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1

A Microkelvin Magnetic Flux Noise Thermometer  

NASA Astrophysics Data System (ADS)

Due to its non-driven nature, noise thermometry intrinsically is the method of choice when minimal heat input during the temperature measurement is required. Our noise thermometer, experimentally characterized for temperatures between 42 K and 0.8 K, is a magnetic Johnson noise thermometer. The noise source is a cold-worked high purity copper cylinder, 5 mm in diameter and 20 mm long. The magnetic flux fluctuations generated by the electrons' Brownian motion is measured inductively by two dc-SQUID magnetometers simultaneously. Cross-correlation of the two channels leads to reduction of parasitic noise by more than one order of magnitude which allows for measuring the tiny noise powers at microkelvin temperatures.

Rothfuß, D.; Reiser, A.; Fleischmann, A.; Enss, C.

2014-06-01

2

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

3

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

4

Adaptive noise cancellation schemes for magnetic flux leakage signals obtained from gas pipeline inspection  

Microsoft Academic Search

Nondestructive evaluation of the gas pipeline system is most commonly performed using magnetic flux leakage (MFL) techniques. A major segment of this network employs seamless pipes. The data obtained From MFL inspection of seamless pipes is contaminated by various sources of noise, including seamless pipe noise due to material properties of the pipe, lift-off variation of MFL sensor due to

Muhammad Afzal; Robi PolikarS; Lalita Udpa; Satish Udpa

2001-01-01

5

Flux noise and flux creep in YBCO thin films  

Microsoft Academic Search

The authors have a dc SQUID to measure the low-frequency magnetic flux noise produced by thin-film rings of YBa\\/sub p2\\/CuâO\\/sub 7-delta\\/ (YBCO) with various microstructures. Below the transition temperature T\\/sub c\\/ of the YBCO, the spectral density of the noise scales as 1\\/f (f is the frequency) from 1 Hz to 1 kHz. This noise generally increases with temperature and

M. J. Ferrari; M. Johnson; F. C. Wellstood; J. Clarke; P. A. Rosenthal; R. H. Hammond; M. R. Beasley

1989-01-01

6

Magnetic Flux Noise of the Vortex Lattice Melting Transition in Bi_2Ba_2CaCu_2O_8+?  

NASA Astrophysics Data System (ADS)

We have measured the spectral density of magnetic flux noise, S_?(f), associated with the vortex lattice melting transition in a single crystal of Bi_2Ba_2CaCu_2O_8+?. The noise was measured by a niobium Superconducting QUantum Interference Device (SQUID) inside a vacuum can and thermally anchored to a liquid helium bath, but thermally isolated from the sample. The sample, of dimensions 1.3 mm × 1.3 mm × 30 ?m, was mounted on a variable temperature stage and placed less than 100 ?m away from the SQUID. The sample was cooled in magnetic fields up to 5 mT, and S_?(f) was measured as a function of field and temperature. In the larger fields, S_?(f) showed a peak at a temperature well below Tc and exhibited qualitatively different spectra above and below the peak. The temperature at which the noise peak occurred shifted towards Tc as the field was lowered, and coincided with the onset of diamagnetic screening of a small applied field. We associate this peak with the vortex melting transition. These results enabled us to study the phdynamic behavior of the phequilibrium melting transition in a range of low magnetic fields previously inaccessible.

Shaw, T. J.; André, M.-O.; Clarke, John; Drost, R. J.; Kes, P. H.

1997-03-01

7

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

8

Investigation of weldments in Victoria-class submarine pressure-hull using magnetic flux leakage and Barkhausen noise  

NASA Astrophysics Data System (ADS)

Evaluation of the stress state within submarine hulls can contribute to risk assessments, which provide assurance that in-service induced stresses will not adversely affect the service life of the naval structure. The purpose of this study was to evaluate the feasibility of using magnetic NDE techniques for identification of stresses associated with weldments in two original pressure hulls of Canada's Victoria class submarines. Magnetic Flux Leakage (MFL) and flux-controlled Barkhausen Noise measurements were investigated for identification of patch boundaries and welds in two sections of Victoria-class submarine-hull steel. While MFL showed clear demarcation of weld boundaries, Barkhausen measurements did not provide sufficiently clear response to identify these features in submarine hull samples. For a better understanding of Barkhausen response, uniaxial tensile stress was investigated on separate samples of submarine steel. A nonlinear dependence of Barkhausen response was observed, with a weaker sensitivity to tensile stresses below 200 MPa. This behavior, combined with the presence of substantial surface compressive stresses, was used to explain the observed insensitivity of Barkhausen measurements to the presence of welds.

Samimi, A. A.; Babbar, V.; Krause, T. W.; Clapham, L.

2014-02-01

9

Magnetic Flux Noise of the Vortex Lattice Melting Transition in Bi_2Ba_2CaCu_2O_8+delta  

Microsoft Academic Search

We have measured the spectral density of magnetic flux noise, S_Phi(f), associated with the vortex lattice melting transition in a single crystal of Bi_2Ba_2CaCu_2O_8+delta. The noise was measured by a niobium Superconducting QUantum Interference Device (SQUID) inside a vacuum can and thermally anchored to a liquid helium bath, but thermally isolated from the sample. The sample, of dimensions 1.3 mm

T. J. Shaw; M.-O. André; John Clarke; R. J. Drost; P. H. Kes

1997-01-01

10

Rejection of seamless pipe noise in magnetic flux leakage data obtained from gas pipeline inspection  

NASA Astrophysics Data System (ADS)

Natural gas is traditionally transmitted from production facilities to customer locations through a vast pipeline network. A major segment of this network employs seamless pipes. This is especially true for smaller diameter transmission and distribution lines. Manufacturing process associated with the production of seamless pipes contribute to a helical variation in the pipe along the axis. The deformation introduces an artifact in the data obtained from MFL inspection of these pipelines. This seamless pipe noise is usually correlated with signals generated by defects and other elements (joints, tees, etc.) in pipelines, and can therefore, mask their indications in MFL data. This warrants the need for methods to improve signal-to-noise ratio (SNR) in MFL data from seamless pipes. This paper presents a technique for detecting signals in MFL data from seamless pipes. The approach processes the data in various steps. First, a wavelet based denoising technique is applied to reduce the noise due to instrumentation and other sources. An adaptive filtering approach is then applied to reject seamless noise in the data. Since the inspection of pipelines typically generates vast amounts of data, it is imperative that the algorithm be computationally efficient. The processing method has to be robust in that it should be data independent. The approach described in this paper meet these criteria. Results from application of the approach to data from field tests are presented.

Afzal, Muhammad; Udpa, Satish; Udpa, Lalita; Lord, William

2000-05-01

11

Solar Magnetic Flux Ropes  

E-print Network

The most probable initial magnetic configuration of a CME is a flux rope consisting of twisted field lines which fill the whole volume of a dark coronal cavity. The flux ropes can be in stable equilibrium in the coronal magnetic field for weeks and even months, but suddenly they loose their stability and erupt with high speed. Their transition to the unstable phase depends on the parameters of the flux rope (i.e., total electric current, twist, mass loading etc.), as well as on the properties of the ambient coronal magnetic field. One of the major governing factors is the vertical gradient of the coronal magnetic field which is estimated as decay index (n). Cold dense prominence material can be collected in the lower parts of the helical flux tubes. Filaments are therefore good tracers of the flux ropes in the corona, which become visible long before the beginning of the eruption. The perspectives of the filament eruptions and following CMEs can be estimated by the comparison of observed filament heights with...

Filippov, Boris; Srivastava, Abhishek K; Uddin, Wahab

2015-01-01

12

Flux Compression Magnetic Nozzle  

NASA Technical Reports Server (NTRS)

In pulsed fusion propulsion schemes in which the fusion energy creates a radially expanding plasma, a magnetic nozzle is required to redirect the radially diverging flow of the expanding fusion plasma into a rearward axial flow, thereby producing a forward axial impulse to the vehicle. In a highly electrically conducting plasma, the presence of a magnetic field B in the plasma creates a pressure B(exp 2)/2(mu) in the plasma, the magnetic pressure. A gradient in the magnetic pressure can be used to decelerate the plasma traveling in the direction of increasing magnetic field, or to accelerate a plasma from rest in the direction of decreasing magnetic pressure. In principle, ignoring dissipative processes, it is possible to design magnetic configurations to produce an 'elastic' deflection of a plasma beam. In particular, it is conceivable that, by an appropriate arrangement of a set of coils, a good approximation to a parabolic 'magnetic mirror' may be formed, such that a beam of charged particles emanating from the focal point of the parabolic mirror would be reflected by the mirror to travel axially away from the mirror. The degree to which this may be accomplished depends on the degree of control one has over the flux surface of the magnetic field, which changes as a result of its interaction with a moving plasma.

Thio, Y. C. Francis; Schafer, Charles (Technical Monitor)

2001-01-01

13

Noise correlations in a flux qubit with tunable tunnel coupling  

E-print Network

We have measured flux-noise correlations in a tunable superconducting flux qubit. The device consists of two loops that independently control the qubit’s energy splitting and tunnel coupling. Low-frequency flux noise in ...

Yoshihara, Fumiki

14

Permanent magnet flux-biased magnetic actuator with flux feedback  

NASA Technical Reports Server (NTRS)

The invention is a permanent magnet flux-biased magnetic actuator with flux feedback for adjustably suspending an element on a single axis. The magnetic actuator includes a pair of opposing electromagnets and provides bi-directional forces along the single axis to the suspended element. Permanent magnets in flux feedback loops from the opposing electromagnets establish a reference permanent magnet flux-bias to linearize the force characteristics of the electromagnets to extend the linear range of the actuator without the need for continuous bias currents in the electromagnets.

Groom, Nelson J. (inventor)

1991-01-01

15

Magnetic balltracking: Tracking the photospheric magnetic flux  

NASA Astrophysics Data System (ADS)

Context. One aspect of understanding the dynamics of the quiet Sun is to quantify the evolution of the flux within small-scale magnetic features. These features are routinely observed in the quiet photosphere and were given various names, such as pores, knots, magnetic patches. Aims: This work presents a new algorithm for tracking the evolution of the broad variety of small-scale magnetic features in the photosphere, with a precision equal to the instrumental resolution. Methods: We have developed a new technique to track the evolution of the individual magnetic features from magnetograms, called "magnetic balltracking". It quantifies the flux of the tracked features, and it can track the footpoints of magnetic field lines inferred from magnetic field extrapolation. The algorithm can detect and quantify flux emergence, as well as flux cancellation. Results: The capabilities of magnetic balltracking are demonstrated with the detection and the tracking of two cases of magnetic flux emergence that lead to the brightening of X-ray loops. The maximum emerged flux ranges from 1018 Mx to 1019 Mx (unsigned flux) when the X-ray loops are observed. Movies associated to Figs. 6 and 18 are available in electronic form at http://www.aanda.org

Attie, R.; Innes, D. E.

2015-02-01

16

Noise properties of high-Tc superconducting flux transformers fabricated using chemical-mechanical polishing  

NASA Astrophysics Data System (ADS)

Reproducible high-temperature superconducting multilayer flux transformers were fabricated using chemical mechanical polishing. The measured magnetic field noise of the flip-chip magnetometer based on one such flux transformer with a 9 × 9 mm2 pickup loop coupled to a bicrystal dc SQUID was 15 fT/Hz1/2 above 2 kHz. We present an investigation of excess 1/f noise observed at low frequencies and its relationship with the microstructure of the interlayer connections within the flux transformer. The developed high-Tc SQUID magnetometers may be advantageous in ultra-low field magnetic resonance imaging and, with improved low frequency noise, magnetoencephalography applications.

Chukharkin, M.; Kalabukhov, A.; Schneiderman, J. F.; Öisjöen, F.; Snigirev, O.; Lai, Z.; Winkler, D.

2012-07-01

17

Simulations of Magnetic Flux Emergence  

NASA Astrophysics Data System (ADS)

Magnetic flux emerges from the solar surface on a wide range of scales. We review recent simulations of both large and small scale flux emergence. In our own simulations, we represent the magnetic flux produced by the global dynamo as uniform, untwisted, horizontal field advected into the simulation domain by supergranule scale inflows at the bottom. Our computational domain extends from the temperature minimum (half a megameter above the visible surface) to 20 Mm below the surface, which is 10% of the depth of the convection zone, but contains 2/3 of its scale heights. We investigate how magnetic flux rises through the upper solar convection zone and emerges through the surface. Convective up-flows and magnetic buoyancy bring field toward the surface. Convective down-flows pin down field and prevent its rise. Most of the field gets pumped downward by the convection, but some field rises to the surface. The convective motions both confine the flux concentrations (without the need for twist) and shred them. This process creates a hierarchy of magnetic loops with smaller loops riding "piggy-back", in a serpentine pattern, on larger loops. As a result, magnetic flux emerges in a mixed polarity, "pepper and salt" pattern. The small loops appear as horizontal field over granules with their vertical legs in the bounding intergranular lanes. The fields are quickly swept into the intergranular lanes. As the larger, parent, flux concentrations reach the surface with their legs rooted in the the downflow boundaries of the underlying, supergranule-scale, convective cells near the bottom of the simulation domain, the surface field counter-streams into separate, opposite polarity concentrations, creating pores and spots. The subsurface magnetic field lines of the pores and spots formed by the magneto-convection (without being imposed as an initial condition) are braided, some tightly, some loosely and they connect in complicated ways to the surrounding field at large depths. The pores evolve on the timescale of the underlying supergranules. Thus, long lives of solar active regions imply that they are rooted in larger, more slowly evolving, deeper convective structures. Based on these simulations we summarize the robust properties of emerging magnetic flux.

Stein, Robert; Nordlund, Aake

18

DISCONNECTING OPEN SOLAR MAGNETIC FLUX  

SciTech Connect

Disconnection of open magnetic flux by reconnection is required to balance the injection of open flux by coronal mass ejections and other eruptive events. Making use of recent advances in heliospheric background subtraction, we have imaged many abrupt disconnection events. These events produce dense plasma clouds whose distinctive shape can now be traced from the corona across the inner solar system via heliospheric imaging. The morphology of each initial event is characteristic of magnetic reconnection across a current sheet, and the newly disconnected flux takes the form of a 'U-'shaped loop that moves outward, accreting coronal and solar wind material. We analyzed one such event on 2008 December 18 as it formed and accelerated at 20 m s{sup -2} to 320 km s{sup -1}, thereafter expanding self-similarly until it exited our field of view 1.2 AU from the Sun. From acceleration and photometric mass estimates we derive the coronal magnetic field strength to be 8 {mu}T, 6 R{sub Sun} above the photosphere, and the entrained flux to be 1.6 Multiplication-Sign 10{sup 11} Wb (1.6 Multiplication-Sign 10{sup 19} Mx). We model the feature's propagation by balancing inferred magnetic tension force against accretion drag. This model is consistent with the feature's behavior and accepted solar wind parameters. By counting events over a 36 day window, we estimate a global event rate of 1 day{sup -1} and a global solar minimum unsigned flux disconnection rate of 6 Multiplication-Sign 10{sup 13} Wb yr{sup -1} (6 Multiplication-Sign 10{sup 21} Mx yr{sup -1}) by this mechanism. That rate corresponds to {approx} - 0.2 nT yr{sup -1} change in the radial heliospheric field at 1 AU, indicating that the mechanism is important to the heliospheric flux balance.

DeForest, C. E.; Howard, T. A.; McComas, D. J. [Southwest Research Institute, 1050 Walnut Street Suite 300, Boulder, CO 80302 (United States)

2012-01-20

19

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

20

Force sensor using changes in magnetic flux  

NASA Technical Reports Server (NTRS)

A force sensor includes a magnetostrictive material and a magnetic field generator positioned in proximity thereto. A magnetic field is induced in and surrounding the magnetostrictive material such that lines of magnetic flux pass through the magnetostrictive material. A sensor positioned in the vicinity of the magnetostrictive material measures changes in one of flux angle and flux density when the magnetostrictive material experiences an applied force that is aligned with the lines of magnetic flux.

Pickens, Herman L. (Inventor); Richard, James A. (Inventor)

2012-01-01

21

Chaos in Magnetic Flux Ropes  

NASA Astrophysics Data System (ADS)

Magnetic Flux Ropes Immersed in a uniform magnetoplasma are observed to twist about themselves, writhe about each other and rotate about a central axis. They are kink unstable and smash into one another as they move. Each collision results in magnetic field line generation and the generation of a quasi-seperatrix layer. Three dimensional magnetic field lines are computed by conditionally averaging the data using correlation techniques. When the currents associated with the ropes are large,this is possible for only a number of rotation cycles as the field line motion becomes chaotic. The permutation entropy1 can be calculated from the the time series of the magnetic field data (this is also done with flows) and used to calculate the positions of the data on a Jensen Shannon complexity map2. The power spectra of much of the magnetic and flow data is exponential and Lorentzian structures in the time domain are embedded in them. The location of data on this map indicates if the magnetic fields are stochastic, or fall into regions of minimal or maximal complexity. The complexity is a function of space and time. The complexity map, and analysis will be explained in the course of the talk. Other types of chaotic dynamical models such as the Lorentz or Gissinger process also fall on the map and can give a clue to the nature of the flux rope turbulence. The ropes fall in the region of the C-H plane where chaotic systems lie. 1 C. Bandt, B. Pompe, Phys. Rev. Lett., 88,174102 (2007) 2 O. Russo et al., Phys. Rev. Lett., 99, 154102 (2007), J. Maggs, G.Morales, “Permutation Entropy analysis of temperature fluctuations from a basic electron heat transport experiment”,submitted PPCF (2013)

Gekelman, Walter; DeHaas, T.; Van Compernolle, B.; Vincena, S.

2013-07-01

22

Chaos in magnetic flux ropes  

NASA Astrophysics Data System (ADS)

Magnetic flux ropes immersed in a uniform magnetoplasma are observed to twist about themselves, writhe about each other and rotate about a central axis. They are kink unstable and smash into one another as they move. Each collision results in magnetic field line reconnection and the generation of a quasi-separatrix layer. Three-dimensional magnetic field lines are computed by conditionally averaging the data using correlation techniques. Conditional averaging is possible for only a number of rotation cycles as the field line motion becomes chaotic. The permutation entropy can be calculated from the time series of the magnetic field data (this is also done with flows) and is used to calculate the positions of the data on a Jensen-Shannon complexity map. The location of data on this map indicates if the magnetic fields are stochastic, or fall into regions of minimal or maximal complexity. The complexity is a function of space and time. The Lyapunov and Hurst exponents are calculated and the complexity and permutation entropy of the flows and field components are shown throughout the volume.

Gekelman, Walter; Van Compernolle, Bart; DeHaas, Tim; Vincena, Stephen

2014-06-01

23

Chaos in Magnetic Flux Ropes  

NASA Astrophysics Data System (ADS)

Magnetic Flux Ropes Immersed in a uniform magnetoplasma are observed to twist about themselves, writhe about each other and rotate about a central axis. They are kink unstable and smash into one another as they move. Full three dimensional magnetic field and flows are measured at thousands of time steps. Each collision results in magnetic field line generation and the generation of a quasi-seperatrix layer and induced electric fields. Three dimensional magnetic field lines are computed by conditionally averaging the data using correlation techniques. The permutation entropy1 ,which is related to the Lyapunov exponent, can be calculated from the the time series of the magnetic field data (this is also done with flows) and used to calculate the positions of the data on a Jensen Shannon complexity map2. The location of data on this map indicates if the magnetic fields are stochastic, or fall into regions of minimal or maximal complexity. The complexity is a function of space and time. The complexity map, and analysis will be explained in the course of the talk. Other types of chaotic dynamical models such as the Lorentz, Gissinger and Henon process also fall on the map and can give a clue to the nature of the flux rope turbulence. The ropes fall in the region of the C-H plane where chaotic systems lie. The entropy and complexity change in space and time which reflects the change and possibly type of chaos associated with the ropes. The maps give insight as to the type of chaos (deterministic chaos, fractional diffusion , Levi flights..) and underlying dynamical process. The power spectra of much of the magnetic and flow data is exponential and Lorentzian structures in the time domain are embedded in them. Other quantities such as the Hurst exponent are evaluated for both magnetic fields and plasma flow. Work Supported by a UC-LANL Lab fund and the Basic Plasma Science Facility which is funded by DOE and NSF. 1) C. Bandt, B. Pompe, Phys. Rev. Lett., 88,174102 (2007) 2) O. Russo et al., Phys. Rev. Lett., 99, 154102 (2007), J. Maggs, G.Morales, 55, 085015 (2013)

Gekelman, W. N.; DeHaas, T.; Van Compernolle, B.

2013-12-01

24

Linear magnetic flux amplifier D. S. Golubovia  

E-print Network

Physics and Chemistry, Nanoscale Superconductivity and Magnetism Group, Laboratory for Solid State PhysicsLinear magnetic flux amplifier D. S. Golubovia and V. V. Moshchalkov INPAC--Institute for Nanoscale that it is feasible to design a linear magnetic flux amplifier for applications in superconducting quantum

Moshchalkov, Victor V.

25

Acoustic noise during functional magnetic resonance imaginga)  

PubMed Central

Functional magnetic resonance imaging (fMRI) enables sites of brain activation to be localized in human subjects. For studies of the auditory system, acoustic noise generated during fMRI can interfere with assessments of this activation by introducing uncontrolled extraneous sounds. As a first step toward reducing the noise during fMRI, this paper describes the temporal and spectral characteristics of the noise present under typical fMRI study conditions for two imagers with different static magnetic field strengths. Peak noise levels were 123 and 138 dB re 20 ?Pa in a 1.5-tesla (T) and a 3-T imager, respectively. The noise spectrum (calculated over a 10-ms window coinciding with the highest-amplitude noise) showed a prominent maximum at 1 kHz for the 1.5-T imager (115 dB SPL) and at 1.4 kHz for the 3-T imager (131 dB SPL). The frequency content and timing of the most intense noise components indicated that the noise was primarily attributable to the readout gradients in the imaging pulse sequence. The noise persisted above background levels for 300-500 ms after gradient activity ceased, indicating that resonating structures in the imager or noise reverberating in the imager room were also factors. The gradient noise waveform was highly repeatable. In addition, the coolant pump for the imager’s permanent magnet and the room air handling system were sources of ongoing noise lower in both level and frequency than gradient coil noise. Knowledge of the sources and characteristics of the noise enabled the examination of general approaches to noise control that could be applied to reduce the unwanted noise during fMRI sessions. PMID:11051496

Ravicz, Michael E.; Melcher, Jennifer R.; Kiang, Nelson Y.-S.

2007-01-01

26

Flux emergence in a magnetized convection zone  

NASA Astrophysics Data System (ADS)

We study the influence of a dynamo magnetic field on the buoyant rise and emergence of twisted magnetic flux-ropes, and their influence on the global external magnetic field. We ran three-dimensional MHD numerical simulations using the ASH code and analysed the dynamical evolution of such buoyant flux-ropes from the bottom of the convection zone until the post-emergence phases. The actual flux-emergence episode is preceded by a localised increase of radial velocity, density and current density at the top of the convection zone. During the buoyant rise, the flux-rope's magnetic field strength and density scale as B~rho(alpha) , with alpha?sssim 1. The properties of initial phases of the buoyant rise are determined essentially by the flux-rope's properties and the convective flows and are, in consequence, in good agreement with previous studies. However, the effects of the interaction of the background dynamo field become increasingly stronger as the flux-ropes evolve. The threshold for the initial magnetic field amplitude is slightly increased by the presence of the background dynamo field, even if it is on average much weaker than the flux-rope's field. The geometry and relative orientation of the magnetic field in the flux-ropes with respect to that in the background magnetic field influences the resulting rise speeds, zonal flows amplitudes (which develop within the flux-ropes) and surface signatures of magnetic flux emergence. This strongly influences the morphology, duration and amplitude of the surface shearing and Poynting flux associated with magnetic flux-rope emergence, which are key ingredients to the current coronal eruption triggering scenarios. The actual magnetic flux emergence is consistently preceded by strong and localised radial velocity enhancements at the place where the flux rope will emerge. The emerged magnetic flux is in most of the cases studied enough to influence the global surface magnetic field. In some cases, the emergence reinforces the system's global polarity reversal while in some others it inhibits the background dynamo from doing so. The fraction of magnetic flux which remains attached to the flux-rope is slowly spread out in latitude, diffused and assimilated by the background dynamo field.

Pinto, Rui; Brun, Allan Sacha

27

Culgoora radio and SKYLAB EUV observations of emerging magnetic flux in the lower corona  

Microsoft Academic Search

Detailed comparisons of Culgoora 160 MHz radioheliograms of solar noise storms and Skylab EUV spectroheliograms of coronal loop structures are presented. It is concluded that: (1) there is a close association between changes in large-scale magnetic fields in the corona and the onset or cessation of noise storms; (2) these coronal changes result from the emergence of new magnetic flux

R. T. Stewart; G. E. Brueckner; K. P. Dere

1986-01-01

28

Culgoora radio and skylab euv observations of emerging magnetic flux in the lower corona  

Microsoft Academic Search

Detailed comparisons of Culgoora 160 MHz radioheliograms of solar noise storms and Skylab EUV spectroheliograms of coronal loop structures are presented. It is concluded that: (1) there is a close association between changes in large-scale magnetic fields in the corona and the onset or cessation of noise storms; (2) these coronal changes result from the emergence of new magnetic flux

R. T. Stewart; G. E. Brueckner; K. P. Dere

1986-01-01

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

Metamaterial anisotropic flux concentrators and magnetic arrays  

E-print Network

A metamaterial magnetic flux concentrator is investigated in detail in combination with a Halbach cylinder of infinite length. A general analytical solution to the field is determined and the magnetic figure of merit is determined for a Halbach cylinder with a flux concentrator. It is shown that an ideal flux concentrator will not change the figure of merit of a given magnet design, while the non-ideal will always lower it. The geometric parameters producing maximum figure of merit, i.e. the most efficient devices, are determined. The force and torque between two concentric Halbach cylinders with flux concentrators is determined and the maximum torque is found. Finally, the effect of non-ideal flux concentrators and the practical use of flux concentrators, as well as demagnetization issues, is discussed.

Bjørk, R; Bahl, C R H

2014-01-01

31

Magnetic Flux Emergence Along the Solar Cycle  

NASA Astrophysics Data System (ADS)

Flux emergence plays an important role along the solar cycle. Magnetic flux emergence builds sunspot groups and solar activity. The sunspot groups contribute to the large scale behaviour of the magnetic field over the 11 year cycle and the reversal of the North and South magnetic polarity every 22 years. The leading polarity of sunspot groups is opposite in the North and South hemispheres and reverses for each new solar cycle. However the hemispheric rule shows the conservation of sign of the magnetic helicity with positive and negative magnetic helicity in the South and North hemispheres, respectively. MHD models of emerging flux have been developed over the past twenty years but have not yet succeeded to reproduce solar observations. The emergence of flux occurs through plasma layers of very high gradients of pressure and changing of modes from a large ? to a low ? plasma (<1). With the new armada of high spatial and temporal resolution instruments on the ground and in space, emergence of magnetic flux is observed in tremendous detail and followed during their transit through the upper atmosphere. Signatures of flux emergence in the corona depend on the pre-existing magnetic configuration and on the strength of the emerging flux. We review in this paper new and established models as well as the recent observations.

Schmieder, B.; Archontis, V.; Pariat, E.

2014-12-01

32

Magnetic bearing. [for supplying magnetic fluxes  

NASA Technical Reports Server (NTRS)

A magnetic bearing is described which includes a pair of coaxial, toroidal, and permanent magnets having axially directed poles. Like poles of the permanent magnets are adjacent to each other, whereby the permanent magnets have a tendency to be urged apart along the common axis. An electromagnet is wound coaxially with the permanent magnets in such a manner that the poles are axially directed. Between the poles of each permanent magnet there is a low magnetic reluctance circuit including two series air gaps. Between the poles of the electromagnet a low reluctance path including only one air gap of each of the low magnetic reluctance circuits is provided. The low reluctance path for the electromagnet includes a ring axially translatable relative to the permanent magnets. The ring forms opposite faces of the air gaps in the magnetic circuits for each permanent magnet.

Studer, P. A. (inventor)

1975-01-01

33

Characteristics of soft magnetic composite material under rotating magnetic fluxes  

Microsoft Academic Search

This paper reports the measurement of magnetic properties of the soft magnetic composite material SOMALOYTM 500 in a square sample under different patterns of flux density with 2D magnetic excitations. The test system, principle of measurement, magnetic power loss calculation, and methods of correction for misalignment of H surface sensing coils are presented. The experimental results show that although nominally

J. J. Zhong; Y. G. Guo; J. G. Zhu; Z. W. Lin

2006-01-01

34

Magnetic flux tubes as coherent structures  

Microsoft Academic Search

\\u000a We perform direct simulations of turbulent MHD in which the initial magnetic field is weak. Just as vortex tubes appear spontaneously\\u000a in hydrodynamic turbulence, we find that the magnetic field occurs primarily in the form of flux tubes. The dynamical interplay\\u000a between the magnetic tubes and the flow is explored during a time interval in which the total magnetic energy

A. Brandenburg; R. L. Jennings; Å. Nordlund; R. F. Stein

35

Flux Emergence in a Magnetized Convection Zone  

NASA Astrophysics Data System (ADS)

We study the influence of a dynamo magnetic field on the buoyant rise and emergence of twisted magnetic flux ropes and their influence on the global external magnetic field. We ran three-dimensional MHD numerical simulations using the ASH code (anelastic spherical harmonics) and analyzed the dynamical evolution of such buoyant flux ropes from the bottom of the convection zone until the post-emergence phases. The global nature of this model can only very crudely and inaccurately represent the local dynamics of the buoyant rise of the implanted magnetic structure, but nonetheless allows us to study the influence of global effects, such as self-consistently generated differential rotation and meridional circulation, and of Coriolis forces. Although motivated by the solar context, this model cannot be thought of as a realistic model of the rise of magnetic structures and their emergence in the Sun, where the local dynamics are completely different. The properties of initial phases of the buoyant rise are determined essentially by the flux-rope's properties and the convective flows and consequently are in good agreement with previous studies. However, the effects of the interaction of the background dynamo field become increasingly strong as the flux ropes evolve. During the buoyant rise across the convection zone, the flux-rope's magnetic field strength scales as Bvprop??, with ? <~ 1. An increase of radial velocity, density, and current density is observed to precede flux emergence at all longitudes. The geometry, latitude, and relative orientation of the flux ropes with respect to the background magnetic field influences the resulting rise speeds, zonal flow amplitudes (which develop within the flux ropes), and the corresponding surface signatures. This influences the morphology, duration and amplitude of the surface shearing, and the Poynting flux associated with magnetic flux-rope emergence. The emerged magnetic flux influences the system's global polarity, leading in some cases to a polarity reversal while inhibiting the background dynamo from doing so in others. The emerged magnetic flux is slowly advected poleward while being diffused and assimilated by the background dynamo field.

Pinto, R. F.; Brun, A. S.

2013-07-01

36

FLUX EMERGENCE IN A MAGNETIZED CONVECTION ZONE  

SciTech Connect

We study the influence of a dynamo magnetic field on the buoyant rise and emergence of twisted magnetic flux ropes and their influence on the global external magnetic field. We ran three-dimensional MHD numerical simulations using the ASH code (anelastic spherical harmonics) and analyzed the dynamical evolution of such buoyant flux ropes from the bottom of the convection zone until the post-emergence phases. The global nature of this model can only very crudely and inaccurately represent the local dynamics of the buoyant rise of the implanted magnetic structure, but nonetheless allows us to study the influence of global effects, such as self-consistently generated differential rotation and meridional circulation, and of Coriolis forces. Although motivated by the solar context, this model cannot be thought of as a realistic model of the rise of magnetic structures and their emergence in the Sun, where the local dynamics are completely different. The properties of initial phases of the buoyant rise are determined essentially by the flux-rope's properties and the convective flows and consequently are in good agreement with previous studies. However, the effects of the interaction of the background dynamo field become increasingly strong as the flux ropes evolve. During the buoyant rise across the convection zone, the flux-rope's magnetic field strength scales as B{proportional_to}{rho}{sup {alpha}}, with {alpha} {approx}< 1. An increase of radial velocity, density, and current density is observed to precede flux emergence at all longitudes. The geometry, latitude, and relative orientation of the flux ropes with respect to the background magnetic field influences the resulting rise speeds, zonal flow amplitudes (which develop within the flux ropes), and the corresponding surface signatures. This influences the morphology, duration and amplitude of the surface shearing, and the Poynting flux associated with magnetic flux-rope emergence. The emerged magnetic flux influences the system's global polarity, leading in some cases to a polarity reversal while inhibiting the background dynamo from doing so in others. The emerged magnetic flux is slowly advected poleward while being diffused and assimilated by the background dynamo field.

Pinto, R. F.; Brun, A. S., E-mail: rui.pinto@cea.fr [Laboratoire AIM Paris-Saclay, CEA/Irfu Universite Paris-Diderot CNRS/INSU, F-91191 Gif-sur-Yvette (France)

2013-07-20

37

Flux Noise Probed with Real Time Qubit Tomography in a Josephson Phase Qubit  

NASA Astrophysics Data System (ADS)

We measure the dependence of qubit phase coherence and flux noise on inductor loop geometry. While wider inductor traces change neither the flux noise power spectrum nor the qubit dephasing time, increased inductance leads to a simultaneous increase in both. Using our new tomographic protocol for measuring low frequency flux noise, we make a direct comparison between the flux noise spectrum and qubit phase decay, finding agreement within 10% of theory.

Sank, Daniel; Barends, R.; Bialczak, Radoslaw C.; Chen, Yu; Kelly, J.; Lenander, M.; Lucero, E.; Mariantoni, Matteo; Megrant, A.; Neeley, M.; O'Malley, P. J. J.; Vainsencher, A.; Wang, H.; Wenner, J.; White, T. C.; Yamamoto, T.; Yin, Yi; Cleland, A. N.; Martinis, John M.

2012-08-01

38

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.

39

Magnetic topology of emerging flux regions  

NASA Astrophysics Data System (ADS)

Coronal magnetic fields structure and governs the dynamics of the solar atmosphere. These magnetic fields are often complex, composed of multiples domains of magnetic-field-lines connectivity. The topology of the magnetic field allows a synthetic description of these complex magnetic field by highlighting the structural elements that are important for the dynamic and the activity of the corona. Topology identifies the key elements where magnetic reconnection will preferentially occurs, and allows to explain and predict the evolution of the coronal plasma. However the topological elements - such as null points, separatrices, separators - do not appear out of thin air. Along with energy, and helicity, the magnetic topology of an active region is build up as the consequence of flux emergence. Some topological elements, such as bald-patches, are even fully part of the mechanism of flux emergence mechanism and drive the evolution and the structuration of the coronal magnetic field as it crosses the lower layer of the solar atmosphere. In the present talk I will therefore review our current understanding of the formation of active region in terms of magnetic topology. I will speak on how the topological structures which are key to solar activity are formed. Meanwhile I'll also discus the topological properties of emerging active region and how topology influences the very process of flux emergence.

Pariat, Etienne

40

Crossed Flux Tubes Magnetic Reconnection Experiment  

NASA Astrophysics Data System (ADS)

The dynamics of arched, plasma-filled flux tubes have been studied in experiments at Caltech. These flux tubes expand, undergo kink instabilities, magnetically reconnect, and are subject to magnetohydrodynamic forces. An upgraded experiment will arrange for two of these flux tubes to cross over each other. It is expected then that the flux tubes will undergo magnetic reconnection at the crossover point, forming one long flux tube and one short flux tube. This reconnection should also result in a half-twist in the flux tubes at the crossover point, which will propagate along each tube as Alfv'en waves. The control circuitry requires two independent floating high energy capacitor power supplies to power the plasma loops, which will be put in series when the plasma loops reconnect. Coordinating these two power supplies requires the building of new systems for controlling plasma generation. Unlike with previous designs, all timing functions are contained on a single printed circuit board, allowing the design to be easily replicated for use with each independent capacitor involved. The control circuit sequencing has been tested successfully in generating a single flux tube. The plasma gun is currently under construction, with its installation pending completion of prior experiments.

Tobin, Zachary; Bellan, Paul

2012-10-01

41

Solar cycle variation of magnetic flux emergence  

NASA Technical Reports Server (NTRS)

The number of X-ray bright points (XBP) has been measured from solar X-ray images obtained during two rocket flights in 1976. When compared with the data obtained during the Skylab mission (1973), the number is found to be higher by a factor of 2. As the probability of obtaining the result by chance is less than 1 in 5 million, it is concluded that the number of XBP has increased in the three year interval. As all other indicators of activity have decreased between 1973 and 1976, the cyclical variation of the short-lifetime end of the magnetic-flux-emergence spectrum is out of phase with the solar cycle as defined by active regions or sunspots. Since XBP in 1973 contributed more to the emerging magnetic flux than did active regions, the possibility exists that the total amount of emerging magnetic flux may be maximized at a sunspot minimum.

Davis, J. M.; Golub, L.; Kreiger, A. S.

1977-01-01

42

Vacuum Currents around a Magnetic Flux String  

NASA Astrophysics Data System (ADS)

We examine vacuum polarization effects around a singular magnetic flux string in 2+1 and 3+1 dimensions. In particular we determine the local effects: charge and current densities and the flux induced by the vacuum current. The analysis is performed entirely on the basis of exact solutions to the pertinent Dirac equations, and we find that the (3+1)-dimensional effects can be described in a simple way as a superposition of the (2+1)-dimensional effects.

Flekkøy, E. G.; Leinaas, J. M.

43

Vacuum Currents around a Magnetic Flux String  

Microsoft Academic Search

We examine vacuum polarization effects around a singular magnetic flux string in 2+1 and 3+1 dimensions. In particular we determine the local effects: charge and current densities and the flux induced by the vacuum current. The analysis is performed entirely on the basis of exact solutions to the pertinent Dirac equations, and we find that the (3+1)-dimensional effects can be

E. G. Flekkøy; J. M. Leinaas

1991-01-01

44

Magnetic flux reconstruction methods for shaped tokamaks  

SciTech Connect

The use of a variational method permits the Grad-Shafranov (GS) equation to be solved by reducing the problem of solving the 2D non-linear partial differential equation to the problem of minimizing a function of several variables. This high speed algorithm approximately solves the GS equation given a parameterization of the plasma boundary and the current profile (p` and FF` functions). The author treats the current profile parameters as unknowns. The goal is to reconstruct the internal magnetic flux surfaces of a tokamak plasma and the toroidal current density profile from the external magnetic measurements. This is a classic problem of inverse equilibrium determination. The current profile parameters can be evaluated by several different matching procedures. Matching of magnetic flux and field at the probe locations using the Biot-Savart law and magnetic Green`s function provides a robust method of magnetic reconstruction. The matching of poloidal magnetic field on the plasma surface provides a unique method of identifying the plasma current profile. However, the power of this method is greatly compromised by the experimental errors of the magnetic signals. The Casing Principle provides a very fast way to evaluate the plasma contribution to the magnetic signals. It has the potential of being a fast matching method. The performance of this method is hindered by the accuracy of the poloidal magnetic field computed from the equilibrium solver. A flux reconstruction package has been implemented which integrates a vacuum field solver using a filament model for the plasma, a multi-layer perception neural network as an interface, and the volume integration of plasma current density using Green`s functions as a matching method for the current profile parameters. The flux reconstruction package is applied to compare with the ASEQ and EFIT data. The results are promising.

Tsui, Chi-Wa

1993-12-01

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

Photospheric Magnetic Flux Transport - Supergranules Rule  

NASA Technical Reports Server (NTRS)

Observations of the transport of magnetic flux in the Sun's photosphere show that active region magnetic flux is carried far from its origin by a combination of flows. These flows have previously been identified and modeled as separate axisymmetric processes: differential rotation, meridional flow, and supergranule diffusion. Experiments with a surface convective flow model reveal that the true nature of this transport is advection by the non-axisymmetric cellular flows themselves - supergranules. Magnetic elements are transported to the boundaries of the cells and then follow the evolving boundaries. The convective flows in supergranules have peak velocities near 500 m/s. These flows completely overpower the superimposed 20 m/s meridional flow and 100 m/s differential rotation. The magnetic elements remain pinned at the supergranule boundaries. Experiments with and without the superimposed axisymmetric photospheric flows show that the axisymmetric transport of magnetic flux is controlled by the advection of the cellular pattern by underlying flows representative of deeper layers. The magnetic elements follow the differential rotation and meridional flow associated with the convection cells themselves -- supergranules rule!

Hathaway, David H.; Rightmire-Upton, Lisa

2012-01-01

47

Magnetic Flux Transients during Solar Flares  

NASA Astrophysics Data System (ADS)

Solar flares result from the sudden release of energy stored in the magnetic field of the solar atmosphere, attributed to magnetic reconnection. In this work, we use line-of-sight magnetograms to study the changes in photospheric magnetic field during large solar flares. The magnetograms are derived from observations using NASA's Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory, and have a cadence of 3 minutes at a 0.5 arcsecond spatial resolution. We studied the inferred magnetic flux changes in 11 X-class flares from (2011-2012) and 26 M-class flares (2011). Of the 37 flares, 32 exhibited short-lived (less than 30 minutes) magnetic flux transients (MFTs) during the progress of the flare, similar to those by Maurya et al. (2012). We note that MFTs were co-temporal with GOES X-ray peaks. Flares with rapid rises (impulsive flares) had stronger transients while those with slower rises (gradual flares) had weak or no MFTs. Finally, flares with stronger GOES X-ray peaks (flare class) showed stronger MFTs. We believe that these changes are non-physical because the changes in the magnetic field are transient (the magnetic field returns to the pre-flare state) and coincide with the impulsive phase of the flare. This work supported by the US Airforce Office of Scientific Research and the AFRL/RV Space Scholar Program.

Balasubramaniam, K. S.; Delgado, F.; Hock, R. A.

2013-12-01

48

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

49

Optimization of magnetic flux density measurement using multiple RF receiver coils and multi-echo in MREIT  

NASA Astrophysics Data System (ADS)

Magnetic Resonance Electrical Impedance Tomography (MREIT) is an MRI method that enables mapping of internal conductivity and/or current density via measurements of magnetic flux density signals. The MREIT measures only the z-component of the induced magnetic flux density B = (Bx, By, Bz) by external current injection. The measured noise of Bz complicates recovery of magnetic flux density maps, resulting in lower quality conductivity and current-density maps. We present a new method for more accurate measurement of the spatial gradient of the magnetic flux density gradient (? Bz). The method relies on the use of multiple radio-frequency receiver coils and an interleaved multi-echo pulse sequence that acquires multiple sampling points within each repetition time. The noise level of the measured magnetic flux density Bz depends on the decay rate of the signal magnitude, the injection current duration, and the coil sensitivity map. The proposed method uses three key steps. The first step is to determine a representative magnetic flux density gradient from multiple receiver coils by using a weighted combination and by denoising the measured noisy data. The second step is to optimize the magnetic flux density gradient by using multi-echo magnetic flux densities at each pixel in order to reduce the noise level of ? Bz and the third step is to remove a random noise component from the recovered ? Bz by solving an elliptic partial differential equation in a region of interest. Numerical simulation experiments using a cylindrical phantom model with included regions of low MRI signal to noise (‘defects’) verified the proposed method. Experimental results using a real phantom experiment, that included three different kinds of anomalies, demonstrated that the proposed method reduced the noise level of the measured magnetic flux density. The quality of the recovered conductivity maps using denoised ? Bz data showed that the proposed method reduced the conductivity noise level up to 3-4 times at each anomaly region in comparison to the conventional method.

Jeong, Woo Chul; Chauhan, Munish; Sajib, Saurav Z. K.; Kim, Hyung Joong; Serša, Igor; In Kwon, Oh; Woo, Eung Je

2014-09-01

50

Electric Flux Tube in Magnetic Plasma  

E-print Network

In this paper we study a methodical problem related to the magnetic scenario recently suggested and initiated by the authors \\cite{Liao_ES_mono} to understand the strongly coupled quark-gluon plasma (sQGP): the electric flux tube in monopole plasma. A macroscopic approach, interpolating between Bose condensed (dual superconductor) and classical gas medium is developed first. Then we work out a microscopic approach based on detailed quantum mechanical calculation of the monopole scattering on electric flux tube, evaluating induced currents for all partial waves. As expected, the flux tube looses its stability when particles can penetrate it: we make this condition precise by calculating the critical value for the product of the flux tube size times the particle momentum, above which the flux tube dissolves. Lattice static potentials indicate that flux tubes seem to dissolve at $T>T_{dissolution} \\approx 1.3 T_c$. Using our criterion one gets an estimate of the magnetic density $n\\approx 4.4 \\sim 6.6 fm^{-3}$ at this temperature.

Jinfeng Liao; Edward Shuryak

2007-12-06

51

Solar cycle variations of heliospheric magnetic flux  

NASA Astrophysics Data System (ADS)

We extend previous studies of BR, the radial component of the heliospheric magnetic field and a measure of heliospheric magnetic flux, to include the maximum in cycle 23 so that most of 4 cycles are covered. Solar rotation averages reveal that BR systematically increases from a minimum value at sunspot minimum to a maximum value during the declining phase. This increase is interrupted by a temporary decrease associated with the disappearance and reversal of the Sun's polar cap fields. We examine variations in BR and field strength, B, and find a close correspondence consistent with the Parker model when allowance is made for the ever-present fluctuations in the heliospheric magnetic field. The solar cycle variation in BR and B is consistent with the widely accepted model of the solar field in which bipolar sunspot fields emerging at solar maximum are responsible for the erosion and reversal of the polar cap fields. Although the solar cycle change in heliospheric magnetic flux is less than a factor of 2, it is very large compared with the average flux transported into the heliosphere by an interplanetary coronal mass ejection (ICME). If the increases in B and BR at solar maximum are attributed to closed ICME fields, several hundred ICMEs must connect to the Sun for about two solar rotations before disconnecting, consistent with a recent model that attributes the increase in heliospheric flux at solar maximum to ICMEs. Future studies must distinguish between the open and closed topologies in order to evaluate their relative contributions.

Zhou, Xiaoyan; Smith, Edward J.

2009-03-01

52

Synthetic magnetic fluxes on the honeycomb lattice  

SciTech Connect

We devise experimental schemes that are able to mimic uniform and staggered magnetic fluxes acting on ultracold two-electron atoms, such as ytterbium atoms, propagating in a honeycomb lattice. The atoms are first trapped into two independent state-selective triangular lattices and then further exposed to a suitable configuration of resonant Raman laser beams. These beams induce hops between the two triangular lattices and make atoms move in a honeycomb lattice. Atoms traveling around each unit cell of this honeycomb lattice pick up a nonzero phase. In the uniform case, the artificial magnetic flux sustained by each cell can reach about two flux quanta, thereby realizing a cold-atom analog of the Harper model with its notorious Hofstadter's butterfly structure. Different condensed-matter phenomena such as the relativistic integer and fractional quantum Hall effects, as observed in graphene samples, could be targeted with this scheme.

Gorecka, Agnieszka [Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543 (Singapore); Gremaud, Benoit [Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543 (Singapore); Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 (Singapore); Laboratoire Kastler Brossel, Ecole Normale Superieure, CNRS, UPMC, 4 Place Jussieu, FR-75005 Paris (France); Miniatura, Christian [Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543 (Singapore); Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 (Singapore); Institut Non Lineaire de Nice, UMR 6618, UNS, CNRS, 1361 Route des Lucioles, FR-06560 Valbonne (France); Institute of Advanced Studies, Nanyang Technological university, 60 Nanyang View, Singapore 639673 (Singapore)

2011-08-15

53

Magnetic flux concentrations in a polytropic atmosphere  

NASA Astrophysics Data System (ADS)

Context. Strongly stratified hydromagnetic turbulence has recently been identified as a candidate for explaining the spontaneous formation of magnetic flux concentrations by the negative effective magnetic pressure instability (NEMPI). Much of this work has been done for isothermal layers, in which the density scale height is constant throughout. Aims: We now want to know whether earlier conclusions regarding the size of magnetic structures and their growth rates carry over to the case of polytropic layers, in which the scale height decreases sharply as one approaches the surface. Methods: To allow for a continuous transition from isothermal to polytropic layers, we employ a generalization of the exponential function known as the q-exponential. This implies that the top of the polytropic layer shifts with changing polytropic index such that the scale height is always the same at some reference height. We used both mean-field simulations (MFS) and direct numerical simulations (DNS) of forced stratified turbulence to determine the resulting flux concentrations in polytropic layers. Cases of both horizontal and vertical applied magnetic fields were considered. Results: Magnetic structures begin to form at a depth where the magnetic field strength is a small fraction of the local equipartition field strength with respect to the turbulent kinetic energy. Unlike the isothermal case where stronger fields can give rise to magnetic flux concentrations at larger depths, in the polytropic case the growth rate of NEMPI decreases for structures deeper down. Moreover, the structures that form higher up have a smaller horizontal scale of about four times their local depth. For vertical fields, magnetic structures of super-equipartition strengths are formed, because such fields survive downward advection that causes NEMPI with horizontal magnetic fields to reach premature nonlinear saturation by what is called the "potato-sack" effect. The horizontal cross-section of such structures found in DNS is approximately circular, which is reproduced with MFS of NEMPI using a vertical magnetic field. Conclusions: Results based on isothermal models can be applied locally to polytropic layers. For vertical fields, magnetic flux concentrations of super-equipartition strengths form, which supports suggestions that sunspot formation might be a shallow phenomenon.

Losada, I. R.; Brandenburg, A.; Kleeorin, N.; Rogachevskii, I.

2014-04-01

54

SYNOPTIC MAPPING OF CHROMOSPHERIC MAGNETIC FLUX  

SciTech Connect

We used daily full-disk Ca II 854.2 nm magnetograms from the Synoptic Optical Long Term Investigations of the Sun (SOLIS) facility to study the chromospheric magnetic field from 2006 April through 2009 November. We determined and corrected previously unidentified zero offsets in the SOLIS magnetograms. By tracking the disk passages of stable unipolar regions, the measured net flux densities were found to systematically decrease from the disk center to the limb by a factor of about two. This decrease was modeled using a thin flux tube model with a difference in signal formation height between the center and limb sides. Comparison of photospheric and chromospheric observations shows that their differences are largely due to horizontal spreading of magnetic flux with increasing height. The north polar magnetic field decreased nearly linearly with time during our study period while the south polar field was nearly constant. We used the annual change in the viewing angle of the polar regions to estimate the radial and meridional components of the polar fields and found that the south polar fields were tilted away from the pole. Synoptic maps of the chromospheric radial flux density distribution were used as boundary conditions for extrapolation of the field from the chromosphere into the corona. A comparison of modeled and observed coronal hole boundaries and coronal streamer positions showed better agreement when using the chromospheric rather than the photospheric synoptic maps.

Jin, C. L. [Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 (China); Harvey, J. W.; Pietarila, A., E-mail: cljin@nao.cas.cn, E-mail: jharvey@nso.edu, E-mail: apietarila@nso.edu [National Solar Observatory, Tucson, AZ 85719 (United States)

2013-03-10

55

What is a flux tube? On the magnetic field topology of buoyant flux structures  

E-print Network

in the language of flux tube models (see e.g. Ossendrijver (2003) and references therein). One of the distinctiveWhat is a flux tube? On the magnetic field topology of buoyant flux structures Fausto Cattaneo, visually resembling idealised magnetic flux tubes, are generated self-consistently by numerical simulation

56

Magnetic Flux Compression Experiments Using Plasma Armatures  

NASA Technical Reports Server (NTRS)

Magnetic flux compression reaction chambers offer considerable promise for controlling the plasma flow associated with various micronuclear/chemical pulse propulsion and power schemes, primarily because they avoid thermalization with wall structures and permit multicycle operation modes. The major physical effects of concern are the diffusion of magnetic flux into the rapidly expanding plasma cloud and the development of Rayleigh-Taylor instabilities at the plasma surface, both of which can severely degrade reactor efficiency and lead to plasma-wall impact. A physical parameter of critical importance to these underlying magnetohydrodynamic (MHD) processes is the magnetic Reynolds number (R(sub m), the value of which depends upon the product of plasma electrical conductivity and velocity. Efficient flux compression requires R(sub m) less than 1, and a thorough understanding of MHD phenomena at high magnetic Reynolds numbers is essential to the reliable design and operation of practical reactors. As a means of improving this understanding, a simplified laboratory experiment has been constructed in which the plasma jet ejected from an ablative pulse plasma gun is used to investigate plasma armature interaction with magnetic fields. As a prelude to intensive study, exploratory experiments were carried out to quantify the magnetic Reynolds number characteristics of the plasma jet source. Jet velocity was deduced from time-of-flight measurements using optical probes, and electrical conductivity was measured using an inductive probing technique. Using air at 27-inHg vacuum, measured velocities approached 4.5 km/s and measured conductivities were in the range of 30 to 40 kS/m.

Turner, M. W.; Hawk, C. W.; Litchford, R. J.

2003-01-01

57

A control method of excitation winding magnetization for a novel HTS magnetic flux pump  

Microsoft Academic Search

The main purpose of magnetic flux pump is to stabilize magnetic field of superconducting magnets. This paper chiefly presents a kind of control method to magnetize excitation winding and control system design for a novel high temperature superconducting(HTS) magnetic flux pump. The basic structure and operation principle of the flux pump is simply described here. According to a stated magnetizing

Chunli Wu; Fengchao Lv; Ying Chen

2011-01-01

58

Magnetic-Flux-Compensated Voltage Divider  

NASA Technical Reports Server (NTRS)

A magnetic-flux-compensated voltage-divider circuit has been proposed for use in measuring the true potential across a component that is exposed to large, rapidly varying electric currents like those produced by lightning strikes. An example of such a component is a lightning arrester, which is typically exposed to currents of the order of tens of kiloamperes, having rise times of the order of hundreds of nanoseconds. Traditional voltage-divider circuits are not designed for magnetic-flux-compensation: They contain uncompensated loops having areas large enough that the transient magnetic fluxes associated with large transient currents induce spurious voltages large enough to distort voltage-divider outputs significantly. A drawing of the proposed circuit was not available at the time of receipt of information for this article. What is known from a summary textual description is that the proposed circuit would contain a total of four voltage dividers: There would be two mixed dividers in parallel with each other and with the component of interest (e.g., a lightning arrester), plus two mixed dividers in parallel with each other and in series with the component of interest in the same plane. The electrical and geometric configuration would provide compensation for induced voltages, including those attributable to asymmetry in the volumetric density of the lightning or other transient current, canceling out the spurious voltages and measuring the true voltage across the component.

Mata, Carlos T.

2005-01-01

59

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

60

Magnetic Flux Transport at the Solar Surface  

NASA Astrophysics Data System (ADS)

After emerging to the solar surface, the Sun's magnetic field displays a complex and intricate evolution. The evolution of the surface field is important for several reasons. One is that the surface field, and its dynamics, sets the boundary condition for the coronal and heliospheric magnetic fields. Another is that the surface evolution gives us insight into the dynamo process. In particular, it plays an essential role in the Babcock-Leighton model of the solar dynamo. Describing this evolution is the aim of the surface flux transport model. The model starts from the emergence of magnetic bipoles. Thereafter, the model is based on the induction equation and the fact that after emergence the magnetic field is observed to evolve as if it were purely radial. The induction equation then describes how the surface flows—differential rotation, meridional circulation, granular, supergranular flows, and active region inflows—determine the evolution of the field (now taken to be purely radial). In this paper, we review the modeling of the various processes that determine the evolution of the surface field. We restrict our attention to their role in the surface flux transport model. We also discuss the success of the model and some of the results that have been obtained using this model.

Jiang, J.; Hathaway, D. H.; Cameron, R. H.; Solanki, S. K.; Gizon, L.; Upton, L.

2014-12-01

61

Slip Running Reconnection in Magnetic Flux Ropes  

NASA Astrophysics Data System (ADS)

Magnetic flux ropes are due to helical currents and form a dense carpet of arches on the surface of the sun. Occasionally one tears loose as a coronal mass ejection and its rope structure can be detected by satellites close to the earth. Current sheets can tear into filaments and these are nothing other than flux ropes. Ropes are not static, they exert mutual ?c{J}×?c{B} forces causing them to twist about each other and eventually merge. Kink instabilities cause them to violently smash into each other and reconnect at the point of contact. We report on experiments on two adjacent ropes done in the large plasma device (LAPD) at UCLA ( ne ˜ 1012, Te ˜ 6 eV, B0z=330G, Brope}\\cong{10G,trep=1 Hz). The currents and magnetic fields form exotic shapes with no ignorable direction and no magnetic nulls. Volumetric space-time data (70,600 spatial locations) show multiple reconnection sites with time-dependent locations. The concept of a quasi-separatrix layer (QSL), a tool to understand and visualize 3D magnetic field lines reconnection without null points is introduced. Three-dimensional measurements of the QSL derived from magnetic field data are presented. Within the QSL field lines that start close to one another rapidly diverge as they pass through one or more reconnection regions. The motion of magnetic field lines are traced as reconnection proceeds and they are observed to slip through the regions of space where the QSL is largest. As the interaction proceeds we double the current in the ropes. This accompanied by intense heating as observed in uv light and plasma flows measured by Mach probes. The interaction of the ropes is clearly seen by vislaulizng magnetic field data , as well as in images from a fast framing camera. Work supported by the Dept. of Energy and The National Science Foundation, done at the Basic Plasma Science Facility at UCLA.Magnetic Field lines (measured) of three flux ropes and the plasma currents associated with them

Gekelman, W. N.; Van Compernolle, B.; Vincena, S. T.; De Hass, T.

2012-12-01

62

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

63

Ultrarelativistic Magnetic Monopole Flux Constraints from RICE  

NASA Astrophysics Data System (ADS)

The Radio Ice Cherenkov Experiment (RICE) is a radio array buried in the Antarctic ice. A Monte Carlo simulation has been developed to model the behavior of an ultrarelativistic magnetic monopole propagating through the ice and RICE's response. With this, we've determined RICE's effective volume for detecting these particles as a function of monopole mass, charge, and gamma-value. As previously reported, RICE failed to detect an in-ice high-energy Cherenkov radiation signal consistent with a relativistic magnetic monopole over roughly two years of livetime. From this null result and the effective volume calculation, we can extrapolate upper bounds on ultrarelativistic magnetic monopole flux. Provisional values are presented.

Hogan, Daniel

2006-04-01

64

Magnetic flux in modeled magnetic clouds at 1 AU and some specific comparisons to associated photospheric flux  

NASA Technical Reports Server (NTRS)

In order to better understand the solar origins of magnetic clouds, statistical distributions of the estimated axial magnetic flux of 30 magnetic clouds at 1 AU, separated according to their occurrence during the solar cycle, were obtained and a comparison was made of the magnetic flux of a magnetic cloud to the aggregate flux of apparently associated photospheric magnetic flux tubes, for some specific cases. The 30 magnetic clouds comprise 12 cases from WIND, and the remainder from IMP-8, earlier IMPs, the International Sun-Earth Explorer (ISEE) 3 and HELIOS. The total magnetic flux along the cloud axis was estimated using a constant alpha, cylindrical, force-free flux rope model to determine cloud diameter and axial magentic field strength. The distribution of magentic fluxes for the 30 clouds is shown to be in the form of a skewed Gaussian.

Lepping, R. P.; Szabo, A.; DeForest, C. E.; Thompson, B. J.

1997-01-01

65

Turbulent Erosion of Magnetic Flux Tubes  

E-print Network

Results from a numerical and analytical investigation of the solution of a nonlinear axially symmetric diffusion equation for the magnetic field are presented for the case when the nonlinear dependence of the diffusivity nu(B) on the magnetic field satisfies basic physical requirements. We find that for sufficiently strong nonlinearity (i.e. for sufficiently strong reduction of nu inside the tube) a current sheet is spontaneously formed around the tube within one diffusion timescale. This sheet propagates inwards with a velocity inversely proportional to the ratio of the field strength just inside the current sheet to the equipartition field strength B0/Be, so the lifetime of a tube with constant internal flux density is increased approximately by a factor not exceeding B0/Be, even for infinitely effective inhibition of turbulence inside the tube. Among the applications of these results we point out that toroidal flux tubes in the solar convective zone are subject to significant flux loss owing to turbulent erosion on a timescale of about 1 month, and that turbulent erosion may be responsible for the formation of a current sheet around a sunspot. It is further proposed that, despite the simplifying assumptions involved, our solutions correctly reflect the essential features of the sunspot decay process.

K. Petrovay; F. Moreno-Insertis

1998-12-29

66

Investigation of 1/f flux noise in SQUIDs and Superconducting Qubits  

NASA Astrophysics Data System (ADS)

Low-frequency 1/f flux noise is a dominant source of dephasing in the Josephson phase and flux qubits. Recent work has revealed the presence of a high density of unpaired spins at the surfaces of superconducting thin films; it is now believed that these spins are the source of the noise, although the microscopic noise mechanism is not understood. We have recently shown that the dielectric encapsulation of the SQUID loop substantially impacts the noise magnitude and noise exponent. Here we describe experiments on SQUIDs and Josephson phase qubits designed to shed light on the underlying noise mechanism, and we describe efforts to develop Josephson phase qubits with reduced levels of 1/f flux noise and improved dephasing times.

Puglielli, Antonio; Sendelbach, Steven; Klaus, Taylor; McDermott, Robert

2013-03-01

67

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

68

Quantitative observation of magnetic flux distribution in new magnetic films for future high  

E-print Network

configuration of magnetic nanoparticles, 15 magnetic films, 16 magnetic tunnel junctions, 17 or even magnetiteQuantitative observation of magnetic flux distribution in new magnetic films for future high

Recanati, Catherine

69

Small scale magnetic flux-averaged magnetohydrodynamics  

SciTech Connect

By relaxing exact magnetic flux conservation below a scale [lambda] a system of flux-averaged magnetohydrodynamic equations are derived from Hamilton's principle with modified constraints. An energy principle can be derived from the linearized averaged system because the total system energy is conserved. This energy principle is employed to treat the resistive tearing instability and the exact growth rate is recovered when [lambda] is identified with the resistive skin depth. A necessary and sufficient stability criteria of the tearing instability with line tying at the ends for solar coronal loops is also obtained. The method is extended to both spatial and temporal averaging in Hamilton's principle. The resulting system of equations not only allows flux reconnection but introduces irreversibility for appropriate choice of the averaging function. Except for boundary contributions which are modified by the time averaging process total energy and momentum are conserved over times much longer than the averaging time [tau] but not for less than [tau]. These modified boundary contributions correspond to the existence, also, of damped waves and shock waves in this theory. Time and space averaging is applied to electron magnetohydrodynamics and in one-dimensional geometry predicts solitons and shocks in different limits.

Pfirsch, D. (Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, D-8046 Garching (Germany)); Sudan, R.N. (Laboratory of Plasma Studies, Cornell University, Ithaca, New York 14853 (United States))

1994-08-01

70

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

NASA Technical Reports Server (NTRS)

A mechanism for the formation of fossil flux transfer events and the low-level boundary layer within the framework of multiple X-line reconnection is proposed. Attention is given to conditions for which the bulk of magnetic flux in a flux rope of finite extent has a simple magnetic topology, where the four possible connections of magnetic field lines are: IMF to MSP, MSP to IMF, IMF to IMF, and MSP to MSP. For a sufficient relative shift of the X lines, magnetic flux may enter a flux rope from the magnetosphere and exit into the magnetosphere. This process leads to the formation of magnetic flux ropes which contain a considerable amount of magnetosheath plasma on closed magnetospheric field lines. This process is discussed as a possible explanation for the formation of fossil flux transfer events in the magnetosphere and the formation of the low-latitude boundary layer.

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

1993-01-01

71

NONLINEAR THREE-DIMENSIONAL MAGNETOCONVECTION AROUND MAGNETIC FLUX TUBES  

SciTech Connect

Magnetic flux in the solar photosphere forms concentrations from small scales, such as flux elements, to large scales, such as sunspots. This paper presents a study of the decay process of large magnetic flux tubes, such as sunspots, on a supergranular scale. Three-dimensional nonlinear resistive magnetohydrodynamic numerical simulations are performed in a cylindrical domain, initialized with axisymmetric solutions that consist of a well-defined central flux tube and an annular convection cell surrounding it. As the nonlinear convection evolves, the annular cell breaks up into many cells in the azimuthal direction, allowing magnetic flux to slip between cells away from the central flux tube (turbulent erosion). This lowers magnetic pressure in the central tube, and convection grows inside the tube, possibly becoming strong enough to push the tube apart. A remnant of the central flux tube persists with nonsymmetric perturbations caused by the convection surrounding it. Secondary flux concentrations form between convection cells away from the central tube. Tube decay is dependent on the convection around the tube. Convection cells forming inside the tube as time-dependent outflows will remove magnetic flux. (This is most pronounced for small tubes.) Flux is added to the tube when flux caught in the surrounding convection is pushed toward it. The tube persists when convection inside the tube is sufficiently suppressed by the remaining magnetic field. All examples of persistent tubes have the same effective magnetic field strength, consistent with the observation that pores and sunspot umbrae all have roughly the same magnetic field strength.

Botha, G. J. J. [Centre for Fusion, Space and Astrophysics, Physics Department, University of Warwick, Coventry CV4 7AL (United Kingdom); Rucklidge, A. M. [Department of Applied Mathematics, University of Leeds, Leeds LS2 9JT (United Kingdom); Hurlburt, N. E., E-mail: G.J.J.Botha@warwick.ac.uk, E-mail: A.M.Rucklidge@leeds.ac.uk, E-mail: hurlburt@lmsal.com [Lockheed Martin Solar and Astrophysics Laboratory, Organization ADBS Building 252, Palo Alto, CA 94304 (United States)

2011-04-20

72

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

73

Noise analysis in fast magnetic resonance electrical impedance tomography (MREIT) based on spoiled multi gradient echo (SPMGE) pulse sequence  

NASA Astrophysics Data System (ADS)

Magnetic resonance electrical impedance tomography (MREIT) is a promising non-invasive method to visualize a static cross-sectional conductivity and/or current density image by injecting low frequency currents. MREIT measures one component of the magnetic flux density caused by the injected current using a magnetic resonance (MR) scanner. For practical in vivo implementations of MREIT, especially for soft biological tissues where the MR signal rapidly decays, it is crucial to develop a technique for optimizing the magnetic flux density signal by the injected current while maintaining spatial-resolution and contrast. We design an MREIT pulse sequence by applying a spoiled multi-gradient-echo pulse sequence (SPMGE) to the injected current nonlinear encoding (ICNE), which fully injects the current at the end of the read-out gradient. The applied ICNE-SPMGE pulse sequence maximizes the duration of injected current almost up to a repetition time by measuring multiple magnetic flux density data. We analyze the noise level of measured magnetic flux density with respect to the pulse width of injection current and T_{2}^{*} relaxation time. In due consideration of the ICNE-SPMGE pulse sequence, using a reference information of T_{2}^{*} values in a local region of interest by a short pre-scan data, we predict the noise level of magnetic flux density to be measured for arbitrary repetition time TR. Results from phantom experiment demonstrate that the proposed method can predict the noise level of magnetic flux density for an appropriate TR = 40 ms using a reference scan for TR = 75 ms. The predicted noise level was compared with the noise level of directly measured magnetic flux density data.

In Oh, Tong; Jeong, Woo Chul; Kim, Ji Eun; Sajib, Saurav Z. K.; Kim, Hyung Joong; In Kwon, Oh; Woo, Eung Je

2014-08-01

74

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

75

Magnetic flux array for spontaneous magnetic reconnection experiments A. Kesich, J. Bonde, J. Egedal,a  

E-print Network

Magnetic flux array for spontaneous magnetic reconnection experiments A. Kesich, J. Bonde, J; published online 24 June 2008 Experimental investigation of reconnection in magnetized plasmas relies on accurate characterization of the evolving magnetic fields. In experimental configurations where the plasma

Egedal, Jan

76

Improving signal-to-noise ratio of fetal magnetocardiograph by third order flux transformer  

NASA Astrophysics Data System (ADS)

The application of SQUIDS and superconducting radiometers in clinical biomagnetic instrumentation for fetal magnetocardiography is presented. The paper focuses on the characteristics and performance of the third order flux transformer for fetal magnetocardiography with a good signal to noise ratio. An optimum figure of merit of the third order flux transformer was determined. The optimal signal-to-noise ratio (SNR) was used as the optimization criterion.

Bachir, Wesam; Dunajski, Zbigniew

2003-10-01

77

Magnetic Flux Loss and Flux Transport in a Decaying Active Region  

E-print Network

We estimate the temporal change of magnetic flux perpendicular to the solar surface in a decaying active region by using a time series of the spatial distribution of vector magnetic fields in the photosphere. The vector magnetic fields are derived from full spectropolarimetric measurements with the Solar Optical Telescope aboard Hinode. We compare a magnetic flux loss rate to a flux transport rate in a decaying sunspot and its surrounding moat region. The amount of magnetic flux that decreases in the sunspot and moat region is very similar to magnetic flux transported to the outer boundary of the moat region. The flux loss rates [$(dF/dt)_{loss}$] of magnetic elements with positive and negative polarities are balanced each other around the outer boundary of the moat region. These results suggest that most of the magnetic flux in the sunspot is transported to the outer boundary of the moat region as moving magnetic features, and then removed from the photosphere by flux cancellation around the outer boundary of the moat region.

M. Kubo; B. W. Lites; T. Shimizu; K. Ichimoto

2008-07-28

78

A magnetic bearing control approach using flux feedback  

NASA Technical Reports Server (NTRS)

A magnetic bearing control approach using flux feedback is described and test results for a laboratory model magnetic bearing actuator are presented. Test results were obtained using a magnetic bearing test fixture, which is also described. The magnetic bearing actuator consists of elements similar to those used in a laboratory test model Annular Momentum Control Device (AMCD).

Groom, Nelson J.

1989-01-01

79

Magnetic Flux Transport by turbulent reconnection in astrophysical flows  

E-print Network

The role of MHD turbulence in astrophysical environments is still highly debated. An important question that permeates this debate is the transport of magnetic flux. This is particularly important, for instance, in the context of star formation. When clouds collapse gravitationally to form stars, there must be some magnetic flux transport. otherwise the new born stars would have magnetic fields several orders of magnitude larger than the observed ones. Also, the magnetic flux that is dragged in the late stages of the formation of a star can remove all the rotational support from the accretion disk that grows around the protostar. The efficiency of the mechanism which is often invoked to allow the transport of magnetic fields in the different stages of star formation, namely, the ambipolar diffusion, has been lately put in check. We here discuss an alternative mechanism for magnetic flux transport which is based on turbulent fast magnetic reconnection. We review recent results obtained from 3D MHD numerical si...

Pino, Elisabete M de Gouveia Dal; Santos-Lima, Reinaldo; Guerrero, Gustavo; Kowal, Grzegorz; Lazarian, Alex

2011-01-01

80

Evidence for temperature-dependent spin diffusion as a mechanism of intrinsic flux noise in SQUIDs  

NASA Astrophysics Data System (ADS)

The intrinsic flux noise observed in superconducting quantum interference devices (SQUIDs) is thought to be due to the fluctuation of electron-spin impurities, but the frequency and temperature dependence observed in experiments do not agree with the usual 1/f models. We present theoretical calculations and experimental measurements of flux noise in rf SQUID flux qubits that show how these observations, and previous reported measurements, can be interpreted in terms of a spin-diffusion constant that increases with temperature. We fit measurements of flux noise in 16 devices, taken in the 20--80 mK temperature range, to the spin-diffusion model. This allows us to extract the spin-diffusion constant and its temperature dependence, suggesting that the spin system is close to a spin-glass phase transition.

Lanting, T.; Amin, M. H.; Berkley, A. J.; Rich, C.; Chen, S.-F.; LaForest, S.; de Sousa, Rogério

2014-01-01

81

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

82

Flux Transport and the Sun's Global Magnetic Field  

NASA Technical Reports Server (NTRS)

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, David H.

2010-01-01

83

Quantitative observation of magnetic flux distribution in new magnetic films for future high  

E-print Network

- tensively studied by Magnetic Force Microscopy (MFM),6,7 micro-magnetic simulations,8 and other magneticQuantitative observation of magnetic flux distribution in new magnetic films for future high.masseboeuf@cemes.fr Abstract Off-axis electron holography was used to observe and quantify the magnetic microstruc- ture

Paris-Sud XI, Université de

84

Vortex Avalanches and Magnetic Flux Fragmentation in Superconductors  

SciTech Connect

We report the results of numerical simulations of nonisothermal dendritic flux penetration in type-II superconductors. We propose a generic mechanism of dynamic branching of a propagating hot spot of a flux flow/normal state triggered by a local heat pulse. The branching occurs when the flux hot spot reflects from inhomogeneities or the boundary on which magnetization currents either vanish, or change direction. The hot spot then undergoes a cascade of successive splittings, giving rise to a dissipative dendritic-type flux structure. This dynamic state eventually cools down, turning into a frozen multifilamentary pattern of magnetization currents.

Aranson, Igor; Gurevich, Alex; Vinokur, Valerii

2001-08-06

85

Signal Photon Flux and Background Noise in a Coupling Electromagnetic Detecting System for High Frequency Gravitational Waves  

E-print Network

A coupling system between Gaussian type-microwave photon flux, 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 pure inverse Gertsenshtein effect (G-effect) caused by the HFGWs in the GHz band, the the electromagnetic (EM) detecting scheme (EDS) proposed by China and the US HFGW groups is based on the composite effect of the synchro-resonance effect and the inverse G-effect. Key parameters in the scheme include 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 $\\sim10^3-10^5$ seconds for the typical laboratory condition and parameters of $h_{r.m.s.}\\sim10^{-26}-10^{-30}$ at 5GHz with bandwidth $\\sim$1Hz. 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.

F. Y. Li; N. Yang; Z. Y. Fang; R. M. L. Baker Jr.; G. V. Stephenson; H. Wen

2010-06-17

86

A novel high temperature superconducting magnetic flux pump for MRI magnets  

NASA Astrophysics Data System (ADS)

This paper presents a kind of minitype magnetic flux pump made of high temperature superconductor. This kind of novel high temperature superconducting (HTS) flux pump has not any mechanical revolving parts or thermal switches. The excitation current of copper coils in magnetic pole system is controlled by a singlechip. The structure design and operational principle have been described. The operating performance of the new model magnetic flux pump has been preliminarily tested. The experiments show that the maximum pumping current is approximately 200 A for Bi2223 flux pump and 80 A for MgB 2 flux pump operating at 20 K. By comparison, it is discovered that the operating temperature range is wider, the ripple is smaller and the pumping frequency is higher in Bi2223 flux pump than those in MgB 2 flux pump. These results indicate that the newly developed Bi2223 magnetic flux pump may efficiently compensate the magnetic field decay in HTS magnet and make the magnet operate in persistent current mode, this point is significant to the magnetic resonance imaging (MRI) magnets. This new flux pump is under construction presently. It is expected that the Bi2223 flux pump would be applied to the superconducting MRI magnets by further optimizing structure and improving working process.

Bai, Zhiming; Yan, Guo; Wu, Chunli; Ding, Shufang; Chen, Chuan

2010-10-01

87

Application of flux reversal principle for axial flux permanent magnet machines  

NASA Astrophysics Data System (ADS)

Applications of flux reversal machine can be found both for linear and radial flux machines. For axial flux machines, only one application is reported considering a two phase machine. In general, flux reversal effect can produce higher back electromotive forces than other topologies for the less amount of permanent magnets and thus can increase the performance of the machine. Combining the axial structure with the flux reversal effect feature allows higher power density machines suitable for high speed or low speed direct drive applications. Proposed model has a 12-slot stator and a 16-pole variable reluctance rotor. Nd-Fe-B magnets are fixed on the surface of the pole of the stator. The objective of this article is to present an effective method for the design and performance prediction for the axial flux reversal configuration. Basic dimension equations are presented and a two-dimensional equivalent model based on finite element analysis is used for the reduction of the simulation time.

Topor, Marcel; Chun, Yon-Do; Koo, Dae-Hyun; Han, Pil-Wan; Woo, Byung-Chul; Boldea, Ion

2008-04-01

88

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

89

The photospheric dynamo. I - Magnetic flux-tube generation  

NASA Astrophysics Data System (ADS)

Velocities of electrons, ions and neutrals are computed, in the three-fluid approximation, for an axisymmetrical magnetic field. By prescribing a radial dependence of the velocity of neutrals, in agreement with a downflow, the radial dependence of the magnetic field energy density is derived. This gives the radial flux of neutrals that is required to increase the magnetic field from 0.01 G to 1000 G inside thin magnetic flux tubes in the photosphere. The hypothesis of local cooling by advection of ionization energy of Schatten and Mayr (1985) is tested, and it is seen that this flux of neutrals is high enough to affect significantly the energy balance in the tube and to maintain the pressure gradient necessary to produce the radial inflow of neutrals. In agreement with recent infrared observations, the inside of the flux-tube is predicted to be cooler than the outside.

Henoux, J. C.; Somov, B. V.

1991-01-01

90

Evolution of the Magnetic Helicity Flux during the Formation and Eruption of Flux Ropes  

NASA Astrophysics Data System (ADS)

We describe the evolution and the magnetic helicity flux for two active regions (ARs) since their appearance on the solar disk: NOAA 11318 and NOAA 11675. Both ARs hosted the formation and destabilization of magnetic flux ropes. In the former AR, the formation of the flux rope culminated in a flare of C2.3 GOES class and a coronal mass ejection (CME) observed by Large Angle and Spectrometric Coronagraph Experiment. In the latter AR, the region hosting the flux rope was involved in several flares, but only a partial eruption with signatures of a minor plasma outflow was observed. We found a different behavior in the accumulation of the magnetic helicity flux in the corona, depending on the magnetic configuration and on the location of the flux ropes in the ARs. Our results suggest that the complexity and strength of the photospheric magnetic field is only a partial indicator of the real likelihood of an AR producing the eruption of a flux rope and a subsequent CME.

Romano, P.; Zuccarello, F. P.; Guglielmino, S. L.; Zuccarello, F.

2014-10-01

91

STUDY OF PERMANENT MAGNET TRANSVERSE FLUX MOTORS WITH SOFT MAGNETIC COMPOSITE CORE  

Microsoft Academic Search

Permanent magnet motors with transverse flux configuration have been developed for application in direct drive systems featuring high torque at low rotational speed. These machines apply simple toroidal windings and a modular construction, which guides the main flux path transversal to the direction of rotation. The three-dimensional nature of magnetic fields, however, makes the magnetic circuit lamination a serious disadvantage.

Y. G. Guo; J. G. Zhu

2004-01-01

92

Limits on Relativistic Magnetic Monopole Flux from RICE  

NASA Astrophysics Data System (ADS)

The Radio Ice Cherenkov Experiment (RICE) is a radio antenna array at the South Pole. A Monte Carlo simulation of magnetic monopole propagation through polar ice is used to determine RICE's cross-section for monopole detection. We present final results for ultrarelativistic (?>=10^7) magnetic monopole flux upper bounds based on RICE observations from 2001 through 2005. This limit is the strongest direct measurement of ultrarelativistic monopole flux.

Hogan, Daniel

2008-04-01

93

Spatial Transport of Magnetic Flux Surfaces in Strongly Anisotropic Turbulence  

NASA Astrophysics Data System (ADS)

Magnetic flux surfaces afford familiar descriptions of spatial structure, dynamics, and connectivity of magnetic fields, with particular relevance in contexts such as solar coronal flux tubes, magnetic field connectivity in the interplanetary and interstellar medium, as well as in laboratory plasmas and dynamo problems [1-4]. Typical models assume that field-lines are orderly, and flux tubes remain identifiable over macroscopic distances; however, a previous study has shown that flux tubes shred in the presence of fluctuations, typically losing identity after several correlation scales [5]. Here, the structure of magnetic flux surfaces is numerically investigated in a reduced magnetohydrodynamic (RMHD) model of homogeneous turbulence. Short and long-wavelength behavior is studied statistically by propagating magnetic surfaces along the mean field. At small scales magnetic surfaces become complex, experiencing an exponential thinning. At large scales, instead, the magnetic flux undergoes a diffusive behavior. The link between the diffusion of the coarse-grained flux and field-line random walk is established by means of a multiple scale analysis. Both large and small scales limits are controlled by the Kubo number. These results have consequences for understanding and interpreting processes such as magnetic reconnection and field-line diffusion in plasmas [6]. [1] E. N. Parker, Cosmical Magnetic Fields (Oxford Univ. Press, New York, 1979). [2] J. R. Jokipii and E. N. Parker, Phys. Rev. Lett. 21, 44 (1968). [3] R. Bruno et al., Planet. Space Sci. 49, 1201 (2001). [4] M. N. Rosenbluth et al., Nuclear Fusion 6, 297 (1966). [5] W. H. Matthaeus et al., Phys. Rev. Lett. 75, 2136 (1995). [6] S. Servidio et al., submitted (2013).

Matthaeus, W. H.; Servidio, S.; Wan, M.; Ruffolo, D. J.; Rappazzo, A. F.; Oughton, S.

2013-12-01

94

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

95

Regression Models for Identifying Noise Sources in Magnetic Resonance Images  

PubMed Central

Stochastic noise, susceptibility artifacts, magnetic field and radiofrequency inhomogeneities, and other noise components in magnetic resonance images (MRIs) can introduce serious bias into any measurements made with those images. We formally introduce three regression models including a Rician regression model and two associated normal models to characterize stochastic noise in various magnetic resonance imaging modalities, including diffusion-weighted imaging (DWI) and functional MRI (fMRI). Estimation algorithms are introduced to maximize the likelihood function of the three regression models. We also develop a diagnostic procedure for systematically exploring MR images to identify noise components other than simple stochastic noise, and to detect discrepancies between the fitted regression models and MRI data. The diagnostic procedure includes goodness-of-fit statistics, measures of influence, and tools for graphical display. The goodness-of-fit statistics can assess the key assumptions of the three regression models, whereas measures of influence can isolate outliers caused by certain noise components, including motion artifacts. The tools for graphical display permit graphical visualization of the values for the goodness-of-fit statistic and influence measures. Finally, we conduct simulation studies to evaluate performance of these methods, and we analyze a real dataset to illustrate how our diagnostic procedure localizes subtle image artifacts by detecting intravoxel variability that is not captured by the regression models. PMID:19890478

Zhu, Hongtu; Li, Yimei; Ibrahim, Joseph G.; Shi, Xiaoyan; An, Hongyu; Chen, Yashen; Gao, Wei; Lin, Weili; Rowe, Daniel B.; Peterson, Bradley S.

2009-01-01

96

Noise of MgO-based magnetic tunnel junctions  

NASA Astrophysics Data System (ADS)

Low-frequency noise has been measured in magnetic tunnel junctions with MgO barriers and magnetoresistance values up to 235%. We present here the noise for different degrees of crystallization and CoFeB/MgO interface quality depending on the annealing temperature. For optimized annealing temperature, an extremely low 1/ f noise, compared to magnetic junctions with Al 2O 3 barriers, has been observed. The origin of the low-frequency noise can be explained in terms of localized charge traps with the MgO barriers. Results for very thin CoFeB are presented in the second part as a function of temperature. Despite the absence of coercivity at room temperature for thinner free-layer structures, an important increase of Hc appears under 180 K. Meanwhile, the random telegraph noise present at room temperature is suppressed due to magnetic domains freezing. These results are discussed in view of various sensors applications of MgO-MTJ, giving advantages and drawbacks in terms of signal-to-noise ratio with respect to the operating temperature.

Polovy, H.; Guerrero, R.; Scola, J.; Pannetier-Lecoeur, M.; Fermon, C.; Feng, G.; Fahy, K.; Cardoso, S.; Almeida, J.; Freitas, P. P.

2010-05-01

97

Noise measurements in a composite niobium/YBCO SQUID and determination of the magnetic noise by direct measurement  

SciTech Connect

We have measured the noise performance of a composite niobium/YBCO point contact 2-hole SQUID, at 4.2 K in both rf and dc bias modes. The noise was measured by a technique which allowed the component which was flux noise to be measured. The flux noise was found to be the same in both bias modes. A direct measurement of the noise in a single hole in the same sample was made as a function of temperature and was found to display two peaks. Possible causes are discussed in this paper.

Harrop, S.P.; Keene, M.N.; Muirhead, C.M.; Gough, C.E. (Superconductivity Research Group, Univ. of Birmingham, Birmingham B15 2TT (GB))

1991-03-01

98

Magnetic flux density reconstruction using interleaved partial Fourier acquisitions in MREIT  

NASA Astrophysics Data System (ADS)

Magnetic resonance electrical impedance tomography (MREIT) has been introduced as a non-invasive modality to visualize the internal conductivity and/or current density of an electrically conductive object by the injection of current. In order to measure a magnetic flux density signal in MREIT, the phase difference approach in an interleaved encoding scheme cancels the systematic artifacts accumulated in phase signals and also reduces the random noise effect. However, it is important to reduce scan duration maintaining spatial resolution and sufficient contrast, in order to allow for practical in vivo implementation of MREIT. The purpose of this paper is to develop a coupled partial Fourier strategy in the interleaved sampling in order to reduce the total imaging time for an MREIT acquisition, whilst maintaining an SNR of the measured magnetic flux density comparable to what is achieved with complete k-space data. The proposed method uses two key steps: one is to update the magnetic flux density by updating the complex densities using the partially interleaved k-space data and the other is to fill in the missing k-space data iteratively using the updated background field inhomogeneity and magnetic flux density data. Results from numerical simulations and animal experiments demonstrate that the proposed method reduces considerably the scanning time and provides resolution of the recovered Bz comparable to what is obtained from complete k-space data.

Park, Hee Myung; Nam, Hyun Soo; In Kwon, Oh

2011-04-01

99

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

2013-01-22

100

Magnetron magnetic priming for rapid startup and noise reduction  

NASA Astrophysics Data System (ADS)

The magnetron is a vacuum electronics crossed-field device: perpendicular electric and magnetic fields determine the electron dynamics. Compactness, efficiency and reliability make magnetrons suitable for a wide range of military and civilian applications: radar, industrial heating, plasma sources, and medical accelerators. The most ubiquitous use of magnetrons is as the microwave power source in microwave ovens, operating at 2.45 GHz and delivering about 800--1000 W. University of Michigan and several other research programs are actively pursuing the development of GW range relativistic magnetrons. This dissertation presents experimental and computational results concerning innovative techniques to improve magnetron noise, startup and mode stability. The DC-operated oven magnetron studies performed at University of Michigan opened new directions by utilizing azimuthally varying magnetic fields (magnetic priming). Magnetic priming for rapid startup in an N-cavity magnetron operating in the pi-mode is based on implementation of an axial magnetic field with N/2 azimuthal periods, to prebunch the electrons in the desired number of spokes (N/2). Experiments with magnetic priming on DC oven magnetrons using perturbing magnets added on the upper existing magnet of the magnetron showed rapid startup (pi-mode oscillation observed at low currents) and up to 35 dB noise reduction (close to the carrier and in sidebands). A complex 3-dimensional (3D) ICEPIC computational model recovered the oven magnetron magnetic priming experimental results: rapid electron prebunching due to presence of perturbing magnets, fast startup and tendency towards a lower noise state. Simulations in 6-cavity relativistic magnetrons show that ideal magnetic priming causes fast startup, rapid mode growth (with radial electron diffusion) and suppression of mode competition. A highly idealized model (planar, crossed-field, non-resonant, non-relativistic structure) using single particle dynamics showed that magnetic priming causes rapid electron prebunching, specific symmetries in the electron cloud and an orbital parametric instability (radial exponential growth).

Neculaes, Vasile Bogdan

101

Electrical low frequency random telegraph noise in magnetic tunnel junctions  

NASA Astrophysics Data System (ADS)

Low frequency random telegraph noise (RTN) of electrical origin is reported in magnetic tunnel junction heads with ultrathin tunnel barriers (<1 nm). The RTN is characterized by abrupt and randomly stepped voltage spikes which modulates 1/f noise property in frequency domain. The appearance of electrical RTN is restricted to limited ranges of bias voltage and temperature. Dependence of RTN statistics on temperature suggests that the dynamic resistance fluctuations could be generally described by a thermal activation model. The extracted activation energy, EA, is found to be 0.3×10-19 J, which is much smaller than reported activation energies for magnetic RTN in early giant magnetoresistive heads.

Liu, Feng; Ding, Yunfei; Kemshetti, Ravindra; Davies, Kenneth; Rana, Paul; Mao, Sining

2009-04-01

102

Noise in small magnetic systems—applications to very sensitive magnetoresistive sensors  

NASA Astrophysics Data System (ADS)

Reduction for 1/ f noise (or random telegraph noise) is a crucial issue for small magnetic sensors which is strongly related to structural properties and magnetic configuration. We show how it is possible to eliminate magnetic noise at low frequency in GMR/TMR sensors by a combination of cross anisotropies, window frame shapes and suitably designed magnetoresisitive stack. These sensors are superior to almost all existing field and flux sensors. Results are presented on a mixed sensor, where a superconducting loop acts as a flux-to-field transformer to the GMR sensor. This device is suitable for detection of biomagnetic signals, such as in magnetocardiography or in magnetoencephalography. Measurements on niobium-based and YBCO-based sensors are presented, leading to sensitivity of 30 fT/?Hz at 77 K for small samples. Sensitivity lower than 1 fT/?(Hz) is expected with appropriate design and use of TMR or CMR layers, which makes these a powerful alternative to SQUIDs.

Pannetier, M.; Fermon, C.; Le Goff, G.; Simola, J.; Kerr, E.; Coey, J. M. D.

2005-04-01

103

Twenty-three cycles of changing open solar magnetic flux  

Microsoft Academic Search

This paper presents a comparison of various estimates of the open solar flux, deduced from measurements of the interplanetary magnetic field, from the aa geomagnetic index and from photospheric magnetic field observations. The first two of these estimates are made using the Ulysses discovery that the radial heliospheric field is approximately independent of heliographic latitude, the third makes use of

M. Lockwood

2003-01-01

104

Siphon flows in isolated magnetic flux tubes. 3: The equilibrium path of the flux tube arch  

NASA Technical Reports Server (NTRS)

The arched equilibrium path of a thin magnetic flux tube in a plane-stratified, nonmagnetic atmosphere is calculated for cases in which the flux tube contains a steady siphon flow. The large scale mechanical equilibrium of the flux tube involves a balance among the magnetic buoyancy force, the net magnetic tension force due to the curvature of the flux tube axis, and the inertial (centrifugal) force due to the siphon flow along curved streamlines. The ends of the flux tube are assumed to be pinned down by some other external force. Both isothermal and adiabatic siphon flows are considered for flux tubes in an isothermal external atmosphere. For the isothermal case, in the absence of a siphon flow the equilibrium path reduces to the static arch calculated by Parker (1975, 1979). The presence of a siphon flow causes the flux tube arch to bend more sharply, so that magnetic tension can overcome the additional straightening effect of the inertial force, and reduces the maximum width of the arch. The curvature of the arch increases as the siphon flow speed increases. For a critical siphon flow, with supercritical flow in the downstream leg, the arch is asymmetric, with greater curvature in the downstream leg of the arch. Adiabatic flow have qualitatively similar effects, except that adiabatic cooling reduces the buoyancy of the flux tube and thus leads to significantly wider arches. In some cases the cooling is strong enough to create negative buoyancy along sections of the flux tube, requiring upward curvature of the flux tube path along these sections and sometimes leading to unusual equilibrium paths of periodic, sinusoidal form.

Thomas, John H.; Montesinis, Benjamin

1989-01-01

105

The equilibrium structure of thin magnetic flux tubes. I  

NASA Technical Reports Server (NTRS)

A model atmosphere within a thin magnetic flux tube that is embedded in an arbitrarily stratified medium is presently constructed by solving the radiative transfer equation in the two-stream approximation for gray opacity, under the assumption that the magnetic field is sufficiently strong to warrant the neglect of both thermal conduction and convective diffusion; energy inside the flux tube therefore being transported solely by radiation. The structure of the internal atmosphere is determined on the basis of the hydrostatic and radiative equilibrium conditions of the tube embedded in an external atmosphere. The gas temperature along the axis of the tube is computed, and the geometry of the flux tube is determined on the basis of magnetic flux conservation and total pressure equilibrium.

Ferrari, A.; Massaglia, S.; Kalkofen, W.; Rosner, R.; Bodo, G.

1985-01-01

106

Sigmoidal equilibria and eruptive instabilities in laboratory magnetic flux ropes  

NASA Astrophysics Data System (ADS)

The Magnetic Reconnection Experiment (MRX) has recently been modified to study quasi-statically driven line-tied magnetic flux ropes in the context of storage-and-release eruptions in the corona. Detailed in situ magnetic measurements and supporting MHD simulations permit quantitative analysis of the plasma behavior. We find that the behavior of these flux ropes depends strongly on the properties of the applied potential magnetic field arcade. For example, when the arcade is aligned parallel to the flux rope footpoints, force free currents induced in the expanding rope modify the pressure and tension in the arcade, resulting in a confined, quiescent discharge with a saturated kink instability. When the arcade is obliquely aligned to the footpoints, on the other hand, a highly sigmoidal equilibrium forms that can dynamically erupt (see Fig. 1 and Fig. 2). To our knowledge, these storage-and-release eruptions are the first of their kind to be produced in the laboratory. A new 2D magnetic probe array is used to map out the internal structure of the flux ropes during both the storage and the release phases of the discharge. The kink instability and the torus instability are studied as candidate eruptive mechanisms--the latter by varying the vertical gradient of the potential field arcade. We also investigate magnetic reconnection events that accompany the eruptions. The long-term objective of this work is to use internal magnetic measurements of the flux rope structure to better understand the evolution and eruption of comparable structures in the corona. This research is supported by DoE Contract Number DE-AC02-09CH11466 and by the Center for Magnetic Self-Organization (CMSO). Qualitative sketches of flux ropes formed in (1) a parallel potential field arcade; and (2) an oblique potential field arcade. One-dimensional magnetic measurements from (1) a parallel arcade discharge that is confined; and (2) an oblique arcade discharge that erupts.

Myers, C. E.; Yamada, M.; Belova, E.; Ji, H.; Yoo, J.

2013-12-01

107

Flux Trapping and Field Magnet Stability of Bulk Superconductors  

NASA Astrophysics Data System (ADS)

The stability and durability of trapped flux is a focus in investigations of melt-growth (RE)Ba2Cu3O7-? (RE=rare earth, e.g., Y, Gd, Sm) bulk high-temperature superconductors. In magnetic flux engineering applications, two important issues affect the expected performance of bulk superconductors: flux trapping and flux stability or durability. The introduction of effective pinning with homogeneous spatial distribution is inevitable. The materials process and durability studies have been pursued in the last decade with either a stationary or a momentary applied external field. In machine applications, the magnetic flux stability is influenced by external perturbations. We topically summarize our progress and current status in relation to past and ongoing work.

Zhang, Y. F.; Zhou, W.; Xu, Y.; Li, B.; Zhou, D.; Tsuzuki, K.; Watasaki, M.; Miki, M.; Izumi, M.

2014-05-01

108

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

109

Local magnetization unit for GMR array based magnetic flux leakage inspection  

NASA Astrophysics Data System (ADS)

GMR sensors are increasingly used for magnetic surface inspection due to their high sensitivity and high spatial resolution. In case of simple planar or cylindrical shaped components, the GMR-based inspection procedure can be automated easily. We present GMR measurements of real fatigue cracks. In addition, we present a probe design using a local magnetization unit and commercially available GMR sensors. The design was carried out by means of finite-element method (FEM) simulations. Using the local probe we measured bearings containing artificial reference cracks of different depths and orientations. Cracks with a depth of 40 ?m could be resolved with a signal-to-noise ratio better than 6. A further reduction of the measuring time can be obtained using a sensor array. For this purpose we present a study of the optimized size of the sensing GMR-layers for a NDE-adapted sensor array. The geometric sensor parameters were investigated through simulations of the magnetic flux leakage of surface cracks using an analytic model.

Pelkner, M.; Neubauer, A.; Reimund, V.; Kreutzbruck, M.

2012-05-01

110

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

111

Transport of magnetic flux from the canopy to the internetwork  

E-print Network

Recent observations have revealed that 8% of linear polarization patches in the internetwork quiet Sun are fully embedded in downflows. These are not easily explained with the typical scenarios for the source of internetwork fields which rely on flux emergence from below. We explore using radiative MHD simulations a scenario where magnetic flux is transported from the magnetic canopy overlying the internetwork into the photosphere by means of downward plumes associated with convective overshoot. We find that if a canopy-like magnetic field is present in the simulation, the transport of flux from the canopy is an important process for seeding the photospheric layers of the internetwork with magnetic field. We propose that this mechanism is relevant for the Sun as well, and it could naturally explain the observed internetwork linear polarization patches entirely embedded in downflows.

Pietarila, A; Danilovic, S; Solanki, S K

2011-01-01

112

Forced three-dimensional magnetic reconnection due to linkage of magnetic flux tubes  

SciTech Connect

During periods of southward interplanetary magnetic field (IMF) orientation the magnetic field geometry at the dayside magnetopause is susceptible to magnetic reconnection. It has been suggested that reconnection may occur in a localized manner at several patches on the magnetopause. A major problem with this picture is the interaction of magnetic flux ropes which are generated by different reconnection processes. An individual flux rope is bent elbowlike where it intersects the magnetopause and the magnetic field changes from magnetospheric to interplanetary magnetic field orientation. Multiple patches of reconnection can lead to the formation of interlinked magnetic flux tubes. Although the corresponding flux is connected to the IMF the northward and southward connected branches are hooked into each other and cannot develop independently. We have studied this problem in the framework of three-dimensional magnetohydrodynamic simulations. The results indicate that a singular current sheet forms at the interface of two interlinked flux tubes if no resistivity is present in the simulation. This current sheet is strongly tilted compared to the original current sheet. In the presence of resistivity the interaction of the two flux tubes forces a fast reconnection process which generates helically twisted closed magnetospheric flux. This linkage induced reconnection generates a boundary layer with layers of open and closed magnetospheric flux and may account for the brightening of auroral arcs poleward of the boundary between open and closed magnetic flux.

Otto, A. [Univ. of Alaska, Fairbanks, AK (United States)

1995-07-01

113

Forced three-dimensional magnetic reconnection due to linkage of magnetic flux tubes  

NASA Technical Reports Server (NTRS)

During periods of southward interplanetary magnetic field (IMF) orientation the magnetic field geometry at the dayside magnetopause is susceptible to magnetic reconnection. It has been suggested that reconnection may occur in a localized manner at several patches on the magnetopause. A major problem with this picture is the interaction of magnetic flux ropes which are generated by different reconnection processes. An individual flux rope is bent elbowlike where it intersects the magnetopause and the magnetic field changes from magnetospheric to interplanetary magnetic field orientation. Multiple patches of reconnection can lead to the formation of interlinked magnetic flux tubes. Although the corresponding flux is connected to the IMF the northward and southward connected branches are hooked into each other and cannot develop independently. We have studied this problem in the framework of three-dimensional magnetohydrodynamic simulations. The results indicate that a singular current sheet forms at the interface of two interlinked flux tubes if no resistivity is present in the simulation. This current sheet is strongly tilted compared to the original current sheet. In the presence of resistivity the interaction of the two flux tubes forces a fast reconnection process which generates helically twisted closed magnetospheric flux. This linkage induced reconnection generates a boundary layer with layers of open and closed magnetospheric flux and may account for the brightening of auroral arcs poleward of the boundary between open and closed magnetic flux.

Otto, A.

1995-01-01

114

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife Infrared Polarimeter  

E-print Network

Recent observations with the HINODE satellite have found abundant horizontal magnetic fields in the internetwork quiet Sun. We compare the results on the horizontal fields with ground-based observations. We obtained 30 sec-integrated data of quiet Sun on disc centre during a period of very good seeing. The data have a rms noise in polarization of around 2 10^-4 of the continuum intensity. The low noise level allowed for an inversion of the spectra. We compare the inversion results with proxies for the determination of magnetic flux. We confirm the presence of the horizontal fields in the quiet Sun internetwork, with voids of some granules extent of nearly zero linear polarization signal. Voids in the circular polarization signal are only of granular scale. More than 60 % of the surface show polarization signals above four times the rms noise level. We find that the total magnetic flux contained in the more inclined to horizontal fields (gamma > 45 deg) is smaller by a factor of around 2 than that of the less ...

Beck, C A R

2009-01-01

115

Magnetic Flux Density in the Heliosphere through Several Solar Cycles  

NASA Astrophysics Data System (ADS)

We studied the magnetic flux density carried by solar wind to various locations in the heliosphere, covering a heliospheric distance range of 0.3-5.4 AU and a heliolatitudinal range from 80° south to 80° north. Distributions of the radial component of the magnetic field, BR , were determined over long intervals from the Helios, ACE, STEREO, and Ulysses missions, as well as from using the 1 AU OMNI data set. We show that at larger distances from the Sun, the fluctuations of the magnetic field around the average Parker field line distort the distribution of BR to such an extent that the determination of the unsigned, open solar magnetic flux density from the average lang|BR |rang is no longer justified. We analyze in detail two methods for reducing the effect of fluctuations. The two methods are tested using magnetic field and plasma velocity measurements in the OMNI database and in the Ulysses observations, normalized to 1 AU. It is shown that without such corrections for the fluctuations, the magnetic flux density measured by Ulysses around the aphelion phase of the orbit is significantly overestimated. However, the matching between the in-ecliptic magnetic flux density at 1 AU (OMNI data) and the off-ecliptic, more distant, normalized flux density by Ulysses is remarkably good if corrections are made for the fluctuations using either method. The main finding of the analysis is that the magnetic flux density in the heliosphere is fairly uniform, with no significant variations having been observed either in heliocentric distance or heliographic latitude.

Erd?s, G.; Balogh, A.

2014-01-01

116

Superpoissonian shot noise in organic magnetic tunnel junctions  

NASA Astrophysics Data System (ADS)

Organic molecules have recently revolutionized ways to create new spintronic devices. Despite intense studies, the statistics of tunneling electrons through organic barriers remains unclear. Here, we investigate conductance and shot noise in magnetic tunnel junctions with 3,4,9,10-perylene-teracarboxylic dianhydride (PTCDA) barriers a few nm thick. For junctions in the electron tunneling regime, with magnetoresistance ratios between 10% and 40%, we observe superpoissonian shot noise. The Fano factor exceeds in 1.5-2 times the maximum values reported for magnetic tunnel junctions with inorganic barriers, indicating spin dependent bunching in tunneling. We explain our main findings in terms of a model which includes tunneling through a two level (or multilevel) system, originated from interfacial bonds of the PTCDA molecules. Our results suggest that interfaces play an important role in the control of shot noise when electrons tunnel through organic barriers.

Cascales, Juan Pedro; Hong, Jhen-Yong; Martinez, Isidoro; Lin, Minn-Tsong; Szczepa?ski, Tomasz; Dugaev, Vitalii K.; Barna?, Józef; Aliev, Farkhad G.

2014-12-01

117

COMPARISON BETWEEN CREEP FAILURE ANALYSIS AND MAGNETIC BARKAHUSEN NOISE RESULTS  

Microsoft Academic Search

On this work we compared the results obtained after a c onventional creep failure analysis and magnetic Barkhausen noise. We chose one region without creep damage that was named as standard. Two other regions with creep damage were named as T1 and T2. The sample T1 after optical metallographic analysis has presented level A and B of creep damage according

Jonhson D. ANGELO; Linilson R. PADOVESE; Manuel ALBERTERIS

118

Topology bifurcation of a magnetic flux surface in toroidal plasmas  

NASA Astrophysics Data System (ADS)

The bifurcation of magnetic topology is identified by a significant change in the heat pulse propagation properties in magnetized plasmas. Clear evidence of stochastization of the magnetic surfaces near a rational surface is observed in the core plasma with weak magnetic shear in the Large Helical Device by slowly decreasing the magnetic shear and measured by applying heat pulses driven by modulated electron cyclotron heating (MECH). Three topologies of the magnetic flux surfaces (a nested magnetic island and partial and full stochastization) are identified by the patterns of heat pulse propagation observed in the flat temperature region in the plasma. Slow heat pulse propagation exhibiting a non-monotonically increasing delay time is evidence of a magnetic island, while the fast heat pulse propagation observed in the plasma with medium magnetic shear is evidence of the stochastization of the magnetic surfaces. The region with the fast heat pulse propagation varies with a slight change of magnetic shear. There are two types of stochastization of the magnetic surfaces. In one, the stochastization region is localized near the rational surface (partial stochastization) and in the other, the region is extended to the magnetic axis (full stochastization). The appearance of a stochastic magnetic field is not caused by MHD instability. The significant increase of the ratio of electron thermal diffusivity to ion thermal diffusivity is consistent with that expected by stochastization of the magnetic field.

Ida, K.; Kobayashi, T.; Inagaki, S.; Suzuki, Y.; Sakakibara, S.; Itoh, K.; Tsuchiya, H.; Suzuki, C.; Yoshinuma, M.; Narushima, Y.; Yokoyama, M.; Itoh, S.-I.

2015-01-01

119

A novel high temperature superconducting magnetic flux pump for MRI magnets  

Microsoft Academic Search

This paper presents a kind of minitype magnetic flux pump made of high temperature superconductor. This kind of novel high temperature superconducting (HTS) flux pump has not any mechanical revolving parts or thermal switches. The excitation current of copper coils in magnetic pole system is controlled by a singlechip. The structure design and operational principle have been described. The operating

Zhiming Bai; Guo Yan; Chunli Wu; Shufang Ding; Chuan Chen

2010-01-01

120

Solar magnetic fields and convection. IV - Magnetic flux ropes and their fibres  

Microsoft Academic Search

The flux-rope theory of solar magnetic fields is based on the concept of a toroidal flux created by differential rotation and simultaneously rolled into helically twisted ropes which later emerge as inverted U's to produce active regions. This theory is further developed by taking account of additional observational data. Evidence is presented to show that sunspot magnetic fields are helically

J. H. Piddington

1976-01-01

121

Equilibrium model of thin magnetic flux tubes. [solar atmosphere  

NASA Technical Reports Server (NTRS)

The existence of a physically realizable domain in which approximations that lead to a self consistent solution for flux tube stratification in the solar atmosphere, without ad hoc hypotheses, is proved. The transfer equation is solved assuming that no energy transport other than radiative is present. Convective motions inside the tube are assumed to be suppressed by magnetic forces. Only one parameter, the plasma beta at tau = 0, must be specified, and this can be estimated from observations of spatially resolved flux tubes.

Bodo, G.; Ferrari, A.; Massaglia, S.; Kalkofen, W.; Rosner, R.

1984-01-01

122

Multiwavelength flux variations induced by stellar magnetic activity: effects on planetary transits  

E-print Network

Stellar magnetic activity is a source of noise in the study of the transits of extrasolar planets. It induces flux variations which affect significantly the transit depth determination and the derivations of planetary and stellar parameters. Furthermore, the colour dependence of stellar activity may significantly influence the characterization of planetary atmospheres. Here we present a systematic approach to quantify the corresponding stellar flux variations as a function of wavelength bands. We consider a star with spots covering a given fraction of its disc and model the variability in the UBVRIJHK photometric system and in the Spitzer/IRAC wavebands for dwarf stars from G to M spectral types. We compare activity-induced flux variations in different passbands with planetary transits and quantify how they affect the determination of the planetary radius and the analysis of the transmission spectroscopy in the study of planetary atmospheres. We suggest that the monitoring of the systems by using broad band p...

Ballerini, P; Lanza, A F; Pagano, I

2012-01-01

123

Lorentz Microscopy of Weak Magnetic Flux Inhomogeneities  

Microsoft Academic Search

The potential of Lorentz microscopy to give quantitative information about weak magnetic field inhomogeneities can be exploited fully only if the classical approximation to the electron contrast is abandoned in favor of the wave mechanical reality. The inhomogeneities cause coordinate dependent phase shifts ?&phgr;m in the coherent electron wavefunction which can be detected in principle by any electron optical method

D. Wohlleben

1968-01-01

124

Magnetic Flux Leakage Method: Large-Scale Approximation  

E-print Network

We consider the application of the magnetic flux leakage (MFL) method to the detection of defects in ferromagnetic (steel) tubulars. The problem setup corresponds to the cases where the distance from the casing and the point where the magnetic field is measured is small compared to the curvature radius of the undamaged casing and the scale of inhomogeneity of the magnetic field in the defect-free case. Mathematically this corresponds to the planar ferromagnetic layer in a uniform magnetic field oriented along this layer. Defects in the layer surface result in a strong deformation of the magnetic field, which provides opportunities for the reconstruction of the surface profile from measurements of the magnetic field. We deal with large-scale defects whose depth is small compared to their longitudinal sizes---these being typical of corrosive damage. Within the framework of large-scale approximation, analytical relations between the casing thickness profile and the measured magnetic field can be derived.

Pimenova, Anastasiya V; Levesley, Jeremy; Ivantsov, Andrey O; Elkington, Peter; Bacciarelli, Mark

2015-01-01

125

Properties of Magnetic Helicity Flux in Turbulent Dynamos  

NASA Astrophysics Data System (ADS)

We study the flux of small-scale magnetic helicity in simulations of driven statistically homogeneous magnetohydrodynamic turbulence in a periodic box with an imposed large-scale shear. The simulations show that in the regime of strong dynamo action the eddy-scale magnetic helicity flux has only two significant terms: advective motion driven by the large-scale velocity field and the Vishniac-Cho (VC) flux which moves helicity across the magnetic field lines. The contribution of all the other terms is negligible. The VC flux is highly correlated with the large-scale electromotive force and is responsible for large-scale dynamo action, while the advective term is not. The VC flux is driven by the anisotropy of the turbulence. We derive analytical expressions for it in terms of the small-scale velocity or magnetic field. These expressions are used to predict the existence and strength of dynamo action for different turbulent anisotropies and tested against the results of the simulations.

Vishniac, Ethan T.; Shapovalov, Dmitry

2014-01-01

126

Sausage Mode Propagation in a Thick Magnetic Flux Tube  

NASA Astrophysics Data System (ADS)

The aim of this paper is to model the propagation of slow magnetohydrodynamic (MHD) sausage waves in a thick expanding magnetic flux tube in the context of the quiescent (VAL-C) solar atmosphere. The propagation of these waves is found to be described by the Klein-Gordon equation. Using the governing MHD equations and the VAL-C atmosphere model we study the variation of the cut-off frequency along and across the magnetic tube guiding the waves. Due to the radial variation of the cut-off frequency the flux tubes act as low frequency filters for the waves.

Pardi, A.; Ballai, I.; Marcu, A.; Orza, B.

2014-04-01

127

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.

128

Magnetic and Electric Dipole Constraints on Extra Dimensions and Magnetic Fluxes  

E-print Network

The propagation of charged particles and gauge fields in a compact extra dimension contributes to $g-2$ of the charged particles. In addition, a magnetic flux threading this extra dimension generates an electric dipole moment for these particles. We present constraints on the compactification size and on the possible magnetic flux imposed by the comparison of data and theory of the magnetic moment of the muon and from limits on the electric dipole moments of the muon, neutron and electron.

Aaron J. Roy; Myron Bander

2008-05-10

129

Magnetic field generation from shear flow in flux ropes  

NASA Astrophysics Data System (ADS)

In the Reconnection Scaling Experiment (RSX) we have measured out of plane quadrupole magnetic field structure in situations where magnetic reconnection was minimal. This quadrupole out of plane magnetic signature has historically been presumed to be the smoking gun harbinger of reconnection. On the other hand, we showed that when flux ropes bounced instead of merging and reconnecting, this signature could evolve. This can follow from sheared fluid flows in the context of a generalized Ohms Law. We reconstruct a shear flow model from experimental data for flux ropes that have been experimentally well characterized in RSX as screw pinch equilibria, including plasma ion and electron flow, with self consistent profiles for magnetic field, pressure, and current density. The data can account for the quadrupole field structure.

Intrator, T. P.; Sears, J.; Gao, K.; Klarenbeek, J.; Yoo, C.

2012-10-01

130

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)] [Department of Physics, The Ohio State University, Columbus, Ohio 43210 (United States)

1999-10-01

131

Relativistic Magnetic Monopole Flux Constraints from RICE  

E-print Network

We report an upper limit on the flux of relativistic monopoles based on the non-observation of in-ice showers by the Radio Ice Cherenkov Experiment (RICE) at the South Pole. We obtain a 95% C.L. limit of order 10^{-18}/(cm^2-s-sr) for intermediate mass monopoles of 10^7

D. P. Hogan; D. Z. Besson; J. P. Ralston; I. Kravchenko; D. Seckel

2008-06-12

132

Relativistic magnetic monopole flux constraints from RICE  

NASA Astrophysics Data System (ADS)

We report an upper limit on the flux of relativistic monopoles based on the nonobservation of in-ice showers by the Radio Ice Cherenkov Experiment (RICE) at the South Pole. We obtain a 95% C.L. limit of order 10-18(cm2ssr)-1 for intermediate-mass monopoles of 107???1012 at the anticipated energy Etot=1016GeV. This bound is over an order of magnitude stronger than all previously published experimental limits for this range of boost parameters ? and exceeds 2 orders of magnitude improvement over most of the range. We review the physics of radio detection, describe a Monte Carlo simulation including continuous and stochastic energy losses, and compare to previous experimental limits.

Hogan, D. P.; Besson, D. Z.; Ralston, J. P.; Kravchenko, I.; Seckel, D.

2008-10-01

133

Magnetic Sensitivity Beyond the Projection Noise Limit by Spin Squeezing  

NASA Astrophysics Data System (ADS)

We report the generation of spin squeezing and entanglement in a magnetically sensitive atomic ensemble, and entanglement-enhanced field measurements with this system. A maximal mf=±1 Raman coherence is prepared in an ensemble of 8.5×105 laser-cooled Rb87 atoms in the f=1 hyperfine ground state, and the collective spin is squeezed by synthesized optical quantum nondemolition measurement. This prepares a state with large spin alignment and noise below the projection-noise level in a mixed alignment-orientation variable. 3.2 dB of noise reduction is observed and 2.0 dB of squeezing by the Wineland criterion, implying both entanglement and metrological advantage. Enhanced sensitivity is demonstrated in field measurements using alignment-to-orientation conversion.

Sewell, R. J.; Koschorreck, M.; Napolitano, M.; Dubost, B.; Behbood, N.; Mitchell, M. W.

2012-12-01

134

Magnetic sensitivity beyond the projection noise limit by spin squeezing.  

PubMed

We report the generation of spin squeezing and entanglement in a magnetically sensitive atomic ensemble, and entanglement-enhanced field measurements with this system. A maximal m(f) = ± 1 Raman coherence is prepared in an ensemble of 8.5 × 10(5) laser-cooled (87)Rb atoms in the f = 1 hyperfine ground state, and the collective spin is squeezed by synthesized optical quantum nondemolition measurement. This prepares a state with large spin alignment and noise below the projection-noise level in a mixed alignment-orientation variable. 3.2 dB of noise reduction is observed and 2.0 dB of squeezing by the Wineland criterion, implying both entanglement and metrological advantage. Enhanced sensitivity is demonstrated in field measurements using alignment-to-orientation conversion. PMID:23368463

Sewell, R J; Koschorreck, M; Napolitano, M; Dubost, B; Behbood, N; Mitchell, M W

2012-12-21

135

Enhanced solar anti-neutrino flux in random magnetic fields  

E-print Network

We discuss the impact of the recent KamLAND constraint on the solar anti-neutrino flux on the analysis of solar neutrino data in the presence of Majorana neutrino transition magnetic moments and solar magnetic fields. We consider different stationary solar magnetic field models, both regular and random, highlighting the strong enhancement in the anti-neutrino production rates that characterize turbulent solar magnetic field models. Moreover, we show that for such magnetic fields inside the Sun, one can constrain the intrinsic neutrino magnetic moment down to the level of mu_nu lessthan few times 10^-12 x mu_B irrespective of details of the underlying turbulence model. This limit is more stringent than all current experimental sensitivities, and similar to the most stringent bounds obtained from stellar cooling. We also comment on the robustness of this limit and show that at most it might be weakened by one order of magnitude, under very unlikely circumstances.

O. G. Miranda; T. I. Rashba; A. I. Rez; J. W. F. Valle

2004-10-13

136

Origin of 1/f noise peaks of YBa{sub 2}Cu{sub 3}O{sub x} films in a magnetic field  

SciTech Connect

The temperature and magnetic field dependence of 1/f noise has been measured in epitaxial YBa{sub 2}Cu{sub 3}O{sub x} films. In a magnetic field, two noise peaks were observed as temperature decreases; one at higher temperature was found to match the thermal fluctuation of the sample resistance and the other near the foot of the transition was found to be magnetic-field dependent. The location of the latter was shifted toward low temperature and the peak height was decreased as a magnetic field increased. In a zero field only one peak from the resistance fluctuation was observed. We show that the field-dependent noises are due to flux motion interacting with the pinning potential. A classical model is used to explain the field-induced noise peaks. We interpret that the temperature dependences of the flux bundle size and the sample resistance are the reasons for the noise peaks, and a rough estimation of the temperature and field dependence of the flux bundle size is presented. Any possible relationship between the noise peaks and the flux-line-lattice phase transition is also discussed.

Kang, W.N.; Kim, D.H.; Park, J.H. [Korea Institute of Science and Technology, Seoul (Korea, Democratic People`s Republic of). Applied Physics Group] [and others

1994-02-01

137

Flux Rope Formation from Magnetic and Velocity Shear  

NASA Astrophysics Data System (ADS)

Spacecraft observations and simulations both suggest that magnetic islands are commonly associated with the onset and nonlinear development of reconnection. While most theoretical efforts have focused on 2D models, in real 3D systems the islands correspond to flux ropes which can form and interact in a variety of complex ways. The most common explanation is the tearing instability driven by the magnetic shear. In large 3D systems, the spectrum of unstable modes can be much richer due to multiple resonance surfaces, but the details depend strongly on the parameter regime. A distinctly different explanation for generating flux ropes is the Kelvin-Helmholtz instability driven by Alfv'enic flow shear. For layers above the ion-scale, the vortex leads to wrapping of the field lines and generation of flux ropes comparable to the size of the vortex. This is in sharp contrast to flux ropes formed from the tearing instability which start on small scales and grow in time to reach larger sizes. Here, we compare and contrast these two mechanisms using 3D fully kinetic simulations for configurations involving various combinations of magnetic and velocity shear. Characteristic properties of the flux ropes, fluctuation spectra and influence on particle acceleration will be discussed.

Daughton, William; Liu, Yi-Hsin; Nakamura, Takuma; Karimabadi, Homa; Roytershteyn, Vadim

2012-10-01

138

The Origin of Mass, Magnetic Flux, and Magnetic Helicity in a Solar Prominence  

Microsoft Academic Search

Solar prominences are cloud-like cool and dense plasma supported by highly non-potential, and very likely twisted magnetic fields in the corona against the gravity. Therefore, the supply of mass, magnetic flux, and magnetic helicity is the necessary condition for the formation of solar prominences. We have been doing a series of case studies on the formation of a prominence in

J. Chae; H. S. Yun; Y.-J. Moon; H. Wang; P. Goode

2002-01-01

139

Active Region EUV Intensities, Magnetic Flux and Coronal Heating  

Microsoft Academic Search

Several authors have previously noted the existence of power law relationships between the total unsigned photospheric magnetic flux and the intensities of a variety of spectral lines emitted by active regions. Applying this approach to more recent datasets, it is shown that power law relationships exist for extreme ultraviolet (EUV) emission data observed by the Coronal Diagnostic Spectrometer (CDS) and

J. Ireland; A. Fludra

2001-01-01

140

Type II superconductivity and magnetic flux transport in neutrons stars  

E-print Network

The transition to a type II proton superconductor which is believed to occur in a cooling neutron star is accompanied by changes in the equation of hydrostatic equilibrium and by the formation of proton vortices with quantized magnetic flux. Analysis of the electron Boltzmann equation for this system and of the proton supercurrent distribution formed at the transition leads to the derivation of a simple expression for the transport velocity of magnetic flux in the liquid interior of a neutron star. This shows that flux moves easily as a consequence of the interaction between neutron and proton superfluid vortices during intervals of spin-down or spin-up in binary systems. The differences between the present analysis and those of previous workers are reviewed and an error in the paper of Jones (1991) is corrected.

P. B. Jones

2005-10-13

141

Plasmas fluxes to surfaces for an oblique magnetic field  

SciTech Connect

The poloidal and toroidal spatial distributions of D{sub {alpha}}, He I and C II emission have been obtained in the vicinity of the TFTR bumper limiter and are compared with models of ion flow to the surface. The distributions are found not to agree with a model (the ``Cosine`` model) which determines the incident flux density using only the parallel fluxes in the scrape-off layer and the projected area of the surface perpendicular to the field lines. In particular, the Cosine model is not able to explain the significant fluxes observed at locations on the surface which are oblique to the magnetic field. It is further shown that these fluxes cannot be explained by the finite Larmor radius of impinging ions. Finally, it is demonstrated, with the use of Monte Carlo codes, that the distributions can be explained by including both parallel and cross-field transport onto the limiter surface.

Pitcher, C.S. [Canadian Fusion Fuels Technology Project, Toronto, ON (Canada); Stangeby, P.C.; Elder, J.D. [Toronto Univ., ON (Canada); Bell, M.G.; Kilpatrick, S.J.; Manos, D.M.; Medley, S.S.; Owens, D.K.; Ramsey, A.T.; Ulrickson, M. [Princeton Univ., NJ (United States). Plasma Physics Lab.

1992-07-01

142

Plasmas fluxes to surfaces for an oblique magnetic field  

SciTech Connect

The poloidal and toroidal spatial distributions of D{sub {alpha}}, He I and C II emission have been obtained in the vicinity of the TFTR bumper limiter and are compared with models of ion flow to the surface. The distributions are found not to agree with a model (the Cosine'' model) which determines the incident flux density using only the parallel fluxes in the scrape-off layer and the projected area of the surface perpendicular to the field lines. In particular, the Cosine model is not able to explain the significant fluxes observed at locations on the surface which are oblique to the magnetic field. It is further shown that these fluxes cannot be explained by the finite Larmor radius of impinging ions. Finally, it is demonstrated, with the use of Monte Carlo codes, that the distributions can be explained by including both parallel and cross-field transport onto the limiter surface.

Pitcher, C.S. (Canadian Fusion Fuels Technology Project, Toronto, ON (Canada)); Stangeby, P.C.; Elder, J.D. (Toronto Univ., ON (Canada)); Bell, M.G.; Kilpatrick, S.J.; Manos, D.M.; Medley, S.S.; Owens, D.K.; Ramsey, A.T.; Ulrickson, M. (Princeton Univ., NJ (United States). Plasma Physics Lab.)

1992-07-01

143

Dynamical constraints from field line topology in magnetic flux tubes  

E-print Network

A topological constraint on the dynamics of a magnetic field in a flux tube arises from the fixed point indices of its field line mapping. This can explain unexpected behaviour in recent resistive-magnetohydrodynamic simulations of magnetic relaxation. Here we present the theory for a general periodic flux tube, representing, for example, a toroidal confinement device or a solar coronal loop. We show how an ideal dynamics on the side boundary of the tube implies that the sum of indices over all interior fixed points is invariant. This constraint applies to any continuous evolution inside the tube, which may be turbulent and/or dissipative. We also consider the analogous invariants obtained from periodic points (fixed points of the iterated mapping). Although there is a countably infinite family of invariants, we show that they lead to at most two independent dynamical constraints. The second constraint applies only in certain magnetic configurations. Several examples illustrate the theory.

A. R. Yeates; G. Hornig

2011-03-11

144

Black hole jets without large-scale net magnetic flux  

NASA Astrophysics Data System (ADS)

We propose a scenario for launching relativistic jets from rotating black holes, in which small-scale magnetic flux loops, sustained by disc turbulence, are forced to inflate and open by differential rotation between the black hole and the accretion flow. This mechanism does not require a large-scale net magnetic flux in the accreting plasma. Estimates suggest that the process could operate effectively in many systems, and particularly naturally and efficiently when the accretion flow is retrograde. We present the results of general-relativistic force-free electrodynamic simulations demonstrating the time evolution of the black hole's magnetosphere, the cyclic formation of jets, and the effect of magnetic reconnection. The jets are highly variable on time-scales ˜10-103rg/c, where rg is the black hole's gravitational radius. The reconnecting current sheets observed in the simulations may be responsible for the hard X-ray emission from accreting black holes.

Parfrey, Kyle; Giannios, Dimitrios; Beloborodov, Andrei M.

2015-01-01

145

Electrostatic manipulation of magnetic flux quanta at the nanoscale  

NASA Astrophysics Data System (ADS)

The electrostatic tuning of physical properties in materials offers significant potential in a large variety of systems. For example, the application of an electric field allows depressing or enhancing superconductivity in certain oxides. Using heterostructures that combine a large-polarization ferroelectric (BiFeO3) and a high-temperature superconductor (YBa2Cu3O7-x), we demonstrate here the nanoscale modulation of the superconducting condensate via ferroelectric field effects [1]. The ability to design the ferroelectric domain structure at will enables us to create nanoscale ``patterns'' of normal regions within the superconductor, in a reversible and modifiable way. This produces an energy landscape for magnetic flux quanta and, in turn, couples the local polarization in the ferroelectric to the local magnetic induction in the superconductor. This new form of magnetoelectric coupling allows the electrostatic manipulation of magnetic flux quanta. [4pt] [1] A. Crassous et al., Phys. Rev. Lett. in press (2011)

Crassous, Arnaud; Bernard, Rozenn; Fusil, Stephane; Bouzehouane, Karim; Briatico, Javier; Bibes, Manuel; Barthelemy, Agnes; Villegas, Javier

2012-02-01

146

Crossed flux tubes 3D magnetic reconnection experiment  

NASA Astrophysics Data System (ADS)

The formation and dynamics of writhing, plasma-filled, twisted open magnetic flux tubes is being investigated using laboratory experiments. The behavior of these flux tubes is relevant to solar corona loops, astrophysical jets, spheromak formation, and open field lines in tokamaks and RFP's. MHD forces have been determined to drive fast axial plasma flows into the flux tube from the boundary it intercepts. These flows fill the flux tubes with plasma while simultaneously injecting linked frozen-in azimuthal flux; helicity injection is thus associated with mass injection. An upgraded experiment under construction will have two adjacent arched plasma-filled flux tubes cross over each other. It is anticipated that a localized 3D reconnection will occur at the cross-over. This reconnection should result in half-twists in the post re-connection topology and subsequent Alfven wave propagation to equilibrate the half-twists along the post-reconnection flux tubes. The electrical circuitry requires two initially independent floating capacitor bank power supplies that become series-connected as a result of reconnection.

Bellan, Paul

2011-11-01

147

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

148

Magnetic Flux Compression with a Gas Puff Z Pinch.  

NASA Astrophysics Data System (ADS)

The dynamics of a hollow gas puff z pinch imploding with a trapped, initially uniform, axial magnetic field is studied on the 0.45 MA, 1.2 mu s rise-time U.C. Irvine z pinch. The primary purpose is to ascertain the feasibility of producing short pulsed ultrahigh magnetic fields with this geometry. Data is taken primarily for Ne and Kr plasmas with a pulsed (7 ns) interferometer, twin pinhole cameras, x-ray diodes, and axial and azimuthal magnetic probes. The spatiotemporal behavior of the plasma is mapped by numerically processed interference patterns. Significant differences between Kr and Ne are recorded with this diagnostic. Kr implodes as a thinner shell which appears more disrupted by spatial nonuniformities of scale-length order 1 mm. Kr, though, achieves a much higher state of compression than Ne. Kr implosions with an initial axial magnetic field of 0.5 T are inferred to produce a field of 150 T at maximum compression assuming the axial magnetic flux interior to the plasma column is conserved during the implosion. This is an order of magnitude higher than can be similarly inferred for Ne implosions. The assumption of flux conservation is reasonably justified for Ne by correlation with on-axis magnetic probe measurements, although the field for Kr rises too quickly for the probe to respond. To help explain these results, the significance of such effects as Rayleigh -Taylor and MHD instabilities, radiation cooling, resistive flux diffusion, and plasma end-loss on flux compression performance are inferred from the data and studied with the help of simple theoretical models.

Ruden, Edward Lee

149

What causes the flux excess in the heliospheric magnetic field?  

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, < r2BR >. However, that result is contrary to a large increase in ? with distance, called the flux excess, that has been reported by Owens et al. (2008a) and attributed to variations in solar wind speed by Lockwood et al. (2009a, 2009b). Ulysses data and a mathematical simulation are used to show that the cause of the flux excess is the replacement of BR by the modulus, |BR|. The modulus rectifies some of the large amplitude magnetic field variations normally present in measurements of BR and increases the mean, < r2 |BR| > relative to < r2BR >. The variance of the magnetic fluctuations, ?, decreases less rapidly with distance than BR and that produces a progressively larger error in < r2 |BR| > resulting in the flux excess. The advisability of defining ? in terms of |BR|, of using < r2 |BR| > beyond 1 AU and the applicability of the Lockwood et al. (2009b) correction to < r2 |BR| > are questioned.

Smith, Edward J.

2011-12-01

150

Twisted Magnetic Flux Tubes in the Solar Wind  

NASA Astrophysics Data System (ADS)

Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surface, where the tubes are twisted due to photospheric motions. It is suggested that the twisted magnetic tubes can be detected as the variation of total (thermal+magnetic) pressure during their passage through the observing satellite. We show that the total pressure of several observed twisted tubes resembles the theoretically expected profile. The twist of the isolated magnetic tube may explain the observed abrupt changes of magnetic field direction at tube walls. We have also found some evidence that the flux tube walls can be associated with local heating of the plasma and elevated proton and electron temperatures. For the tubes aligned with the Parker spiral, the twist angle can be estimated from the change of magnetic field direction. Stability analysis of twisted tubes shows that the critical twist angle of the tube with a homogeneous twist is 70°, but the angle can further decrease due to the motion of the tube with respect to the solar wind stream. The tubes with a stronger twist are unstable to the kink instability, therefore they probably cannot reach 1 AU.

Zaqarashvili, Teimuraz V.; Vörös, Zoltán; Narita, Yasuhito; Bruno, Roberto

2014-03-01

151

MHD waves on solar magnetic flux tubes - Tutorial review  

NASA Technical Reports Server (NTRS)

Some of the highly simplified models that have been developed for solar magnetic flux tubes, which are intense photospheric-level fields confined by external gas pressure but able to vary rapidly with height, are presently discussed with emphasis on the torsional Alfven mode's propagation, reflection, and non-WKB properties. The 'sausage' and 'kink' modes described by the thin flux-tube approximation are noted. Attention is also given to the surface waves and resonance absorption of X-ray-emitting loops, as well as to the results of recent work on the resonant instabilities that occur in the presence of bulk flows.

Hollweg, Joseph V.

1990-01-01

152

Twisted magnetic flux tubes in the solar wind  

NASA Astrophysics Data System (ADS)

Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surface, where the tubes are twisted owing to photospheric motions. The twisted magnetic tubes can be detected as the variation of total (thermal+magnetic) pressure during their passage through observing satellite. The twist of isolated magnetic tube may explain the observed abrupt changes of magnetic field direction at tube walls. For the tubes aligned with the Parker spiral, the twist angle can be estimated from the change of magnetic field direction. The twisted tubes are unstable to kink instability when the twist exceeds a critical value. It is shown that the critical twist angle of the tube with a homogeneous twist is 70 degree, but the angle can be decreased owing to the motion of the tube with regards to the solar wind stream. Tangential velocity discontinuity near the boundaries of individual tubes may also result in the Kelvin-Helmholtz instability. It is shown that the axial magnetic field stabilizes the instability in the case of sub-Alfvenic speeds. But even small twist in the external magnetic field allows the Kelvin-Helmholtz instability to be developed for any speed. Therefore, twisted magnetic flux tubes can be unstable to Kelvin-Helmholtz instability when they move with small speed relative to main solar wind stream. The Kelvin-Helmholtz vortices may significantly contribute into the solar wind turbulence. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement N 313038/STORM and from the Austrian 'Fonds zur Förderung der wissenschaftlichen Forschung' under project P24740-N27.

Zaqarashvili, Teimuraz; Voros, Zoltan; Zhelyazkov, Ivan; Narita, Yasuhito; Bruno, Roberto

2014-05-01

153

RETRACTED ARTICLE: Effect of magnetic relaxation on flux jump in high temperature superconductors  

NASA Astrophysics Data System (ADS)

The effect of magnetic relaxation on flux jump is studied in high temperature superconducting slab. The magnetothermal diffusion equations based on the Kim model are presented in this paper to estimate the effect of magnetic relaxation on flux jump inside the slab in the flux dynamics process. Numerical results obtained show that the flux-jump phenomenon could be suppressed by the magnetic relaxation and the dependence of the number of flux jumps on the relaxation time are obvious.

Zhao, Y.-F.; He, T.-H.

2010-10-01

154

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

155

Reconnection experiments with flux ropes near 3D magnetic nulls  

NASA Astrophysics Data System (ADS)

Depending on the topology and geometry of the magnetic field, a rich collection of magnetic reconnection scenarios is possible in 3D including reconnection at magnetic nulls. Nulls have been reported in the solar corona [1] and in Earth's magnetosphere [2], yet there are a limited number of laboratory observations. At the Versatile Toroidal Facility (VTF) we have implemented a new magnetic geometry with a pair of 3D null points in the background toroidal field. In the nominal symmetric configuration a field line connects the nulls. We form a flux rope along this field line and observe the rope rapidly restructuring and rewiring as the nulls develop. A suit of diagnostics will be deployed and results presented for the dynamics of the geometry. [1ex] [1] Fletcher et al., Astrophys. J. 554, 451(2001).[0ex] [2] Xiao et al., Nat. Phys. 2, 478 (2006).

Vrublevskis, A.; Egedal, J.; Le, A.

2012-10-01

156

Magnetic Flux Compression Concept for Aerospace Propulsion and Power  

NASA Technical Reports Server (NTRS)

The objective of this research is to investigate system level performance and design issues associated with magnetic flux compression devices for aerospace power generation and propulsion. The proposed concept incorporates the principles of magnetic flux compression for direct conversion of nuclear/chemical detonation energy into electrical power. Specifically a magnetic field is compressed between an expanding detonation driven diamagnetic plasma and a stator structure formed from a high temperature superconductor (HTSC). The expanding plasma cloud is entirely confined by the compressed magnetic field at the expense of internal kinetic energy. Electrical power is inductively extracted, and the detonation products are collimated and expelled through a magnetic nozzle. The long-term development of this highly integrated generator/propulsion system opens up revolutionary NASA Mission scenarios for future interplanetary and interstellar spacecraft. The unique features of this concept with respect to future space travel opportunities are as follows: ability to implement high energy density chemical detonations or ICF microfusion bursts as the impulsive diamagnetic plasma source; high power density system characteristics constrain the size, weight, and cost of the vehicle architecture; provides inductive storage pulse power with a very short pulse rise time; multimegajoule energy bursts/terawatt power bursts; compact pulse power driver for low-impedance dense plasma devices; utilization of low cost HTSC material and casting technology to increase magnetic flux conservation and inductive energy storage; improvement in chemical/nuclear-to-electric energy conversion efficiency and the ability to generate significant levels of thrust with very high specific impulse; potential for developing a small, lightweight, low cost, self-excited integrated propulsion and power system suitable for space stations, planetary bases, and interplanetary and interstellar space travel; potential for attaining specific impulses approaching 10 (exp 6) seconds, which would enable missions to the outer planets within ten years and missions at interstellar distances within fifty years.

Litchford, Ron J.; Robertson, Tony; Hawk, Clark W.; Turner, Matt; Koelfgen, Syri

2000-01-01

157

Magnetic perturbation effects on noise and startup in DC-operating oven magnetrons  

Microsoft Academic Search

Previous experiments demonstrated that imposing an azimuthally varying axial magnetic field, axially asymmetric, in dc-operating oven magnetrons causes rapid mode growth (by magnetic priming) and significant noise reduction. This configuration was previously implemented by adding five perturbing magnets on the upper existing magnet of the magnetron. Experiments reported here add five perturbing magnets on each of the two existing magnets

V. Bogdan Neculaes; Michael C. Jones; Ronald M. Gilgenbach; Y. Y. Lau; J. W. Luginsland; B. W. Hoff; W. M. White; N. M. Jordan; P. Pengvanich; Y. Hidaka; H. L. Bosman

2005-01-01

158

Reconnection Experiments with Flux Ropes near 3D Magnetic Nulls  

NASA Astrophysics Data System (ADS)

Magnetic reconnection has been predominantly investigated in two dimensions. However, depending on the topology and geometry of the magnetic field, a rich collection of magnetic reconnection scenarios is possible in 3D including reconnection at magnetic nulls. Nulls have been reported in the solar corona [1] and in Earth's magnetosphere [2], yet there are a limited number of laboratory observations. At the Versatile Toroidal Facility (VTF) we have implemented a new magnetic geometry with a pair of 3D null points in the background toroidal field. We form a flux rope along the background field and observe it to rapidly restructure and rewire as the nulls develop. We can adjust the topology of the configuration from one where a field line connects the nulls to one where the nulls are no longer linked. A suit of diagnostics will be deployed and results presented for the dynamics of the flux rope. [1] Fletcher et al., Astrophys. J. 554, 451(2001) [2] Xiao et al., Nat. Phys. 2, 478 (2006)

Vrublevskis, A.; Egedal, J.; Le, A.

2012-12-01

159

High power pulse compression using magnetic flux compression  

NASA Astrophysics Data System (ADS)

A design of high power pulse compression by (axial) magnetic flux compression is presented. Optimal model parameters are presented for liner plasma using the thin-shell model code and finite element analyses for stator coils design. Typical model simulation results indicated that a 1.5 ?s rise-time pulse with 7.5 MA peak value could be compressed to 20 MA peak value fast rise pulse (˜40 ns rise-time) in the secondary circuit with a 0.5 ? external load. The limitations resulting from Joule heating and extra high magnetic field diffusion on stator coils are discussed.

Xu, Fu-Kai; Ding, Wu

2012-05-01

160

Helical motion of magnetic flux tubes in the solar atmosphere  

E-print Network

Photospheric granulation may excite transverse kink pulses in anchored vertical magnetic flux tubes. The pulses propagate upwards along the tubes with the kink speed, while oscillating wakes are formed behind the wave front in a stratified atmosphere. The wakes oscillate at the kink cut-off frequency of stratified medium and gradually decay in time. When two or more consecutive kink pulses with different polarizations propagate in the same thin tube, then the wakes corresponding to different pulses may superimpose. The superposition sets up helical motions of magnetic flux tubes in the photosphere/chromosphere as seen by recent Hinode movies. The energy carried by the pulses is enough to heat the solar chrmosphere/corona and accelerate the solar wind.

T. V. Zaqarashvili; N. Skhirtladze

2008-07-02

161

Residual Magnetic Flux Leakage: A Possible Tool for Studying Pipeline Defects  

E-print Network

Residual Magnetic Flux Leakage: A Possible Tool for Studying Pipeline Defects Vijay Babbar1 weaker flux signals. KEY WORDS: Magnetic flux leakage; residual magnetization; pipeline defects; pipeline pipelines, which may develop defects such as corrosion pits as they age in service.(1) Under the ef- fect

Clapham, Lynann

162

Direct study of magnetic flux penetration and trapping in HTSC  

Microsoft Academic Search

A method for visualization of the magnetic flux penetration and trapping in high-temperature superconductors (HTSC) using magneto-uniaxial ferrimagnetic films is described. The difference in the character of these processes in a number of materials (HTSC single crystals, films and ceramics) is studied. Specific features of remagnetization of Tl2Ba2CaCu2Ox and YBa2Cu3O7-delta single crystals in the superconducting state which do not fit

M. V. Indenbom; N. N. Kolesnikov; M. P. Kulakov; I. G. Naumenko; V. I. Nikitenko; A. A. Polyanskii; N. F. Vershinin; V. K. Vlasko-Vlasov

1990-01-01

163

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] [Iowa State Univ., Ames, IA (United States). Dept. of Electrical and Computer Engineering

1996-05-01

164

Stability of magnetic flux ropes with background flow  

NASA Astrophysics Data System (ADS)

MHD stability of magnetic flux ropes is usually studied from the view point of perturbing a static equilibrium background, whereas the significant background flow that is usually present completely modifies the stability of such systems. A new theory, based on energy conservation and self-adjoint operators, permits the computation of the full spectrum of waves and instabilities of stationary plasmas. It involves the construction of a network of curves (the spectral web) in the complex omega-plane associated with the complex complementary energy, which is the energy needed to maintain harmonic time dependence in an open system. Vanishing of that energy, at the intersections of the mentioned curves, yields the eigenvalues of the closed system. Thus, for the first time, knowledge of the full complex spectrum of modes together with a connecting structure is obtained. This theory is applied to compute the complete spectrum of waves and instabilities of flux ropes in a thin accretion disk and of the rotating magnetized jets emitted from those disks. It yields specific stability criteria in terms of the helicities of the magnetic field and of the flow velocity that may be compared with observable parameters of the flux ropes.

Goedbloed, Hans; Keppens, Rony

2012-10-01

165

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

166

Variability of Open Magnetic Flux in the Heliosphere  

NASA Astrophysics Data System (ADS)

The Ulysses result that r2 BR is independent of heliographic latitude at solar minimum and maximum implies that BR at any latitude is a measure the total open magnetic flux in the heliosphere. Simultaneous agreement between Ulysses and in-ecliptic data indicates that in-ecliptic BR is a faithful measure of open flux. In- ecliptic measurements extend over 4 sunspot cycles and reveal that open flux has varied by less than a factor of 2 or about one-tenth of the change in total photospheric flux. BR is a minimum at solar minimum and, in the past 3 cycles (20, 21 22), has returned to nearly the same value leading to speculation that it was invariant. However, recent observations by Ulysses in 2006-07 show a decrease in r2 BR from 3.6 to 2.3 nT (AU)2 that coincides with a decrease in the Sun's polar cap field. BR gradually ascends from solar minimum into the declining phase before decreasing to the following solar minimum. The increase is interrupted when the polar cap fields disappear and reverse sign. The interpretation of this variation is complicated by the simultaneous presence of several possible contributors to the open flux including Coronal Mass Ejecta.

Smith, E. J.

2008-05-01

167

Generation of a flare loop structure and ejection of magnetic flux from an erupting laboratory arched magnetic flux rope  

NASA Astrophysics Data System (ADS)

A laboratory plasma experiment has been built to generate an arched magnetic flux rope (AMFR) which is essentially an arch-shaped, current-carrying, magnetized plasma structure. Coronal loops and prominences are the main examples of solar AMFRs that frequently erupt and evolve into more energetic events such as flares and coronal mass ejections. Numerous small-scale AMFRs are also observed in the solar corona. In order to capture the important micro-physics of an erupting AMFR, the laboratory experiment has been designed by careful scaling of the solar plasma parameters. The laboratory AMFR (n ~ 1019 m-3, Te ~ 10 eV, L ~ 0.5 m) is produced using a LaB6 plasma source in presence of an arched vacuum magnetic field (B ~ 1 kG) and it evolves in a large magnetized plasma (1.0 m diameter, 4.5 m long, n ~ 1018 m-3, Te ~ 4 eV, B = 25-150 G). Two laser beams (1064 nm, ~0.5 J/pulse) strike movable carbon targets placed behind the electrodes to generate controlled plasma flows from the footpoints of the AMFR. The laser generated flows can mimic a variety of plasma flow conditions that exist on the sun and they can trigger the AMFR eruption by injecting dense plasma and magnetic flux in the AMFR. The experiment runs continuously with a 0.5 Hz repetition rate and is highly reproducible. Thus, several thousands of identical eruptions are routinely generated and evolution of the magnetic field, density, and plasma temperature is recorded in 3D with a high spatiotemporal resolution ( dx = 1 mm, dt= 20 ns) using movable diagnostic probes. Fast-camera images of the erupting AMFR demonstrate striking similarities between laboratory and solar plasma structures, most notably the observation of a flare-loop like structure following the eruption of the laboratory AMFR. The eruption of the AMFR can be initiated either by the laser generated intense flows or by the presence of a strong background magnetic field (B > 50 G ~ magnetic field at the leading edge of the AMFR). In both scenarios, the AMFR plasma is released and oscillatory plasma motions are observed in the AMFR. Dramatic release of the magnetic flux from the AMFR occurs only when the AMFR is destabilized by plasma flows. We will present experimental data that captures the dynamics of the 3D magnetic field and associated current channels of the laboratory flare loop. Reference: S. K. P. Tripathi and W. Gekelman, Phys. Rev. Lett. 105, 075005 *Work performed at Basic Plasma Science Facility, UCLA and supported by US Department of Energy and National Science Foundation

Tripathi, S.; Gekelman, W. N.

2011-12-01

168

Flux lattice relaxation, noise and symmetry-breaking in frustrated Josephson junction arrays  

SciTech Connect

The purpose of the present report is to study: (1) Relaxation from an initially random flux state. We wish to exhibit the multiple length and time scales involved in this relaxation, and the influence of thermal fluctuations. This noisy relaxation is controlled by the dynamics of various mesoscopic defect'' structures defined with respect to the underlying ground-state flux structure. The defects, their mesoscopic collective patterns, and their dynamics control a complex macroscopic response, but are themselves microscopically controlled by the competitions producing the ground-state flux completely. Such multiscaleresponses have been observed in other competing interaction systems e.g., spin glasses, random field magnets, and weakly pinned charge-density waves, as well as noisy responses in high-temperature superconductors. Various creep'' and stretched-exponential'' regimes have been proposed, as well as phenomenological scaling theories attempting to relate spatial domain sizes with temporal scales. (2) Noisy (i.e.,multitime-scale) voltages responses, and their relation to multilength scales, in current driven JJA's in the presence of a uniform extended magnetic field which frustrates the flux order.

Gronbech-Jensen, N.; Bishop, A.R.; Lomdahl, P.S.

1993-01-01

169

Nonlinear Parker instability of isolated magnetic flux in a plasma  

NASA Technical Reports Server (NTRS)

The nonlinear evolution of the Parker instability in an isolated horizontal magnetic-flux sheet embedded in a two-temperature layer atmosphere is studied by using a two-dimensional MHD code. In the solar case, this two-layer model is regarded as a simplified abstraction of the sun's photosphere/chromosphere and its overlying much hotter (coronal) envelope. The horizontal flux sheet is initially located in the lower temperature atmosphere so as to satisfy magnetostatic equilibrium under a constant gravitational acceleration. Ideal MHD is assumed, and only perturbations with k parallel to the magnetic-field lines are investigated. As the instability develops, the gas slides down the expanding loop, and the evacuated loop rises as a result of enhanced magnetic buoyancy. In the nonlinear regime of the instability, both the rise velocity of a magnetic loop and the local Alfven velocity at the top of the loop increase linearly with height and show self-similar behavior with height as long as the wavelength of the initial perturbation is much smaller than the horizontal size of the computing domain.

Shibata, K.; Tajima, T.; Matsumoto, R.; Hanawa, T.; Horiuchi, T.

1989-01-01

170

Magnetic Flux Compression Concept for Nuclear Pulse Propulsion and Power  

NASA Technical Reports Server (NTRS)

The desire for fast, efficient interplanetary transport requires propulsion systems having short acceleration times and very high specific impulse attributes. Unfortunately, most highly efficient propulsion systems which are within the capabilities of present day technologies are either very heavy or yield very low impulse such that the acceleration time to final velocity is too long to be of lasting interest, One exception, the nuclear thermal thruster, could achieve the desired acceleration but it would require inordinately large mass ratios to reach the range of desired final velocities. An alternative approach, among several competing concepts that are beyond our modern technical capabilities, is a pulsed thermonuclear device utilizing microfusion detonations. In this paper, we examine the feasibility of an innovative magnetic flux compression concept for utilizing microfusion detonations, assuming that such low yield nuclear bursts can be realized in practice. In this concept, a magnetic field is compressed between an expanding detonation driven diamagnetic plasma and a stationary structure formed from a high temperature superconductor (HTSC). In general, we are interested in accomplishing two important functions: (1) collimation of a hot diamagnetic plasma for direct thrust production; and (2) pulse power generation for dense plasma ignition. For the purposes of this research, it is assumed that rnicrofusion detonation technology may become available within a few decades, and that this approach could capitalize on recent advances in inertial confinement fusion ICF) technologies including magnetized target concepts and antimatter initiated nuclear detonations. The charged particle expansion velocity in these detonations can be on the order of 10 (exp 6)- 10 (exp 7) meters per second, and, if effectively collimated by a magnetic nozzle, can yield the Isp and the acceleration levels needed for practical interplanetary spaceflight. The ability to ignite pure fusion micro-bursts with reasonable levels of input energy is an equally challenging scientific problem. It remains to be seen, however, whether an effective ignition driver can be developed which meets the requirements for practical spaceflight application (namely high power density, compactness, low weight, and low cost). In this paper, system level performance and design issues are examined including generator performance, magnetic flux compression processes, magnetic diffusion processes, high temperature superconductor (HTSC) material properties, plasmadynamic processes, detonation plasma expansion processes, magnetohydrodynamic instabilities, magnetic nozzle performance, and thrust production performance. Representative generator performance calculations based on a simplified skin layer formulation are presented as well as the results of exploratory small-scale laboratory experiments on magnetic flux diffusion in HTSC materials. In addition, planned follow-on scientific feasibility experiments are described which utilize high explosive detonations and high energy gas discharges to simulate the plasma conditions associated with thermonuclear micro-detonations.

Litchford, Ronald J.

2000-01-01

171

A dispersive nanoSQUID magnetometer for ultra-low noise, high bandwidth flux detection  

NASA Astrophysics Data System (ADS)

We describe a dispersive nanoSQUID (nanoscale superconducting quantum interference device) magnetometer comprised of two variable thickness aluminum weak-link Josephson junctions shunted in parallel with an on-chip capacitor. This arrangement forms a nonlinear oscillator with a tunable 4-8 GHz resonant frequency with a quality factor Q = 30 when coupled directly to a 50 ? transmission line. In the presence of a near-resonant microwave carrier signal, a low frequency flux input generates sidebands that are readily detected using microwave reflectometry. If the carrier excitation is sufficiently strong, then the magnetometer also exhibits parametric gain, resulting in a minimum effective flux noise of 30 n?0 Hz-1/2 with 20 MHz of instantaneous bandwidth. If the magnetometer is followed with a near-quantum-noise-limited Josephson parametric amplifier, we can increase the bandwidth to 60 MHz without compromising sensitivity. This combination of high sensitivity and wide bandwidth with no on-chip dissipation makes this device ideal for local sensing of spin dynamics, both classical and quantum.

Levenson-Falk, E. M.; Vijay, R.; Antler, N.; Siddiqi, I.

2013-05-01

172

Induced current in the presence of a magnetic flux tube of small radius  

Microsoft Academic Search

The induced current density, corresponding to the massless Dirac equation in (2+1) dimensions in a magnetic flux tube of small radius is considered. This problem is important for graphene. In the case when an electron cannot penetrate the region of nonzero magnetic field, this current is the odd periodical function of the magnetic flux. If the region inside the magnetic

Alexander I. Milstein; Ivan S. Terekhov

2011-01-01

173

Mode-hopping mechanism generating colored noise in a magnetic tunnel junction based spin torque oscillator  

NASA Astrophysics Data System (ADS)

The frequency noise spectrum of a magnetic tunnel junction based spin torque oscillator is examined where multiple modes and mode-hopping events are observed. The frequency noise spectrum is found to consist of both white noise and 1/f frequency noise. We find a systematic and similar dependence of both white noise and 1/f frequency noise on bias current and the relative angle between the reference and free layers, which changes the effective damping and hence the mode-hopping behavior in this system. The frequency at which the 1/f frequency noise changes to white noise increases as the free layer is aligned away from the anti-parallel orientation w.r.t the reference layer. These results indicate that the origin of 1/f frequency noise is related to mode-hopping, which produces both white noise as well as 1/f frequency noise similar to the case of ring lasers.

Sharma, Raghav; Dürrenfeld, P.; Iacocca, E.; Heinonen, O. G.; Åkerman, J.; Muduli, P. K.

2014-09-01

174

Stochastic Flux-Freezing and Turbulent Magnetic Dynamo  

NASA Astrophysics Data System (ADS)

“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 be ``frozen-in’’ to a turbulent MHD fluid (plasma or liquid metal) in the original sense of Alfvén, even at infinite conductivity if also the kinetic Reynolds number is large. We show that systems described by resistive nonlinear hydromagnetic equations (MHD, Hall MHD, etc.) satisfy a stochastic Alfvén Theorem and we use this result to argue that flux-conservation must remain stochastic at infinite Reynolds numbers. The predictions of standard flux-freezing are thus found to be wrong---by many orders of magnitude---in high-Reynolds-number MHD turbulence. Stochastic flux-freezing has fundamental consequences for many astrophysical problems, such as planetary and solar dynamos, star formation, solar flares, etc. As one example, we present numerical results on the kinematic, fluctuation dynamo in non-helical, incompressible turbulence at magnetic Prandtl number Pr=1, using a Lagrangian particle method with a hydrodynamic turbulence database at Re_?=433. We find that Richardson diffusion and stochasticity of field-line motion play an essential role in magnetic energy growth. The Lagrangian mechanisms of small-scale dynamo are found to be very similar to those in the soluble Kazantsev model at Pr=0. We also discuss briefly the application of stochastic flux-freezing to the problem of fast magnetic reconnection. We use the phenomenological Goldreich-Sridhar 1995 theory to estimate the dispersion of particle-pairs in strong MHD turbulence with an imposed magnetic field. We then recover the Lazarian-Vishniac 1999 theory of fast reconnection as a natural generalization of the laminar Sweet-Parker model to high-Reynolds-number MHD turbulence.

Eyink, G. L.

2010-12-01

175

Flux buildup in field reversed configurations using rotating magnetic fields  

NASA Astrophysics Data System (ADS)

Rotating magnetic field (RMF) current drive is a very attractive method for both increasing the flux and sustaining the current in field reversed configurations (FRC). It has been demonstrated in low temperature, low field rotamaks, and will now be applied to a new translation, confinement, and sustainment (TCS) experiment attached to the LSX/mod (Large s field-reversed configuration Experiment) facility [Hoffman et al. Fusion Technol. 23, 185 (1993)]. Previous RMF calculations have been concerned primarily with the plasma currents and particle orbits produced in one-dimensional cylinders with the rotating field strength of near equal magnitude to the confining axial field. Both fluid current and particle orbits are calculated here in the more interesting regime appropriate to TCS and reactors where the confinement field far exceeds the rotating field strength. New insight is gained into both the flux buildup requirements for two-dimensional equilibria and into the limits on ion rotation in this high confinement field regime.

Hoffman, Alan L.

1998-04-01

176

Elementary heating events - magnetic interactions between two flux sources. II. Rates of flux reconnection  

NASA Astrophysics Data System (ADS)

Magnetic fragments in the photosphere are in continuous motion and, due to the complex nature of the magnetic field in the solar atmosphere, these motions are likely to drive a lucrative coronal energy source: the passing of initially-unconnected opposite-polarity fragments that release energy through both closing and then re-opening the same fieldlines. Three-dimensional, time-dependent MHD and potential models are used to investigate the passing of fragments in an overlying field. The processes of closing and opening the field generally occur through separator and separatrix reconnection, respectively. The rates of flux reconnection in these processes are determined. They are found to be dependent on the direction of the surrounding magnetic field relative to the motion of the fragments and the velocity of the sources. In particular, separator reconnection rates (closing) and separatrix-surface reconnection rates (opening) are directly related to the rate of flux transport perpendicular to the current sheet (overlying field). The results suggest that both types of reconnection are fast with the peak rates of separator and separatrix reconnection occurring at 58% and 29% of the peak potential reconnection rate, respectively, when the sources are driven at a hundredth of the peak Alfvén velocity in the box. Moreover, the slower the system is driven the closer the flux reconnection rates are to the instantaneous potential rates. Furthermore, there is a maximum reconnection rate for both types of reconnection as the driving speed tends to the Alfvén speed with the separatrix reconnection rate typically half that of separator reconnection. These results suggest that, on the Sun, reconnection driven by the passing of small-scale network and intranetwork fragments is a highly efficient process that is very likely to contribute significantly to the heating of the background solar corona. The three-dimensional reconnection processes are efficient because, unlike in two-dimensions, there are many places within the current sheets where reconnection can take place simultaneously giving rise to fine-scale structure along the boundaries between the open, closed and re-opened flux. Furthermore, due to the complexity of the magnetic field above the photosphere the reconnection all takes place low down at less than a quarter of the separation of the initial fragments above the photosphere.

Parnell, C. E.; Galsgaard, K.

2004-12-01

177

Magnetic-field-sensing mechanism based on dual-vortex motion and magnetic noise  

NASA Astrophysics Data System (ADS)

In this study, we report two novel field sensing mechanisms using elliptical permalloy single layer. Using micromagnetic modeling, dual-vortex structure is observed and stabilized in elliptical permalloy single layer by applying hard bias field (along the y-axis) and vertical axis field (perpendicular to plane). During the increasing or decreasing of the hard bias field within certain range, the dual vortices would move away from or approach to each other at a constant velocity, leading to a positive correlation between the hard bias field and the vortex gap. By exploring the magnetic noise properties of the elliptical permalloy single layer under various vortex gap, the vortex gap is found to be positively correlated with both the FMR (Ferromagnetic Resonance) peak positions and the integrated thermally excited mag-noise. Therefore, the combination of the dual-vortex motion and the magnetic noise properties make it possible to measure external field (along hard bias direction) through measuring the FMR peak positions or integrated thermally mag-noise. This FMR-peak-based field sensing mechanism and integrated-noise-based field sensing introduce a simple field sensor structure with expected highest sensitivity to 1.1%/Oe and field detectable range over 1000 Oe, which is promising for potential sensor applications.

Zeng, Tui; Zhou, Yan; Lin, Ko-Wei; Lai, Pui-To; Pong, Philip W. T.

2014-05-01

178

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

179

Effect of the magnetic-flux variations on SQUID stability  

SciTech Connect

The oscillation dynamics of a nonhysteretic, one-contact superconducting quantum interferometer under low-frequency modulation of an external magnetic flux is studied. It is shown that even in the case of adiabatic, equilibrium, and nonhysteretic operation modes of a superconducting ring enclosed by a weak link, there can appear chaotic oscillations. The conditions of chaos arising are considered analytically and numerically; mechanisms and destruction scenarios of periodic oscillations are studied. The stochastic instability could impose limitations on the increase in sensitivity of nonhysteretic SQUIDs, which was earlier predicted to be high enough.

Bulgakov, S.A.; Ryabov, V.B.; Shnyrkov, V.I.; Vavriv, D.M. (Inst. of Radio Astronomy, Kharkov (Ukrainian SSR))

1991-06-01

180

Decoupling Suspension Controller Based on Magnetic Flux Feedback  

PubMed Central

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

2013-01-01

181

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

2013-01-01

182

Numerical Modeling of a Magnetic Flux Compression Experiment  

NASA Astrophysics Data System (ADS)

A possible plasma target for Magnetized Target Fusion (MTF) is a stable diffuse z-pinch in a toroidal cavity, like that in MAGO experiments. To examine key phenomena of such MTF systems, a magnetic flux compression experiment with this geometry is under design. The experiment is modeled with 3 codes: a slug model, the 1D Lagrangian RAVEN code, and the 1D or 2D Eulerian Magneto-Hydro-Radiative-Dynamics-Research (MHRDR) MHD simulation. Even without injection of plasma, high- Z wall plasma is generated by eddy-current Ohmic heating from MG fields. A significant fraction of the available liner kinetic energy goes into Ohmic heating and compression of liner and central-core material. Despite these losses, efficiency of liner compression, expressed as compressed magnetic energy relative to liner kinetic energy, can be close to 50%. With initial fluctuations (1%) imposed on the liner and central conductor density, 2D modeling manifests liner intrusions, caused by the m = 0 Rayleigh-Taylor instability during liner deceleration, and central conductor distortions, caused by the m = 0 curvature-driven MHD instability. At many locations, these modes reduce the gap between the liner and the central core by about a factor of two, to of order 1 mm, at the time of peak magnetic field.

Makhin, Volodymyr; Bauer, Bruno S.; Awe, Thomas J.; Fuelling, Stephan; Goodrich, Tasha; Lindemuth, Irvin R.; Siemon, Richard E.; Garanin, Sergei F.

2007-06-01

183

Time-Varying Magnetic Field Coupled Noise Reduction in Low-Voltage Measurements in Superconductors  

Microsoft Academic Search

Noise generated by time-varying magnetic fields in- terferes in the measurements of low voltages particularly with superconductor materials. The measured values can alter sig- nificantly if it is not compensated appropriately or if suitable protocols are not followed. Measurements on inductive loads, like superconducting wires and coils, considerably generate high magnetic field in the surrounding, which couples electromagnetic noise through

Kalpesh Doshi; Yohan Khristi; Sunil Kedia; Subrata Pradhan

2011-01-01

184

Thermal magnetic noise control in the ultra-high-density read head  

Microsoft Academic Search

In order to reduce the resistance of tunnel magnetoresistive (TMR) read heads, a large stripe height sensor structure was proposed. The thermal magnetic noise, called as mag-noise, in this type of TMR heads was simulated by micromagnetic modeling using the Landau Lifshitz Gilbert (LLG) gyro-magnetic equation. It is found that for the same hard bias strength, both the sensitivity and

Y. K. Zheng; G. C. Han; B. Liu

2008-01-01

185

Thermal magnetic noise control in the ultra-high-density read head  

Microsoft Academic Search

In order to reduce the resistance of tunnel magnetoresistive (TMR) read heads, a large stripe height sensor structure was proposed. The thermal magnetic noise, called as mag-noise, in this type of TMR heads was simulated by micromagnetic modeling using the Landau–Lifshitz–Gilbert (LLG) gyro-magnetic equation. It is found that for the same hard bias strength, both the sensitivity and the mag-noise

Y. K. Zheng; G. C. Han; B. Liu

2008-01-01

186

MAGNETIC NULL POINTS DUE TO MULTIPLE SOURCES OF SOLAR PHOTOSPHERIC FLUX  

E-print Network

MAGNETIC NULL POINTS DUE TO MULTIPLE SOURCES OF SOLAR PHOTOSPHERIC FLUX G. W. INVERARITY and E. R are magnetic null points in the highly complex magnetic field of the solar atmosphere? In this work we seek to model the magnetic structure of quiet regions by placing magnetic sources and sinks on a hexagonal

Priest, Eric

187

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

188

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

189

A new stator-flux orientation strategy for flux-switching permanent magnet motor based on current-hysteresis control  

NASA Astrophysics Data System (ADS)

A stator-flux orientation strategy based on current hysteresis for the flux-switching permanent magnet (FSPM) motor is proposed, in which the stator-PM FSPM motor is considered as a conventional rotor-PM surface-mounted motor and an equivalent rotor-orientated dq-axes synchronous reference frame is built although there are actually no rotary magnetic motive force produced by the stator magnets in the FSPM motor. Based on the proposed model, a vector-control strategy with current hysteresis for the FSPM motor drive is investigated and implemented on a dSPACE-based platform, and both the simulated and experimental results validate the effectiveness. It should be emphasized that the proposed stator-flux orientation strategy can be applied to other stator-PM machines (including doubly salient and flux-reversal PM machines) and other control methods (including space-vector pulsed-width-modification and direct torque control).

Hua, Wei; Cheng, Ming; Lu, Wei; Jia, Hongyun

2009-04-01

190

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

191

Buoyant magnetic flux ropes in a magnetized stellar envelope: Idealized numerical 2.5-D MHD simulations  

E-print Network

Context: The context of this paper is buoyant toroidal magnetic flux ropes, which is a part of flux tube dynamo theory and the framework of solar-like magnetic activity. Aims: The aim is to investigate how twisted magnetic flux ropes interact with a simple magnetized stellar model envelope--a magnetic "convection zone"--especially to examine how the twisted magnetic field component of a flux rope interacts with a poloidal magnetic field in the convection zone. Method: Both the flux ropes and the atmosphere are modelled as idealized 2.5-dimensional concepts using high resolution numerical magneto-hydrodynamic (MHD) simulations. Results: It is illustrated that twisted toroidal magnetic flux ropes can interact with a poloidal magnetic field in the atmosphere to cause a change in both the buoyant rise dynamics and the flux rope's geometrical shape. The details of these changes depend primarily on the polarity and strength of the atmospheric field relative to the field strength of the flux rope. It is suggested that the effects could be verified observationally.

S. B. F. Dorch

2006-09-22

192

Anthropogenic electromagnetic noise disrupts magnetic compass orientation in a migratory bird.  

PubMed

Electromagnetic noise is emitted everywhere humans use electronic devices. For decades, it has been hotly debated whether man-made electric and magnetic fields affect biological processes, including human health. So far, no putative effect of anthropogenic electromagnetic noise at intensities below the guidelines adopted by the World Health Organization has withstood the test of independent replication under truly blinded experimental conditions. No effect has therefore been widely accepted as scientifically proven. Here we show that migratory birds are unable to use their magnetic compass in the presence of urban electromagnetic noise. When European robins, Erithacus rubecula, were exposed to the background electromagnetic noise present in unscreened wooden huts at the University of Oldenburg campus, they could not orient using their magnetic compass. Their magnetic orientation capabilities reappeared in electrically grounded, aluminium-screened huts, which attenuated electromagnetic noise in the frequency range from 50?kHz to 5?MHz by approximately two orders of magnitude. When the grounding was removed or when broadband electromagnetic noise was deliberately generated inside the screened and grounded huts, the birds again lost their magnetic orientation capabilities. The disruptive effect of radiofrequency electromagnetic fields is not confined to a narrow frequency band and birds tested far from sources of electromagnetic noise required no screening to orient with their magnetic compass. These fully double-blinded tests document a reproducible effect of anthropogenic electromagnetic noise on the behaviour of an intact vertebrate. PMID:24805233

Engels, Svenja; Schneider, Nils-Lasse; Lefeldt, Nele; Hein, Christine Maira; Zapka, Manuela; Michalik, Andreas; Elbers, Dana; Kittel, Achim; Hore, P J; Mouritsen, Henrik

2014-05-15

193

Three-dimensional magnetohydrodynamical simulation of expanding magnetic flux ropes  

SciTech Connect

Three-dimensional, time-dependent numerical simulations of the dynamics of magnetic flux ropes are presented. The simulations are targeted towards an experiment previously conducted at California Institute of Technology [P. M. Bellan and J. F. Hansen, Phys. Plasmas 5, 1991 (1998)] which aimed at simulating solar prominence eruptions in the laboratory. The plasma dynamics is described by ideal magnetohydrodynamics using different models for the evolution of the mass density. The initial current distribution represents the situation at the plasma creation phase, while it is not increased during the simulation. Key features of the reported experimental observations like pinching of the current loop, its expansion and distortion into helical shape are reproduced in the numerical simulations. Details of the final structure depend on the choice of a specific model for the mass density.

Arnold, L.; Dreher, J.; Grauer, R. [Theoretische Physik I, Ruhr-Universitaet, 44780 Bochum (Germany); Soltwisch, H.; Stein, H. [Experimentalphysik V, Ruhr-Universitaet, 44780 Bochum (Germany)

2008-04-15

194

Magnetohydrostatic equilibrium. II. Three-dimensional multiple open magnetic flux tubes in the stratified solar atmosphere  

E-print Network

A system of multiple open magnetic flux tubes spanning the solar photosphere and lower corona is modelled analytically, within a realistic stratified atmosphere subject to solar gravity. This extends results for a single magnetic flux tube in magnetohydrostatic equilibrium, described in Gent et al. (MNRAS, 435, 689, 2013). Self-similar magnetic flux tubes are combined to form magnetic structures, which are consistent with high-resolution observations. The observational evidence supports the existence of strands of open flux tubes and loops persisting in a relatively steady state. Self-similar magnetic flux tubes, for which an analytic solution to the plasma density and pressure distribution is possible, are combined. We calculate the appropriate balancing forces, applying to the equations of momentum and energy conservation to preserve equilibrium. Multiplex flux tube configurations are observed to remain relatively stable for up to a day or more, and it is our aim to apply our model as the background conditi...

Gent, Frederick A; Erd'elyi, Rebertus

2014-01-01

195

Validation Tests of Data Driven Magnetic Flux Emergence  

NASA Astrophysics Data System (ADS)

The emergence of magnetic flux through the solar photosphere into the corona is a key problem to understanding the energization of the solar corona. With the advent of high spatial and temporal resolution solar vector magnetic field measurements, it may now be possible to simulate the dynamical evolution of the corona by using these measurements as the driving boundary condition for magnetohydrodynamical (MHD) simulations of the corona. First, however, methods for using this data to drive simulations must be validated via quantitative tests. We report here on a series of such tests wherein the driving inputs are taken from self-consistent simulations of the emergence of flux ropes from the upper convection zone through the photosphere and chromosphere into the low corona. Photospheric MHD output from these simulations is then used to drive new simulations, and the driven results are compared against the original results for a variety of driving assumptions and algorithms. We will focus here on the relative advantages and disadvantages of data driving a higher order finite-element-based MHD code (HiFi) versus data driving a lower order Lagrangian remap-based MHD code (LARE3D). For each code, we will report on the necessary input conditions (spatial and temporal resolution, specification of MHD variables, and specification of vertical gradients) needed to reproduce a high level of agreement between the original and the driven simulations. Finally, we will report on the level of agreement achieved when using driving input equivalent to that of current solar observations. Based on these tests, we will address the prospects for using high time and spatial resolution vector magnetogram observations to drive MHD simulations of the solar chromosphere and corona.This work was supported by the ONR 6.1 and the NASA LWS programs.

Linton, Mark; Lukin, Vyacheslav; Leake, James Edward; Schuck, Peter W.

2014-06-01

196

Simulations of Emerging Magnetic Flux. II. The Formation of Unstable Coronal Flux Ropes and the Initiation of Coronal Mass Ejections  

NASA Astrophysics Data System (ADS)

We present results from three-dimensional magnetohydrodynamic simulations of the emergence of a twisted convection zone flux tube into a pre-existing coronal dipole field. As in previous simulations, following the partial emergence of the sub-surface flux into the corona, a combination of vortical motions and internal magnetic reconnection forms a coronal flux rope. Then, in the simulations presented here, external reconnection between the emerging field and the pre-existing dipole coronal field allows further expansion of the coronal flux rope into the corona. After sufficient expansion, internal reconnection occurs beneath the coronal flux rope axis, and the flux rope erupts up to the top boundary of the simulation domain (~36 Mm above the surface). We find that the presence of a pre-existing field, orientated in a direction to facilitate reconnection with the emerging field, is vital to the fast rise of the coronal flux rope. The simulations shown in this paper are able to self-consistently create many of the surface and coronal signatures used by coronal mass ejection (CME) models. These signatures include surface shearing and rotational motions, quadrupolar geometry above the surface, central sheared arcades reconnecting with oppositely orientated overlying dipole fields, the formation of coronal flux ropes underlying potential coronal field, and internal reconnection which resembles the classical flare reconnection scenario. This suggests that proposed mechanisms for the initiation of a CME, such as "magnetic breakout," are operating during the emergence of new active regions.

Leake, James E.; Linton, Mark G.; Antiochos, Spiro K.

2014-05-01

197

Slow twists of solar magnetic flux tubes and the polar magnetic field of the sun  

NASA Technical Reports Server (NTRS)

The solar wind model of Weber and Davis (1967) is generalized to compute the heliospheric magnetic field resulting from solar rotation or a steady axisymmetric twist including a geometrical expansion which is more rapid than spherical. The calculated increase in the ratio of the toroidal to poloidal field components with heliocentric radial distance r clarifies an expression derived recently by Jokipii and Kota (1989). Magnetic-field components transverse to r do not in general grow to dominate the radial component at large r. The analysis also yields expressions for the Poynting flux associated with the steady twists.

Hollweg, Joseph V.; Lee, Martin A.

1989-01-01

198

Slow twists of solar magnetic flux tubes and the polar magnetic field of the sun  

NASA Astrophysics Data System (ADS)

The solar wind model of Weber and Davis (1967) is generalized to compute the heliospheric magnetic field resulting from solar rotation or a steady axisymmetric twist including a geometrical expansion which is more rapid than spherical. The calculated increase in the ratio of the toroidal to poloidal field components with heliocentric radial distance r clarifies an expression derived recently by Jokipii and Kota (1989). Magnetic-field components transverse to r do not in general grow to dominate the radial component at large r. The analysis also yields expressions for the Poynting flux associated with the steady twists.

Hollweg, Joseph V.; Lee, Martin A.

1989-08-01

199

Enhancement of magnetic flux distribution in a DC superconducting electric motor  

NASA Astrophysics Data System (ADS)

Most motor designs require an air gap between the rotor and stator to enable the armature to rotate freely. The interaction of magnetic flux from rotor and stator within the air gap will provide the thrust for rotational motion. Thus, the understanding of magnetic flux in the vicinity of the air gap is very important to mathematically calculate the magnetic flux generated in the area. In this work, a finite element analysis was employed to study the behavior of the magnetic flux in view of designing a synchronous DC superconducting electric motor. The analysis provides an ideal magnetic flux distribution within the components of the motor. From the flux plot analysis, it indicates that flux losses are mainly in the forms of leakage and fringe effect. The analysis also shows that the flux density is high at the area around the air gap and the rotor. The high flux density will provide a high force area that enables the rotor to rotate. In contrast, the other parts of the motor body do not show high flux density indicating low distribution of flux. Consequently, a bench top model of a DC superconducting motor was developed where by motor with a 2-pole type winding was chosen. Each field coil was designed with a racetrack-shaped double pancake wound using DI-BSCCO Bi-2223 superconducting tapes. The performance and energy efficiency of the superconducting motor was superior when compared to the conventional motor with similar capacity.

Hamid, N. A.; Ewe, L. S.; Chin, K. M.

2013-06-01

200

A chain of magnetic flux ropes in the magnetotail of Mars J. P. Eastwood,1  

E-print Network

A chain of magnetic flux ropes in the magnetotail of Mars J. P. Eastwood,1 J. J. H. Videira,1 D. A of magnetic flux ropes in the magneto- tail of Mars, Geophys. Res. Lett., 39, L03104, doi:10.1029/ 2011GL; published 7 February 2012. [1] The interaction of Mars with the solar wind leads to the formation

California at Berkeley, University of

201

New models of Jupiter's magnetic field constrained by the Io flux tube footprint  

Microsoft Academic Search

Spherical harmonic models of the planetary magnetic field of Jupiter are obtained from in situ magnetic field measurements and remote observations of the position of the foot of the Io flux tube in Jupiter's ionosphere. The Io flux tube (IFT) footprint locates the ionospheric footprint of field lines traced from Io's orbital radial distance in the equator plane (5.9 Jovian

J. E. P. Connerney; M. H. Acufia; N. F. Ness; T. Satoh

1998-01-01

202

Magnetohydrostatic Equilibrium. II. Three-dimensional Multiple Open Magnetic Flux Tubes in the Stratified Solar Atmosphere  

NASA Astrophysics Data System (ADS)

A system of multiple open magnetic flux tubes spanning the solar photosphere and lower corona is modeled analytically, within a realistic stratified atmosphere subject to solar gravity. This extends results for a single magnetic flux tube in magnetohydrostatic equilibrium, described in Gent et al. Self-similar magnetic flux tubes are combined to form magnetic structures, which are consistent with high-resolution observations. The observational evidence supports the existence of strands of open flux tubes and loops persisting in a relatively steady state. Self-similar magnetic flux tubes, for which an analytic solution to the plasma density and pressure distribution is possible, are combined. We calculate the appropriate balancing forces, applying to the equations of momentum and energy conservation to preserve equilibrium. Multiplex flux tube configurations are observed to remain relatively stable for up to a day or more, and it is our aim to apply our model as the background condition for numerical studies of energy transport mechanisms from the solar surface to the corona. We apply magnetic field strength, plasma density, pressure, and temperature distributions consistent with observational and theoretical estimates for the lower solar atmosphere. Although each flux tube is identical in construction apart from the location of the radial axis, combinations can be applied to generate a non-axisymmetric magnetic field with multiple non-uniform flux tubes. This is a considerable step forward in modeling the realistic magnetized three-dimensional equilibria of the solar atmosphere.

Gent, F. A.; Fedun, V.; Erdélyi, R.

2014-07-01

203

Modelling the initiation of coronal mass ejections: magnetic flux emergence versus shearing motions  

Microsoft Academic Search

Context: Coronal mass ejections (CMEs) are enormous expulsions of magnetic flux and plasma from the solar corona into the interplanetary space. These phenomena release a huge amount of energy. It is generally accepted that both photospheric motions and the emergence of new magnetic flux from below the photosphere can put stress on the system and eventually cause a loss of

F. P. Zuccarello; C. Jacobs; A. Soenen; S. Poedts; B. van der Holst

2009-01-01

204

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

205

Magnetic helicity analysis of an interplanetary twisted flux tube  

E-print Network

, and magnetic clouds; 2134 Interplanetary Physics: Interplanetary magnetic fields; 7524 Solar Physics, Astrophysics, and Astronomy: Magnetic fields; 7513 Solar Physics, Astrophysics, and Astronomy: Coronal mass, which are characterized by enhanced magnetic field strength with respect to ambient values, a large

Dasso, Sergio

206

Fine resolution soil water fluxes measured with a small Smart Field Lysimeter: The noise removal and further interpretation  

NASA Astrophysics Data System (ADS)

A weighable Smart Field Lysimeter (30 cm diameter, 30 cm depth) with an adaptively regulated suction at its bottom was used to measure soil water fluxes at the surface and at the 30 cm depth of a short grass stand. No overland flow or accumulation of water at the surface were observed and there was no groundwater table within the soil profile. Appropriate distinction between the fluxes of different directions made it possible to separately estimate actual evapotranspiration (upward surface flux), precipitation and condensation (downward surface flux and dew on grass leaves), percolation (downward flux at 30 cm) and capillary rise (upward flux at 30 cm). The primary data were collected at 1 minute intervals but required digital filtering to remove the information noise. Various methods of filtering were tested, with a special regard to intensive rain events. The resulting data have a 10-minute resolution. The lysimeter is capable of self-recovery after a period of drought but the noise of percolation and capillary rise estimates is enhanced for some time during, before and after this period. In these situations, it is important that a porous matrix sensor measures the suction in parallel to the reference tensiometer. Both the precipitation and the actual evapotranspiration derived from the lysimeter data alone are in absolute values higher than the analogous quantities obtained with the help of the directly measured tipping bucket precipitation. These discrepancies are probably due to the rain gauge underestimating true precipitation, but partly also due to numerical noise, however smoothed. If the rain gauge data are used only to distinguish the periods of rain from the rainless periods, than the condensation of water in the soil and on the grass leaves can be estimated. The actual evapotranspiration measured by the lysimeter has a diurnal patterns depending on actual weather. The maximum occurs, on average, shortly after the noon. The percolation curves after rain events make it possible to estimate the soil hydraulic properties on the principle of the outflow method.

Dolezal, Frantisek; Bekere Mekonnen, Getu; Matula, Svatopluk; Mihalikova, Marketa; Fisak, Jaroslav; Teressa Chala, Ayele; Hrkalova, Marketa; Moreira Barradas, Joao Manuel

2014-05-01

207

Three-dimensional Simulation of Magnetic Flux Dynamics and Temperature Rise in HTSC Bulk during Pulsed Field Magnetization  

NASA Astrophysics Data System (ADS)

We have performed a three-dimensional (3D) numerical simulation of the dynamical motion of the magnetic flux and the heat propagation in the superconducting bulk after applying a pulsed magnetic field. An inhomogeneous Jc distribution was supposed in the bulk; the Jc in the growth sector boundary (GSB) is four times higher than that in the growth sector region (GSR). For lower applied pulsed field, magnetic flux was penetrated and trapped in the GSR, and for higher applied pulsed field, the magnetic flux was trapped more preferentially in the GSB. These results of the simulation reproduce the experimental ones and are valuable for the understanding the flux dynamics in the bulk during pulsed field magnetization.

Fujishiro, H.; Naito, T.; Oyama, M.

208

Induced current in the presence of magnetic flux tube of small radius  

Microsoft Academic Search

The induced current density, corresponding to the massless Dirac equation in\\u000a(2+1) dimensions in a magnetic flux tube of small radius is considered. This\\u000aproblem is important for graphene. In the case, when an electron can not\\u000apenetrate the region of nonzero magnetic field, this current is the odd\\u000aperiodical function of the magnetic flux. If the region inside the

Alexander I. Milstein; Ivan S. Terekhov

2010-01-01

209

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

Microsoft Academic Search

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 interacting with the figure-eight-shaped null-flux coils in the guideway. Closed-form formulas for the magnetic damping coefficient as functions of heave-and-sway displacements

Jianliang He; H. Coffey

1997-01-01

210

How is open solar magnetic flux lost over the solar cycle? M. J. Owens,1,2  

E-print Network

How is open solar magnetic flux lost over the solar cycle? M. J. Owens,1,2 N. U. Crooker,3 and M 2011. [1] The Sun's open magnetic field, magnetic flux dragged out into the heliosphere by the solar by nearSun magnetic reconnection between open solar magnetic field lines, be they open or closed

Lockwood, Mike

211

Structures of Interplanetary Magnetic Flux Ropes and Comparison with Their Solar Sources  

NASA Astrophysics Data System (ADS)

Whether a magnetic flux rope is pre-existing or formed in situ in the Sun's atmosphere, there is little doubt that magnetic reconnection is essential to release the flux rope during its ejection. During this process, the question remains: how does magnetic reconnection change the flux-rope structure? In this work, we continue with the original study of Qiu et al. by using a larger sample of flare-coronal mass ejection (CME)-interplanetary CME (ICME) events to compare properties of ICME/magnetic cloud (MC) flux ropes measured at 1 AU and properties of associated solar progenitors including flares, filaments, and CMEs. In particular, the magnetic field-line twist distribution within interplanetary magnetic flux ropes is systematically derived and examined. Our analysis shows that, similar to what was found before, for most of these events, the amount of twisted flux per AU in MCs is comparable with the total reconnection flux on the Sun, and the sign of the MC helicity is consistent with the sign of the helicity of the solar source region judged from the geometry of post-flare loops. Remarkably, we find that about half of the 18 magnetic flux ropes, most of them associated with erupting filaments, have a nearly uniform and relatively low twist distribution from the axis to the edge, and the majority of the other flux ropes exhibit very high twist near the axis, up to >~ 5 turns per AU, which decreases toward the edge. The flux ropes are therefore not linearly force-free. We also conduct detailed case studies showing the contrast of two events with distinct twist distribution in MCs as well as different flare and dimming characteristics in solar source regions, and discuss how reconnection geometry reflected in flare morphology may be related to the structure of the flux rope formed on the Sun.

Hu, Qiang; Qiu, Jiong; Dasgupta, B.; Khare, A.; Webb, G. M.

2014-09-01

212

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

213

Development of a PM transverse flux motor with soft magnetic composite core  

Microsoft Academic Search

This paper reports the design, performance analysis, fabrication, and experimental results of a three-phase, three-stack permanent magnet transverse flux motor with a soft magnetic composite stator core, which was designed to take advantage of the unique properties of the new material. Parameter computations by finite element analysis of the magnetic field and performance prediction by the equivalent electric circuit are

YouGuang Guo; Jian Guo Zhu; Peter A. Watterson; Wei Wu

2006-01-01

214

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

215

Investigation of Flux Linkage Profile Measurement Methods for Switched Reluctance Motors and Permanent Magnet  

E-print Network

and Permanent Magnet Motors K. Y. Lu, P. O. Rasmussen, A. E. Ritchie Abstract ­ Knowledge of actual flux linkage for switched reluctance motors (SRM's) and permanent magnet motors (PMM's). Various measurement methods have reluctance motors, permanent magnet motors. I. INTRODUCTION Switched Reluctance Motors (SRM's) have very

Lu, Kaiyuan

216

Low frequency noise peak near magnon emission energy in magnetic tunnel junctions  

NASA Astrophysics Data System (ADS)

We report on the low frequency (LF) noise measurements in magnetic tunnel junctions (MTJs) below 4 K and at low bias, where the transport is strongly affected by scattering with magnons emitted by hot tunnelling electrons, as thermal activation of magnons from the environment is suppressed. For both CoFeB/MgO/CoFeB and CoFeB/AlOx/CoFeB MTJs, enhanced LF noise is observed at bias voltage around magnon emission energy, forming a peak in the bias dependence of noise power spectra density, independent of magnetic configurations. The noise peak is much higher and broader for unannealed AlOx-based MTJ, and besides Lorentzian shape noise spectra in the frequency domain, random telegraph noise (RTN) is visible in the time traces. During repeated measurements the noise peak reduces and the RTN becomes difficult to resolve, suggesting defects being annealed. The Lorentzian shape noise spectra can be fitted with bias-dependent activation of RTN, with the attempt frequency in the MHz range, consistent with magnon dynamics. These findings suggest magnon-assisted activation of defects as the origin of the enhanced LF noise.

Liu, Liang; Xiang, Li; Guo, Huiqiang; Wei, Jian; Li, D. L.; Yuan, Z. H.; Feng, J. F.; Han, X. F.; Coey, J. M. D.

2014-12-01

217

A coupled model of magnetic flux generation and transport in stars  

NASA Astrophysics Data System (ADS)

We present a combined model for magnetic field generation and transport in cool stars with outer convection zones. The mean toroidal magnetic field, which is generated by a cyclic thin-layer \\alpha\\Omega dynamo at the bottom of the convection zone is taken to determine the emergence probability of magnetic flux tubes in the photosphere. Following the nonlinear rise of the unstable thin flux tubes, emergence latitudes and tilt angles of bipolar magnetic regions are determined. These quantities are put into a surface flux transport model, which simulates the surface evolution of magnetic flux under the effects of large-scale flows and turbulent diffusion. First results are discussed for the case of the Sun and for more rapidly rotating solar-type stars. Movies are available via http://www.aip.de/AN/movies

I?ik, E.; Schmitt, D.; Schüssler, M.

2007-12-01

218

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

NASA Astrophysics Data System (ADS)

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 at both solar minimum and maximum. Measurements of r2 BR contain information about the average polar cap field strength when allowance is made for the non-radial expansion of the magnetic field and solar wind near the Sun that eliminates the latitude gradient in magnetic pressure. Recent Earth-based magnetograph observations indicate that the Sun's polar cap field strength, BP, has decreased by a factor of about two between the previous and present latitude scans. Ulysses measurements show that the average value of r2 BR has decreased from 3.6 nT (AU)2 in 1993.5 to 1995.0 to 2.3 nT (AU)2 in 2006.1 to 2007.4, a decrease by 0.64. The two Ulysses scans are not precisely at solar minimum. However, in-ecliptic BR is highly correlated with the Ulysses measurements at all latitudes and can be used to determine the open flux at the two solar minima. Averages of BR at the two solar minima are 2.82 and 2.45 nT. This decrease is contrary to the suggestion based on previous solar cycles that BR returns to the same value of ~ 3 nT at solar minimum. The ratio of BP to the expansion factor, fE, is proportional to the measured open flux and observed and assumed values of BP are used to determine the corresponding values of fE. Another property of the fast solar wind is that it is highly turbulent compared to lower latitudes. To determine if the decrease in r2 BR and BP has affected the intensity of the magnetic fluctuations, the total variances in the magnetic field fluctuations are derived and found to decrease by a factor of 0.75.

Smith, Edward J.; Balogh, Andre

2008-11-01

219

Laboratory Simulation of Arched Magnetic Flux Rope Eruptions in the Solar Atmosphere S. K. P. Tripathi* and W. Gekelman  

E-print Network

Laboratory Simulation of Arched Magnetic Flux Rope Eruptions in the Solar Atmosphere S. K. P loops, prominences, and filaments are arched magnetic flux rope (AMFR) structures in the solar atmo magnetic flux rope in a large ambient plasma has been studied in a laboratory experiment that simulates

California at Los Angles, University of

220

A low-noise ferrite magnetic shield T. W. Kornack,a  

E-print Network

A low-noise ferrite magnetic shield T. W. Kornack,a S. J. Smullin, S.-K. Lee, and M. V. Romalis April 2007; published online 29 May 2007 Ferrite materials provide magnetic shielding performance by thermal Johnson currents due to their high electrical resistivity. Measurements inside a ferrite shield

Romalis, Mike

221

Fermionic condensate in a conical space with a circular boundary and magnetic flux  

SciTech Connect

The fermionic condensate is investigated in a (2+1)-dimensional conical spacetime in the presence of a circular boundary and a magnetic flux. It is assumed that on the boundary the fermionic field obeys the MIT bag boundary condition. For irregular modes, we consider a special case of boundary conditions at the cone apex, when the MIT bag boundary condition is imposed at a finite radius, which is then taken to zero. The fermionic condensate is a periodic function of the magnetic flux with the period equal to the flux quantum. For both exterior and interior regions, the fermionic condensate is decomposed into boundary-free and boundary-induced parts. Two integral representations are given for the boundary-free part for arbitrary values of the opening angle of the cone and magnetic flux. At distances from the boundary larger than the Compton wavelength of the fermion particle, the condensate decays exponentially, with the decay rate depending on the opening angle of the cone. If the ratio of the magnetic flux to the flux quantum is not a half-integer number for a massless field the boundary-free part in the fermionic condensate vanishes, whereas the boundary-induced part is negative. For half-integer values of the ratio of the magnetic flux to the flux quantum, the irregular mode gives a nonzero contribution to the fermionic condensate in the boundary-free conical space.

Bellucci, S. [INFN, Laboratori Nazionali di Frascati, Via Enrico Fermi 40, 00044 Frascati (Italy); Bezerra de Mello, E. R. [Departamento de Fisica, Universidade Federal da Paraiba 58.059-970, Caixa Postal 5.008, Joao Pessoa, PB (Brazil); Saharian, A. A. [Department of Physics, Yerevan State University, Alex Manoogian Street, 0025 Yerevan (Armenia)

2011-04-15

222

Magnetic flux conversion and relaxation toward a minimum-energy state in S-1 spheromak plasmas  

SciTech Connect

S-1 Spheromak currents and magnetic fluxes have been measured with Rogowski coils and flux loops external to the plasma. Toroidal plasma currents up to 350 kA and spheromak configuration lifetimes over 1.0 msec have been achieved at moderate power levels. The plasma formation in the S-1 Spheromak device is based on an inductive transfer of poloidal and toroidal magnetic flux from a toroidal ''flux core'' to the plasma. Formation is programmed to guide the configuration into a force-free, minimum-energy Taylor state. Properly detailed programming of the formation process is found not to be essential since plasmas adjust themselves during formation to a final equilibrium near the Taylor state. After formation, if the plasma evolves away from the stable state, then distinct relaxation oscillation events occur which restore the configuration to that stable state. The relaxation process involves reconnection of magnetic field lines, and conversion of poloidal to toroidal magnetic flux (and vice versa) has been observed and documented. The scaling of toroidal plasma current and toroidal magnetic flux in the plasma with externally applied currents is consistent with the establishment of a Taylor state after formation. In addition, the magnetic helicity is proportional to that injected from the flux core, independent of how that helicity is generated.

Janos, A.

1985-09-01

223

Plasma signatures in large Martian magnetic flux ropes: MARSIS/ASPERA-3 observations  

NASA Astrophysics Data System (ADS)

Cylindrical structures of highly twisted magnetic field (flux ropes) have been observed at Mars, using measurements by the MAG-ER magnetometer-electron reflectometer onboard Mars Global Surveyor (MGS) and by the MARSIS radar sounder onboard Mars Express (MEX). Signatures of flux ropes are spikes of magnetic field strength and magnetic field rotations. Both small scale flux ropes (diameters of a few tens of km) and large scale flux ropes (diameters of around 100 km) have been found at Mars. We look at times of presumed flux ropes on the dayside of Mars, detected in the local magnetic field strength given by MARSIS. The signatures in MARSIS are magnetic field strength increases (peak strength reaches several tens to hundred nT) for several minutes (size of hundreds of km along the spacecraft track), found outside but near crustal magnetic field regions. Although we cannot determine the presence of a magnetic field rotation because of the lack of a magnetometer onboard MEX, we assume that these magnetic field increases are large flux ropes. There are indeed large flux ropes with similar characteristics which were established by the magnetometer data from MGS, and thought to form by stretching and reconnection of crustal magnetic field by the solar wind. On the other hand, MEX possesses in situ ion measurements, unlike MGS. We will use the ion and electron data from the ASPERA-3 particle instrument onboard MEX in order to characterize the plasma (ionospheric only or mixing with shocked plasma?) inside the flux ropes, which will give hints on their origin.

Diéval, Catherine; Morgan, David; Duru, Firdevs; Gurnett, Donald

2014-05-01

224

LARGE-SCALE MAGNETIC HELICITY FLUXES ESTIMATED FROM MDI MAGNETIC SYNOPTIC CHARTS OVER THE SOLAR CYCLE 23  

SciTech Connect

To investigate the characteristics of large-scale and long-term evolution of magnetic helicity with solar cycles, we use the method of Local Correlation Tracking to estimate the magnetic helicity evolution over solar cycle 23 from 1996 to 2009 using 795 MDI magnetic synoptic charts. The main results are as follows: the hemispheric helicity rule still holds in general, i.e., the large-scale negative (positive) magnetic helicity dominates the northern (southern) hemisphere. However, the large-scale magnetic helicity fluxes show the same sign in both hemispheres around 2001 and 2005. The global, large-scale magnetic helicity flux over the solar disk changes from a negative value at the beginning of solar cycle 23 to a positive value at the end of the cycle, while the net accumulated magnetic helicity is negative in the period between 1996 and 2009.

Yang Shangbin; Zhang Hongqi, E-mail: yangshb@nao.cas.cn [Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing (China)

2012-10-10

225

Classification and performance of denoising algorithms for low signal-to-noise ratio magnetic resonance images  

NASA Astrophysics Data System (ADS)

The generation of magnitude magnetic resonance images comprises a sequence of data encodings or transformations, from detection of an analog electrical signal to a digital phase/frequency k-space to a complex image space via an inverse Fourier transform and finally to a magnitude image space via a magnitude transformation and rescaling. Noise present in the original signal is transformed at each step of this sequence. Denoising MR images from low field strength scanners is important because such images exhibit low signal to noise ratio. Algorithms that perform denoising of magnetic resonance images may be usefully classified according to the data domain on which they operate (i.e. at which step of the sequence of transformations they are applied) and the underlying statistical distribution of the noise they assume. This latter dimension is important because the noise distribution for low SNR images may be decidedly non-Gaussian. Examples of denoising algorithms include 2D wavelet thresholding (operates on the wavelet transform of the magnitude image; assumes Gaussian noise), Nowak's 2D wavelet filter (operates on the squared wavelet transform of the magnitude image; assumes Rician noise), Alexander et. al.'s complex 2D filters (operates on the wavelet transform of the complex image space; assumes Gaussian noise), wavelet packet denoising (wavelet packet transformation of magnitude image; assumes Rician noise) and anisotropic diffusion filtering (operates directly on magnitude image; no assumptions on noise distribution). Effective denoising of MR images must take into account both the availability of the underlying data, and the distribution of the noise to be removed. We classify a number of recently published denoising algorithms and compare their performance on images from a 0.35T permanent magnet MR scanner.

Rosenbaum, Wilfred L.; Atkins, M. Stella; Sarty, Gordon E.

2000-06-01

226

Low-frequency and shot noises in CoFeB/MgO/CoFeB magnetic tunneling junctions  

NASA Astrophysics Data System (ADS)

The low-frequency and shot noises in spin-valve CoFeB/MgO/CoFeB magnetic tunneling junctions were studied at low temperature. The measured 1/f noise around the magnetic hysteresis loops of the free layer indicates that the main origin of the 1/f noise is the magnetic fluctuation, which is discussed in terms of a fluctuation-dissipation relation. Random telegraph noise (RTN) is observed to be symmetrically enhanced in the hysteresis loop with regard to the two magnetic configurations. We found that this enhancement is caused by the fluctuation between two magnetic states in the free layer. Although the 1/f noise is almost independent of the magnetic configuration, the RTN is enhanced in the antiparallel configuration. These findings indicate the presence of spin-dependent activation of RTN. Shot noise reveals the spin-dependent coherent tunneling process via a crystalline MgO barrier.

Arakawa, Tomonori; Tanaka, Takahiro; Chida, Kensaku; Matsuo, Sadashige; Nishihara, Yoshitaka; Chiba, Daichi; Kobayashi, Kensuke; Ono, Teruo; Fukushima, Akio; Yuasa, Shinji

2012-12-01

227

Morphology and magnetic flux distribution in superparamagnetic, single-crystalline Fe3O4 nanoparticle rings  

NASA Astrophysics Data System (ADS)

This study reports on the correlation between crystal orientation and magnetic flux distribution of Fe3O4 nanoparticles in the form of self-assembled rings. High-resolution transmission electron microscopy demonstrated that the nanoparticles were single-crystalline, highly monodispersed, (25 nm average diameter), and showed no appreciable lattice imperfections such as twins or stacking faults. Electron holography studies of these superparamagnetic nanoparticle rings indicated significant fluctuations in the magnetic flux lines, consistent with variations in the magnetocrystalline anisotropy of the nanoparticles. The observations provide useful information for a deeper understanding of the micromagnetics of ultrasmall nanoparticles, where the magnetic dipolar interaction competes with the magnetic anisotropy.

Takeno, Yumu; Murakami, Yasukazu; Sato, Takeshi; Tanigaki, Toshiaki; Park, Hyun Soon; Shindo, Daisuke; Ferguson, R. Matthew; Krishnan, Kannan M.

2014-11-01

228

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

229

Morphology and magnetic flux distribution in superparamagnetic, single-crystalline Fe3O4 nanoparticle rings.  

PubMed

This study reports on the correlation between crystal orientation and magnetic flux distribution of Fe3O4 nanoparticles in the form of self-assembled rings. High-resolution transmission electron microscopy demonstrated that the nanoparticles were single-crystalline, highly monodispersed, (25?nm average diameter), and showed no appreciable lattice imperfections such as twins or stacking faults. Electron holography studies of these superparamagnetic nanoparticle rings indicated significant fluctuations in the magnetic flux lines, consistent with variations in the magnetocrystalline anisotropy of the nanoparticles. The observations provide useful information for a deeper understanding of the micromagnetics of ultrasmall nanoparticles, where the magnetic dipolar interaction competes with the magnetic anisotropy. PMID:25422526

Takeno, Yumu; Murakami, Yasukazu; Sato, Takeshi; Tanigaki, Toshiaki; Park, Hyun Soon; Shindo, Daisuke; Ferguson, R Matthew; Krishnan, Kannan M

2014-11-01

230

Avalanche dynamics of magnetic flux in a two-dimensional discrete superconductor  

SciTech Connect

The critical state of a two-dimensional discrete superconductor in an external magnetic field is studied. This state is found to be self-organized in the generalized sense, i.e., is a set of metastable states that transform to each other by means of avalanches. An avalanche is characterized by the penetration of a magnetic flux to the system. The sizes of the occurring avalanches, i.e., changes in the magnetic flux, exhibit the power-law distribution. It is also shown that the size of the avalanche occurring in the critical state and the external magnetic field causing its change are statistically independent quantities.

Ginzburg, S. L.; Nakin, A. V.; Savitskaya, N. E. [Russian Academy of Sciences, Petersburg Nuclear Physics Institute (Russian Federation)], E-mail: savitska@thd.pnpi.spb.ru

2006-11-15

231

Development of an isolated flyback converter employing boundary-mode operation and magnetic flux sensing feedback  

E-print Network

This thesis focuses on the marriage of magnetic-flux-sensing feedback and boundary-mode operation in a flyback converter to create a simple, small, low-cost, isolated, and tightly regulated power supply. Although each ...

Kenia, Mayur V. (Mayur Vasant), 1981-

2004-01-01

232

Electrical resistance of gases in explosive magnetic flux compression generator environments  

E-print Network

Gases that are electrical insulators at STP can become conductors when subjected to the high pressure and temperature environment in explosive magnetic flux compression generators (FCGs). This thesis describes experiments performed to determine...

Dorsey, Daniel John

2012-06-07

233

Cluster electric current density measurements within a magnetic flux rope in the plasma sheet  

NASA Technical Reports Server (NTRS)

On August 22, 2001 all 4 Cluster spacecraft nearly simultaneously penetrated a magnetic flux rope in the tail. The flux rope encounter took place in the central plasma sheet, Beta(sub i) approx. 1-2, near the leading edge of a bursty bulk flow. The "time-of-flight" of the flux rope across the 4 spacecraft yielded V(sub x) approx. 700 km/s and a diameter of approx.1 R(sub e). The speed at which the flux rope moved over the spacecraft is in close agreement with the Cluster plasma measurements. The magnetic field profiles measured at each spacecraft were first modeled separately using the Lepping-Burlaga force-free flux rope model. The results indicated that the center of the flux rope passed northward (above) s/c 3, but southward (below) of s/c 1, 2 and 4. The peak electric currents along the central axis of the flux rope predicted by these single-s/c models were approx.15-19 nA/sq m. The 4-spacecraft Cluster magnetic field measurements provide a second means to determine the electric current density without any assumption regarding flux rope structure. The current profile determined using the curlometer technique was qualitatively similar to those determined by modeling the individual spacecraft magnetic field observations and yielded a peak current density of 17 nA/m2 near the central axis of the rope. However, the curlometer results also showed that the flux rope was not force-free with the component of the current density perpendicular to the magnetic field exceeding the parallel component over the forward half of the rope, perhaps due to the pressure gradients generated by the collision of the BBF with the inner magnetosphere. Hence, while the single-spacecraft models are very successful in fitting flux rope magnetic field and current variations, they do not provide a stringent test of the force-free condition.

Slavin, J. A.; Lepping, R. P.; Gjerloev, J.; Goldstein, M. L.; Fairfield, D. H.; Acuna, M. H.; Balogh, A.; Dunlop, M.; Kivelson, M. G.; Khurana, K.

2003-01-01

234

Generating buoyant magnetic flux ropes in solar-like convective dynamos  

NASA Astrophysics Data System (ADS)

Our Sun exhibits strong convective dynamo action which results in magnetic flux bundles emerging through the stellar surface as magnetic spots. Global-scale dynamo action is believed to generate large-scale magnetic structures in the deep solar interior through the interplay of convection, rotation and shear. Portions of these large-scale magnetic structures are then believed to rise through the convective layer, forming magnetic loops which then pierce the photosphere as sunspot pairs. Previous global simulations of three-dimensional magnetohydrodynamic convection in rotating spherical shells have demonstrated mechanisms whereby large-scale magnetic wreaths can be generated in the bulk of the convection zone. Our recent simulations have achieved sufficiently high levels of turbulence to permit portions of these wreaths to become magnetically buoyant and rise through the simulated convective layer through a combination of magnetic buoyancy and advection by convective giant cells. These buoyant magnetic loops are created in the bulk of the convective layer as strong Lorentz force feedback in the cores of the magnetic wreaths dampen small-scale convective motions, permitting the amplification of local magnetic energies to over 100 times the local kinetic energy. While the magnetic wreaths are largely generated the shearing of axisymmetric poloidal magnetic fields by axisymmetric rotational shear (the ?-effect), the loops are amplified to their peak field strengths before beginning to rise by non-axisymmetric processes. This further extends and enhances a new paradigm for the generation of emergent magnetic flux bundles, which we term turbulence-enabled magnetic buoyancy.

Nelson, N. J.; Miesch, M. S.

2014-06-01

235

SOLAR MAGNETIC TRACKING. III. APPARENT UNIPOLAR FLUX EMERGENCE IN HIGH-RESOLUTION OBSERVATIONS  

Microsoft Academic Search

Understanding the behavior of weak magnetic fields near the detection limit of current instrumentation is important for determining the flux budget of the solar photosphere at small spatial scales. Using 0.''3-resolution magnetograms from the Solar Optical Telescope's Narrowband Filter Imager (NFI) on the Hinode spacecraft, we confirm that the previously reported apparent unipolar magnetic flux emergence seen in intermediate-resolution magnetograms

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

2010-01-01

236

Solar Magnetic Tracking. III. Apparent Unipolar Flux Emergence in High-resolution Observations  

Microsoft Academic Search

Understanding the behavior of weak magnetic fields near the detection limit of current instrumentation is important for determining the flux budget of the solar photosphere at small spatial scales. Using 0farcs3-resolution magnetograms from the Solar Optical Telescope's Narrowband Filter Imager (NFI) on the Hinode spacecraft, we confirm that the previously reported apparent unipolar magnetic flux emergence seen in intermediate-resolution magnetograms

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

2010-01-01

237

A MTPA control scheme for an IPM synchronous motor considering magnet flux variation caused by temperature  

Microsoft Academic Search

In a hybrid electric vehicle (HEV), an interior permanent magnet (Nd-Fe-B) synchronous motor is widely used. However, the flux density of the Nd-Fe-B magnet is sensitive to the temperature variation. Particularly, in a HEV, the heat coming from internal combustion engine causes high temperature rise of the motor. It degrades the flux density and output torque. In this paper, a

Gubae Rang; Jaesang Lim; Kwanghee Nam; Hyung-Bin Ihm; Ho-Gi Kim

2004-01-01

238

A Prediction Model Of Solar Euv Irradiance On The Basis Of Solar Magnetic Flux Evolution  

Microsoft Academic Search

Harvey (1991, 1994) determined the correlation between total solar surface magnetic flux and solar irradiance (i.e. F10.7, 1-8 Å and Lyman alpha). This implies that if one could predict the solar magnetic flux as it changes over time, then Harvey's correlation could be used to also predict solar EUV irradiance. Wu et al. (1993) have developed a magnetohydrodynamic (MHD) photospheric

S. T. Wu; A. H. Wang; C. D. Fry; W. K. Tobiska; J. Pap

2004-01-01

239

Comparative study of 3D flux electrical machines with soft magnetic composite cores  

Microsoft Academic Search

This paper compares two types of three-dimensional (3D) flux electrical machines with soft magnetic composite (SMC) cores, namely claw pole and transverse flux machines. 3D electromagnetic field analysis is conducted for the computation of some important parameters and optimization of the machine structures. An equivalent electric circuit is derived to calculate the machine performances. The analysis methods are validated by

Y. G. Guo; J. G. Zhu; P. A. Watterson; W. Wu

2002-01-01

240

Design and construction of a permanent magnet axial flux synchronous generator  

Microsoft Academic Search

This paper presents the design characteristics, special features, and manufacturing aspects of a permanent magnet axial flux synchronous generator. This machine is aimed at applications in the low power low speed range (5 kVA, 500 rpm). A central stator configuration is selected for the construction of the axial flux generator, so that the stator leakage inductance is significantly reduced

Rogel R. Wallace; Thomas A. Lipo; Luis A. Moran; Juan A. Tapia

1997-01-01

241

Simulation of the outer radiation belt electron flux decrease during the March 26, 1995, magnetic storm  

Microsoft Academic Search

In this paper we study the variation of the relativistic electron fluxes in the Earth's outer radiation belt during the March 26, 1995, magnetic storm. Using observations by the radiation environment monitor (REM) on board the space technology research vehicle (STRV-1b), we discuss the flux decrease and possible loss of relativistic electrons during the storm main phase. In order to

L. Desorgher; P. Bühler; A. Zehnder; E. O. Flückiger

2000-01-01

242

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

243

Line-of-sight magnetic flux imbalances caused by electric currents  

NASA Technical Reports Server (NTRS)

Several physical and observational effects contribute to the significant imbalances of magnetic flux that are often observed in active regions. We consider an effect not previously treated: the influence of electric currents in the photosphere. Electric currents can cause a line-of-sight flux imbalance because of the directionality of the magnetic field they produce. Currents associated with magnetic flux tubes produce larger imbalances than do smoothly-varying distributions of flux and current. We estimate the magnitude of this effect for current densities, total currents, and magnetic geometry consistent with observations. The expected imbalances lie approximately in the range 0-15%, depending on the character of the current-carying fields and the angle from which they are viewed. Observationally, current-induced flux imbalances could be indicated by a statistical dependence of the imbalance on angular distance from disk center. A general study of magnetic flux balance in active regions is needed to determine the relative importance of other- probably larger- effects such as dilute flux (too weak to measure or rendered invisible by radiative transfer effects), merging with weak background fields, and long-range connections between active regions.

Gary, G. Allen; Rabin, Douglas

1995-01-01

244

A moving hum filter to suppress rotor noise in high-resolution airborne magnetic data  

USGS Publications Warehouse

A unique filtering approach is developed to eliminate helicopter rotor noise. It is designed to suppress harmonic noise from a rotor that varies slightly in amplitude, phase, and frequency and that contaminates aero-magnetic data. The filter provides a powerful harmonic noise-suppression tool for data acquired with modern large-dynamic-range recording systems. This three-step approach - polynomial fitting, bandpass filtering, and rotor-noise synthesis - significantly reduces rotor noise without altering the spectra of signals of interest. Two steps before hum filtering - polynomial fitting and bandpass filtering - are critical to accurately model the weak rotor noise. During rotor-noise synthesis, amplitude, phase, and frequency are determined. Data are processed segment by segment so that there is no limit on the length of data. The segment length changes dynamically along a line based on modeling results. Modeling the rotor noise is stable and efficient. Real-world data examples demonstrate that this method can suppress rotor noise by more than 95% when implemented in an aeromagnetic data-processing flow. ?? 2005 Society of Exploration Geophysicists. All rights reserved.

Xia, J.; Doll, W.E.; Miller, R.D.; Gamey, T.J.; Emond, A.M.

2005-01-01

245

Direct control of air gap flux in permanent magnet machines  

DOEpatents

A method and apparatus for field weakening in PM machines uses field weakening coils (35, 44, 45, 71, 72) to produce flux in one or more stators (34, 49, 63, 64), including a flux which counters flux normally produced in air gaps between the stator(s) (34, 49, 63, 64) and the rotor (20, 21, 41, 61) which carries the PM poles. Several modes of operation are introduced depending on the magnitude and polarity of current in the field weakening coils (35, 44, 45, 71, 72). The invention is particularly useful for, but not limited to, the electric vehicle drives and PM generators.

Hsu, John S. (Oak Ridge, TN)

2000-01-01

246

Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind  

NASA Astrophysics Data System (ADS)

Context. Tangential velocity discontinuity near the boundaries of solar wind magnetic flux tubes results in Kelvin-Helmholtz instability, which might contribute to solar wind turbulence. While the axial magnetic field stabilizes the instability, a small twist in the magnetic field may allow sub-Alfvénic motions to be unstable. Aims: We aim to study the Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind with different configurations of the external magnetic field. Methods: We use magnetohydrodynamic equations in cylindrical geometry and derive the dispersion equations governing the dynamics of twisted magnetic flux tubes moving along its axis in the cases of untwisted and twisted external fields. Then, we solve the dispersion equations analytically and numerically and find thresholds for Kelvin-Helmholtz instability in both cases of the external field. Results: Both analytical and numerical solutions show that the Kelvin-Helmholtz instability is suppressed in the twisted tube by the external axial magnetic field for sub-Alfvénic motions. However, even a small twist in the external magnetic field allows the Kelvin-Helmholtz instability to be developed for any sub-Alfvénic motion. The unstable harmonics correspond to vortices with high azimuthal mode numbers that are carried by the flow. Conclusions: Twisted magnetic flux tubes can be unstable to Kelvin-Helmholtz instability when they move with small speed relative to the main solar wind stream, then the Kelvin-Helmholtz vortices may significantly contribute to the solar wind turbulence.

Zaqarashvili, T. V.; Vörös, Z.; Zhelyazkov, I.

2014-01-01

247

Flux growth and magnetic properties of FeVO{sub 4} single crystals  

SciTech Connect

FeVO{sub 4} (I) single crystals are grown by the flux method using V{sub 2}O{sub 5} as the self-flux. The grown crystals exhibit a characteristic morphology with natural facets. The quality of the crystals is confirmed by X-ray diffraction and EPMA techniques. Magnetic properties are investigated by means of magnetic susceptibility, magnetization, and heat capacity measurements. Two magnetic phase transitions are observed at {approx}13 and {approx}20 K. Such unusual magnetic behaviors are suggested to originate from two different Fe ligand environments of octahedral FeO{sub 6} and trigonal bipyramidal FeO{sub 5} in a six-column doubly bent chain. - Graphical abstract: FeVO{sub 4} (I) single crystals are grown by the flux method using V{sub 2}O{sub 5} as the self-flux. Magnetic properties are investigated by means of magnetic susceptibility, magnetization, and heat capacity measurements, showing two magnetic phase transitions at {approx}13 and {approx}20 K.

He Zhangzhen [Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581 (Japan)], E-mail: hezz@fjirsm.ac.cn; Yamaura, Jun-Ichi; Ueda, Yutaka [Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581 (Japan)

2008-09-15

248

The application of wavelet shrinkage denoising to magnetic Barkhausen noise measurements  

NASA Astrophysics Data System (ADS)

The application of Magnetic Barkhausen Noise (MBN) as a non-destructive method of defect detection has proliferated throughout the manufacturing community. Instrument technology and measurement methodology have matured commensurately as applications have moved from the R&D labs to the fully automated manufacturing environment. These new applications present a new set of challenges including a bevy of error sources. A significant obstacle in many industrial applications is a decrease in signal to noise ratio due to (i) environmental EMI and (II) compromises in sensor design for the purposes of automation. The stochastic nature of MBN presents a challenge to any method of noise reduction. An application of wavelet shrinkage denoising is proposed as a method of decreasing extraneous noise in MBN measurements. The method is tested and yields marked improvement on measurements subject to EMI, grounding noise, and even measurements in ideal conditions.

Thomas, James

2014-02-01

249

The application of wavelet shrinkage denoising to magnetic Barkhausen noise measurements  

SciTech Connect

The application of Magnetic Barkhausen Noise (MBN) as a non-destructive method of defect detection has proliferated throughout the manufacturing community. Instrument technology and measurement methodology have matured commensurately as applications have moved from the R and D labs to the fully automated manufacturing environment. These new applications present a new set of challenges including a bevy of error sources. A significant obstacle in many industrial applications is a decrease in signal to noise ratio due to (i) environmental EMI and (II) compromises in sensor design for the purposes of automation. The stochastic nature of MBN presents a challenge to any method of noise reduction. An application of wavelet shrinkage denoising is proposed as a method of decreasing extraneous noise in MBN measurements. The method is tested and yields marked improvement on measurements subject to EMI, grounding noise, and even measurements in ideal conditions.

Thomas, James [American Stress Technologies, Inc., 540 Alpha Drive, Pittsburgh, PA 15238 (United States)

2014-02-18

250

22 year cycle in the imbalance of the photospheric magnetic fluxes  

NASA Astrophysics Data System (ADS)

The manifestation of the 22 year solar magnetic cycle in the imbalance of positive and negative photospheric magnetic fluxes is studied. For the analysis we use synoptic maps of the photospheric magnetic field of Kitt Peak Observatory (1976 - 2003) and John Wilcox Observatory in Stanford (1976 - 2012). We consider strong magnetic fields for the heliolatitudes in the interval from +40° to -40°. It is shown that the sign of the imbalance between positive and negative fluxes remains constant during 11 years from one inversion of the Sun’s global magnetic field to the next one and always coincides with the sign of the polar field in the Northern hemisphere. Thus, the imbalance between the magnetic fluxes of different polarities changes according to the 22 year cycle. The sign of the imbalance is determined both by the phase of the solar cycle (before or after the inversion) and by the parity of the solar cycle. The imbalance of positive and negative magnetic fluxes can be observed not only for the strong fields in the sunspot zone. The mean magnetic field of the Sun (Sun as a star), which is determined by the net flux of the background fields, changes according to the same pattern as the imbalance of the strong fields. The regular changes of the imbalance of the photospheric magnetic fields are reflected also in the parameters of heliosphere. We show the connection of the imbalance with the quadrupole component of the photospheric magnetic field and with the imbalance of the interplanetary magnetic field (the difference between the numbers of the days with positive and negative polarities of the interplanetary magnetic field near Earth).

Vernova, Elena; Baranov, Dmitrii; Tyasto, Marta

251

On the role of crystal and stress anisotropy in magnetic Barkhausen noise  

NASA Astrophysics Data System (ADS)

The article describes the micromagnetic behavior of non- and pre-plastically deformed high strength steel samples under applied stress using different magnetic nondestructive methods such as magnetic Barkhausen noise analysis and hysteresis measurements. It was found that the maximum amplitude of Barkhausen noise (MMAX) increases with applied stress up to a certain point and then decreases again (so-called MMAX(?)-curve). Changes of magnetostriction, hysteresis curves and magnetic domain structures have been measured and have been further investigated to find out the reasons with respect to macro- and microscopic material behavior. The results obtained are mainly discussed on the basis of the Villari effect and the relation between applied stress and the Barkhausen noise parameters is described. It is concluded that the interaction between crystal and stress anisotropy is the main reason of the specific MMAX(?)-curve observed.

Sheikh Amiri, Meisam; Thielen, Matthias; Rabung, Madalina; Marx, Michael; Szielasko, Klaus; Boller, Christian

2014-12-01

252

Laboratory study of arched magnetic flux ropes formed within a solar-relevant potential field configuration  

NASA Astrophysics Data System (ADS)

Solar eruptive events such as coronal mass ejections (CMEs) are thought to be driven by a sudden release of magnetic energy stored in the solar corona. In many cases, the pre-eruptive configuration is a non-potential magnetic structure that can be modeled as a line-tied magnetic flux rope. In spite of ever-improving observations, directly studying these coronal flux ropes remains a significant challenge. As an alternative, we have designed a laboratory experiment to produce low-? arched magnetic flux ropes similar to those found in the corona. These line-tied flux ropes are formed as a magnetized arc discharge between two electrodes and they evolve quasi-statically over hundreds of Alfv'en times. Recently, we have constructed a new set of magnetic field coils to produce an active-region-like potential field configuration. Initial results from plasmas formed in this configuration are presented, including fast camera images and internal magnetic measurements. These discharges are expected to access a regime where a slowly evolving flux rope can suddenly undergo a dynamic eruption due to a loss-of-equilibriumfootnotetextForbes & Isenberg, ApJ 373, 294 (1991) or the torus instability.footnotetextKliem & T"or"ok, PRL 96, 255002 (2006)

Myers, C. E.; Yamada, M.; Ji, H.; Yoo, J.; Jara-Almonte, J.; Lawrence, E. E.

2012-10-01

253

Dualspacecraft observation of largescale magnetic flux ropes in the Martian ionosphere  

E-print Network

nearly coincident magnetic field measurements from the Mars Advanced Radar for Subsurface and IonosphereDualspacecraft observation of largescale magnetic flux ropes in the Martian ionosphere D. D. Morgan in the ionosphere of Mars. The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), on board Mars

Gurnett, Donald A.

254

A fast method for rectangular crack sizes reconstruction in magnetic flux leakage testing  

Microsoft Academic Search

Magnetic flux leakage (MFL) testing is widely used to examine ferromagnetic materials. For the reason of estimating the sizes of cracks in metals is important in piping industries, a fast method based on particle swarm optimization algorithm is proposed for reconstructing the sizes of rectangular crack in this article. Considering the magnetic leakage field intensity is related to the air

Yong Zhang; Zhongfu Ye; Chong Wang

2009-01-01

255

Vortices with half magnetic flux quanta in ``heavy-fermion'' superconductors  

Microsoft Academic Search

It is shown that in ``heavy-fermion'' superconductors a new vortex state can occur characterized by the existence of half magnetic flux quanta. Vortices in polycrystals should exist even in the absence of an externally applied magnetic field. The internal structure of the vortices is also investigated.

V. B. Geshkenbein; A. I. Larkin; A. Barone

1987-01-01

256

Axial-flux PM wind generator with a soft magnetic composite core  

Microsoft Academic Search

This paper presents the potential application of soft magnetic composite (SMC) material in low speed, directly driven, axial-flux permanent magnet (PM) wind generators. Comparative design studies are conducted on PM wind generators of different configurations with both lamination cores and SMC core. Finite element analysis is used to enhance the design precision, from which analytical formulas are modified. Through careful

Yicheng Chen; Pragasen Pillay

2005-01-01

257

Design and analysis of a transverse flux machine with soft magnetic composite core  

Microsoft Academic Search

This paper presents the design and performance analysis of a three phase, three stack permanent magnet transverse flux motor with soft magnetic composite core. To predict and optimize the major parameters, three-dimensional finite element analysis is performed. The performance is calculated when the motor operates with a brushless DC drive.

Youguang Guo; Jian guo Zhu; P. A. Watterson; Wei Wu

2003-01-01

258

Fault-Tolerant Flux-Switching Permanent Magnet Brushless AC Machines  

Microsoft Academic Search

Flux-switching permanent magnet (FSPM) brushless machines have emerged as an attractive machine type by virtue of their high torque densities, simple and robust rotor structure and the fact that permanent magnets and coils are both located on the stator. Both 2D and 3D finite element analyses (FEA) are employed to compare the performance of a standard topology with three modular,

R. L. Owen; Z. Q. Zhu; A. S. Thomas; G. W. Jewell; D. Howe

2008-01-01

259

Modelling of the outer electron belt flux dropout and losses during magnetic storm main phase  

Microsoft Academic Search

In this paper we analyze the flux dropout and loss of outer belt electrons during magnetic storms. Using observations of the outer belt variation during the 26 March 1995 magnetic storm by the REM detector onboard the STRV-1b satellite, we show that a real loss of electrons occurred during the storm main phase. In order to simulate the outer belt

L. Desorgher; E. Flückiger; P. Bühler; A. Zehnder

2000-01-01

260

Flux Jumps” in Cable-in-Conduit Conductors Induced by Transient Magnetic Field  

Microsoft Academic Search

A jump-like temperature increase that can not be explained only by AC losses was observed during the tests of Nb-Ti CICC model samples in transient magnetic fields. The samples were placed between two racetracks that generate background transient magnetic field (TMF). The flux jumps occurred in CICC samples even during the slow discharge of racetrack coils. A physical model was

V. E. Keilin; I. A. Kovalev; S. L. Kruglov; S. A. Lelekhov; P. G. Naumov; V. I. Shcherbakov

2006-01-01

261

Maintenance of magnetic flux density standards on the basis of proton gyromagnetic ratio at KRISS  

Microsoft Academic Search

The proton gyromagnetic ratio (?'p) plays a fundamental role in achieving high accuracy in magnetic measurements. We have improved our magnetic flux density standards using the ?'p-value and related techniques developed at Korea Research Institute of Standards and Science (KRISS). The standards are used for the calibration of magnetometers and support the test of sensors and materials in the range

Po Gyu Park; Young Gyun Kim; V. Ya. Shifrin

2005-01-01

262

Magnetic flux relaxation in YBa2Cu3O(7-x) thin film: Thermal or athermal  

NASA Technical Reports Server (NTRS)

The magnetic flux relaxation behavior of YBa2Cu3O(7-x) thin film on LaAlO3 for H parallel c was studied in the range of 4.2-40 k and 0.2-1.0 T. Both the normalized flux relaxation rate (S) and the net flux pinning energy (U) increase continuously from 1.3 x 10 exp -2 to 3.0 x 10 exp -2 and from 70-240 meV respectively, as the temperature (T) increases from 10 to 40 K. This behavior is consistent with the thermally activated flux motion model. At low temperatures, however, S is found to decrease much more slowly as compared with kT, in contradiction to the thermal activation model. This behavior is discussed in terms of the athermal quantum tunneling of flux lines. The magnetic field dependence of U, however, is not completely understood.

Vitta, Satish; Stan, M. A.; Warner, Joseph D.; Alterovitz, Samuel A.

1992-01-01

263

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

NASA Technical Reports Server (NTRS)

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.

Mikic, Zoran; Schnack, Dalton D.; Van Hoven, Gerard

1990-01-01

264

Mean-field and direct numerical simulations of magnetic flux concentrations from vertical field  

NASA Astrophysics Data System (ADS)

Context. Strongly stratified hydromagnetic turbulence has previously been found to produce magnetic flux concentrations if the domain is large enough compared with the size of turbulent eddies. Mean-field simulations (MFS) using parameterizations of the Reynolds and Maxwell stresses show a large-scale negative effective magnetic pressure instability and have been able to reproduce many aspects of direct numerical simulations (DNS) regarding growth rate, shape of the resulting magnetic structures, and their height as a function of magnetic field strength. Unlike the case of an imposed horizontal field, for a vertical one, magnetic flux concentrations of equipartition strength with the turbulence can be reached, resulting in magnetic spots that are reminiscent of sunspots. Aims: We determine under what conditions magnetic flux concentrations with vertical field occur and what their internal structure is. Methods: We use a combination of MFS, DNS, and implicit large-eddy simulations (ILES) to characterize the resulting magnetic flux concentrations in forced isothermal turbulence with an imposed vertical magnetic field. Results: Using DNS, we confirm earlier results that in the kinematic stage of the large-scale instability the horizontal wavelength of structures is about 10 times the density scale height. At later times, even larger structures are being produced in a fashion similar to inverse spectral transfer in helically driven turbulence. Using ILES, we find that magnetic flux concentrations occur for Mach numbers between 0.1 and 0.7. They occur also for weaker stratification and larger turbulent eddies if the domain is wide enough. Using MFS, the size and aspect ratio of magnetic structures are determined as functions of two input parameters characterizing the parameterization of the effective magnetic pressure. DNS, ILES, and MFS show magnetic flux tubes with mean-field energies comparable to the turbulent kinetic energy. These tubes can reach a length of about eight density scale heights. Despite being ?1% equipartition strength, it is important that their lower part is included within the computational domain to achieve the full strength of the instability. Conclusions: The resulting vertical magnetic flux tubes are being confined by downflows along the tubes and corresponding inflow from the sides, which keep the field concentrated. Application to sunspots remains a viable possibility.

Brandenburg, A.; Gressel, O.; Jabbari, S.; Kleeorin, N.; Rogachevskii, I.

2014-02-01

265

Effect of the proximity of the Heliospheric Current Sheet on the heliospheric magnetic flux density  

NASA Astrophysics Data System (ADS)

In recent work (Erd?s & Balogh, ApJ., 781, 50, 2014) it has been shown that the magnetic flux density measured by the Ulysses probe around its solar polar orbit from 1990 to 2009 matches that calculated from the OMNI in-ecliptic, 1 AU data set over the same interval. This result has shown that the sun's magnetic flux is generally distributed uniformly in the heliosphere and depends only on the total open magnetic flux of the sun. We have nevertheless examined those two intervals in the nearly 20-year long observations, when the flux density measured at Ulysses appears to be greater than that observed at 1 AU. The two intervals are near the minima in solar activity in 1995-96 and 2007-08, with the difference in the first interval noticeably larger than in the second. We show that the difference between the Ulysses and the OMNI results are strongly correlated to the distance between the Heliospheric Current Sheet (HCC) and the 1 AU observation point, with the flux density smaller at the in ecliptic observation point than at Ulysses which during those intervals was generally well north or south of the HCC. A corresponding result can also be identified as a longitude dependence in the magnetic flux density when comparing the observations by the STEREO-A and -B spacecraft which are close to 1 AU in the ecliptic, but are widely separated in heliolongitude. We discuss possible causes of the reduced flux density near the HCC in terms of the geometric effects on the uniform spreading of the solar magnetic flux and the possible effect of magnetic reconnection associated with the HCC and the properties of the slow wind in which it is embedded.

Erdös, Géza; Balogh, André

2014-05-01

266

Magnetic flux transport and the sun's dipole moment - New twists to the Babcock-Leighton model  

NASA Technical Reports Server (NTRS)

The mechanisms that give rise to the sun's large-scale poloidal magnetic field are explored in the framework of the Babcock-Leighton (BL) model. It is shown that there are in general two quite distinct contributions to the generation of the 'alpha effect': the first is associated with the axial tilts of the bipolar magnetic regions as they erupt at the surface, while the second arises through the interaction between diffusion and flow as the magnetic flux is dispersed over the surface. The general relationship between flux transport and the BL dynamo is discussed.

Wang, Y.-M.; Sheeley, N. R., Jr.

1991-01-01

267

Why Solar Magnetic Flux Concentrations Are Bright in Molecular Bands  

Microsoft Academic Search

Using realistic ab initio simulations of radiative magnetoconvection, we show that the bright structures in images taken in the ``G band,'' a spectral band dominated by lines of the CH molecule, precisely outline small-scale concentrations of strong magnetic fields on the visible solar surface. The brightening is caused by a depletion of CH molecules in the hot and tenuous magnetic

M. Schüssler; S. Shelyag; S. Berdyugina; A. Vögler; S. K. Solanki

2003-01-01

268

Evaluation of magnetic flux distribution from magnetic domains in [Co/Pd] nanowires by magnetic domain scope method using contact-scanning of tunneling magnetoresistive sensor  

SciTech Connect

Current-driven magnetic domain wall motions in magnetic nanowires have attracted great interests for physical studies and engineering applications. The magnetic force microscope (MFM) is widely used for indirect verification of domain locations in nanowires, where relative magnetic force between the local domains and the MFM probe is used for detection. However, there is an occasional problem that the magnetic moments of MFM probe influenced and/or rotated the magnetic states in the low-moment nanowires. To solve this issue, the “magnetic domain scope for wide area with nano-order resolution (nano-MDS)” method has been proposed recently that could detect the magnetic flux distribution from the specimen directly by scanning of tunneling magnetoresistive field sensor. In this study, magnetic domain structure in nanowires was investigated by both MFM and nano-MDS, and the leakage magnetic flux density from the nanowires was measured quantitatively by nano-MDS. Specimen nanowires consisted from [Co (0.3)/Pd (1.2)]{sub 21}/Ru(3) films (units in nm) with perpendicular magnetic anisotropy were fabricated onto Si substrates by dual ion beam sputtering and e-beam lithography. The length and the width of the fabricated nanowires are 20??m and 150?nm. We have succeeded to obtain not only the remanent domain images with the detection of up and down magnetizations as similar as those by MFM but also magnetic flux density distribution from nanowires directly by nano-MDS. The obtained value of maximum leakage magnetic flux by nano-MDS is in good agreement with that of coercivity by magneto-optical Kerr effect microscopy. By changing the protective diamond-like-carbon film thickness on tunneling magnetoresistive sensor, the three-dimensional spatial distribution of leakage magnetic flux could be evaluated.

Okuda, Mitsunobu, E-mail: okuda.m-ky@nhk.or.jp; Miyamoto, Yasuyoshi; Miyashita, Eiichi; Hayashi, Naoto [NHK Science and Technology Research Laboratories, 1-10-11 Kinuta Setagaya, Tokyo 157-8510 (Japan)

2014-05-07

269

Equilibrium features and eruptive instabilities in laboratory magnetic flux rope plasmas  

NASA Astrophysics Data System (ADS)

One avenue for connecting laboratory and solar plasma studies is to carry out laboratory plasma experiments that serve as a well-diagnosed model for specific solar phenomena. In this paper, we present the latest results from one such laboratory experiment that is designed to address ideal instabilities that drive flux rope eruptions in the solar corona. The experiment, which utilizes the existing Magnetic Reconnection Experiment (MRX) at Princeton Plasma Physics Laboratory, generates a quasi-statically driven line-tied magnetic flux rope in a solar-relevant potential field arcade. The parameters of the potential field arcade (e.g., its magnitude, orientation, and vertical profile) are systematically scanned in order to study their influence on the evolution and possible eruption of the line-tied flux rope. Each flux rope discharge is diagnosed using a combination of fast visible light cameras and an in situ 2D magnetic probe array that measures all three components of the magnetic field over a large cross-section of the plasma. In this paper, we present the first results obtained from this new 2D magnetic probe array. With regard to the flux rope equilibrium, non-potential features such as the formation of a characteristic sigmoid shape and the generation of core toroidal field within the flux rope are studied in detail. With regard to instabilities, the onset and evolution of two key eruptive instabilities—the kink and torus instabilities—are quantitatively assessed as a function of the potential field arcade parameters and the amount of magnetic energy stored in the flux rope.This research is supported by DoE Contract Number DE-AC02-09CH11466 and by the NSF/DoE Center for Magnetic Self-Organization (CMSO).

Myers, Clayton E; Yamada, Masaaki; Belova, Elena V; Ji, Hantao; Yoo, Jongsoo; Fox, William

2014-06-01

270

RMHD simulations of collision-induced magnetic dissipations in Poynting flux dominated jets  

NASA Astrophysics Data System (ADS)

We perform a 3D relativistic ideal MHD simulation to study the collision between high-? magnetic blobs which contain both poloidal and toroidal magnetic field components, which can mimic the interactions inside a highly variable Poynting flux dominated jet. We discover a significant Poynting flux energy dissipation component in addition to the dissipation due to the evolution of the blob without collision. We identify this additional Poynting flux energy dissipation as being mainly facilitated by the collision driven magnetic reconnections, through both observing the reconnection events in the simulations and quantitative calculations of the energy dissipation rate. Additional resolution and parameter studies show a robust result that our results are nearly independent of the numerical resolution or most physical parameters in the relevant parameter range. Our results give a good support to some theories in astrophysical systems, such as the internal collision-induced magnetic reconnection and turbulence (ICMART) model for GRBs, and some reconnection triggered mini-jet model for AGNs.

Deng, Wei; Li, Hui; Zhang, Bing; Li, Shengtai

2015-01-01

271

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

272

Twist Accumulation and Topology Structure of a Solar Magnetic Flux Rope  

NASA Astrophysics Data System (ADS)

To study the buildup of a magnetic flux rope before a major flare and coronal mass ejection (CME), we compute the magnetic helicity injection, twist accumulation, and topology structure of the three-dimensional (3D) magnetic field, which is derived by the nonlinear force-free field model. The Extreme-ultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory observed a series of confined flares without any CME before a major flare with a CME at 23:02 UT on 2005 January 15 in active region NOAA 10720. We derive the vector velocity at eight time points from 18:27 UT to 22:20 UT with the differential affine velocity estimator for vector magnetic fields, which were observed by the Digital Vector Magnetograph at Big Bear Solar Observatory. The injected magnetic helicity is computed with the vector magnetic and velocity fields. The helicity injection rate was (- 16.47 ± 3.52) × 1040 Mx2 hr-1. We find that only about 1.8% of the injected magnetic helicity became the internal helicity of the magnetic flux rope, whose twist increasing rate was -0.18 ± 0.08 Turns hr-1. The quasi-separatrix layers (QSLs) of the 3D magnetic field are computed by evaluating the squashing degree, Q. We find that the flux rope was wrapped by QSLs with large Q values, where the magnetic reconnection induced by the continuously injected magnetic helicity further produced the confined flares. We suggest that the flux rope was built up and heated by the magnetic reconnection in the QSLs.

Guo, Y.; Ding, M. D.; Cheng, X.; Zhao, J.; Pariat, E.

2013-12-01

273

Adaptive cancellation of geomagnetic background noise for magnetic anomaly detection using coherence  

NASA Astrophysics Data System (ADS)

Magnetic anomaly detection (MAD) is an effective method for the detection of ferromagnetic targets against background magnetic fields. Currently, the performance of MAD systems is mainly limited by the background geomagnetic noise. Several techniques have been developed to detect target signatures, such as the synchronous reference subtraction (SRS) method. In this paper, we propose an adaptive coherent noise suppression (ACNS) method. The proposed method is capable of evaluating and detecting weak anomaly signals buried in background geomagnetic noise. Tests with real-world recorded magnetic signals show that the ACNS method can excellently remove the background geomagnetic noise by about 21?dB or more in high background geomagnetic field environments. Additionally, as a general form of the SRS method, the ACNS method offers appreciable advantages over the existing algorithms. Compared to the SRS method, the ACNS algorithm can eliminate the false target signals and represents a noise suppressing capability improvement of 6.4?dB. The positive outcomes in terms of intelligibility make this method a potential candidate for application in MAD systems.

Liu, Dunge; Xu, Xin; Huang, Chao; Zhu, Wanhua; Liu, Xiaojun; Yu, Gang; Fang, Guangyou

2015-01-01

274

3D Visualization Analysis of Magnetic Flux Rope in the Earth's Magnetotail  

NASA Astrophysics Data System (ADS)

With the recent development of super-computers, large scale 3D computer simulations for space plasmas are getting practical. We obtain variety of 3D structures or configurations from 3D numerical simulations. However, techniques and environments for analyzing the 3D space plasma simulation data have not been established. We have been developing a data analysis environment using various types of 3D visualization techniques and virtual reality systems. It is well known that the Earth's magnetosphere shows complex 3D magnetic topologies when the IMF By component is predominant. We used 3D Global MHD simulations with southward IMF and non-zero By to study interactions between the IMF and the Earth's magnetosphere. We found that magnetic flux ropes in the Earth's magnetotail is generated and propagates tailward. However, the process of origin and generation of 3D magnetic topologies of magnetic flux ropes, especially the source of the By components in the flux ropes, is not clarified. In this study, we analyze 3D magnetic field topology of magnetic flux ropes by using 3D visualization analysis environment such as virtual reality (VR) system and 3D haptic device "PHANToM". These systems provide us with force feedback and 3D object manipulation capability in 3D virtual space. First, we classified magnetic field topology of magnetic flux ropes (and vicinity of the magnetic flux ropes) into the following ten types: (1)IMF-IMF (helical), (2)IMF-Earth (helical), (3)Earth-Earth (helical), (4)IMF-IMF (not helical), (5)IMF-Earth (not helical), (6)Earth-Earth (not helical), (7)closed (not helical), (8)IMF-IMF (not flux rope), (9)IMF-Earth (not flux rope), (10)Earth-Earth (not flux rope). This classification are carried out by using the "PHANToM" Next, we analyzed the time-dependent profiles of each magnetic field topology. As a result, we found the following processes taking place to produce a magnetic flux rope: (1)Earth-Earth (helical) field lines are reconnected to Earth-IMF (helical) field lines, (2)Earth-IMF (helical) field lines are reconnected to IMF-IMF (helical) field lines, (3)magnetic flux ropes separated from the Earth field line and move tailward because all of the Earth-Earth (helical) field lines are reconnected. In this presentation, we discuss the process of magnetic flux rope's generation and suggest the 3D time- dependent model of magnetic topology including all other magnetic reconnections (such as at dayside magnetosphere, magnetopause flank and magnetotail). We also discuss the efficiency of our 3D visualization analysis environment using visual and haptic virtual reality system. We believe that this analysis environment provides Earth and space science researchers with new research method to achieve new scientific findings.

Matsuoka, D.; Murata, K.; Fujita, S.; Tanaka, T.; Yamamoto, K.

2006-12-01

275

Noise-Enhanced Synchronization of Stochastic Magnetic Oscillators  

NASA Astrophysics Data System (ADS)

We present an experimental study of phase locking in a stochastic magnetic oscillator. The system comprises a magnetic tunnel junction with a superparamagnetic free layer, whose magnetization dynamics is driven with spin torques through an external periodic driving current. We show that synchronization of this stochastic oscillator to the input current is possible for current densities below 3×106 A/cm2, and occurs for input frequencies lower than the natural mean frequency of the stochastic oscillator. We show that such injection locking is robust and leads to a drastic reduction in the phase diffusion of the stochastic oscillator, despite the presence of a frequency mismatch between the oscillator and the excitation.

Locatelli, N.; Mizrahi, A.; Accioly, A.; Matsumoto, R.; Fukushima, A.; Kubota, H.; Yuasa, S.; Cros, V.; Pereira, L. G.; Querlioz, D.; Kim, J.-V.; Grollier, J.

2014-09-01

276

Elementary heating events - magnetic interactions between two flux sources. II. Rates of flux reconnection  

Microsoft Academic Search

Magnetic fragments in the photosphere are in continuous motion and, due to the complex nature of the magnetic field in the solar atmosphere, these motions are likely to drive a lucrative coronal energy source: the passing of initially-unconnected opposite-polarity fragments that release energy through both closing and then re-opening the same fieldlines. Three-dimensional, time-dependent MHD and potential models are used

C. E. Parnell; K. Galsgaard

2004-01-01

277

Characterizing and modeling magnetic flux transport in the sun's photosphere and determining its impact on the sunspot cycle  

NASA Astrophysics Data System (ADS)

The characterization and modeling of magnetic flux transport within the surface layers of the Sun are vital to explaining the sunspot cycle. The Sun's polar fields at solar cycle minimum are the seeds of the next solar cycle: weak polar fields produce weak cycles. Magnetic flux transport is key to the buildup of the polar fields and the subsequent magnetic reversals that are essential to modulating the sunspot cycle. The primary goals of this dissertation are threefold: 1. Make precise measurements of the Sun's axisymmetric flows (i.e., differential rotation and meridional flow). 2. Create a realistic surface flux transport model that reproduces the magnetic field evolution at the surface by incorporating the observed flows. 3. Investigate the role of flux transport in modulating the polar fields, and thereby the solar activity cycle. This work has been done in collaboration with Dr. David H. Hathaway of NASA Marshall Space Flight Center. In Chapter 1, I provide an introduction to the Sun as a star. I begin with a discussion on stellar structure and evolution. I then discuss the techniques and instruments that have been used to study the Sun. I conclude Chapter 1 with a section on magnetic activity cycles on the Sun and in other stars. Magnetic flux on the Sun is transported by supergranular flows and the axisymmetric flows of differential rotation (DR) and meridional flow (MF). In Chapter 2, I introduce these flows. I then show a derivation of the Surface Flux Transport equation starting from Maxwell's equations and Ohm's Law. I conclude this chapter with an introduction to prior Surface Flux Transport models. In Chapter 3, I discuss a cross-correlation technique that we have used on magnetograms (maps of the magnetic field strengths over the surface of the Sun) to characterize the DR and MF and their variations from 1996 to present. Results show that while variability in DR is negligible, the MF varies in two fundamental ways: over the course of a solar cycle and from one cycle to the next. The MF is faster at solar cycle minimum and slower at maximum. Furthermore, the MF speeds that preceded the Solar Cycle 23/24 minimum were ˜ 20% faster than the MF speeds that preceded the prior minimum. This faster MF may have led to weaker polar field strengths and thus the subsequent extended solar minimum and an unusually weak Cycle 24. To understand the impact flux transport on the evolution of the polar fields, I have conducted experiments using a surface magnetic flux transport model, introduced in Chapter 4, that we have developed. This model advects the magnetic flux emerging in active regions (sunspots) using detailed observations of the near-surface flows that transport the magnetic elements. These flows include the axisymmetric differential rotation and meridional flow and the non-axisymmetric cellular convective flows (supergranules), all of which vary in time in the model as indicated by direct observations. At each time step, magnetic maps of the entire Sun are created. These maps are used to create plots of the Sun's axial dipole moment, a measure of the polar field. Also in Chapter 4, I illustrate how this model is used to create a baseline. In this regime, the model assimilates (i.e. continually adds in data weighted by its noise level) magnetic data from magnetograms at all available latitudes. This ensures that it accurately represents the magnetic fields observed on the surface of the Sun. This baseline is used to illustrate the difference in the timing of the polar field reversals based on four different definitions of polar fields. Advantages and disadvantages of each of these definitions are discussed. In Chapter 5, I discuss how the model is used in a predictive or simulation regime. I have tested the predictability of this model using a baseline map as an initial condition, but with daily sunspot area data used to give the sources of new magnetic flux. I found that the strength of the polar fields at cycle minimum and the polar field reversals at cycle maximum can be reliably predicted up to 3 years in adv

Upton, Lisa A.

278

The spatial structure of Martian magnetic flux ropes recovered by the Grad-Shafranov reconstruction technique  

NASA Astrophysics Data System (ADS)

We applied the Grad-Shafranov (GS) reconstruction technique to Martian magnetic flux ropes observed by Mars Global Surveyor in order to estimate their spatial structures. This technique can provide a magnetic field map of their cross section from single spacecraft data, under the assumption that the structure is two-dimensional, magnetohydrostatic, and time independent. We succeeded in recovering the spatial structure for 70 events observed between April 1999 and November 2006. The reconstruction results indicate that the flux rope axes were mostly oriented horizontal to the Martian surface and were randomly distributed with respect to the typical plasma streamline. A subset of events with duration longer than 240 s was observed at solar zenith angles larger than 75°. These events all occur downstream from strong crustal magnetic field in the southern hemisphere, indicating an association between the crustal fields and the detected flux ropes. Using the shape and size of the flux ropes obtained from the GS reconstruction, we estimate lower limits on their volume that span 2-3 orders of magnitude, with larger flux ropes observed downstream from strong crustal magnetic fields. Estimated ion escape rates associated with flux ropes are of the order of 1022-1023 ion/s, being approximately 10% of previously estimated escape rates during solar minimum.

Hara, Takuya; Seki, Kanako; Hasegawa, Hiroshi; Brain, David A.; Matsunaga, Kazunari; Saito, Miho H.

2014-02-01

279

ELF-magnetic flux densities measured in a city environment in summer and winter.  

PubMed

Epidemiological studies have indicated a connection between extremely low frequency magnetic flux densities above 0.4 microT (time weighted average) and childhood leukemia risks. This conclusion is based mainly on indoor exposure measurements. We therefore regarded it important to map outdoor magnetic flux densities in public areas in Trondheim, Norway. Because of seasonal power consumption variations, the fields were measured during both summer and winter. Magnetic flux density was mapped 1.0 m above the ground along 17 km of pavements in downtown Trondheim. The spectrum was measured at some spots and the magnetic flux density emanated mainly from the power frequency of 50 Hz. In summer less than 4% of the streets showed values exceeding 0.4 microT, increasing to 29% and 34% on cold and on snowy winter days, respectively. The average levels were 0.13 microT (summer), 0.85 microT (winter, cold), and 0.90 microT (winter, snow), with the highest recorded value of 37 microT. High spot measurements were usually encountered above underground transformer substations. In winter electric heating of pavements also gave rise to relatively high flux densities. There was no indication that the ICNIRP basic restriction was exceeded. It would be of interest to map the flux density situation in other cities and towns with a cold climate. PMID:17786926

Straume, Aksel; Johnsson, Anders; Oftedal, Gunnhild

2008-01-01

280

On Polar Magnetic Field Reversal and Surface Flux Transport During Solar Cycle 24  

NASA Astrophysics Data System (ADS)

As each solar cycle progresses, remnant magnetic flux from active regions (ARs) migrates poleward to cancel the old-cycle polar field. We describe this polarity reversal process during Cycle 24 using four years (2010.33-2014.33) of line-of-sight magnetic field measurements from the Helioseismic and Magnetic Imager. The total flux associated with ARs reached maximum in the north in 2011, more than two years earlier than the south; the maximum is significantly weaker than Cycle 23. The process of polar field reversal is relatively slow, north-south asymmetric, and episodic. We estimate that the global axial dipole changed sign in 2013 October; the northern and southern polar fields (mean above 60° latitude) reversed in 2012 November and 2014 March, respectively, about 16 months apart. Notably, the poleward surges of flux in each hemisphere alternated in polarity, giving rise to multiple reversals in the north. We show that the surges of the trailing sunspot polarity tend to correspond to normal mean AR tilt, higher total AR flux, or slower mid-latitude near-surface meridional flow, while exceptions occur during low magnetic activity. In particular, the AR flux and the mid-latitude poleward flow speed exhibit a clear anti-correlation. We discuss how these features can be explained in a surface flux transport process that includes a field-dependent converging flow toward the ARs, a characteristic that may contribute to solar cycle variability.

Sun, Xudong; Hoeksema, J. Todd; Liu, Yang; Zhao, Junwei

2015-01-01

281

Magnetized ion flux to arbitrary-shaped objects  

E-print Network

The interaction of magnetized plasma ions with a collecting object of arbitrary shape such as a Langmuir probe is analyzed by drift approximation. This solution provides the theoretical basis for interpreting "perpendicular" ...

Hutchinson, Ian H.

282

Shot noise of charge current in a quantum dot responded by rotating and oscillating magnetic fields  

NASA Astrophysics Data System (ADS)

We have investigated the shot noise and Fano factor of the dynamic spin-polarized quantum dot under the perturbations of a rotating magnetic field (RMF), and an oscillating magnetic field (OMF) by employing the non-equilibrium Green's function approach. The shot noise is enhanced from sub-Poissonian to super-Poissonian due to the application of RMF and OMF, and it is controlled sensitively by the tilt angle ? of RMF. The magnitude of shot noise increases as the photon energy ?? of OMF increases, and its valley eventually is reversed to peaks as the photon energy is large enough. Double-peak structure of Fano factor is exhibited as the frequency of OMF increases to cover a large regime. The Zeeman energy ?0B0 acts as an effective gate bias to exhibit resonant behavior, and novel peak emerges associated with the applied OMF.

Zhao, Hong-Kang; Zou, Wei-Ke; Chen, Qiao

2014-09-01

283

Leak current estimated from the shot noise in magnetic tunneling junctions  

NASA Astrophysics Data System (ADS)

We performed the shot noise measurement in epitaxial Fe/MgO/Fe-based magnetic tunneling junctions (MTJs) with various MgO thicknesses between 1.1 nm and 1.625 nm. While the Fano factor to characterize the shot noise is very close to 1 in MTJs with MgO barrier thicker than 1.2 nm, the magnetic configuration-dependent reduction of the Fano factor for MTJs with thin MgO barrier was observed, which is mainly due to the existence of leakage current. By using a simple parallel circuit model, we demonstrated that the contribution of the leak current can be sensitively derived from the shot noise.

Tanaka, Takahiro; Arakawa, Tomonori; Maeda, Masahiro; Kobayashi, Kensuke; Nishihara, Yoshitaka; Ono, Teruo; Nozaki, Takayuki; Fukushima, Akio; Yuasa, Shinji

2014-07-01

284

The Feynman integrand for the Charged Particle in a Constant Magnetic field as White Noise Distribution  

E-print Network

The concepts of Feynman integrals in white noise analysis are used to realize the Feynman integrand for a charged particle in a constant magnetic field as a Hida distribution. For this purpose we identify the velocity dependent potential as a so called generalized Gauss kernel.

Wolfgang Bock; Martin Grothaus; Sebastian Jung

2012-03-01

285

Noise temperature improvement for magnetic fusion plasma millimeter wave imaging systems  

SciTech Connect

Significant progress has been made in the imaging and visualization of magnetohydrodynamic and microturbulence phenomena in magnetic fusion plasmas [B. Tobias et al., Plasma Fusion Res. 6, 2106042 (2011)]. Of particular importance have been microwave electron cyclotron emission imaging and microwave imaging reflectometry systems for imaging T{sub e} and n{sub e} fluctuations. These instruments have employed heterodyne receiver arrays with Schottky diode mixer elements directly connected to individual antennas. Consequently, the noise temperature has been strongly determined by the conversion loss with typical noise temperatures of ?60?000 K. However, this can be significantly improved by making use of recent advances in Monolithic Microwave Integrated Circuit chip low noise amplifiers to insert a pre-amplifier in front of the Schottky diode mixer element. In a proof-of-principle design at V-Band (50–75 GHz), significant improvement of noise temperature from the current 60?000 K to measured 4000 K has been obtained.

Lai, J.; Domier, C. W.; Luhmann, N. C. [Department of Electrical and Computer Engineering, University of California at Davis, Davis, California 95616 (United States)] [Department of Electrical and Computer Engineering, University of California at Davis, Davis, California 95616 (United States)

2014-03-15

286

Magnetization-noise-induced collapse and revival of Rabi oscillations in circuit QED  

NASA Astrophysics Data System (ADS)

We use a quasi-Hamiltonian formalism to describe the dissipative dynamics of a circuit-QED qubit that is affected by several fluctuating two-level systems with a 1/f-noise power spectrum. The qubit-resonator interactions are described by the Jaynes-Cummings model. We argue that the presence of pure dephasing noise in such a qubit-resonator system will also induce an energy relaxation mechanism via a fluctuating dipole-coupling term. This random modulation of the coupling is seen to lead to rich physical behavior. For non-Markovian noise, the coupling can either worsen or alleviate decoherence, depending on the initial conditions. The magnetization noise leads to behavior resembling the collapse and revival of Rabi oscillations if the photons are in a coherent state. For a broad distribution of noise couplings, the frequency of these oscillations depends on the mean noise strength. We describe this behavior semianalytically and find it to be independent of the number of fluctuators. This phenomenon could be used as an in situ probe of the noise characteristics.

De, Amrit; Joynt, Robert

2013-04-01

287

Qualities of Cosmic Rays Flux, Crossing the Boundary between Magnetic Cloud and Solar Wind  

NASA Astrophysics Data System (ADS)

The model of cosmic ray (CR) Forbush-decrease is researched in a magnetic cloud propagating in interplanetary space. The magnetic cloud is presented as the part of a torus in initial time. The characteristic type of the cloud magnetic field is taken into consideration in the calculation. The change of the torus shape in the following time depends on radial distribution of flow velocity and is described by the kinematic model. The magnetic field is determined from the condition of the freezing-in. The kinetic method of CR flux calculation crossing the boundary between the magnetic cloud and solar wind in view of magnetic field properties is developed. The CR flux arriving in the cloud from interplanetary space and escaping from it, depending on particle energy, magnetic cloud properties and cloud orientation in interplanetary space is determined. It is found: 1) the spectrum value of CR flux coming into the magnetic cloud is higher than the spectrum value of CR existing in interplanetary space; 2) the fastest escaping occurs through the front part of the torus surface; 3) the considerable part of the particles come to its butt-ends during propagation inside the torus.

Petukhov, Ivan; Petukhov, Stanislav

288

Catastrophic eruption of magnetic flux rope in the corona and solar wind with and without magnetic reconnection  

E-print Network

It is generally believed that the magnetic free energy accumulated in the corona serves as a main energy source for solar explosions such as coronal mass ejections (CMEs). In the framework of the flux rope catastrophe model for CMEs, the energy may be abruptly released either by an ideal magnetohydrodynamic (MHD) catastrophe, which belongs to a global magnetic topological instability of the system, or by a fast magnetic reconnection across preexisting or rapidly-developing electric current sheets. Both ways of magnetic energy release are thought to be important to CME dynamics. To disentangle their contributions, we construct a flux rope catastrophe model in the corona and solar wind and compare different cases in which we either prohibit or allow magnetic reconnection to take place across rapidly-growing current sheets during the eruption. It is demonstrated that CMEs, even fast ones, can be produced taking the ideal MHD catastrophe as the only process of magnetic energy release. Nevertheless, the eruptive speed can be significantly enhanced after magnetic reconnection sets in. In addition, a smooth transition from slow to fast eruptions is observed when increasing the strength of the background magnetic field, simply because in a stronger field there is more free magnetic energy at the catastrophic point available to be released during an eruption. This suggests that fast and slow CMEs may have an identical driving mechanism.

Y. Chen; Y. Q. Hu; S. J. Sun

2007-05-26

289

Performance comparison of three-phase flux reversal permanent magnet motors in BLDC and BLAC operation mode  

NASA Astrophysics Data System (ADS)

The paper presents a comparison of torque capability and flux-weakening performance of three-phase flux reversal permanent magnet motors with surface and inset permanent magnets. Finite element analysis is employed to determine the performance of each motor in BLDC and BLAC operation mode. It is shown that the torque capability and flux-weakening performance of surface or inset permanent magnet configuration is strongly dependent on the stator teeth number/rotor pole number combination.

Štumberger, B.; Štumberger, G.; Hadžiselimovi?, M.; Hamler, A.; Gori?an, V.; Jesenik, M.; Trlep, M.

290

Performance study of two axial-flux permanent-magnet machine topologies with soft magnetic composite cores  

Microsoft Academic Search

This paper compares two stator-to-rotor pole ratio topologies, namely 12\\/10 and 12\\/8, of surface mounted axial-flux permanent-magnet machines with soft magnetic composite (SMC) cores. A quasi three-dimensional (3-D) analytical model is first developed for the derivation of machine static parameters. Two 5-kW @1000 rpm surface mounted AFPM segmented armature torus (SAT) machines are designed for direct-drive electric vehicle propulsion. The

W. Fei; P. Luk

2009-01-01

291

Simulation of the outer radiation belt electron flux decrease during the March 26, 1995, magnetic storm  

NASA Astrophysics Data System (ADS)

In this paper we study the variation of the relativistic electron fluxes in the Earth's outer radiation belt during the March 26, 1995, magnetic storm. Using observations by the radiation environment monitor (REM) on board the space technology research vehicle (STRV-Ib), we discuss the flux decrease and possible loss of relativistic electrons during the storm main phase. In order to explain the observations we have performed fully adiabatic and guiding center simulations for relativistic equatorial electrons in the nonstationary Tsygarienko96 magnetospheric magnetic field model. In our simulations the drift of electrons through the magnetopause was considered as a loss process. We present our model results and discuss their dependence on the magnetospheric magnetic and electric field model, as well as on the prestorm fluxes used in the simulations.

Desorgher, L.; Bühler, P.; Zehnder, A.; Flückiger, E. O.

2000-09-01

292

Parallel heat flux and flow acceleration in open field line plasmas with magnetic trapping  

NASA Astrophysics Data System (ADS)

The magnetic field strength modulation in a tokamak scrape-off layer (SOL) provides both flux expansion next to the divertor plates and magnetic trapping in a large portion of the SOL. Previously, we have focused on a flux expander with long mean-free-path, motivated by the high temperature and low density edge anticipated for an absorbing boundary enabled by liquid lithium surfaces. Here, the effects of magnetic trapping and a marginal collisionality on parallel heat flux and parallel flow acceleration are examined. The various transport mechanisms are captured by kinetic simulations in a simple but representative mirror-expander geometry. The observed parallel flow acceleration is interpreted and elucidated with a modified Chew-Goldberger-Low model that retains temperature anisotropy and finite collisionality.

Guo, Zehua; Tang, Xian-Zhu; McDevitt, Chris

2014-10-01

293

Parallel heat flux and flow acceleration in open field line plasmas with magnetic trapping  

SciTech Connect

The magnetic field strength modulation in a tokamak scrape-off layer (SOL) provides both flux expansion next to the divertor plates and magnetic trapping in a large portion of the SOL. Previously, we have focused on a flux expander with long mean-free-path, motivated by the high temperature and low density edge anticipated for an absorbing boundary enabled by liquid lithium surfaces. Here, the effects of magnetic trapping and a marginal collisionality on parallel heat flux and parallel flow acceleration are examined. The various transport mechanisms are captured by kinetic simulations in a simple but representative mirror-expander geometry. The observed parallel flow acceleration is interpreted and elucidated with a modified Chew-Goldberger-Low model that retains temperature anisotropy and finite collisionality.

Guo, Zehua; Tang, Xian-Zhu; McDevitt, Chris [Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2014-10-15

294

Comparative study of 3-D flux electrical machines with soft magnetic composite cores  

Microsoft Academic Search

This paper compares two types of three-dimensional (3-D) flux electrical machines with soft magnetic composite (SMC) cores, namely, claw-pole and transverse-flux machines. 3-D electromagnetic field analysis is conducted for the computation of some important parameters and optimization of the machine structures. An equivalent electric circuit is derived to calculate the machine performance. The analysis methods are validated by experimental results

YouGuang Guo; Jian Guo Zhu; Peter A. Watterson; Wei Wu

2003-01-01

295

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

Microsoft Academic Search

The paper discuses 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; E. Spooner

1999-01-01

296

Exact scattering matrix of graphs in magnetic field and quantum noise  

NASA Astrophysics Data System (ADS)

We consider arbitrary quantum wire networks modelled by finite, noncompact, connected quantum graphs in the presence of an external magnetic field. We find a general formula for the total scattering matrix of the network in terms of its local scattering properties and its metric structure. This is applied to a quantum ring with N external edges. Connecting the external edges of the ring to heat reservoirs, we study the quantum transport on the graph in ambient magnetic field. We consider two types of dynamics on the ring: the free Schrödinger and the free massless Dirac equations. For each case, a detailed study of the thermal noise is performed analytically. Interestingly enough, in presence of a magnetic field, the standard linear Johnson-Nyquist law for the low temperature behaviour of the thermal noise becomes nonlinear. The precise regime of validity of this effect is discussed and a typical signature of the underlying dynamics is observed.

Caudrelier, Vincent; Mintchev, Mihail; Ragoucy, Eric

2014-08-01

297

Magnetic-field decay of three interlocked flux rings with zero linking number  

SciTech Connect

The resistive decay of chains of three interlocked magnetic flux rings is considered. Depending on the relative orientation of the magnetic field in the three rings, the late-time decay can be either fast or slow. Thus, the qualitative degree of tangledness is less important than the actual value of the linking number or, equivalently, the net magnetic helicity. Our results do not suggest that invariants of higher order than that of the magnetic helicity need to be considered to characterize the decay of the field.

Del Sordo, Fabio; Candelaresi, Simon; Brandenburg, Axel [NORDITA, AlbaNova University Center, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden and Department of Astronomy, Stockholm University, SE 10691 Stockholm (Sweden)

2010-03-15

298

Cross-tail magnetic flux ropes as observed by the GEOTAIL spacecraft  

SciTech Connect

Ten transient magnetic structures in Earth`s magnetotail, as observed in GEOTAIL measurements, selected for early 1993 [at({minus}) X{sub GSM}=90-130 R{sub E}], are shown to have helical magnetic field configurations similar to those of interplanetary magnetic clouds at 1 AU but smaller in size by a factor of {approx} 700. Such structures are shown to be well approximated by a comprehensive magnetic force-free flux-rope model. For this limited set of 10 events the rope axes and the average diameter of these structures is {approx} 15 R{sub E}. 18 refs., 2 figs., 1 tab.

Leppintg, R.P.; Fairfield, D.H. [NASA/GSFC, Greenbelt, MD (United States)] [NASA/GSFC, Greenbelt, MD (United States); Jones, J. [Hughes STX Corporation, Greenbelt, MD (United States)] [and others] [Hughes STX Corporation, Greenbelt, MD (United States); and others

1995-05-15

299

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

300

Flux Reversal by Noncoherent Rotation in Magnetic Films  

Microsoft Academic Search

A model of magnetization reversal in thin ferromagnetic films is proposed. The model is based on a small angular dispersion (?3°) in the film plane of the axis of planar anisotropy. The dynamical effects of this dispersion are important when the switching field (Hs) is of the order of the anisotropy field, fall off rapidly with increasing Hs, and include

K. J. Harte

1960-01-01

301

Magnetic helicity fluxes in an ? dynamo embedded in a halo  

Microsoft Academic Search

We present the results of simulations of forced turbulence in a slab where the mean kinetic helicity has a maximum near the mid-plane, generating gradients of magnetic helicity of both large and small-scale fields. We also study systems that have poorly conducting buffer zones away from the midplane in order to assess the effects of boundaries. The dynamical ? quenching

Alexander Hubbard; Axel Brandenburg

2010-01-01

302

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

303

Comparisons of Earthward Poynting flux and the kinetic energy flux of up-flowing transversely heated ions from the Polar spacecraft on cusp magnetic field lines  

NASA Astrophysics Data System (ADS)

This paper presents estimates of the Poynting flux flowing along magnetic field lines in the Earth's cusp region over altitudes from 0.8 Re to 7 Re using measurements during several passes from the Polar spacecraft. The Poynting flux is calculated from measurements of electric fields from the University of California, Berkeley double probe electric field instrument, and from magnetic field measurements from the U.C.L.A. fluxgate magnetometer. The estimates of Poynting flux are of special interest because the high altitude mapping of the cusp magnetic flux tubes may connect to newly reconnected field lines and the low altitude mapping of these field lines is the scene of powerful acceleration processes, most notably transverse heating and outflow of ions. The data show that the Poynting flux is predominantly downward over the frequency range from 1 mHz to 1 Hz . This frequency range includes the Poynting flux due to steady state convection and field-aligned current systems, Alfven waves, and kinetic Alfven waves. Measurement of transversely heated ions over the energy ranges from 10 eV to several keV and their associated ion kinetic energy flux are presented from the University of Iowa Hydra instrument and compared to the values of the downward Poynting flux. Generally the downward Poynting flux exceeds the upward kinetic energy flux of the ions.

Tian, S.; Wygant, J. R.; Cattell, C. A.; Scudder, J. D.; Mozer, F.; Russell, C. T.

2013-12-01

304

The equilibrium structure of thin magnetic flux tubes. II. [in sun and late stars  

NASA Technical Reports Server (NTRS)

The thermal structure of the medium inside thin, vertical magnetic flux tubes embedded in a given external atmosphere is investigated, assuming cylindrical symmetry and a depth-independent plasma beta. The variation with tube radius of the temperature on the tube axis is computed and the temperature on the tube wall is estimated. The temperature variation across the flux tube is found to be due to the depth variation of the intensity and to the density stratification of the atmosphere. Since the temperature difference between the axis and the wall is small in thin flux tubes (of the order of 10 percent), the horizontal temperature gradient may often be neglected and the temperature in a tube of given radius may be described by a single function of depth. Thus, a more detailed numerical treatment of the radiative transfer within thin flux tubes can be substantially simplified by neglecting horizontal temperature differences within the flux tube proper.

Kalkofen, W.; Rosner, R.; Ferrari, A.; Massaglia, S.

1986-01-01

305

Bound oscillations on thin magnetic flux tubes - Convective instability and umbral oscillations  

NASA Technical Reports Server (NTRS)

The possibility that 'tube waves' can be trapped on slender solar magnetic flux tubes is investigated. For rigid isothermal flux tubes, it is found that the flux tube geometry can by itself lead to waves which are trapped on the part of the tube that expands with height. Some geometries lead to trapped modes with eigenperiods near 180 s, if parameters appropriate to sunspot umbrae are chosen. It is possible that the umbral oscillations are a manifestation of such trapped waves, if sunspot umbrae consist of an assembly of slender flux tubes, as in the spaghetti model of Parker (1979). For flux tubes which have a constant ratio of Alfven speed to sound speed, it is found that it is primarily the variation of temperature with height which determines whether trapped waves can exist. Certain temperature profiles lead to disturbances for which omega squared is less than zero, corresponding to convective instability or Rayleigh-Taylor instability.

Hollweg, J. V.; Roberts, B.

1981-01-01

306

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

307

Occurrence rates of magnetic activities and flux maintenance in quiet regions  

NASA Astrophysics Data System (ADS)

We investigate occurrence rates of magnetic activities, namely merging, splitting, cancellation, and emergence as functions of flux content in quiet regions by means of feature tracking technique. The structure of magnetic field on the solar surface is important not only because it is the but also because it is actual magneto-convection on the stellar surface which we can observe for the most details. Recent satellite observation reveals scaling nature of magnetic patches on the solar surface. Parnell et al. (2009) found a power-law distribution with an index of -1.85 for flux content in each magnetic patch and Thornton & Parnell (2011) found a power-law distribution with an index of -2.7 for flux content of emerging flux. However, it is not still understood what mechanism makes these power-law nature. We try this issue by developing auto-tracking technique and quantization of patch activities. We use two data sets of line-of-sight magnetograms by Solar Optical Telescope (SOT)/ Narrowband Filter Imager (NFI) on board Hinode satellite. One has the high temporal cadence, 1 minute, and the other has the longest observational period, 140 hours. More than 3000 and 40000 patches are tracked in the data sets, respectively. We found that the occurrence of merging and splitting is larger than those of emergence and cancellation by one-order of magnitude, which means that the frequency distribution of flux content is maintained by the surface activities not by flux supply itself. The frequency dependence of cancellation on flux amount is also investigated. We found that a power-law distribution with an index of -2.48 with an error bar of 0.24, which is same as that of emergence in Thornton & Parnell (2011). In the discussion, we want to suggest flux re-cycling in quiet regions, which can explain these characters. begin{enumerate} Frequency distribution of flux content is formed to a power-law distribution by merging and splitting on the solar surface. Transport of patches of surface convection results in steep power-law distribution of flux content of cancellation. The submerged fluxes though cancellations re-appear to the solar surface, which results in small-scale flux emergences on the solar surface.

Iida, Yusuke

308

Topological quantum phase transitions in a spin-orbit coupled electron system with staggered magnetic fluxes  

NASA Astrophysics Data System (ADS)

We investigate the topological properties of a spin-orbit coupled electron system in the presence of staggered magnetic fluxes. A series of topological quantum phase transitions occurs by tuning the magnetic fluxes, and some topological phases with high Chern numbers and high spin Chern numbers are present. We also find that in a certain region of parameters, the system becomes a hybrid topological insulator, where quantum spin Hall and quantum anomalous Hall phases coexist. We further demonstrate that the topological characterization based on the Chern number and the spin Chern number fully agrees with the characteristic spectrum of the edge states in different regimes.

Yang, Yuan; Zhang, Y. F.; Sheng, L.; Xing, D. Y.

2014-01-01

309

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

310

Magnetic Flux Effect on a Kondo-Induced Electric Polarization in a Triangular Triple Quantum Dot  

NASA Astrophysics Data System (ADS)

A magnetic flux effect is studied theoretically on an electric polarization induced by the Kondo effect in a triangular triple-quantum-dot system, where one of the three dots is connected to a metallic lead. This electric polarization exhibits an Aharonov–Bohm oscillation as a function of the magnetic flux penetrating through the triangular loop. The numerical renormalization group analysis reveals how the oscillation pattern depends on the Kondo coupling of a local spin with lead electrons, which is sensitive to the point contact with the lead. It provides an experimental implication that the Kondo effect is the origin of the emergent electric polarization.

Koga, Mikito; Matsumoto, Masashige; Kusunose, Hiroaki

2014-08-01

311

SOLAR MAGNETIC TRACKING. III. APPARENT UNIPOLAR FLUX EMERGENCE IN HIGH-RESOLUTION OBSERVATIONS  

SciTech Connect

Understanding the behavior of weak magnetic fields near the detection limit of current instrumentation is important for determining the flux budget of the solar photosphere at small spatial scales. Using 0.''3-resolution magnetograms from the Solar Optical Telescope's Narrowband Filter Imager (NFI) on the Hinode spacecraft, we confirm that the previously reported apparent unipolar magnetic flux emergence seen in intermediate-resolution magnetograms is indeed the coalescence of previously existing flux. We demonstrate that similar but smaller events seen in NFI magnetograms are also likely to correspond to the coalescence of previously existing weak fields. The uncoalesced flux, detectable only in the ensemble average of hundreds of these events, accounts for 50% of the total flux within 3 Mm of the detected features. The spatial scale at which apparent unipolar emergence can be directly observed as coalescence remains unknown. The polarity of the coalescing flux is more balanced than would be expected given the imbalance of the data set, however without further study we cannot speculate whether this implies that the flux in the apparent unipolar emergence events is produced by a granulation-scale dynamo or is recycled from existing field.

Lamb, D. A. [Catholic University of America, NASA Goddard Space Flight Center, Code 671, Greenbelt, MD 20771 (United States); DeForest, C. E. [Southwest Research Institute, 1050 Walnut Street Suite 300, Boulder, CO 80302 (United States); Hagenaar, H. J. [Lockheed Martin Advanced Technology Center, Org. ADBS, Bldg 252, Palo Alto, CA 94304 (United States); Parnell, C. E. [School of Mathematics and Statistics, University of St. Andrews, St. Andrews, Scotland, KY16 9SS (United Kingdom); Welsch, B. T., E-mail: dlamb@spd.aas.or [University of California, Berkeley Space Sciences Laboratory, 7 Gauss Way, Berkeley, CA 94720 (United States)

2010-09-10

312

Evidence in Magnetic Clouds for Systematic Open Flux Transport on the Sun  

NASA Technical Reports Server (NTRS)

Most magnetic clouds encountered by spacecraft at 1 AU display a mix of unidirectional suprathermal electrons signaling open field lines and counterstreaming electrons signaling loops connected to the Sun at both ends. Assuming the open fields were originally loops that underwent interchange reconnection with open fields at the Sun, we determine the sense of connectedness of the open fields found in 72 of 97 magnetic clouds identified by the Wind spacecraft in order to obtain information on the location and sense of the reconnection and resulting flux transport at the Sun. The true polarity of the open fields in each magnetic cloud was determined from the direction of the suprathermal electron flow relative to the magnetic field direction. Results indicate that the polarity of all open fields within a given magnetic cloud is the same 89% of the time, implying that interchange reconnection at the Sun most often occurs in only one leg of a flux rope loop, thus transporting open flux in a single direction, from a coronal hole near that leg to the foot point of the opposite leg. This pattern is consistent with the view that interchange reconnection in coronal mass ejections systematically transports an amount of open flux sufficient to reverse the polarity of the heliospheric field through the course of the solar cycle. Using the same electron data, we also find that the fields encountered in magnetic clouds are only a third as likely to be locally inverted as not. While one might expect inversions to be equally as common as not in flux rope coils, consideration of the geometry of spacecraft trajectories relative to the modeled magnetic cloud axes leads us to conclude that the result is reasonable.

Crooker, N. U.; Kahler, S. W.; Gosling, J. T.; Lepping, R. P.

2008-01-01

313

Torsional Alfven Waves in Solar Magnetic Flux Tubes of Axial Symmetry  

E-print Network

Aims: Propagation and energy transfer of torsional Alfv\\'en waves in solar magnetic flux tubes of axial symmetry is studied. Methods: An analytical model of a solar magnetic flux tube of axial symmetry is developed by specifying a magnetic flux and deriving general analytical formulae for the equilibrium mass density and a gas pressure. The main advantage of this model is that it can be easily adopted to any axisymmetric magnetic structure. The model is used to simulate numerically the propagation of nonlinear Alfv\\'en waves in such 2D flux tubes of axial symmetry embedded in the solar atmosphere. The waves are excited by a localized pulse in the azimuthal component of velocity and launched at the top of the solar photosphere, and they propagate through the solar chromosphere, transition region, and into the solar corona. Results: The results of our numerical simulations reveal a complex scenario of twisted magnetic field lines and flows associated with torsional Alfv\\'en waves as well as energy transfer to t...

Murawski, K; Musielak, Z E; Srivastava, A K; Kraskiewicz, J

2015-01-01

314

The Interaction of New and Old Magnetic Fluxes at the Beginning of Solar Cycle 23  

E-print Network

The 11-year cycle of solar activity follows Hale's law by reversing the magnetic polarity of leading and following sunspots in bipolar regions during the minima of activity. In the 1996-97 solar minimum, most solar activity emerged in narrow longitudinal zones - `active longitudes' but over a range in latitude. Investigating the distribution of solar magnetic flux, we have found that the Hale sunspot polarity reversal first occurred in these active zones. We have estimated the rotation rates of the magnetic flux in the active zones before and after the polarity reversal. Comparing these rotation rates with the internal rotation inferred by helioseismology, we suggest that both `old' and `new' magnetic fluxes were probably generated in a low-latitude zone near the base of the solar convection zone. The reversal of active region polarity observed in certain longitudes at the beginning of a new solar cycle suggests that the phenomenon of active longitudes may give fundamental information about the mechanism of the solar cycle. The non-random distribution of old-cycle and new-cycle fluxes presents a challenge for dynamo theories, most of which assume a uniform longitudinal distribution of solar magnetic fields.

E. E. Benevolenskaya; J. T. Hoeksema; A. G. Kosovichev; P. H. Scherrer

1999-03-26

315

Micromechanical Devices to Reduce 1/f Noise in Magnetic Field and Electric Charge Sensors  

NASA Astrophysics Data System (ADS)

1/f noise is present in every aspect of nature. Sensors and read-out electronics have the ultimate detection limit set by the noise floor of the white noise. In order to increase signal-to-noise ratio (SNR) of low frequency signals buried by high 1/f noise, the signal can be up-converted to a high frequency signal that lies in the lower white noise regime of the sensing device. Mechanical modulation can be employed to move low frequency electronic signals to higher frequency region through the use of microresonators. This thesis has two goals: (1) develop and fabricate a hybrid micromechanical-magnetoresistive magnetic field sensor; and (2) design an electrometer to measure currents collected from air streams containing ionized nano-particles. First, we designed magnetoresistive-microelectromechanical systems (MR-MEMS) hybrid devices based on the monolithic integration of magnetic thin films and silicon-on-insulator (SOI) MEMS fabrication techniques. We used MgO-based magnetic tunnel junctions (MTJ) placed on a bulk micromachined silicon MEMS device to form a hybrid sensing device. The MEMS device was used to mechanically modulate the magnetic field signal detected by the MTJ, thereby reducing the effects of 1/f noise on the MTJ's output. Two actuator designs were investigated: cantilever and electrostatic comb-drive. The second component of the thesis presents a MEMS-based electrometer for the detection of small currents from ionized particles in a particle detection system for air-quality monitoring. One method of particle detection ionizes particles and then feeds a stream of charged particles into a Faraday cup electrometer. We replaced the Faraday cup with a filtering porous mesh sensing-electrode coupled to a MEMS electrometer with a noise floor below 1 fA rms. Experiments were conducted with fA level currents produced by 10 nm diameter particles within an airflow of 1.0 L/min. The MEMS electrometer was compared and calibrated using commercial electrometers and particle counters.

Jaramillo, Gerardo

316

Numerical Simulation of Magnetic Flux Emerging Through a Model Solar Atmosphere: Density Gradient, Magnetic Field, and Mach Number  

NSDL National Science Digital Library

This animation is one of a series depicting the results of a two-dimensional ideal magnetohydrodynamic simulation of magnetic flux emerging through a solar atmosphere. The simulation has a resolution of 300x500 cells and a length scale of 16 Mm x 6.8 Mm. The simulation depicts 1730 seconds in the evolution of the model.

Krauss, Liam; Acuna, Andy; Rilee, M.; Spicer, Dan; Sudan, R.

1996-12-12

317

Numerical Simulation of Magnetic Flux Emerging Through a Model Solar Atmosphere: Density, Magnetic Field, and Mach Number  

NSDL National Science Digital Library

This animation is one of a series depicting the results of a two-dimensional ideal magnetohydrodynamic simulation of magnetic flux emerging through a solar atmosphere. The simulation has a resolution of 300x500 cells and a length scale of 16 Mm x 6.8 Mm. The simulation depicts 1730 seconds in the evolution of the model.

Krauss, Liam; Acuna, Andy; Rilee, M.; Spicer, Dan; Sudan, R.

1996-12-12

318

Geometrical investigation of the kinetic evolution of the magnetic field in a periodic flux rope  

SciTech Connect

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.; Lapenta, G. [Afdeling Plasma-astrofysica, Departement Wiskunde, KULeuven, University of Leuven, Leuven (Belgium)] [Afdeling Plasma-astrofysica, Departement Wiskunde, KULeuven, University of Leuven, Leuven (Belgium); Markidis, S. [High Performance Computing and Visualization (HPCViz) Department, KTH Royal Institute of Technology, Stockholm (Sweden)] [High Performance Computing and Visualization (HPCViz) Department, KTH Royal Institute of Technology, Stockholm (Sweden); Intrator, T. [Los Alamos National Laboratory, M.S. E526, Los Alamos, New Mexico 87545 (United States)] [Los Alamos National Laboratory, M.S. E526, Los Alamos, New Mexico 87545 (United States)

2013-08-15

319

Sensitivity of a Babcock-Leighton Flux-Transport Dynamo to Magnetic Diffusivity Profiles  

E-print Network

We study the influence of various magnetic diffusivity profiles on the evolution of the poloidal and toroidal magnetic fields in a kinematic flux transport dynamo model for the Sun. The diffusivity is a poorly understood ingredient in solar dynamo models. We mathematically construct various theoretical profiles of the depth-dependent diffusivity, based on constraints from mixing length theory and turbulence, and on comparisons of poloidal field evolution on the Sun with that from the flux-transport dynamo model. We then study the effect of each diffusivity profile in the cyclic evolution of the magnetic fields in the Sun, by solving the mean-field dynamo equations. We investigate effects on the solar cycle periods, the maximum tachocline field strengths, and the evolution of the toroidal and poloidal field structures inside the convection zone, due to different diffusivity profiles. We conduct three experiments: (I) comparing very different magnetic diffusivity profiles; (II) comparing different locations of ...

Zita, E J

2010-01-01

320

A high flux of ultra-cold chromium atoms in a magnetic guide  

Microsoft Academic Search

We report the observation of a very high flux of ultra-cold bosonic chromium atoms in a magnetic guide. The beam is created by operating a magneto-optical trap\\/moving optical molasses within the magnetic field of the guide. A relative detuning between two pairs of the cooling lasers cools the atoms into a frame moving along the axes of the guide. When

Axel Griesmaier; Anoush Aghajani-Talesh; Markus Falkenau; Jimmy Sebastian; Alexander Greiner; Tilman Pfau

2009-01-01

321

Alternate Poles Wound Flux-Switching Permanent-Magnet Brushless AC Machines  

Microsoft Academic Search

Flux-switching permanent-magnet (FSPM) brushless machines have emerged as an attractive machine type by virtue of their high torque densities, simple and robust rotor structure, and the fact that permanent magnets and coils are both located on the stator. Both 2-D and 3-D finite element analyses are employed to compare the performance of a conventional all poles wound (double-layer winding) topology

Richard L. Owen; Z. Q. Zhu; Arwyn S. Thomas; Geraint W. Jewell; David Howe

2010-01-01

322

Maximising the flux-weakening capability of permanent magnet brushless AC machines and drives  

Microsoft Academic Search

The behaviour of permanent magnet brushless AC machines, having alternative rotor topologies, viz. surface mounted, inset, interior radial and circumferential magnets, is investigated, when they are designed and operated under the `infinite' flux-weakening capability condition. The corresponding torque\\/power-speed characteristics, the demagnetisation withstand capability, the variation of iron losses, and the fault management of the drive and\\/or machine etc. are analysed

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

2000-01-01

323

Torque quality and comparison of internal and external rotor axial flux surface-magnet disc machines  

Microsoft Academic Search

In this paper, pulsating torque components of permanent magnet machines and pulsating torque minimization techniques are discussed for axial flux surface-magnet disc-type PM machines. The pulsating torque analysis describing general instantaneous electromagnetic torque equation and torque ripple factor is briefly provided in order to analyze torque ripple component. Detailed finite-element analyses focusing on the minimization of cogging and torque ripple

Metin Aydin; Surong Huang; Thomas A. Lipo

2006-01-01

324

On the Determination of Magnetic Field Strength and Flux in Inter-Network  

Microsoft Academic Search

The results of the determination of magnetic field strength and flux from weak polarimetric signals in solar inter-network regions are contradictory. We investigate the origin of this contradiction with the help of MHD simulations. It is shown that the Stokes-V line ratio of the Fe I 5247\\/5250 Å and 15652\\/15648 Å line pairs is a good indicator of kG magnetic

E. Khomenko; M. Collados

2006-01-01

325

Magnetic flux of progenitor stars sets gamma-ray burst luminosity and variability  

NASA Astrophysics Data System (ADS)

Long-duration gamma-ray bursts (GRBs) are thought to come from the core collapse of Wolf-Rayet stars. Whereas their stellar masses M* have a rather narrow distribution, the population of GRBs is very diverse, with gamma-ray luminosities L? spanning several orders of magnitude. This suggests the existence of a `hidden' stellar variable whose burst-to-burst variation leads to a spread in L?. Whatever this hidden variable is, its variation should not noticeably affect the shape of GRB light curves, which display a constant luminosity (in a time-average sense) followed by a sharp drop at the end of the burst seen with Swift/XRT. We argue that such a hidden variable is progenitor star's large-scale magnetic flux. Shortly after the core collapse, most of stellar magnetic flux accumulates near the black hole (BH) and remains there. The flux extracts BH rotational energy and powers jets of roughly a constant luminosity, Lj. However, once BH mass accretion rate dot{M} falls below ˜Lj/c2, the flux becomes dynamically important and diffuses outwards, with the jet luminosity set by the rapidly declining mass accretion rate, L_j˜ dot{M}c^2. This provides a potential explanation for the sharp end of GRBs and the universal shape of their light curves. During the GRB, gas infall translates spatial variation of stellar magnetic flux into temporal variation of Lj. We make use of the deviations from constancy in Lj to perform stellar magnetic flux `tomography'. Using this method, we infer the presence of magnetized tori in the outer layers of progenitor stars for GRB 920513 and GRB 940210.

Tchekhovskoy, Alexander; Giannios, Dimitrios

2015-02-01

326

Magnetic buoyancy instabilities in the presence of magnetic flux pumping at the base of the solar convection zone  

E-print Network

We perform idealised numerical simulations of magnetic buoyancy instabilities in a model of the solar tachocline. We introduce a simplified model of magnetic flux pumping in an upper layer (the convection zone), and study the effects of its inclusion on the evolution of buoyancy instabilities in a lower layer (the radiative interior). We study its effects on the instability of both a preconceived magnetic slab and of a shear-generated magnetic layer. In the former, we find that in the regime in which the downward pumping velocity is comparable with the Alfven speed of the magnetic layer, flux pumping is able to hold back the bulk of the magnetic field, with only small pockets of strong field able to rise into the upper layer. In simulations in which the magnetic layer is generated by shear, we find that the shear velocity is not necessarily required to exceed that of the pumping (therefore the kinetic energy of the shear is not required to exceed that of the overlying convection), for strong localised pockets...

Barker, Adrian J; Proctor, Michael R E; Weiss, Nigel O

2012-01-01

327

Characterization of magnetic degradation mechanism in a high-neutron-flux environment  

NASA Astrophysics Data System (ADS)

Radiation-induced demagnetization of permanent magnets can result in the failure of magnet-based devices operating in high-radiation environments. To understand the mechanism underlying demagnetization, Nd-Fe-B magnets were irradiated with fast and fast plus thermal neutrons at fluences of 1012, 1013, 1014, and 1015 n/cm2, respectively. After irradiation, magnetic flux losses were shown to increase with the fluence. Compared with samples irradiated only with fast neutrons, the samples exposed to the fast plus thermal neutrons have higher magnetic flux losses, which is attributed to the thermal neutron capture reaction of boron. Hysteresis loops of the Nd-Fe-B magnets reveal a slightly increase in the coercivity after irradiation. Full remagnetization of the samples after irradiation was possible, which indicates that structural damage is unlikely an important factor in the demagnetization process at these levels of neutron flux and fluence. Finally, we performed a preliminary Molecular Dynamic (MD) simulation on a cube of ions to obtain a better understanding of the thermal spike mechanism.

Samin, Adib; Qiu, Jie; Hattrick-Simpers, Jason; Dai-Hattrick, Liyang; Zheng, Yuan F.; Cao, Lei

2014-09-01

328

Observation of the magnetic flux and three-dimensional structure of skyrmion lattices by electron holography.  

PubMed

Skyrmions are nanoscale spin textures that are viewed as promising candidates as information carriers in future spintronic devices. Skyrmions have been observed using neutron scattering and microscopy techniques. Real-space imaging using electrons is a straightforward way to interpret spin configurations by detecting the phase shifts due to electromagnetic fields. Here, we report the first observation by electron holography of the magnetic flux and the three-dimensional spin configuration of a skyrmion lattice in Fe(0.5)Co(0.5)Si thin samples. The magnetic flux inside and outside a skyrmion was directly visualized and the handedness of the magnetic flux flow was found to be dependent on the direction of the applied magnetic field. The electron phase shifts ? in the helical and skyrmion phases were determined using samples with a stepped thickness t (from 55 nm to 510 nm), revealing a linear relationship (? = 0.00173 t). The phase measurements were used to estimate the three-dimensional structures of both the helical and skyrmion phases, demonstrating that electron holography is a useful tool for studying complex magnetic structures and for three-dimensional, real-space mapping of magnetic fields. PMID:24727689

Park, Hyun Soon; Yu, Xiuzhen; Aizawa, Shinji; Tanigaki, Toshiaki; Akashi, Tetsuya; Takahashi, Yoshio; Matsuda, Tsuyoshi; Kanazawa, Naoya; Onose, Yoshinori; Shindo, Daisuke; Tonomura, Akira; Tokura, Yoshinori

2014-05-01

329

Slipping Magnetic Reconnection Triggering a Solar Eruption of a Triangle-shaped Flag Flux Rope  

NASA Astrophysics Data System (ADS)

We report the first simultaneous activities of the slipping motion of flare loops and a slipping eruption of a flux rope in 131 Å and 94 Å channels on 2014 February 2. The east hook-like flare ribbon propagated with a slipping motion at a speed of about 50 km s-1, which lasted about 40 minutes and extended by more than 100 Mm, but the west flare ribbon moved in the opposite direction with a speed of 30 km s-1. At the later phase of flare activity, there was a well developed "bi-fan" system of flare loops. The east footpoints of the flux rope showed an apparent slipping motion along the hook of the ribbon. Simultaneously, the fine structures of the flux rope rose up rapidly at a speed of 130 km s-1, much faster than that of the whole flux rope. We infer that the east footpoints of the flux rope are successively heated by a slipping magnetic reconnection during the flare, which results in the apparent slippage of the flux rope. The slipping motion delineates a "triangle-shaped flag surface" of the flux rope, implying that the topology of a flux rope is more complex than anticipated.

Li, Ting; Zhang, Jun

2014-08-01

330

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

331

Stellar activity cycles in a model for magnetic flux generation and transport  

NASA Astrophysics Data System (ADS)

We present results from a model for magnetic flux generation and transport in cool stars and a qualitative comparison of models with observations. The method combines an ??-type dynamo at the base of the convection zone, buoyant rise of magnetic flux tubes, and a surface flux transport model. Based on a reference model for the Sun, numerical simulations were carried out for model convection zones of G- and K-type main sequence and subgiant stars. We investigate magnetic cycle properties for stars with different rotation periods, convection zone depths, and dynamo strengths. For a Sun-like star with P rot=9 d, we find that a cyclic dynamo can underly an apparently non-cyclic, `flat' surface activity, as observed in some stars. For a subgiant K1 star with P rot=2.8 d the long-term activity variations resemble the multi-periodic cycles observed in V711 Tau, owing to high-latitude flux emergence, weak transport effects and stochastic processes of flux emergence.

I?ik, Emre

2012-07-01

332

Influence of growth and annealing conditions on low-frequency magnetic 1/f noise in MgO magnetic tunnel junctions  

NASA Astrophysics Data System (ADS)

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 °C. The variation of the magnetic noise parameter (?mag) of the reference layer with annealing temperature mainly reflects the variation of the pinning effect of the exchange-bias layer. A reduction in ?mag with bias is associated with the bias dependence of the tunneling magnetoresistance. The related magnetic losses are parameterized by a phase lag ?, 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; Stearrett, Ryan; Singh, A.; Nowak, Edmund R.; Coey, J. M. D.

2012-11-01

333

Temperature Evolution of a Magnetic Flux Rope in a Failed Solar Eruption  

NASA Astrophysics Data System (ADS)

In this paper, we report for the first time the detailed temperature evolution process of the magnetic flux rope in a failed solar eruption. Occurring on 2013 January 05, the flux rope was impulsively accelerated to a speed of ~400 km s-1 in the first minute, then decelerated and came to a complete stop in two minutes. The failed eruption resulted in a large-size high-lying (~100 Mm above the surface), high-temperature "fire ball" sitting in the corona for more than two hours. The time evolution of the thermal structure of the flux rope was revealed through the differential emission measure analysis technique, which produced temperature maps using observations of the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. The average temperature of the flux rope steadily increased from ~5 MK to ~10 MK during the first nine minutes of the evolution, which was much longer than the rise time (about three minutes) of the associated soft X-ray flare. We suggest that the flux rope is heated by the energy release of the continuing magnetic reconnection, different from the heating of the low-lying flare loops, which is mainly produced by the chromospheric plasma evaporation. The loop arcade overlying the flux rope was pushed up by ~10 Mm during the attempted eruption. The pattern of the velocity variation of the loop arcade strongly suggests that the failure of the eruption was caused by the strapping effect of the overlying loop arcade.

Song, H. Q.; Zhang, J.; Cheng, X.; Chen, Y.; Liu, R.; Wang, Y. M.; Li, B.

2014-03-01

334

NUMERICAL SIMULATIONS OF MAGNETIC FLUX SHEETS OSKAR STEINER and MICHAEL KN  

E-print Network

modification or suppression of the convective energy transport by strong mag­ netic fields; \\Gamma global interest, namely \\Gamma concentration of magnetic flux into structures of intense field strength; \\Gamma); \\Gamma transport of mechanical energy into the chromosphere and corona. \\Lambda The The National Center

Steiner, Oskar

335

Excess open solar magnetic flux from satellite data: 1. Analysis of the third perihelion Ulysses pass  

E-print Network

Excess open solar magnetic flux from satellite data: 1. Analysis of the third perihelion Ulysses; published 12 November 2009. [1] We use the third perihelion pass by the Ulysses spacecraft to illustrate. Analysis of the third perihelion Ulysses pass, J. Geophys. Res., 114, A11103, doi:10.1029/2009JA014449. 1

Lockwood, Mike

336

Particle swarm optimisation algorithm for crack shape reconstruction in magnetic flux leakage nondestructive testing  

Microsoft Academic Search

Magnetic flux leakage testing is widely used to examine ferromagnetic materials. Considering the importance of estimating the size of surface crack in metals, which is used in nuclear power, railway and piping industries, a method based on particle swarm optimisation algorithm is proposed in this article. This approach maps the size of crack to the location of particles. After initialising

Yong Zhang; Zhongfu Ye

2009-01-01

337

Adaptive Wavelets for Characterizing Magnetic Flux Leakage Signals From Pipeline Inspection  

Microsoft Academic Search

Natural gas transmission pipelines are commonly inspected using magnetic flux leakage (MFL) method for detecting cracks and corrosion in the pipewall. Traditionally the MFL data obtained is processed to estimate an equivalent length (L), width (W), and depth (D) of defects. This information is then used to predict the maximum safe operating pressure (MAOP). In order to obtain a more

Ameet Joshi; Lalita Udpa; Satish Udpa; Antonello Tamburrino

2006-01-01

338

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. , XXXX, DOI:10.1029/, Magnetic Flux Emergence in the Sun  

E-print Network

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. , XXXX, DOI:10.1029/, Magnetic Flux Emergence in the Sun V­dimensional evolution of solar eruptions as they leave the Sun and move into the interplanetary space. One of the most important processes, responsible for many dynamical phenomena ob­ served in the Sun, is the emergence

Sengun, Mehmet Haluk

339

Rail stator augmentation magnetic flux compression linear generator used in electric guns  

Microsoft Academic Search

In order to find an applicable power supply used in electric guns, there have been two types of the magnetic flux compression linear generator (MFCLG) that use the solenoids as the stators. That is to say, a piston armature or a solenoid armature is used within the stator. Although they can amplify the current theoretically, they can only compress the

Duan Xiaojun; Liu Zhengguo; Pang Guohua; Wang Ying; Li Zhiyuan

2004-01-01

340

FEM modelling techniques of magnetic flux leakage-type NDT for ferromagnetic plate inspections  

Microsoft Academic Search

The magnetic flux leakage (MFL) method is commonly used as an inspection technique in the petrochemical, transportation, energy and metal industries. This paper shows how the finite element method techniques can be utilized to model a MFL system. It compares the predicted MFL signals from 2D and 3D models and shows that a signal obtained from 2D models is much

F. I. Al-Naemi; J. P. Hall; A. J. Moses

2006-01-01

341

A new axial flux surface mounted permanent magnet machine capable of field control  

Microsoft Academic Search

This paper presents a new axial flux surface mounted permanent magnet (PM) field controlled TORUS type (FCT) machine. Machine structure and principles are explored and the field weakening feature of the topology as well as the advantages of the machine are presented in the first part. The second section introduces the linear model and sizing analysis using generalized sizing equations.

Metin Aydin; Surong Huang; Thomas A. Lipo

2002-01-01

342

A Method for Embedding Circular Force-Free Flux Ropes in Potential Magnetic Fields  

NASA Astrophysics Data System (ADS)

We propose a method for constructing approximate force-free equilibria in pre-eruptive configurations that locally are a bipolar-type potential magnetic field with a thin force-free flux rope embedded inside it. The flux rope is assumed to have a circular-arc axis, circular cross-section, and electric current that is either concentrated in a thin layer at the boundary of the rope or smoothly distributed across it with a maximum of the current density at the center.The entire solution is described in terms of the magnetic vector potential in order to facilitate the implementation of the method in numerical magnetohydrodynamic (MHD) codes that evolve the vector potential rather than the magnetic field itself. The parameters of the flux rope can be chosen so that its subsequent MHD relaxation under photospheric line-tied boundary conditions leads to nearly exact numerical equilibria. To show the capabilities of our method, we apply it to several cases with different ambient magnetic fields and internal flux-rope structures. These examples demonstrate that the proposed method is a useful tool for initializing data-driven simulations of solar eruptions.

Titov, Viacheslav; Torok, Tibor; Mikic, Zoran; Linker, Jon A.

2014-06-01

343

Radial flux high temperature superconductor motor using bulk trapped field magnets  

Microsoft Academic Search

This team has designed, built, and successfully run a radial flux high temperature superconductor (HTS) trapped field magnet (TFM) motor. The motor was run in a zero field cooled mode. This paper conveys several lessons learned about the use of TFMs in motor and generator applications. The state of the art of bulk TFM material is reviewed. Attention is given

X. Feng; G. Gao; K. Davey; M. Werst; R. Hebner; R. Weinstein; D. Parks; R. Sawh

2009-01-01

344

A Flux Tube Tectonics Model for Solar Coronal Heating Driven by the Magnetic Carpet.  

E-print Network

A Flux Tube Tectonics Model for Solar Coronal Heating Driven by the Magnetic Carpet. Eric R. Priest heating. The dissipation of energy along sharp boundaries we call, by analogy with geophysi- cal plate tectonics, the tectonics model of coronal heating. Similar to the case on Earth, the relative motions

Priest, Eric

345

A FLUX-TUBE TECTONICS MODEL FOR SOLAR CORONAL HEATING DRIVEN BY THE MAGNETIC CARPET  

E-print Network

. The dissipation of energy along sharp boundaries we call, by analogy with geophysical plate tectonicsA FLUX-TUBE TECTONICS MODEL FOR SOLAR CORONAL HEATING DRIVEN BY THE MAGNETIC CARPET Eric R. Priest, the tectonics model of coronal heating. Simi- lar to the case on Earth, the relative motions of the photospheric

Priest, Eric

346

Extension of the Parker bound on the flux of magnetic monopoles  

NASA Technical Reports Server (NTRS)

The Parker bound on the flux of magnetic monopoles is extended and strengthened. A new limit on the survival and growth of a small galactic seed field is obtained which rules out the possibility that monopoles much lighter than 10 exp 17 GeV/sq c can provide the closure density of the universe.

Adams, Fred C.; Fatuzzo, Marco; Freese, Katherine; Tarle, Gregory; Watkins, Richard; Turner, Michael S.

1993-01-01

347

Numerical Simulation of Magnetic Flux Emerging Through a Model Solar Atmosphere: Density Gradient and Velocities  

NSDL National Science Digital Library

This animation is one of a series depicting the results of a two-dimensional ideal magnetohydrodynamic simulation of magnetic flux emerging through a solar atmosphere. The simulation has a resolution of 300x500 cells and a length scale of 16 Mm x 6.8 Mm. The simulation depicts 1730 seconds in the evolution of the model.

Krauss, Liam; Acuna, Andy; Rilee, M.; Spicer, Dan; Sudan, R.

1996-12-12

348

Numerical Simulation of Magnetic Flux Emerging Through a Model Solar Atmosphere: Density Gradient  

NSDL National Science Digital Library

This animation is one of a series depicting the results of a two-dimensional ideal magnetohydrodynamic simulation of magnetic flux emerging through a solar atmosphere. The simulation has a resolution of 300x500 cells and a length scale of 16 Mm x 6.8 Mm. The simulation depicts 1730 seconds in the evolution of the model.

Krauss, Liam; Acuna, Andy; Rilee, M.; Spicer, Dan; Sudan, R.

1996-12-12

349

Numerical Simulation of Magnetic Flux Emerging Through a Model Solar Atmosphere: Density  

NSDL National Science Digital Library

This animation is one of a series depicting the results of a two-dimensional ideal magnetohydrodynamic simulation of magnetic flux emerging through a solar atmosphere. The simulation has a resolution of 300x500 cells and a length scale of 16 Mm x 6.8 Mm. The simulation depicts 1730 seconds in the evolution of the model.

Krauss, Liam; Acuna, Andy; Rilee, M.; Spicer, Dan; Sudan, R.

1996-12-12

350

Light ion concentrations and fluxes in the polar regions during magnetically quiet times  

Microsoft Academic Search

Ion concentrations, flow velocities, and fluxes, measured by the ion mass spectrometer on the Isis 2 satellite for periods around the solstices and equinoxes of 1971 and 1972, have been used in a study of the polar wind morphology during magnetically quiet times. The results confirm the major tenets of polar wind theories; namely, that an upward flow of both

J. H. Hoffman; W. H. Dodson

1980-01-01

351

Magnetic Flux Expulsion in the Powerful Superbubble Explosions and the Alpha-Omega Dynamo  

E-print Network

The possibility of the magnetic flux expulsion from the Galaxy in the superbubble (SB) explosions, important for the Alpha-Omega dynamo, is considered. Special emphasis is put on the investigation of the downsliding of the matter from the top of the shell formed by the SB explosion which is able to influence the kinematics of the shell. It is shown that either Galactic gravity or the development of the Rayleigh-Taylor instabilities in the shell limit the SB expansion, thus, making impossible magnetic flux expulsion. The effect of the cosmic rays in the shell on the sliding is considered and it is shown that it is negligible compared to Galactic gravity. Thus, the question of possible mechanism of flux expulsion in the Alpha-Omega dynamo remains open.

R. R. Rafikov; R. M. Kulsrud

2000-04-06

352

Thermal noise of mechanical oscillators in steady states with a heat flux  

NASA Astrophysics Data System (ADS)

We present an experimental investigation of the statistical properties of the position fluctuations of low-loss oscillators in nonequilibrium steady states. The oscillators are coupled to a heat bath, and a nonequilibrium steady state is produced by flowing a constant heat flux, setting a temperature difference across the oscillators. We investigated the distribution of the measurements of the square of the oscillator position and searched for signs of changes with respect to the equilibrium case. We found that, after normalization by the mean value, the second, third, and fourth standardized statistical moments are not modified by the underlying thermodynamic state. This differs from the behavior of the absolute, i.e., not normalized, second moment, which is strongly affected by temperature gradients and heat fluxes. We illustrate this with a numerical experiment in which we study via molecular dynamics the fluctuations of the length of a one-dimensional chain of identical particles interacting via anharmonic interparticle potentials, with the extremes thermostated at different temperatures: we use the variance of the length in correspondence to its first elastic mode of resonance to define an effective temperature which we observe to depart from the thermodynamic one in the nonequilibrium states. We investigate the effect of changing the interparticle potential and show that the qualitative behavior of the nonequilibrium excess is unchanged. Our numerical results are consistent with the chain length being Gaussian distributed in the nonequilibrium states. Our experimental investigation reveals that the position variance is the only, and crucially easily accessible, observable for distinguishing equilibrium from nonequilibrium steady states. The consequences of this fact for the design of interferometric gravitational wave detectors are discussed.

Conti, Livia; Lazzaro, Claudia; Karapetyan, Gagik; Bonaldi, Michele; Pegoraro, Matteo; Thakur, Ram-Krishna; De Gregorio, Paolo; Rondoni, Lamberto

2014-09-01

353

Convective radial energy flux due to resonant magnetic perturbations and magnetic curvature at the tokamak plasma edge  

NASA Astrophysics Data System (ADS)

With the resonant magnetic perturbations (RMPs) consolidating as an important tool to control the transport barrier relaxation, the mechanism on how they work is still a subject to be clearly understood. In this work, we investigate the equilibrium states in the presence of RMPs for a reduced MHD model using 3D electromagnetic fluid numerical code with a single harmonic RMP (single magnetic island chain) and multiple harmonics RMPs in cylindrical and toroidal geometry. Two different equilibrium states were found in the presence of the RMPs with different characteristics for each of the geometries used. For the cylindrical geometry in the presence of a single RMP, the equilibrium state is characterized by a strong convective radial thermal flux and the generation of a mean poloidal velocity shear. In contrast, for toroidal geometry, the thermal flux is dominated by the magnetic flutter. For multiple RMPs, the high amplitude of the convective flux and poloidal rotation are basically the same in cylindrical geometry, but in toroidal geometry the convective thermal flux and the poloidal rotation appear only with the islands overlapping of the linear coupling between neighbouring poloidal wavenumbers m, m - 1, and m + 1.

Marcus, F. A.; Beyer, P.; Fuhr, G.; Monnier, A.; Benkadda, S.

2014-08-01

354

Convective radial energy flux due to resonant magnetic perturbations and magnetic curvature at the tokamak plasma edge  

SciTech Connect

With the resonant magnetic perturbations (RMPs) consolidating as an important tool to control the transport barrier relaxation, the mechanism on how they work is still a subject to be clearly understood. In this work, we investigate the equilibrium states in the presence of RMPs for a reduced MHD model using 3D electromagnetic fluid numerical code with a single harmonic RMP (single magnetic island chain) and multiple harmonics RMPs in cylindrical and toroidal geometry. Two different equilibrium states were found in the presence of the RMPs with different characteristics for each of the geometries used. For the cylindrical geometry in the presence of a single RMP, the equilibrium state is characterized by a strong convective radial thermal flux and the generation of a mean poloidal velocity shear. In contrast, for toroidal geometry, the thermal flux is dominated by the magnetic flutter. For multiple RMPs, the high amplitude of the convective flux and poloidal rotation are basically the same in cylindrical geometry, but in toroidal geometry the convective thermal flux and the poloidal rotation appear only with the islands overlapping of the linear coupling between neighbouring poloidal wavenumbers m, m – 1, and m?+?1.

Marcus, F. A., E-mail: albertus@if.usp.br [Institute of Physics at University of São Paulo, C.P. 66318, 05315-970 São Paulo, S.P. (Brazil); Aix-Marseille Université, CNRS, PIIM UMR 7345, 13397 Marseille Cedex 20 (France); Beyer, P.; Fuhr, G.; Monnier, A.; Benkadda, S. [Aix-Marseille Université, CNRS, PIIM UMR 7345, 13397 Marseille Cedex 20 (France)

2014-08-15

355

Feasibility study on 3 axis magnetic sensor for flux leakage method  

NASA Astrophysics Data System (ADS)

Most of NDT system by magnetic field sensing has employed coil or semiconductor as sensor which has one axis sensitivity. Recent development of semiconductor technology can makes a chip that enable us to measure 3 axis magnetic field in a 1mm square. This vector information is expected to show a new insight in NDT testing. This presentation will show a basic experimental feasibility study for application of magnetic sensor to flux leakage and eddy current testing method by using a sensing system with the chip.

Sasamoto, Akira

2014-04-01

356

Analytical Study of Optimal Layout of Permanent Magnets in IPMSM for Generating Sinusoidal Flux-Density Distribution in Air Gap  

NASA Astrophysics Data System (ADS)

This paper analytically demonstrates the optimal layout of permanent magnets for generating a sinusoidal flux density distribution in the air gap of the interior permanent-magnet synchronous machines with multiple flux barriers. In addition, practical design methods for realizing the optimal layout are proposed, and the designs are verified by conducting finite element analysis.

Kondo, Minoru

357

QUIET SUN MAGNETIC HELICITY TRANSPORT: I. MUTUAL HELICITY FLUX B. T. Welsch 1 and D. W. Longcope 2  

E-print Network

­emerging fields that emerged and submerged at a previous time, then # 10 25 Mx of flux emerges in the quiet sun1 QUIET SUN MAGNETIC HELICITY TRANSPORT: I. MUTUAL HELICITY FLUX B. T. Welsch 1 and D. W. Longcope do the same for quiet­sun magnetic fields. We derive a theoretical expression for the total helicity

Welsch, Brian

358

Noise reduction control strategy of a permanent magnet synchronous machine for vehicle applications  

E-print Network

NOISE REDUCTION CONTROL STRATEGY OF A PERMANENT MAGNET SYNCHRONOUS MACHINE FOR VEHICLE APPLICATIONS A Thesis by RANDY GENE DOOLITTLE Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment... Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Approved by: Chair of Committee, Mehrdad Ehsani Committee Members, Prasad Enjeti Jim Xiuquan Ji...

Doolittle, Randy Gene

2009-05-15

359

MAGNETIC FLUX DENSITY MEASURED IN FAST AND SLOW SOLAR WIND STREAMS  

SciTech Connect

The radial component of the heliospheric magnetic field vector is used to estimate the open magnetic flux density of the Sun. This parameter has been calculated using observations from the Ulysses mission that covered heliolatitudes from 80 Degree-Sign S to 80 Degree-Sign N, from 1990 to 2009 and distances from 1 to 5.4 AU, the Advanced Composition Explorer mission at 1 AU from 1997 to 2010, the OMNI interplanetary database from 1971, and the Helios 1 and 2 missions that covered the distance range from 0.3 to 1 AU. The flux density was found to be much affected by fluctuations in the magnetic field which make its calculated value dependent on heliospheric location, type of solar wind (fast or slow), and the level of solar activity. However, fluctuations are distributed symmetrically perpendicular to the average Parker direction. Therefore, distributions of the field vector in the two-dimensional plane defined by the radial and azimuthal directions in heliospheric coordinates provide a way to reduce the effects of the fluctuations on the measurement of the flux density. This leads to a better defined flux density parameter; the distributions modified by removing the effects of fluctuations then allow a clearer assessment of the dependence of the flux density on heliospheric location, solar wind type, and solar activity. This assessment indicates that the flux density normalized to 1 AU is independent of location and solar wind type (fast or slow). However, there is a residual dependence on solar activity which can be studied using the modified flux density measurements.

Erdos, G. [Wigner Research Centre for Physics of the Hungarian Academy of Sciences, H-1525 Budapest, POB 49 (Hungary); Balogh, A., E-mail: erdos.geza@wigner.mta.hu [The Blackett Laboratory, Imperial College, London SW7 2BZ (United Kingdom)

2012-07-10

360

Noise performance of a precision pulsed electromagnet power supply for magnetic resonance imaging.  

PubMed

Prepolarized magnetic resonance imaging (PMRI) uses two pulsed electromagnets to achieve high-field image quality with the benefits of low-field data acquisition. The principal challenge with all resistive MRI systems is the implementation of a highly precise magnet current supply. The noise current through the magnet is fundamentally limited by the current transducer used to provide feedback and the voltage reference used to generate the demand signal. Field instability in the main field magnet can both corrupt the received data and degrade the robustness of Carr¿Purcell¿Meiboom¿Gill (CPMG) echo trains, which are paramount to efficient imaging in PMRI. In this work, we present the magnet control system that achieved sufficient field stability for PMRI at $0.5/0.13$ T, identify the dominant sources of noise in the control system, examine the imaging artifacts that can occur if the field stability is insufficient, and identify how the design can be improved for better field stability, should it be required for future implementations of PMRI. PMID:18270064

Matter, N I; Chronik, B; Pauly, J M; Macovski, A; Conolly, S M; Scott, G C

2008-01-01

361

Numerical computation for a new way to reduce vibration and noise due to magnetostriction and magnetic forces of transformer cores  

NASA Astrophysics Data System (ADS)

Magnetostriction (MS) caused by the global magnetization of limbs and yokes and magnetic forces are the undisputed causes of the vibration and noise in power transformer cores. This paper presents a novel way to reduce the vibration and noise, in which nanocrystalline soft magnetic composite (NSMC) material with high permeability is used to fill the step-lap joint gaps of the power transformer magnetic cores. In order to numerically predict the effectiveness of the proposed method, a 3-D magneto-mechanical strong coupled model including MS and magnetic anisotropy of steel sheet was founded. Then, the numerical model was applied to analyze the step-lap joint region of the corner of magnetic cores. The analysis results illustrated that the deformation and noise of core with NSMC are lower than with the traditional epoxy damping material. Moreover, the validity of the proposed new way was verified by the simplified step-lap joint cores, which were achieved based on Epstein Frames.

Zhu, Lihua; Yang, Qingxin; Yan, Rongge; Li, Yongjian; Zhang, Xian; Yan, Weili; Zhu, Jianguo

2013-05-01

362

Anomalous electron trapping by magnetic flux tubes and electric current vortices  

E-print Network

We consider an electron with an anomalous magnetic moment, g>2, confined to a plane and interacting with a nonhomogeneous magnetic field B, and investigate the corresponding Pauli Hamiltonian. We prove a lower bound on the number of bound states for the case when B is of a compact support and the related flux is $N+\\epsilon, \\epsilon\\in(0,1]$. In particular, there are at least N+1 bound states if B does not change sign. We also consider the situation where the magnetic field is due to a localized rotationally symmetric electric current vortex in the plane. In this case the flux is zero; there is a pair of bound states for a weak coupling, and higher orbital-momentum "spin-down" states appearing as the current strength increases.

F. Bentosela; P. Exner; V. A. Zagrebnov

1998-10-23

363

Vortex zero modes, large flux limit and Ambjørn-Nielsen-Olesen magnetic instabilities  

NASA Astrophysics Data System (ADS)

In the large flux limit vortices become flux tubes with almost constant magnetic field in the interior region. This occurs in the case of non-Abelian vortices as well, and the study of such configurations allows us to reveal a close relationship between vortex zero modes and the gyromagnetic instabilities of vector bosons in a strong background magnetic field discovered by Nielsen, Olesen and Ambjørn. The BPS vortices are exactly at the onset of this instability, and the dimension of their moduli space is precisely reproduced in this way. We present a unifying picture in which, through the study of the linear spectrum of scalars, fermions and W bosons in the magnetic field background, the expected number of translational, orientational, fermionic as well as semilocal zero modes is correctly reproduced in all cases.

Bolognesi, Stefano; Chatterjee, Chandrasekhar; Gudnason, Sven Bjarke; Konishi, Kenichi

2014-10-01

364

Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere.  

PubMed

The structure of Earth's magnetosphere is poorly understood when the interplanetary magnetic field is northward. Under this condition, uncharacteristically energetic plasma is observed in the magnetotail lobes, which is not expected in the textbook model of the magnetosphere. Using satellite observations, we show that these lobe plasma signatures occur on high-latitude magnetic field lines that have been closed by the fundamental plasma process of magnetic reconnection. Previously, it has been suggested that closed flux can become trapped in the lobe and that this plasma-trapping process could explain another poorly understood phenomenon: the presence of auroras at extremely high latitudes, called transpolar arcs. Observations of the aurora at the same time as the lobe plasma signatures reveal the presence of a transpolar arc. The excellent correspondence between the transpolar arc and the trapped closed flux at high altitudes provides very strong evidence of the trapping mechanism as the cause of transpolar arcs. PMID:25525244

Fear, R C; Milan, S E; Maggiolo, R; Fazakerley, A N; Dandouras, I; Mende, S B

2014-12-19

365

Depinning of flux lines and AC losses in magnet-superconductor levitation system  

SciTech Connect

The AC loss characteristics of a magnet-superconductor system were studied with the magnet fixed to the free end of an oscillating cantilever located near a stationary melt-textured YBCO pellet. Below a threshold AC field amplitude {approx}2Oe, the dissipation of the oscillator is amplitude-independent, characteristic of a linear, non-hysteretic regime. Above threshold,dissipation increases with amplitude, reflecting the depinning and hysteretic motion of flux lines. The threshold AC field is an order of magnitude higher than that measured for the same YBCO material via AC susceptometry in a uniform DC magnetic field, A partial lock-in of flux lines between YBCO ab planes is proposed as the mechanism for the substantial increase of the depinning threshold.

Terentiev, A. N.; Hull, J. R.; De Long, L. E.

1999-11-29

366

Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere  

NASA Astrophysics Data System (ADS)

The structure of Earth’s magnetosphere is poorly understood when the interplanetary magnetic field is northward. Under this condition, uncharacteristically energetic plasma is observed in the magnetotail lobes, which is not expected in the textbook model of the magnetosphere. Using satellite observations, we show that these lobe plasma signatures occur on high-latitude magnetic field lines that have been closed by the fundamental plasma process of magnetic reconnection. Previously, it has been suggested that closed flux can become trapped in the lobe and that this plasma-trapping process could explain another poorly understood phenomenon: the presence of auroras at extremely high latitudes, called transpolar arcs. Observations of the aurora at the same time as the lobe plasma signatures reveal the presence of a transpolar arc. The excellent correspondence between the transpolar arc and the trapped closed flux at high altitudes provides very strong evidence of the trapping mechanism as the cause of transpolar arcs.

Fear, R. C.; Milan, S. E.; Maggiolo, R.; Fazakerley, A. N.; Dandouras, I.; Mende, S. B.

2014-12-01

367

Sensitivity of a Babcock-Leighton Flux-transport Dynamo to Magnetic Diffusivity Profiles  

NASA Astrophysics Data System (ADS)

We studied the influence of various magnetic diffusivity profiles on the evolution of poloidal and toroidal magnetic fields in a flux transport dynamo model for the Sun. The diffusivity (magnetic permeability/resistivity) is the least known ingredient in kinematic, flux-transport dynamo models. We mathematically constructed various theoretical profiles of the depth-dependent diffusivity, based on constraints from mixing length theory and turbulence, and on comparisons of observed solar poloidal field evolution with results from flux-transport dynamo modeling. We then studied the effect of each profile in the cyclic evolution of dynamo fields in the Sun, by solving the mean-field dynamo equations. We compared our theoretical time-latitude diagrams from model output, with solar synoptic maps. Finally, we investigated how the toroidal and poloidal field structures inside the convection zone are influenced by different diffusivity profiles. Based on these simulations, we study which depth dependent diffusivity profiles may be most relevant for magnetic flux evolution in the Sun. We found, surprisingly, that the solar dynamo depends relatively weakly on the *shape* of the magnetic diffusivity profile; and, for a fixed location of diffusivity gradient, the evolution of magnetic field and dynamo are slightly sensitive to the actual *slope* of the gradient. Most interestingly, we find that the actual *location* of a given diffusivity gradient can have profound influences. In particular, without a gradient close enough to the tachocline, insufficient toroidal field may be carried up by the meridional circulation to sustain normal dynamo cycles. We gratefully acknowledge the use of Mausumi Dikpati's dynamo model, and critical guidance and helpful feedback from Dr. Dikpati and Peter Gilman (HAO/NCAR, Boulder CO, 80301). This work was supported by NASA grants NNH05AB521, NNH06AD51I, the NCAR Director's opportunity fund, the HAO/NCAR Visitor Program, and TESC's Sponsored Research program. The National Center for Atmospheric Research is sponsored by the NSF.

Zita, E. J.

2009-12-01

368

Acoustic Noise of Switched Reluctance and Permanent Magnet Motors: A Comparison in the Context of Electric Brakes  

Microsoft Academic Search

It is held as fact that Permanent Magnet (PM) brushless motors are less noisy than Switched Reluctance (SR) motors, and many methods have been published to reduce the noise of SR drives. This paper provides a systematic experimental comparison of both drives, and of proposed noise- mitigation strategies, in the context of a specific mass-market application: Automotive Electro Mechanical Brakes

Avoki M. Omekanda; Suresh Gopalakrishnan; Harald Klode

2007-01-01

369

Asymmetric magnetic flux generation, m = 1 activity, and edge phenomena on a reversed-field pinch. [ZT-40M  

SciTech Connect

The ZT-40M (Fusion Tech. 8, 1571 (1985)) reversed-field pinch has been used to study magnetic flux perturbations during high-theta (theta = B/sub theta/(a)/>1.6) discharges. Asymmetric toroidal magnetic flux perturbations are found to be associated with magnetic flux emerging through the toroidal shell gap and with m = 0 magnetic disturbances moving toroidally. Ramping current discharges, which are a special case of high-theta operation, show the most robust self-generation of toroidal flux. The electron density fluctuations on the inside major radius and associated m = 1 and m = 0 fluctuations seen on edge magnetic field probes provide a clearer picture of activity during a soft x-ray sawtooth crash. During the sawtooth crash, significant magnetic energy is apparently converted into kinetic energy of the particles.

Howell, R.B.; Ingraham, J.C.; Wurden, G.A.; Weber, P.G.; Buchenauer, C.J.

1987-06-01

370

On the influence of open magnetic flux on substorm intensity: Ground- and space-based observations  

NASA Astrophysics Data System (ADS)

Using the location of maximum region 1 current determined by the Active Magnetosphere and Planetary Electrodynamics Response Experiment as a proxy for the open/closed field line boundary, we monitor the evolution of the amount of open magnetic flux inside the magnetosphere during 772 substorms. We then divide all substorms into three classes, depending on the amount of open flux at expansion phase onset. Studying the temporal variations during the substorms of each class for a number of related geophysical parameters, we find that substorms occurring while the amount of open flux is large are generally more intense. By intense we mean that the auroral electrojet, region 1 current, auroral brightness, tail dipolarization and flow speed, ground magnetic signatures, Pi2 wave power, as well as the intensity and extent of the substorm current wedge (SCW) are all larger than during substorms that occur on a contracted polar cap. The SCW manifests itself as an intensification of the nightside R1 and R2 current system after onset. Our analysis shows that to dispose of large amounts of accumulated open magnetic flux, large substorms are triggered in the terrestrial magnetosphere.

Clausen, L. B. N.; Milan, S. E.; Baker, J. B. H.; Ruohoniemi, J. M.; Glassmeier, K.-H.; Coxon, J. C.; Anderson, B. J.

2013-06-01

371

Plasma dynamics on current-carrying magnetic flux tubes. II - Low potential simulation  

NASA Technical Reports Server (NTRS)

The evolution of plasma in a current-carrying magnetic flux tube of variable cross section is investigated using a one-dimensional numerical simulation. The flux tube is narrow at the two ends and broad in the middle. The middle part of the flux tube is loaded with a hot, magnetically trapped population, and the two ends have a more dense, gravitationally bound population. A potential difference larger than the gravitational potential but less than the energy of the hot population is applied across the domain. The general result is that the potential change becomes distributed along the anode half of the domain, with negligible potential change on the cathode half. The potential is supported by the mirror force of magnetically trapped particles. The simulations show a steady depletion of plasma on the anode side of the flux tube. The current steadily decreases on a time scale of an ion transit time. The results may provide an explanation for the observed plasma depletions on auroral field lines carrying upward currents.

Swift, Daniel W.

1992-01-01

372

A FLUX ROPE NETWORK AND PARTICLE ACCELERATION IN THREE-DIMENSIONAL RELATIVISTIC MAGNETIC RECONNECTION  

SciTech Connect

We investigate magnetic reconnection and particle acceleration in relativistic pair plasmas with three-dimensional particle-in-cell simulations of a kinetic-scale current sheet in a periodic geometry. We include a guide field that introduces an inclination between the reconnecting field lines and explore outside-of-the-current sheet magnetizations that are significantly below those considered by other authors carrying out similar calculations. Thus, our simulations probe the transitional regime in which the magnetic and plasma pressures are of the same order of magnitude. The tearing instability is the dominant mode in the current sheet for all guide field strengths, while the linear kink mode is less important even without the guide field, except in the lower magnetization case. Oblique modes seem to be suppressed entirely. In its nonlinear evolution, the reconnection layer develops a network of interconnected and interacting magnetic flux ropes. As smaller flux ropes merge into larger ones, the reconnection layer evolves toward a three-dimensional, disordered state in which the resulting flux rope segments contain magnetic substructure on plasma skin depth scales. Embedded in the flux ropes, we detect spatially and temporally intermittent sites of dissipation reflected in peaks in the parallel electric field. Magnetic dissipation and particle acceleration persist until the end of the simulations, with simulations with higher magnetization and lower guide field strength exhibiting greater and faster energy conversion and particle energization. At the end of our largest simulation, the particle energy spectrum attains a tail extending to high Lorentz factors that is best modeled with a combination of two additional thermal components. We confirm that the primary energization mechanism is acceleration by the electric field in the X-line region. The highest-energy positrons (electrons) are moderately beamed with median angles {approx}30 Degree-Sign -40 Degree-Sign relative to (the opposite of) the direction of the initial current density, but we speculate that reconnection in more highly magnetized plasmas would give rise to stronger beaming. Finally, we discuss the implications of our results for macroscopic reconnection sites, and which of our results may be expected to hold in systems with higher magnetizations.

Kagan, Daniel; Milosavljevic, Milos [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Spitkovsky, Anatoly [Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States)

2013-09-01

373

ON THE CAUSES OF PLASMOID ACCELERATION AND CHANGES IN MAGNETIC FLUX IN A RESISTIVE MAGNETOHYDRODYNAMIC PLASMA  

SciTech Connect

Observationally, the change of acceleration of coronal mass ejections is commonly attributed to the change of the reconnection rate. In this study, we use a two-dimensional magnetohydrodynamic simulation with finite resistivity to study: (1) the forces that lead to the acceleration of the plasma and plasmoid and (2) the time evolution of the topological change of the magnetic flux across the current sheet. Our results show that the fast flows are not limited to the direction perpendicular to the local magnetic field. The fast parallel flows are accelerated by the parallel component of the pressure gradient force. The net force perpendicular to the magnetic field can accelerate the plasma and the plasmoid along the current sheet. The acceleration of the plasmoid is also controlled by the mass contained in the plasmoid. We find that the fast ejection of the plasmoid can stretch the current sheet and consequently reduce the magnetic reconnection/reconfiguration rate temporally before a new plasmoid is formed. We show that the topological change of the magnetic flux is due to the non-uniform magnetic annihilation rate along the current sheet. Therefore, the reconnection/reconfiguration site does not necessarily stay at the neutral point. It can move with the Y-line next to the bifurcated current sheets.

Yu, H. S.; Lyu, L. H. [Institute of Space Science, National Central University, Chung-Li 32001, Taiwan (China); Wu, S. T., E-mail: hsyu@jupiter.ss.ncu.edu.t, E-mail: Lyu@jupiter.ss.ncu.edu.t [Center for Space Plasma and Aeronomic Research, Department of Mechanical and Aerospace Engineering, University of Alabama in Huntsville, Huntsville, AL 35899 (United States)

2011-01-10

374

A flux-mnemonic permanent magnet brushless motor for electric vehicles  

NASA Astrophysics Data System (ADS)

In this paper, a new permanent magnet (PM) brushless motor is proposed for electric vehicles. The key is to incorporate the concept of memory motors, namely, the online tunable flux-mnemonic PMs, into the stator doubly fed doubly salient PM motor, hence achieving effective air-gap flux control. By further employing the outer-rotor and double-layer-stator topology, the proposed motor takes the definite advantages of compact structure, low armature reaction, and direct-drive capability. Increasingly, this motor can offer the unique features of pole dropping and pole reversing. Finite element analysis and, hence, computer simulation are given to verify the validity of the proposed motor.

Yu, Chuang; Chau, K. T.; Liu, Xinhua; Jiang, J. Z.

2008-04-01

375

Fast Solar Wind from Slowly Expanding Magnetic Flux Tubes (P54)  

NASA Astrophysics Data System (ADS)

aks.astro.itbhu@gmail.com We present an empirical model of the fast solar wind, emanating from radially oriented slowly expanding magnetic flux tubes. We consider a single-fluid, steady state model in which the flow is driven by thermal and non-thermal pressure gradients. We apply a non-Alfvénic energy correction at the coronal base and find that specific relations correlate solar wind speed and non-thermal energy flux with the aerial expansion factor. The results are compared with the previously reported ones.

Srivastava, A. K.; Dwivedi, B. N.

2006-11-01

376

Flux Consumption and Poloidal Magnetic Field Measurements in the MEDUSA TOKAMAK  

NASA Astrophysics Data System (ADS)

The Madison EDUcational Small-Aspect-ratio (MEDUSA) tokamak is a small (R=12 cm, a=8 cm) spherical tokamak designed to investigate features of ST plasmas on a laboratory scale. Recent experiments have been performed on MEDUSA to determine the consumption of ohmic flux during startup for several different operating scenarios. The primary diagnostic tool for these experiments was a low profile (0.3 cm diameter) internal magnetic probe array that provided poloidal field measurements crucial to the determination of the plasma internal inductance l_i. This probe consists of five fifteen-turn, 0.9 mm diameter coils. The coils are spaced at 1.5 cm intervals in the z direction at an installed major radius of 12 cm, and measure the radial component of the poloidal field (B_R). The flux consumption analysis was performed using the Poynting method, using the magnetic probe and the MEDUSA external magnetics to reconstruct the evolution of the MHD equilibria during startup. Results will be presented that identify the partition of poloidal flux into inductive and dissipative components. The magnetic probe array was also used to observe the redistribution of plasma current and the corresponding change in li caused by internal reconnection events. Results from these measurements will be presented.

Garstka, G. D.; Fonck, R. J.; Intrator, T.

1996-11-01

377

Relationship Between the Magnetic Flux of Solar Eruptions and the Ap Index of Geomagnetic Storms  

NASA Astrophysics Data System (ADS)

Solar coronal mass ejections (CMEs) are the main drivers of the most powerful non-recurrent geomagnetic storms. In the extreme-ultraviolet range, CMEs are accompanied by bright post-eruption arcades and dark dimmings. The analysis of events of Solar Cycle 23 (Chertok et al. in Solar Phys. 282, 175, 2013) revealed that the summarized unsigned magnetic flux in the arcades and dimming regions at the photospheric level, ?, is significantly related to the intensity (Dst index) of geomagnetic storms. This provides the basis for the earliest diagnostics of geoefficiency of solar eruptions. In the present article, using the same data set, we find that a noticeable correlation also exists between the eruptive magnetic flux, ?, and another geomagnetic index, Ap. As the magnetic flux increases from some tens to ? 500 (in units of 1020 Mx), the geomagnetic storm intensity measured by the three-hour Ap index increases on average from Ap ? 50 to a formal upper limit of 400 (in units of 2 nT). The established relationship shows that the real value of the Ap index is not limited and during the most severe magnetic storms may significantly exceed 400.

Chertok, I. M.; Abunina, M. A.; Abunin, A. A.; Belov, A. V.; Grechnev, V. V.

2015-02-01

378

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

379

Reducing magnetic field induced noise in broad-band seismic recordings  

NASA Astrophysics Data System (ADS)

Seismic broad-band sensors are known to be sensitive to the magnetic field. Magnetic storms and man-made disturbances of the magnetic field can produce significant noise in seismic recordings. I show that variations in the magnetic field translate directly into apparent acceleration of the seismic sensor within the period range from 60 to 1200s for all leaf-spring sensors under investigation. For a Streckeisen STS-1V this is shown even for periods down to 1s. The sensitivity is quantified in magnitude and direction. Both are quite stable over many time windows and signal periods. The sensitivities obtained by linear regression of the acceleration signal on magnetic field recordings during a magnetic storm can effectively be applied to reduce noise in seismic signals. The sensitivity varies in magnitude from sensor to sensor but all are in the range from 0.05 to 1.2ms-2T-1. Seismograms from sensors at Black Forest Observatory (BFO) and stations of the German Regional Seismic Network were investigated. Although these are mainly equipped with leaf-spring sensors, the problem is not limited to this type of instrument. The effect is not observable on the horizontal component STS-1s at BFO while it is significant in the recordings of the vertical STS-1. The main difference between these instruments is the leaf-spring suspension in the vertical component that appears to be the source of the trouble. The suspension springs are made of temperature compensated Elinvar alloys that inherently are ferromagnetic and may respond to the magnetic field in various ways. However, the LaCoste Romberg ET-19 gravimeter at BFO, which uses this material too, does not respond to magnetic storms at a similar magnitude neither do the Invar-wire strainmeters. An active shielding, composed of three Helmholtz coils and a feedback system, is installed at station Stuttgart and provides an improvement of signal-to-noise ratio by almost a factor of 20 at this particular station. The passive Permalloy shielding commonly installed with STS-1V sensors performs similarly well.

Forbriger, Thomas

2007-04-01

380

Light ion concentrations and fluxes in the polar regions during magnetically quiet times  

NASA Technical Reports Server (NTRS)

Ion concentrations, flow velocities, and fluxes, measured by the ion mass spectrometer on the Isis 2 satellite for periods around the solstices and equinoxes of 1971 and 1972, have been used in a study of the polar wind morphology during magnetically quiet times. The results confirm the major tenets of polar wind theories; namely, that an upward flow of both H(+) and He(+) ions exists at all times over the entire polar region; H(+) fluxes are of the order of 10 to the 8th ions/sq cm s in both summer and winter polar regions. O(+) is the dominant ion specie at 1400-km altitude in regions of strong H(+) flows, and H(+) is depleted over the entire region where the polar wind flows. H(+) fluxes tend to be lower in winter than in summer, whereas He(+) fluxes are a factor of 10 higher in winter and tend to follow the neutral helium concentration near the F2 maximum. The latter agree well with model calculations of flux in the winter but lie above the model values by a factor of 2 in the summer. H(+) fluxes also agree well with ion flow models.

Hoffman, J. H.; Dodson, W. H.

1980-01-01

381

Procedure to Perform Real-Time Reconstruction of the Magnetic Flux in FTU Using RTAI Virtual Machine  

NASA Astrophysics Data System (ADS)

One of the important topics of plasma equilibrium issue in a tokamak is to determine and reconstruct the magnetic iso-flux surfaces using plasma boundary condition (in Shafranov, Sov Phys JETP Engl Transl 37:775, 1960). The equilibrium code ODIN is based on the technique using the multi-polar moments method (in Alladio and Crisanti, Nuclear Fusion 26:1143, 1986) which results from homogeneous solution of the Grad-Shafranov equation. This method is used to reconstruct the magnetic flux and equilibrium in the Frascati Tokamak Upgrade experiment. The real-time reconstruction of the magnetic field map is important to compute quantities necessary to control the plasma. In this paper we address the procedure to perform real-time reconstruction of the magnetic flux (based on ODIN) on RTAI virtual machine. As result of the real-time implementation, we will show the time evolution of the reconstructed magnetic iso-flux surfaces.

Sadeghi, Yahya

2012-06-01

382

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

SciTech Connect

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 into a low-mass coil that surrounds the laser target. A >15 T target field has been demonstrated using a <100 J capacitor bank, a laser-triggered switch, and a low-impedance (<1 {Omega}) strip line. The device has been integrated into a series of magnetic-flux-compression experiments on the 60 beam, 30 kJ OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The initial application is a novel magneto-inertial fusion approach [O. V. Gotchev et al., J. Fusion Energy 27, 25 (2008)] to inertial confinement fusion (ICF), where the amplified magnetic field can inhibit thermal conduction losses from the hot spot of a compressed target. This can lead to the ignition of massive shells imploded with low velocity--a way of reaching higher gains than is possible with conventional ICF.

Gotchev, O. V. [Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623 (United States); Fusion Science Center for Extreme States of Matter and Fast Ignition Physics, University of Rochester, 250 East River Road, Rochester, New York 14623 (United States); Department of Mechanical Engineering, University of Rochester, Rochester, New York 14627 (United States); Knauer, J. P.; Shoup, M. J. III [Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623 (United States); Chang, P. Y. [Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623 (United States); Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627 (United States); Jang, N. W. [Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623 (United States); Department of Mechanical Engineering, University of Rochester, Rochester, New York 14627 (United States); Meyerhofer, D. D.; Betti, R. [Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623 (United States); Fusion Science Center for Extreme States of Matter and Fast Ignition Physics, University of Rochester, 250 East River Road, Rochester, New York 14623 (United States); Department of Mechanical Engineering, University of Rochester, Rochester, New York 14627 (United States); Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627 (United States)

2009-04-15

383

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

PubMed

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 into a low-mass coil that surrounds the laser target. A >15 T target field has been demonstrated using a <100 J capacitor bank, a laser-triggered switch, and a low-impedance (<1 Omega) strip line. The device has been integrated into a series of magnetic-flux-compression experiments on the 60 beam, 30 kJ OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The initial application is a novel magneto-inertial fusion approach [O. V. Gotchev et al., J. Fusion Energy 27, 25 (2008)] to inertial confinement fusion (ICF), where the amplified magnetic field can inhibit thermal conduction losses from the hot spot of a compressed target. This can lead to the ignition of massive shells imploded with low velocity-a way of reaching higher gains than is possible with conventional ICF. PMID:19405657

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

2009-04-01

384

The role of the ejecta magnetic flux on the two-step Forbush decreases  

NASA Astrophysics Data System (ADS)

A Forbush Decrease (FD) is a depression in the Galactic Cosmic Ray (GCR) background intensity, and are usually associated to the passage of an Interplanetary Coronal Mass Ejection (ICME). Magnetic Clouds (MCs) are a subset of ICMEs that are well studied, and are known to cause the deepest FDs. Usually, FDs present two steps in the depression profile, one associated to the shock arrival, and a steeper one restricted to the duration of the ejecta passage. There is a wide variety of processes responsible for the GCR depressions. For instance: the enhanced solar wind (SW) convection, reduced diffusion coefficients, enhanced adiabatic cooling, increase of the coherent magnetic field, etc. Our aim is to make a selection of FD events filtering those associated to well studied magnetic structures, such as Magnetic Clouds (MCs), in order to minimize the mixing processes involved in the ICME-GCR interactions in the resulting sample, and to study statistical properties. We determine the parameters of each FD profile and look for correlations with the associated MC parameters. We propose a method to decompose the FD profile into shock and ejecta components, and investigate correlations with the associated amplitudes of the ejecta components. We introduce the parameter, ``magnetic flux per unit length F/L'', and investigate its importance in the context of a simple ``diffusive barrier'' model. According to the correlations found, the two-step events are better represented by the ``diffusive barrier'' model, and the flux F/L is the parameter that better correlates with these events.

Masías Meza, Jimmy; Dasso, Sergio

385

Noise-Sustained Convective Instability in a Magnetized Taylor-Couette Flow  

E-print Network

The helical magnetorotational instability of the magnetized Taylor-Couette flow is studied numerically in a finite cylinder. A distant upstream insulating boundary is shown to stabilize the convective instability entirely while reducing the growth rate of the absolute instability. The reduction is less severe with larger height. After modeling the boundary conditions properly, the wave patterns observed in the experiment turn out to be a noise-sustained convective instability. After the source of the noise resulted from unstable Ekman and Stewartson layers is switched off, a slowly-decaying inertial oscillation is observed in the simulation. We reach the conclusion that the experiments completed to date have not yet reached the regime of absolute instability.

Wei Liu

2008-10-13

386

Noise-sustained convective instability in a magnetized Taylor-Couette flow  

SciTech Connect

The helical magnetorotational instability of the magnetized Taylor-Couette flow is studied numerically in a finite cylinder. A distant upstream insulating boundary is shown to stabilize the convective instability entirely while reducing the growth rate of the absolute instability. The reduction is less severe with larger height. After modeling the boundary conditions properly, the wave patterns observed in the experiment turn out to be a noise-sustained convective instability. After the source of the noise resulted from unstable Ekman and Stewartson layers is switched off, a slowly-decaying inertial oscillation is observed in the simulation. We reach the conclusion that the experiments completed to date have not yet reached the regime of absolute instability.

Liu, Wei [Los Alamos National Laboratory

2008-01-01

387

Dynamic and Stagnating Plasma Flow Leading to Magnetic-Flux-Tube Collimation S. You, G. S. Yun, and P. M. Bellan  

E-print Network

Dynamic and Stagnating Plasma Flow Leading to Magnetic-Flux-Tube Collimation S. You, G. S. Yun February 2005; published 22 July 2005) Highly collimated, plasma-filled magnetic-flux tubes are frequently magnetohydrodynamic pumping process explains why such collimated, plasma-filled magnetic-flux tubes are ubiquitous

Bellan, Paul M.

388

Self-consistent solutions for vacuum currents around a magnetic flux string  

Microsoft Academic Search

We consider the feedback effects of the vacuum currents induced by a magnetic flux string in 2+1 dimensions. Self-consistent induced charge and current densities that satisfy both the Dirac and Maxwell equations are given for the dimensionless coupling constant [ital g]==[ital e][sup 2]\\/[ital m][r arrow]0 and [ital g][r arrow][infinity]. These quantities for arbitrary [ital g] are inferred from the results

Hsiang-nan Li; David Coker; Alfred Goldhaber

1993-01-01

389

Self-consistent solutions for vacuum currents around a magnetic flux string  

NASA Astrophysics Data System (ADS)

We consider the feedback effects of the vacuum currents induced by a magnetic flux string in 2+1 dimensions. Self-consistent induced charge and current densities that satisfy both the Dirac and Maxwell equations are given for the dimensionless coupling constant g==e2/m-->0 and g-->?. These quantities for arbitrary g are inferred from the results of the two limiting cases, and also studied numerically.

Li, Hsiang-Nan; Coker, David A.; Goldhaber, Alfred S.

1993-01-01

390

Self-consistent solutions for vacuum currents around a magnetic flux string  

Microsoft Academic Search

We consider the feedback effects of the vacuum currents induced by a magnetic flux string in 2+1 dimensions. Self-consistent induced charge and current densities that satisfy both the Dirac and Maxwell equations are given for the dimensionless coupling constant g==e2\\/m-->0 and g-->?. These quantities for arbitrary g are inferred from the results of the two limiting cases, and also studied

Hsiang-Nan Li; David A. Coker; Alfred S. Goldhaber

1993-01-01

391

Dual-rotor, radial-flux, toroidally-wound, permanent-magnet machines  

Microsoft Academic Search

A novel machine family-dual-rotor, radial-flux, toroidally-wound, permanent-magnet (RFTPM) machines-is proposed in order to substantially improve machine torque density and efficiency. After the principles of operation, configurations, and features are discussed, the machine design and optimization guidelines are given. A prototype has been designed, built, and tested. The measured torque density of the prototype, which well matches the design value, is

Ronghai Qu; Thomas A. Lipo

2002-01-01

392

Dual-rotor, radial-flux, toroidally wound, permanent-magnet machines  

Microsoft Academic Search

A novel machine family-dual-rotor, radial-flux, toroidally wound, permanent-magnet (RFTPM) machines-is proposed in order to substantially improve machine torque density and efficiency. After the principles of operation, configurations, and features are discussed, the machine design and optimization guidelines are given. A prototype has been designed, built, and tested. The measured torque density of the prototype, which well matches the design value,

Ronghai Qu; Thomas A. Lipo

2003-01-01

393

Brightness and size of small-scale solar magnetic flux concentrations  

Microsoft Academic Search

The new 1 m Swedish Solar Telescope SST on La Palma allows to observe inter-granular G-band bright points (igBP) in solar active regions at an unprecedented spatial resolution. The igBP are reasonably assumed to be small-scale magnetic flux-concentrations. A sample of more than 1500 igBP shows tight relations of diameter and brightness in the G-band and in the continuum; it

E. Wiehr; B. Bovelet; J. Hirzberger

2004-01-01

394

Enhanced dc magnetic field sensitivity by improved flux concentration in magnetoelectric laminates  

NASA Astrophysics Data System (ADS)

In this letter, we present magnetostatic modeling results that show significant magnetic field concentration tunability through geometric modification of high-mu magnetostatic Metglas layers of laminate magnetoelectric (ME) sensors. Based on the modeling results, composite ME sensors were fabricated with longer Metglas foils and found to exhibit notably higher ME voltage coefficients at smaller DC magnetic biases in response to a 1 kHz driving signal. Such ME sensors have been used to detect DC magnetic field changes as small as 6 nT at 1 kHz, while maintaining a signal-to-noise ratio greater than 10. This represents an enhancement of ˜250% relative to values previously reported for Metglas/Pb(Zr,Ti)O3 laminates.

Gao, Junqi; Gray, David; Shen, Ying; Li, Jiefang; Viehland, D.

2011-10-01

395

Statistical analysis of sunspot area and magnetic flux variations in 2001-2003  

NASA Astrophysics Data System (ADS)

This paper utilizes a searchable Solar Feature Catalogue (SFC) for sunspots created from the SOHO/MDI whitelight images and magnetograms in 2001-2003 using the automated pattern recognition techniques described at http://www.cyber.brad.ac.uk/egso/. Comparison of sunspot areas taklen from the SFC with the daily sunspot numbers published in the Sunspot Index Data Centre (SIDC) shows very good correlation of 78-86%. The total magnetic flux from sunspot areas, measured from a single solar image, is shown to have a strong northern-southern asymmetry revealing magnetic flux decrease in the northern hemisphere and an increase in the southern one with the solar cycle descendence towards the period end (year 2003). The resulting, or excess, flux has also demonstrated very significant asymmetry being negative in the southern hemisphere and positive in the northern at the beginning of the period (solar maximum, 2001)with its amplitude descreasing towards the the period end (2003), or the solar minimum. Also, in 2003 there are signs of change in the excess flux polarity between the northern and southern hemispheres.

Zharkov, S. I.; Zharkova, V. V.

396

Mars Magnetic Data: The Impact of Noise on the Vertical Extrapolation of Fields and Methods of Suppression  

Microsoft Academic Search

Fourier analysis of Mars magnetic data along 180 shows an exponential decrease in amplitudes. Removal of noise and vertical extrapolation suggests a major source near 50 S. The magnetization pattern does not require reversed lineations, but does not exclude them either.

D. M. Jurdy; M. Stefanick

2003-01-01

397

On the estimation of the magnetic easy axis in pipeline steels using magnetic Barkhausen noise  

NASA Astrophysics Data System (ADS)

A method for determination of the magnetic easy axis of the Roll Magnetic Anisotropy in API-5L steels is proposed. The method is based on the fact that the angular dependence of the energy corresponding to the main peak of the Magnetic Barkhausen signal presents uniaxial anisotropy with its easy axis parallel to the rolling direction, independently of the angular dependence of the magnetocrystalline energy in the materials. The proposal is also justified based on the analysis of the influence of microstructural changes, produced by hot-rolling on the domain wall dynamics.

Martínez-Ortiz, P.; Pérez-Benitez, J. A.; Espina-Hernández, J. H.; Caleyo, F.; Hallen, J. M.

2015-01-01

398

An improved measurement of the flux distribution of the Ly? forest in QSO absorption spectra: the effect of continuum fitting, metal contamination and noise properties  

NASA Astrophysics Data System (ADS)

We have performed an extensive Voigt profile analysis of the neutral hydrogen (HI) and metal absorption present in a sample of 18 high-resolution, high signal-to-noise ratio quasi-stellar object (QSO) spectra observed with the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph. We use this analysis to separate the metal contribution from the HI absorption and present an improved measurement of the flux probability distribution function (PDF) due to HI absorption alone at = 2.07, 2.52 and 2.94. The flux PDF is sensitive to the continuum fit in the normalized flux range 0.8 < F < 1.0 and to metal absorption at 0.2 < F < 0.8. Our new measurements of the flux PDF due to HI absorption alone are systematically lower at 0.2 < F < 0.8 by up to 30 per cent compared to the widely used measurement of McDonald et al., based on a significantly smaller sample of Keck High Resolution Echelle Spectrometer data. This discrepancy is probably due to a combination of our improved removal of the metal absorption and cosmic variance, since variations in the flux PDF between different lines-of-sight are large. The HI effective optical depth ?effHI at 1.7 < z < 4 is best fit with a single power law, , in good agreement with previous measurements from comparable data. As also found previously, the effect of noise on the flux distribution is not significant in high-resolution, high signal-to-noise ratio data. Based on data taken from ESO archive obtained with UVES at VLT, Paranal, Chile. E-mail: tkim@aip.de

Kim, T.-S.; Bolton, J. S.; Viel, M.; Haehnelt, M. G.; Carswell, R. F.

2007-12-01

399

Solar wind elemental abundances related to the Sun's open magnetic flux  

NASA Astrophysics Data System (ADS)

Context: We present an investigation of the effects of solar magnetic field on the elemental abundance ratios of heavy ions in the solar wind. Aims: We compare the elemental abundance ratios Fe/O, Si/O, Mg/O, Ne/O, and C/O over the 23rd solar cycle, attention being given to the varying fraction of open magnetic flux on the visible side of the solar disc. Methods: We use ACE solar wind data, the current sheet source surface (CSSS) model of the corona, and SoHO/MDI data to relate solar wind compositional properties with the fraction of open magnetic flux on the Sun. We include both a quasi-stationary and intermittent solar wind. Results: For the six investigated solar wind speed ranges, the elemental composition shows a similar FIP-bias of the quasi-stationary solar wind composition with respect to the photosphere but with three plateaus separating low-, mid-, and high-FIP elements. The average enrichment factor for the low-FIP element group (Mg, Fe, Si) with respect to the photospheric values is between 1.67 and 2.47 when the fraction of open magnetic flux on the visible side of the solar disc is above 14%, and it ranges from 2.78 to 4.15 when this fraction is less than 7%. They are close to the enrichment of low-FIP elements with respect to the photospheric values in quasi-stationary fast- and slow- solar wind, which implies similar fractionation mechanisms for the stationary solar wind and the intermittent solar wind. In the low alpha (<7%) case (alpha is the fraction of open magnetic flux on the visible side of the solar disc), the enrichment of heavy elements for the fast solar wind is well correlated with the high fraction of CMEs. In addition, both the charge state ratios and the elemental abundance ratios exhibit a lower dependence on alpha in a slow solar wind than in a fast solar wind. The dependence of the charge state ratios on alpha is correlated with FIP, which implies that solar magnetic effects play a non-negligible role in the elemental fractionation of the solar wind. We suggest that lower hybrid waves play an important role in the possible linkage between solar magnetic field and elemental fractionation.

Wang, X.; Klecker, B.; Wurz, P.

2009-10-01

400

The revised electromagnetic fields directive and worker exposure in environments with high magnetic flux densities.  

PubMed

Some of the strongest electromagnetic fields (EMF) are found in the workplace. A European Directive sets limits to workers' exposure to EMF. This review summarizes its origin and contents and compares magnetic field exposure levels in high-risk workplaces with the limits set in the revised Directive. Pubmed, Scopus, grey literature databases, and websites of organizations involved in occupational exposure measurements were searched. The focus was on EMF with frequencies up to 10 MHz, which can cause stimulation of the nervous system. Selected studies had to provide individual maximum exposure levels at the workplace, either in terms of the external magnetic field strength or flux density or as induced electric field strength or current density. Indicative action levels and the corresponding exposure limit values for magnetic fields in the revised European Directive will be higher than those in the previous version. Nevertheless, magnetic flux densities in excess of the action levels for peripheral nerve stimulation are reported for workers involved in welding, induction heating, transcranial magnetic stimulation, and magnetic resonance imaging (MRI). The corresponding health effects exposure limit values for the electric fields in the worker's body can be exceeded for welding and MRI, but calculations for induction heating and transcranial magnetic stimulation are lacking. Since the revised European Directive conditionally exempts MRI-related activities from the exposure limits, measures to reduce exposure may be necessary for welding, induction heating, and transcranial nerve stimulation. Since such measures can be complicated, there is a clear need for exposure databases for different workplace scenarios with significant EMF exposure and guidance on good practices. PMID:24557933

Stam, Rianne

2014-06-01

401

Investigation of the magnetic field and stress dependence of 180° domain wall motion in pipeline steel using magnetic Barkhausen noise  

NASA Astrophysics Data System (ADS)

Magnetic Barkhausen noise (MBN) measurements were performed on 2% Mn pipeline steel samples as a function of magnetic field ( H) and as a function of orientation of an applied sweep field for various tensile and compressive stress values. For the magnetic field dependent measurements, the H field and stress were applied along the same axis, which was aligned within 10° of the sample's magnetic easy axis. Tensile stresses between 210 MPa and 0 and compressive stresses between 0 and -200 MPa were applied. The MBN signal was observed to initially increase with H followed by a decrease and eventual suppression of the MBN signal at higher H values. The application of tensile stress increased the total MBN signal and increased the field at which suppression occurred, while the application of compressive stress decreased the total signal with a corresponding decrease in the suppression field value. The stress and low-field behaviour of the MBN signal was correlated with the irreversible differential permeability ( ?IDP) measured in a similar 2% Mn pipeline steel sample. The behaviour of the MBN signal as a function of H and stress was attributed to the effect of these parameters on the relative population and/or size of 180° domain walls, that is, the total 180° domain wall area, within the sample.

Krause, Thomas W.; Makar, J. M.; Atherton, D. L.

1994-10-01

402

Coronal Heating and the Increase of Coronal Luminosity with Magnetic Flux  

NASA Technical Reports Server (NTRS)

We present the observed scaling of coronal luminosity with magnetic flux in a set of quiet regions. Comparison of this with the observed scaling found for active regions suggests an underlying difference between coronal heating in active regions and quiet regions. From SOHO/EIT coronal images and SOHO/MDI magnetograms of four similar large quiet regions, we measure L(sub corona) and Phi(sub total) in random subregions ranging in area from about four supergranules [(70,000 km)(exp 2)] to about 100 supergranules [(0.5 R(sub sun))(exp 2)], where L(sub corona) is the luminosity of the corona in a subregion and Phi(sub total) is the flux content of the magnetic network in the subregion. This sampling of our quiet regions yields a correlation plot of Log L(sub corona) vs Log Phi(sub total) appropriate for comparison with the corresponding plot for active regions. For our quiet regions, the mean values of L(sub corona) and Phi(sub total) both increase linearly with area (simply because each set of subregions of the same area has very nearly the same mean coronal luminosity per unit area and mean magnetic flux per unit area), and in each constant-area set the values of L(sub corona) and Phi(sub total) 'scatter' about their means for that area. This results in the linear least-squares fit to the Log ((L (sub corona)), vs Log ((Phi (sub total)) plot having a slope somewhat less than one. If active regions mimicked our quiet regions in that all large sets of same-area active regions had the same mean coronal luminosity per unit area and same mean magnetic flux per unit area, then the least-squares fit to their Log((L (sub corona)) vs Log((Phi (sub total)) plot would also have a slope of less than one. Instead, the slope for active regions is 1.2. Given the observed factor of three scatter about the least-squares linear fit, this slope is consistent with Phi(sub total) on average increasing linearly with area (A) as in quiet regions, but L(sub corona) on average increasing as the volume (A(exp 1.5)) of the active region instead of as the area. This possibility is reasonable if the heating in active regions is a burning down of previously-stored coronal magnetic energy rather than a steady dissipation of energy flux from below as expected in quiet regions.

Moore, R. L.; Falconer, D. A.; Porter, J. G.; Hathaway, D. H.; Six, N. Frank (Technical Monitor)

2002-01-01

403

A dual-channel flux-switching permanent magnet motor for hybrid electric vehicles  

NASA Astrophysics Data System (ADS)

The flux-switching permanent magnet (FSPM) motor is a relatively novel brushless machine having both magnets and concentrated windings in the stator, which exhibits inherently sinusoidal PM flux-linkage, back-EMF waveforms, and high torque capability. However, in the application of hybrid electric vehicles, it is essential to prevent magnets and armature windings moving in radial direction due to the possible vibration during operation, and to ensure fault-tolerant capability. Hence, in this paper based on an original FSPM motor, a dual-channel FSPM (DC-FSPM) motor with modified structure to fix both armature windings and magnets and improved reliability is proposed for a practical 10 kW integral starter/generator (ISG) in hybrid electric vehicles. The influences of different solutions and the end-effect on the static characteristics, are evaluated based on the 2D and 3D finite element analysis, respectively. Finally, both the predicted and experimental results, compared with a prototype DC-FSPM motor and an interior PM motor used in Honda Civic, confirm that the more sinusoidal back-EMF waveform and lower torque ripple can be achieved in the DC-FSPM motor, whereas the torque is smaller under the same coil current.

Hua, Wei; Wu, Zhongze; Cheng, Ming; Wang, Baoan; Zhang, Jianzhong; Zhou, Shigui

2012-04-01

404

Optimization of magnetic flux density in electrical steels: Slater-Pauling pattern repetition in multicomponent alloys  

NASA Astrophysics Data System (ADS)

By investigating the magnetization variation of Fe-rich multicomponent alloys with solute concentration, the magnetic flux density of Si electrical steel is optimized with average valency. For binary alloys of Fe-X [X=d-5d transition-metal (TM) and 3sp-6sp elements], the usual mountain-shape behavior of the Slater-Pauling curve is produced even for late 4d-5d TM, and the monotonically decreasing behavior for 3sp-5sp elements. Anomalously, a rise-and-fall pattern is found for the 6sp element of X=Bi. For ternary alloys of Fe-Si-X (X=3d-5d TM and 3sp-6sp elements), the role of Si is shown to shift the starting point of moment variation and the magnetic flux density of Fe-Si-X alloys is found to repeat the Si-absent binary pattern at the shifted reference moment. On the basis of the calculated magnetic moments, Fe-Si-X-Y (X=Co, Pd, Pt and Y=Al, Sb, Bi) is proposed as a viable candidate for the optimum products of electrical steels.

Baik, Seung Su; Kwon, S. K.; Min, B. I.

2012-02-01

405

Riemannian isometries of twisted magnetic flux tubes and stable current-carrying solar loops  

E-print Network

Two examples of the use of differential geometry in plasma physics are given: The first is the computation and solution of the constraint equations obtained from the Riemann metric isometry of the twisted flux tube. In this case a constraint between the Frenet torsion and curvature is obtained for inhomogeneous helical magnetic flux tube axis. In the second one, geometrical and topological constraints on the current-carrying solar loops are obtained by assuming that the plasma filament is stable. This is analogous to early computations by Liley [(Plasma Physics (1964)] in the case of hydromagnetic equilibria of magnetic surfaces. It is shown that exists a relationship between the ratio of the current components along and cross the plasma filament and the Frenet torsion and curvature. The computations are performed for the helical plasma filaments where torsion and curvature are proportional. The constraints imposed on the electric currents by the energy stability condition are used to solve the remaining magnetohydrodynamical (MHD) equations which in turn allows us to compute magnetic helicity and from them the twist and writhe topological numbers. Magnetic energy is also computed from the solutions of MHD equations.

Garcia de Andrade

2007-02-12

406

Study on Method to Suppress Decay of Trapped Magnetic Fluxes in the HTS Bulk Subjected to Perturbation of External Magnetic Field by Use of Shielding Coil Wound of HTS Wire  

Microsoft Academic Search

Trapped magnetic fluxes in HTS bulks are decayed when the bulks are subjected to perturbation of external magnetic fields. In HTS bulk apparatuses such as HTS bulk motors and magnetic levitation bearings, there is a possibility that the trapped magnetic fluxes in the bulks disappear because the bulks are in- evitably subjected to perturbations of the external magnetic fields. For

K. Yamagishi; O. Tsukamoto; J. Ogawa; D. Miyagi

2011-01-01

407

EFFECT OF POLARIMETRIC NOISE ON THE ESTIMATION OF TWIST AND MAGNETIC ENERGY OF FORCE-FREE FIELDS  

SciTech Connect

The force-free parameter {alpha}, also known as helicity parameter or twist parameter, bears the same sign as the magnetic helicity under some restrictive conditions. The single global value of {alpha} for a whole active region gives the degree of twist per unit axial length. We investigate the effect of polarimetric noise on the calculation of global {alpha} value and magnetic energy of an analytical bipole. The analytical bipole has been generated using the force-free field approximation with a known value of constant {alpha} and magnetic energy. The magnetic parameters obtained from the analytical bipole are used to generate Stokes profiles from the Unno-Rachkovsky solutions for polarized radiative transfer equations. Then we add random noise of the order of 10{sup -3} of the continuum intensity (I {sub c}) in these profiles to simulate the real profiles obtained by modern spectropolarimeters such as Hinode (SOT/SP), SVM (USO), ASP, DLSP, POLIS, and SOLIS etc. These noisy profiles are then inverted using a Milne-Eddington inversion code to retrieve the magnetic parameters. Hundred realizations of this process of adding random noise and polarimetric inversion is repeated to study the distribution of error in global {alpha} and magnetic energy values. The results show that (1) the sign of {alpha} is not influenced by polarimetric noise and very accurate values of global twist can be calculated, and (2) accurate estimation of magnetic energy with uncertainty as low as 0.5% is possible under the force-free condition.

Tiwari, Sanjiv Kumar; Venkatakrishnan, P.; Gosain, Sanjay; Joshi, Jayant [Udaipur Solar Observatory, Physical Research Laboratory, Dewali, Bari Road, Udaipur-313 001 (India)], E-mail: stiwari@prl.res.in, E-mail: pvk@prl.res.in, E-mail: sgosain@prl.res.in, E-mail: jayant@prl.res.in

2009-07-20

408

Influence of stator slots on the development of noise-generating magnetic force waves  

NASA Astrophysics Data System (ADS)

The calculation of noise in electrical machines is important in the design of these machines. It is shown that the magnetic anisotropy of a laminated stator core caused by stator slots has a substantial effect on the force waves acting on the yoke. The force waves are described simply using a modulation function and the Fourier coefficients of the waves. The effects of the amplitude of the magnetic force waves are discussed, and the effectiveness of stator slot skewing is determined. It is shown that subharmonics can be generated by the slotting (affecting the harmonic number of the force waves), and that for specific harmonic numbers, a reversal in the direction of rotation of force waves takes place. The effectiveness of the method is substantiated by a practical example.

Wachta, B.

409

Spin relaxometry of single nitrogen-vacancy defects in diamond nanocrystals for magnetic noise sensing  

NASA Astrophysics Data System (ADS)

We report an experimental study of the longitudinal relaxation time (T1) of the electron spin associated with single nitrogen-vacancy (NV) defects hosted in nanodiamonds (NDs). We first show that T1 decreases over three orders of magnitude when the ND size is reduced from 100 to 10 nm owing to the interaction of the NV electron spin with a bath of paramagnetic centers lying on the ND surface. We next tune the magnetic environment by decorating the ND surface with Gd3+ ions and observe an efficient T1 quenching, which demonstrates magnetic noise sensing with a single electron spin. We estimate a sensitivity down to ?14 electron spins detected within 10 s, using a single NV defect hosted in a 10-nm-size ND. These results pave the way towards T1-based nanoscale imaging of the spin density in biological samples.

Tetienne, J.-P.; Hingant, T.; Rondin, L.; Cavaillès, A.; Mayer, L.; Dantelle, G.; Gacoin, T.; Wrachtrup, J.; Roch, J.-F.; Jacques, V.

2013-06-01

410

A permendur-piezoelectric multiferroic composite for low-noise ultrasensitive magnetic field sensors  

NASA Astrophysics Data System (ADS)

Low-frequency and resonance magnetoelectric (ME) effects have been studied for a trilayer of permendur (alloy of Fe-Co-V) and lead zirconate titanate (PZT). The high permeability and high magnetostriction for permendur, key ingredients for magnetic field confinement, and ME response result in ME voltage coefficient of 23 V/cm Oe at low-frequency and 250 V/cm Oe at electromechanical resonance (EMR) for a sample with PZT fibers and inter-digital-electrodes. Theoretical ME coefficients are in agreement with the data. Measured magnetic noise floor of 25 pT/?Hz at 1 Hz and 100 fT/?Hz at EMR are comparable to best values reported for Metglas-PZT fiber sensors.

Sreenivasulu, G.; Laletin, U.; Petrov, V. M.; Petrov, V. V.; Srinivasan, G.

2012-04-01

411

Magnetic Field Delocalization and Flux Inversion in Fractional Vortices in Two-Component Superconductors  

NASA Astrophysics Data System (ADS)

We demonstrate that, in contrast with the single-component Abrikosov vortex, in two-component superconductors vortex solutions with an exponentially screened magnetic field exist only in exceptional cases: in the case of vortices carrying an integer number of flux quanta and in a special parameter limit for half-quantum vortices. For all other parameters, the vortex solutions have a delocalized magnetic field with a slowly decaying tail. Furthermore, we demonstrate a new effect which is generic in two-component systems but has no counterpart in single-component systems: on exactly half of the parameter space of the U(1)×U(1) Ginzburg-Landau model, the magnetic field of a generic fractional vortex inverts its direction at a certain distance from the vortex core.

Babaev, Egor; Jäykkä, Juha; Speight, Martin

2009-12-01

412

Modelling of the Outer Electron Belt Flux Dropout and Losses During Magnetic Storm Main Phase  

NASA Astrophysics Data System (ADS)

In this paper we analyze the flux dropout and loss of outer belt electrons during magnetic storms. Using observations of the outer belt variation during the 26 March 1995 magnetic storm by the REM detector onboard the STRV-lb satellite, we show that a real loss of electrons occurred during the storm main phase. In order to simulate the outer belt variation during this storm, fully adiabatic motion of equatorial electrons was simulated with the non stationary Tsyganenko96 magnetic field model. We discuss the effect of adiabatic deceleration during the storm main phase and conclude that drift of particles into the magnetopause may be a possible mechanism. Simulations with different field configurations show that the result are sensitive to the various field components

Desorgher, L.; Flückiger, E.; Bühler, P.; Zehnder, A.

413

Reconciling Models of Luminous Blazars with Magnetic Fluxes Determined by Radio Core-shift Measurements  

NASA Astrophysics Data System (ADS)

Estimates of magnetic field strength in relativistic jets of active galactic nuclei, obtained by measuring the frequency-dependent radio core location, imply that the total magnetic fluxes in those jets are consistent with the predictions of the magnetically arrested disk (MAD) scenario of jet formation. On the other hand, the magnetic field strength determines the luminosity of the synchrotron radiation, which forms the low-energy bump of the observed blazar spectral energy distribution (SED). The SEDs of the most powerful blazars are strongly dominated by the high-energy bump, which is most likely due to the external radiation Compton mechanism. This high Compton dominance may be difficult to reconcile with the MAD scenario, unless (1) the geometry of external radiation sources (broad-line region, hot-dust torus) is quasi-spherical rather than flat, or (2) most gamma-ray radiation is produced in jet regions of low magnetization, e.g., in magnetic reconnection layers or in fast jet spines.

Nalewajko, Krzysztof; Sikora, Marek; Begelman, Mitchell C.

2014-11-01

414

The magnetic structure of surges in small-scale emerging flux regions  

E-print Network

Aims: To investigate the relationship between surges and magnetic reconnection during the emergence of small-scale active regions. In particular, to examine how the large-scale geometry of the magnetic field, shaped by different phases of reconnection, guides the flowing of surges. Methods: We present three flux emergence models. The first model, and the simplest, consists of a region emerging into a horizontal ambient field that is initially parallel to the top of the emerging region. The second model is the same as the first but with an extra smaller emerging region which perturbs the main region. This is added to create a more complex magnetic topology and to test how this complicates the development of surges compared to the first model. The last model has a non-uniform ambient magnetic field to model the effects of emergence near a sunspot field and impose asymmetry on the system through the ambient magnetic field. At each stage, we trace the magnetic topology to identify the locations of reconnection. T...

MacTaggart, David; Haynes, Andrew; Simitev, Radostin; Zuccarello, Francesca

2015-01-01

415

DECREASING SUNSPOT MAGNETIC FIELDS EXPLAIN UNIQUE 10.7 cm RADIO FLUX  

SciTech Connect

Infrared spectral observations of sunspots from 1998 to 2011 have shown that on average sunspots changed, the magnetic fields weakened, and the temperatures rose. The data also show that sunspots or dark pores can only form at the solar surface if the magnetic field strength exceeds about 1500 G. Sunspots appear at the solar surface with a variety of field strengths, and during the period from 1998 to 2002 a histogram of the sunspot magnetic fields shows a normal distribution with a mean of 2436 {+-} 26 G and a width of 323 {+-} 20 G. During this observing period the mean of the magnetic field distribution decreased by 46 {+-} 6 G per year, and we assume that as the 1500 G threshold was approached, magnetic fields appeared at the solar surface which could not form dark sunspots or pores. With this assumption we propose a quantity called the sunspot formation fraction and give an analytical form derived from the magnetic field distribution. We show that this fraction can quantitatively explain the changing relationship between sunspot number and solar radio flux measured at 10.7 cm wavelengths.

Livingston, W.; Penn, M. J. [National Solar Observatory, 950 North Cherry Avenue, Tucson, AZ 85718 (United States); Svalgaard, L. [HEPL, Via Ortega, Stanford University, Stanford, CA 94305 (United States)

2012-09-20

416

Pulsating nighttime magnetic background noise in the upper ULF band at low latitudes  

NASA Astrophysics Data System (ADS)

model long-period (~2 h) irregular pulsations in the ellipticity of magnetic background noise (MBN) in the upper ULF band which were frequently observed during nighttime at a low-latitude site on the Island of Crete. It is shown that such pulsations cannot be reproduced in the calculations when using the ionosphere parameters from the statistical IRI (International Reference Ionosphere) model, while regular diurnal signatures of the ellipticity spectrum at sunset and sunrise are successfully reproduced. We apply the same approach to the location of the Arecibo incoherent scatter radar and show that using actually measured ionosphere profiles (up to a height of 400 km) instead of IRI profiles produces the ellipticity pulsations very similar to those observed at Crete. Comparison of model results with the calculated behavior of Alfvén mode refractive index allows us to conclude that the observed nighttime long-period irregular pulsations in the MBN ellipticity are caused by dynamic processes at the upper boundary of the ionospheric E-F valley which serves as a subionospheric Alfvén resonator. Irregular widening, shrinking, and/or deepening of the valley with time scales of 1 to 4 h affect the electrodynamical properties of this resonator and manifest themselves in the magnetic background noise properties.

Bösinger, T.; Demekhov, A. G.; Ermakova, E. N.; Haldoupis, C.; Zhou, Q.

2014-05-01

417

Tidal Disruption and Magnetic Flux Capture: Powering a Jet from a Quiescent Black Hole  

E-print Network

The transient Swift J1644+57 is believed to have been produced by an unlucky star wandering too close to a supermassive black hole (BH) leading to a tidal disruption event. This unusual flare displayed highly super-Eddington X-ray emission which likely originated in a relativistic, collimated jet. This presents challenges to modern accretion and jet theory as upper limits of prior BH activity, which we obtain from the radio afterglow of this event, imply that both the pre-disruption BH and stellar magnetic fluxes fall many orders of magnitude short of what is required to power the observed X-ray luminosity. We argue that a pre-existing, "fossil" accretion disc can contain a sufficient reservoir of magnetic flux and that the stellar debris stream is capable of dragging this flux into the BH. To demonstrate this, we perform local, 3D magnetohydrodynamic simulations of the disc--stream interaction and demonstrate that the interface between the two is unstable to mixing. This mixing entrains a sufficient amount o...

Kelley, Luke Zoltan; Narayan, Ramesh

2014-01-01

418

ON THE ANISOTROPY IN EXPANSION OF MAGNETIC FLUX TUBES IN THE SOLAR CORONA  

SciTech Connect

Most one-dimensional hydrodynamic models of plasma confined to magnetic flux tubes assume circular tube cross sections. We use potential field models to show that flux tubes in circumstances relevant to the solar corona do not, in general, maintain the same cross-sectional shape through their length and therefore the assumption of a circular cross section is rarely true. We support our hypothesis with mathematical reasoning and numerical experiments. We demonstrate that lifting this assumption in favor of realistic, non-circular loops makes the apparent expansion of magnetic flux tubes consistent with that of observed coronal loops. We propose that in a bundle of ribbon-like loops, those that are viewed along the wide direction would stand out against those that are viewed across the wide direction due to the difference in their column depths. That result would impose a bias toward selecting loops that appear not to be expanding, seen projected in the plane of sky. An implication of this selection bias is that the preferentially selected non-circular loops would appear to have increased pressure scale heights even if they are resolved by current instruments.

Malanushenko, A. [Department of Physics, Montana State University, Bozeman, MT (United States); Schrijver, C. J. [Lockheed Martin Advanced Technology Center, Palo Alto, CA (United States)

2013-10-01

419

The role of electron heat flux in guide-field magnetic reconnection  

SciTech Connect

A combination of analytical theory and particle-in-cell simulations are employed in order to investigate the electron dynamics near and at the site of guide field magnetic reconnection. A detailed analysis of the contributions to the reconnection electric field shows that both bulk inertia and pressure-based quasiviscous processes are important for the electrons. Analytic scaling demonstrates that conventional approximations for the electron pressure tensor behavior in the dissipation region fail, and that heat flux contributions need to be accounted for. Based on the evolution equation of the heat flux three tensor, which is derived in this paper, an approximate form of the relevant heat flux contributions to the pressure tensor is developed, which reproduces the numerical modeling result reasonably well. Based on this approximation, it is possible to develop a scaling of the electron current layer in the central dissipation region. It is shown that the pressure tensor contributions become important at the scale length defined by the electron Larmor radius in the guide magnetic field.

Hesse, Michael; Kuznetsova, Masha; Birn, Joachim [NASA Goddard Space Flight Center, Greenbelt, Maryland 20771 (United States); Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2004-12-01

420

Signature of Coherent Transport in Epitaxial Spinel-based Magnetic Tunnel Junctions Probed by Shot Noise Measurement  

E-print Network

We measured the shot noise in fully epitaxial Fe/MgAl2OX/Fe-based magnetic tunneling junctions (MTJs). While the Fano factor to characterize the shot noise is very close to unity in the antiparallel configuration, it is reduced to 0.98 in the parallel configuration. This observation shows the sub-Poissonian process of electron tunneling in the parallel configuration, indicating the coherent tunneling through the spinel-based tunneling barrier of the MTJ.

Tanaka, Takahiro; Chida, Kensaku; Nishihara, Yoshitaka; Chiba, Daichi; Kobayashi, Kensuke; Ono, Teruo; Sukegawa, Hiroaki; Kasai, Shinya; Mitani, Seiji

2012-01-01