novel flux switching transverse flux permanent magnet wind generator with 3-D FEM  

Microsoft Academic Search

Transverse flux permanent magnet machine (TFPMM) offers a higher power density than the conventional radial and axial ones. Based on the principle of TFPMM, a novel flux switching transverse flux PM generator (FS-TFPMG) with a unique structure is presented for low speed wind power applications in this paper. The stator space utilization can be improved by arranging more stator cores

Jianhu Yan; Heyun Lin; Yunkai Huang; Hengchuan Liu; Z. Q. Zhu

2009-01-01

100

Investigation of Low-Frequency Excess Flux Noise in DC SQUIDs at mK Temperatures  

Microsoft Academic Search

The excess low-frequency flux noise in dc super- conducting quantum interference devices (SQUIDs) operated at ultra-low temperatures was studied. A large number of single SQUIDs as well as SQUID arrays from 16 wafers fabricated over a period of six years were characterized at 4.2 K and 320 mK. Considering the large spread in the low-frequency noise at 4.2 K, there

Dietmar Drung; Jörn Beyer; Jan-Hendrik Storm; Margret Peters; Thomas Schurig

2011-01-01

101

Fluctuation-dissipation theorem and flux noise in overdamped Josephson-junction arrays  

Microsoft Academic Search

The form of the fluctuation-dissipation theorem for a resistively shunted\\u000aJosephson juction array is derived with the help of the method which\\u000aexplicitely takes into acoount screening effects. This result is used to\\u000aexpress the flux noise power spectrum in terms of frequency dependent sheet\\u000aimpedance of the array. The relation between noise amplitude and parameters of\\u000athe detection coil

S. E. Korshunov; L. D. Landau

2002-01-01

102

Flux Transport and the Sun's Global Magnetic Field (Invited)  

NASA Astrophysics Data System (ADS)

The Sun’s global magnetic field is produced and evolved through the emergence of magnetic flux in active regions and its transport across the solar surface by the axisymmetric differential rotation and meridional flow and the non-axisymmetric convective flows of granulation, supergranulation, and giant cell convection. Maps of the global magnetic field serve as the inner boundary condition for space weather. The photospheric magnetic field and its evolution determine the coronal and solar wind structures through which CMEs must propagate and in which solar energetic particles are accelerated and propagate. Producing magnetic maps which best represent the actual field configuration at any instant requires knowing the magnetic field over the observed hemisphere as well as knowing the flows that transport flux. From our Earth-based vantage point we only observe the front-side hemisphere and each pole is observable for only six months of the year at best. Models for the surface magnetic flux transport can be used to provide updates to the magnetic field configuration in those unseen regions. In this presentation I will describe successes and failures of surface flux transport and present new observations on the structure, the solar cycle variability, and the evolution of the flows involved in magnetic flux transport. I find that supergranules play the dominant role due to their strong flow velocities and long lifetimes. Flux is transported by differential rotation and meridional flow only to the extent that the supergranules participate in those two flows.

Hathaway, D. H.

2010-12-01

103

Solar noise storms and magnetic sector structures  

Microsoft Academic Search

A synoptic study of the occurrence and polarization of 160 MHz noise storms recorded at Culgoora during the current solar cycle shows that the storm sources occur in large unipolar cells extending >90° in solar longitude and ?60° in latitude, with lifetimes of ~ 1 yr. From solar maximum onwards these large cells stretch across the solar equator to form

R. T. Stewart

1985-01-01

104

OBSERVATIONS OF A SMALL INTERPLANETARY MAGNETIC FLUX ROPE ASSOCIATED WITH A MAGNETIC RECONNECTION EXHAUST  

SciTech Connect

A small interplanetary magnetic flux rope prior to an X-line magnetic reconnection exhaust was observed on 1998 March 25 at 1 AU. The X-line magnetic reconnection exhaust has been identified and reported by Gosling et al. The duration of this small magnetic flux rope is about 2 hr. We fitted the constant alpha force-free model to the observed magnetic fields. The model fitting results show that the spacecraft crosses the magnetic flux rope well away from the axis, with d {sub 0}/R {sub 0} being 0.76. The fitting results also show that its magnetic configuration is a right-handed helical flux rope, that the estimated field intensity at the axis is 16.3 nT, and that its diameter is 0.0190 AU. In addition, the axial direction of this rope is (theta = 6 deg., phi = 214 deg.), namely, this magnetic flux rope is lying nearly in the ecliptic plane. According to the geometric relation of the small flux rope and the reconnection exhaust, it is very possible that the small magnetic flux rope has a larger scale initially and comes from the corona; its magnetic fields are peeled off when moving from the Sun to the Earth and at last it reaches a small scale. Though magnetic reconnection can produce a flux-rope topology, in this case the X-line magnetic reconnection is destroying rather than generating the small magnetic flux rope.

Feng, H. Q. [College of Physics and Electronics Science, Luoyang Normal University, Luoyang (China); Wu, D. J., E-mail: hengqiangfeng@gmail.co [Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008 (China)

2009-11-10

105

Magnetic Field Noise from High Permeability Magnetic Shields for Precision Measurements  

NASA Astrophysics Data System (ADS)

High permeability magnetic shields often generate magnetic field noise that can limit the sensitivity of precision measurements. We show that calculations based on the fluctuation-dissipation theorem allow evaluation of magnetic field noise, either by current or spin fluctuations, from high permeability metals and ferrites over a broad frequency range. For example, the noise spectrum of a mu-metal shield generally exhibits three distinct frequency dependent behaviors: low frequency 1/f spin noise, white noise due to Johnson noise current, and high frequency roll-off due to self-shielding. To reduce the effect of Johnson noise current, we built a multi-layer shield for a potassium SERF atomic magnetometer using ferrite for the innermost layer. We found that the white noise was reduced from about 20 fT/Hz^1/2, as expected for an all-mu metal shield, to 0.75 fT/Hz^1/2, limited by laser noise. The low frequency 1/f noise agreed well with calculation based on the measured complex permeability of the ferrite. Our method can be used to identify low noise shielding materials for further suppression of shield-generated noise for compact atomic magnetometers.

Lee, S.-K.; Smullin, S. J.; Kornack, T. W.; Romalis, M. V.

2007-06-01

106

Two-dimensional magnetic flux shell model for magnetohydrodynamic simulations  

Microsoft Academic Search

A magnetic flux shell model is described which is suitable for numerical simulations of cylindrically symmetric plasmas. The magnetohydrodynamic equations in magnetic field line coordinates are derived including the effects of finite electrical conductivity and anisotropic heat conductivity. Subsequently, the equations of motion of coaxial plasma shells of constant density, temperature, axial velocity, and axial magnetic field are derived including

J. N. McMullin; R. D. Milroy; C. E. Capjack

1979-01-01

107

Magnetic Field Noise Changes Effect of Combined Magnetic Field on Gravitropic Reaction of Cress Roots.  

NASA Astrophysics Data System (ADS)

The gravitropic reaction of cress roots in combined magnetic field was studied in details. It was shown that the negative roots gravitropism observed at the frequency of alternating component of combined magnetic field adjusted to the Ca ion cyclotron frequency could be observed only at Nayquist magnetic field noise level under 5 nT/Hz. While the magnetic noise level was increasing the negative gravitropism was disappearing. The inhibition of gravitropic reaction was observed in all cases. The effect was accompanied by the changes in the noise spectrum generated by cress roots.

Bogatina, Nina; Kordyum, Elizabeth; Sheykina, Nadezhda

108

Low-noise nano superconducting quantum interference device operating in Tesla magnetic fields.  

PubMed

Superconductivity in the cuprate YBa(2)Cu(3)O(7) (YBCO) persists up to huge magnetic fields (B) up to several tens of Teslas, and sensitive direct current (dc) superconducting quantum interference devices (SQUIDs) can be realized in epitaxially grown YBCO films by using grain boundary Josephson junctions (GBJs). Here we present the realization of high-quality YBCO nanoSQUIDs, patterned by focused ion beam milling. We demonstrate low-noise performance of such a SQUID up to B = 1 T applied parallel to the plane of the SQUID loop at the temperature T = 4.2 K. The GBJs are shunted by a thin Au layer to provide nonhysteretic current voltage characteristics, and the SQUID incorporates a 90 nm wide constriction which is used for on-chip modulation of the magnetic flux through the SQUID loop. The white flux noise of the device increases only slightly from 1.3 ??(0)/(Hz)(1/2) at B = 0 to 2.3 ??(0)/(Hz))(1/2) at 1 T. Assuming that a point-like magnetic particle with magnetization in the plane of the SQUID loop is placed directly on top of the constriction and taking into account the geometry of the SQUID, we calculate a spin sensitivity S(?)(1/2) = 62 ?(B)/(Hz))(1/2) at B = 0 and 110 ?(B)/(Hz))(1/2) at 1 T. The demonstration of low noise of such a SQUID in Tesla fields is a decisive step toward utilizing the full potential of ultrasensitive nanoSQUIDs for direct measurements of magnetic hysteresis curves of magnetic nanoparticles and molecular magnets. PMID:23252846

Schwarz, Tobias; Nagel, Joachim; Wölbing, Roman; Kemmler, Matthias; Kleiner, Reinhold; Koelle, Dieter

2012-12-27

109

MAGNETIC HELICITY FLUX IN THE PRESENCE OF SHEAR  

SciTech Connect

Magnetic helicity has risen to be a major player in dynamo theory, with the helicity of the small-scale field being linked to the dynamo saturation process for the large-scale field. It is a nearly conserved quantity, which allows its evolution equation to be written in terms of production and flux terms. The flux term can be decomposed in a variety of fashions. One particular contribution that has been expected to play a significant role in dynamos in the presence of mean shear was isolated by Vishniac and Cho. Magnetic helicity fluxes are explicitly gauge dependent however, and the correlations that have come to be called the Vishniac-Cho flux were determined in the Coulomb gauge, which turns out to be fraught with complications in shearing systems. While the fluxes of small-scale helicity are explicitly gauge dependent, their divergences can be gauge independent. We use this property to investigate magnetic helicity fluxes of the small-scale field through direct numerical simulations in a shearing-box system and find that in a numerically usable gauge the divergence of the small-scale helicity flux vanishes, while the divergence of the Vishniac-Cho flux remains finite. We attribute this seeming contradiction to the existence of horizontal fluxes of small-scale magnetic helicity with finite divergences.

Hubbard, Alexander; Brandenburg, Axel, E-mail: alex.i.hubbard@gmail.com [NORDITA, AlbaNova University Center, Roslagstullsbacken 23, SE 10691 Stockholm (Sweden)

2011-01-20

110

Magnetic Flux Paradigm for Radio Loudness of Active Galactic Nuclei  

NASA Astrophysics Data System (ADS)

We argue that the magnetic flux threading the black hole (BH), rather than BH spin or Eddington ratio, is the dominant factor in launching powerful jets and thus determining the radio loudness of active galactic nuclei (AGNs). Most AGNs are radio quiet because the thin accretion disks that feed them are inefficient in depositing magnetic flux close to the BH. Flux accumulation is more likely to occur during a hot accretion (or thick disk) phase, and we argue that radio-loud quasars and strong emission-line radio galaxies occur only when a massive, cold accretion event follows an episode of hot accretion. Such an event might be triggered by the merger of a giant elliptical galaxy with a disk galaxy. This picture supports the idea that flux accumulation can lead to the formation of a so-called magnetically choked accretion flow. The large observed range in radio loudness reflects not only the magnitude of the flux pressed against the BH, but also the decrease in UV flux from the disk, due to its disruption by the "magnetosphere" associated with the accumulated flux. While the strongest jets result from the secular accumulation of flux, moderate jet activity can also be triggered by fluctuations in the magnetic flux deposited by turbulent, hot inner regions of otherwise thin accretion disks, or by the dissipation of turbulent fields in accretion disk coronae. These processes could be responsible for jet production in Seyferts and low-luminosity AGNs, as well as jets associated with X-ray binaries.

Sikora, Marek; Begelman, Mitchell C.

2013-02-01

111

Unsteady wandering magnetic field lines, turbulence and laboratory flux ropes  

NASA Astrophysics Data System (ADS)

We describe earth bound laboratory experiment investigations of patchy, unsteady, bursty, patchy magnetic field structures that are unifying features of magnetic reconnection and turbulence in helio, space and astro physics. Macroscopic field lines occupy cross sectional areas, fill up three dimensional (3D) volumes as flux tubes. They contain mass with Newtonian dynamics that follow magneto-hydro-dynamic (MHD) equations of motion. Flux rope geometry can be ubiquitous in laminar reconnection sheet geometries that are themselves unstable to formation of secondary "islands" that in 3D are really flux ropes. Flux ropes are ubiquitous structures on the sun and the rest of the heliosphere. Understanding the dynamics of flux ropes and their mutual interactions offers the key to many important astrophysical phenomena, including magnetic reconnection and turbulence. We describe laboratory investigations on RSX, where 3D interaction of flux ropes can be studied in great detail. We use experimental probes inside the the flux ropes to measure the magnetic and electric fields, current density, density, temperatures, pressure, and electrostatic and vector plasma potentials. Macroscopic magnetic field lines, unsteady wandering characteristics, and dynamic objects with structure down to the dissipation scale length can be traced from data sets in a 3D volume. Computational approaches are finally able to tackle simple 3D systems and we sketch some intriguing simulation results that are consistent with 3D extensions of typical 2D cartoons for magnetic reconnection and turbulence.

Intrator, T.; Sears, J.; Weber, T.; Liu, D.; Pulliam, D.; Lazarian, A.

2011-12-01

112

Magnetic flux pumping and the structure of a sunspot penumbra  

NASA Astrophysics Data System (ADS)

We propose an overall scenario for the development and maintenance of a sunspot penumbra, in which turbulent magnetic flux pumping plays a key role. Recent high-resolution observations have revealed arched, "returning" magnetic flux tubes that emerge in the inner or middle penumbra, dive back down below the solar surface near the outer edge of the penumbra, and carry much of the Evershed flow. Some mechanism is required to keep the outer parts of the returning flux tubes submerged in spite of their magnetic buoyancy. We have proposed that the relevant mechanism is downward turbulent pumping of magnetic flux by granular convection in the moat outside the sunspot. This mechanism is demonstrated by means of an appropriate three-dimensional numerical simulation of turbulent compressible convection in the strongly superadiabatic granulation layer. We suggest that a penumbra first forms through a convectively driven instability at the outer edge of a growing pore. The nonlinear development of this instability pUSA)roduces the filamentary penumbra with its interlocking-comb magnetic field geometry. Downward flux pumping of some of the nearly horizontal magnetic flux in the dark filaments produces the returning flux tubes, with their associated Evershed flow, and also establishes the subcritical nature of the bifurcation that produces the filamentary penumbra, which explains why there are pores larger than the smallest sunspots.

Thomas, J. H.; Weiss, N. O.; Tobias, S. M.; Brummell, N. H.

2002-07-01

113

Characteristics of Ionospheric Magnetic Flux Ropes on Venus  

NASA Astrophysics Data System (ADS)

Because Venus has no intrinsic magnetic field, its atmosphere is more directly exposed to the solar wind than an atmosphere of a magnetized planet such as Earth. One observed consequence of this solar wind-atmosphere interaction is the presence of magnetic flux ropes, approximately cylindrical structures consisting of twisted magnetic field lines. The central region of a flux rope contains current that can transport charged particles and may therefore aid in atmospheric escape from Venus. Flux ropes in the ionosphere are observed more often during solar maximum periods, when increased photoionization creates an ionospheric thermal pressure sufficient to exclude the solar wind magnetic field. Despite the discovery of flux ropes more than 30 years ago and the availability of new observations since the arrival of Venus Express (VEX) in 2006, the formation mechanism for ionospheric flux ropes is still unresolved. We present the results of a manual survey of magnetic field data from the VEX magnetometer (MAG) for magnetic flux ropes, which present as localized peaks in magnetic field strength with field rotations consistent with flux rope geometry. We survey data from evenly spaced month-long time intervals from 2006 - 2012 to examine the effects of different stages of the solar cycle on flux rope properties such as location, half-length, and orientation relative to the planet. We present trends in the properties of observed flux ropes, how they are affected by the solar wind, and how they compare to previous results. This research is supported by a NASA Venus Express Supporting Investigator grant.

Steichen, Elise; McEnulty, T. R.; Molaverdikhani, K.; Brain, D.; Zhang, T.

2013-10-01

114

Solar Magnetic Tracking. II. The Apparent Unipolar Origin of Quiet-Sun Flux  

NASA Astrophysics Data System (ADS)

We investigate the origin of small-scale flux concentrations in the quiet Sun. In apparent violation of the physical requirement for flux balance, 94% of the features containing newly detected flux are unipolar at a resolution of 1.2". We analyze 2619 of these apparent unipolar emergences in an image sequence from the SOHO MDI magnetograph and compare the ensemble average to a model of asymmetric bipolar emergence that could in principle hide opposing flux under the noise floor of MDI. We examine the statistical consequences of this mechanism and find that it cannot be responsible for more than a small fraction of the unipolar emergences. We conclude that the majority of the newly detected flux in the quiet Sun is instead due to the coalescence of previously existing but unresolved flux into concentrations that are large and strong enough to be detected. We estimate the rate of coalescence into arcsecond-scale magnetic features averaged over the solar surface to be 7×1021 Mx hr-1, comparable to the reported flux injection rate due to ephemeral regions. This implies that most flux in the solar network has been processed by very small scale shredding, emergence, cancellation, and/or coalescence that is not resolved at 1.2", and it suggests that currently unresolved emergences may be at least as important as ephemeral region emergences to the overall flux budget.

Lamb, D. A.; DeForest, C. E.; Hagenaar, H. J.; Parnell, C. E.; Welsch, B. T.

2008-02-01

115

Localization of Metal-Induced Gap States at the Metal-Insulator Interface: Origin of Flux Noise in SQUIDs and Superconducting Qubits  

NASA Astrophysics Data System (ADS)

The origin of magnetic flux noise in dc Superconducting Quantum Interference Devices (SQUIDs) with a power spectrum scaling as 1/f (f is frequency) has been a puzzle for over 25 years. This noise limits both the low frequency performance of SQUIDs and the decoherence time of flux-sensitive superconducting qubits, making scaling-up for quantum computing problematic. Recent calculations and experiments indicate that the noise is generated by electrons that randomly reverse their spin directions. Their areal density of ˜ 5 x 10^17 m-2 is relatively insensitive to the nature of the superconductor and substrate. Here, we propose that the local magnetic moments originate in metal-induced gap states (MIGSs) localized by potential disorder at the metal-insulator interface. MIGSs are particularly sensitive to such disorder, so that the localized states have a Coulomb repulsion sufficiently large to make them singly occupied. Our calculations demonstrate that a modest level of disorder generates the required areal density of localized moments. This result suggests that magnetic flux noise could be reduced by fabricating superconductor-insulator interfaces with less disorder. Support: NSF DMR07-05941, US DOE De-AC02-05CH11231, Samsung Foundation, Teragrid, NERSC.

Choi, Sangkook; Lee, Dung-Hai; Louie, Steven G.; Clarke, John

2010-03-01

116

Characteristics and prediction of magnetic noise due to DC electric railcars for biomagnetic measurements  

Microsoft Academic Search

A magnetically shielded room for biomagnetism should be designed taking account of the S\\/N ratio of the biomagnetic signal to the environmental magnetic noise generated mainly by dc electric railcars at an installed location. This paper describes the characteristics of the fluctuation of magnetic noise and a method of prediction of its amplitude. The distance dependence of the magnetic noise

Keita Yamazaki; Kazuo Kato; Koichiro Kobayashi; Yoshinori Uchikawa; Yoji Kumagai; Akira Haga; Koji Fujiwara

2001-01-01

117

The classical and quantum theory of thermal magnetic noise, with applications in spintronics and quantum microscopy  

Microsoft Academic Search

Thermal fluctuations generate magnetic noise in the vicinity of any conductive and\\/or magnetically permeable solid. This magnetic noise plays a fundamental role in the design of spintronic devices: namely, it sets the time scale during which electron spins retain their coherence. This paper presents a rigorous classical and quantum analysis of thermal magnetic noise, together with practical engineering examples. Starting

JOHN A. SIDLES; JOSEPH L. GARBINI; WILLIAM M. DOUGHERTY; Shih-Hui Chao

2003-01-01

118

Photospheric and Subphotospheric Dynamics of Emerging Magnetic Flux  

NASA Astrophysics Data System (ADS)

Magnetic fields emerging from the Sun's interior carry information about physical processes of magnetic field generation and transport in the convection zone. Soon after appearance on the solar surface the magnetic flux gets concentrated in sunspot regions and causes numerous active phenomena on the Sun. This paper discusses some properties of the emerging magnetic flux observed on the solar surface and in the interior. A statistical analysis of variations of the tilt angle of bipolar magnetic regions during the emergence shows that the systematic tilt with respect to the equator (the Joy's law) is most likely established below the surface. However, no evidence of the dependence of the tilt angle on the amount of emerging magnetic flux, predicted by the rising magnetic flux rope theories, is found. Analysis of surface plasma flows in a large emerging active region reveals strong localized upflows and downflows at the initial phase of emergence but finds no evidence for large-scale flows indicating future appearance a large-scale magnetic structure. Local helioseismology provides important tools for mapping perturbations of the wave speed and mass flows below the surface. Initial results from SOHO/MDI and GONG reveal strong diverging flows during the flux emergence, and also localized converging flows around stable sunspots. The wave speed images obtained during the process of formation of a large active region, NOAA 10488, indicate that the magnetic flux gets concentrated in strong field structures just below the surface. Further studies of magnetic flux emergence require systematic helioseismic observations from the ground and space, and realistic MHD simulations of the subsurface dynamics.

Kosovichev, A. G.

119

Photospheric and Subphotospheric Dynamics of Emerging Magnetic Flux  

NASA Astrophysics Data System (ADS)

Magnetic fields emerging from the Sun’s interior carry information about physical processes of magnetic field generation and transport in the convection zone. Soon after appearance on the solar surface the magnetic flux gets concentrated in sunspot regions and causes numerous active phenomena on the Sun. This paper discusses some properties of the emerging magnetic flux observed on the solar surface and in the interior. A statistical analysis of variations of the tilt angle of bipolar magnetic regions during the emergence shows that the systematic tilt with respect to the equator (the Joy’s law) is most likely established below the surface. However, no evidence of the dependence of the tilt angle on the amount of emerging magnetic flux, predicted by the rising magnetic flux rope theories, is found. Analysis of surface plasma flows in a large emerging active region reveals strong localized upflows and downflows at the initial phase of emergence but finds no evidence for large-scale flows indicating future appearance a large-scale magnetic structure. Local helioseismology provides important tools for mapping perturbations of the wave speed and mass flows below the surface. Initial results from SOHO/MDI and GONG reveal strong diverging flows during the flux emergence, and also localized converging flows around stable sunspots. The wave speed images obtained during the process of formation of a large active region, NOAA 10488, indicate that the magnetic flux gets concentrated in strong field structures just below the surface. Further studies of magnetic flux emergence require systematic helioseismic observations from the ground and space, and realistic MHD simulations of the subsurface dynamics.

Kosovichev, A. G.

2009-04-01

120

Magnetohydrodynamic simulations of the ejection of a magnetic flux rope  

NASA Astrophysics Data System (ADS)

Context. Coronal mass ejections (CME's) are one of the most violent phenomena found on the Sun. One model to explain their occurrence is the flux rope ejection model. In this model, magnetic flux ropes form slowly over time periods of days to weeks. They then lose equilibrium and are ejected from the solar corona over a few hours. The contrasting time scales of formation and ejection pose a serious problem for numerical simulations. Aims: We simulate the whole life span of a flux rope from slow formation to rapid ejection and investigate whether magnetic flux ropes formed from a continuous magnetic field distribution, during a quasi-static evolution, can erupt to produce a CME. Methods: To model the full life span of magnetic flux ropes we couple two models. The global non-linear force-free field (GNLFFF) evolution model is used to follow the quasi-static formation of a flux rope. The MHD code ARMVAC is used to simulate the production of a CME through the loss of equilibrium and ejection of this flux rope. Results: We show that the two distinct models may be successfully coupled and that the flux rope is ejected out of our simulation box, where the outer boundary is placed at 2.5 R?. The plasma expelled during the flux rope ejection travels outward at a speed of 100 km s-1, which is consistent with the observed speed of CMEs in the low corona. Conclusions: Our work shows that flux ropes formed in the GNLFFF can lead to the ejection of a mass loaded magnetic flux rope in full MHD simulations. Coupling the two distinct models opens up a new avenue of research to investigate phenomena where different phases of their evolution occur on drastically different time scales. Movies are available in electronic form at http://www.aanda.org

Pagano, P.; Mackay, D. H.; Poedts, S.

2013-06-01

121

Resistive magnetic flux emergence and formation of solar active regions  

NASA Astrophysics Data System (ADS)

Magnetic flux emergence as the mechanism leading to the formation of magnetized structures in the solar atmosphere plays a key role in the dynamic of the Sun. Observed as a whole, emerging flux regions show clear signs of twisted structure, bearing the magnetic free energy necessary to power active events. The high resolution observations of the recent solar observatories (e.g. Hinode, SDO) have revealed how intermittent the magnetic field appears and how various active events induced by flux emergence are. Magnetic field reconstructions methods show that the topology of the field in interspot regions presents a serpentine structure, i.e. field lines having successive U and ? parts. Associated with the appearance of magnetic polarities, a tremendous number of brief small scale brightening are observed in different photospheric and chromospheric lines, e.g. Ellerman Bombs, along with small scale jet-like structures. These events are believed to be the observational signatures of the multiple magnetic reconnections which enable the magnetic field to emerge further up and magnetically structure the corona above active region. Meanwhile a world-wide effort to numerically model the emergence of magnetic field forming solar active region is been carried on. Using different types of physical paradigm - e.g. idealized magnetohydrodynamic model, advanced treatment of the physical equations, data-driven simulations - these numerical experiments highlight how electric currents can build-up during flux emergence, lead to reconnection and thus explain the formation of the different observed transients.

Pariat, E.; Schmieder, B.; Masson, S.; Aulanier, G.

2012-06-01

122

Recent Developments of the Electro-Magnetic Flux Compression  

NASA Astrophysics Data System (ADS)

We have established a world record for the highest indoor magnetic field by the electro-magnetic flux compression (EMFC) technique; currently the maximum magnetic field is 730 T. The copper lined primary coil, which is a composite of steel and copper was found to be very efficient for generating a higher maximum magnetic field with a stabilized liner implosion. We present some of the recent results of our experiments of the EMFC.

Takeyama, S.; Sawabe, H.; Kojima, E.

2010-04-01

123

Observations of a Small Interplanetary Magnetic Flux Rope Associated with a Magnetic Reconnection Exhaust  

Microsoft Academic Search

A small interplanetary magnetic flux rope prior to an X-line magnetic reconnection exhaust was observed on 1998 March 25 at 1 AU. The X-line magnetic reconnection exhaust has been identified and reported by Gosling et al. The duration of this small magnetic flux rope is about 2 hr. We fitted the constant alpha force-free model to the observed magnetic fields.

H. Q. Feng; D. J. Wu

2009-01-01

124

OBSERVATIONS OF A SMALL INTERPLANETARY MAGNETIC FLUX ROPE ASSOCIATED WITH A MAGNETIC RECONNECTION EXHAUST  

Microsoft Academic Search

A small interplanetary magnetic flux rope prior to an X-line magnetic reconnection exhaust was observed on 1998 March 25 at 1 AU. The X-line magnetic reconnection exhaust has been identified and reported by Gosling et al. The duration of this small magnetic flux rope is about 2 hr. We fitted the constant alpha force-free model to the observed magnetic fields.

H. Q. Feng; D. J. Wu

2009-01-01

125

Circular quantum billiard with a singular magnetic flux line  

Microsoft Academic Search

We discuss the application of Gutzwiller's semiclassical theory to a circular billiard with a singular magnetic flux line added at its center. The Aharonov-Bohm effect manifests itself through the cancellation of periodic orbits for particular flux strengths. Diffraction phenomena affect the gross-shell structure of the level density and require corrections of higher order in h. The full quantization of the

S. M. Reimann; M. Brack; A. G. Magner; J. Blaschke; M. V. N. Murthy

1996-01-01

126

Rotor Losses Measurements in an Axial Flux Permanent Magnet Machine  

Microsoft Academic Search

MMF space harmonics and slot openings produce considerable rotor losses in permanent magnet (PM) machines, especially if fractional-slot windings are adopted. This paper aims to measure the rotor losses of a 12-slot 10-pole axial flux phase modulation machine. Both MMF space harmonics and slot open- ings are considered. The prototype is an axial flux machine with open slots, equipped with

Luigi Alberti; Emanuele Fornasiero; Nicola Bianchi; Silverio Bolognani

2011-01-01

127

Three-Dimensional Magnetic Field Line Reconnection involving Magnetic Flux Ropes (Invited)  

Microsoft Academic Search

We report on two experiments in which three dimensional magnetic field line reconnection plays a role. Magnetic field line reconnection is a processes in which the magnetic field energy is converted to particle energy and heating accompanied by changes in the magnetic topology. In the first experiment two magnetic flux ropes are generated from initially adjacent pulsed current channels in

W. N. Gekelman; B. van Compernolle; E. Lawrence; S. T. Vincena

2010-01-01

128

Stress effects on magnetic Barkhausen noise measurements from magnetized pipeline steel  

Microsoft Academic Search

The magnetic flux leakage (MFL) technique is used to determine nondestructively the maximum allowable operating pressure of steel pipelines for oil and gas [1,2]. This method involves magnetically saturating the pipe wall and measuring the leakage flux near the pipe wall surface with Hall probes or induction coils. The goal of the nondestructive testing (NDT) industry is to estimate the

Annie Hsu

2000-01-01

129

Sudden flux change studies in high field superconducting accelerator magnets  

SciTech Connect

As part of the High Field Magnet Program at Fermilab many magnets have been tested which utilize multi strand Rutherford type cable made of state-of-the art Nb{sub 3}Sn strands. During these magnet tests we observed sudden flux changes by monitoring coil voltages and the magnetic field close to the magnets. These flux changes might be linked to magnet instabilities. The voltage spike signals were correlated with quench antenna signals, a strong indication that these are magnet phenomena. With a new high resolution voltage spike detection system, we were able to observe the detailed structure of the spikes. Two fundamentally different signal shapes were distinguished, most likely generated by different mechanisms.

Feher, S.; Bordini, B.; Carcagno, R.; Makulski, A.; Orris, D.F.; Pischalnikov, Y.M.; Sylvester, C.; Tartaglia, M.; Tompkins, J.C.; Zlobin, A.V.; /Fermilab

2004-10-01

130

Magnetic flux generation and transport in cool stars  

NASA Astrophysics Data System (ADS)

Context. The Sun and other cool stars harbouring outer convection zones manifest magnetic activity in their atmospheres. The connection between this activity and the properties of a deep-seated dynamo generating the magnetic flux is not well understood. Aims: By employing physical models, we study the spatial and temporal characteristics of the observable surface field for various stellar parameters. Methods: We combine models for magnetic flux generation, buoyancy instability, and transport, which encompass the entire convection zone. The model components are: (i) a thin-layer ?? dynamo at the base of the convection zone; (ii) buoyancy instabilities and the rise of flux tubes through the convection zone in 3D, which provides a physically consistent determination of emergence latitudes and tilt angles; and (iii) horizontal flux transport at the surface. Results: For solar-type stars and rotation periods longer than about 10 days, the latitudinal dynamo waves generated by the deep-seated ?? dynamo are faithfully reflected by the surface distribution of magnetic flux. For rotation periods of the order of two days, however, Coriolis acceleration of rising flux loops leads to surface flux emergence at much higher latitudes than the dynamo waves at the bottom of the convection zone reach. A similar result is found for a K0V star with a rotation period of two days. In the case of a rapidly rotating K1 subgiant, overlapping dynamo waves lead to noisy activity cycles and mixed-polarity fields at high latitudes. Conclusions: The combined model reproduces the basic observed features of the solar cycle. The differences between the latitude distributions of the magnetic field at the bottom of the convection zone and the emerging surface flux grow with increasing rotation rate and convection zone depth, becoming quite substantial for rapidly rotating dwarfs and subgiants. The dynamical evolution of buoyantly rising magnetic flux should be considered as an essential ingredient in stellar dynamo models. 3 movies are only available in electronic form at http://www.aanda.org

I??k, E.; Schmitt, D.; Schüssler, M.

2011-04-01

131

A Note on Computation of Relative Magnetic-Helicity Flux Across the Photosphere  

NASA Astrophysics Data System (ADS)

A number of investigations of the rate of relative magnetic-helicity transport across the photosphere [{{d H}/{dt}}|S] have reported differences in the estimates computed from two different formulations of the relative-helicity flux-density proxy G A and G ? . There have been suggestions that G ? is a more robust helicity-flux density proxy and that the differences in the estimates of {{d H}/{dt}}|S are caused by biases in G A, noise, and/or the boundary conditions. In this note, we prove that the differences are caused by the inconsistent choice of boundary conditions in the explicit or implicit Green's function [{G}({x},{x}')] used for computing G A and G ? when comparing the helicity-flux estimates based on G A and G ? . When the boundary conditions in {G} are chosen consistently, the two helicity-flux density proxies, [ G A and G ? ] produce essentially identical results for the rate of helicity transport across the photosphere. They also yield essentially identical results for the rate of helicity transport of the shearing and advection terms separately. Using MHD simulation, HMI observational data, and Monte Carlo simulations of noise we show that this result is robust. Neither the shape of the active region, nor the shape of the boundary, nor data noise causes any difference in the rate of helicity transport computed via G A and G ? .

Liu, Yang; Schuck, P. W.

2013-04-01

132

SIGNATURES OF MAGNETIC RECONNECTION AT BOUNDARIES OF INTERPLANETARY SMALL-SCALE MAGNETIC FLUX ROPES  

SciTech Connect

The interaction between interplanetary small-scale magnetic flux ropes and the magnetic field in the ambient solar wind is an important topic in the understanding of the evolution of magnetic structures in the heliosphere. Through a survey of 125 previously reported small flux ropes from 1995 to 2005, we find that 44 of them reveal clear signatures of Alfvenic fluctuations and thus classify them as Alfven wave trains rather than flux ropes. Signatures of magnetic reconnection, generally including a plasma jet of {approx}30 km s{sup -1} within a magnetic field rotational region, are clearly present at boundaries of about 42% of the flux ropes and 14% of the wave trains. The reconnection exhausts are often observed to show a local increase in the proton temperature, density, and plasma beta. About 66% of the reconnection events at flux rope boundaries are associated with a magnetic field shear angle larger than 90{sup 0} and 73% of them reveal a decrease of 20% or more in the magnetic field magnitude, suggesting a dominance of anti-parallel reconnection at flux rope boundaries. The occurrence rate of magnetic reconnection at flux rope boundaries through the years 1995-2005 is also investigated and we find that it is relatively low around the solar maximum and much higher when approaching solar minima. The average magnetic field depression and shear angle for reconnection events at flux rope boundaries also reveal a similar trend from 1995 to 2005. Our results demonstrate for the first time that boundaries of a substantial fraction of small-scale flux ropes have properties similar to those of magnetic clouds, in the sense that both of them exhibit signatures of magnetic reconnection. The observed reconnection signatures could be related either to the formation of small flux ropes or to the interaction between flux ropes and the interplanetary magnetic fields.

Tian Hui; Yao Shuo; Zong Qiugang; Qi Yu [School of Earth and Space Sciences, Peking University, 100871 Beijing (China); He Jiansen, E-mail: tianhui924@pku.edu.c [Max-Planck-Institut fuer Sonnensystemforschung, 37191 Katlenburg-Lindau (Germany)

2010-09-01

133

On the magnetic flux conservation in the partially ionized plasma  

NASA Astrophysics Data System (ADS)

Ohm, Hall, and ambipolar diffusions in the partially ionized plasma are considered. The statement of Pandey & Wardle (2011,2012) that only Ohmic diffusion is capable of destroying the magnetic flux is not sufficiently correct due to the formal dependence of the magnetic diffusion on a frame of reference.

Tsap, Yuriy

2013-07-01

134

Axial-flux permanent magnet machines for micropower generation  

Microsoft Academic Search

This paper reports on the design, fabrication, and testing of an axial-flux permanent magnet electromagnetic generator. The generator comprises a polymer rotor with embedded permanent magnets sandwiched between two silicon stators with electroplated planar coils. Finite element simulations have been carried out using ANSYS to determine the effects on performance of design parameters such as the number of layers in

Andrew S. Holmes; Guodong Hong; Keith R. Pullen

2005-01-01

135

Exact static equilibrium of vertically oriented magnetic flux tubes  

Microsoft Academic Search

A method is prescribed for generating exact solutions of magnetostatic equilibrium describing a cylindrically symmetric magnetic flux tube oriented vertically in a stratified medium. Given the geometric shape of the field lines, compact formulae are presented for the direct calculation of all the possible distributions of pressure, density, temperature and magnetic field strength compatible with these field lines under the

B. C. Low

1980-01-01

136

Advection of magnetic flux by accretion disks around neutron stars  

Microsoft Academic Search

The aim of our research is to address why millisecond pulsars have relatively weak surface magnetic fields, of about 10^8 G, with a narrow spread. We propose that the accretion of plasma from the companion star fully screens the original neutron star field, but the accretion disk carries additional magnetic flux from the companion star, or itself can generate field

S. Flores-Tulian; A. Reisenegger

2007-01-01

137

Standard measuring system for calibration of magnetic flux density gradiometers  

Microsoft Academic Search

A standard measuring system (setup) has been developed at VNIIM to be used for the calibration of magnetic flux density gradient (MFDG) coils, sensors and magnetic gradiometers (MG). This measurement standard is based on the calculable quartz coil used for the reproduction of gradient which is uniform in a specified working space. It also includes measuring equipment to compare coils,

V. N. Khorev; V. Y. Shifrin; S. A. Shubin; P. G. Park

2010-01-01

138

A dynamo effect due to instability of magnetic flux tubes  

Microsoft Academic Search

We show that a non-axisymmetric instability of toroidal magnetic flux tubes in a rotating star provides a dynamo effect. The instability occurs in the form of propagating helical waves; their growth in amplitude causes a phase shift between the perturbations of magnetic field and velocity, which leads to an electric field (anti)parallel to the direction of the unperturbed field. Together

A. Ferriz-Mas; D. Schmitt; M. Schuessler

1994-01-01

139

Flow instabilities of magnetic flux tubes. IV. Flux storage in the solar overshoot region  

NASA Astrophysics Data System (ADS)

Context: Flow-induced instabilities of magnetic flux tubes are relevant to the storage of magnetic flux in the interiors of stars with outer convection zones. The stability of magnetic fields in stellar interiors is of importance to the generation and transport of solar and stellar magnetic fields. Aims: We consider the effects of material flows on the dynamics of toroidal magnetic flux tubes located close to the base of the solar convection zone, initially within the overshoot region. The problem is to find the physical conditions in which magnetic flux can be stored for periods comparable to the dynamo amplification time, which is of the order of a few years. Methods: We carry out nonlinear numerical simulations to investigate the stability and dynamics of thin flux tubes subject to perpendicular and longitudinal flows. We compare the simulations with the results of simplified analytical approximations. Results: The longitudinal flow instability induced by the aerodynamic drag force is nonlinear in the sense that the growth rate depends on the perturbation amplitude. This result is consistent with the predictions of linear theory. Numerical simulations without friction show that nonlinear Parker instability can be triggered below the linear threshold of the field strength, when the difference in superadiabaticity along the tube is sufficiently large. A localised downflow acting on a toroidal tube in the overshoot region leads to instability depending on the parameters describing the flow, as well as the magnetic field strength. We determined ranges of the flow parameters for which a linearly Parker-stable magnetic flux tube is stored in the middle of the overshoot region for a period comparable to the dynamo amplification time. Conclusions: The longitudinal flow instability driven by frictional interaction of a flux tube with its surroundings is relevant to determining the storage time of magnetic flux in the solar overshoot region. The residence time for magnetic flux tubes with 2 × 1021 Mx in the convective overshoot layer can be comparable to the dynamo amplification time, provided that the average speed and the duration of an external downflow do not exceed about 50 m s -1 and 100 days, respectively, and that the lateral extension of the flow is smaller than about 10°. Appendix C and movies are only available in electronic form at http://www.aanda.org

I??k, E.; Holzwarth, V.

2009-12-01

140

Influence of Noise Temporal Correlation on Magnetization Spectra and Thermal Relaxations in Soft Magnetic Materials  

Microsoft Academic Search

The analysis of the noise temporal correlation effects on spectral properties and relaxation phenomena in hysteretic systems driven by arbitrary colored noise processes is presented. The calculations are performed under the framework of Monte Carlo techniques for simulating the stochastic processes and Preisach formalism for modeling magnetic materials. It is found that the correlation time of the input signal significantly

Mihai Dimian; Ayodeji Adedoyin; Anca Gindulescu; Petru Andrei

2010-01-01

141

Magnetic flux dynamics in a hexagonal network of superconducting islands  

NASA Astrophysics Data System (ADS)

Honeycomb arrays (cells ~ 50 ?m in size) were patterned in a YBa2Cu3Oy thin film in order to model magnetization and transport current processes in granular materials. The samples were studied magnetically and magneto-optically (MO). The induced inter-granular currents exhibited an anomaly similar to that observed in Bi-based tapes. The magneto-optical observation showed a flux penetration/reversal process consisting of two Bean critical states, of inter- and intra-granular screening currents. The rate of flux penetration into the inter-granular space was an inversal function of the bridge width and was significantly faster than penetration into the grains. The grain magnetization was nonuniform and asymmetrical. Although grains stood up to maximum field of 57.7 mT incompletely penetrated, after the field reversal the opposite magnetic flux fully penetrated the grains far before reaching remnant state.

Jirsa, Milos; Yurchenko, Vitaliy; Bobyl, Alexander V.; Johansen, Tom H.; Shantsev, Daniel V.; Wördenweber, Roger

2004-05-01

142

Magnetic flux emergence in the Sun  

Microsoft Academic Search

Space weather research is closely connected with the study of the solar magnetic activity. In past years, many solar missions (e.g., YOHKOH, SOHO, TRACE, and RHESSI) have provided outstanding observations, which have been used to improve our understanding of the structure and the dynamical evolution of solar magnetic fields. In addition, the newly launched solar missions (e.g., Hinode and STEREO)

V. Archontis

2008-01-01

143

Ultra-low-noise magnetic sensing with long Josephson tunnel junctions  

NASA Astrophysics Data System (ADS)

We study how the magnetic field dependence of the Eck step voltage in long Josephson tunnel junctions (LJTJs) can allow for ultra-low-noise magnetic sensing. The field to be measured is applied perpendicular to a continuous superconducting pickup loop. Wherever the loop has a narrow constriction, the density of the flux-restoring circulating currents will become relatively high and will locally create a magnetic field large enough to bring a biased LJTJ into the flux-flow state, i.e., at a finite voltage proportional to the field strength. This method allows the realization of a novel family of robust and general-purpose superconducting devices which, despite their simplicity, function as ultra-low-noise, wide-band and high-dynamics magnetometers. The performance of low-Tc sensor prototypes, including a highly linear voltage responsivity and an intrinsic voltage spectral density {S}_{V}^{1/2} in the pV Hz?1/2 range, promises to be competitive with that of the best superconducting quantum interference devices.

Monaco, Roberto; Granata, Carmine; Russo, Roberto; Vettoliere, Antonio

2013-12-01

144

Geometric considerations of the evolution of magnetic flux ropes.  

PubMed

We use flux conservation and magnetohydrodynamics (MHD) theory to discuss essential differences in the nature of the evolution of two analytical solutions describing magnetic flux tubes evolving in time. The first of these maintains the elongation of the tube, while the second maintains a constant angular extension with respect to a possible pointlike source. In the first case, free expansion of the plasma (density N) occurs only in a direction perpendicular to the flux-tube x axis. In the second case, isotropic evolution is considered. In both cases it is assumed that at initial time t(0) the flux-tube B field is the force-free magnetostatic Lundquist solution, which energetically corresponds to the most stable state for any flux-tube structure. We show that for each case conservation of magnetic flux is enough to establish the scaling with time of the B field. While both expansions may correspond to the evolution of observed flux tubes in the heliosphere, the isotropic expansion appears to capture consistently essential features associated with the actual observations of expanding coronal mass ejections within 30 solar radii. For isotropic expansion of the plasma the force-free nature of the B field is preserved for all time. As an example the MHD solutions are applied to an interplanetary magnetic cloud observed with the spacecraft Wind, which passed Earth's vicinity on June 2, 1998. PMID:12689169

Berdichevsky, D B; Lepping, R P; Farrugia, C J

2003-03-19

145

Magnetic flux rope formation within a magnetosheath hot flow anomaly  

NASA Astrophysics Data System (ADS)

We report observations on 1 March 2004 by the Cluster spacecraft of a hot flow anomaly (HFA) encountered in the dayside magnetosheath near Earth's bow shock. Embedded within the HFA was a magnetic flux rope with a diameter of a few thousand km, which was moving sunward and was presumably expanding. The pristine upstream solar wind seen by the ACE spacecraft contains an interplanetary current sheet favorable for the HFA formation, but shows no flux rope signatures. The properties of the flux rope, such as its slow speed, magnetic field variations, and the absence of magnetospheric electrons, are not likely to be due to magnetopause flux transfer events. These results suggest that the flux rope was created in the magnetosheath, rather than in the solar wind, in the foreshock, or on the magnetopause, through magnetic reconnection initiated in the course of the HFA development. Interestingly, energetic (˜100 keV) electron fluxes were enhanced in and around this HFA-associated flux rope. The observations indicate that reconnection can occur within the magnetosheath part of HFAs and that such reconnection may play a role in electron acceleration, which is a common feature of HFAs.

Hasegawa, H.; Zhang, H.; Lin, Y.; Sonnerup, B. U. Ö.; Schwartz, S. J.; Lavraud, B.; Zong, Q.-G.

2012-09-01

146

Flux noise resulting from vortex avalanches using a simple kinetic model  

SciTech Connect

We have carried out a model calculation of the flux noise produced by vortex avalanches in a type-II superconductor, using a simple kinetic model proposed by Bassler and Paczuski. Over a broad range of frequencies, we find that the flux noise S{sub {Phi}}({omega}) has a power-law dependence on frequency, S{sub {Phi}}({omega}){approximately}{omega}{sup {minus}s}, with s{approximately}1.4 in reasonable agreement with experiment. In addition, for small lattices, the calculated S{sub {Phi}}({omega}) has a high-frequency knee, which is seen in some experiments, and which is due to the finite lattice size. Deviations between calculation and experiment are attributed mostly to uncertainties in the measured critical current densities and pinning strengths of the experimental samples. {copyright} {ital 1999} {ital The American Physical Society}

Mohler, G.; Stroud, D. [Department of Physics, The Ohio State University, Columbus, Ohio 43210 (United States)

1999-10-01

147

Reconstruction of Open Solar Magnetic Flux and Interplanetary Magnetic Field in the 20Th Century  

Microsoft Academic Search

We reconstruct mean magnitudes of the open solar magnetic field since 1915 using ? magnetic synoptic charts of the Sun. The\\u000a obtained series allows estimation of the interplanetary magnetic field. They also confirm the known conclusion about the secular\\u000a increase of the solar open magnetic flux in the first half of the 20th century.

V. G. Ivanov; E. V. Miletsky

2004-01-01

148

Absolute determination of the magnetic flux quantum using superconducting magnetic levitation  

Microsoft Academic Search

The current status of our superconducting magnetic levitation experiment for determining the magnetic flux quantum is described. The flux-up system has been improved significantly by using a Josephson voltage standard. Studies are also in progress to improve the mechanical measurement relevant to the floating body with the goal of reducing the uncertainty to less than 1 ppm

F. Shiota; Y. Miki; A. Namba; Y. Nezu; Y. Sakamoto; T. Morokuma; K. Hara

1995-01-01

149

Inference of physical phenomena from FFTF (Fast Flux Test Facility) noise analysis  

SciTech Connect

The source of features observed in noise spectra collected by an automated data collection system operated by the Oak Ridge National Laboratory at the Fast Flux Test Facility (FFTF) can be identified using a methodology based on careful data observation and intuition. When a large collection of data is available, as in this case, automatic pattern recognition and parameter storage and retrieval using a data base can be used to extract useful information. However, results can be limited to empirical signature comparison monitoring unless an effort is made to determine the noise sources. This paper describes the identification of several FFTF noise data phenomena and suggests how this understanding may lead to new or enhanced monitoring. 13 refs., 4 figs.

Thie, J.A.; Damiano, B.; Campbell, L.R.

1989-01-01

150

Magnetic Flux Emergence Observed with the Advanced Stokes Polarimeter  

NASA Astrophysics Data System (ADS)

We have carried out quantitative observations of the vector magnetic field during the emergence of three small bipolar active regions in June, 1992, July 1993, and September 1994 using the Advanced Stokes Polarimeter (ASP). The region of horizontal magnetic field at the actual site of emergence is always characterized by low magnetic field strength (i.e. considerably less than 1000 Gauss). We find a strong relationship between field strength and inclination in these regions. This suggests that 1) flux emerging from below the photosphere does not coalesce into strong flux tubes until it reaches the photosphere, becomes nearly vertical as a result of magnetic buoyancy, and is then acted upon by convective collapse, and 2) the field strength of flux rising through the convection zone may be in rough equipartition with the fluid motions. We find the flux emergence zone to be characterized by highly variable (both spatially and temporally) fill factors for the magnetic field, suggesting that the flux below the surface is filamentary, that it rises rapidly through the photosphere to form a magnetic canopy above the emergence region. Sequences of H? on- and off-band images obtained with the ASP reveal the accompanying development of the arch-filament system, and suggest that the material within the H? structures is supplied by a siphon flow as evidenced by apparent chromospheric red shifts on the sides of the loops closest to a large pore, and blue shifts where the fields anchor in plage regions. Proper motions of the magnetic flux images throughout a day's observation indicate the presence of a persistent vortex flow on a small scale (a few arcseconds). The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Lites, B.; Martinez Pillet, V.

1996-05-01

151

Controlling magnetic flux motion by arrays of zigzag-arranged magnetic bars  

NASA Astrophysics Data System (ADS)

Recent advances in manufacturing arrays of artificial pinning sites, i.e., antidots, blind holes and magnetic dots, allowed an effective control of magnetic flux in superconductors. An array of magnetic bars deposited on top of a superconducting film was shown to display different pinning regimes depending on the direction of the in-plane magnetization of the bars. Changing the sign of their magnetization results in changes in the induced magnetic pinning potentials. By numerically solving the time-dependent Ginzburg-Landau equations in a superconducting film with periodic arrays of zigzag-arranged magnetic bars, we revealed various flux dynamics regimes. In particular, we demonstrate flux pinning and flux flow, depending on the direction of the magnetization of the magnetic bars. Remarkably, the revealed different flux-motion regimes are associated with different mechanisms of vortex-antivortex dynamics. For example, we found that for an ‘antiparallel’ configuration of magnetic bars this dynamics involves a repeating vortex-antivortex generation and annihilation. We show that the depinning transition and the onset of flux flow can be manipulated by the magnetization of the bars and the geometry of the array. This provides an effective control of the depinning critical current that can be useful for possible fluxonics applications.

Kapra, A. V.; Misko, V. R.; Peeters, F. M.

2013-02-01

152

The Evolution of Open Magnetic Flux Driven by Photospheric Dynamics  

NASA Astrophysics Data System (ADS)

The coronal magnetic field is of paramount importance in solar and heliospheric physics. Two profoundly different views of the coronal magnetic field have emerged. In quasi-steady models, the predominant source of open magnetic field is in coronal holes. In contrast, in the interchange model, the open magnetic flux is conserved, and the coronal magnetic field can only respond to the photospheric evolution via interchange reconnection. In this view, the open magnetic flux diffuses through the closed, streamer belt fields, and substantial open flux is present in the streamer belt during solar minimum. However, Antiochos and coworkers, in the form of a conjecture, argued that truly isolated open flux cannot exist in a configuration with one heliospheric current sheet—it will connect via narrow corridors to the polar coronal hole of the same polarity. This contradicts the requirements of the interchange model. We have performed an MHD simulation of the solar corona up to 20 R sun to test both the interchange model and the Antiochos conjecture. We use a synoptic map for Carrington rotation 1913 as the boundary condition for the model, with two small bipoles introduced into the region where a positive polarity extended coronal hole forms. We introduce flows at the photospheric boundary surface to see if open flux associated with the bipoles can be moved into the closed-field region. Interchange reconnection does occur in response to these motions. However, we find that the open magnetic flux cannot be simply injected into closed-field regions—the flux eventually closes down and disconnected flux is created. Flux either opens or closes, as required, to maintain topologically distinct open- and closed-field regions, with no indiscriminate mixing of the two. The early evolution conforms to the Antiochos conjecture in that a narrow corridor of open flux connects the portion of the coronal hole that is nearly detached by one of the bipoles. In the later evolution, a detached coronal hole forms, in apparent violation of the Antiochos conjecture. Further investigation reveals that this detached coronal hole is actually linked to the extended coronal hole by a separatrix footprint on the photosphere of zero width. Therefore, the essential idea of the conjecture is preserved, if we modify it to state that coronal holes in the same polarity region are always linked, either by finite width corridors or separatrix footprints. The implications of these results for interchange reconnection and the sources of the slow solar wind are briefly discussed.

Linker, Jon A.; Lionello, Roberto; Miki?, Zoran; Titov, Viacheslav S.; Antiochos, Spiro K.

2011-04-01

153

THE EVOLUTION OF OPEN MAGNETIC FLUX DRIVEN BY PHOTOSPHERIC DYNAMICS  

SciTech Connect

The coronal magnetic field is of paramount importance in solar and heliospheric physics. Two profoundly different views of the coronal magnetic field have emerged. In quasi-steady models, the predominant source of open magnetic field is in coronal holes. In contrast, in the interchange model, the open magnetic flux is conserved, and the coronal magnetic field can only respond to the photospheric evolution via interchange reconnection. In this view, the open magnetic flux diffuses through the closed, streamer belt fields, and substantial open flux is present in the streamer belt during solar minimum. However, Antiochos and coworkers, in the form of a conjecture, argued that truly isolated open flux cannot exist in a configuration with one heliospheric current sheet-it will connect via narrow corridors to the polar coronal hole of the same polarity. This contradicts the requirements of the interchange model. We have performed an MHD simulation of the solar corona up to 20 R{sub sun} to test both the interchange model and the Antiochos conjecture. We use a synoptic map for Carrington rotation 1913 as the boundary condition for the model, with two small bipoles introduced into the region where a positive polarity extended coronal hole forms. We introduce flows at the photospheric boundary surface to see if open flux associated with the bipoles can be moved into the closed-field region. Interchange reconnection does occur in response to these motions. However, we find that the open magnetic flux cannot be simply injected into closed-field regions-the flux eventually closes down and disconnected flux is created. Flux either opens or closes, as required, to maintain topologically distinct open- and closed-field regions, with no indiscriminate mixing of the two. The early evolution conforms to the Antiochos conjecture in that a narrow corridor of open flux connects the portion of the coronal hole that is nearly detached by one of the bipoles. In the later evolution, a detached coronal hole forms, in apparent violation of the Antiochos conjecture. Further investigation reveals that this detached coronal hole is actually linked to the extended coronal hole by a separatrix footprint on the photosphere of zero width. Therefore, the essential idea of the conjecture is preserved, if we modify it to state that coronal holes in the same polarity region are always linked, either by finite width corridors or separatrix footprints. The implications of these results for interchange reconnection and the sources of the slow solar wind are briefly discussed.

Linker, Jon A.; Lionello, Roberto; Mikic, Zoran; Titov, Viacheslav S. [Predictive Science, Inc., 9990 Mesa Rim Road, Suite 170, San Diego, CA 92121-2910 (United States); Antiochos, Spiro K., E-mail: linkerj@predsci.com, E-mail: lionel@predsci.com, E-mail: mikicz@predsci.com, E-mail: titovv@predsci.com, E-mail: spiro.antiochos@nasa.gov [NASA Goddard Space Flight Center, NASA/GSFC Code 674, Greenbelt, MD 20771 (United States)

2011-04-20

154

A generalized flux function for three-dimensional magnetic reconnection  

SciTech Connect

The definition and measurement of magnetic reconnection in three-dimensional magnetic fields with multiple reconnection sites is a challenging problem, particularly in fields lacking null points. We propose a generalization of the familiar two-dimensional concept of a magnetic flux function to the case of a three-dimensional field connecting two planar boundaries. In this initial analysis, we require the normal magnetic field to have the same distribution on both boundaries. Using hyperbolic fixed points of the field line mapping, and their global stable and unstable manifolds, we define a unique flux partition of the magnetic field. This partition is more complicated than the corresponding (well-known) construction in a two-dimensional field, owing to the possibility of heteroclinic points and chaotic magnetic regions. Nevertheless, we show how the partition reconnection rate is readily measured with the generalized flux function. We relate our partition reconnection rate to the common definition of three-dimensional reconnection in terms of integrated parallel electric field. An analytical example demonstrates the theory and shows how the flux partition responds to an isolated reconnection event.

Yeates, A. R.; Hornig, G. [Division of Mathematics, University of Dundee, Dundee DD1 4HN (United Kingdom)

2011-10-15

155

Auroral streamers and magnetic flux closure  

NASA Astrophysics Data System (ADS)

On 7 December 2000 at 2200 UT an auroral streamer was observed to develop above Scandinavia with the IMAGE-FUV global imagers. The ionospheric equivalent current deduced from the MIRACLE-IMAGE Scandinavian ground-based network of magnetometers is typical of a substorm-time streamer. Observations of the proton aurora using the SI12 imager onboard the IMAGE satellite are combined with measurements of the ionospheric convection obtained by the SuperDARN radar network to compute the dayside merging and nightside flux closure rates. On the basis of this and other similar events, it is found that auroral streamers appear during the period of most intense flux closure in the magnetotail, most often shortly after substorm onset. The ionospheric convection velocity, as measured by SuperDARN, appears to be reduced in the vicinity of the streamer, suggesting de-coupling of magnetospheric and ionospheric plasma flows in the region of enhanced ionospheric conductance.

Hubert, B.; Kauristie, K.; Amm, O.; Milan, S. E.; Grocott, A.; Cowley, S. W. H.; Pulkkinen, T. I.

2007-08-01

156

Non-geometric magnetic flux and crossed modules  

Microsoft Academic Search

It is shown that the BRST operator of twisted N=4 Yang–Mills theory in four dimensions is locally the same as the BRST operator of a fully decomposed non-Abelian gerbe. Using locally defined Yang–Mills theories we describe non-perturbative backgrounds that carry a novel magnetic flux. Given by elements of the crossed module G?AutG, these non-geometric fluxes can be classified in terms

Jussi Kalkkinen

2006-01-01

157

Design Considerations for Tubular Flux-Switching Permanent Magnet Machines  

Microsoft Academic Search

This paper describes a tubular, three-phase, flux-switching permanent magnet (PM) brushless machine that combines salient features from switched reluctance (SR) and conventional PM brushless machines. Feasible slot-pole number combinations, which are also applicable to rotary flux-switching machines, are derived. This paper also examines an alternative stator winding configuration, which is unique to the tubular machine topology. It is shown that

Jiabin Wang; Weiya Wang; Kais Atallah; David Howe

2008-01-01

158

Noise-induced magnetization in sigma-delta data converters  

Microsoft Academic Search

We study the behavior of sigma-delta data converters using the Ising model description. In particular, we demonstrate how the magnetization diagram gives an estimate of the input stability boundary, which is an important engineering specification. In sigma-delta data converters, the presence of a small amount of thermal noise can lead to long-range order via a first order phase transition. By

Douglas J. Mar; M. K. Stephen Yeung; Carson C. Chow; Robert W. Adams; James J. Collins

2001-01-01

159

Tubes of Magnetic Flux and Electric Current in Space Physics  

NASA Astrophysics Data System (ADS)

The singularities of an irrotational magnetic field are lines of electric current. This property derives from the relationship between vector fields and the topology of the underlying three-space and allows for a definition of cosmic field flux tubes and flux ropes as cores (in the sense of the physics of defects) of helical singularities. When applied to force-free flux ropes, and assuming current conservation, an interesting feature is the quantization of the radii, pitches, and helicities. One expects similar quantization effects in the general case. In the special case when the total electric current vanishes, a force-free rope embedded in a medium devoid of magnetic field is nonetheless topologically stable, because it is the core of a singularity of the vector potential. Magnetic merging is also briefly discussed in the same framework.

Kleman, M.; Robbins, J. M.

2013-09-01

160

On the Dynamics of Emerging Toroidal Magnetic Flux Tubes  

NASA Astrophysics Data System (ADS)

We study the dynamic evolution of emerging toroidal magnetic flux rings in the solar convective envelope by carrying out 3D numerical simulations based on the thin flux tube approximation of Spruit. We find: 1)For an axisymmetric flux ring, the aerodynamic drag force experienced by the ring when moving with respect to the ambient fluid transfers no angular momentum to the ring. Therefore in both cases, with or without the drag force, the ring moves nearly parallel to the rotational axis of the sun and emerges at a latitude significantly poleward of sunspot zones, as pointed out by Choudhuri and Gilman. However, for a non-axisymmetric flux ring (i.e. with wave-like undulations along its circumference), the aerodynamic drag force can transfer angular momentum to the flux ring, and therefore reduces the latitude of flux emergence to within the observed sunspot latitudes. 2)As each apex of a flux loop rises due to the magnetic buoyancy force, gas inside the flux tube tends to diverge from the apex. In the meantime, however, the Coriolis force drives a flow within the flux tube opposite to the direction of rotation. Thus the point of maximum divergence in the flow within the tube is shifted from the apex into the leading side (in the direction of rotation) of the emerging loop. The evacuation of plasma from the leading side of the loop results in a much lower internal gas pressure there as compared to that in the following side at the same depth. Therefore, the magnetic field strength is stronger on the leading side. The numerical simulations show that the field strength in the leading side of the loop can be twice as large as that of the following side at the same depth. This result offers a simple explanation for the observed fact that the leading polarity of an active region is more compact, forms sunspots more easily, and has a longer life time than does the following polarity.

Fan, Y.; Fisher, G. H.; Deluca, E. E.

1992-05-01

161

Improved interactive visualization of magnetic flux lines in 3-D space using edge finite elements  

Microsoft Academic Search

An improved method for interactive visualization of magnetic flux lines in 3-D space is presented. The problem of determining the number of flux lines and their starting position is treated and solved using a simple and computationally efficient algorithm which incorporates the intensity values of the magnetic flux-density and several appropriate weighted functions. The solenoidal characteristic of the magnetic flux

Vlatko Cingoski; Tsuyoshi Kuribayashi; Kazufumi Kaneda; Hideo Yamashita

1996-01-01

162

Advection of magnetic flux by accretion disks around neutron stars  

NASA Astrophysics Data System (ADS)

The aim of our research is to address why millisecond pulsars have relatively weak surface magnetic fields, of about 10^8 G, with a narrow spread. We propose that the accretion of plasma from the companion star fully screens the original neutron star field, but the accretion disk carries additional magnetic flux from the companion star, or itself can generate field by means of dynamo processes. For a strongly magnetized star, the field prevents the disk from approaching the star. The accretion is along the field lines and deposits the matter on the polar cap. Then, the accreted plasma flows, dragging with itself the magnetic field lines, from the pole to the equator (Payne & Melatos 2004). In a following stage, when the star becomes non-magnetic, because the field has been buried, the disk touches the star. We suggest that some effective mechanism of magnetic flux transport such as that proposed by Spruit & Uzdensky 2005 (or Bisnovatyi-Kogan & Lovelace 2007), operates and necessarily leads to a "strongly magnetized disk''. It becomes laminar because the magneto-rotational instability saturates (it is considered to be responsible for turbulence in the disk), and the magnetic difussivity is negligible. Then, the loss of angular momentum allowing the accretion is only caused by the magneto-centrifugal disk-wind (Blandford & Payne 1982). Meanwhile, the wind-driven transport of the magnetic flux by the disk re-magnetizes the star. This process continues until the Lorentz force due to the star's magnetic field forbids any further accretion of matter and magnetic flux, in the Ideal Magneto-Hydro-Dynamics approach. Additional of material can fall onto the star (but at lower rate) if some instability process sets in, allowing the diffusion of mass through the magnetic field lines (e.g the Interchange Instability, Spruit & Taam 1990). All these processes might lead to an asymptotic magnetic field of 10^8 G,as is inferred from observations. We are developing a self-consistent theoretical model to describe the above dynamical processes taking into account the interaction among the star, the strongly magnetized disk, and the disk-wind, justifying our hypothesis.

Flores-Tulian, S.; Reisenegger, A.

163

A flux-coupled ac/dc magnetizing device.  

PubMed

We report on an instrument for applying ac and dc magnetic fields by capturing the flux from a rotating permanent magnet and projecting it between two adjustable pole pieces. This can be an alternative to standard electromagnets for experiments with small samples or in probe stations in which an applied magnetic field is needed locally, with advantages that include a compact form-factor, very low power requirements and dissipation as well as fast field sweep rates. This flux capture instrument (FLUXCAP) can produce fields from -400 to +400 mT, with field resolution less than 1 mT. It generates static magnetic fields as well as ramped fields, with ramping rates as high as 10 T/s. We demonstrate the use of this apparatus for studying the magnetotransport properties of spin-valve nanopillars, a nanoscale device that exhibits giant magnetoresistance. PMID:23822372

Gopman, D B; Liu, H; Kent, A D

2013-06-01

164

A flux-coupled ac/dc magnetizing device  

NASA Astrophysics Data System (ADS)

We report on an instrument for applying ac and dc magnetic fields by capturing the flux from a rotating permanent magnet and projecting it between two adjustable pole pieces. This can be an alternative to standard electromagnets for experiments with small samples or in probe stations in which an applied magnetic field is needed locally, with advantages that include a compact form-factor, very low power requirements and dissipation as well as fast field sweep rates. This flux capture instrument (FLUXCAP) can produce fields from -400 to +400 mT, with field resolution less than 1 mT. It generates static magnetic fields as well as ramped fields, with ramping rates as high as 10 T/s. We demonstrate the use of this apparatus for studying the magnetotransport properties of spin-valve nanopillars, a nanoscale device that exhibits giant magnetoresistance.

Gopman, D. B.; Liu, H.; Kent, A. D.

2013-06-01

165

Flux Rope Acceleration and Enhanced Magnetic Reconnection Rate  

Microsoft Academic Search

A physical mechanism of flares, in particular for the flare rise phase, has emerged from our 2-1\\/2D resistive MHD simulations. The dynamical evolution of current sheet formation and magnetic reconnection and flux rope acceleration subject to continuous, slow increase of magnetic shear in the arcade are studied by employing a non-uniform anomalous resistivity in the reconnecting current sheet under gravity.

C. Z. Cheng; G. S. Choe; Y. Ren; Y.-J. Moon

2003-01-01

166

Observation of Convergent Oscillations of the Flux Line Lattice as a Result of Magnetic Flux Jumping in Hard Superconductors  

Microsoft Academic Search

We have monitored new peculiarities of the dynamics of catastrophic avalanches of the magnetic flux in superconducting Nb, Nb-Ti, and YBaCuO samples: i) convergent oscillations of the magnetic flux; ii) a threshold for entering the huge flux avalanches in the shielding experiments; iii) a threshold for the exit of a residual flux in the trapping experiments. The observed phenomena are

Victor V. Chabanenko; V. F. Rusakov; V. A. Yampol'skii; Stanislaw Piechota; Adam Nabialek; Sergei V. Vasiliev; Henryk Szymczak

2001-01-01

167

Distribution of Magnetic Helicity Flux with Solar Cycles  

NASA Astrophysics Data System (ADS)

It is normally believed that a magnetic field transfers helicity from the solar subatmosphere into interplanetary space. This is based on the calculation of the injected magnetic helicity near the center of the solar disk between latitude ±30° of both solar hemispheres in the period 1996-2009. As one follows the long-term injection of magnetic helicity, one finds that the transfer of magnetic helicity does not have a monotonic sign in the northern and southern hemispheres, and that the bulk of the helicity contributed goes to the active region, while the contribution to the quiet Sun is insignificant. The consistency between the total injected magnetic helicity and the sunspot numbers has also been found statistically in the solar cycle. The estimated total injected magnetic helicity flux in our calculation is of the order of or larger than 5.0 × 1046 Mx2 in the 23rd solar cycle.

Zhang, Hongqi; Yang, Shangbin

2013-02-01

168

Human powered axial flux permanent magnet machines: Review and comparison  

Microsoft Academic Search

This paper presents a review of concepts for harvesting power from human movement and comparison of three axial flux permanent magnet machine (AFPM) designs. Some schemes for human power generating systems have been presented and compared in order to choose the most suitable configuration for our application. From the concepts evaluated, choices were made based on project specifications in the

S. O. Ani; D. Bang; H. Polinder; J. Y. Lee; S. R. Moon; D. H. Koo

2010-01-01

169

Electromagnetic field of a rotating closed singular magnetic flux line  

Microsoft Academic Search

Summary  The electromagnetic field due to the rotation of a circular singular magnetic flux line is calculated. By averaging the resulting\\u000a electric field over the period of rotation it is shown that by this procedure neither a static Coulomb charge nor an electric\\u000a dipole moment can be generated.

H. Rupertsberger

1982-01-01

170

Pulsed power conversion with inductive storage. [PULSAR compressed magnetic flux  

Microsoft Academic Search

The PULSAR compressed magnetic flux, pulse power generator system has applications as a topping stage in commercial power generation. A plasma armature mode of operation is described which could be powered by an inertially confined fusion reactor. The paper summarizes the development of a plasma armature and experimental results from its application in a PULSAR generator which utilizes a 0.5

E. C. Cnare; M. Cowan; T. P. Wright; W. K. Tucker

1977-01-01

171

Resistance calculation of the reusable linear magnetic flux compressor coil  

Microsoft Academic Search

The reusable linear magnetic flux compressor has been regarded as a preferred pulsed power supply for the electric gun. Some papers have discussed models for optimal design of this compressor. However, none of them have discussed the resistance calculation of the compressor coil particularly, which is the key issue of the compressor design. Based on a two-dimensional finite element model

Jun Li; Yinchun Gui; Qibin Deng; Chenda Yu; Ping Yan; Jiansheng Yuan

2005-01-01

172

Resistance calculation of the reusable linear magnetic flux compressor coil  

Microsoft Academic Search

The reusable linear magnetic flux compressor has been regarded as a preferred pulsed power supply for the electric gun. Some papers have discussed models for optimal design of this compressor. However, none of them have discussed the resistance calculation of the compressor coil particularly, which is the key issue of the compressor design. Based on a two-dimensional finite element model

Jun Li; Yinchun Gui; Jiansheng Yuan; Zanji Wang; Ping Yan

2004-01-01

173

Vertical motions in an intense magnetic flux tube  

Microsoft Academic Search

Radiative damping of waves is important in the upper photosphere. It is thus of interest to examine the effect of radiative relaxation on the propagation of waves in an intense magnetic flux tube embedded in a uniform atmosphere. Assuming Newton's law of cooling, it is shown that the radiative energy loss leads to wave damping. Both the ‘damping per wavelength’

A. R. Webb; B. Roberts

1980-01-01

174

Slow magnetic flux from human frontal cortex.  

PubMed

Slow magnetic fields concurrent with two successive contingent negative variations (CNVs) were elicited in 8 subjects during visual recognition tasks involving pattern versus place discrimination. All stimuli were presented as a rectangular array of lights with various patterns of 6 lights at the center and, simultaneously, with places indicated by missing lights at the periphery. One of two possible stimuli (warning) started each trial, indicating whether pattern or place recognition should be performed on the following two stimuli. The purposes of the experiment were to localize the sources of the slow magnetic fields equivalent to the CNVs and to address the issue of regional specialization of prefrontal cortical function. Results indicated that the equivalent current dipoles (ECDs) found as solutions for the measured slow fields were indeed localized in the prefrontal cortex of each hemisphere. Also, in the right hemisphere, the source location of the CNVs was dependent on task, which supported the hypothesis of specialization of prefrontal function. The place recognition task was associated with more anterior and inferior CNV sources than the pattern recognition task. Finally, it was observed that ECDs for the warning period CNVs were indistinguishable from those for the test period of the tasks. PMID:7510630

Basile, L F; Rogers, R L; Bourbon, W T; Papanicolaou, A C

1994-02-01

175

Fractional quantization of the magnetic flux in cylindrical unconventional superconductors.  

PubMed

The magnetic flux threading a conventional superconducting ring is typically quantized in units of ?0=hc/2e. The factor of 2 in the denominator of ?0 originates from the existence of two different types of pairing states with minima of the free energy at even and odd multiples of ?0. Here we show that spatially modulated pairing states exist with energy minima at fractional flux values, in particular, at multiples of ?0/2. In such states, condensates with different center-of-mass momenta of the Cooper pairs coexist. The proposed mechanism for fractional flux quantization is discussed in the context of cuprate superconductors, where hc/4e flux periodicities were observed. PMID:23931397

Loder, F; Kampf, A P; Kopp, T

2013-07-23

176

Magnetic Flux Emergence and Evolution in the New Solar Cycle  

NASA Astrophysics Data System (ADS)

Cycle 24 follows the longest and deepest solar minimum in about a century. The first spot of cycle 24 appeared in Jan 2008 but so far the new cycle has shown a relatively slow rise in magnetic activity and a noticeable asymmetry between the South and North hemisphere, with the North significantly more active than the South. We will examine the emergence and evolution of magnetic flux during the early rising phase of solar cycle 24 and compare it with previous cycles. In particular, we will discuss the consequences of the hemispheric asymmetry and the progress of the polar magnetic field reversal.

de Toma, G.

2011-12-01

177

Materials for efficient high-flux magnetic bearing actuators  

NASA Astrophysics Data System (ADS)

Magnetic bearings have demonstrated the capability for achieving positioning accuracies at the nanometer level in precision motion control stages. This makes possible the positioning of a wafer in six degrees of freedom with the precision necessary for photolithography. To control the position of an object at the nanometer level, a model of the magnetic bearing actuator force-current-airgap relationship must be accurately obtained. Additionally, to reduce thermal effects the design of the actuator should be optimized to achieve maximum power efficiency and flux density. Optimization of the actuator is accomplished by proper pole face sizing and utilizing a magnetic core material which can be magnetized to the highest flux density with low magnetic loss properties. This paper describes the construction of a magnetic bearing calibration fixture designed for experimental measurement of the actuator force characteristics. The results of a material study that review the force properties of nickel-steel, silicon-steel, and cobalt-vanadium-iron, as they apply to magnetic bearing applications are also presented.

Williams, M. E.; Trumper, D. L.

1994-05-01

178

Magnetic flux leakage inspection of tailor-welded blanks  

NASA Astrophysics Data System (ADS)

A feasibility study was conducted on the application of magnetic flux leakage (MFL) inspection to the evaluation of weld quality in automotive tailor-welded blanks (TWB). Using a permanent magnet configuration, magnetic flux was directed through the weld region of a TWB. A Hall effect sensor was coupled to the movement of a digital plotter and was, thereby, scanned around the weld region. Signals from the Hall effect sensor were processed and correlated with defects to determine corresponding MFL signatures. Simulated through-hole defects as small as 0.34 mm in diameter were readily detected. Furthermore, there was a reasonably linear relationship between the MFL signals associated with these defects and the diameter of the defect hole. Preliminary tests with specimens having naturally occurring defects such as concavity, pinholes, and undercutting, indicate that the MFL technique has excellent potential as an inspection method in this application.

O'Connor, S.; Clapham, L.; Wild, P.

2002-02-01

179

GRANULAR-SCALE MAGNETIC FLUX CANCELLATIONS IN THE PHOTOSPHERE  

SciTech Connect

We investigate the evolution of five granular-scale magnetic flux cancellations just outside the moat region of a sunspot by using accurate spectropolarimetric measurements and G-band images with the Solar Optical Telescope (SOT) aboard Hinode. The opposite-polarity magnetic elements approach a junction of the intergranular lanes and then collide with each other there. The intergranular junction has strong redshifts, darker intensities than the regular intergranular lanes, and surface converging flows. This clearly confirms that the converging and downward convective motions are essential for the approaching process of the opposite-polarity magnetic elements. However, the motion of the approaching magnetic elements does not always match with their surrounding surface flow patterns in our observations. This suggests that, in addition to the surface flows, subsurface downward convective motions and subsurface magnetic connectivities are important for understanding the approach and collision of the opposite-polarity elements observed in the photosphere. We find that the horizontal magnetic field appears between the canceling opposite-polarity elements in only one event. The horizontal fields are observed along the intergranular lanes with Doppler redshifts. This cancellation is most probably a result of the submergence (retraction) of low-lying photospheric magnetic flux. In the other four events, the horizontal field is not observed between the opposite-polarity elements at any time when they approach and cancel each other. These approaching magnetic elements are more concentrated rather than gradually diffused, and they have nearly vertical fields even while they are in contact each other. We thus infer that the actual flux cancellations are highly time-dependent events at scales less than a pixel of Hinode SOT (about 200 km) near the solar surface.

Kubo, M.; Low, B. C.; Lites, B. W. [High Altitude Observatory, National Center for Atmospheric Research P.O. Box 3000, Boulder, CO 80307 (United States)

2010-04-01

180

Simulations of Turbulent Dynamos Driven by the Magnetic Helicity Flux  

NASA Astrophysics Data System (ADS)

We present results of numerical simulations of driven magnetohydrodynamic turbulence in a periodic box with an imposed large-scale sinusoidal shear. We produce strong large-scale magnetic fields through dynamo action. This dynamo action is highly dependent on the spectral properties of the forcing function. In particular, we can produce either dynamo or antidynamo action by choosing particular forcing functions for the small-scale turbulence. Our results show that the large-scale electromotive force is well correlated with the local accumulation of eddy-scale magnetic helicity. Conversely, the electromotive force is very weakly correlated with the large-scale magnetic field strength or its product with kinetic helicity. We argue that in these simulations large-scale dynamo action is driven by an inverse cascade of magnetic helicity, ultimately caused by the tendency of anisotropic turbulence to create a magnetic helicity flux.

Shapovalov, Dmitry S.; Vishniac, Ethan T.

2011-09-01

181

SIMULATIONS OF TURBULENT DYNAMOS DRIVEN BY THE MAGNETIC HELICITY FLUX  

SciTech Connect

We present results of numerical simulations of driven magnetohydrodynamic turbulence in a periodic box with an imposed large-scale sinusoidal shear. We produce strong large-scale magnetic fields through dynamo action. This dynamo action is highly dependent on the spectral properties of the forcing function. In particular, we can produce either dynamo or antidynamo action by choosing particular forcing functions for the small-scale turbulence. Our results show that the large-scale electromotive force is well correlated with the local accumulation of eddy-scale magnetic helicity. Conversely, the electromotive force is very weakly correlated with the large-scale magnetic field strength or its product with kinetic helicity. We argue that in these simulations large-scale dynamo action is driven by an inverse cascade of magnetic helicity, ultimately caused by the tendency of anisotropic turbulence to create a magnetic helicity flux.

Shapovalov, Dmitry S. [Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 (United States); Vishniac, Ethan T., E-mail: dsh@pha.jhu.edu, E-mail: ethan@mcmaster.ca [Department of Physics and Astronomy, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4M1 (Canada)

2011-09-01

182

Thermal magnetic field noise limits resolution in transmission electron microscopy.  

PubMed

The resolving power of an electron microscope is determined by the optics and the stability of the instrument. Recently, progress has been obtained towards subångström resolution at beam energies of 80 kV and below but a discrepancy between the expected and achieved instrumental information limit has been observed. Here we show that magnetic field noise from thermally driven currents in the conductive parts of the instrument is the root cause for this hitherto unexplained decoherence phenomenon. We demonstrate that the deleterious effect depends on temperature and at least weakly on the type of material. PMID:23931384

Uhlemann, Stephan; Müller, Heiko; Hartel, Peter; Zach, Joachim; Haider, Max

2013-07-22

183

On the dynamics of emerging toroidal magnetic flux tubes  

SciTech Connect

In the present work the authors study the dynamic evolution of emerging toroidal magnetic flux rings in the solar convective envelope by carrying out 3D numerical simulations based on the thin flux tube approximation of Spruit. They find: (1) For an axisymmetric flux ring, the aerodynamic drag force experienced by the ring when moving with respect to the ambient fluid transfers no angular momentum to the ring. Therefore in both cases, with or without the drag force, the ring moves nearly parallel to the rotational axis of the sun and emerges at a latitude significantly poleward of sunspot zones, as pointed out by Choudhuri and Gilman. However, for a non-axisymmetric flux ring (i.e., with wave-like undulations along its circumference), the aerodynamic drag force can transfer angular momentum to the flux ring, and therefore reduces the latitude of flux emergence to within the observed sunspot latitudes. (2) As each apex of a flux loop rises due to the magnetic buoyancy force, gas inside the flux tube tends to diverge from the apex. In the meantime, however, the Coriolis force drives a flow within the flux tube opposite to the direction of rotation. Thus, the point of maximum divergence in the flow within the tube is shifted from the apex into the leading side (in the direction of rotation) of the emerging loop. The evacuation of plasma from the leading side of the loop results in a much lower internal gas pressure there as compared to that in the following side at the same depth. Therefore, the magnetic field strength is stronger on the leading side. The numerical simulations show that the field strength in the leading side of the loop can be twice as large as that of the following side at the same depth. This result offers a simple explanation for the observed fact that the leading polarity of an active region is more compact, forms sunspots more easily, and has a longer life time than does the following polarity.

Fan, Y.; Fisher, G.H.; Deluca, E.E.

1992-01-01

184

Observational consequences of a magnetic flux rope topology  

NASA Astrophysics Data System (ADS)

We consider the implications of a magnetic flux rope topology for the interpretation of observations of sigmoidal active regions. A region of tangential magnetic discontinuities can be identified using techniques that determine a bald patch (BP) and corresponding separatrices or a quasi-separatrix layer (QSL) -- for a flux rope this region can be S-shaped, or sigmoidal. If such a region is physically driven, current sheets can form yielding conditions appropriate for reconnective heating. Using a numerical simulation of an emerging flux rope driven by the kink instability, Fan and Gibson (ApJL, 2003) showed that current sheets indeed formed a sigmoidal surface. In this poster we will demonstrate that the current sheets formed on the BP and BP separatrices. Moreover, we will use the results of the numerical simulation as proxies for observations: specifically the simulated field at the photosphere as proxy for the magnetic boundary condition, the sigmoidal current sheets as proxy for the X-ray active region emission, and the location of dipped magnetic field lines as proxy for a filament. We will then consider to what extent such observations might be used to understand and constrain the basic properties of the coronal field.

Gibson, S.; Barnes, G.; Demoulin, P.; Fan, Y.; Fisher, G.; Leka, K.; Longcope, D.; Mandrini, C.; Metcalf, T.

2003-12-01

185

Current in Hubbard rings manipulated via magnetic flux.  

PubMed

We study currents in a quantum ring threaded by a magnetic flux which is varied in an arbitrary way from an initial constant value ?(1) at time t(1) to a final constant value ?(2) at time t(2). We analyze how the induced currents for t > t(2) can be controlled by the rate of flux variation [Formula: see text]. The dynamics of electrons in the ring is described using the Hubbard and the extended Hubbard models. In the Hubbard model with infinite on-site repulsion the current for t > t(2) is shown to be independent of the flux variation before t(2) and is fully determined by a solution of the initial equilibrium problem and by the value ?(2) of the flux. For intermediate values of the interaction strength the current displays regular or irregular time oscillations and the amplitude of oscillations is sensitive to the rate of the flux changing [Formula: see text]: slow changes of the flux result in small amplitudes of the current oscillations and vice versa. We demonstrate that the time dependence of the induced current bears information on electronic correlations. Our results have important implications for not only mesoscopic rings but also the designing of quantum motors built out of ring-shaped optical lattices. PMID:21393779

Mierzejewski, M; ?uczka, J; Dajka, J

2010-05-26

186

Models of the Large-Scale Corona. II. Magnetic Connectivity and Open Flux Variation  

Microsoft Academic Search

In this paper the changing connectivity of the coronal magnetic field during the formation and ejection of magnetic flux ropes is considered. Using recent simulations of the coronal field, it is shown that reconnection may occur both above and below the flux ropes. Those occurring above slowly strip away coronal arcades overlying the flux ropes and allow the flux ropes

D. H. Mackay; A. A. van Ballegooijen

2006-01-01

187

Open Magnetic Flux and Magnetic Reconnection During Steady Magnetic Convection Intervals.  

NASA Astrophysics Data System (ADS)

The Imager for Magnetopause to Aurora Global Exploration (IMAGE) spacecraft was launched in 2000 with several imaging instruments onboard. The Far UltraViolet (FUV) experiment imaged the N2 LBH (Wideband Imaging Camera - WIC-), OI 135.6 nm (Spectrographic Imager -SI13-) and Doppler-shifted Lyman alpha auroral emission (SI12). The Doppler-shifted Lyman-alpha emission allows to monitor the auroral oval both on the day and night sides. Remote sensing of the polar aurora is completed by ground based data of the Super Dual Auroral Radar Network (SuperDARN) that monitors the ionospheric convection flow pattern in the polar region. In the present study, SI12 images are used to estimate the open/closed (o/c) field line boundary location, and monitor its movement. The SuperDARN data are used to compute the electric field of the polar cap at the location of the o/c boundary. The total electric field is then computed along the boundary accounting for its movement applying Faraday's law, so that the dayside and nightside reconnection voltages can be retrieved. We apply this method to the study of several intervals of steady magnetic convection (SMC). SMC events are intervals of enhanced convection without classical substorm signatures. During these intervals, it is expected that the amount of open magnetic flux remains fairly constant, and it has been suggested that the rate of opening (at the magnetopause) and closure (in the magnetotail) of magnetic flux balance each other. These rates can be expressed as voltages with a positive sign for the opening and a negative sign for closure. The net reconnection voltage then represents the net rate of accumulation of open flux by the magnetosphere. We find that, during SMC intervals, the open magnetic flux varies only slowly, and sometimes remains stationary during several hours. As a consequence, the net voltage often remains close to zero during SMC intervals. Occasionally, we find that an increase in the opening voltage is followed by a similar intensification of the closure voltage after downtail convection of the newly created open flux. The convection time can be roughly estimated and ranges between 20 and 40 minutes, i.e. the typical order of magnitude of the convection time in the magnetosphere.

Hubert, B.; Milan, S. E.; Cowley, S. W.

2009-05-01

188

Magnetic force microscopy signal of flux line above a semi-infinite type II-superconductor  

Microsoft Academic Search

We have examined a single flux line in the semi-infinite type-II superconductor. The stray magnetic field of the flux line has been calculated. We have found that the vertical force exerted on a magnetic force microscopy (MFM) tip from the flux line is measurable by currently existing MFM. Two types of magnetic tips were taken into consideration, solid and thin

A. Wadas; O. Fritz; H. J. Hug; H.-J. Güntherodt

1992-01-01

189

Magnetic flux distribution around BSCCO single crystal d-dot  

NASA Astrophysics Data System (ADS)

We have proposed a composite structure named as d-dot where a d-wave superconducting square dot is embedded in s-wave superconductor matrix. Spontaneous vortices with a half flux quantum should appear in the four corners of the d-dot because ? phase shift of d-wave order parameter occurs at around corner junctions. The d-wave cuprate superconductor BSCCO dot surrounded by a conventional s-wave superconductor Pb is prepared by using a photolithography process, an Ar ion-milling method, a vacuum evaporation and a lift-off technique. Local magnetic flux distribution around the BSCCO single crystal d-dot is investigated by using a scanning SQUID microscope. The spontaneous half flux quantum is observed at one corner of the d-dot.

Kawamata, Shuichi; Yamamoto, Masanori; Uno, Mayumi; Satoh, Kazuo; Yuki, Taeko; Yotsuya, Tsutomu; Ishida, Takekazu

2010-12-01

190

The Effect of a Magnetic Flux Line in Quantum Theory  

Microsoft Academic Search

The nonlocal exchange of the conserved, gauge invariant quantity ei\\/ℏ(pk-e\\/cAk)Lk, Lk=const, k=1,2 between the charged particle and the magnetic flux line (in the k=3 direction) is responsible for the Aharonov-Bohm effect. This exchange occurs at a definite time, before the wave packets are brought together to interfere, and can be verified experimentally.

Y. Aharonov; T. Kaufherr

2004-01-01

191

Alternative magnetic flux leakage modalities for pipeline inspection  

SciTech Connect

Increasing quality consciousness is placing higher demands on the accuracy and reliability of inspection systems used in defect detection and characterization. Nondestructive testing techniques often rely on using multi-transducer approaches to obtain greater defect sensitivity. This paper investigates the possibility of taking advantage of alternative modalities associated with the standard magnetic flux leakage tool to obtain additional defect information, while still using a single excitation source.

Katragadda, G.; Lord, W.; Sun, Y.S.; Udpa, S.; Udpa, L. [Iowa State Univ., Ames, IA (United States). Dept. of Electrical and Computer Engineering

1996-05-01

192

Ion Viscous Heating and the Destruction of Magnetic Flux  

SciTech Connect

Further work on ion heating through viscous dissipation of vorticity generated by short wavelength m=0 instabilities is reported. Both heavy tungsten wire arrays and light stainless steel arrays are considered, the latter leading to record ion temperatures in the 200-300keV range. The problem of magnetic flux destruction is addressed, and this is solved by a reversible mechanism involving axially asymmetric m=0 hot spots on the axis.

Haines, M.G. [Blackett Laboratory, Imperial College, London (United Kingdom)

2006-01-05

193

Current-driven plasma turbulence in a magnetic flux tube  

Microsoft Academic Search

The behaviour of a multi-component anisotropic plasma in a magnetic flux tube is studied in the presence of current-driven electrostatic ion-cyclot r on turbulence. The plasma transport is considered in both parallel and perpendicular directions with respect to the given tube. As one of the sources of the parallel electric field, the anomalous resistivity of the plasma caused by the

V. E. Zakharov; C.-V. Meister

1999-01-01

194

Optimization of multiply acquired magnetic flux density B(z) using ICNE-Multiecho train in MREIT.  

PubMed

The aim of magnetic resonance electrical impedance tomography (MREIT) is to visualize the electrical properties, conductivity or current density of an object by injection of current. Recently, the prolonged data acquisition time when using the injected current nonlinear encoding (ICNE) method has been advantageous for measurement of magnetic flux density data, Bz, for MREIT in the signal-to-noise ratio (SNR). However, the ICNE method results in undesirable side artifacts, such as blurring, chemical shift and phase artifacts, due to the long data acquisition under an inhomogeneous static field. In this paper, we apply the ICNE method to a gradient and spin echo (GRASE) multi-echo train pulse sequence in order to provide the multiple k-space lines during a single RF pulse period. We analyze the SNR of the measured multiple B(z) data using the proposed ICNE-Multiecho MR pulse sequence. By determining a weighting factor for B(z) data in each of the echoes, an optimized inversion formula for the magnetic flux density data is proposed for the ICNE-Multiecho MR sequence. Using the ICNE-Multiecho method, the quality of the measured magnetic flux density is considerably increased by the injection of a long current through the echo train length and by optimization of the voxel-by-voxel noise level of the B(z) value. Agarose-gel phantom experiments have demonstrated fewer artifacts and a better SNR using the ICNE-Multiecho method. Experimenting with the brain of an anesthetized dog, we collected valuable echoes by taking into account the noise level of each of the echoes and determined B(z) data by determining optimized weighting factors for the multiply acquired magnetic flux density data. PMID:20400810

Nam, Hyun Soo; Kwon, Oh In

2010-04-19

195

Solar Magnetic Flux Transport Modeling with Far-side Data  

NASA Astrophysics Data System (ADS)

Observed global solar photospheric magnetic field distribution maps, of high quality and cadence, are critical for coronal and heliospheric models to forecast and estimate the severity of geoeffective space weather events. Until STEREO-like observations of the solar magnetic field are available on the solar far-side, only the Earth-side of the solar surface is measured without large temporal gaps. This results in global maps which include data more than 13 days old, since the solar rotational period relative to Earth is approximately 27 days. The boundaries between old and new observations typically lead to large discontinuities that result in monopole signals. To avoid these large discontinuities, along with the lack of or poor polar measurements which can also introduce artificial monopoles, we have developed the ADAPT (Air Force Data Assimilative Photospheric flux Transport) model. ADAPT incorporates data assimilation using an Ensemble Least Squares (EnLS) estimation method with photospheric magnetic flux transport. The transport model is a modified version of the Worden and Harvey (WH) model that evolves the solar magnetic flux by incorporating rotational, meridional, and super-granular diffusive transport processes during periods that lack observations for a given solar region. The modified model utilizes an ensemble of model realizations using different model parameters. In this presentation, the methodology for incorporating solar magnetic far-side data, inferred from helioseismology, in the ADAPT model will be discussed. This will include our approach for representing the nominal distribution and morphology of the far-side active region polarity (e.g., Hale rule) and orientation (e.g., Joy's rule) within the ADAPT ensemble. For example, though any single choice for the far-side flux distribution and organization is expected to be amiss at best, a distribution of morphologies within the estimated far-side signal area can be generated using the ADAPT ensemble of global maps to represent nominal active region characteristics. In addition, the need for realistic error estimations of the far-side field strength and area to better estimate the model uncertainties will be discussed. Besides providing an improved estimate of the global magnetic field, the far-side signal allows for a more accurate representation of the uncertainty within the model ensemble for more rapid data assimilation once the region is observed. Preliminary model comparison between ADAPT maps with and without far-side data for forecast parameters (e.g., the solar 10.7 cm radio flux) will be presented. This work is supported by a grant from the Air Force Office of Scientific Research (AFOSR) and by the AFRL Space Weather Forecasting Laboratory (SWFL).

Henney, C. J.; Arge, C. N.; Gonzalez Hernandez, I.; Koller, J.; Toussaint, W. A.; Godinez, H. C.

2011-12-01

196

THREE-DIMENSIONAL RECONNECTION INVOLVING MAGNETIC FLUX ROPES  

SciTech Connect

Two and three magnetic flux ropes are created and studied in a well-diagnosed laboratory experiment. The twisted helical bundles of field lines rotate and collide with each other over time. In the two rope case, reverse current layers indicative of reconnection are observed. Using a high spatial and temporal resolution three-dimensional volume data set in both cases, quasi-separatrix layers (QSLs) are identified in the magnetic field. Originally developed in the context of solar magnetic reconnection, QSLs are thought to be preferred sites for reconnection. This is verified in these studies. In the case of three flux ropes there are multiple QSLs, which come and go in time. The divergence of the field lines within the QSLs and the field line motion is presented. In all cases, it is observed that the reconnection is patchy in space and bursty in time. Although it occurs at localized positions it is the result of the nonlocal behavior of the flux ropes.

Gekelman, W.; Van Compernolle, B. [Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 (United States); Lawrence, E. [Princeton Plasma Physics Laboratory, Princeton, NJ (United States)

2012-07-10

197

Flux Rope Acceleration and Enhanced Magnetic Reconnection Rate  

SciTech Connect

A physical mechanism of flares, in particular for the flare rise phase, has emerged from our 2-1/2-dimensional resistive MHD simulations. The dynamical evolution of current-sheet formation and magnetic reconnection and flux-rope acceleration subject to continuous, slow increase of magnetic shear in the arcade are studied by employing a non-uniform anomalous resistivity in the reconnecting current sheet under gravity. The simulation results directly relate the flux rope's accelerated rising motion with an enhanced magnetic reconnection rate and thus an enhanced reconnection electric field in the current sheet during the flare rise phase. The simulation results provide good quantitative agreements with observations of the acceleration of flux rope, which manifests in the form of SXR ejecta or erupting filament or CMEs, in the low corona. Moreover, for the X-class flare events studied in this paper the peak reconnection electric field is about O(10{sup 2} V/m) or larger, enough to accelerate p articles to over 100 keV in a field-aligned distance of 10 km. Nonthermal electrons thus generated can produce hard X-rays, consistent with impulsive HXR emission observed during the flare rise phase.

C.Z. Cheng; Y. Ren; G.S. Choe; Y.-J. Moon

2003-03-25

198

Innovating approaches to the generation of intense magnetic fields : design and optimization of a 4 Tesla permanent magnet flux source  

Microsoft Academic Search

An original permanent magnet flux source is designed in order to generate a magnetic field of several Tesla. The magnet configuration and discretization of the structure are optimized with the help of numerical simulation software developed at LEG (DIPOLE-3D, FLUX2D & FLUX3D). The model of spheroidal flux source presented in the paper creates a field in excess of 4.3 T

F. Bloch; O. Cugat; G. Meunier; J. C. Toussaint

1998-01-01

199

Magnetic Flux Circulation During Dawn-Dusk Oriented Interplanetary Magnetic Field  

NASA Astrophysics Data System (ADS)

Magnetic flux circulation is a primary mode of energy transfer from the solar wind into the ionosphere and inner magnetosphere. For southward interplanetary magnetic field (IMF), magnetic flux circulation is described by the Dungey cycle (dayside merging, night side reconnection, and magnetospheric convection), and both the ionosphere and inner magnetosphere receive energy. For dawn-dusk oriented IMF, magnetic flux circulation is not well understood, and the inner magnetosphere does not receive energy. Several models have been suggested for possible reconnection patterns; the general pattern is: dayside merging; reconnection on the dayside or along the dawn/dusk regions; and, return flow on dayside only. These models are consistent with the lack of energy in the inner magnetosphere. We will present evidence that the Dungey cycle does not explain the energy transfer during dawn-dusk oriented IMF. We will also present evidence of how magnetic flux does circulate during dawn-dusk oriented IMF, specifically how the magnetic flux reconnects and circulates back.

Mitchell, E. J.; Lopez, R. E.; Fok, M. H.; Deng, Y.; Wiltberger, M. J.; Lyon, J.

2010-12-01

200

Intensification of magnetic field by concentrate-and-stretch of magnetic flux lines  

Microsoft Academic Search

The mechanism of generation of magnetic field by thermal convection is investigated by analyzing the numerical solution to the Boussinesq magneto-hydrodynamic equations in a rotating spherical shell. The magnetic flux lines are intensified by concentrate-and-stretch around sinks in the outer boundary layer, on the equatorial plane and in the inner boundary layer, and the strong magnetic field is generated downstream

Hideaki Kitauchi; Shigeo Kida

1998-01-01

201

Spectral features of background ULF noise during magnetic storms  

NASA Astrophysics Data System (ADS)

The monitoring of tangential components of ULF noise below first Schumann resonance was permanently provided since 2003 up to now at mid-latitude station New Life (56N, 44,7E). The regular structure in ULF spectrograms named SRS -spectral resonance structure could be observed practically every night excepting years of solar activity maximum. The SRS appears in the ULF noise due existing the resonator for Alfven waves in the ionosphere at heights from the bottom up to 600 -1000 km [1]. It was noticed that during magnetically disturbed time the strong anti-correlation between the intensity of SRS events and K index from station on neighbor magnetic meridian [2]. For more detail investigation it was choused several magnetic storms. First one is storm witch has began at November 20, 2003 with maximum Kp=9 (strong), second -at March 19, 2006 with Kp=6+ (moderate) and the third -at November 25, 2008 with Kp=5 (weak). The spectra of magnetic component amplitude and polarization parameter were analyzed. The comparison with ionosond and magnetic field data as well as world TEC maps for mentioned storm periods were made. The dynamics of changing the properties of ULF spectra during the storms were explored. The general regularities of these changes were displayed. One of the prominent feathers appeared at the end of the strong and moderate storms looked like fish shoals. Perhaps this kind of ULF bursts is the same nature as well known pearls pulsation but displayed in broad frequency range 1-6 Hz. May be it could be classified as special spectral structure. The analysis of changing the properties of ULF spectra during the storms together with geo-physical data has shown that the main regularities in changes the SRS caused by changing of the ionospheric parameters above the observation point. But some of ULF spectral features are results of interaction waves and particles in the magnetosphere. 1. Belyaev P.P., Polyakov S.V., Ermakova E.N., Isaev S.V. Solar cycle variations in the iono-spheric Alfven resonator1985-1995, J. Atmos. Solar-Terr. Phys. 2000. V.62, No 4. P.239-248 2. D.S. Kotik, E.N. Ermakova, The anticorrelation between SRS and geomagnetic activity levels at mid latitudes. Abstracts of XXX Annual Seminar "Physics of Auroral phenomena", 28 February 2007, p. 18.

Kotik, Dmitry; Ermakova, Elena; Ryabov, Alexander; Shchecoldin, Aexander

202

Laboratory simulation of solar magnetic flux rope eruptions  

NASA Astrophysics Data System (ADS)

A laboratory plasma experiment has been constructed to simulate the eruption of arched magnetic flux ropes (AMFRs e.g., coronal loops, solar prominences) in an ambient magnetized plasma. The laboratory AMFR is produced using an annular hot LaB6 cathode and an annular anode in a vacuum chamber which has additional electrodes to produce the ambient magnetized plasma. Two laser beams strike movable carbon targets placed behind the annular electrodes to generate controlled plasma flows from the AMFR footpoints that drives the AMFR eruption. The experiment operates with a 0.5 Hz repetition rate and is highly reproducible. Thus, time evolution of the AMFR is recorded in three-dimensions with high spatio-temporal resolutions using movable diagnostic probes. Experimental results demonstrate outward expansion of the AMFR, release of its plasma to the background, and excitation of fast magnetosonic waves during the eruption.

Tripathi, S. K. P.; Gekelman, W.

2011-08-01

203

Reconstruction of a magnetic flux rope from THEMIS observations  

NASA Astrophysics Data System (ADS)

We investigate a magnetic flux rope (MFR) observed by THEMIS near the duskside magnetopause on 20 May 2007 using the reconstruction technique based on solving the Grad-Shafranov equation. The MFR has characteristics distinct from the adjacent magnetosheath and magnetosphere regions. In spite of these differences, the reconstruction result shows that the MFR is connected simultaneously with both the magnetosheath and the magnetosphere in terms of the magnetic vector potential characteristics. This result provides strong evidence that the MFR represents the union of these two regions. It has a small spatial dimension of ~0.5 R E, a strong core magnetic field of >50 nT, and an intense axial current density of >40 nA/m2 with non-negligible current densities transverse to its axis.

Lui, A. T. Y.; Sibeck, D. G.; Phan, T.; Angelopoulos, V.; McFadden, J.; Carlson, C.; Larson, D.; Bonnell, J.; Glassmeier, K.-H.; Frey, S.

2008-04-01

204

Internal structure of a magnetic flux rope from Cluster observations  

NASA Astrophysics Data System (ADS)

We investigate a magnetic flux rope (MFR) observed by Cluster in the magnetotail during a substorm on 2001 August 22. The MFR was aligned with its principal axis closely along the dawn-dusk direction and had a small size of ~2 R E with a total current of ~0.8 MA. The four spacecraft traversed the MFR at different distances from its center based on the magnetic field signature. This fortuitous situation reveals the irregular magnetic field structure in its inner core, which is a feature reported here for the first time. At the leading edge, the y-component of the electric field was dawnward against the current density direction (dynamo action) and the x-component of the Lorentz force was Earthward. These parameters reversed in direction at its trailing edge (load).

Lui, A. T. Y.; Dunlop, M. W.; Rème, H.; Kistler, L. M.; Gustafsson, G.; Zong, Q.-G.

2007-04-01

205

Low-noise magnetic sensing for marine munitions characterization  

NASA Astrophysics Data System (ADS)

Because the recovery of underwater munitions is many times more expensive than recovering the same items on dry land, there is a continuing need to advance marine geophysical characterization methods. To efficiently and reliably conduct surveying in marine environments, low-noise geophysical sensors are being configured to operate close to the sea bottom. We describe systems that are deployed from surface vessels via rigid or flexible tow cables or mounted directly to submersible platforms such as unmanned underwater vehicles. Development and testing of a towed configuration has led to a 4 meter wide hydrodynamically stable tow wing with an instrumented top-side assembly mounted on the stern of a surface survey vessel. An integrated positioning system combined with an instrumented cable management system, vessel and wing attitude and wing depth measurements to provide sub-meter positional accuracy in up to 25 meter water depths and within 1 to 2 meters of the seafloor. We present the results of data collected during an instrument validation survey over a series of targets emplaced at measured locations. Performance of the system was validated through analyses of data collected at varying speeds, headings, and height above the seafloor. Implementation of the system during live-site operations has demonstrated its capability to survey hundreds of acres of marine or lacustrine environment. Unique deployment concepts that utilize new miniaturized and very low noise sensors show promise for expanding the applicability of magnetic sensing at marine sites.

Schultz, Gregory; Keranen, Joe; Billings, Stephen; Fonda, Raul; Foley, Jack; Bassani, Chet

2011-05-01

206

Investigation of the magnetic flux leakage signatures of dents and gouges  

NASA Astrophysics Data System (ADS)

A study of gouges and dents in the context of pipelines has been completed, using the non-destructive evaluation (NDE) techniques of magnetic flux leakage (MFL) and magnetic Barkhausen noise (MBN). The research is part of an ongoing effort by the Applied Magnetics Group (AMG) at Queen's University to improve the interpretation of the MFL signal, which is used extensively by industry for defect detection and evaluation. The gouges were found to have distinctive MFL signatures depending on their orientation relative to the magnetization axis. Features in the MFL signal were identified as superpositions of geometry-related effects and strain or work hardening of the surface material. A qualitative magnetic permeability distribution in the material near a gouge has been proposed. The distribution is expected to vary in magnitude and extent according to the defect severity. The MFL results of the dent studies, on samples made available by Gaz de France (GdF), largely agreed qualitatively with previous research of dents. However, the differences pointed to the need for study of more varied dent shapes; new signal features were observed that suggested tensile residual strain in the dent rim is more prominent than earlier studies and modeling have predicted. Additionally, upgrades made to the MFL scanning system used by the AMG and a novel approach for building computer models are detailed.

Marble, Kris

207

Laser switch for stroboscopic read-out of magnetic flux  

NASA Astrophysics Data System (ADS)

We have realized and tested a fast stroboscopic detector for magnetic flux measurements. The key element of our detector is a hysteretic dc superconducting quantum interference device (SQUID). Stroboscopic read-out of the magnetic flux coupled with the SQUID is accomplished by biasing the SQUID with fast current pulses. The shorter these pulses, the more stroboscopic and less invasive the measurement we are performing. In order to reduce the duration of the current pulses, we take advantage of the superconducting-normal transition induced by laser light in thin superconducting films. The interaction of laser light with superconducting thin films has been investigated thoroughly in the past and many applications have been proposed which rely on the fast typical times with which superconductivity is broken and a resistive behavior arises. We have measured a threshold resolution of 6.9m?0 at 4.2 K, and this value corresponds to the thermodynamic limit of the SQUID. The detector has been accurately characterized: An improved and more sensitive version might prove useful for quantum mechanics and quantum computation experiments, for example, in detecting the state of flux qubits.

Ferrara, Marco; Carelli, Pasquale; Chiarello, Fabio; Castellano, Maria Gabriella; Torrioli, Guido; Cosmelli, Carlo

2004-06-01

208

Magnetic reconnection and particle accelerationinitiated by flux emergence  

NASA Astrophysics Data System (ADS)

So as to perform an MHD simulation of the evolution of the corona driven by the evolution of the photosphere, a key aspect is the definition of the boundary conditions for reaching a good compromise between physical conditions and numerical constraints. In this work, we focused on the simulation of a confined flare observed on Nov 16, 2002. As initial configuration, we considered a uniform temperature corona, with a magnetic field resulting from a 3D potential field extrapolation from a SOHO/MDI magnetogram. We prescribed a velocity field at the photospheric boundary of the domain, so as to mimic the observed flow pattern associated to a flux emergence. This resulted in a combination of ``slipping reconnection'' in a halo of QSLs surrounding a 3D null point, through which a ``fan reconnection'' regime took place. This simplified approach of flux emergence has successfully reproduced the main characteristics of the observed flare: the flare ribbons observed in the EUV with TRACE being due to the chromospheric impact of particles accelerated along reconnecting field lines, this bimodal regime could explain both the shapes and dynamics of these ribbons. We foresee that this kind of modeling should be able to simulate the evolution of slipping magnetic flux tubes in open configurations, allowing to predict the spatio-temporal evolution of particle beams injected into the heliosphere.

Masson, S.; Aulanier, G.; Pariat, E.; Klein, K.-L.; Schrijver, C. J.

2008-11-01

209

Absence of magnetic state dependent low-frequency noise in spin-valve systems  

NASA Astrophysics Data System (ADS)

The low-frequency noise in magnetic tunnel junctions and giant magnetoresistance devices in various magnetic states has been investigated. The noise measurements in both types of devices show a 1/f spectrum in either the parallel or antiparallel magnetic state. The noise as the magnetization switches from parallel to antiparallel and vice versa in the transition regions (characterized by a large dR/dH), in which the magnetic aftereffect is present and the system is out of equilibrium, also exhibits a 1/f spectrum of a magnitude consistent with that in the parallel and antiparallel states where the system relaxes to the equilibrium state. Included is how even small magnetic aftereffect signals can replicate the results of many previous studies. Two additional experiments are described which set an upper limit for the magnetic noise in the devices investigated.

Guo, Feng; McKusky, Greg; Dahlberg, E. Dan

2013-07-01

210

The Role of Electron Heat Flux in Magnetic Reconnection  

NASA Astrophysics Data System (ADS)

Particle-in-Cell (PIC) and hybrid simulations (kinetic ions, fluid massless electrons) have been used to investigate magnetic reconnection in 2-D with no guide field. Both simulations are initialized with a Harris sheet equilibrium and the magnetic field is perturbed in order to excite a linear tearing instability. The electron momentum equation is used to calculate the electric field in the hybrid simulation, and the divergence of the full pressure tensor is included in order to break the frozen-in condition at the X-point. In order to evolve the full pressure tensor, we multiply the Vlasov equation by vivj in order to obtain an evolution equation for Pij. However, this scheme requires knowledge of the divergence of the heat flux (Q), which leads to the well known closure problem in plasma fluid theory. The Hybrid code currently solves the full evolution equation of the electron pressure tensor with the divergence of the heat flux term set to zero. In this paper, we compare the results from the hybrid code with results from the PIC code. Generally the results of the two codes agree, consistent with earlier work. However, we find differences in the evolution of the electron heating and also in the location of the heating. For example, in the hybrid code, the electrons mainly heat in the center of the diffusion region, whereas in the PIC code, the electrons heat at the edge of the diffusion region. To show the effects of the electron heat flux, we calculate it directly from a PIC code and compare with the other source terms in \\frac{? P}{? t}. We show that the heat flux term is as important in determining the electric field as all the other source terms, and therefore cannot be neglected in the calculation of the pressure tensor. Distribution functions are then presented which demonstrate kinetically the source of the heat flux. A scheme is then presented for including heat flux in the hybrid code which does not rely on taking the third moment of the Vlasov equation. Preliminary hybrid results are presented which includes heat flux in the calculation of the momentum equation.

Main, D. S.; Yin, L.; Winske, D.; Bowers, K.

2007-12-01

211

Effects and Compensation of Magnetic Saturation in Flux-Weakening Controlled Permanent Magnet Synchronous Motor Drives  

Microsoft Academic Search

Permanent magnet synchronous (PM) motors can be applied to applications requiring constant power operation, such as traction and spindIe drives by means of flux-weakening control. In a PM motor drive system with flux-weakening con- trol, the motor parameters are used to produce the current vector command. The motor parameters vary because of mag- netic saturation and as a result, the

Shigeo Morimoto; Masayuki Sanada; Yoji Takeda

1994-01-01

212

Direct Observation of Magnetic Flux Behavior in High-Tc Oxide Superconductors  

Microsoft Academic Search

The magnetic flux behavior in YBa2Cu3Ox specimens under various applied fields was observed at 10K and 77K using the Faraday effect of an iron garnet film with stripe magnetic domains. The magnetic flux distribution was determined by optical observation of the domain structure in the iron garnet film placed on the sample. The magnetic flux gradients in a crystal fabricated

S. Gotoh; N. Koshizuka; M. Yoshida; H. Fujimoto; M. Murakami; H. Izumi; K. Ohata

1992-01-01

213

Magnetic Reconnection During Flux Conversion in a Driven Spheromak  

SciTech Connect

During buildup of a spheromak by helicity injection, magnetic reconnection converts toroidal flux into poloidal flux. This physics is explored in the resistive magnetohydrodynamic code, NIMROD [C.R. Sovinec, A.H. Glasser, T.A. Gianakon, D.C. Barnes, R.A. Nebel, S.E. Kruger, D.D. Schnack, S.J. Plimpton, A. Tarditi, and M.S. Chu, J. Comp. Phys., 195, 355-386 (2004)], which reveals negative current sheets with {lambda} = {mu}{sub 0}j {center_dot} B/B{sup 2}reversed relative to the applied current. The simulated event duration is consistent with magnetic diffusion on the sheet thickness and is accompanied by cathode voltage spikes and poloidal field increases similar to those seen in the Sustained Spheromak Physics Experiment, SSPX [E. B. Hooper, L. D. Pearlstein, and R. H. Bulmer, Nucl. Fusion 39, 863 (1999)]. All magnetic fieldlines are open during reconnection and their trajectories are very sensitive to their starting points, resulting in chaos. The current sheets are most intense inside the separatrix near the X-point of the mean-field spheromak, suggesting that the reconnection occurs near fieldlines which are closed in the azimuthal average.

Hooper, E B; Kopriva, T A; Cohen, B I; Hill, D N; McLean, H S; Wood, R D; Woodruff, S; Sovinec, C R

2005-06-07

214

SIMULATING THE 'SLIDING DOORS' EFFECT THROUGH MAGNETIC FLUX EMERGENCE  

SciTech Connect

Recent Hinode photospheric vector magnetogram observations have shown that the opposite polarities of a long arcade structure move apart and then come together. In addition to this 'sliding doors' effect, orientations of horizontal magnetic fields along the polarity inversion line on the photosphere evolve from a normal-polarity configuration to an inverse one. To explain this behavior, a simple model by Okamoto et al. suggested that it is the result of the emergence of a twisted flux rope. Here, we model this scenario using a three-dimensional megnatohydrodynamic simulation of a twisted flux rope emerging into a pre-existing overlying arcade. We construct magnetograms from the simulation and compare them with the observations. The model produces the two signatures mentioned above. However, the cause of the 'sliding doors' effect differs from the previous model.

MacTaggart, David; Hood, Alan W., E-mail: dm428@st-andrews.ac.u [School of Mathematics and Statistics, University of St Andrews, St Andrews, Fife KY16 9SS (United Kingdom)

2010-06-20

215

Inertia Wheel on Low-Noise Active Magnetic Suspension  

NASA Astrophysics Data System (ADS)

Magnetic bearings are particularly suited for space applications for a number of reasons: - they are ideally suited for vacuum applications; - the lack of lubrication and wear enhances the reliability and guaranties a long maintenance-free operation - the low drag torque decreases power consumption and reduces the torque exerted on the stator of the machine. - the possibility of insulating actively the spacecraft from the excitation due to unbalance of the rotating system In the case of reaction wheels, a well designed magnetic suspension allows high speed operation with a very low power consumption and vibration level. Conversely, microgravity (and possibly vacuum) operation is an advantage for magnetic bearings. The absence of static forces allows to operate with low current levels, thus reducing electrical noise and allowing to reach even lower vibration levels than in Earth applications of magnetic bearings. Active magnetic bearings (AMB) allow to adapt the working characteristics of the system to the operating needs: it is possible to use the actuators to lock the system during launch (absence of grabbers) and to stiffen the suspension when the spacecraft is accelerated (impulsive phases), while working in conditions optimised for microgravity when this is needed. Magnetic suspension systems designed for microgravity environment cannot be correctly tested on the ground. Testing in ground conditions results in the need of grossly overdesigning the levitation device; furthermore, in some cases ground testing is completely impossible, if not by introducing devices which compensate for the Earth gravitational field. If the compensation for the gravitational force is supplied by the same actuators used for microgravity operation, the actuators and the power amplifiers must be overdesigned and in some cases the suspension can be altogether impossible. They work in conditions which are much different from nominal ones and, above all, it is impossible to reach the precision in force measuring or vibration isolation which are required. Note that the stiffness of a magnetic suspension usually increases when it must compensate for a large static force and the increase of stiffness changes drastically the vibration isolation characteristics. It is also possible to support the rotor using a separate controlled electromagnet, but the latter will introduce disturbances which make impossible to evaluate the performances of the magnetic levitation system. Moreover, the sensitivity of the device to the operating conditions makes testing in conditions so different from the actual ones of very little significance. This is particularly true when accurate force measuring or vibration isolation is required or when low power consumption is one of the design specifications. Finally, if an external electromagnetic device is used for compensating for weight, its presence changes the stiffness of the system, to the point of altering drastically its stability characteristics. Parabolic flight is not a solution for this problem: the duration of low gravity conditions during parabolic flights is too short to perform significant experiments on magnetic suspension systems, particularly if the natural frequency of the suspension is very low as is typical of devices aimed at the isolation from low frequency vibrations. The environment in which parabolic flight testing is performed is also too rough for accurate testing. The availability of the space station changes deeply this situation: magnetic levitation systems built for space application can be tested in conditions which are very close to the operating ones. Although the space station environment is not vibrationally so clean as it would be necessary for some application, it is nevertheless far better than any simulated environment on the ground. The present paper deals with the design and construction of an engineering model of an inertia wheel on AMB. The aim of the project is to test the performance of the inertia wheel, particularly for what the disturbances caused by the rotor on the suppor

Carabelli, S.; Genta, G.; Silvagni, M.; Tonoli, A.

2002-01-01

216

System having unmodulated flux locked loop for measuring magnetic fields  

DOEpatents

A system (10) for measuring magnetic fields, wherein the system (10) comprises an unmodulated or direct-feedback flux locked loop (12) connected by first and second unbalanced RF coaxial transmission lines (16a, 16b) to a superconducting quantum interference device (14). The FLL (12) operates for the most part in a room-temperature or non-cryogenic environment, while the SQUID (14) operates in a cryogenic environment, with the first and second lines (16a, 16b) extending between these two operating environments.

Ganther, Jr., Kenneth R. (Olathe, KS); Snapp, Lowell D. (Blue Springs, MO)

2006-08-15

217

Landau, Abrikosov, Hofstadter: Magnetic flux penetration in a lattice superconductor  

SciTech Connect

Magnetic flux penetration in superconductors involves a rich variety of subtle phenomena, much of which is still poorly understood. Here these complexities are studied by formulating the Ginzburg-Landau equations as a lattice gauge theory. Their solutions are compared and contrasted with the (heuristic) Landau model of type I superconductivity, and the (perturbative) Abrikosov model for type II superconductors. Novelties arise as the continuum limit is approached, related to an effect discovered by Hofstadter. Various cautionary remarks pertinent to large-scale simulations are made. 28 refs., 6 figs., 1 tab.

Callaway, D.J.E. (Rockefeller Univ., New York (United States))

1993-06-01

218

Magnetic Reconnection Indicated in Jupiter's H3+ Auroral Flux Variations  

NASA Astrophysics Data System (ADS)

Due to its complexity, the production mechanism of Jupiter's powerful aurora is to date not very well understood. Possible correlation with the solar wind has been one of such unsolved problems (Prange et al. 1993; Baron et al., 1996; Gurnet et al., 2002). We analyzed several sets of ground-based infrared data of Jupiter's H3+ aurora, acquired at NASA/IRTF atop Mauna Kea, Hawaii during 1998-2000 seasons. Night-to-night variations of total auroral flux are measured in images and are compared with the solar wind parameters at Jupiter's orbit. The solar wind parameters used in this study have been numerically inferred using a MHD tomography based on the interplanetary scintillation (IPS) observations (Hayashi et al., 2003).This method reconstructs the global structure of corotating solar wind assuming that such structure exists steadily during one Carrington rotation. Because of this assumption, transient changes of the solar wind can not be reproduced. As Jupiter's H3+ aurora is believed to reflect "time-averaged" magnetospheric activities, the solar wind parameters with 1-day time resolution is still a useful index. We evaluated the solar-wind dynamic pressure P and the reconnection voltage ? (Nichols et al., 2006) for the period of auroral observations. These two quantities are then converted to possible changes of magnetic flux density in Jupiter's magnetosphere. Neither of these two can explain the auroral flux vatiations solely. However, it is found that combining these two quantities (with slight adjustments) could better explain the increases/decreases of auroral flux. Amplitudes of the auroral flux variations, as well as uncertainties due to "extrapolation" of solar wind parameters to Jupiter's orbit will be discussed.

Satoh, Takehiko; Connerney, J. E.; Morioka, A.; Tokumaru, M.; Hayashi, K.

2007-10-01

219

Original superconducting quantum interference device (SQUID) design and measurement technique for flux noise source localization in SQUID systems  

Microsoft Academic Search

We present an original method for studying the low frequency flux noise due to vortices in superconducting quantum interference device (SQUID) systems. We use two SQUIDs connected to the same washer in order to study the correlation of their outputs. A dedicated electronic system has been built so as to operate both SQUIDs at the same time. It was thus

M. Lam Chok Sing; S. Flament; X. Ridereau; C. Gunther; L. Méchin; D. Bloyet

2003-01-01

220

Noise-reduction and fast-startup induced in magnetrons by magnetic priming  

NASA Astrophysics Data System (ADS)

Azimuthally-varying-axial magnetic fields (Â"magnetic primingÂ") in magnetrons yield fast startup and low noise. Optimal magnetic priming of N-cavity magnetrons uses N/2 azimuthal magnetic periods, to pre-bunch electrons in N/2 spokes. Magnetic priming experiments on DC oven magnetrons show rapid mode growth and (-30 dB) noise reduction in sidebands and close-to-carrier noise. Azimuthally-varying-axial magnetic fields in 10-vane oven magnetrons add five perturbing magnets on the existing upper annular magnet. Simulations by 3D ICEPIC confirm magnetic priming experimental results: fast startup and lower noise. By adding five perturbing magnets on both annular permanent magnets of the magnetron, an axially-symmetric, azimuthally-varying, magnetic field is created, causing (-20 dB) noise reduction close to carrier. This research was supported by AFOSR and DUST (S) under the Innovative Microwave Vacuum Electronics MURI Program managed by AFOSR under Grant F49620-99-1-1297. 1. V. B. Neculaes et al., Appl. Phys. Lett. 83, 1938 (2003); IEEE T-PS (6/2004). 2. M. C. Jones et al., Appl. Phys. Lett. 84, 1016 (2003); J. W. Luginsland et al., ibid, 84, 5425 (2004).

Neculaes, V. B.; Gilgenbach, R. M.; Lau, Y. Y.; Jones, M. C.; White, W. M.; Luginsland, J. W.

2004-11-01

221

Decoupling suspension controller based on magnetic flux feedback.  

PubMed

The suspension module control system model has been established based on MIMO (multiple input and multiple output) state feedback linearization. We have completed decoupling between double suspension points, and the new decoupling method has been applied to CMS04 magnetic suspension vehicle in national mid-low-speed maglev experiment field of Tangshan city in China. Double suspension system model is very accurate for investigating stability property of maglev control system. When magnetic flux signal is taken back to the suspension control system, the suspension module's antijamming capacity for resisting suspension load variety has been proved. Also, the external force interference has been enhanced. As a result, the robustness and stability properties of double-electromagnet suspension control system have been enhanced. PMID:23844415

Zhang, Wenqing; Li, Jie; Zhang, Kun; Cui, Peng

2013-06-11

222

Langevin Molecular Dynamics of Driven Magnetic Flux Lines  

NASA Astrophysics Data System (ADS)

The characterization of type-II superconducting materials and their technological applications in external magnetic fields require a thorough understanding of the stationary and dynamical properties of vortex matter. The competition of repulsive interactions and attractive material defects renders the physics of externally driven magnetic flux lines very rich. We study the non-equilibrium steady states as well as transient relaxation properties of driven vortex lines in the presence of randomly distributed point pinning centers. We model the vortices as interacting elastic lines and employ a Langevin Molecular Dynamics (LMD) algorithm to extract steady-state and non-stationary time-dependent behavior. We compare the efficiency and accuracy of LMD to previously obtained Metropolis Monte Carlo steady-state force-velocity and gyration radius data. In future work we intend investigate the transient two-time height-height correlation and response functions.

Dobramysl, Ulrich; Pleimling, Michel; Täuber, Uwe C.

2011-10-01

223

The nature of magnetic fields and the flux quantum  

NASA Astrophysics Data System (ADS)

The understanding of electromagnetic phenomena is based on the historic landmark A Treatise on Electricity & Magnetism of 1873 by James Clerk Maxwell. The form in which the theory is most commonly expressed is in the form of the vector equations introduced by Gibbs. There is another more general formulation due to Élie Cartan, in terms of external differential forms. We will develop the description of electromagnetic phenomena using these concepts. It is a pre-metric description of conservation laws, that will highlight the basic properties of the electromagnetic phenomena. We will in particular demonstrate the fundamental role of the flux quantum ?0 = h/2e, and point to ways to calibrate magnetic fields.

Perenboom, J. A. A. J.; Peters, C. M. E. E.

2006-11-01

224

Decrease in heliospheric magnetic flux in this solar minimum: Recent Ulysses magnetic field observations  

Microsoft Academic Search

The Ulysses spacecraft has traveled from the solar equator at 1.3 and 5.3 AU to above the polar caps at 2.2 AU three times during the last 17 years and has provided measurements of the solar-heliospheric magnetic field. The open magnetic flux, i.e., the radial component, BR, multiplied by the square of the radial distance, r, is independent of latitude

Edward J. Smith; Andre Balogh

2008-01-01

225

Analysis and Solution on Squeak Noise of Small Permanent-Magnet DC Brush Motors in Variable Speed Applications  

Microsoft Academic Search

The phenomenon of squeaking noise, based on noise spectra data during the running up of a small permanent-magnet DC (PMDC) brush motor, is analyzed. From the analysis, it is found that apart from some mechanical issues, the major contributor of the squeaking noise of PMDC is the unbalanced magnetic radial force. In this study, the unbalanced magnetic radial force is

Y. B. Li; S. L. Ho; W. N. Fu; B. F. Xue

2009-01-01

226

The continuum intensity as a function of magnetic field. II. Local magnetic flux and convective flows  

NASA Astrophysics Data System (ADS)

Context. To deepen our understanding of the role of small-scale magnetic fields in active regions (ARs) and in the quiet Sun (QS) on the solar irradiance, it is fundamental to investigate the physical processes underlying their continuum brightness. Previous results showed that magnetic elements in the QS reach larger continuum intensities than in ARs at disk center, but left this difference unexplained. Aims: We use Hinode/SP disk center data to study the influence of the local amount of magnetic flux on the vigour of the convective flows and the continuum intensity contrasts. Methods: The apparent (i.e. averaged over a pixel) longitudinal field strength and line-of-sight (LOS) plasma velocity were retrieved by means of Milne-Eddington inversions (VFISV code). We analyzed a series of boxes taken over AR plages and the QS, to determine how the continuum intensity contrast of magnetic elements, the amplitude of the vertical flows and the box-averaged contrast were affected by the mean longitudinal field strength in the box (which scales with the total unsigned flux in the box). Results: Both the continuum brightness of the magnetic elements and the dispersion of the LOS velocities anti-correlate with the mean longitudinal field strength. This can be attributed to the "magnetic patches" (here defined as areas where the longitudinal field strength is above 100 G) carrying most of the flux in the boxes. There the velocity amplitude and the spatial scale of convection are reduced. Due to this hampered convective transport, these patches appear darker than their surroundings. Consequently, the average brightness of a box decreases as the the patches occupy a larger fraction of it and the amount of embedded flux thereby increases. Conclusions: Our results suggest that as the magnetic flux increases locally (e.g. from weak network to strong plage), the heating of the magnetic elements is reduced by the intermediate of a more suppressed convective energy transport within the larger and stronger magnetic patches. This, together with the known presence of larger magnetic features, could explain the previously found lower contrasts of the brightest magnetic elements in ARs compared to the QS. The inhibition of convection also affects the average continuum brightness of a photospheric region, so that at disk center, an area of photosphere in strong network or plage appears darker than a purely quiet one. This is qualitatively consistent with the predictions of 3D MHD simulations.

Kobel, P.; Solanki, S. K.; Borrero, J. M.

2012-06-01

227

INTERPLANETARY MAGNETIC FLUX DEPLETION DURING PROTRACTED SOLAR MINIMA  

SciTech Connect

We examine near-Earth solar wind observations as assembled within the Omni data set over the past 15 years that constitute the latest solar cycle. We show that the interplanetary magnetic field continues to be depleted at low latitudes throughout the protracted solar minimum reaching levels below previously predicted minima. We obtain a rate of flux removal resulting in magnetic field reduction by 0.5 nT yr{sup -1} at 1 AU when averaged over the years 2005-2009 that reduces to 0.3 nT yr{sup -1} for 2007-2009. We show that the flux removal operates on field lines that follow the nominal Parker spiral orientation predicted for open field lines and are largely unassociated with recent ejecta. We argue that the field line reduction can only be accomplished by ongoing reconnection of nominally open field lines or very old closed field lines and we contend that these two interpretations are observationally equivalent and indistinguishable.

Connick, David E.; Smith, Charles W.; Schwadron, Nathan A., E-mail: davideconnick@gmail.com, E-mail: Charles.Smith@unh.edu, E-mail: N.Schwadron@unh.edu [Physics Department, Space Science Center, University of New Hampshire, Durham, NH (United States)

2011-01-20

228

Implications of Non-cylindrical Flux Ropes for Magnetic Cloud Reconstruction Techniques and the Interpretation of Double Flux Rope Events  

NASA Astrophysics Data System (ADS)

Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) which exhibit signatures consistent with a magnetic flux rope structure. Techniques for reconstructing flux rope orientation from single-point in situ observations typically assume the flux rope is locally cylindrical, e.g., minimum variance analysis (MVA) and force-free flux rope (FFFR) fitting. In this study, we outline a non-cylindrical magnetic flux rope model, in which the flux rope radius and axial curvature can both vary along the length of the axis. This model is not necessarily intended to represent the global structure of MCs, but it can be used to quantify the error in MC reconstruction resulting from the cylindrical approximation. When the local flux rope axis is approximately perpendicular to the heliocentric radial direction, which is also the effective spacecraft trajectory through a magnetic cloud, the error in using cylindrical reconstruction methods is relatively small (? 10?). However, as the local axis orientation becomes increasingly aligned with the radial direction, the spacecraft trajectory may pass close to the axis at two separate locations. This results in a magnetic field time series which deviates significantly from encounters with a force-free flux rope, and consequently the error in the axis orientation derived from cylindrical reconstructions can be as much as 90?. Such two-axis encounters can result in an apparent `double flux rope' signature in the magnetic field time series, sometimes observed in spacecraft data. Analysing each axis encounter independently produces reasonably accurate axis orientations with MVA, but larger errors with FFFR fitting.

Owens, M. J.; Démoulin, P.; Savani, N. P.; Lavraud, B.; Ruffenach, A.

2012-06-01

229

A laboratory study of arched magnetic flux rope eruptions*  

NASA Astrophysics Data System (ADS)

Arched magnetic flux ropes (AMFRs) are arch-shaped twisted magnetic-structures that confine plasma and carry electrical current. Coronal loops and solar prominences are the main examples of AMFRs in the solar atmosphere. Solar AMFRs appear stable for long duration (several Alfven transit times) and then suddenly erupt due to occurrence of instabilities (e.g., kink instability). Solar AMFR eruptions have been frequently observed to evolve into more energetic events such as solar flares and coronal mass ejections. A laboratory plasma experiment has been constructed to simulate such eruptions in an ambient magnetized plasma. The laboratory AMFR (n ~ 1019 m-3 , Te ~ 10 eV, B ~ 1 kG, L ~ 0.5 m) is produced using an annular LaB6 cathode and an annular anode mounted on two movable shafts in a vacuum chamber (1.0 m diameter, 4.5 m long). Each AMFR electrode has an electromagnet to produce a vacuum magnetic field along the curved axis of the AMFR. The vacuum chamber has an additional plasma source and electromagnets to produce the ambient magnetized plasma (n ~ 1018 m-3, Te ~ 4 eV, B ~ 25 G). Two laser beams (1064 nm, ~0.5 J/pulse) strike movable carbon targets placed behind the orifices of the electrodes to generate controlled plasma flows from the AMFR footpoints that drives the eruption. The experiment operates with a 0.5 Hz repetition rate and is highly reproducible. Thus, time evolution of the AMFR is recorded in three-dimensions with high spatio-temporal resolutions using movable diagnostic probes. Initial results on the dramatic eruption of an arched magnetic flux rope will be presented that demonstrate outward expansion of the AMFR, release of the AMFR plasma to the background, and excitation of magnetosonic waves in the ambient plasma. Reference: S. K. P. Tripathi and W. Gekelman, Phys. Rev. Lett. 105, 075005 *Work supported by US DOE and NSF Fast camera image of a laboratory AMFR. Laser generated flows can be seen emanating from the both footpoints of the AMFR.

Tripathi, S.; Gekelman, W. N.

2010-12-01

230

Analysis of local projected current density from one component of magnetic flux density in MREIT  

NASA Astrophysics Data System (ADS)

Magnetic resonance electrical impedance tomography is a new modality capable of imaging the static electrical conductivity of an object by measuring Bz data, a component of the magnetic flux density B = (Bx, By, Bz), perturbed by an external injection current. In an imaging area, the current density J induced by the external injection current can be uniquely decomposed into a recoverable component JP and an invisible component from the measured Bz data. In the case of in vivo animal and human imaging experiments, the imaging area frequently includes local defective regions with a low signal-to-noise ratio. As a result, the measured Bz data in the defective regions include serious noise due to rapid T2 decay, a small amount of internal current density and weak MR signals. In this paper, we propose an algorithm to reconstruct a recoverable current density \\mathbf {J}^R_{ {D}} from the measured Bz data in a local region {D} avoiding the defective regions. We estimate the L2-norm of the difference between the induced internal current density J and the locally recovered \\mathbf {J}^R_{ {D}} from the measured Bz data in the local region {D}. The difference only depends on the z-components of J and J0 and the values of Bx and By on the boundary \\partial {D}, where J0 is the background current density by the injected current. Numerical simulations and phantom experiments demonstrate that the proposed method directly reconstructs a local current density avoiding noise effects in defective regions.

Kim, Hyung Joong; Sajib, Saurav Z. K.; Jeong, Woo Chul; Nyoun Kim, Myoung; In Kwon, Oh; Woo, Eung Je

2013-07-01

231

Flux-Weakening Regime Operation of an Interior Permanent-Magnet Synchronous Motor Drive  

Microsoft Academic Search

The interior permanent magnet (IPM) synchronous motor is compatible with extended-speed-range constant-power operation by means of flux-weakening control. Flux weakening uses stator current components to counter the fixed-amplitude magnetic airgap flux generated by the rotor magnets, performing a role similar to field weakening in a separately excited dc motor. The nature of current regulator saturation caused by the finite inverter

Thomas M. Jahns

1987-01-01

232

Magnetic-flux penetration and critical currents in superconducting strips with slits.  

PubMed

We theoretically investigate transport-current-induced magnetic-flux penetration into superconducting strip lines with slits. Even when the individual strips have no bulk pinning, geometrical barriers prevent penetration of magnetic flux into the innermost strips while flux quasistatically penetrates into the outermost slits. The critical current of strip lines with 2N slits at zero applied magnetic field is found to be enhanced by a factor of (N+1)(1/2) above that of a single strip line without slits. Under suitable conditions, a domelike flux distribution due to the geometrical barrier can appear in the individual strips even in the absence of an applied magnetic field. PMID:11290060

Mawatari, Y; Clem, J R

2001-03-26

233

Emergence of undulatory magnetic flux tubes by small scale reconnections  

NASA Astrophysics Data System (ADS)

With Flare Genesis Experiment (FGE), a balloon borne observatory launched in Antarctica on January 2000, series of high spatial resolution vector magnetograms, Dopplergrams, and H? filtergrams have been obtained in an emerging active region (AR 8844). Previous analyses of this data revealed the occurence of many short-lived and small-scale H ? brightenings called 'Ellerman bombs' (EBs) within the AR. We performed an extrapolation of the field above the photosphere using the linear force-free field approximation. The analysis of the magnetic topology reveals a close connexion between the loci of EBs and the existence of "Bald patches" (BP) regions (BPs are regions where the vector magnetic field is tangential to the photosphere). Some of these EBs/BPs are magnetically connected by low-lying field lines, presenting a serpentine shape. This results leads us to conjecture that arch filament systems and active regions coronal loops do not result from the smooth emergence of large scale ?-loops, but rather from the rise of flat undulatory flux tubes which get released from their photospheric anchorage by reconnection at BPs, which observational signature is Ellerman bombs.

Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N.

2006-01-01

234

Effect of Interfacial Disorder on 1/f Noise in Magnetic Tunnel Junctions  

NASA Astrophysics Data System (ADS)

Magnetic tunnel junctions (MTJs) have the potential for low field ( 1pT/Hz^0.5 @ 1 Hz) magnetic sensors. However, 1/f noise limits their performance. Here we correlate measured 1/f noise with dynamic Lorentz imaging and high-frequency ferromagnetic resonance (FMR) measurements. The measurements show that a large fraction of the 1/f noise is due to thermal fluctuations of nano-scale magnetic ripple structure which arises from a combination of disorder in the antiferromagnetic exchange bias layer and interfacial roughness in the tunnel barrier. We have changed the interfacial properties by varying growth conditions and by inserting nano-oxides. The samples show varying amounts of disorder that manifests itself as increased ripple structure, increased 1/f noise, and a broadened FMR linewidth. Time dependent Lorentz imaging has been used to directly observe nano-scale thermal fluctuations that give rise to 1/f noise.

Russek, Stephen; Shaw, Justin; Francisco Sierra, Juan

2007-03-01

235

Hard Bias Effect on Magnetic Noise in Different Types of Tunnel Magnetoresistive Heads  

Microsoft Academic Search

Hard bias effect on thermally activated magnetic noise has been investigated through changing the biasing direction in three types of tunnel magnetoresistive (TMR) heads with different head structures and sensor widths. It is found that characteristic variation in the noise spectra is different for different TMR heads. The results can be explained through stiffness field difference in different TMR heads.

G. C. Han; B. Y. Zong; P. Luo; L. Wang; Z. B. Guo; S. N. Mao

2008-01-01

236

Riemannian geometry of twisted magnetic flux tubes in almost helical plasma flows  

SciTech Connect

Riemannian geometry of curves applied recently by Ricca [Fluid Dyn. Res 36, 319 (2005)] in the case of inflectional disequilibrium of twisted magnetic flux tubes is used here to compute the magnetic helicity force-free field case. Here the application of Lorentz force-free to the magnetic flux tube in tokamaks allows one to obtain an equation that generalizes the cylindrical tokamak equation by a term that contains the curvature of the magnetic flux tube. Another example of the use of the magnetic flux tube is done by taking the electron magnetohydrodynamics (MHD) fluid model (EMHD) of plasma physics that allows one to compute the velocity of the fluid in helical and almost helical flows in terms of the Frenet torsion of thin magnetic flux tubes. The cases of straight and curved twisted tubes are examined. Second-order effects on the Frenet torsion arise on the poloidal component of the magnetic field, while curvature effects appear in the toroidal component. The magnetic fields are computed in terms of the penetration depth used in superconductors. The ratio between poloidal and toroidal components of the magnetic field depends on the torsion and curvature of the magnetic flux tube. It is shown that the rotation of the almost helical plasma flow contributes to the twist of the magnetic flux tube through the total Frenet torsion along the tube.

Garcia de Andrade, L.C. [Departamento de Fisica Teorica, Instituto de Fisica-UERJ Rua Sao Fco. Xavier 524, Rio de Janeiro, RJ Maracana, CEP:20550-003 (Brazil)

2006-02-15

237

Properties of diverted plasmas with magnetically expanded flux surfaces  

SciTech Connect

Vertical elongated 1.3:1 elliptical plasmas in which the outermost flux surfaces are magnetically expanded and diverted by external coils into the lower half of the Doublet III vacuum vessel are described. Approximately 5 cm of the nominal 45 cm plasma minor radius is diverted to the lower chamber. The diverted flux is expanded by more than a factor of ten before reaching the vessel wall. Photographic measurements show diffused hydrogen recycling light in the lower half of the vessel, with greatly reduced recycling at the normal limiters, and no evidence of localized particle flow to the vessel wall. A significant amount (approx. 50%) of the ohmic power is radiated in the expanded boundary region. Comparison of similar low density plasmas (approx. 2 x 10/sup 13/ cm/sup -3/) with and without the expanded boundary shows that (1) the expanded boundary reduces the influx of nickel and oxygen impurities by an order of magnitude, (2) the boundary also reduces the influx of injected argon and helium by a similar factor, and (3) the concentration of argon in a non-diverted plasma is reduced by a factor of 10 when the expanded boundary is turned on. The central radiated power falls by an order of magnitude, to less than 0.01 W/cm/sup 3/. Similarly, Z/sub eff/(0) drops from 3.2 to 2.1.

Ali Mahdavi, M.; Ohyabu, N.; Baker, D.R.

1980-07-01

238

Dynamics of Magnetic Flux Ropes in a Laboratory Plasma  

NASA Astrophysics Data System (ADS)

The behavior and interaction of magnetic flux ropes has long been a topic of interest to solar and space plasma physicists. (Gekelman, et al. IEEE Trans. Plasma Sci. 20, 614. Furno, et al. Phys. Plasmas 12, 055702.) Very few laboratory experiments have been performed as it is necessary to have a relatively collisionless plasma and currents with significant self-generated fields. Movable lanthanum hexaboride (LaB6) cathodes have been developed to study the 3D dynamics of flux ropes in the Large Plasma Device (LaPD). The background plasma (n ~ 2 × 1012 cm -3, d ~ 60 cm, L ~ 18 m, and ?rep= 1 s) is produced with a DC discharge using a pulsed barium oxide-coated cathode. The two or more current channels are created by biasing the LaB6 disks with respect to a grid anode at the opposite end of the chamber. They are emitted parallel to each other and the guide field. J × B forces cause the currents to move across the field and interact. Each 2.5 cm LaB6 disk can produce current densities of 5-10 A/cm2, and can be positioned freely within a transverse plane of the cylindrical LaPD. We plan to make detailed volumetric measurements of the magnetic fields and flows generated by the current channels. Diagnostics include ?, Langmuir, and Mach probes, and laser induced fluorescence. Research is supported by the Department of Energy (grant DE-FG02-03ER54717) and conducted at the Basic Plasma Science Facility at UCLA.

Lawrence, E. E.; Gekelman, W.

2007-05-01

239

Topological Structure of Magnetic Flux Lines Generated by Thermal Convection in a Rotating Spherical Shell  

Microsoft Academic Search

Topological structure and reconnection of magnetic flux lines are investigated by analyzing the numerical solutionof the Boussinesq magneto-hydrodynamic equationsin a rotating spherical shell.Five pairs of Taylor-Proudman vortex columns are generatedand drift westward steadily.Magnetic field is intensified around the tops of these vortex columns.Magnetic flux linesconnect east-west adjacent domains of intense magnetic field,which migrate eastward relative to the vortex columns.We describe

Hideaki Kitauchi

1998-01-01

240

Signal extraction using ensemble empirical mode decomposition and sparsity in pipeline magnetic flux leakage nondestructive evaluation  

NASA Astrophysics Data System (ADS)

The commonly used and cost effective corrosion inspection tools for the evaluation of pipelines utilize the magnetic flux leakage (MFL) technique. The MFL signal is usually contaminated by various noise sources. In this paper, we propose that the pipeline flaw MFL signal is extracted using the ensemble empirical mode decomposition (EEMD) and the sparsity. At first, we introduce the EEMD method. The EEMD defines the true intrinsic mode function (IMF) components as the mean of an ensemble of trials, each consisting of the signal plus a white noise of finite amplitude. Moreover, sparsity selection restriction was defined. Then, The MFL signal is decomposed into several IMFs used for signal reconstruction. Some modes are selected to reconstruct a new signal considering their sparsity. Finally, the comparison is made with the empirical mode decomposition. At the same time, the comparison of the selection restriction between the sparsity and the energy is described. The results show that the EEMD and the sparsity is an efficient technology with the pipeline flaw extraction.

Chen, Liang; Li, Xing; Li, Xun-Bo; Huang, Zuo-Ying

2009-02-01

241

Signal extraction using ensemble empirical mode decomposition and sparsity in pipeline magnetic flux leakage nondestructive evaluation.  

PubMed

The commonly used and cost effective corrosion inspection tools for the evaluation of pipelines utilize the magnetic flux leakage (MFL) technique. The MFL signal is usually contaminated by various noise sources. In this paper, we propose that the pipeline flaw MFL signal is extracted using the ensemble empirical mode decomposition (EEMD) and the sparsity. At first, we introduce the EEMD method. The EEMD defines the true intrinsic mode function (IMF) components as the mean of an ensemble of trials, each consisting of the signal plus a white noise of finite amplitude. Moreover, sparsity selection restriction was defined. Then, The MFL signal is decomposed into several IMFs used for signal reconstruction. Some modes are selected to reconstruct a new signal considering their sparsity. Finally, the comparison is made with the empirical mode decomposition. At the same time, the comparison of the selection restriction between the sparsity and the energy is described. The results show that the EEMD and the sparsity is an efficient technology with the pipeline flaw extraction. PMID:19256676

Chen, Liang; Li, Xing; Li, Xun-bo; Huang, Zuo-ying

2009-02-01

242

Grad-Shafranov Reconstruction of Magnetic Flux Ropes in the Near-Earth Space  

NASA Astrophysics Data System (ADS)

Electric currents permeate space plasmas and often have a significant component along the magnetic field to form magnetic flux ropes. A larger spatial perspective of these structures than from the direct observation along the satellite path is crucial in visualizing their role in plasma dynamics. For magnetic flux ropes that are approximately two-dimensional equilibrium structures on a certain plane, Grad-Shafranov reconstruction technique, developed by Bengt Sonnerup and his colleagues (see Sonnerup et al. in J. Geophys. Res. 111:A09204, 2006), can be used to reveal two-dimensional maps of associated plasma and field parameters. This review gives a brief account of the technique and its application to magnetic flux ropes near the Earth's magnetopause, in the solar wind, and in the magnetotail. From this brief survey, the ranges of the total field-aligned current and the total magnetic flux content for these magnetic flux ropes are assessed. The total field-aligned current is found to range from ˜0.14 to ˜9.7×104 MA, a range of nearly six orders of magnitude. The total magnetic flux content is found to range from ˜0.25 to ˜2.3×106 MWb, a range of nearly seven orders of magnitude. To the best of our knowledge, this review reports the largest range of both the total field-aligned current and the total magnetic flux content for magnetic flux ropes in space plasmas.

Lui, A. T. Y.

2011-01-01

243

Three-Dimensional Magnetic Field Line Reconnection involving Magnetic Flux Ropes (Invited)  

NASA Astrophysics Data System (ADS)

We report on two experiments in which three dimensional magnetic field line reconnection plays a role. Magnetic field line reconnection is a processes in which the magnetic field energy is converted to particle energy and heating accompanied by changes in the magnetic topology. In the first experiment two magnetic flux ropes are generated from initially adjacent pulsed current channels in a background magnetoplasma in the LAPD device at UCLA. The currents exert mutual jXB forces causing them to twist about each other and merge. The currents are not static but move towards or away from each other in time. In addition the currents are observed to filament after merging. Volumetric space-time data show multiple reconnection sites with time-dependent locations. The quasi-separatrix layer (QSL) is a narrow region between the flux ropes. Two field lines on either side of the QSL will have closely spaced foot-points at on end of the flux ropes, but a very different separation at the other end. Outside the QSL, neighboring field lines do not diverge. The QSL has been measured, for the first time in this experiment [1] and its three dimensional development will be shown in movies made from the data. A system involving the reconnection of three flux ropes will also be presented. Three flux ropes are generated by drawing currents through apertures in a carbon shield located in front of a 10 cm diameter cathode immersed in the background magnetoplasma. The currents are observed to twist about themselves, writhe about each other and thrash about due to kink the kink instability. Multiple reconnection regions (which are three dimensional) and a complex QSL are observed. The magnetic helicity is evaluated from volumetric data in both cases and its rate of change is used to estimate the plasma resistivity. These measurements lead one to suspect that magnetic field line reconnection is not an independent topic, which can be studied in isolation, but part of the phenomena associated with broader subject of 3D waves and current systems in plasmas. [1] E. Lawrence, W. Gekelman, Phys. Rev. Lett., 103, 105002 (2009). This work was done at the Basic Plasma Science Facility at UCLA and funded by NSF and DOE.

Gekelman, W. N.; van Compernolle, B.; Lawrence, E.; Vincena, S. T.

2010-12-01

244

Testing of the Mark 101 magnetic flux compression generator  

SciTech Connect

The Mark 101 explosive flux compression generator is a line-initiated, vacuum/magnetically insulated, helical generator. This device offered some unique challenges in transforming the theoretical design into a testable experiment. The two main reasons for this are that in theory an eight-turn, four-wire Mark 101 possesses a terminal dL/dt of approx.0.5 ..cap omega.. and operates with electric fields which are greater than the threshold for electron field emission. With this in mind, we designed an integral vacuum-jacket-generator configuration with a passive load inductance of less than or equal to0.5 ..mu..H. The generator contained approx.8 ..mu..H of initial inductance. The field emission required the stator to be entirely sealed within the vacuum jacket. The open, helical stator resulted in the presence of non-trivial leakage fields and voltages. To accommodate these fields, the vacuum chamber for the generator was segmented and axially insulated with rings of acrylic, similar to stacked-ring diodes. We made no attempt to break the azimuthal metal surfaces due to the physical difficulty this would incur. Diagnostics included an input current Rogowski loop, a load Rogowski loop, two dB/dt probes in the load, a Faraday fiber-optic current sensor, and two dB/dt probes in the region between the stator winding and the vacuum jacket to measure the leakage azimuthal and axial magnetic fields. The results of explosive tests are presented.

Freeman, B.L.; Fowler, C.M.; King, J.C.; Martinez, A.R.

1986-01-01

245

Emergence of undulatory magnetic flux tubes by small scale reconnections  

NASA Astrophysics Data System (ADS)

With Flare Genesis Experiment (FGE), a balloon borne observatory launched in Antarctica on January 2000, series of high spatial resolution vector magnetograms, Dopplergrams, and H? filtergrams have been obtained in an emerging active region (AR 8844). Previous analyses of this data revealed the occurence of many short-lived and small-scale H? brightenings called 'Ellerman bombs' (EBs) within the AR. We performed an extrapolation of the field above the photosphere using the linear force-free field approximation. The analysis of the magnetic topology reveals a close connexion between the loci of EBs and the existence of ``Bald patches'' regions (BPs are regions where the vector magnetic field is tangential to the photosphere). Among 47 identified EBs, we found that 23 are co-spatial with a BP, while 19 are located at the footpoint of very flat separatrix field lines passing throught a distant BP. We reveal for the first time that some of these EBs/BPs are magneticaly connected by low-lying lines, presenting a 'sea-serpent' shape. This results leads us to conjecture that arch filament systems and active regions coronal loops do not result from the smooth emergence of large scale ? loops, but rather from the rise of flat undulatory flux tubes which get released from their photospheric anchorage by reconnection at BPs, whose observational signature is Ellerman bombs.

Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N.

246

Evaluating the Limits of Shock Wave Magnetic Flux Compression in Solids  

Microsoft Academic Search

In this paper we present the results of an investigation of the interaction of ionizing shock waves in solids with strong magnetic fields in the Shock Wave Magnetic Flux Compression (SWMFC) generator. Deceleration of the ionizing shock waves by magnetic pressure is calculated for cesium iodide (CsI). It will be shown that it is possible to evaluate the limiting magnetic

S. V. Kolosenok; V. S. Soukhomlinov; Yu. A. Tolmachev; L. L. Altgilbers; D. J. Hemmert

247

Speed control of interior permanent magnet synchronous motor drive for the flux weakening operation  

Microsoft Academic Search

A novel flux-weakening scheme for the interior permanent magnet synchronous motor (IPMSM) is proposed. This is implemented based on the output of the synchronous PI current regulator-reference voltage to the PWM inverter. The onset of flux weakening and the flux level are adjusted inherently by the outer voltage regulation loop to prevent saturation of the current regulator. Attractive features of

Jang-Mok Kim; Seung-Ki Sul

1997-01-01

248

Comparison of the dynamic response of radial and tangential magnetic flux thrust bearings  

Microsoft Academic Search

Theoretical predictions were made for the dynamic performance of a tangential flux magnetic thrust bearing. A prototype bearing was built with the stators and rotors made from tape wound strip. The performance of this bearing was measured and compared to the theoretical predictions and also to the performance of a radial flux thrust bearing. Tangential flux bearings are intrinsically amenable

Andrew Kenny; Alan B. Palazzolo

2002-01-01

249

Magnetic Filed Analysis of Magnetic Flux Concentration Type Hybrid Surface Magnet Motor  

NASA Astrophysics Data System (ADS)

This paper presents results of magnet characteristic analysis of a concentration type hybrid surface magnetic motor by using the finite element method. This motor has a structure combined neodymium magnets and ferrite magnets. We investigated optimal thickness of the magnets and their effectiveness.

Eguchi, Keisuke; Zeze, Shingo; Todaka, Takashi; Enokizono, Masato

250

Magnetic flux-line lattices and vortices in the copper oxide superconductors  

Microsoft Academic Search

A variety of recent experiments on both the static and the dynamic properties of vortices and flux-line lattices in the mixed state of the copper oxide superconductors are discussed. The experiments are of two basic types: (1) experiments that image the magnetic flux patterns either with magnetic decoration or neutrons and give information about static structures, and (2) experiments that

D. J. Bishop; P. L. Gammel; D. A. Huse; C. A. Murray

1992-01-01

251

A high performance position sensorless Surface Permanent Magnet Synchronous Motor drive based on flux angle  

Microsoft Academic Search

A position sensorless Surface Permanent Magnet Synchronous Motor (SPMSM) drive based on flux angle is presented in this paper. The motor equations are written in rotor fixed d-q reference frame. A PID controller is used to process the speed error to generate the reference torque current keeping the magnetizing current fixed. The estimated stator flux using Recurrent Neural Network (RNN)

Kalyan Kumar Halder; N. K. Roy; B. C. Ghosh

2010-01-01

252

Evidence for Sterilization of Saccharomyces Cerevisiae K 7 by an External Magnetic Flux  

Microsoft Academic Search

A new process of sterilization on Saccharomyces cerevisiae proposed and experimentally demonstrated. This process consists of external magnetic flux and ferrite. Consequently, an alteration of yeast cells, caused by treatment with 2000 G magnetic flux and 6 g of ferrite, was detected through observation of the release of materials from yeast cells absorbing at 260 nm and microscopy of disrupted

Tatsuya Ito; Yuzo Murayama; Masafumi Suzuki; Noboru Yoshimura; Kimio Iwano; Kozo Kudo

1992-01-01

253

A Transverse Flux Permanent Magnet machine for micro-wind generation application  

Microsoft Academic Search

A new Transverse Flux Permanent Magnet (TFPM) generator topology is proposed for direct-drive wind turbine application. The peculiarity of TFPM machines is their high torque density, and attention is mainly focused on their configuration at concentrated flux. The main objective is to find an innovative structure for simplification of the constructive process with consequently reduced production costs. Many magnetic circuit

M. Bellucci; V. Isastia Cimino; R. Rizzo

2011-01-01

254

Magnetic Flux Leakage Detection Technology for Well Casing on Neural Network  

Microsoft Academic Search

Well casing integrity is vital for the safe operations of oil wells, and also significant to detect well casing defects. Magnetic flux leakage (MFL) detection technology is widely-used in detecting the defects of various pipelines. Owing to the very complicated environment where well casing is laid in, the system which based on magnetic flux leakage technology is not mature yet

Jinzhong Chen; Lin Li; Jinan Shi

2008-01-01

255

Low-frequency noise measurements on commercial magnetoresistive magnetic field sensors  

NASA Astrophysics Data System (ADS)

Low-frequency noise was measured in the frequency range from 0.1 Hz to 10 kHz on a variety of commercially available magnetic sensors. The types of sensors investigated include anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR) effect devices. The 1/f noise components of electronic and magnetic origin are identified by measuring sensor noise and sensitivity at various applied magnetic fields. Commercial magnetometers typically consist of four elements in a Wheatstone bridge configuration and are biased with either a constant voltage or current. Voltage fluctuations at the sensor output are amplified by a pair of battery powered low-noise preamplifiers and input to a spectrum analyzer. A two-channel cross-correlation technique is used when the performance of a single preamplifier is not sufficient. For the AMR and GMR sensors investigated, both electronic and magnetic components contribute to the overall sensor noise. Maximum noise occurs at the bias field which gives maximum sensitivity. The noise of TMR based sensors is primarily due to resistance fluctuations in the tunnel barrier, having little to no field dependence. The best low-field detectivity of the sensors that have been measured is on the order of 100 pT/Hz0.5 at 1 Hz.

Stutzke, Nathan A.; Russek, Stephen E.; Pappas, David P.; Tondra, Mark

2005-05-01

256

HOW MUCH DOES A MAGNETIC FLUX TUBE EMERGE INTO THE SOLAR ATMOSPHERE?  

SciTech Connect

The emergence process of the magnetic field into the solar atmosphere plays an essential role in determining the configuration of the magnetic field and its activity on the Sun. This paper focuses on how much the magnetic flux contained by a flux tube emerges into the solar atmosphere, which is the key to understanding the physical mechanism of solar eruptions. By comparing a kinematic model of an emerging flux tube to a series of magnetohydrodynamic simulations, we derive the characteristics of the emergence process, showing how the process depends on the pre-emerged state of the magnetic field such as the radius of a flux tube, field strength, field-line twist, and wavelength of undulation assumed by the flux tube. We also discuss the relationship between magnetic configurations and their stability on the Sun.

Magara, T., E-mail: magara@khu.ac.kr [Department of Astronomy and Space Science, School of Space Research, Kyung Hee University, 1 Seocheon-dong, Giheung-gu, Yongin, Gyeonggi-do 446-701 (Korea, Republic of)

2012-03-20

257

Analysis of interior permanent magnet synchronous motor designed for flux weakening operation  

Microsoft Academic Search

The paper presents a finite element method (FEM) based analysis of interior permanent magnet synchronous motor, designed for flux-weakening operation. Flux-weakening performance determined from the FEM analysis on the proposed permanent magnet motor structure is presented in order to verify the suitability of the design concept for flux-weakening operation. The influence of iron losses on the output power capability is

B. Stumberger; A. Hamler; M. Trlep; M. Jesenik

2001-01-01

258

Least-Squares Fitting Methods for Estimating the Winding Rate in Twisted Magnetic-Flux Tubes  

NASA Astrophysics Data System (ADS)

We investigate least-squares fitting methods for estimating the winding rate of field lines about the axis of twisted magnetic-flux tubes. These methods estimate the winding rate by finding the values for a set of parameters that correspond to the minimum of the discrepancy between vector magnetic-field measurements and predictions from a twisted flux-tube model. For the flux-tube model used in the fitting, we assume that the magnetic field is static, axisymmetric, and does not vary in the vertical direction. Using error-free, synthetic vector magnetic-field data constructed with models for twisted magnetic-flux tubes, we test the efficacy of fitting methods at recovering the true winding rate. Furthermore, we demonstrate how assumptions built into the flux-tube models used for the fitting influence the accuracy of the winding-rate estimates. We identify the radial variation of the winding rate within the flux tube as one assumption that can have a significant impact on the winding-rate estimates. We show that the errors caused by making a fixed, incorrect assumption about the radial variation of the winding rate can be largely avoided by fitting directly for the radial variation of the winding rate. Other assumptions that we investigate include the lack of variation of the field in the azimuthal and vertical directions in the magnetic-flux tube model used for the fitting, and the inclination, curvature, and location of the flux-tube axis. When the observed magnetic field deviates substantially from the flux-tube model used for the fitting, we find that the winding-rate estimates can be unreliable. We conclude that the magnetic-flux tube models used in this investigation are probably too simple to yield reliable estimates for the winding rate of the field lines in solar magnetic structures in general, unless additional information is available to justify the choice of flux-tube model used for the fitting.

Crouch, A. D.

2012-12-01

259

Magnetic flux pinning in superconductors with hyperbolic-tessellation arrays of pinning sites  

NASA Astrophysics Data System (ADS)

We study magnetic flux interacting with arrays of pinning sites (APSs) placed on vertices of hyperbolic tessellations (HTs). We show that, due to the gradient in the density of pinning sites, HT APSs are capable of trapping vortices for a broad range of applied magnetic fluxes. Thus, the penetration of magnetic field in HT APSs is essentially different from the usual scenario predicted by the Bean model. We demonstrate that, due to the enhanced asymmetry of the surface barrier for vortex entry and exit, this HT APS could be used as a “capacitor” to store magnetic flux.

Misko, V. R.; Nori, Franco

2012-05-01

260

Measurements and Phenomenological Modeling of Magnetic FluxBuildup in Spheromak Plasmas  

SciTech Connect

Internal magnetic field measurements and high-speed imaging at the Sustained Spheromak Physics Experiment (SSPX) [E. B. Hooper, L. D. Pearlstein, R. H. Bulmer, Nucl. Fusion 39, 863 (1999)] are used to study spheromak formation and field buildup. The measurements are analyzed in the context of a phenomenological model of magnetic helicity based on the topological constraint of minimum helicity in the open flux before reconnecting and linking closed flux. Two stages are analyzed: (1) the initial spheromak formation, i. e. when all flux surfaces are initially open and reconnect to form open and closed flux surfaces, and (2) the stepwise increase of closed flux when operating the gun on a new mode that can apply a train of high-current pulses to the plasma. In the first stage, large kinks in the open flux surfaces are observed in the high-speed images taken shortly after plasma breakdown, and coincide with large magnetic asymmetries recorded in a fixed insertable magnetic probe that spans the flux conserver radius. Closed flux (in the toroidal average sense) appears shortly after this. This stage is also investigated using resistive magnetohydrodynamic simulations. In the second stage, a time lag in response between open and closed flux surfaces after each current pulse is interpreted as the time for the open flux to build helicity, before transferring it through reconnection to the closed flux. Large asymmetries are seen during these events, which then relax to a slowly decaying spheromak before the next pulse.

Romero-Talamas, C A; Hooper, E B; Jayakumar, R; McLean, H S; Wood, R D; Moller, J M

2007-12-14

261

The Aharonov-Anandan current induced by a time-dependent magnetic flux in graphene rings  

NASA Astrophysics Data System (ADS)

We investigate the current response of a graphene ring in the presence of a time-dependent magnetic flux. Using the linear response theory, we find a nonzero Aharonov-Anandan (AA) phase of carriers in the ring. When the flux increases, the current responses to the flux nonlinearly, which can be applied to translate the external flux signal to the electronic signal. Especially for the huge flux with low frequency, this system can be used as an electronic current square-wave generator. Moreover, a resonant transition can occur in the case of the flux with the proper frequency. And then a current beat frequency phenomenon is observed.

Zhang, Shengli; Chen, Huawei; Zhang, Erhu; Liu, Daqing

2013-09-01

262

Effect of magnetic-vortex interaction on the kinetics of magnetic-flux creep in type II superconductors  

SciTech Connect

Magnetic vortices localized by the lattice`s pinning potential in type II superconductors are used to study the repulsive interaction of quasiparticles on the kinetics of magnetic vortex creep. A microscopic model is developed in which the interaction of the quasiparticles (vortices) is allowed for in addition to their thermal activation through the pinning-potential barriers. When the localization of magnetic vortices is strong and their number density low, allowing for the repulsive interaction between vortices leads to a considerable increase in magnetic vortex diffusion, a situation ignored in previous models of magnetic flux creep. A kinetic equation describing the dynamics of magnetic flux creep under these conditions is derived. Finally, numerical and analytic solutions of the equation that describes the kinetics of magnetic flux creep are given. These solutions illustrate the variation of creep kinetics due to vortex interaction. 24 refs., 4 figs.

Genkin, G.M.; Okomel`kov, A.V. [Institute of Microstructure Physics, Novgorod (Russian Federation)

1995-03-01

263

Flux-invading behaviors and temperature changes in Dy123 bulk magnet in iterative pulse field magnetization process  

NASA Astrophysics Data System (ADS)

The flux invasion behaviors into the Dy123 based on HTS bulk magnet were precisely measured during the iterative pulsed field magnetizations operated at 31 K. The values of magnetic flux densities were measured at various positions on the bulk magnet sample during and after six successive applications of a peak applied field of 5T. The temperature changes were measured on the sample surface to compare them to the magnetic flux motions in the same manner. The invading flux were apparently repelled by the remaining trapped flux due to the former field application, and the domain where the magnetic flux reached was pushed back outward the sample with increasing the pulse number. The heat generation decreased with the iteration of field applications as well as the magnetic flux invasion expressing the same behavior, and saturated by the sixth field application. The time evolution of temperature measured at various positions on the sample surface showed that the heat generation occurs at the periphery of the sample and propagates toward the center of the sample.

Oka, T.; Ishiduka, D.; Ogawa, J.; Fukui, S.; Sato, T.; Yokoyama, K.; Murakami, A.; Stopfel, H.

2013-11-01

264

A sensor measuring the Fourier coefficients of the magnetic flux density for pipe crack detection using the magnetic flux leakage method  

NASA Astrophysics Data System (ADS)

A simple sensor for magnetic flux leakage methods of detecting cracks on the surfaces of ferromagnetic pipes is proposed. We show that the crack position can be determined by the Fourier coefficients of the leakage magnetic flux density on a circle inside the pipe in the pipe cross-sectional plane. Coils that directly output the Fourier cosine and sine coefficients were made. The experimental results showed that, using only these two coils rather than dozens of magnetic sensors, the center position of a crack on the inside/outside surface of the pipe could be localized.

Nara, T.; Takanashi, Y.; Mizuide, M.

2011-04-01

265

Starting Torque of Single-Phase Flux-Switching Permanent Magnet Motors  

Microsoft Academic Search

Flux-switching permanent magnet motors are a form of brushless dc motor, with a simple salient-pole rotor, similar to that of a switched reluctance motor, and the excitation coils and permanent magnets on the stator. The influence of the rotor pole-arc and asymmetry on the torque-rotor position characteristic of a single-phase flux-switching permanent magnet motor is investigated by finite-element analysis, and

Y. Chen; S. Chen; Z. Q. Zhu; D. Howe; Y. Y. Ye

2006-01-01

266

An Experimental Determination of Static Magnetic Fields Induced Noise in Living Systems  

NASA Astrophysics Data System (ADS)

Living systems are constantly exposed to static magnetic fields (SMFs) from both natural and man-made sources. Exposures vary in dose and duration ranging from geomagnetic (˜50?T) to residential and industrial (˜10s of mT) fields. Efforts to characterize responses to SMFs have yielded conflicting results, showing a dependence on experimental variables used. Here we argue that low to moderate SMF exposure is a sub-threshold perturbation operating below thermal noise, and assays that evaluate statistical characteristics of a single cell may identify responses not consistently found by population averaging approaches. Recent studies of gene expression show that it is a stochastic process capable of producing bursting dynamics. Moreover, theoretical and experimental methods have also been developed to allow quantitative estimates of the associated biophysical parameters. These developments provide a new way to assess responses of living systems to SMFs. In this work, we report on our efforts to use single molecule fluorescence in situ hybridization to assess responses of NIH-3T3 cells to SMF exposure at flux densities ranging from 1 to 440 mT for 48 hours. Results will contribute to determining mechanisms by which SMF exposure influences gene expression.

Brady, Megan; Laramee, Craig

2013-03-01

267

EFFECT OF FINITE LARMOR RADIUS ON COSMIC-RAY PENETRATION INTO AN INTERPLANETARY MAGNETIC FLUX ROPE  

SciTech Connect

We discuss a mechanism for cosmic-ray penetration into an interplanetary magnetic flux rope, particularly the effect of the finite Larmor radius and magnetic field irregularities. First, we derive analytical solutions for cosmic-ray behavior inside a magnetic flux rope, on the basis of the Newton-Lorentz equation of a particle, to investigate how cosmic rays penetrate magnetic flux ropes under an assumption of there being no scattering by small-scale magnetic field irregularities. The results show that the behavior of a particle is determined by only one parameter f{sub 0}, that is, the ratio of the Larmor radius at the flux rope axis to the flux rope radius. The analytical solutions show that cosmic rays cannot penetrate into the inner region of a flux rope by only gyration and gradient-curvature drift in the case of small f{sub 0}. Next, we perform a numerical simulation of a cosmic-ray penetration into an interplanetary magnetic flux rope by adding small-scale magnetic field irregularities. The results show that cosmic rays can penetrate into a magnetic flux rope even in the case of small f{sub 0} because of the effect of small-scale magnetic field irregularities. This simulation also shows that a cosmic-ray density distribution is greatly different from that deduced from a guiding center approximation because of the effect of the finite Larmor radius and magnetic field irregularities for the case of a moderate to large Larmor radius compared to the flux rope radius.

Kubo, Yuki [Space Environment Group, National Institute of Information and Communications Technology, Tokyo 184-8795 (Japan); Shimazu, Hironori, E-mail: kubo@nict.go.j [Department of Information System Fundamentals, University of Electro-Communications, Tokyo 182-8585 (Japan)

2010-09-01

268

The magnetic, basal, and radiative-equilibrium components in Mount Wilson Ca II H + K fluxes  

SciTech Connect

Mount Wilson Ca II H + K flux measurements of cool dwarf stars are analyzed and compared with stellar Mg II h + k fluxes, variability amplitudes, rotation rates, and solar data. It is concluded that the Mount Wilson Ca II H + K fluxes comprise three principal parts: (1) a photospheric contribution in the line wings, (2) a basal chromospheric component that appears to be unrelated to stellar magnetic activity and is, therefore, possibly nonmagnetic in origin, and (3) a chromospheric component which is associated with magnetically active regions and the (quiet and active) network. The basal chromosphere appears to cover the entire surface of magnetically inactive stars. The basal Ca II H + K flux density for solar-type stars equals the average emission observed in the centers of solar supergranulation cells, where the magnetic flux density is small. 27 refs.

Schrijver, C.J.; Dobson, A.K.; Radick, R.R. (National Solar Observatory, Sunspot, NM (USA); Joint Institute for Laboratory Astrophysics, Boulder, CO (USA))

1989-06-01

269

Magnetic flux emergence in granular convection: radiative MHD simulations and observational signatures  

NASA Astrophysics Data System (ADS)

Aims:We study the emergence of magnetic flux from the near-surface layers of the solar convection zone into the photosphere. Methods: To model magnetic flux emergence, we carried out a set of numerical radiative magnetohydrodynamics simulations. Our simulations take into account the effects of compressibility, energy exchange via radiative transfer, and partial ionization in the equation of state. All these physical ingredients are essential for a proper treatment of the problem. Furthermore, the inclusion of radiative transfer allows us to directly compare the simulation results with actual observations of emerging flux. Results: We find that the interaction between the magnetic flux tube and the external flow field has an important influence on the emergent morphology of the magnetic field. Depending on the initial properties of the flux tube (e.g. field strength, twist, entropy etc.), the emergence process can also modify the local granulation pattern. The emergence of magnetic flux tubes with a flux of 1019 Mx disturbs the granulation and leads to the transient appearance of a dark lane, which is coincident with upflowing material. These results are consistent with observed properties of emerging magnetic flux. Movies are only available in electronic form at http://www.aanda.org

Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F.

2007-05-01

270

Study on Enhancement and Weakening of Permanent Magnet Air-Gap Flux Density  

SciTech Connect

In a conventional permanent magnet (PM) machine, the air gap flux produced by the PM is fixed. It is difficult to enhance the air gap flux density because of limitations of the PM in a series magnetic circuit. However, the air gap flux density can be weakened by using power electronic field weakening to the limit of demagnetization of the PMs. This paper presents an analytical study for controlling the PM air gap flux density through a stationary brushless excitation coil. The air gap flux density can be either enhanced or weakened. There is no concern with demagnetizing the PMs during field weakening. The leakage flux of the excitation coil through the PMs is blocked. The prototype motors built on the principle of this analytical study confirm the capabilities of significant flux enhancement and weakening.

Hsu, John S [ORNL

2007-01-01

271

Potential Magnetic Field around a Helical Flux Rope Current Structure in the Solar Corona  

NASA Astrophysics Data System (ADS)

We consider the potential magnetic field associated with a helical electric line current flow, idealizing the near-potential coronal field within which a highly localized twisted current structure is embedded. It is found that this field has a significant axial component off the helical magnetic axis where there is no current flow, such that the flux winds around the axis. The helical line current field, in including the effects of flux rope writhe, is therefore more topologically complex than the straight line and ring current fields sometimes used in solar flux rope models. The axial flux in magnetic fields around confined current structures may be affected by the writhe of these current structures such that the field twists preferentially with the same handedness as the writhe. This property of fields around confined current structures with writhe may be relevant to classes of coronal magnetic flux ropes, including structures observed to have sigmoidal forms in soft X-rays and prominence magnetic fields. For example, ``bald patches'' and the associated heating by Parker current sheet dissipation seem likely. Thus, some measurements of flux rope magnetic helicities may derive from external, near-potential fields. The predicted hemispheric preference for positive and negative magnetic helicities is consistent with observational results for prominences and sigmoids and past theoretical results for flux rope internal fields.

Petrie, G. J. D.

2007-05-01

272

Analytical computation and visualization of magnetic flux lines in 3-D space from hexahedral edge finite element results  

Microsoft Academic Search

This paper describes an analytical method for the computation of magnetic flux lines in three-dimensional (3-D) space from hexahedral edge finite element results and its visualization. Using the results obtained from the edge finite element analysis by hexahedral mesh and the magnetic flux line equation in 3-D space, the sequence of line segments that construct the magnetic flux line is

S. Noguchi; Takahiro Yoshigai; Hideo Yamashita

2005-01-01

273

Effects of Magnetic Flux Circulation on Radiation Belt and Ring Current Populations  

NASA Astrophysics Data System (ADS)

The orientation of the interplanetary magnetic field (IMF) determines the location of the dayside merging line and the magnetic flux circulation patterns. Magnetic flux circulation determines the amount of energy which enters the magnetosphere and ionosphere. We use the Lyon-Fedder-Mobarry (LFM) global Magneto-Hydro-Dynamic (MHD) code to simulate both idealized and real solar wind cases. We use several satellites to validate the LFM simulation results for the real solar wind case studies. With these cases, we examine the magnetic flux circulation under differing IMF orientations. We also use the Comprehensive Ring Current Model (CRCM) and Radiation Belt Environment (RBE) model to examine the inner magnetospheric response to the orientation of the IMF. We will present the different magnetic flux circulation patterns and the resulting effects on the radiation belt and ring current population.

Mitchell, E. J.; Fok, M. H.

2011-12-01

274

Thrust Force of Novel PM Transverse Flux Linear Oscillating Actuators With Moving Magnet  

Microsoft Academic Search

This paper investigates the thrust forces of two novel permanent magnet transverse flux linear oscillating actuators (TFLOAs), one with moving magnet and the other moving both magnet and iron core, for pump and compressor drives. Different from the conventional linear actuators, the lamination construction of the TFLOAs is similar to that of a normalrotating motor, which is easy to stack.

Qinfen Lu; Minghu Yu; Yunyue Ye; Youtong Fang; Jianguo Zhu

2011-01-01

275

Improvement of the sensor system in magnetic flux leakage-type nondestructive testing (NDT)  

Microsoft Academic Search

In magnetic flux leakage (MFL)-type nondestructive testing (NDT) system, the sensitivity of MFL sensor depends on the change of the magnetic field, not just on the magnitude of the field. In this paper, the leakage parameter was defined to determine the operating point in saturation curves, and an optimum design method to determine the size of the magnet to maximize

Gwan Soo Park; Eun Sik Park

2002-01-01

276

Effects of creep damage, shot peening, and case hardening on magnetic Barkhausen noise analysis  

SciTech Connect

The micromagnetic emissions, commonly known as Barkhausen noise, are very sensitive to variations in the microstructure and sub-surface stress states of magnetic materials. Steel pipelines at power plants often have creep damage due to microstructural changes in their service life. Early detection of this damage will prevent costly failures. There is also an increasing demand to characterize the sub-surface stress states in structural materials such as high strength materials used in landing gear components in the aerospace industry. Shot peening is used to improve the fatigue strength of these components by the introduction of residual compressive stresses to the surface. Because the magnitude of Barkhausen noise varies with the magnitude of compressive stress, these noise measurements can be used for in-situ evaluation of the effectiveness of the shot peening process. Furthermore, surface modification such as case hardened magnetic samples can be easily observed using micromagnetic Barkhausen noise (MBE) to determine further modification needs.

Sipahi, L.B. (Ames Lab., IA (United States) Iowa State Univ., Ames, IA (United States). Inst. for Physical Research and Technology)

1994-11-01

277

Shot noise probing of magnetic ordering in zigzag graphene nanoribbons  

Microsoft Academic Search

The nonequilibrium time-dependent fluctuations of charge current have recently emerged as a sensitive experimental tool to probe ballistic transport through evanescent wave functions introduced into clean, wide, and short graphene strips by the attached metallic electrodes. We demonstrate that such ``pseudodiffusive'' shot noise can be substantially modified in zigzag graphene nanoribbon (ZGNR) due to the topology of its edges responsible

Ralitsa L. Dragomirova; Denis A. Areshkin; Branislav K. Nikolic

2009-01-01

278

Evolution of Open Magnetic Flux Driven by Photospheric Dynamics.  

National Technical Information Service (NTIS)

The coronal magnetic field is of paramount importance in solar and heliospheric physics. Two profoundly different views of the coronal magnetic field have emerged. In quasi-steady models, the predominant source of open magnetic field is in coronal holes. ...

J. A. Linker R. Lionello S. K. Antiochos V. S. Titov Z. Mikic

2010-01-01

279

Noise Reduction for Magnetic Resonance Images via Adaptive Multiscale Products Thresholding  

Microsoft Academic Search

Edge-preserving denoising is of great interest in medical image processing. This paper presents a wavelet-based multiscale products thresholding scheme for noise suppression of magnetic resonance images. A Canny edge detector-like dyadic wavelet transform is employed. This results in the significant features in images evolving with high magnitude across wavelet scales, while noise decays rapidly. To exploit the wavelet interscale dependencies

Paul Bao; Lei Zhang

2003-01-01

280

Numerical Simulations of 3D Coronal Magnetic Fields Resulting from the Emergence of Twisted Magnetic Flux Tubes  

NASA Astrophysics Data System (ADS)

We present MHD simulations in the low-? regime of the evolution of the 3D coronal magnetic field as an arched, twisted magnetic flux tube is transported into a pre-existing coronal potential magnetic arcade. It is found that the line-tied emerging flux tube becomes kink unstable when a sufficient amount of twist is transported into the corona. For an emerging flux tube with a left-handed twist (which is the preferred sense of twist for active region flux tubes in the northern hemisphere), the kink motion of the tube and its interaction with the ambient coronal magnetic field lead to the formation of an intense current layer which displays an inverse-S shape, consistent with the X-ray sigmoid morphology preferentially seen in the northern hemisphere. Our simulation results may explain the X-ray sigmoid brightenings that are observed during eruptive flares and confirm the prediction by previous topological studies that magnetic tangential discontinuities (or current sheets) should form along the so called ``bald-patch'' separatrix surface, across which the connectivity of the coronal magnetic field with the dense photosphere undergoes a sharp transition. Finally, we will also present simulations in a 3D spherical geometry of a CME-like eruption of the coronal magnetic field due to the kink instability of a twisted magnetic flux rope emerging into the corona.

Fan, Y.; Gibson, S. E.

2004-05-01

281

Magnetic Flux Changes and Cancellation Associated with X-Class and M-Class Flares  

NASA Astrophysics Data System (ADS)

We perform a statistical study of permanent changes in longitudinal fields associated with solar flares by tracking magnetic features. The YAFTA feature tracking algorithm is applied to GONG++ 1-minute magnetograms for 77 X-class and M-class flares to analyze the evolution and interaction of the magnetic features and to estimate the amount of canceled magnetic flux. We find that significantly more magnetic flux decreases than increases occurred during the flares, consistent with a model of collapsing loop structure for flares. Correlations between both total (unsigned) and net (signed) flux changes and the GOES peak X-ray flux are dominated by X-class flares at limb locations. The flux changes were accompanied in most cases by significant cancellation, most of which occurred during the flares. We find that the field strength and complexity near the polarity inversion line are approximately equally important in the flux cancellation processes that accompany the flares. We do not find a correlation between the flux cancellation events and the stepwise changes in the magnetic flux in the region.

Burtseva, Olga; Petrie, Gordon

2013-04-01

282

Influence of quantum and thermal noise on spin-torque-driven magnetization switching  

NASA Astrophysics Data System (ADS)

We apply a recently developed quantum theory of spin torque to study the effect of the quantum noise on the magnetization switching. The quantum noise induces considerable fluctuation of the switching time at zero temperature. By including the thermal noise, the temperature dependence of the expectation value and standard deviation of the switching time are obtained, and the results are fitted to an effective first passage model. Our theoretical predictions can be examined in single-shot experiments of spin-torque-driven switching.

Wang, Yong; Zhou, Yan; Zhang, Fu-Chun

2013-07-01

283

Reducing media noise of perpendicular magnetic recording tape for over-50 TB class data cartridge  

NASA Astrophysics Data System (ADS)

To reduce medium noise inherent in a perpendicular magnetic recording tape, which is deposited by facing targets sputtering, we have examined an epitaxial double-layered structure consisting of a soft magnetic underlayer (SUL), bcc-FeCoB/fcc-NiFe/Si/bcc-FeCoB, on a 4.5 ?m para-aromatic polyamide (aramid) film. An epitaxial soft magnetic bilayer reduces broadband noise by 5.6 dB at 337 kilo fluxchanges per inch (kfci) compared to a single SUL medium (Tape S). This is due to the reduction in the low-frequency noise, which originates from the domain walls of the SUL. Improvement of the crystal orientation of the magnetic layer hcp-CoPtCr-SiO2 and the intermediate layer hcp-Ru by inserting a fcc-NiFe spacer in laminated SULs extends the roll-off curve toward high linear density. When each SUL layer of the bilayer was made thinner, from 25 nm (Tape D2) to 10 nm (Tape D1), the medium noise decreased, improving the signal-to-noise ratio by 8.6 dB at 337 kfci compared to Tape S. In a high resolution playback test of Tape D1, an areal density of 45.0 gigabits per square inch (Gb/in.2) was confirmed. Achieved areal density means the capability of an over-50 terabyte (TB) capacity for a typical linear-formatted data cartridge.

Matsunuma, S.; Inoue, T.; Watanabe, T.; Doi, T.; Gomi, S.; Mashiko, Y.; Hirata, K.; Nakagawa, S.

2011-04-01

284

SR Torque Ripple Control Based on Power Spectrum Evaluation of Magnetic Flux Density of Air Gap  

Microsoft Academic Search

This paper has introduced the SR motor torque ripple control scheme based on power spectrum evaluation of magnetic flux density in air gap. We analyze the feasibility of its realization theoretically and give specific control tactfully and principle block diagram

Guangzhao Cuil; Xianghong Cao; Hua Zhang

2005-01-01

285

BaBar technical design report: Chapter 9, Magnet coil and flux return  

SciTech Connect

The BaBar magnet is a thin, 1.5 T superconducting solenoid with a hexagonal flux return. This chapter discusses the physics requirements and performance goals for the magnet, describes key interfaces, and summarizes the projected magnet performance. It also presents the design of the superconducting solenoid, including magnetic design, cold mass design, quench protection and stability, cold mass cooling, cryostat design, and coil assembly and transportation. The cryogenic supply system and instrumentation are described briefly, and the flux return is described.

O`Connor, T.; The BaBar Collaboration

1995-03-01

286

New Magnetic Field Sensor Based on Combined FluxGate\\/Hall-Effect Arrangement  

Microsoft Academic Search

A novel planar magnetic field sensor is presented that is based on a combined flux-gate\\/Hall-effect arrangement capable of measuring the three components of a three-dimensional quasi-static magnetic field. The proposed device involves a thin, isotropic, circular magnetic core, the magnetization of which is driven to saturation by means of a rotating excitation-field, produced by four printed planar coils. That way,

Chris Petridis; Panagiotis D. Dimitropoulos; Evangelos Hristoforou

2009-01-01

287

A study of noise as a function of magnetic state in thin film metal evaporated 8 mm video tape  

Microsoft Academic Search

Noise from thin film 8 mm video tapes has been investigated as a function of the magnetic state produced by bulk magnetisation and demagnetisation. It is found that the noise exhibits a maximum around the dc-demagnetised state consistent with thin film behaviour but in the ac-demagnetised state the noise is lower than at saturation. The results are compared with those

P. R. Bissell; R. W. Chantrell; M. D. Clarke; M. S. Araghi

1992-01-01

288

Spectral line radiation from solar small-scale magnetic flux tubes  

Microsoft Academic Search

We consider spectral line radiation from small-scale magnetic model flux tubes in the solar atmosphere. The structure of the tube is determined from the magnetostatic equations in the thin flux tube approximation. We assume that the tube is in energy equilibrium and pressure balance with the ambient medium. For the latter, we construct a quiet sun model with an artificial

F. Kneer; S. S. Hasan; W. Kalkofen

1996-01-01

289

An introduction to the propellant-driven magnetic flux compression generator  

SciTech Connect

An introduction to the concept of a propellant-driven magnetic flux compression generator is presented, together with the theory of its operation. The principles of operation of the propellant flux compression generator combine generator principles, derived from lumped parameter circuit theory, and interior ballistic principles.

Williams, P.E. [Los Alamos National Lab., NM (United States)

1995-12-01

290

Analysis and Test of a High-Speed Axial Flux Permanent Magnet Synchronous Motor  

Microsoft Academic Search

This paper gives the analysis and test of a surface mounted axial flux permanent magnet synchronous motor (PMSM) for a high-speed centrifugal compressor drive. Both analytical analysis and 3-D electromagnetic finite element method (FEM) are used to calculate flux density in the airgap, rotor back iron, and stator iron. 3-D FEM is used to simulate the electromotive force (back EMF)

L. Zheng; T. X. Wu; K. B. Sundaram; J. Vaidya; L. Zhao; D. Acharya; C. H. Ham; J. Kapat; L. Chow

2005-01-01

291

A Coreless Axial-Flux Permanent-Magnet Generator for Automotive Applications  

Microsoft Academic Search

We present a modified structure of an axial flux permanent magnet generator for an automotive application. The generator has a coreless double-stator structure; one is stationary, and the other can be moved to achieve the desired performance. This provides a simple mechanical way to weaken flux in order to extend the speed range of the generator for a constant output

Saeid Javadi; Mojtaba Mirsalim

2008-01-01

292

Equations for the motion of an isolated thin magnetic flux tube  

NASA Astrophysics Data System (ADS)

The kinematics of thin flux tubes is investigated, with the arc length as the only independent space variable. From the equation of motion of magnetic flux tubes obtained by Spruit (1981), the intrinsic form of the equation is derived. A correction is made to Spruit's equation in calculating the backreaction on the tube by the surrounding fluid flowing around it.

Cheng, J.

1992-10-01

293

Evidence of an Erupting Magnetic Flux Rope: LASCO Coronal Mass Ejection of 1997 April 13  

Microsoft Academic Search

A coronal mass ejection (CME) observed by LASCO exhibits evidence that its magnetic field geometry is that of a flux rope. The dynamical properties throughout the fields of view of C2 and C3 telescopes are examined. The results are compared with theoretical predictions based on a model of solar flux ropes. It is shown that the LASCO observations are consistent

J. Chen; G. E. Brueckner; R. Santoro; J. Krall; S. E. Paswaters; O. C. St. Cyr; R. Schwenn; P. Lamy; G. M. Simnett

1997-01-01

294

Superhigh-vacuum linear asynchronous motor with rolling rotor and magnetic-flux concentrators  

Microsoft Academic Search

The analytical method permitting to calculate the super-vacuum linear induction motor with rolling rotor and magnetic flux\\u000a concentrators has been presented. It is based on analogue approximation of orthotropic mediums. The electromagnetic torque\\u000a is calculated according to differential performances of electromagnetic field obtained by considering the configuration of\\u000a magnetic flux concentrators.

V. I. Polevskii; A. V. Sapsalev

2009-01-01

295

Interplanetary small- and intermediate-sized magnetic flux ropes during 1995–2005  

Microsoft Academic Search

We present a comprehensive survey of 125 small- and intermediate-sized interplanetary magnetic flux ropes during solar cycle 23 (1995–2005) using Wind in situ observations near 1 AU. As a result, we found the following: (1) The annual number of small- and intermediate-sized interplanetary magnetic flux ropes is not very sensitive to the solar cycle, but its trend is very similar

H. Q. Feng; D. J. Wu; C. C. Lin; J. K. Chao; L. C. Lee; L. H. Lyu

2008-01-01

296

Evidence for Sterilization of Saccharomyces Cerevisiae K 7 by an External Magnetic Flux  

NASA Astrophysics Data System (ADS)

A new process of sterilization on Saccharomyces cerevisiae proposed and experimentally demonstrated. This process consists of external magnetic flux and ferrite. Consequently, an alteration of yeast cells, caused by treatment with 2000 G magnetic flux and 6 g of ferrite, was detected through observation of the release of materials from yeast cells absorbing at 260 nm and microscopy of disrupted cells, cell debris, ghost cells and intracellular components. The same results were seen at 28°C and 4°C.

Ito, Tatsuya; Murayama, Yuzo; Suzuki, Masafumi; Yoshimura, Noboru; Iwano, Kimio; Kudo, Kozo

1992-06-01

297

Emergence of Embedded Magnetic Flux and Its Connections to the Solar Dynamo  

NASA Astrophysics Data System (ADS)

A strong (~few×105 G) and slightly twisted magnetic field (of flux 1023 Mx) can be stored in a dynamo flux tube within the overshoot region provided that the tube has a slightly lower temperature than the background gas, which has a stratification profile ?(dln?/dlnP)-1>=B2/8?P, where P, ?, B and ? are the gas pressure, density, magnetic field strength, and adiabatic index. Pumped by the intense vortical downdraft, the outer layer of dynamo flux tube is proposed to possess opposite magnetic helicity from that in the tube interior, as a direct result of the conservation of magnetic helicity. When the field-line twist in the outer layer exceeds some threshold, we show that the helicity-reversal layer can be subject to MHD instabilities that trigger partial flux eruption. In the nonlinear stage, flux ropes of 1022 Mx with magnetic helicity of a definite sign are expected to be ejected into the convection zone, leaving magnetic helicity of the opposite sign to accumulate in the main flux tube. Such a helicity removal mechanism breaks the conservation of magnetic helicity within the dynamo flux tube and can circumvent the recently discovered difficulty (the so-called ? quench) for the generation of large-scale fields. We argue that the dynamo ? is likely to resume a value of the same order as that of the kinematic turbulent dynamo. We further suggest that the helical flux expulsion is an indispensable component for the solar dynamo to operate properly. A natural prediction of this scenario is that strong sunspots in the same hemisphere should possess magnetic helicity of the same sign, regardless of the solar cycles. Specifically, the helical field lines in strong sunspots should obey the left-hand rule in the northern hemisphere and the right-hand rule in the southern hemisphere.

Chiueh, Tzihong

2000-08-01

298

Computer model simulation of null-flux magnetic suspension and guidance  

SciTech Connect

This paper discusses the magnetic force computations in a null-flux suspension system using dynamic circuit theory. A computer simulation model that can be used to compute magnetic forces and predict the system performance is developed on the basis of dynamic circuit theory. Numerical examples are presented to demonstrate the application of the model. The performance of the null-flux suspension system is simulated and discussed. 8 refs.

He, Jianliang; Rote, D.M.

1992-06-01

299

Computer model simulation of null-flux magnetic suspension and guidance  

SciTech Connect

This paper discusses the magnetic force computations in a null-flux suspension system using dynamic circuit theory. A computer simulation model that can be used to compute magnetic forces and predict the system performance is developed on the basis of dynamic circuit theory. Numerical examples are presented to demonstrate the application of the model. The performance of the null-flux suspension system is simulated and discussed. 8 refs.

He, Jianliang; Rote, D.M.

1992-01-01

300

Microcontroller based grid connected inverter for axial flux permanent magnet generator  

Microsoft Academic Search

This paper presents a single-phase grid connected inverter for an axial flux permanent magnet generator. The power is transferred by using a microprocessor based IGBT inverter to a 220V, 50Hz single-phase supply via a transformer. The five phase axial flux permanent magnet generator provides dc voltage through five bridge diode rectifier sets. The converter circuit operation is described. The performance

P. Wannakarn; V. Kinnares

2011-01-01

301

Infrared lines as probes of solar magnetic features. XIII. The relative flux in weak and strong quiet-sun magnetic fields  

NASA Astrophysics Data System (ADS)

An estimate of the fraction of magnetic flux in intrinsically weak-field form, i.e. fields with less than 1 kG intrinsic strength, in the quiet Sun is presented. We find that on average approximately 2/3 of the flux is in weak-field form, although our data allow a range of values between 25%\\ and 85%. These estimates have been derived with the help of radiative transfer model calculations from low-noise scans through the quiet Sun at fixed wavelengths within a Zeeman sensitive spectral line at 1.56 mu m. They represent the first rapidly modulated polarimetric observations of solar 1.56 mu m radiation. These scans show that the polarimetric signature near the core of the line exhibits a very different spatial structure than in the outer line wings. Since the outer part of the line profile is only sensitive to intrinsically strong magnetic fields, while the central part reacts also to intrinsically weak fields, this implies that intrinsically weak fields are distributed differently on the surface than strong fields are (weak-field features lie closer together). The spatial distribution suggests that the strong fields we are observing are network elements, whereas the weak fields are intranetwork features. Further results, such as the average distance between weak and strong field features, flux distribution, spatial power spectra and the Stokes asymmetry of intrinsically weak fields are also presented. In particular, we find that the flux per magnetic feature is distributed lognormally, in agreement with a similar finding for sunspot umbral areas by Bogdan et al. (1988).

Meunier, N.; Solanki, S. K.; Livingston, W. C.

1998-03-01

302

Plasma diagnosis from thermal noise and limits on dust flux or mass in comet Giacobini-Zinner  

NASA Astrophysics Data System (ADS)

Thermal noise spectroscopy was used to measure the density and temperature of the main (cold) electron plasma population during two hours around the point of closest approach of the International Cometary Explorer (ICE) to comet Giacobini-Zinner. The time resolution was 18 seconds in the plasma tail and 54 seconds elsewhere. Near the tail axis, the maximum plasma density was 670/cu cm and the temperature slightly above one volt. Away from the axis, the plasma density dropped to 100/cu cm over 2000 km, then decreased to 10/cu cm over 15,000 km; at the plasma tail, the density fluctuated between 10 and 30/cu cm, and the temperature, between 100,000 and 400,000 K. No evidence was found of grain impact on the spacecraft or antennas in the plasma tail. This yields an upper limit for the dust flux or particle mass, indicating either fluxes or masses in the tail smaller than those implied by models or an anomalous grain structure. Outside the tail, and particularly near 100,000 km from its axis, impulsive noises indicating plasma turbulence were observed.

Meyer-Vernet, N.; Couturier, P.; Hoang, S.; Perche, C.; Steinberg, J. L.; Fainberg, J.; Meetre, C.

1986-04-01

303

A COMPARISON OF THE INITIAL SPEED OF CORONAL MASS EJECTIONS WITH THE MAGNETIC FLUX AND MAGNETIC HELICITY OF MAGNETIC CLOUDS  

SciTech Connect

To investigate the relationship between the speed of a coronal mass ejection (CME) and the magnetic energy released during its eruption, we have compared the initial speed of CMEs (V{sub CME}) and the two parameters of their associated magnetic clouds (MC), magnetic flux (F{sub MC}), and magnetic helicity per unit length (|H{sub MC}|/L), for 34 pairs of CMEs and MCs. The values of these parameters in each MC have been determined by fitting the magnetic data of the MC to the linear force-free cylindrical model. As a result, we found that there are positive correlations between V {sup 2}{sub CME} and F{sub MC}, and between V{sup 2} {sub CME} and |H{sub MC}|/L. It is also found that the kinetic energy of CMEs (E{sub CME}) is correlated with F{sub MC} and |H{sub MC}|/L of the associated MC. In contrast, we found no significant correlation between (V{sub MC}){sup 2} and F{sub MC}, nor between (V{sub MC}){sup 2} and |H{sub MC}|/L. Our results support the notion that the eruption of a CME is related to the magnetic helicity of the source active region.

Sung, S.-K.; Marubashi, K.; Cho, K.-S.; Kim, Y.-H.; Kim, K.-H. [Korea Astronomy and Space Science Institute, Space Science Division, Solar and Space Weather Research Group, Daejeon (Korea, Republic of); Chae, J. [Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul (Korea, Republic of); Moon, Y.-J.; Kim, I.-H. [School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-701 (Korea, Republic of)], E-mail: sksung@khu.ac.kr

2009-07-01

304

Observation of an evolving magnetic flux rope before and during a solar eruption.  

PubMed

Explosive energy release is a common phenomenon occurring in magnetized plasma systems ranging from laboratories, Earth's magnetosphere, the solar corona and astrophysical environments. Its physical explanation is usually attributed to magnetic reconnection in a thin current sheet. Here we report the important role of magnetic flux rope structure, a volumetric current channel, in producing explosive events. The flux rope is observed as a hot channel before and during a solar eruption from the Atmospheric Imaging Assembly telescope on board the Solar Dynamic Observatory. It initially appears as a twisted and writhed sigmoidal structure with a temperature as high as 10 MK, and then transforms toward a semi-circular shape during a slow-rise phase, which is followed by fast acceleration and onset of a flare. The observations suggest that the instability of the magnetic flux rope triggers the eruption, thus making a major addition to the traditional magnetic-reconnection paradigm. PMID:22434190

Zhang, Jie; Cheng, Xin; Ding, Ming-de

2012-03-20

305

Observation of an evolving magnetic flux rope before and during a solar eruption  

NASA Astrophysics Data System (ADS)

Explosive energy release is a common phenomenon occurring in magnetized plasma systems ranging from laboratories, Earth's magnetosphere, the solar corona and astrophysical environments. Its physical explanation is usually attributed to magnetic reconnection in a thin current sheet. Here we report the important role of magnetic flux rope structure, a volumetric current channel, in producing explosive events. The flux rope is observed as a hot channel before and during a solar eruption from the Atmospheric Imaging Assembly telescope on board the Solar Dynamic Observatory. It initially appears as a twisted and writhed sigmoidal structure with a temperature as high as 10 MK, and then transforms toward a semi-circular shape during a slow-rise phase, which is followed by fast acceleration and onset of a flare. The observations suggest that the instability of the magnetic flux rope triggers the eruption, thus making a major addition to the traditional magnetic-reconnection paradigm.

Zhang, Jie; Cheng, Xin; Ding, Ming-De

2012-03-01

306

Comparing a current-carrying circular wire with polygons of equal perimeter: magnetic field versus magnetic flux  

NASA Astrophysics Data System (ADS)

We compare the magnetic field at the centre and the self-magnetic flux through a current-carrying circular loop, with those obtained for current-carrying polygons with the same perimeter. As the magnetic field diverges at the position of the wires, we compare the self-fluxes utilizing several regularization procedures. The calculation is best performed utilizing the vector potential, thus highlighting its usefulness in practical applications. Our analysis answers some of the intuition challenges students face when they encounter a related simple textbook example. These results can be applied directly to the determination of mutual inductances in a variety of situations.

Silva, J. P.; Silvestre, A. J.

2005-09-01

307

Investigation of the dependence of Barkhausen noise on stress and the angle between the stress and magnetization directions  

Microsoft Academic Search

The dependence of Barkhausen noise amplitude on stress, and on the angle between the stress and magnetization directions, has been investigated in specimens of AISI\\/SAE 4340, HY-80 and ASTM A-36 steels. In all three materials it was found that the Barkhausen noise amplitude measured with the magnetic field applied at 60° to the stress axis is independent of stress. This

H. Kwun

1985-01-01

308

Thermal magnetization noise as a function of frequency in amorphous ferromagnets  

Microsoft Academic Search

We report the observation of 1\\/frequency magnetization noise in toroidal strip-wound cores of amorphous ferromagnetic alloys at liquid-helium temperatures and in the frequency range 0.1 Hz–1 kHz. The materials tested are Co-based alloys, and exhibit very low magnetostriction and good soft magnetic properties. The experimental apparatus consists of a radio frequency SQUID magnetometer coupled to the sample by a superconducting

G. A. Prodi; S. Vitale; M. Cerdonio; P. Falferi

1989-01-01

309

Non-Abelian magnetic flux at high temperatures  

NASA Astrophysics Data System (ADS)

We study by Monte Carlo methods the behavior of non-Abelian magnetic fields at high temperatures in the Mack-Petkova modified SU(2) lattice gauge theory which does not contain dynamical magnetic monopoles. Our results indicate that magnetic screening at high temperatures is a genuine phenomenon which survives in the continuum limit and is not an artifact of the standard Wilson lattice gauge theory caused by the artificial dynamical magnetic monopoles contained in this theory.

Lazarides, G.; Sarantakos, Stavros

1985-01-01

310

Active control of the volume acquisition noise in functional magnetic resonance imaging: Method and psychoacoustical evaluation  

NASA Astrophysics Data System (ADS)

Functional magnetic resonance imaging (fMRI) provides a noninvasive tool for observing correlates of neural activity in the brain while a subject listens to sound. However, intense acoustic noise is generated in the process of capturing MR images. This noise stimulates the auditory nervous system, limiting the dynamic range available for displaying stimulus-driven activity. The noise is potentially damaging to hearing and is distracting for the subject. In an active noise control (ANC) system, a reference sample of a noise is processed to form a sound which adds destructively with the noise at the listener's ear. We describe an implementation of ANC in the electromagnetically hostile and physically compact MRI scanning environment. First, a prototype system was evaluated psychoacoustically in the laboratory, using the electrical drive to a noise-generating loudspeaker as the reference. This system produced 10-20 dB of subjective noise-reduction between 250 Hz and 1 kHz, and smaller amounts at higher frequencies. The system was modified to operate in a real MR scanner where the reference was obtained by recording the acoustic scanner noise. Objective reduction by 30-40 dB of the most intense component in scanner noises was realized between 500 Hz and 3500 Hz, and subjective reduction of 12 dB and 5 dB in tests at frequencies of 600 Hz and at 1.9 kHz, respectively. Although the benefit of ANC is limited by transmission paths to the cochlea other than air-conduction routes from the auditory meatus, ANC achieves worthwhile attenuation even in the frequency range of maximum bone conduction (1.5-2 kHz). ANC should, therefore, be generally useful during auditory fMRI.

Chambers, John; Akeroyd, Michael A.; Summerfield, A. Quentin; Palmer, Alan R.

2001-12-01

311

An Analysis of Magnetohydrodynamic Invariants of Magnetic Fluctuations within Interplanetary Flux Ropes  

NASA Astrophysics Data System (ADS)

A statistical analysis of magnetic flux ropes, identified by large-amplitude, smooth rotations of the magnetic field vector and a low level of both proton density and temperature, has been performed by computing the invariants of the ideal magnetohydrodynamic (MHD) equations, namely the magnetic helicity, the cross-helicity, and the total energy, via magnetic field and plasma fluctuations in the interplanetary medium. A technique based on the wavelet spectrograms of the MHD invariants allows the localization and characterization of those structures in both scales and time: it has been observed that flux ropes show, as expected, high magnetic helicity states (|? m | in [0.6: 1]), but extremely variable cross-helicity states (|? c | in [0: 0.8]), which, however, are not independent of the magnetic helicity content of the flux rope itself. The two normalized MHD invariants observed within the flux ropes tend indeed to distribute, neither trivially nor automatically, along the \\sqrt{\\sigma _{m}^{2}+\\sigma _{c}^{2}}=1 curve, thus suggesting that some constraint should exist between the magnetic and cross-helicity content of the structures. The analysis carried out has further showed that the flux rope properties are totally independent of their time duration and that they are detected either as a sort of interface between different portions of solar wind or as isolated structures embedded in the same stream.

Telloni, D.; Perri, S.; Bruno, R.; Carbone, V.; Amicis, R. D.

2013-10-01

312

Dual-spacecraft observation of large-scale magnetic flux ropes in the Martian ionosphere  

NASA Astrophysics Data System (ADS)

We here report the first dual-spacecraft detection of planetary flux ropes in the ionosphere of Mars. The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), on board Mars Express, can measure the magnetic field magnitude near the spacecraft. Typically, these measurements track the known crustal magnetic field strength very well; however, occasionally, MARSIS detects transient, intense magnetic fields that deviate significantly from the known crustal fields. Two such magnetic field enhancements occur in near-coincidence with flux rope detections by the Mars Global Surveyor Magnetometer and Electron Reflectometer, which provides vector magnetic field measurements, allowing us to clearly identify the enhancements as flux ropes. The flux ropes detected are quasi-stable for at least a half hour, have peak magnetic field strengths of ˜50 and 90 nT, and are ˜650-700 km in diameter. Both occur downstream of the region of strong crustal fields. In addition, MARSIS has detected 13 other magnetic enhancements over a 5 year period, which we infer to be flux ropes. These structures have peak field strengths up to 130 nT and measured horizontal dimensions of several hundred to over a thousand kilometers. They are clustered around the intense crustal fields in the southern hemisphere of Mars. The large spatial scale of these flux ropes distinguishes them from small-scale flux ropes, with diameters of tens of kilometers, that have been seen in the ionospheres of Venus and Mars. These large-scale flux ropes are believed to be caused by solar wind stretching and shearing of the Martian crustal fields.

Morgan, D. D.; Gurnett, D. A.; Akalin, F.; Brain, D. A.; Leisner, J. S.; Duru, F.; Frahm, R. A.; Winningham, J. D.

2011-02-01

313

A study of noise as a function of magnetic state in thin film metal evaporated 8 mm video tape  

NASA Astrophysics Data System (ADS)

Noise from thin film 8 mm video tapes has been investigated as a function of the magnetic state produced by bulk magnetisation and demagnetisation. It is found that the noise exhibits a maximum around the dc-demagnetised state consistent with thin film behaviour but in the ac-demagnetised state the noise is lower than at saturation. The results are compared with those of a particulate medium and the behaviour interpreted in terms of the magnetic microstructure.

Bissell, P. R.; Chantrell, R. W.; Clarke, M. D.; Araghi, M. S.

1992-07-01

314

Synchronized and noise-robust audio recordings during realtime magnetic resonance imaging scans (L)  

PubMed Central

This letter describes a data acquisition setup for recording, and processing, running speech from a person in a magnetic resonance imaging (MRI) scanner. The main focus is on ensuring synchronicity between image and audio acquisition, and in obtaining good signal to noise ratio to facilitate further speech analysis and modeling. A field-programmable gate array based hardware design for synchronizing the scanner image acquisition to other external data such as audio is described. The audio setup itself features two fiber optical microphones and a noise-canceling filter. Two noise cancellation methods are described including a novel approach using a pulse sequence specific model of the gradient noise of the MRI scanner. The setup is useful for scientific speech production studies. Sample results of speech and singing data acquired and processed using the proposed method are given.

Bresch, Erik; Nielsen, Jon; Nayak, Krishna; Narayanan, Shrikanth

2007-01-01

315

MODELING THE SUN'S OPEN MAGNETIC FLUX AND THE HELIOSPHERIC CURRENT SHEET  

SciTech Connect

By coupling a solar surface flux transport model with an extrapolation of the heliospheric field, we simulate the evolution of the Sun's open magnetic flux and the heliospheric current sheet (HCS) based on observational data of sunspot groups since 1976. The results are consistent with measurements of the interplanetary magnetic field near Earth and with the tilt angle of the HCS as derived from extrapolation of the observed solar surface field. This opens the possibility for an improved reconstruction of the Sun's open flux and the HCS into the past on the basis of empirical sunspot data.

Jiang, J.; Cameron, R.; Schmitt, D.; Schuessler, M., E-mail: jiang@mps.mpg.d [Max-Planck-Institut fuer Sonnensystemforschung, 37191 Katlenburg-Lindau (Germany)

2010-01-20

316

Superconducting Film Flux Transformer for a Sensor of a Weak Magnetic Field  

NASA Astrophysics Data System (ADS)

The object of study is a superconducting film flux transformer in the form of a square shaped loop with the tapering operative strip used in a sensor of a weak magnetic field. The magnetosensitive film element based on the giant magnetoresistance effect is overlapped with the tapering operative strip of the flux transformer; it is separated from the latter by the insulator film. It is shown that the topological nanostructuring of the operative strip of the flux transformer increases its gain factor by one or more orders of magnitude, i.e. increases its efficiency, which leads to a significant improvement of important parameters of a magnetic-field sensor.

Ichkitidze, L.; Mironyuk, A.

2012-12-01

317

Magnetic Johnson noise constraints on electron electric dipole moment experiments  

SciTech Connect

Magnetic fields from statistical fluctuations in currents in conducting materials broaden atomic linewidths by the Zeeman effect. The constraints imposed by this broadening on the design of experiments that measure the electric dipole moment of the electron are analyzed. Contrary to the predictions of Lamoreaux [S. K. Lamoreaux, Phys. Rev. A 60, 1717 (1999)], the standard material for high-permeability magnetic shields proves to be as significant a source of broadening as is an ordinary metal. A scheme that would replace this standard material with ferrite is proposed.

Munger, Charles T. Jr. [Stanford Linear Accelerator Center, 2575 Sand Hill Road, Mail Stop 59, Menlo Park, California 94025 (United States) and Lawrence Berkeley National Laboratory, Mail Stop 71-259, Berkeley, California 94320 (United States)

2005-07-15

318

Magnetic Johnson Noise Constraints on Electron Electric Dipole Moment Experiments  

SciTech Connect

Magnetic fields from statistical fluctuations in currents in conducting materials broaden atomic linewidths by the Zeeman effect. The constraints so imposed on the design of experiments to measure the electric dipole moment of the electron are analyzed. Contrary to the predictions of Lamoreaux [S.K. Lamoreaux, Phys. Rev. A60, 1717(1999)], the standard material for high-permeability magnetic shields proves to be as significant a source of broadening as an ordinary metal. A scheme that would replace this standard material with ferrite is proposed.

Munger, C.

2004-11-18

319

Structural studies of magnetic flux line lattices near critical transitions  

Microsoft Academic Search

Small Angle Neutron Scattering (SANS) studies are an essential tool for studying vortices in the bulk of type II superconductors,\\u000a providing deep understanding of their three dimensional microscopic structure in a wide range of fields and temperatures.\\u000a This talk will summarize detailed studies of flux lattices in the vicinity of two critical transitions: (1) flux lattices\\u000a in the vicinity of

Uri Yaron

1997-01-01

320

Signal to noise ratio scaling and density limit estimates in longitudinal magnetic recording  

Microsoft Academic Search

A simplified general expression is given for SNR for digital magnetic recording for transition noise dominant systems. High density media are assumed in which the transition parameter scales with the in-plane grain diameter. At a fixed normalized code density, the SNR varies as the square of the bit spacing times the read track width divided by the grain diameter cubed.

H. N. Bertram; H. Zhou; R. Gustafson

1998-01-01

321

Study on control system of permanent magnet loader for gearbox noise test  

Microsoft Academic Search

In order to change noise problems of the existing gearbox loader, a new type of Permanent magnet loader is designed by using water cooling. The control system is realized mainly by using PLC (Programmable Logic Controller) and MCU (Micro Control Unit), and obtaining signals from multi-sensors. Meanwhile, fuzzy parameters self-tuning PID algorithm and automatic location algorithm have been studied, and

Lezhi Ye; Desheng Li; Jiliang Liu; Xiaobo Hou

2010-01-01

322

Investigation of Optimum Field Amplitude for Stress Dependence of Magnetic Barkhausen Noise  

Microsoft Academic Search

An investigation of the sensitivity of magnetic Barkhausen noise (MBN) to changes in uniaxial tensile stress applied to mild steel under conditions of low, medium, and high excitation field amplitudes is presented. Field excitation in the lowest range that was tested produced the most sensitive response to change in stress for the squared-voltage-signal output integrated with respect to time (MBNenergy).

C. Mandache; T. W. Krause; L. Clapham

2007-01-01

323

Optimal Slot Numbers for Magnetic Noise Reduction in Variable-Speed Induction Motors  

Microsoft Academic Search

Although many empirical rules have been established for correctly choosing the number of stator and rotor slots so as to limit the audible magnetic noise level radiated by induction machines, these rules never take into account the stator natural frequencies or the fact that the motor is run at variable speed. In this paper, we present a fast simulation tool

Jean Le Besnerais; Vincent Lanfranchi; Michel Hecquet; Pascal Brochet

2009-01-01

324

DRIFT ORBITS OF ENERGETIC PARTICLES IN AN INTERPLANETARY MAGNETIC FLUX ROPE  

SciTech Connect

Interplanetary magnetic flux ropes have significant effects on the distribution of energetic particles in space. Flux ropes can confine solar energetic particles (SEPs) for hours, and have relatively low densities of Galactic cosmic rays (GCRs), as seen during second-stage Forbush decreases. As particle diffusion is apparently inhibited across the flux rope boundary, we suggest that guiding center drifts could play a significant role in particle motion into and out of the flux ropes. We develop an analytic model of the magnetic field in an interplanetary magnetic flux rope attached to the Sun at both ends, in quasi-toroidal coordinates, with the realistic features of a flux rope cross section that is small near the Sun, expanding with distance from the Sun, and field lines that are wound less tightly close to the Sun due to stretching by the solar wind. We calculate the particle drift velocity field due to the magnetic field curvature and gradient as a function of position and pitch-angle cosine, and trace particle guiding center orbits numerically, assuming conservation of the first adiabatic invariant. We find that SEPs in the interior of a flux rope can have drift orbits that are trapped for long times, as in a tokamak configuration, with resonant escape features as a function of the winding number. For Forbush decreases of GCRs, the drifts should contribute to a unidirectional anisotropy and net flow from one leg of the loop to the other, in a direction determined by the poloidal field direction.

Krittinatham, W.; Ruffolo, D., E-mail: watcharawuth.krittinatham@gmail.co, E-mail: scdjr@mahidol.ac.t [Department of Physics, Faculty of Science, Mahidol University, Rama VI Road, Bangkok 10400 (Thailand)

2009-10-10

325

Inertia Wheel on Low-Noise Active Magnetic Suspension  

Microsoft Academic Search

Magnetic bearings are particularly suited for space applications for a number of reasons: - they are ideally suited for vacuum applications; - the lack of lubrication and wear enhances the reliability and guaranties a long maintenance-free operation - the low drag torque decreases power consumption and reduces the torque exerted on the stator of the machine. - the possibility of

S. Carabelli; G. Genta; M. Silvagni; A. Tonoli

2002-01-01

326

Dynamical evolution of twisted magnetic flux tubes. I. Equilibrium and linear stability  

SciTech Connect

The three-dimensional dynamical evolution of twisted magnetic flux tubes is studied using a time-dependent magnetohydrodynamic (MHD) model. The flux tubes are intended to model solar coronal loops, and include the stabilizing effect of photospheric line tying. The model permits the complete evolution of flux tubes to be followed self-consistently, including the formation, equilibrium, linear instability, and nonlinear behavior. Starting from an initial uniform background magnetic field, a twisted flux tube is created by the application of slow, localized photospheric vortex flows. The flux tube evolves quasi-statically through sequences of equilibria with increasing twist, until it becomes linearly unstable to an ideal MHD kink mode. In this paper, the equilibrium properties and the linear stability behavior are discussed. The application of the method to the uniform-twist, Gold-Hoyle field confirms the previous stability threshold for kink instability and provides estimates of the resulting growth rate. 29 refs.

Mikic, Z.; Schnack, D.D.; Van Hoven, G. (Science Applications International Corp., San Diego, CA (USA) California Univ., Irvine (USA))

1990-10-01

327

SCATTERING OF THE f-MODE BY SMALL MAGNETIC FLUX ELEMENTS FROM OBSERVATIONS AND NUMERICAL SIMULATIONS  

SciTech Connect

The scattering of f-modes by magnetic tubes is analyzed using three-dimensional numerical simulations. An f-mode wave packet is propagated through a solar atmosphere embedded with three different flux tube models that differ in radius and total magnetic flux. A quiet-Sun simulation without a tube present is also performed as a reference. Waves are excited inside the flux tube and propagate along the field lines, and jacket modes are generated in the surroundings of the flux tube, carrying 40% as much energy as the tube modes. The resulting scattered wave is mainly an f-mode composed of a mixture of m = 0 and m = {+-}1 modes. The amplitude of the scattered wave approximately scales with the magnetic flux. A small amount of power is scattered into the p{sub 1}-mode. We have evaluated the absorption and phase shift from a Fourier-Hankel decomposition of the photospheric vertical velocities. They are compared with the results obtained from the ensemble average of 3400 small magnetic elements observed in high-resolution MDI Doppler datacubes. The comparison shows that the observed dependence of the phase shift with wavenumber can be matched reasonably well with the simulated flux tube model. The observed variation of the phase shifts with the azimuthal order m appears to depend on details of the ensemble averaging, including possible motions of the magnetic elements and asymmetrically shaped elements.

Felipe, T.; Braun, D.; Crouch, A.; Birch, A., E-mail: tobias@cora.nwra.com [NorthWest Research Associates, Colorado Research Associates, Boulder, CO 80301 (United States)

2012-10-01

328

MAGNETAR GIANT FLARES-FLUX ROPE ERUPTIONS IN MULTIPOLAR MAGNETOSPHERIC MAGNETIC FIELDS  

SciTech Connect

We address a primary question regarding the physical mechanism that triggers the energy release and initiates the onset of eruptions in the magnetar magnetosphere. Self-consistent stationary, axisymmetric models of the magnetosphere are constructed based on force-free magnetic field configurations that contain a helically twisted force-free flux rope. Depending on the surface magnetic field polarity, there exist two kinds of magnetic field configurations, inverse and normal. For these two kinds of configurations, variations of the flux rope equilibrium height in response to gradual surface physical processes, such as flux injections and crust motions, are carefully examined. We find that equilibrium curves contain two branches: one represents a stable equilibrium branch, and the other an unstable equilibrium branch. As a result, the evolution of the system shows a catastrophic behavior: when the magnetar surface magnetic field evolves slowly, the height of the flux rope would gradually reach a critical value beyond which stable equilibriums can no longer be maintained. Subsequently, the flux rope would lose equilibrium and the gradual quasi-static evolution of the magnetosphere will be replaced by a fast dynamical evolution. In addition to flux injections, the relative motion of active regions would give rise to the catastrophic behavior and lead to magnetic eruptions as well. We propose that a gradual process could lead to a sudden release of magnetosphere energy on a very short dynamical timescale, without being initiated by a sudden fracture in the crust of the magnetar. Some implications of our model are also discussed.

Yu Cong, E-mail: cyu@ynao.ac.cn [National Astronomical Observatories/Yunnan Astronomical Observatory, Chinese Academy of Sciences, Kunming, 650011 (China)

2012-09-20

329

Magnetar Giant Flares—Flux Rope Eruptions in Multipolar Magnetospheric Magnetic Fields  

NASA Astrophysics Data System (ADS)

We address a primary question regarding the physical mechanism that triggers the energy release and initiates the onset of eruptions in the magnetar magnetosphere. Self-consistent stationary, axisymmetric models of the magnetosphere are constructed based on force-free magnetic field configurations that contain a helically twisted force-free flux rope. Depending on the surface magnetic field polarity, there exist two kinds of magnetic field configurations, inverse and normal. For these two kinds of configurations, variations of the flux rope equilibrium height in response to gradual surface physical processes, such as flux injections and crust motions, are carefully examined. We find that equilibrium curves contain two branches: one represents a stable equilibrium branch, and the other an unstable equilibrium branch. As a result, the evolution of the system shows a catastrophic behavior: when the magnetar surface magnetic field evolves slowly, the height of the flux rope would gradually reach a critical value beyond which stable equilibriums can no longer be maintained. Subsequently, the flux rope would lose equilibrium and the gradual quasi-static evolution of the magnetosphere will be replaced by a fast dynamical evolution. In addition to flux injections, the relative motion of active regions would give rise to the catastrophic behavior and lead to magnetic eruptions as well. We propose that a gradual process could lead to a sudden release of magnetosphere energy on a very short dynamical timescale, without being initiated by a sudden fracture in the crust of the magnetar. Some implications of our model are also discussed.

Yu, Cong

2012-09-01

330

Magnetic flux inversion in charged BPS vortices in a Lorentz-violating Maxwell-Higgs framework  

NASA Astrophysics Data System (ADS)

We demonstrate for the first time the existence of electrically charged BPS vortices in a Maxwell-Higgs model supplemented with a parity-odd Lorentz-violating (LV) structure belonging to the CPT-even gauge sector of the standard model extension and a fourth order potential (in the absence of the Chern-Simons term). The modified first order BPS equations provide charged vortex configurations endowed with some interesting features: localized and controllable spatial thickness, integer flux quantization, electric field inversion and localized magnetic flux reversion. This model could possibly be applied on condensed matter systems which support charged vortices carrying integer quantized magnetic flux, endowed with localized flipping of the magnetic flux.

Casana, R.; Ferreira, M. M.; da Hora, E.; Miller, C.

2012-12-01

331

Feasibility Study of an Absolute Determination of the Magnetic Flux Quantum.  

National Technical Information Service (NTIS)

The principle and an experimental study of a new method to determine the magnetic flux quantum (phi sub 0) is described. Magnetic energy proportional to (phi sub 0) squared is substituted for and measured in terms of gravitational potential energy. A supe...

K. Hara F. Shiota T. Kubota

1984-01-01

332

Models of the Large-Scale Corona. I. Formation, Evolution, and Liftoff of Magnetic Flux Ropes  

Microsoft Academic Search

The response of the large-scale coronal magnetic field to transport of magnetic flux in the photosphere is investigated. In order to follow the evolution on long timescales, the coronal plasma velocity is assumed to be proportional to the Lorentz force (magnetofriction), causing the coronal field to evolve through a series of nonlinear force-free states. Magnetofrictional simulations are used to study

D. H. Mackay; A. A. van Ballegooijen

2006-01-01

333

Compression of the magnetic flux by imploding ionizing shock waves [pulsed power  

Microsoft Academic Search

Magnetic flux compression (MFC) by a cylindrically symmetric imploding ionizing shock front in media with variable conductivity is studied. It is shown that the maximum achievable magnetic field is limited mainly by two factors: by the effect of finite conductivity of the matter behind the shock wave (SW) and by electrical breakdown of the nonconductive media ahead of the SW.

G. Bjarnholt; S. M. Golberg; S. E. Nyholm

1997-01-01

334

Reduction of the unbalanced magnetic force of a transverse flux machine by using symmetric multipair cores  

Microsoft Academic Search

This paper investigates the characteristics of the magnetic force and the torque in the conventional rotatory two-phase transverse flux machine (TFM) by using the three-dimensional finite element method. This research shows that the unbalanced magnetic force is one of the dominant excitation forces in this machine, and it proposes a TFM with symmetric multipair cores in which each stator core

G. H. Jang; N. K. Park; C. I. Lee; J. H. Chang; S. W. Jeong; D. H. Kang

2008-01-01

335

Superconducting flux pump for high-temperature superconductor insert coils of NMR magnets  

NASA Astrophysics Data System (ADS)

This paper describes a prototype flux pump recently operated at the MIT Francis Bitter Magnet Laboratory. The results of the prototype flux pump will be used in the development of a full-scale flux pump that will be coupled to a high-temperature superconductor (HTS) insert coil of a high-field NMR magnet. Such an HTS insert is unlikely to operate in persistent mode because of the conductor's low index (n). The flux pump can compensate for field decay in the HTS insert coil and make the insert operate effectively in persistent mode. The flux pump, comprised essentially of a transformer and two switches, all made of superconductor, transfers into the insert coil a fraction of a magnetic energy that is first introduced in the secondary circuit of the transformer by a current supplied to the primary circuit. A prototype flux pump has been designed, fabricated, and operated to demonstrate that a flux pump can indeed supply a small metered current into a load superconducting magnet. A current increment in the range of microamperes has been measured in the magnet after each pumping action. The superconducting prototype flux pump is made of Nb3Sn tape. The pump is placed in a gaseous environment above the liquid helium level to keep its heat dissipation from directly discharged in the liquid; the effluent helium vapor maintains the thermal stability of the flux pump. [This paper is also published in Advances in Cryogenic Engineering Volume 47A, AIP Conference Proceedings Volume 613, pp. 441-448.] .

Jeong, S.; Lee, H.; Iwasa, Y.

2002-05-01

336

A Magnetic Flux Leakage and Magnetostrictive Guided Wave Hybrid Transducer for Detecting Bridge Cables  

PubMed Central

Condition assessment of cables has gained considerable attention for the bridge safety. A magnetic flux leakage and magnetostrictive guided wave hybrid transducer is provided to inspect bridge cables. The similarities and differences between the two methods are investigated. The hybrid transducer for bridge cables consists of an aluminum framework, climbing modules, embedded magnetizers and a ribbon coil. The static axial magnetic field provided by the magnetizers meets the needs of the magnetic flux leakage testing and the magnetostrictive guided wave testing. The magnetizers also provide the attraction for the climbing modules. In the magnetic flux leakage testing for the free length of cable, the coil induces the axial leakage magnetic field. In the magnetostrictive guided wave testing for the anchorage zone, the coil provides a pulse high power variational magnetic field for generating guided waves; the coil induces the magnetic field variation for receiving guided waves. The experimental results show that the transducer with the corresponding inspection system could be applied to detect the broken wires in the free length and in the anchorage zone of bridge cables.

Xu, Jiang; Wu, Xinjun; Cheng, Cheng; Ben, Anran

2012-01-01

337

A magnetic flux leakage and magnetostrictive guided wave hybrid transducer for detecting bridge cables.  

PubMed

Condition assessment of cables has gained considerable attention for the bridge safety. A magnetic flux leakage and magnetostrictive guided wave hybrid transducer is provided to inspect bridge cables. The similarities and differences between the two methods are investigated. The hybrid transducer for bridge cables consists of an aluminum framework, climbing modules, embedded magnetizers and a ribbon coil. The static axial magnetic field provided by the magnetizers meets the needs of the magnetic flux leakage testing and the magnetostrictive guided wave testing. The magnetizers also provide the attraction for the climbing modules. In the magnetic flux leakage testing for the free length of cable, the coil induces the axial leakage magnetic field. In the magnetostrictive guided wave testing for the anchorage zone, the coil provides a pulse high power variational magnetic field for generating guided waves; the coil induces the magnetic field variation for receiving guided waves. The experimental results show that the transducer with the corresponding inspection system could be applied to detect the broken wires in the free length and in the anchorage zone of bridge cables. PMID:22368483

Xu, Jiang; Wu, Xinjun; Cheng, Cheng; Ben, Anran

2012-01-05

338

Observational Consequences of a Magnetic Flux Rope Emerging into the Corona  

NASA Astrophysics Data System (ADS)

We show that a numerical simulation of a magnetic flux rope emerging into a coronal magnetic field predicts solar structures and dynamics consistent with observations. We first consider the structure, evolution, and relative location and orientation of S-shaped, or sigmoid, active regions and filaments. The basic assumptions are that (1) X-ray sigmoids appear at the regions of the flux rope known as ``bald-patch-associated separatrix surfaces (BPSSs), where, under dynamic forcing, current sheets can form, leading to reconnection and localized heating, and that (2) filaments are regions of enhanced density contained within dips in the magnetic flux rope. We demonstrate that the shapes and relative orientations and locations of the BPSS and dipped field are consistent with observations of X-ray sigmoids and their associated filaments. Moreover, we show that current layers indeed form along the sigmoidal BPSS as the flux rope is driven by the kink instability. Finally, we consider how apparent horizontal motions of magnetic elements at the photosphere caused by the emerging flux rope might be interpreted. In particular, we show that local correlation tracking analysis of a time series of magnetograms for our simulation leads to an underestimate of the amount of magnetic helicity transported into the corona by the flux rope, largely because of undetectable twisting motions along the magnetic flux surfaces. Observations of rotating sunspots may provide better information about such rotational motions, and we show that if we consider the separated flux rope legs as proxies for fully formed sunspots, the amount of rotation that would be observed before the region becomes kink unstable would be in the range 40°-200° per leg/sunspot, consistent with observations.

Gibson, S. E.; Fan, Y.; Mandrini, C.; Fisher, G.; Demoulin, P.

2004-12-01

339

Two-step Emergence of the Magnetic Flux Sheet from the Solar Convection Zone  

NASA Astrophysics Data System (ADS)

We perform two-dimensional magnetodydrodynamic simulations of the flux emergence from the solar convection zone to the corona. The flux sheet is initially located moderately deep in the adiabatically stratified convection zone (-20,000 km) and is perturbed to trigger the Parker instability. The flux rises through the solar interior due to the magnetic buoyancy, but suffers a gradual deceleration and a flattening in the middle of the way to the surface since the plasma piled on the emerging loop cannot pass through the convectively stable photosphere. As the magnetic pressure gradient enhances, the flux becomes locally unstable to the Parker instability so that the further evolution to the corona occurs. The second-step nonlinear emergence is well described by the expansion law by Shibata et al. To investigate the condition for this "two-step emergence" model, we vary the initial field strength and the total flux. When the initial field is too strong, the flux exhibits the emergence to the corona without a deceleration at the surface and reveals an unrealistically strong flux density at each footpoint of the coronal loop, while the flux either fragments within the convection zone or cannot pass through the surface when the initial field is too weak. The condition for the "two-step emergence" is found to be 1021-1022 Mx with 104 G at z = -20,000 km. We present some discussions in connection with recent observations and the results of the thin-flux-tube model.

Toriumi, S.; Yokoyama, T.

2010-05-01

340

Dynamics of local isolated magnetic flux tubes in a fast-rotating stellar atmosphere  

SciTech Connect

Dynamics of magnetic flux tubes in the fast rotating stellar atmosphere is studied. We focus on the effects and signatures of the instability of the flux tube emergence influenced by the Coriolis force. We present the result from a linear stability analysis and discuss its possible signatures in the course of the evolution of G-type and M-type stars. We present a three dimensional magnetohydrodynamical simulation of local isolated magnetic flux tubes under a magnetic buoyancy instability in co-rotating Cartesian coordinates. We find that the combination of the buoyancy instability and the Coriolis effect gives rise to a mechanism, to twist the emerging magnetic flux tube into a helical structure. The tilt angle, east-west asymmetry and magnetic helicity of the Twisted flux tubes in the simulations are studied in detail. The linear and nonlinear analyses provide hints as to what kind of pattern of large spots in young M-type main-sequence stars might be observed. We find that young and old G-type stars may have different distributions of spots while M-type stars may always have low latitudes spots. The size of stellar spots may decrease when a star becomes older, due to the decreasing of magnetic field. A qualitative comparison with solar observations is also presented.

Chou, W.; Tajima, C.T. [Univ. of Texas, Austin, TX (United States). Dept. of Physics; Matsumoto, R. [Chiba Univ. (Japan)]|[ASRC, JAERI, Naka (Japan); Shibata, K. [National Astronomical Observatory, Mitaka (Japan)

1998-01-01

341

Photospheric Injection of Magnetic Helicity: Connectivity-Based Flux Density Method  

NASA Astrophysics Data System (ADS)

Magnetic helicity quantifies the degree to which the magnetic field in a volume is globally sheared and/or twisted. This quantity is believed to play a key role in solar activity due to its conservation property. Helicity is continuously injected into the corona during the evolution of active regions (ARs). To better understand and quantify the role of magnetic helicity in solar activity, the distribution of magnetic helicity flux in ARs needs to be studied. The helicity distribution can be computed from the temporal evolution of photospheric magnetograms of ARs such as the ones provided by SDO/HMI and Hinode/SOT. Most recent analyses of photospheric helicity flux derived a proxy to the helicity-flux density based on the relative rotation rate of photospheric magnetic footpoints. Although this proxy allows a good estimate of the photospheric helicity flux, it is still not a true helicity flux density because it does not take into account the connectivity of the magnetic field lines. For the first time, we implement a helicity density that takes this connectivity into account. To use it for future observational studies, we tested the method and its precision on several types of models involving different patterns of helicity injection. We also tested it on more complex configurations - from magnetohydrodynamics (MHD) simulations - containing quasi-separatrix layers. We demonstrate that this connectivity-based proxy is best-suited to map the true distribution of photospheric helicity injection.

Dalmasse, K.; Pariat, E.; Démoulin, P.; Aulanier, G.

2013-06-01

342

Laboratory simulation of arched magnetic flux rope eruptions in the solar atmosphere.  

PubMed

Dramatic eruption of an arched magnetic flux rope in a large ambient plasma has been studied in a laboratory experiment that simulates coronal loops. The eruption is initiated by laser generated plasma flows from the footpoints of the rope that significantly modify the magnetic-field topology and link the magnetic-field lines of the rope with the ambient plasma. Following this event, the flux rope erupts by releasing its plasma into the background. The resulting impulse excites intense magnetosonic waves that transfer energy to the ambient plasma and subsequently decay. PMID:20868055

Tripathi, S K P; Gekelman, W

2010-08-13

343

Laboratory Simulation of Arched Magnetic Flux Rope Eruptions in the Solar Atmosphere  

SciTech Connect

Dramatic eruption of an arched magnetic flux rope in a large ambient plasma has been studied in a laboratory experiment that simulates coronal loops. The eruption is initiated by laser generated plasma flows from the footpoints of the rope that significantly modify the magnetic-field topology and link the magnetic-field lines of the rope with the ambient plasma. Following this event, the flux rope erupts by releasing its plasma into the background. The resulting impulse excites intense magnetosonic waves that transfer energy to the ambient plasma and subsequently decay.

Tripathi, S. K. P.; Gekelman, W. [Physics and Astronomy, University of California at Los Angeles, Los Angeles, California 90095 (United States)

2010-08-13

344

Modification of solar wind flux by the Reiner Gamma magnetic anomaly  

NASA Astrophysics Data System (ADS)

The Moon possess regions of localized surface magnetic field called magnetic anomalies. The albedo of the lunar surface at some of these magnetic anomalies is much higher than the surrounding region. These high albedo regions are referred to as lunar swirls. One of the proposed methods for swirl formation is through the deflection of charged particles that can weather the surface by the anomalous magnetic field. I will present results from particle tracking studies for the Reiner Gamma magnetic anomaly. Launching of thousands of solar wind protons produces noticeable deflection by the anomalous magnetic field in a manner consistent with the high albedo regions. I will also present particle and energy flux calculations at the surface and infer how the spatial variation in particle and energy flux would modulate the space weathering of the surface.

Harnett, E. M.; Kramer, G. Y.

2011-12-01

345

Buried magnetic flux tubes in giant stars near the ``Coronal Dividing Line''  

NASA Astrophysics Data System (ADS)

We apply the ``solar paradigm'' for stellar magnetic activity to the post-main-sequence evolution of stars in the mass range 1 Msun <= Mstar <= 3 Msun. The model starts from a strong toroidal magnetic field generated by a dynamo working in the overshoot layer below the convection envelope. Once a critical field strength is exceeded, an undulatory (Parker-type) instability leads to rising flux loops. Upon emergence at the stellar surface, they form bipolar magnetic regions and large-scale coronal loops. By considering the stability, dynamics, and rise of magnetic flux tubes along evolutionary sequences of stellar models, we find that the flux loops become trapped in the stellar interior when the depth of convective envelope exceeds about 80% of the stellar radius. Trapping is caused by an increase of field line curvature at the loop summit, so that eventually the magnetic tension force dominates over the buoyancy force. The magnetic loops find a stable equilibrium configuration within the convection zone and do not emerge at the stellar surface. The transition from emerging to trapped flux tubes falls in the range of spectral types G7 to K0 for luminosity class III giants, which is close to the observed ``coronal dividing line'' in the Hertzsprung-Russell diagram. This result is remarkably stable within large ranges of stellar parameters (mass, rotation) and flux tube parameters (field strength, magnetic flux) and depends practically exclusively on the fractional radius of the stellar radiative core. We suggest that flux tube trapping is the cause for the strong decline of stellar X-ray emission across the ``coronal dividing line''.

Holzwarth, V.; Schüssler, M.

2001-10-01

346

Noise annoyance caused by magnetic levitation train passbys  

Microsoft Academic Search

In a laboratory study, the annoyance caused by the passby sounds from a magnetic levitation (maglev) train was investigated. The outdoor A-weighted sound exposure level (ASEL) of the maglev sounds varied from 65 to 90 dB. The driving speed of the maglev train varied from 100 to 400 km\\/h. Four important results were obtained. Provided that the outdoor ASELs were

Joos Vos

2001-01-01

347

Magnetic and spontaneous Barkhausen noise techniques used in investigation of a martensitic transformation  

NASA Astrophysics Data System (ADS)

Magnetic Barkhausen noise (MBN) was used to characterize the progress of austenite to martensite phase transformation while cooling steel specimens, using a conventional Barkhausen noise emission setup stimulated by an alternating magnetic field. The phase transformation was also followed by electrical resistivity measurements and by optical and scanning electron microscopy. MBN measurements on a AISI D2 tool steel austenitized at 1473 K and cooled to liquid nitrogen temperature presented a clear change near 225 K during cooling, corresponding to the MS (martensite start) temperature, as confirmed by resistivity measurements. Analysis of the resulting signals suggested a novel experimental technique that measures spontaneous magnetic emission during transformation, in the absence of any external field. Spontaneous magnetic noise emission measurements were registered in situ while cooling an initially austenitic sample in liquid nitrogen, showing that local microstructural changes, corresponding to an avalanche or ``burst'' phenomena, could be detected. This spontaneous magnetic emission (SME) can thus be considered a new experimental tool for the study of martensite transformations in ferrous alloys, at the same level as acoustic emission.

Capò Sànchez, J.; Huallpa, E.; Farina, P.; Padovese, L. R.; Goldenstein, H.

2011-10-01

348

Dynamics of magnetic flux loops in cooling-flow clusters of galaxies  

NASA Astrophysics Data System (ADS)

We study the dynamics of magnetic flux loops embedded in an intracluster medium. In order to perform the calculations semi-analytically we make several simplifying assumptions, which include a treatment of only the two ends of the magnetic flux loops, assuming a simple relation between the density and the magnetic field and neglecting heat conduction. Our results indicate that the existence of a large number of magnetic flux loops can naturally lead to a multiphase intracluster medium. A multiphase intracluster medium is inferred from observations of the mass deposition rate in many cooling-flow clusters. The loops will be observed as `filaments' having different density and temperature from their surroundings, namely X-ray filaments. X-ray filaments have been discovered in a few cases, although their discovery is controversial. Our model predicts that many such X-ray filaments will be discovered with future high-spatial-resolution X-ray telescopes, such as AXAF.

Zoabi, Essam; Soker, Noam; Regev, Oded

1998-05-01

349

Magnetic damping forces in figure-eight-shaped null-flux coil suspension systems  

SciTech Connect

This paper discusses magnetic damping forces in figure-eight-shaped null-flux coil suspension systems, focusing on the Holloman maglev rocket system. The paper also discusses simulating the damping plate, which is attached to the superconducting magnet by two short-circuited loop coils in the guideway. Closed-form formulas for the magnetic damping coefficient as functions of heave-and-sway displacements are derived by using a dynamic circuit model. These formulas are useful for dynamic stability studies.

He, Jianliang; Coffey, H.

1997-08-01

350

Seeding magnetic fields for laser-driven flux compression in high-energy-density plasmas  

Microsoft Academic Search

A compact, self-contained magnetic-seed-field generator (5 to 16 T) is the enabling technology for a novel laser-driven flux-compression scheme in laser-driven targets. A magnetized target is directly irradiated by a kilojoule or megajoule laser to compress the preseeded magnetic field to thousands of teslas. A fast (300 ns), 80 kA current pulse delivered by a portable pulsed-power system is discharged

O. V. Gotchev; J. P. Knauer; P. Y. Chang; N. W. Jang; M. J. Shoup; D. D. Meyerhofer; R. Betti

2009-01-01

351

Gaussian approximation and single-spin measurement in magnetic resonance force microscopy with spin noise  

SciTech Connect

A promising technique for measuring single electron spins is magnetic resonance force microscopy (MRFM), in which a microcantilever with a permanent magnetic tip is resonantly driven by a single oscillating spin. The most effective experimental technique is the oscillating cantilever-driven adiabatic reversals (OSCAR) protocol, in which the signal takes the form of a frequency shift. If the quality factor of the cantilever is high enough, this signal will be amplified over time to the point where it can be detected by optical or other techniques. An important requirement, however, is that this measurement process occurs on a time scale that is short compared to any noise which disturbs the orientation of the measured spin. We describe a model of spin noise for the MRFM system and show how this noise is transformed to become time dependent in going to the usual rotating frame. We simplify the description of the cantilever-spin system by approximating the cantilever wave function as a Gaussian wave packet and show that the resulting approximation closely matches the full quantum behavior. We then examine the problem of detecting the signal for a cantilever with thermal noise and spin with spin noise, deriving a condition for this to be a useful measurement.

Raghunathan, Shesha; Brun, Todd A. [Center for Quantum Information Science and Technology, Communication Sciences Institute, Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089 (United States); Goan, Hsi-Sheng [Department of Physics and Center for Theoretical Sciences, National Taiwan University, Taipei 10617, Taiwan (China); Center for Quantum Science and Engineering, National Taiwan University, Taipei 10617, Taiwan (China)

2010-11-15

352

Gaussian approximation and single-spin measurement in magnetic resonance force microscopy with spin noise  

NASA Astrophysics Data System (ADS)

A promising technique for measuring single electron spins is magnetic resonance force microscopy (MRFM), in which a microcantilever with a permanent magnetic tip is resonantly driven by a single oscillating spin. The most effective experimental technique is the oscillating cantilever-driven adiabatic reversals (OSCAR) protocol, in which the signal takes the form of a frequency shift. If the quality factor of the cantilever is high enough, this signal will be amplified over time to the point where it can be detected by optical or other techniques. An important requirement, however, is that this measurement process occurs on a time scale that is short compared to any noise which disturbs the orientation of the measured spin. We describe a model of spin noise for the MRFM system and show how this noise is transformed to become time dependent in going to the usual rotating frame. We simplify the description of the cantilever-spin system by approximating the cantilever wave function as a Gaussian wave packet and show that the resulting approximation closely matches the full quantum behavior. We then examine the problem of detecting the signal for a cantilever with thermal noise and spin with spin noise, deriving a condition for this to be a useful measurement.

Raghunathan, Shesha; Brun, Todd A.; Goan, Hsi-Sheng

2010-11-01

353

A chain of magnetic flux ropes in the magnetotail of Mars  

NASA Astrophysics Data System (ADS)

The interaction of Mars with the solar wind leads to the formation of a magnetotail through which significant quantities of planetary plasma are transported. Of particular interest is the extent to which this plasma transport could be affected by magnetotail dynamics, for example by magnetic reconnection and flux rope formation. Here we show observations from Mars Global Surveyor of multiple flux ropes observed in Mars' magnetotail current sheet. A chain of at least three flux ropes is observed; based on the geometry of the encounter, the flux ropes are all being ejected in the same direction from a single dominant site and modeling shows that at least two of the flux ropes are close to being in a force free condition. Given geometrical considerations, it is likely that the flux ropes are generated sequentially rather than simultaneously, suggesting periodic generation via secondary instabilities at the reconnection site.

Eastwood, J. P.; Videira, J. J. H.; Brain, D. A.; Halekas, J. S.

2012-02-01

354

Vector magnetic fields of emerging solar flux. I. Properties at the site of emergence  

NASA Astrophysics Data System (ADS)

Several small emerging bipolar regions have been observed with the Advanced Stokes Polarimeter (ASP), including extensive time series measurements of one small region. Both new and previously recognized properties of the actual site of first emergence, where the magnetic field is nearly horizontal to the surface, are revealed by these observations. They provide the most complete and accurate observational description to date of newly emerging vector magnetic fields. We find that: 1) the strength of the magnetic field at the site of the emergence (where the vector field is nearly parallel to the solar surface) ranges from about 200 to 600 G, 2) as individual flux elements migrate rapidly away from the emergence zone, they attain kiloGauss strengths only after becoming oriented nearly vertically, 3) the emergence zone is dotted by small, transient, upward rising ( ~ 1 km s(-1) ) horizontal magnetic elements as indicated by the Doppler shift of the polarized spectral profiles, 4) the leading polarity flux coalesces immediately into a compact region which forms a pore, but the emerging following polarity flux is spatially much less compact, 5) some ``moving magnetic features'' having the same magnetic polarity as the growing pore, but on the opposite side of the pore from the emergence zone, coalesce with the pore during the observation period, and 6) the observations suggest a low canopy of weak horizontal magnetic fields arches over the emergence zone. These observations support a widely accepted picture of emerging bipolar flux: the buoyantly rising flux transports mass from the photosphere into the chromosphere, where it then may drain downward along arched magnetic loops. The observed formation of a pore suggests that emergence of subsurface structure, not organized flows near the surface, is largely responsible for the apparent coalescence of sunspots from more diffuse fields viewed at the solar surface. These observations neither confirm nor refute the operation of convective collapse of flux tubes.

Lites, B. W.; Skumanich, A.; Martinez Pillet, V.

1998-05-01

355

Quantitative modeling of magnetic reconnection creating a twisted flux rope  

Microsoft Academic Search

Coronal mass ejections (CMEs) are one of the key elements coupling solar activity throughout the heliosphere. A two-dimensional model, called CSHKP has been used to explain the observed sequence of events in a typical CME. They are launched from the solar corona above reversals in photospheric magnetic polarity. Chromospheric emission during the accompanying two-ribbon flare provides evidence of magnetic reconnection.

D. Longcope

2008-01-01

356

An accurate symplectic calculation of the inboard magnetic footprint from statistical topological noise and field errors in the DIII-D  

SciTech Connect

Any canonical transformation of Hamiltonian equations is symplectic, and any area-preserving transformation in 2D is a symplectomorphism. Based on these, a discrete symplectic map and its continuous symplectic analog are derived for forward magnetic field line trajectories in natural canonical coordinates. The unperturbed axisymmetric Hamiltonian for magnetic field lines is constructed from the experimental data in the DIII-D [J. L. Luxon and L. E. Davis, Fusion Technol. 8, 441 (1985)]. The equilibrium Hamiltonian is a highly accurate, analytic, and realistic representation of the magnetic geometry of the DIII-D. These symplectic mathematical maps are used to calculate the magnetic footprint on the inboard collector plate in the DIII-D. Internal statistical topological noise and field errors are irreducible and ubiquitous in magnetic confinement schemes for fusion. It is important to know the stochasticity and magnetic footprint from noise and error fields. The estimates of the spectrum and mode amplitudes of the spatial topological noise and magnetic errors in the DIII-D are used as magnetic perturbation. The discrete and continuous symplectic maps are used to calculate the magnetic footprint on the inboard collector plate of the DIII-D by inverting the natural coordinates to physical coordinates. The combination of highly accurate equilibrium generating function, natural canonical coordinates, symplecticity, and small step-size together gives a very accurate calculation of magnetic footprint. Radial variation of magnetic perturbation and the response of plasma to perturbation are not included. The inboard footprint from noise and errors are dominated by m=3, n=1 mode. The footprint is in the form of a toroidally winding helical strip. The width of stochastic layer scales as (1/2) power of amplitude. The area of footprint scales as first power of amplitude. The physical parameters such as toroidal angle, length, and poloidal angle covered before striking, and the safety factor all have fractal structure. The average field diffusion near the X-point for lines that strike and that do not strike differs by about three to four orders of magnitude. The magnetic footprint gives the maximal bounds on size and heat flux density on collector plate.

Punjabi, Alkesh; Ali, Halima [Hampton University, Hampton, Virginia 23668 (United States)

2011-02-15

357

Use of superconducting plates and shells to deflect magnetic noise fields: Application to MEG (magnetoencephalography)  

SciTech Connect

Insertion of a superconducting plate or shell in a magnetic field causes a surface current distribution which opposes the applied field. Resultant fields near plate edges are higher while those near centers are much lower than applied fields. We make use of these principles in MEG by placing suitably oriented gradiometers at points where noise fields are smallest. Signals from nearby brain wave sources are enhanced because the net signal is a combination of that from the sources and that from its image. The principles of noise deflection and source imaging are applied to new concepts of gradiometry. 4 refs., 2 figs.

Overton, W.C. Jr.; van Hulsteyn, D.B.; Flynn, E.R. (Los Alamos National Lab., NM (USA))

1989-01-01

358

Use of superconducting plates and shells to deflect magnetic noise fields: Application to MEG  

NASA Astrophysics Data System (ADS)

Insertion of a superconducting plate or shell in a magnetic field causes a surface current distribution which opposes the applied field. Resultant fields near plate edges are higher while those near centers are much lower than applied fields. Use is made of these principles in magnetoencephalography by placing suitably oriented gradiometers at points where noise fields are smallest. Signals from nearby brain wave sources are enhanced because the net signal is a combination of that from the sources and that from its image. The principles of noise deflection and source imaging are applied to new concepts of gradiometry.

Overton, William Calvin, Jr.; Vanhulsteyn, David B.; Flynn, Edward R.

359

Magnetic flux shields of superconducting niobium-titanium alloys  

SciTech Connect

The flux shielding characteristics of NbTi alloy tubes are investigated in both axial and transverse applied fields by a field mapping technique using miniature Hall generators. Bulk Nb65w/oTi tubes without cold work are shown to be able to exclude axial applied fields of over 1.73 tesla and transverse applied fields of up to 0.52 tesla. A jelly-roll tube of NbTi and Cu is successful in shielding a transverse field of 0.72 tesla without flux jumps. The distribution of shielding supercurrents in the tube wall are analytically derived from the field profile measured inside or around the flux exclusion tube. It is found that the induced supercurrents in an axial applied field always concentrate near the tube ends and cause these end areas to be prone to partial or complete flux jumps. The effectiveness of the critical state concept and Kim's critical current model is fully verified in our NbTi alloy tubular system. A phenomenological model of the supercurrent distribution in the tube wall is proposed to interpret the experimental results in the transverse fields. The influence of alloy composition and heat treatments on the shielding performance of NbTi alloy tubes is experimentally studied.

Yu, D.

1983-01-01

360

Colliding Magnetic Flux Ropes and Quasi-Separatrix Layers in a Laboratory Plasma  

NASA Astrophysics Data System (ADS)

An experimental study of the dynamics of colliding magnetic flux ropes and the magnetic reconnection that occurs during these collisions is presented. A magnetic flux rope is a bundle of twisted magnetic field lines that is ubiquitous in space and solar plasmas. The flux ropes are created in the Large Plasma Device (LAPD) using two heated lanthanum hexaboride (LaB6) cathodes that inject currents into the background plasma. The currents are initially parallel to the background magnetic field. The azimuthal field of each current together with the background axial field create helical twisted flux ropes. It is found that the flux ropes rotate in time (corkscrew) and collide with each other. During a collision, antiparallel magnetic fields can undergo magnetic reconnection. When these collisions occur, we observe current layers flowing in the opposite direction of the injected current, a signatuare of reconnection. Analysis of the three-dimensional magnetic field lines shows the existence of quasi-separatrix layers (QSLs). These are regions in the magnetic configuration where there are large spatial gradients in the connectivity of field line footpoints in the boundary surfaces. QSLs are thought to be favorable sites for magnetic reconnection. It is shown that the location and shape of the QSL is similar to what is seen in simulations of merging flux ropes. Furthermore, the field line structure of the QSL is similar to that of a twisted hyperbolic flux tube (HFT). An HFT is a type of QSL that has been shown to be a preferred site for current sheet formation in simulations of interacting coronal loops. The HFT in this experiment is found to be generally near the reverse current layers, although the agreement is not perfect. Looking at the time evolution of the QSL, we find that the QSL cross-sectional area grows and contracts at the same time that the flux ropes collide and that the reverse current layers appear. Analysis of the field line motion shows that, during reconnection, bundles of field lines rapidly flip across the QSLs. This is analagous to the way that field lines are pushed across a separatrix in 2D reconnection.

Lawrence, Eric Eugene

361

Calculation of Boozer magnetic coordinates for multiple plasma regions (with either closed or open flux surfaces) connected by magnetic separatrices  

SciTech Connect

Magnetic coordinates ({psi}{sub T}=radial label of flux surfaces, {theta}=poloidal, and {phi}=toroidal angle) are introduced in toroidal magnetoplasma equilibria in order to straighten the field lines [described by: {theta}-{iota}({psi}{sub T}){phi}=constant on any flux surface, {iota}/({psi}{sub T}) being the rotational transform]. The simplest method for analyzing the ideal magnetohydrodynamic (MHD) stability expands the perturbed plasma displacement {xi}-vector in magnetic coordinates and solves the normal mode equation through one-dimensional (1D) radial finite elements. This paper extends the calculation of (Boozer) magnetic coordinates to simply connected equilibria that embed a magnetic separatrix, with regular X-points (B-vector{ne}0), and reach the symmetry axis, with singular magnetic X-points (B-vector=0). These configurations include multiple plasma regions, whose outermost one (surrounding plasma) is not composed by toroidal surfaces closed around a single magnetic axis. Two examples are chosen: (i) flux-core-spheromak (FCS) configurations, where the surrounding plasma is a screw pinch, with open flux surfaces; (ii) Chandrasekhar-Kendall-Furth (CKF) configurations, where it is a toroidal shell, carved by multiple toroidal plasma regions. This paper shows that a proper ordering of the radial coordinate {psi}{sub T}, the requirement of continuity for {theta} and {phi} and an {iota} matching condition (between neighboring mesh points on opposite sides of the connecting separatrix) resolve the ambiguities in the definition of magnetic coordinates in both CKF and FCS cases. However, a few metric coefficients diverge at the separatrices; therefore, often numerical MHD stability codes do not use magnetic coordinates there, but adopt local two-dimensional (2D) finite elements. This paper instead investigates all the divergences, in order to allow for the asymptotic analysis of {xi}-vector near the separatrices, with the purpose of maintaining the magnetic coordinate method and the 1D radial finite elements in the ideal MHD stability analysis.

Alladio, F.; Mancuso, A.; Micozzi, P.; Rogier, F. [Associazione EURATOM-ENEA sulla Fusione, CR Frascati, C.P. 65-00044, Frascati, Rome (Italy); ONERA-CERT/DTIM/M2SN 2, avenue Edouard Belin-BP 4025-31055, Toulouse Cedex 4 (France)

2005-11-15

362

Tunable magnetic flux sensor using a metallic Rashba ring with half-metal electrodes  

SciTech Connect

We propose a magnetic field sensor consisting of a square ring made of metal with a strong Rashba spin-orbital coupling (RSOC) and contacted to half-metal electrodes. Due to the Aharonov-Casher effect, the presence of the RSOC imparts a spin-dependent geometric phase to conduction electrons in the ring. The combination of the magnetic flux emanating from the magnetic sample placed below the ring, and the Aharonov-Casher effect due to RSOC results in spin interference, which modulates the spin transport in the ring nanostructure. By using the tight-binding nonequilibrium Green's function formalism to model the transport across the nanoring detector, we theoretically show that with proper optimization, the Rashba ring can function as a sensitive and tunable magnetic probe to detect magnetic flux.

Chen, J. [Computational Nanoelectronics and Nano-Device Laboratory, Electrical and Computer Engineering Department, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 (Singapore); Jalil, M. B. A. [Computational Nanoelectronics and Nano-Device Laboratory, Electrical and Computer Engineering Department, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 (Singapore); Information Storage Materials Laboratory, Electrical and Computer Engineering Department, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 (Singapore); Tan, S. G. [Computational Nanoelectronics and Nano-Device Laboratory, Electrical and Computer Engineering Department, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 (Singapore); Data Storage Institute, A-STAR (Agency for Science, Technology and Research) DSI Building, 5 Engineering Drive 1, Singapore 117608 (Singapore)

2011-04-01

363

Numerical Prediction of Signal for Magnetic Flux Leakage Benchmark Task  

NASA Astrophysics Data System (ADS)

Numerical results predicted by the finite element method based code are presented. The nonlinear magnetic time-dependent benchmark problem proposed by the World Federation of Nondestructive Evaluation Centers, involves numerical prediction of normal (radial) component of the leaked field in the vicinity of two practically rectangular notches machined on a rotating steel pipe (with known nonlinear magnetic characteristic). One notch is located on external surface of pipe and other is on internal one, and both are oriented axially.

Lunin, V.; Alexeevsky, D.

2003-03-01

364

A tubular flux-switching permanent magnet machine  

NASA Astrophysics Data System (ADS)

The paper describes a novel tubular, three-phase permanent magnet brushless machine, which combines salient features from both switched reluctance and permanent magnet machine technologies. It has no end windings and zero net radial force and offers a high power density and peak force capability, as well as the potential for low manufacturing cost. It is, therefore, eminently suitable for a variety of applications, ranging from free-piston energy converters to active vehicle suspensions.

Wang, J.; Wang, W.; Clark, R.; Atallah, K.; Howe, D.

2008-04-01

365

Design of the Mark 101 magnetic flux compression generator  

SciTech Connect

The Mark 101 explosive flux compression generator was designed to generate high voltages into external loads. The stators are helically wound but the armature explosive is simultaneously initiated along its axis. Thus, the generator can possess the large inductance of conventional helical generators, but the burntime is substantially reduced. This leads to large generator impedances that lead to high voltage generation. The armature wall-thickness is varied in such a fashion that it leaves the load output aperture open as the armature starts contact with the stator. Calculations are given for a number of designs in which both the number of stator turns and load inductances are varied. It is also noted that flux losses in the generator skin can be significant.

Fowler, C.M.; Caird, R.S.; Freeman, B.L.; Marsh, S.P.

1986-01-01

366

Modification of the Magnetic Flux-Line Interaction at a Superconductor's Surface  

Microsoft Academic Search

The pair interaction between magnetic flux lines in a semi-infinite slab of an anisotropic type-II superconductor in an external field is derived in the London limit. The case where the applied field is normal to the superconductor\\/vacuum interface is considered. The presence of stray fields near the surface leads to an additional contribution to the repulsive interaction between flux lines

M. Cristina Marchetti

1992-01-01

367

Electronic states in continuous random networks: Rivier lines as half-integer magnetic flux lines  

Microsoft Academic Search

We develop a simple effective-mass formalism for studying the effect of the Rivier lines (odd lines) on the electronic states in a tight-binding model defined for continuous random networks. Rivier lines act like infinitely thin fictitious solenoids carrying half-integer magnetic flux quanta for states close to the antibonding end of the band. The electron feels the effect of these flux

G. Baskaran

1986-01-01

368

Bulk properties, structure and pinning of magnetic flux-line arrays in high-temperature superconductors  

Microsoft Academic Search

A serious impediment to many potential applications of the high-Tsb{c} copper oxide superconductors is the relative ease with which magnetic flux-lines move within these materials, thereby producing finite electrical resistance. To devise methods for rigidly fixing flux-lines in these materials, which is necessary to achieve a truly superconducting state, requires an understanding of their fundamental structural properties. The most direct

Zhen Yao

1997-01-01

369

Modification of the magnetic flux-line interaction at a superconductor's surface  

Microsoft Academic Search

The pair interaction between magnetic flux-lines in a semi-infinite slab of an anisotropic type-II superconductor in an external field is derived in the London limit. The case where the applied field is normal to the superconductor\\/vacuum interface is considered. The presence of stray fields near the surface leads to an additional contribution to the repulsive interaction between flux lines that

M. Cristina Marchetti

1992-01-01

370

Methods for Computation and Visualization of Magnetic Flux Lines in 3-D  

Microsoft Academic Search

Field line visualization provides a valuable insight into complex flow patterns in electromagnetic devices. In this paper, a method for the construction of closed flux lines in 3-D quasi-static electromagnetic fields is presented. The seeding, i.e., the selection of which field line to represent is performed by weighting the magnetic flux on a specified cutting surface. Due to the discretization

Martin Hafner; Marc Schoning; Marcin Antczak; Andrzej Demenko; Kay Hameyer

2010-01-01

371

Path of magnetic flux lines through high-Tc copper oxide superconductors  

Microsoft Academic Search

A SERIOUS impediment to many potential applications of the high-transition-temperature (high-Tc) copper oxide superconductors is the relative ease with which magnetic flux lines move within these materials, thereby producing finite electrical resistance1,2. To devise methods for rigidly fixing flux lines in these materials, which is necessary to achieve a truly superconducting (zero resistance) state, requires an understanding of their fundamental

Zhen Yao; Seokwon Yoon; Hongjie Dai; Shoushan Fan; Charles M. Lieber

1994-01-01

372

Simulation of a radial flux permanent magnet generator with two contra-rotating rotors  

Microsoft Academic Search

In this paper, the no-load operational characteristics of radial-flux permanent magnet generators with two contra-rotating rotors are addressed. Three different radial flux machine topologies have been qualitatively studied through a sequence of stationary two dimensional finite element field solutions. It is shown that the peak value of the induced phase electromotive force during synchronous operation depend on the angular positions

Martin Ranlöf; Gary Connor; Urban Lundin

2008-01-01

373

An axial-flux permanent-magnet generator for a gearless wind energy system  

Microsoft Academic Search

The paper discusses the development of an axial-flux permanent-magnet generator for a gearless wind energy system which aims to demonstrate the feasibility of integrating wind and photovoltaic energy converters for the generation of electricity and to achieve optimum exploitation of the two energy sources. The merits of an axial-flux generator topology are discussed with reference to the particular requirements of

B. J. Chalmers; W. Wu; E. Spooner

1996-01-01

374

An outer-rotor flux-switching permanent magnet machine for traction applications  

Microsoft Academic Search

A flux-switching permanent magnet (FSPM) machine with an outer-rotor configuration is proposed for traction applications. The effects of the pole and slot numbers on back-EMF and cogging torque are studied, and sizing equations are derived to determine the machine dimensions. Finite element analysis (FEA) models are developed to optimize the machine performance including flux-weakening capability, and core losses are also

Yu Wang; Meng-Jia Jin; Jian-Xin Shen; W. Z. Fei; P. C. K. Luk

2010-01-01

375

Minimization of Cogging Torque in Axial-Flux Permanent-Magnet Machines: Design Concepts  

Microsoft Academic Search

Various techniques exist for reducing the cogging torque in radial-flux permanent-magnet (RFPM) machines. However, although some of these can be applied to axial-flux PM (AFPM) machines, the additional manufacturing complexity and cost may be prohibitive. Therefore, alternative low-cost techniques are desirable for use in AFPM machines. In this paper, the utility of employing cogging torque minimization techniques that have been

M. Aydin; Z. Q. Zhu; T.A. Lipo; D. Howe

2007-01-01

376

NUMERICAL EXPERIMENTS ON THE TWO-STEP EMERGENCE OF TWISTED MAGNETIC FLUX TUBES IN THE SUN  

SciTech Connect

We present the new results of the two-dimensional numerical experiments on the cross-sectional evolution of a twisted magnetic flux tube rising from the deeper solar convection zone (-20,000 km) to the corona through the surface. The initial depth is 10 times deeper than most of the previous calculations focusing on the flux emergence from the uppermost convection zone. We find that the evolution is illustrated by the following two-step process. The initial tube rises due to its buoyancy, subject to aerodynamic drag due to the external flow. Because of the azimuthal component of the magnetic field, the tube maintains its coherency and does not deform to become a vortex roll pair. When the flux tube approaches the photosphere and expands sufficiently, the plasma on the rising tube accumulates to suppress the tube's emergence. Therefore, the flux decelerates and extends horizontally beneath the surface. This new finding owes to our large-scale simulation, which simultaneously calculates the dynamics within the interior as well as above the surface. As the magnetic pressure gradient increases around the surface, magnetic buoyancy instability is triggered locally and, as a result, the flux rises further into the solar corona. We also find that the deceleration occurs at a higher altitude than assumed in our previous experiment using magnetic flux sheets. By conducting parametric studies, we investigate the conditions for the two-step emergence of the rising flux tube: field strength {approx}> 1.5 x 10{sup 4} G and the twist {approx}> 5.0 x 10{sup -4} km{sup -1} at -20,000 km depth.

Toriumi, S.; Yokoyama, T., E-mail: toriumi@eps.s.u-tokyo.ac.jp [Department of Earth and Planetary Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)

2011-07-10

377

Moving magnetic tubes: fragmentation, vortex streets and the limit of the approximation of thin flux tubes  

NASA Astrophysics Data System (ADS)

Aims.We study the buoyant rise of magnetic flux tubes in a stratified layer over a range of Reynolds numbers (25 ? Re ? 2600) by means of numerical simulations. Special emphasis is placed on studying the fragmentation of the rising tube, its trailing wake and the formation of a vortex street in the high-Reynolds number regime. Furthermore, we evaluate the relevance of the thin flux tube approximation with regard to describing the evolution of magnetic flux tubes in the simulations. Methods: .We used the FLASH code, which has an adaptive mesh refinement (AMR) algorithm, thus allowing the simulations to be carried out at high Reynolds numbers. Results: .The evolution of the magnetic flux tube and its wake depends on the Reynolds number. At Re up to a few hundred, the wake consists of two counter-rotating vortex rolls. At higher Re, the vortex rolls break up and the shedding of flux into the wake occurs in a more intermittent fashion. The amount of flux retained by the central portion of the tube increases with the field line twist (in agreement with previous literature) and with Re. The time evolution of the twist is compatible with a homologous expansion of the tube. The motion of the central portion of the tube in the simulations is very well described by the thin flux tube model whenever the effects of flux loss or vortex forces can be neglected. If the flux tube has an initial net vorticity, it undergoes asymmetric vortex shedding. In this case, the lift force accelerates the tube in such a way that an oscillatory horizontal motion is super-imposed on the vertical rise of the tube, which leaves behind a vortex street. This last result is in accordance with previous simulations reported in the literature, which were carried out at lower Reynolds number.

Cheung, M. C. M.; Moreno-Insertis, F.; Schüssler, M.

2006-05-01

378

Noise annoyance caused by magnetic levitation train passbys  

NASA Astrophysics Data System (ADS)

In a laboratory study, the annoyance caused by the passby sounds from a magnetic levitation (maglev) train was investigated. The outdoor A-weighted sound exposure level (ASEL) of the maglev sounds varied from 65 to 90 dB. The driving speed of the maglev train varied from 100 to 400 km/h. Four important results were obtained. Provided that the outdoor ASELs were the same, (1) the annoyance was independent of the driving speed of the maglev train, (2) the annoyance caused by the maglev train was considerably higher than that caused by intercity trains, (3) the annoyance caused by the maglev train was hardly different from that caused by road traffic (passenger cars and trucks), and (4) the results (1)-(3) held true both for open or closed windows. On the basis of the present results, it might be expected that the sounds are equally annoying if the ASELs of the maglev-train passbys are at least 5 dB lower than those of the intercity train passbys. Consequently, the results of the present experiment do not support application of a railway bonus to the maglev-train sounds. Issues for future research, such as exploring further contributions of nonacoustic factors, will be discussed.

Vos, Joos

2001-05-01

379

ABSORPTION OF p MODES BY THIN MAGNETIC FLUX TUBES  

SciTech Connect

We study the interaction between p modes and the many magnetic fibrils that lace the solar convection zone. In particular, we investigate the resulting absorption of p-mode energy by the fibril magnetic field. Through mechanical buffeting, the p modes excite tube waves on the magnetic fibrils-in the form of longitudinal sausage waves and transverse kink waves. The tube waves propagate up and down the magnetic fibrils and out of the p-mode cavity, thereby removing energy from the incident acoustic waves. We compute the absorption coefficient associated with this damping mechanism and model the absorption that would be observed for magnetic plage. We compare our results to the absorption coefficient that is measured using the local-helioseismic technique of ridge-filtered holography. We find that, depending on the mode order and the photospheric boundary conditions, we can achieve absorption coefficients for simulated plage that exceed 50%. The observed increase of the absorption coefficient as a function of frequency is reproduced for all model parameters.

Jain, Rekha [Applied Mathematics Department, University of Sheffield, Sheffield S3 7RH (United Kingdom); Hindman, Bradley W. [JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0440 (United States); Braun, Doug C.; Birch, Aaron C. [NWRA CoRA Division, Boulder, CO 80301 (United States)], E-mail: R.Jain@sheffield.ac.uk

2009-04-10

380

Aurora and open magnetic flux during isolated substorms, sawteeth, and SMC events  

NASA Astrophysics Data System (ADS)

Using Polar UVI LBHl and IMAGE FUV WIC data, we have compared the auroral signatures and polar cap open flux for isolated substorms, sawteeth oscillations, and steady magnetospheric convection (SMC) events. First, a case study of each event type is performed, comparing auroral signatures and open magnetic fluxes to one another. The latitude location of the auroral oval is similar during isolated substorms and SMC events. The auroral intensity during SMC events is similar to that observed during the expansion phase of an isolated substorm. Examination of an individual sawtooth shows that the auroral intensity is much greater than the SMC or isolated substorm events and the auroral oval is displaced equatorward making a larger polar cap. The temporal variations observed during the individual sawtooth are similar to that observed during the isolated substorm, and while the change in polar cap flux measured during the sawtooth is larger, the percent change in flux is similar to that measured during the isolated substorm. These results are confirmed by a statistical analysis of events within these three classes. The results show that the auroral oval measured during individual sawteeth contains a polar cap with, on average, 150% more magnetic flux than the oval measured during isolated substorms or during SMC events. However, both isolated substorms and sawteeth show a 30% decrease in polar cap magnetic flux during the dipolarization (expansion) phase.

Dejong, A. D.; Cai, X.; Clauer, R. C.; Spann, J. F.

2007-08-01

381

An approach to improving the signal-to-optical-noise ratio of pulsed magnetic field photonic sensors  

Microsoft Academic Search

During last years, interest in pulsed magnetic field sensors has widely increased. In fact, magnetic field measurement has a critical part in various scientific and technical areas. In order to research on pulsed magnetic field characteristic and corresponding measuring and defending means, a sensor with high immunity to electrical noise, high sensitivity, high accuracy and wide dynamic range is needed.

Jiang-Ping Wang; Yu-Quan Li

2008-01-01

382

Profile structure of magnetic flux lines in type-II superconductor from a rectangular electron hologram.  

PubMed

Profile magnetic configuration of a quantized flux line and flux-line lattice penetrating a type-II superconductor thin foil, niobium, was observed by electron holography and Lorentz microscopy using a 300 kV field-emission electron microscope. Each single flux line was distributed periodically as lattice structure in the mixed state just below its critical temperature of 8.5 K, while at low temperature of 5 K the flux lines were weakly bound as bundles. In order to observe the flux-line distribution over a wide area, discrete Fourier transform reconstruction in the holography was extended for a rectangular area without loss of information and data precision. PMID:14599098

Harada, Ken; Beleggia, Marco; Endo, Junji; Kasai, Hiroto; Togawa, Yoshihiko; Matsuda, Tsuyoshi; Tonomura, Akira

2003-01-01

383

Multiple Triangulation Analysis: another approach to determine the orientation of magnetic flux ropes  

NASA Astrophysics Data System (ADS)

Another approach (Multiple Triangulation Analysis, MTA) is presented to determine the orientation of magnetic flux rope, based on 4-point measurements. A 2-D flux rope model is used to examine the accuracy of the MTA technique in a theoretical way. It is found that the precision of the estimated orientation is dependent on both the spacecraft separation and the constellation path relative to the flux rope structure. However, the MTA error range can be shown to be smaller than that of the traditional MVA technique. As an application to real Cluster data, several flux rope events on 26 January 2001 are analyzed using MTA, to obtain their orientations. The results are compared with the ones obtained by several other methods which also yield flux rope orientation. The estimated axis orientations are shown to be fairly close, suggesting the reliability of the MTA method.

Zhou, X.-Z.; Zong, Q.-G.; Pu, Z. Y.; Fritz, T. A.; Dunlop, M. W.; Shi, Q. Q.; Wang, J.; Wei, Y.

2006-07-01

384

Disc formation in turbulent cloud cores: is magnetic flux loss necessary to stop the magnetic braking catastrophe or not?  

NASA Astrophysics Data System (ADS)

Recent numerical analysis of Keplerian disc formation in turbulent, magnetized cloud cores by Santos-Lima et al. demonstrated that reconnection diffusion is an efficient process to remove the magnetic flux excess during the buildup of a rotationally supported disc. This process is induced by fast reconnection of the magnetic fields in a turbulent flow. In a similar numerical study, Seifried et al. concluded that reconnection diffusion or any other non-ideal magnetohydrodynamic effects would not be necessary and turbulence shear alone would provide a natural way to build up a rotating disc without requiring magnetic flux loss. Their conclusion was based on the fact that the mean mass-to-flux ratio (?) evaluated over a spherical region with a radius much larger than the disc is nearly constant in their models. In this paper, we compare the two sets of simulations and show that this averaging over large scales can mask significant real increases of ? in the inner regions where the disc is built up. We demonstrate that turbulence-induced reconnection diffusion of the magnetic field happens in the initial stages of the disc formation in the turbulent envelope material that is accreting. Our analysis is suggestive that reconnection diffusion is present in both sets of simulations and provides a simple solution for the `magnetic braking catastrophe' which is discussed in the literature in relation to the formation of protostellar accretion discs.

Santos-Lima, R.; de Gouveia Dal Pino, E. M.; Lazarian, A.

2013-03-01

385

Magnetic energy fluxes in sub-Alfvénic planet star and moon planet interactions  

NASA Astrophysics Data System (ADS)

Context. Electromagnetic coupling of planetary moons with their host planets is well observed in our solar system. Similar couplings of extrasolar planets with their central stars have been studied observationally on an individual as well as on a statistical basis. Aims: We aim to model and to better understand the energetics of planet star and moon planet interactions on an individual and as well as on a statistical basis. Methods: We derived analytic expressions for the Poynting flux communicating magnetic field energy from the planetary obstacle to the central body for sub-Alfvénic interaction. We additionally present simplified, readily useable approximations for the total Poynting flux for small Alfvén Mach numbers. These energy fluxes were calculated near the obstacles and thus likely present upper limits for the fluxes arriving at the central body. We applied these expressions to satellites of our solar system and to HD 179949 b. We also performed a statistical analysis for 850 extrasolar planets. Results: Our derived Poynting fluxes compare well with the energetics and luminosities of the satellites' footprints observed at Jupiter and Saturn. We find that 295 of 850 extrasolar planets are possibly subject to sub-Alfvénic plasma interactions with their stellar winds, but only 258 can magnetically connect to their central stars due to the orientations of the associated Alfvén wings. The total energy fluxes in the magnetic coupling of extrasolar planets vary by many orders of magnitude and can reach values larger than 1019 W. Our calculated energy fluxes generated at HD 179949 b can only explain the observed energy fluxes for exotic planetary and stellar magnetic field properties. In this case, additional energy sources triggered by the Alfvén wave energy launched at the extrasolar planet might be necessary. We provide a list of extrasolar planets where we expect planet star coupling to exhibit the largest energy fluxes. As supplementary information we also attach a table of the modeled stellar wind plasma properties and possible Poynting fluxes near all 850 extrasolar planets included in our study. Conclusions: The orders of magnitude variations in the values for the total Poynting fluxes even for close-in extrasolar planets provide a natural explanation why planet star coupling might have been only observable on an individual basis but not on a statistical basis. Estimated plasma parameters and their associated Poynting fluxes are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/552/A119

Saur, J.; Grambusch, T.; Duling, S.; Neubauer, F. M.; Simon, S.

2013-04-01

386

Local Observation of Field Polarity Dependent Flux Pinning by Magnetic Dipoles.  

PubMed

A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field. PMID:11136117

Van Bael MJ; Bekaert; Temst; Van Look L; Moshchalkov; Bruynseraede; Howells; Grigorenko; Bending; Borghs

2001-01-01

387

Influence of growth and annealing conditions on low-frequency magnetic 1/f noise in MgO magnetic tunnel junctions  

SciTech Connect

Magnetic 1/f noise is compared in magnetic tunnel junctions with electron-beam evaporated and sputtered MgO tunnel barriers in the annealing temperature range 350 - 425 Degree-Sign C. The variation of the magnetic noise parameter ({alpha}{sub mag}) of the reference layer with annealing temperature mainly reflects the variation of the pinning effect of the exchange-bias layer. A reduction in {alpha}{sub mag} with bias is associated with the bias dependence of the tunneling magnetoresistance. The related magnetic losses are parameterized by a phase lag {epsilon}, which is nearly independent of bias especially below 100 mV. The similar changes in magnetic noise with annealing temperature and barrier thickness for two types of MgO magnetic tunnel junctions indicate that the barrier layer quality does not affect the magnetic losses in the reference layer.

Feng Jiafeng; Diao Zhu; Kurt, Huseyin; Singh, A.; Coey, J. M. D. [CRANN and School of Physics, Trinity College, Dublin 2 (Ireland); Stearrett, Ryan; Nowak, Edmund R. [Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716 (United States)

2012-11-01

388

Magnetic field in an elliptic flux rope: a generalization of the Lundquist solution  

NASA Astrophysics Data System (ADS)

Magnetic fields in interplanetary flux ropes are commonly described by a force-free solution with constant alpha in a circular cylinder found by Lundquist (1950). However, both MHD simulations (Vandas et al., 1995, 2002; Cargill et al., 1996) and detailed analyses of spacecraft measurements (Mulligan & Russell, 2001) indicate that interplanetary flux ropes have not a circular cross section, but they are quite oblate. We present a solution for force-free magnetic field with constant alpha in an elliptic cylinder. This analytical solution can be regarded as a generalization of the Lundquist solution.

Vandas, M.; Romashets, E. P.

2002-12-01

389

Statistics of magnetic fields and fluxes of massive OB stars and the origin of neutron star magnetic fields  

NASA Astrophysics Data System (ADS)

Based on the newest measurements, statistical properties of rms mean magnetic fields of OB and neutron stars (NSs) were investigated. The magnetic field distribution function f(B) for OB stars was determined and a sharp decrease of f(B) for weak magnetic fields was found. The mean magnetic fluxes F for all massive stars and NSs with measured magnetic fields was estimated, and it was found that log F = 27.7 for OB stars and log F = 24.5 for NSs. To explain the large differences of the fluxes from normal and neutron stars we studied the birth and evolution of isolated neutron stars in the whole volume of our Galaxy with our new code of population synthesis. We started modeling %with our code from the birth of massive OB stars and followed their motion within the spiral arms to the point of supernova explosion. Next we considered the evolution of NS up to the death line with considering the magnetic field decay. We found that a significant magnetic field decay occurs during the first million years of a NS's life. We have estimated the mean time of the Ohmic decay for NS. We modeled the distributions of pulsar periods P, of period derivatives \\dot P, and of pulsar magnetic fields B, and found that they are in a good agreement with those taken from \\cite{ATNF}.

Igoshev, A. P.; Kholtygin, A. F.

2011-12-01

390

Flux Transport and the Sun's Polar Magnetic Fields at Cycle 23/24 Minimum  

NASA Astrophysics Data System (ADS)

The polar magnetic fields observed on the Sun during the Cycle 23/24 minimum were substantially weaker than those observed for the previous three minima. In most dynamo models weak polar fields result in weak following cycles, which are observed to start late and leave behind a long low minimum. While this accounts for the peculiarities observed during the Cycle 23/24 minimum, it begs an explanation for why the polar magnetic fields were so weak. Here we model the transport of magnetic flux from active region sources via the observed magnetic element differential rotation and meridional flow from 1996 to 2011 (Hathaway & Rightmire ApJ 729:80, 2011). The active region sources are characterized by the observed sizes and locations of the leading and following polarity magnetic flux. Both the sources of the flux and the flows that transport them are fully constrained by observations. Comparisons will be made between the observed and modeled polar field reversals and the final polar field strengths. Conclusions will be drawn concerning the need for additional flux transport processes.

Upton, L. A.; Hathaway, D. H.

2011-12-01

391

The nature of magnetic fields and the flux quantum  

Microsoft Academic Search

The understanding of electromagnetic phenomena is based on the historic landmark A Treatise on Electricity & Magnetism of 1873 by James Clerk Maxwell. The form in which the theory is most commonly expressed is in the form of the vector equations introduced by Gibbs. There is another more general formulation due to Élie Cartan, in terms of external differential forms.

J. A. A. J. Perenboom; C. M. E. E. Peters

2006-01-01

392

Stochastic Flux-Freezing and Turbulent Magnetic Dynamo  

Microsoft Academic Search

``Spontaneous stochasticity'' of Lagrangian particle trajectories is a long-overlooked consequence of the explosive separation of particles undergoing turbulent Richardson diffusion. The effect implies a breakdown of Laplacian determinism for classical dynamics, with infinitely many (random) trajectories for the same initial particle position. We discuss the theoretical basis and empirical evidence for the phenomenon. Spontaneous stochasticity implies that magnetic field-lines cannot

G. L. Eyink

2010-01-01

393

A very low noise preamplifier for extremely low frequency magnetic antenna  

NASA Astrophysics Data System (ADS)

Besides the electrode-pair antenna, the magnetic antenna is also used for the extremely low frequency (ELF) submarine communication. To receive the weak ELF signals, the structure of a small sized magnetic antenna determines its specific electrical characteristics. The ELF magnetic antenna shows high internal resistance, alternating-current impedance, and a resonance frequency near the operating bandwidth. In accordance with the electrical characteristics of ELF magnetic antenna, a low noise preamplifier and frequency compensation circuit were designed and realized. The preamplifier is a three-stage negative feedback circuit, which is composed of parallel JFET, common-emitter amplifier with a Darlington structure and a common-collector amplifier in push-pull connection. And a frequency compensation circuit is cascaded to compensate the characteristic in low frequency range. In the operating bandwidth f = 30-200 Hz, the circuit has a gain of 39.4 dB. The equivalent input noise is 1.97 nV/?Hz and the frequency response keeps flat in operating bandwidth. The proposed preamplifier of the ELF magnetic antenna performs well in receiving ELF signals.

Shimin, Feng; Suihua, Zhou; Zhiyi, Chen

2013-07-01

394

The Evolution of Vector Magnetic Fields in an Emerging Flux Region  

NASA Astrophysics Data System (ADS)

Collaborative observations of NOAA Active Region 9231 were carried out during 9 days in 2000 November using the Advanced Stokes Polarimeter (ASP), Yohkoh/SXT, TRACE, and SOHO/MDI, in order to record the evolution of the photospheric magnetic field and its related coronal response. During this period an emerging flux region (EFR) appeared in the photosphere near the well-developed leading sunspot of this region, and subsequently bright bundles of coronal loops formed between the main concentrations of opposite magnetic polarity. The structure of the photospheric field comprising the EFR is classified into three regions: (1) the main bipolar magnetic flux of the EFR; (2) two small, rapidly emerging bipoles within the EFR; and (3) the remainder of the EFR excluding the other two regions. Two small, rapidly emerging bipoles are observed within a few hours of their first appearance at the photosphere. Examination of the vector magnetic field, its filling factor, and Doppler motion within the EFR shows that the young emerging magnetic field is nearly horizontal, the intrinsic field strength is weaker than that of the surrounding magnetic field (~500 G), and the weak field has a high filling factor (>80%) and upward motion (<1 km s-1). At both ends of the horizontal field structure we find that the magnetic field strength increases to about 1500 G and the filling factor drops to about 40% as the magnetic field becomes vertical in orientation during its first 12 hr. This field strength is typical of the field within the main bipolar magnetic flux, but the filling factor increases to 80% during the following 2 days. The process for organizing magnetic field configuration including convective collapse and flux concentration provides one possible explanation of the evolution of the field strength and the filling factor in the EFR. In addition, aymmetric surface distributions of magnetic field inclination were observed in the horizontal magnetic field area in the EFR. These asymmetric distributions were also observed in the small, young, emerging bipoles. This may mean that the magnetic field of the EFR is affected by the preexisting magnetic environment surrounding the EFR and that the emerging magnetic loops are deformed before or at the time they reach the photospheric level.

Kubo, M.; Shimizu, T.; Lites, B. W.

2003-09-01

395

THE INTERNAL STRUCTURE OF CORONAL MASS EJECTIONS: ARE ALL REGULAR MAGNETIC CLOUDS FLUX ROPES?  

SciTech Connect

In this Letter, we investigate the internal structure of a coronal mass ejection (CME) and its dynamics by invoking a realistic initiation mechanism in a quadrupolar magnetic setting. The study comprises a compressible three-dimensional magnetohydrodynamics simulation. We use an idealized model of the solar corona, into which we superimpose a quadrupolar magnetic source region. By applying shearing motions resembling flux emergence at the solar boundary, the initial equilibrium field is energized and it eventually erupts, yielding a fast CME. The simulated CME shows the typical characteristics of a magnetic cloud (MC) as it propagates away from the Sun and interacts with a bimodal solar wind. However, no distinct flux rope structure is present in the associated interplanetary ejection. In our model, a series of reconnection events between the eruptive magnetic field and the ambient field results in the creation of significant writhe in the CME's magnetic field, yielding the observed rotation of the magnetic field vector, characteristic of an MC. We demonstrate that the magnetic field lines of the CME may suffer discontinuous changes in their mapping on the solar surface, with footpoints subject to meandering over the course of the eruption due to magnetic reconnection. We argue that CMEs with internal magnetic structure such as that described here should also be considered while attempting to explain in situ observations of regular MCs at L1 and elsewhere in the heliosphere.

Jacobs, C.; Poedts, S. [Centrum voor Plasma-Astrofysica, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Leuven (Belgium); Roussev, I. I.; Lugaz, N. [Institute for Astronomy, University of Hawai'i, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States)], E-mail: Carla.Jacobs@wis.kuleuven.be, E-mail: Stefaan.Poedts@wis.kuleuven.be, E-mail: iroussev@ifa.hawaii.edu, E-mail: nlugaz@ifa.hawaii.edu

2009-04-20

396

High Frequency Magnetization Dynamics and Popcorn Noise in Thin Film Heads.  

NASA Astrophysics Data System (ADS)

As the areal density and data rates in magnetic recording continue to increase, it is important to limit the occurrences of popcorn noise in thin film inductive recording heads, especially in disk drives utilizing sector servo schemes. In this dissertation, the mechanisms of popcorn noise are described. In particular, the effects of thermal transients after write, changes in stress-magnetostriction products, dynamic domain structures and dynamic domain instabilities on popcorn noise probabilities in thin film heads are discussed. A new and simple inductance fluctuation measurement technique using DC bias current to the head is shown to be useful in quickly detecting heads that are susceptible to popcorn noise. Finally, based upon the improved understanding of popcorn noise mechanisms, various criteria for head design, processing, and operating conditions are suggested to ensure low probabilities of popcorn noise in thin film heads. Delayed-relaxation Barkhausen wall jumps which cause popcorn noise have been modelled by Klaassen and van Peppen (IEEE Trans. Magn. MAG-5, 3212 (1989)) as Poisson processes. However, it has been found that additional factors, such as external stress gradients, dynamic domain structures and domain instabilities, also can strongly affect the occurrences of popcorn noise in thin film heads. In the cases where the stress-magnetostriction products in the top yokes are negative, popcorn noise probabilities were observed to increase as functions of applied stress gradients in micro Flexhead^{rm TM} components. On the other hand, applied stress gradients have negligible effect when the resultant stress-magnetostriction products in the top yokes are positive. This sign-dependent effect has been explained to be caused by negative-magnetoelastic-energy induced Bloch domain wall instabilities. Thin film heads with unstable domain structures near the backgap closure of top yokes, either in the configuration of edge closure domains of spike-like domains, have also been found to be particular noisy. Using a 0.5-nsec exposure time wide-field Kerr effect microscope, occurrences of delayed-relaxation Barkhausen wall jumps were actually imaged after specific write current excitations in noisy heads. A quantitative correlation between the probabilities of popcorn noise and dynamic domain instabilities was also observed in a noisy head. In addition to the improved understanding of popcorn noise mechanisms, it has also been found that the susceptibilities of thin film heads to Barkhausen wall jumps, thus to popcorn noise, can be measured electrically as inductance fluctuations under DC bias current. A quantitative correlation was found between the peak popcorn noise probabilities and peak inductance fluctuations of 12 different thin film heads. This inductance characterization technique was thus shown to be useful in quickly detecting heads that are susceptible to popcorn noise. Finally, several criteria in head design, processing and operating conditions were proposed to ensure low probabilities of popcorn noise in thin film heads.

Liu, Francis H.

397

Geometrical investigation of the kinetic evolution of the magnetic field in a periodic flux rope  

NASA Astrophysics Data System (ADS)

Flux ropes are bundles of magnetic field wrapped around an axis. Many laboratory, space, and astrophysics processes can be represented using this idealized concept. Here, a massively parallel 3D kinetic simulation of a periodic flux rope undergoing the kink instability is studied. The focus is on the topology of the magnetic field and its geometric structures. The analysis considers various techniques such as Poincaré maps and the quasi-separatrix layer (QSL). These are used to highlight regions with expansion or compression and changes in the connectivity of magnetic field lines and consequently to outline regions where heating and current may be generated due to magnetic reconnection. The present study is, to our knowledge, the first QSL analysis of a fully kinetic 3D particle in cell simulation and focuses the existing QSL method of analysis to periodic systems.

Restante, A. L.; Markidis, S.; Lapenta, G.; Intrator, T.

2013-08-01

398

Background acoustic noise and the hemispheric lateralization of speech processing in the human brain: magnetic mismatch negativity study  

Microsoft Academic Search

The present study explored effects of background noise on the cerebral functional asymmetry of speech perception. The magnetic equivalent (MMNm) of mismatch negativity (MMN) elicited by consonant-vowel syllable change presented in silence and during background white noise was measured with a whole-head magnetometer. It was found that in silence MMNm to speech stimuli, registered from the auditory cortex, was stronger

Yury Shtyrov; Teija Kujala; Jyrki Ahveninen; Mari Tervaniemi; Paavo Alku; Risto J Ilmoniemi; Risto Näätänen

1998-01-01

399

Low frequency noise in La0.7Sr0.3MnO3 based magnetic tunnel junctions  

NASA Astrophysics Data System (ADS)

Magnetic tunnel junctions based on manganites can exhibit a high tunneling magnetoresistance ratio due to the almost full spin polarization at the Fermi level. However, the performances of magnetic tunnel junction devices are also strongly linked to their noise characteristics. Here, we present a low frequency noise study on fully epitaxial La0.7Sr0.3MnO3/SrTiO3/La0.7Sr0.3MnO3/La0.66Sr0.33Mn0.995Ru0.005O3 tunnel spin valves with tunneling magnetoresistance ratios larger than 100%. We evidence non-conventional low frequency noise dependence on temperature related to the magnetic fluctuations and structural phase transitions in the structure. We present also a comparison with the low frequency noise exhibited in Fe/MgO/Fe magnetic tunnel junctions.

Guerrero, R.; Solignac, A.; Fermon, C.; Pannetier-Lecoeur, M.; Lecoeur, Ph.; Fernández-Pacheco, R.

2012-04-01

400

Helical instability of a magnetic flux line in a current-carrying superconducting film  

Microsoft Academic Search

An exact solution for helical magnetic vortex structure in a type-II superconductor strip subjected to a magnetic field parallel to a transport current is obtained. A field-dependent characteristic current of flux-line instability with respect to helical distortions is found to be comparable to experimental values of the critical current of dissipation onset in thin films. A resistive picture following from

Yuri A. Genenko

1996-01-01

401

Factors Affecting Magnetic Flux Leakage Inspection of Tailor-Welded Blanks  

Microsoft Academic Search

The development of a laboratory-based tailor-welded blank (TWB) inspection system using the principles of magnetic flux leakage (MFL) is presented. The effects of variations in inspection system operating parameters are quantified to allow for optimized system performance. The parameters examined included the applied magnetic field strength, inspection scanning velocity, spatial resolution of acquired signals, specimen end effects, and pole-piece lift-off.

A. Montgomery; P. Wild; L. Clapham

2006-01-01

402

Modeling of electromagnetic phenomena in soft magnetic materials under unidirectional time periodic flux excitations  

Microsoft Academic Search

We report on recent advances in the modeling of magnetic losses in steel laminations used in electromagnetic devices. The integrated-lamination moving dynamic Preisach model, used to evaluate the dynamic magnetization loops under distorted unidirectional flux patterns, is described. The main goal is the comparison of two numerical procedures, using the finite element-finite difference technique and the finite element-fixed point technique,

L. R. Dupre; Oriano Bottauscio; M. Chiampi; M. Repetto; J. A. A. Melkebeek

1999-01-01

403

Axial-Flux Permanent-Magnet Generator for Induction Heating Gensets  

Microsoft Academic Search

This paper presents a single-phase slotless axial-flux permanent-magnet synchronous machine for induction heating gensets. A full-scale prototype of the machine (110 kVA, 400 Hz, 690 A) has been designed and subsequently analyzed through finite element analysis (FEA). Induced current distributions in the permanent magnets and in the rotors have also been calculated through FEA, showing that the resulting losses are

Federico Caricchi; Francescaromana Maradei; Giulio De Donato; Fabio Giulii Capponi

2010-01-01

404

Axial Flux Permanent Magnet Generator with Concentrated Winding for Small Wind Power Applications  

Microsoft Academic Search

An axial flux permanent magnet machine, designed to operate as a generator in a small-scale wind-power applications, is described in this paper. The machine is realized by using consecrated stator winding with open slots and surface mounted permanent magnets on rotor disk. Such a novel generator structure is simple to construct and its performance is good; it offers sinusoidal back-emf

A. Parviainen; J. Pyrhonen; P. Kontkanen

2005-01-01

405

Design, Prototyping, and Analysis of a Low Cost Axial-Flux Coreless Permanent-Magnet Generator  

Microsoft Academic Search

This paper presents the design, prototyping, and analysis of a relatively small and cheap axial-flux three-phase coreless permanent-magnet generator. The excitation of the machine is done by rectangular flat shaped neodymium-iron-boron magnets. A two-dimensional model of the machine is analyzed with finite-element software to obtain the machine parameters. One special feature of the constructed generator is in the design and

Seyed Mohsen Hosseini; Mojtaba Agha-Mirsalim; Mehran Mirzaei

2008-01-01

406

A property of electric and magnetic flux in non-abelian gauge theories  

Microsoft Academic Search

Pure non-Abelian gauge models with gauge group SU(N) are considered in a box with periodic boundary conditions at various temperatures ?1. Electric and magnetic flux are defined in a gauge-invariant way. The free energy of the system satisfies an exact duality equation, following from Euclidean invariance. The equation relates properties of the electric and the magnetic fields. Conclusions that can

1979-01-01

407

Flux line decoration and magnetic properties of YBa2Cu3O7 single crystals  

NASA Astrophysics Data System (ADS)

Magnetic properties of YBa2Cu3O7 single crystals, grown either in alumina or gold crucibles, have been studied in the superconducting state with a vibrating sample magnetometer. The flux lines present in these crystals have been visualised by the high resolution Bitter pattern technique. Decoration experiments have been done in different conditions on the same crystals in field cooling mode at 4.2 K. Vortex patterns and magnetic measurements are used to correlate crystal defects and vortex structures.

Blum, R.; Rouault, A.; Senateur, J. P.; Thomas, O.; Weiss, F.; Audier, M.; Agarwal, S. K.; Schlenker, C.; Wolf, T.

1991-12-01

408

The photospheric dynamo. I. Physics of thin magnetic flux tubes  

Microsoft Academic Search

In a previous paper (Henoux and Somov, 1991A&A...241..613H) it has been shown that, in an initially weak magnetic field, a radial inflow of neutrals can generate azimuthal DC currents, and that an azimuthal velocity field can create radial DC currents leading to the circulation of vertical currents. The effects of such azimuthal velocity field on the intensity and topology of

J. C. Henoux; B. V. Somov

1997-01-01

409

Sensing magnetic flux density of artificial neurons with a MEMS device.  

PubMed

We describe a simple procedure to characterize a magnetic field sensor based on microelectromechanical systems (MEMS) technology, which exploits the Lorentz force principle. This sensor is designed to detect, in future applications, the spiking activity of neurons or muscle cells. This procedure is based on the well-known capability that a magnetic MEMS device can be used to sense a small magnetic flux density. In this work, an electronic neuron (FitzHugh-Nagumo) is used to generate controlled spike-like magnetic fields. We show that the magnetic flux density generated by the hardware of this neuron can be detected with a new MEMS magnetic field sensor. This microdevice has a compact resonant structure (700?×?600?×?5 ?m) integrated by an array of silicon beams and p-type piezoresistive sensing elements, which need an easy fabrication process. The proposed microsensor has a resolution of 80 nT, a sensitivity of 1.2 V.T(-1), a resonant frequency of 13.87 kHz, low power consumption (2.05 mW), quality factor of 93 at atmospheric pressure, and requires a simple signal processing circuit. The importance of our study is twofold. First, because the artificial neuron can generate well-controlled magnetic flux density, we suggest it could be used to analyze the resolution and performance of different magnetic field sensors intended for neurobiological applications. Second, the introduced MEMS magnetic field sensor may be used as a prototype to develop new high-resolution biomedical microdevices to sense magnetic fields from cardiac tissue, nerves, spinal cord, or the brain. PMID:21113665

Tapia, Jesus A; Herrera-May, Agustin L; García-Ramírez, Pedro J; Martinez-Castillo, Jaime; Figueras, Eduard; Flores, Amira; Manjarrez, Elías

2011-04-01

410

Experimental measurement of quasi-separatrix layers in magnetic flux ropes  

NASA Astrophysics Data System (ADS)

A quasi-separatrix layer (QSL) is a region in a magnetic configuration where there are strong spatial gradients in the field line connectivity, and are thought to be favorable sites for 3D magnetic reconnection. Solar physicists have used QSLs extensively to identify reconnection sites in the complicated 3D magnetic configurations in solar flares, but we present the the first use of the technique in an experimental setting. In this experiment, performed in the Large Plasma Device (LAPD) at UCLA, two lanthanum hexaboride cathodes produce current channels initially parallel to the background magnetic field. The current channels create twisted helical structures, or flux ropes, in the magnetic field. The flux ropes rotate about their central axes, which causes them to periodically collide. Three dimensional magnetic measurements at 20000 spatial locations make the QSL calculation possible. During these collisions, QSLs, as well as reverse current sheets, are observed to form between the flux ropes. The structure of these QSLs is very similar to those seen in MHD simulations of merging currents.

Lawrence, Eric

2009-11-01

411

TWO-STEP EMERGENCE OF THE MAGNETIC FLUX SHEET FROM THE SOLAR CONVECTION ZONE  

SciTech Connect

We perform two-dimensional magnetodydrodynamic simulations of the flux emergence from the solar convection zone to the corona. The flux sheet is initially located moderately deep in the adiabatically stratified convection zone (-20,000 km) and is perturbed to trigger the Parker instability. The flux rises through the solar interior due to the magnetic buoyancy, but suffers a gradual deceleration and a flattening in the middle of the way to the surface since the plasma piled on the emerging loop cannot pass through the convectively stable photosphere. As the magnetic pressure gradient enhances, the flux becomes locally unstable to the Parker instability so that the further evolution to the corona occurs. The second-step nonlinear emergence is well described by the expansion law by Shibata et al. To investigate the condition for this 'two-step emergence' model, we vary the initial field strength and the total flux. When the initial field is too strong, the flux exhibits the emergence to the corona without a deceleration at the surface and reveals an unrealistically strong flux density at each footpoint of the coronal loop, while the flux either fragments within the convection zone or cannot pass through the surface when the initial field is too weak. The condition for the 'two-step emergence' is found to be 10{sup 21}-10{sup 22} Mx with 10{sup 4} G at z = -20,000 km. We present some discussions in connection with recent observations and the results of the thin-flux-tube model.

Toriumi, S.; Yokoyama, T., E-mail: toriumi@eps.s.u-tokyo.ac.j [Department of Earth and Planetary Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033 (Japan)

2010-05-01

412

On the stability of stationary flows in thin magnetic flux tubes  

NASA Astrophysics Data System (ADS)

Based on the almost fibril state of magnetic flux in astrophysical bodies we present a study of the stability of magnetic flux tubes which are described by the thin flux tube approximation. A stationary flow is allowed in the tube which in equilibrium lies in a vertical plane, while the surroundings are assumed to be static. This complements earlier studies with external but no internal flows. The equations governing small adiabatic displacements are given in a canonical form which allows the proof that operators are Hermitian. This form is derived for the first time. The character of operators allows powerful conclusions about the eigensolutions, sufficient conditions for stability as well as the application of variational techniques. The velocity may potentially lead to the phenomenon of overstability. Special cases are discussed analytically. These cover the local approximation, binormal perturbations, static equilibria, and horizontal and vertical flux tubes. Stability of siphon flows along arch-shaped flux tubes describing the Evershed effect in penumbrae of sunspots is analyzed numerically. A standard model is found to be stable. Increasing the flow speed or decreasing the magnetic field strength however leads to monotonic instability. Further astrophysical applications are mentioned.

Schmitt, D.

413

Downward pumping of magnetic flux as the cause of filamentary structures in sunspot penumbrae.  

PubMed

The structure of a sunspot is determined by the local interaction between magnetic fields and convection near the Sun's surface. The dark central umbra is surrounded by a filamentary penumbra, whose complicated fine structure has only recently been revealed by high-resolution observations. The penumbral magnetic field has an intricate and unexpected interlocking-comb structure and some field lines, with associated outflows of gas, dive back down below the solar surface at the outer edge of the spot. These field lines might be expected to float quickly back to the surface because of magnetic buoyancy, but they remain submerged. Here we show that the field lines are kept submerged outside the spot by turbulent, compressible convection, which is dominated by strong, coherent, descending plumes. Moreover, this downward pumping of magnetic flux explains the origin of the interlocking-comb structure of the penumbral magnetic field, and the behaviour of other magnetic features near the sunspot. PMID:12459777

Thomas, John H; Weiss, Nigel O; Tobias, Steven M; Brummell, Nicholas H

2002-11-28

414

Downward pumping of magnetic flux as the cause of filamentary structures in sunspot penumbrae  

NASA Astrophysics Data System (ADS)

The structure of a sunspot is determined by the local interaction between magnetic fields and convection near the Sun's surface. The dark central umbra is surrounded by a filamentary penumbra, whose complicated fine structure has only recently been revealed by high-resolution observations. The penumbral magnetic field has an intricate and unexpected interlocking-comb structure and some field lines, with associated outflows of gas, dive back down below the solar surface at the outer edge of the spot. These field lines might be expected to float quickly back to the surface because of magnetic buoyancy, but they remain submerged. Here we show that the field lines are kept submerged outside the spot by turbulent, compressible convection, which is dominated by strong, coherent, descending plumes. Moreover, this downward pumping of magnetic flux explains the origin of the interlocking-comb structure of the penumbral magnetic field, and the behaviour of other magnetic features near the sunspot.

Thomas, John H.; Weiss, Nigel O.; Tobias, Steven M.; Brummell, Nicholas H.

2002-11-01

415

Flux Pumping and Magnetic Fields in the Outer Penumbra of a Sunspot  

NASA Astrophysics Data System (ADS)

The filamentary structure of a sunspot penumbra is believed to be magnetoconvective in origin. In the outer penumbra there is a difference in inclination of up to 30°-40° between the magnetic fields associated with bright and dark filaments, and the latter fields plunge downward below the surface toward the edge of the spot. We have proposed that these fields are dragged downward by magnetic pumping caused by the external granular convection. In this paper we model this process in a more elaborate idealized configuration that includes the curvature force exerted by an arched magnetic field in addition to magnetic buoyancy, and demonstrate that magnetic pumping remains an efficient mechanism for holding flux submerged. We discuss the implications of these results for the magnetic structure of the outer penumbra.

Brummell, Nicholas H.; Tobias, Steven M.; Thomas, John H.; Weiss, Nigel O.

2008-10-01

416

Flux Ropes and CMEs: The Kink and Torus Instabilities, Catastrophe, and Magnetic Reconnection  

NASA Astrophysics Data System (ADS)

Prior to eruption, the coronal magnetic field evolves along an equilibrium sequence due to slow photospheric changes. Configurations containing a flux rope can erupt when the rope passes the threshold of an instability in the sequence or when the rope is driven beyond an end point of the sequence in parameter space (catastrophe). The kink instability of a flux rope yields quantitative agreement with characteristic properties of many CMEs during their onset and early evolution: development of helical shape and exponential-to-linear rise profiles. The large-scale evolution of kinking flux ropes is governed by the torus (expansion) instability (TI). This instability yields a unified description of fast and slow CMEs, the preferred occurrence of very fast CMEs in quadrupolar active regions, and an indication why the minor flux rope radius expands overproportionally in the course of the eruption, creating or deepening the cavity seen in three-part CMEs. If an eruption is triggered by a flux rope catastrophe, we expect its evolution to possess characteristics similar to the TI-driven case. The magnetic reconnection that commences in the wake of a rising unstable flux rope is an integral part of the eruption and proceeds in a highly dynamic and complex manner, forming many intermittent X- and O-type structures along a vertical current sheet.

Kliem, Bernhard; Toeroek, T.

2006-06-01

417

Reducing the Effects of Background Noise during Auditory Functional Magnetic Resonance Imaging of Speech Processing: Qualitative and Quantitative Comparisons between Two Image Acquisition Schemes and Noise Cancellation  

ERIC Educational Resources Information Center

|Purpose: The intense sound generated during functional magnetic resonance imaging (fMRI) complicates studies of speech and hearing. This experiment evaluated the benefits of using active noise cancellation (ANC), which attenuates the level of the scanner sound at the participant's ear by up to 35 dB around the peak at 600 Hz. Method: Speech and…

Blackman, Graham A.; Hall, Deborah A.

2011-01-01

418

Why current-carrying magnetic flux tubes gobble up plasma and become thin as a result  

NASA Astrophysics Data System (ADS)

Suppose an electric current I flows along a magnetic flux tube that has axial flux ? and a radius a=a(z) where z is the axial position along the flux tube. It is shown that, in such a case, non-linear, non-conservative J× B forces accelerate plasma axially from regions of small a to regions of large a and that this acceleration is proportional to partial I^2/partial z˜=2I( partial I/partial?) ( partial?/partial z) . Thus, if a current-carrying flux tube is bulged at, say, z=0 and constricted at, say, z= h, then plasma will be accelerated from z= h towards z=0 resulting in a situation similar to two water jets pointed at each other. The ingested plasma rotates to follow the twisted magnetic field and also convects embedded, frozen-in azimuthal magnetic flux ? from z= h to z=0; this azimuthal flux is associated with the current I. The counterstreaming, counter-rotating flows collide and stagnate at z=0 and in so doing (i) convert their translational kinetic energy into heat, (ii) increase the plasma density at z0, and (iii) increase the embedded azimuthal flux density at z0. The increase in azimuthal flux density at z0 increases the magnetic pinch force at z0 and so causes a reduction of a(0). Thus, the flux tube develops an axially uniform cross-section, a decreased volume, an increased density, and an increased temperature. This model is proposed as an explanation for the long-standing mystery of why solar coronal loops are observed to be axially uniform, bright, and hot. It is also argued that a small number of tail particles bouncing between the approaching counterstreaming plasma jets should be Fermi accelerated to extreme energies. Finally, analytic solution of the Grad-Shafranov equation predicts that a flux tube becomes axially uniform when the ingested plasma becomes hot and dense enough to have 2?_0n? T/B_axial^2= (?_0Ia(0)/? )^2/2; observed coronal loop parameters are in reasonable agreement with this relationship which is analogous to having ?_pol=1 in a tokamak.

Bellan, Paul

2002-11-01

419

Design and application of axial-flux permanent magnet wheel motors for an electric vehicle  

Microsoft Academic Search

This paper introduces an optimal design of an axial-flux permanent magnet wheel motor for a small electric vehicle. This vehicle is driven directly by two wheel motors installed on its rear wheels, which are compact, low weight, and highly efficient. The optimal current waveform of the same pattern as the back electromagnetic force is used to increase the output torque

Yee-Pien Yang; Jia-Yuan Liang; Xian-Yee Xing

2009-01-01

420

The effect of quantized magnetic flux lines on the dynamics of superfluid neutron star cores  

Microsoft Academic Search

We investigate dynamical coupling time-scales of a neutron star's superfluid core, taking into account the interactions of quantized neutron vortices with quantized flux lines of the proton superconductor in addition to the previously considered scattering of the charged components against the spontaneous magnetization of the neutron vortex line. We compare the cases where vortex motion is constrained in different ways

T. Sidery; M. A. Alpar

2009-01-01