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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 copper oxide superconductors  

SciTech Connect

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

Ferrari, M.J.

1991-11-01

3

Magnetic-Field Dependence of the Noise in a Magnetoresistive Sensor Having MEMS Flux Concentrators  

Microsoft Academic Search

We report the dc and ac magnetic field dependence of the low-frequency noise in a microelectromechanical system (MEMS) flux concentrator device containing a giant magnetoresistance spin valve (SV). The noise is dominated by resistance fluctuations having a magnetic origin. Under nominally zero magnetic-field biasing conditions, the noise power is large and varies rapidly with small changes in magnetic field. Metastability

Arif Ozbay; E. R. Nowak; A. S. Edelstein; G. A. Fischer; C. A. Nordman; Shu Fan Cheng

2006-01-01

4

Magnetic Flux Noise in dc SQUIDs: Temperature and Geometry Dependence S. M. Anton,1  

E-print Network

Magnetic Flux Noise in dc SQUIDs: Temperature and Geometry Dependence S. M. Anton,1 J. S. BirenbaumĂ? of magnetic flux noise in ten dc superconducting quantum interference devices (SQUIDs) with systematically of the outer dimension of a given SQUID. These results are incompatible with a model based on the random

Clarke, John

5

Magnetic flux noise in q-bits and SQUIDS from primordial density fluctuations  

E-print Network

The understanding of the origin of $1/f$ magnetic flux noise commonly observed in superconducting devices such as SQUIDS and q-bits is still a major unsolved puzzle. Here we show that a significant part of the flux noise in superconducting Josephson devices may have a cosmological origin: As the Earth moves through a surrounding axionic dark matter condensate that has conserved the primordial power spectrum of density fluctuations, spatial fluctuations of the axion misalignment angle generate mirror temporal fluctuations of the superconducting phase difference in the junction. Our theory predicts the existence of flux noise with a power spectrum given by an $1/f^{2-n_s}$ spectrum, where $n_s=0.96$ is the spectral index of the near-scale invariant primordial density fluctuations. For the typical amplitude of this cosmologically generated universal flux noise we theoretically calculate the average value $\\delta \\Phi /\\Phi_0 =3.37 \\cdot 10^{-6}$ at 1Hz. These theoretical predictions are in excellent agreement with recent low-frequency flux noise measurements of various experimental groups. Magnetic flux noise, so far mainly considered as a nuisance for electronic devices, may thus contain valuable information about fluctuation spectra in the very early universe.

Christian Beck

2014-09-16

6

Magnetic flux noise in q-bits and SQUIDS from primordial density fluctuations  

E-print Network

The understanding of the origin of $1/f$ magnetic flux noise commonly observed in superconducting devices such as SQUIDS and q-bits is still a major unsolved puzzle. Here we show that a significant part of the flux noise in superconducting Josephson devices may have a cosmological origin: As the Earth moves through a surrounding axionic dark matter condensate that has conserved the primordial power spectrum of density fluctuations, spatial fluctuations of the axion misalignment angle generate mirror temporal fluctuations of the superconducting phase difference in the junction. Our theory predicts the existence of flux noise with a power spectrum given by an $1/f^{2-n_s}$ spectrum, where $n_s=0.96$ is the spectral index of the near-scale invariant primordial density fluctuations. For the typical amplitude of this cosmologically generated universal flux noise we theoretically calculate the average value $\\delta \\Phi /\\Phi_0 =3.37 \\cdot 10^{-6}$ at 1Hz. These theoretical predictions are in excellent agreement wi...

Beck, Christian

2014-01-01

7

Minimizing 1/f Noise in Magnetic Sensors Using MEMS Flux Concentrators  

NASA Astrophysics Data System (ADS)

The 1/f noise of new types of magnetoresistance sensors based on GMR and MTJ limits their sensitivities at low frequencies. Our approach for dealing with this problem is to shift the operating frequency to higher frequencies where the 1/f noise is much lower. The shift is accomplished by placing flux concentrators on MEMS flaps. Springs connecting the flaps are used to establish the proper normal mode. The motion of the MEMS structure, driven to oscillate at 15 kHz by electrostatic comb drives, modulates the field at the position of the sensor. The device was fabricated using SOI wafers, deep reactive ion etching (DRIE), and flip chip bonding. The motion of the permalloy on the MEMS flaps modulates the field by a factor of 2. Driving the motion only requires microwatts of power. Noise measurements indicate that the device is likely to increase the sensitivity of many magnetic sensors at low frequencies by orders of magnitude.

Edelstein, Alan; Fischer, Greg; Pedersen, Michael; Bernard, William; Cheng, Shu-Fan; Nowak, Edmund

2006-03-01

8

Minimizing 1\\/f Noise in Magnetic Sensors Using MEMS Flux Concentrators  

Microsoft Academic Search

The 1\\/f noise of new types of magnetoresistance sensors based on GMR and MTJ limits their sensitivities at low frequencies. Our approach for dealing with this problem is to shift the operating frequency to higher frequencies where the 1\\/f noise is much lower. The shift is accomplished by placing flux concentrators on MEMS flaps. Springs connecting the flaps are used

Alan Edelstein; Greg Fischer; Michael Pedersen; William Bernard; Shu-Fan Cheng; Edmund Nowak

2006-01-01

9

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

10

Magnetic flux compression  

NASA Astrophysics Data System (ADS)

Magnetic Flux Compression, as treated in this paper, is accomplished by high explosives. Flux is first captured in a closed conducting circuit, of which some or all of the conducting elements are overlaid with high explosives. Upon detonation of the explosives, these elements are driven in such a fashion as to compress the flux into regions of smaller areas or, in engineering terminology, into regions of lower inductance. The magnetic energy associated with the flux is increased by the flux compression. The additional energy is ultimately supplied by the explosive as it drives the conductors against the magnetic field pressure, which in some cases may be in the megabar range. Various names in common use for flux compression devices are flux compression generators (FCG), magneto-cumulative generators (MCG), particularly in the USSR, or simply, flux compressors. FCGs are generally used in two broadly defined categories: as compact, high-power sources to drive various loads and as generators of very large magnetic fields. In this talk, general principles of flux compression are first discussed. This is followed by a description of several applications in which different types of FCGs are used to supply pulsed power to various devices.

Fowler, C. M.

1989-08-01

11

Modeling Magnetic Flux Ropes  

NASA Astrophysics Data System (ADS)

The magnetic configuration hosting prominences can be a large-scale helical magnetic flux rope. As a necessary step towards future prominence formation studies, we report on a stepwise approach to study flux rope formation. We start with summarizing our recent three-dimensional (3D) isothermal magnetohydrodynamic (MHD) simulation where a flux rope is formed, including gas pressure and gravity. This starts from a static corona with a linear force-free bipolar magnetic field, altered by lower boundary vortex flows around the main polarities and converging flows towards the polarity inversion. The latter flows induce magnetic reconnection and this forms successive new helical loops so that a complete flux rope grows and ascends. After stopping the driving flows, the system relaxes to a stable helical magnetic flux rope configuration embedded in an overlying arcade. Starting from this relaxed isothermal endstate, we next perform a thermodynamic MHD simulation with a chromospheric layer inserted at the bottom. As a result of a properly parametrized coronal heating, and due to radiative cooling and anisotropic thermal conduction, the system further relaxes to an equilibrium where the flux rope and the arcade develop a fully realistic thermal structure. This paves the way to future simulations for 3D prominence formation.

Xia, Chun; Keppens, Rony

2014-01-01

12

Magnetic flux compression  

SciTech Connect

Magnetic Flux Compression, as treated in this paper, is accomplished by high explosives. Flux is first captured in a closed conducting circuit, of which some or all of the conducting elements are overlaid with high explosives. Upon detonation of the explosives, these elements are driven in such a fashion as to compress the flux into regions of smaller areas or, in engineering terminology, into regions of lower inductance. The magnetic energy associated with the flux is increased by the flux compression. The additional energy is ultimately supplied by the explosive as it drives the conductors against the magnetic field pressure, which in some cases may be in the megabar range. Various names in common use for flux compression devices are flux compression generators (FCG), magneto-cumulative generators (MCG), particularly in the USSR, or simply, flux compressors. FCGs are generally used in two broadly defined categories: as compact, high-power sources to drive various loads: and as generators of very large magnetic fields. In this talk, general principles of flux compression are first discussed. This is followed by a description of several applications in which different types of FCGs are used to supply pulsed power to various devices. The talk closes with a discussion of results obtained from a number of experiments done to explore the properties of materials in very large magnetic fields or under nearly isentropic compression. As requested, the work reported here surveys the Los Alamos program. However, sources cited in the bibliography contain much of the extensive literature in the field. Individual papers cited have been selected partly to highlight other groups that have been active in the field. 25 refs., 15 figs.

Fowler, C.M.

1989-01-01

13

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

14

Magnetic flux transport by dipolarizing flux bundles  

NASA Astrophysics Data System (ADS)

dipolarizing flux bundle (DFB) is a small magnetotail flux tube (typically < ~3 RE in XGSM and YGSM) with a significantly more dipolar magnetic field than its background. Dipolarizing flux bundles typically propagate earthward at a high speed from the near-Earth reconnection region. Knowledge of a DFB's flux transport properties leads to better understanding of near-Earth (X = -6 to -30 RE) magnetotail flux transport and thus conversion of magnetic energy to kinetic and thermal plasma energy following magnetic reconnection. We explore DFB properties with a statistical study using data from the Time History of Events and Macroscale Interactions during Substorms mission. To establish the importance of DFB flux transport, we compare it with transport by bursty bulk flows (BBFs) that typically envelop DFBs. Because DFBs coexist with flow bursts inside BBFs, they contribute >65% of BBF flux transport, even though they last only ~30% as long as BBFs. The rate of DFB flux transport increases with proximity to Earth and to the premidnight sector, as well as with geomagnetic activity and distance from the neutral sheet. Under the latter two conditions, the total flux transport by a typical DFB also increases. Dipolarizing flux bundles appear more often during increased geomagnetic activity. Since BBFs have been previously shown to be the major flux transporters in the tail, we conclude that DFBs are the dominant drivers of this transport. The occurrence rate of DFBs as a function of location and geomagnetic activity informs us about processes that shape global convection and energy conversion.

Liu, Jiang; Angelopoulos, V.; Zhou, Xu-Zhi; Runov, A.

2014-02-01

15

Fast Flux Test Facility noise data management  

SciTech Connect

An extensive collection of spectra from an automated data collection system at the Fast Flux Test Facility has features from neutron data extracted and managed by database software. Inquiry techniques, including screening, applied to database results show the influences of control rods on wideband noise and, more generally, abilities to detect diverse types of off-normal noise. Uncovering a temporary 0.1-Hz resonance shift gave additional diagnostic information on a 13-Hz mechanical motion characterized by the interference of two resonances. The latter phenomenon is discussed generically for possible application to other reactor types.

Not Available

1987-01-01

16

Model for l/f Flux Noise in SQUIDs and Qubits  

SciTech Connect

We propose a model for 1/f flux noise in superconducting devices (f is frequency). The noise is generated by the magnetic moments of electrons in defect states which they occupy for a wide distribution of times before escaping. A trapped electron occupies one of the two Kramers-degenerate ground states, between which the transition rate is negligible at low temperature. As a result, the magnetic moment orientation is locked. Simulations of the noise produced by a plausible density of randomly oriented defects yield 1/f noise magnitudes in good agreement with experiments.

Koch, Roger H.; DiVincenzo, David P.; Clarke, John

2007-01-19

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

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

20

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

NASA Astrophysics Data System (ADS)

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

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

2013-10-01

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 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

22

Basal magnetic flux and the local solar dynamo  

NASA Astrophysics Data System (ADS)

The average unsigned magnetic flux density in magnetograms of the quiet Sun is generally dominated by instrumental noise. Due to the entirely different scaling behavior of the noise and the solar magnetic pattern it has been possible to determine the standard deviation of the Gaussian noise distribution and remove the noise contribution from the average unsigned flux density for the whole 15-yr SOHO/MDI data set and for a selection of SDO/HMI magnetograms. There is a very close correlation between the MDI disk-averaged unsigned vertical flux density and the sunspot number, and regression analysis gives a residual level of 2.7 G when the sunspot number is zero. The selected set of HMI magnetograms, which spans the most quiet phase of solar activity, has a lower limit of 3.0 G to the noise-corrected average flux density. These apparently cycle-independent levels may be identified as a basal flux density, which represents an upper limit to the possible flux contribution from a local dynamo, but not evidence for its existence. The 3.0 G HMI level, when scaled to the Hinode spatial resolution, translates to 3.5 G, which means that the much higher average flux densities always found by Hinode in quiet regions do not originate from a local dynamo. The contributions to the average unsigned flux density come almost exclusively from the extended wings of the probability density function, also in the case of HMI magnetograms with only basal-level magnetic flux. These wings represent intermittent magnetic flux. As the global dynamo continually feeds flux into the small scales at a fast rate through turbulent shredding, a hypothetical local dynamo may only be relevant to the Sun if its rate of flux build-up can be competitive. While the global dynamo appears to dominate the magnetic energy spectrum at all the resolved spatial scales, there are indications from the observed Hanle depolarization in atomic lines that the local dynamo may dominate the spectrum at scales of order 1-10 km and below.

Stenflo, J. O.

2012-11-01

23

One-sided fluxes -- A magnetic curiosity?  

Microsoft Academic Search

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

John C. Mallinson

1973-01-01

24

Low Noise and Smooth Torque Permanent Magnet Propulsion Motors: Comparison of Non-slotted and Slotted Radial and Axial Flux Topologies  

Microsoft Academic Search

In this paper, torque ripple factor (TRF) is introduced to analyze pulsating torque component of surface mounted permanent magnet (PM) machines. Based on the torque and sizing analysis, optimum design can be achieved for minimum ripple torque and maximum torque density. PM pole arc ratio and magnet skew angle are chosen both to minimize the ripple torque and to optimize

Surong Huang; Metin Aydin; Thomas A. Lipo

25

Secular variation of the Sun's magnetic flux  

NASA Astrophysics Data System (ADS)

We present an extension of the model of \\citet{Solanki:etal:2000} that allows us to reconstruct the time evolution of both the total and the open magnetic flux at the solar surface since 1700. The flux emerging in large active regions is determined using the sunspot number as a proxy, while the flux emergence in small ephemeral regions is described by an extended cycle whose amplitude and length are related to the corresponding sunspot cycle. Both types of regions contribute to the open flux, which is the source of the heliospheric field. The overlap of the activity cycles of ephemeral regions leads to a secular variation of the total cycle-related magnetic flux (active region flux + ephemeral region flux + open flux). The model results indicate that the total surface flux has doubled in the first half of the last century. The evolution of the open flux is in good agreement with the reconstruction by \\citet{Lockwood:etal:1999}.

Solanki, S. K.; Schüssler, M.; Fligge, M.

2002-02-01

26

The turbulent twisted magnetic flux tube gas  

NASA Technical Reports Server (NTRS)

The kinematic behavior of a dilute, two-dimensional 'gas' of parallel twisted magnetic flux tubes in a highly conducting fluid is considered. Collisions between flux tubes with the same sense of twist are inelastic due to the reconnection of their oppositely directed azimuthal magnetic field components at impact. In some collisions, the tension in the reconnected magnetic flux is sufficient to bind the two flux tubes together. Collisions between flux tubes with the opposite sense of twist are elastic. It is shown that a tenuous gas of individual twisted flux tubes condenses into two large-scale regions of magnetic field with opposite twists. This calculation illustrates the results of Montgomery and co-workers on the migration of magnetic energy towards small wavenumbers in two-dimensional magnetohydrodynamic (MHD) turbulence. In particular, this problem illustrates the dynamical nonequilibrium of the relaxation phase in turbulent MHD systems.

Bogdan, T. J.

1984-01-01

27

Self-organization in magnetic flux ropes  

NASA Astrophysics Data System (ADS)

This cross-disciplinary special issue on 'Self-organization in magnetic flux ropes' follows in the footsteps of another collection of manuscripts dedicated to the subject of magnetic flux ropes, a volume on 'Physics of magnetic flux ropes' published in the American Geophysical Union's Geophysical Monograph Series in 1990 [1]. Twenty-four years later, this special issue, composed of invited original contributions highlighting ongoing research on the physics of magnetic flux ropes in astrophysical, space and laboratory plasmas, can be considered an update on our state of understanding of this fundamental constituent of any magnetized plasma. Furthermore, by inviting contributions from research groups focused on the study of the origins and properties of magnetic flux ropes in a variety of different environments, we have attempted to underline both the diversity of and the commonalities among magnetic flux ropes throughout the solar system and, indeed, the universe. So, what is a magnetic flux rope? The answer will undoubtedly depend on whom you ask. A flux rope can be as narrow as a few Larmor radii and as wide as the Sun (see, e.g., the contributions by Heli Hietala et al and by Angelous Vourlidas). As described below by Ward Manchester IV et al , they can stretch from the Sun to the Earth in the form of interplanetary coronal mass ejections. Or, as in the Swarthmore Spheromak Experiment described by David Schaffner et al , they can fit into a meter-long laboratory device tended by college students. They can be helical and line-tied (see, e.g., Walter Gekelman et al or J Sears et al ), or toroidal and periodic (see, e.g., John O'Bryan et al or Philippa Browning et al ). They can form in the low plasma beta environment of the solar corona (Tibor Török et al ), the order unity beta plasmas of the solar wind (Stefan Eriksson et al ) and the plasma pressure dominated stellar convection zones (Nicholas Nelson and Mark Miesch). In this special issue, Setthivoine You describes how canonical helicity can determine the result of reconnection and merging of multiple magnetic flux ropes, John Finn et al focus on diagnosing flux rope reconnection using quasi-separatrix layers, and Stefano Markidis et al investigate how a single flux rope can become unstable and begin to fall apart. With these many examples of different magnetized plasma structures, which can all be called magnetic flux ropes, the question still stands: just what is it that makes a volume of magnetized plasma a magnetic flux rope? There may not be a strict definition of a magnetic flux rope that everyone can agree on. Nonetheless, the ingredient common to all magnetic flux ropes is that the magnetic field lines that thread nearby plasma elements at one location along the flux rope must wind around and not diverge away from each other over a sufficiently long distance to look like a piece of an ordinary rope. In a way, it is similar to turbulence—you know it when you see it. The figures and illustrations included in this special issue provide plenty of examples of observed, measured, modeled and imagined magnetic flux ropes for you, the reader, to develop an appreciation of what different members of our research community mean by a magnetic flux rope. If you have never studied magnetic flux ropes, we hope that this special issue inspires you to look into their many mysteries. If magnetic flux ropes are already an integral part of your research, we hope the diversity of perspectives presented herein refresh your interest in the underlying plasma physics of whichever kind of magnetic flux rope you happen to be working with. References [1] Russell C T, Priest E R and Lee L-C 1990 Physics of Magnetic Flux Ropes AGU Geophysical Monograph Series vol 58 (Washington, DC: American Geophysical Union)

Lukin, Vyacheslav S.

2014-06-01

28

Magnetic Flux Through a Superconducting Ring  

Microsoft Academic Search

The observed quantization of magnetic flux enclosed in a superconducting ; annulus is discussed. It is shown that the size of the quantum unit (hc\\/2e) ; implies a pairing effect between the electrons. The suggestion that flux ; quantization is an intrinsic property of electromagnetic fields is analyzed. The ; effect of flux quantization on the observed fact, that the

Lars Onsager

1961-01-01

29

Secular variation of the Sun's magnetic flux  

Microsoft Academic Search

We present an extension of the model of \\\\citet{Solanki:etal:2000} that allows us to reconstruct the time evolution of both the total and the open magnetic flux at the solar surface since 1700. The flux emerging in large active regions is determined using the sunspot number as a proxy, while the flux emergence in small ephemeral regions is described by an

S. K. Solanki; M. Schüssler; M. Fligge

2002-01-01

30

Analysis of Vibration and Acoustic Noise in Permanent Magnet Motors.  

NASA Astrophysics Data System (ADS)

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

Hwang, Sangmoon

31

Theoretic and Experimental Investigation of FluxGate Magnetometer Noise  

Microsoft Academic Search

The problems of calculation and measurement of magnetic noise (MN) are considered, the results of research in this branch are described. It is shown that the amplitude distribution of MN is the more deviating from the normal distribution the wider is the frequency band and the lower is the signal frequency. The rise of MN density in the direction of

R. Berkman

32

Photospheric processes and magnetic flux tubes  

NASA Astrophysics Data System (ADS)

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

Steiner, Oskar

2007-07-01

33

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

34

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

35

Magnetic refrigeration using flux compression in superconductors  

NASA Technical Reports Server (NTRS)

The feasibility of using flux compression in high-temperature superconductors to produce the large time-varying magnetic fields required in a field cycled magnetic refrigerator operating between 20 K and 4 K is presently investigated. This paper describes the refrigerator concept and lists limitations and advantages in comparison with conventional refrigeration techniques. The maximum fields obtainable by flux compression in high-temperature supercoductor materials, as presently prepared, are too low to serve in such a refrigerator. However, reports exist of critical current values that are near usable levels for flux pumps in refrigerator applications.

Israelsson, U. E.; Strayer, D. M.; Jackson, H. W.; Petrac, D.

1990-01-01

36

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

Microsoft Academic Search

Brushless permanent magnet motors have been widely used in home applications and industrial fields. These days, high efficiency and low noise motors are demanded from the view point of environment. Electromagnetic noise and iron loss of the motor are produced by the harmonic fluxes and electromagnetic forces. However, order and space pattern of these have not been discussed in detail.

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

2007-01-01

37

Modeling Magnetic Flux Emergence in HD 106225  

NASA Astrophysics Data System (ADS)

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

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

1996-05-01

38

Magnetic Flux Quantization of the Landau Problem  

NASA Astrophysics Data System (ADS)

Landau problem has a very important application in modern physics, in which two-dimensional electron gas system and quantum Hall effect are outstanding. In this paper, first we review the solution of the Pauli equation, then using the single electron wave function, we calculate moving area expectations of the ideal 2-dimensional electron gas system and the per unit area's degeneracy of the electron gas system. As a result, how to calculate the magnetic flux of the electron gas system is given. It shows that the magnetic flux of 2-dimensional electron gas system in magnetic field is quantized, and magnetic flux quantization results from the quantization of the moving area expectations of electron gas system.

Wang, Jianhua; Li, Kang; Long, Shuming; Yuan, Yi

2014-08-01

39

Flux-Feedback Magnetic-Suspension Actuator  

NASA Technical Reports Server (NTRS)

Flux-feedback magnetic-suspension actuator provides magnetic suspension and control forces having linear transfer characteristics between force command and force output over large range of gaps. Hall-effect devices used as sensors for electronic feedback circuit controlling currents flowing in electromagnetic windings to maintain flux linking suspended element at substantially constant value independent of changes in length of gap. Technique provides effective method for maintenance of constant flux density in gap and simpler than previous methods. Applications include magnetic actuators for control of shapes and figures of antennas and of precise segmented reflectors, magnetic suspensions in devices for storage of angular momentum and/or kinetic energy, and systems for control, pointing, and isolation of instruments.

Groom, Nelson J.

1990-01-01

40

Galactic dynamos supported by magnetic helicity fluxes  

NASA Astrophysics Data System (ADS)

We present a simple semi-analytical model of non-linear, mean field galactic dynamos and use it to study the effects of various magnetic helicity fluxes. The dynamo equations are reduced using the `no-z' approximation to a non-linear system of ordinary differential equations in time; we demonstrate that the model reproduces accurately earlier results, including those where non-linear behaviour is driven by a magnetic helicity flux. We discuss the implications and interplay of two types of magnetic helicity flux, one produced by advection (e.g. due to the galactic fountain or wind) and the other, arising from anisotropy of turbulence as suggested by Vishniac & Cho. We argue that the latter is significant if the galactic differential rotation is strong enough: in our model, for R? <~ -10 in terms of the corresponding turbulent magnetic Reynolds number. We confirm that the intensity of gas outflow from the galactic disc optimal for the dynamo action is close to that expected for normal spiral galaxies. The steady-state strength of the large-scale magnetic field supported by the helicity advection is still weaker than that corresponding to equipartition with the turbulent energy. However, the Vishniac-Cho helicity flux can boost magnetic field further to achieve energy equipartition with turbulence. For stronger outflows that may occur in starburst galaxies, the Vishniac-Cho flux can be essential for the dynamo action. However, this mechanism requires a large-scale magnetic field of at least ~=1?G to be launched, so that it has to be preceded by a conventional dynamo assisted by the advection of magnetic helicity by the fountain or wind.

Sur, Sharanya; Shukurov, Anvar; Subramanian, Kandaswamy

2007-05-01

41

Equilibrium of Twisted Horizontal Magnetic Flux Tubes  

Microsoft Academic Search

The equilibrium of non-force-free twisted horizontal magnetic flux tubes is studied including gravity and an arbitrary pressure perturbation on the tube boundary. To solve this free-boundary problem, we use general nonorthogonal flux coordinates and consider the two-dimensional case in which there is no variation of the physical quantities along the tube axis. For the applications in the convection zone and

T. Emonet; F. Moreno-Insertis

1996-01-01

42

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

43

Torque ripple reduction of axial flux permanent magnet synchronous machine with segmented and laminated stator  

Microsoft Academic Search

Torque ripple is the parasitic element that causes mechanical vibration, acoustic noise and even latent operational failure in permanent magnet (PM) machines. Consequently, considerable design efforts should be made to minimize torque ripple of the machines for high performance applications. In this paper, one cost effective torque ripple reduction technique is proposed and investigated for an axial flux permanent magnet

W. Fei; P. C. K. Luk

2009-01-01

44

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

45

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

46

Magnetic Barkhausen Noise in quenched carburized steels  

NASA Astrophysics Data System (ADS)

Steels with different carbon content, 0.11%C and 0.48%C were submitted to a heat treatment for carburization in the surface. The samples were analyzed after several types of heat treatment, including quenching for producing martensite. The Magnetic Barkhausen Noise (MBN) is directly related to the microstructure. Samples with lower carbon content, have ferrite, a constituent where domain walls can move freely and present higher amplitude in the envelope of MBN. It is also found that the MBN peaks are quite distinct for the samples with martensite, which have lower permeability, and the results suggest that domain rotation contributes as mechanism for reversal of magnetization in martensite. The results also indicate that MBN is very suitable for monitoring the carburizing heat treatment.

de Campos, M. F.; Franco, F. A.; Santos, R.; da Silva, F. S.; Ribeiro, S. B.; Lins, J. F. C.; Padovese, L. R.

2011-07-01

47

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

48

Magnetic flux evolution in highly shaped plasmas  

NASA Astrophysics Data System (ADS)

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

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

2001-06-01

49

Magnetic merging in colliding flux tubes  

NASA Technical Reports Server (NTRS)

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

Zweibel, Ellen G.; Rhoads, James E.

1995-01-01

50

Magnetic Merging in Colliding Flux Tubes  

E-print Network

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

Ellen G. Zweibel; James E. Rhoads

1994-08-14

51

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

52

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

53

Effects of MHD slow shocks propagating along magnetic flux tubes in a dipole magnetic field  

Microsoft Academic Search

Variations of the plasma pressure in a magnetic flux tube can produce MHD waves evolving into shocks. In the case of a low plasma beta, plasma pressure pulses in the magnetic flux tube generate MHD slow shocks propagating along the tube. For converging magnetic field lines, such as in a dipole magnetic field, the cross section of the magnetic flux

N. V. Erkaev; V. A. Shaidurov; V. S. Semenov; H. K. Biernat

2002-01-01

54

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

55

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

56

The interchange instability of stellar magnetic flux tubes  

Microsoft Academic Search

We investigate the stability of magnetic flux tubes in the surface layers of late-type stars, concentrating on the interchange (fluting) instability. Flux tubes on low gravity stars (log g ≲ 3.5) are generally unaffected by this instability. On stars with higher surface gravity, tubes are stable if their magnetic flux exceeds 1019 - 1021 Mx (the precise value depending on

M. Bunte; S. H. Saar

1993-01-01

57

Devices and methods for redistributing magnetic flux density  

US Patent & Trademark Office Database

Redistributing magnetic flux density within electro-magnetic or permanent magnet devices, as described herein, causes the device to increase its utilization of its magnetic core material and thereby increase its power density (Watts/volume). The preferred embodiment uses magnetic core bias currents, synchronized to the device's magnetizing current, through uniform, longitudinally isolated, magnetic core sections. The preferred embodiment can be complemented with local core bias currents that generate magnetic flux that oppose the incident magnetizing flux in local magnetic core sections with high flux density concentrations such as core corners. An alternative embodiment longitudinally interlaces magnetically isolated core sections of equal magnetic path length and uniform areal cross section. Another alternative embodiment redirects the magnetic flux in spiral wound inductors and transformers to the circumferential direction used in toroids. All magnetic core shapes, materials, and sizes can be modified to accommodate bias currents; however, the tape wound toroidal core featured mostly in transformers and inductors, is the easiest core to modify. Examples of the types of electro-magnetic and permanent magnet devices that benefit from the appropriate application of magnetic flux density redistribution include electrical devices such as transformers, inductors, delay lines, and electromechanical devices such as motors, generators, relays, solenoids, and rail guns.

2011-01-04

58

Magnetic flux leakage inspection of tailor-welded blanks  

Microsoft Academic Search

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

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

2002-01-01

59

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

60

Analysis of flux leakage in a brushless permanent-magnet motor with embedded magnets  

Microsoft Academic Search

This paper analyzes the magnetic characteristics of brushless permanent-magnet motors with embedded magnets. It shows that the flux leakage has a substantial effect on the air gap flux density interacting directly with the armature current to produce torque. The flux leakage parameters ? and ? are expressed analytically in terms of the magnetic material properties and the motor dimensions. They

Wen-Bin Tsai; Ting-Yu Chang

1999-01-01

61

Magnetic Flux Concentration and Zonal Flows in Magnetorotational Instability Turbulence  

NASA Astrophysics Data System (ADS)

Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies to the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.

Bai, Xue-Ning; Stone, James M.

2014-11-01

62

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

63

On magnetic reconnection and flux rope topology in solar flux emergence  

NASA Astrophysics Data System (ADS)

We present an analysis of the formation of atmospheric flux ropes in a magnetohydrodynamic solar flux emergence simulation. The simulation domain ranges from the top of the solar interior to the low corona. A twisted magnetic flux tube emerges from the solar interior and into the atmosphere where it interacts with the ambient magnetic field. By studying the connectivity of the evolving magnetic field, we are able to better understand the process of flux rope formation in the solar atmosphere. In the simulation, two flux ropes are produced as a result of flux emergence. Each has a different evolution resulting in different topological structures. These are determined by plasma flows and magnetic reconnection. As the flux rope is the basic structure of the coronal mass ejection, we discuss the implications of our findings for solar eruptions.

MacTaggart, D.; Haynes, A. L.

2014-02-01

64

Suppression of Magnetic Flux Diffusion in Reduced 3D MHD  

Microsoft Academic Search

The important impact of small scale magnetic fields on self-organization (i.e., dynamo) in MHD turbulence was originally pin-pointed by the observation that magnetic flux diffusion (anomalous resistivity) is drastically reduced in 2D MHD turbulence. This reduction is a consequence of mean square magnetic potential in two dimensions. It is natural, then, to investigate magnetic flux diffusion in 3D reduced MHD;

A. Bayliss; A. S. Ware; P. H. Diamond; E.-J. Kim

1999-01-01

65

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

66

Complexity and Diffusion of Magnetic Flux Surfaces in Anisotropic Turbulence  

NASA Astrophysics Data System (ADS)

The complexity of magnetic flux surfaces is investigated analytically and numerically in static homogeneous magnetic turbulence. Magnetic surfaces are computed to large distances in magnetic fields derived from a reduced magnetohydrodynamic model. The question addressed is whether one can define magnetic surfaces over large distances when turbulence is present. Using a flux surface spectral analysis, we show that magnetic surfaces become complex at small scales, experiencing an exponential thinning that is quantified here. The computation of a flux surface is of either exponential or nondeterministic polynomial complexity, which has the conceptual implication that global identification of magnetic flux surfaces and flux exchange, e.g., in magnetic reconnection, can be intractable in three dimensions. The coarse-grained large-scale magnetic flux experiences diffusive behavior. The link between the diffusion of the coarse-grained flux and field-line random walk is established explicitly through multiple scale analysis. The Kubo number controls both large and small scale limits. These results have consequences for interpreting processes such as magnetic reconnection and field-line diffusion in astrophysical plasmas.

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

2014-04-01

67

Development of Computational Technique for Labeling Magnetic Flux-Surfaces  

NASA Astrophysics Data System (ADS)

In recent Large Helical Device (LHD) experiments, radial profiles of ion temperature, electric field, etc. have been measured in the m/n = 1/1 magnetic island produced by island control coils, where m is the poloidal mode number and n the toroidal mode number. When the plasma transport in radial profiles is numerically analyzed, an average over a magnetic flux-surface in the island is a very useful concept to understand the transport. When averaging, a proper labeling of the flux-surfaces is necessary. In general, it is not easy to label the flux-surfaces in a magnetic field containing the island, compared with the case of a magnetic field configuration having nested flux-surfaces. In the present paper, we have developed a new computational technique to label the magnetic flux-surfaces. This technique uses an optimization algorithm called the simulated annealing method. The flux-surfaces are discerned by using two labels: one is classification of the magnetic field structure, i.e., core, island, ergodic, and outside regions, and the other depends on the value of the toroidal magnetic flux. We have applied this technique to an LHD configuration with the m/n = 1/1 island, and successfully discriminated of the magnetic field structure.

Nunami, Masanori; Kanno, Ryutaro; Satake, Shinsuke; Takamaru, Hisanori; Hayashi, Takaya

68

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

2009-09-23

69

Localization of metal-induced gap states at the metal-insulator interface: Origin of flux noise in SQUIDs and superconducting qubits  

SciTech Connect

The origin of magnetic flux noise in Superconducting Quantum Interference Devices with a power spectrum scaling as 1/f (f is frequency) has been a puzzle for over 20 years. This noise limits the decoherence time of superconducting qubits. A consensus has emerged that the noise arises from fluctuating spins of localized electrons with an areal density of 5 x 10(17)m(-2). We show that, in the presence of potential disorder at the metal-insulator interface, some of the metal-induced gap states become localized and produce local moments. A modest level of disorder yields the observed areal density.

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

2009-10-10

70

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

71

Helicity charging and eruption of magnetic flux from the Sun  

NASA Technical Reports Server (NTRS)

The ejection of helical toroidal fields from the solar atmosphere and their detection in interplanetary space are described. The discovery that solar magnetic fields are twisted and that they are segregated by hemisphere according to their chirality has important implications for the escape process. The roles played by erupting prominences, coronal mass ejections (CME's) and active region (AR) loops in expressing the escape of magnetic flux and helicity are discussed. Sporadic flux escape associated with filament eruptions accounts for less than one-tenth the flux loss. Azimuthal flux loss by CME's could account for more, but the major contributor to flux escape may be AR loop expansion. It is shown how the transfer of magnetic helicity from the sun's interior into emerged loops ('helicity charging') could be the effective driver of solar eruptions and of flux loss from the sun.

Rust, David M.; Kumar, A.

1994-01-01

72

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

73

Controlling the flux dynamics in superconductors by nanostructured magnetic arrays  

NASA Astrophysics Data System (ADS)

In this thesis we investigate theoretically how the critical current jc of nano-engineered mesoscopic superconducting film can be improved and how one can control the dynamics of the magnetic flux, e.g., the transition from flux-pinned to flux-flow regime, using arrays of magnetic nanostructures. In particularly we investigate: (1) Vortex transport phenomena in superconductors with deposited ferromagnetic structures on top, and the influence of the sample geometry on the critical parameters and on the vortex configurations. Changing geometry of the magnetic bars and magnetization of the bars will affect the critical current jc of the superconducting film. Such nanostructured ferromagnets strongly alter the vortex structure in its neighborhood. The influence of geometry, position and magnetization of the ferromagnet (single bar or regular lattice of the bars) on the critical parameters of the superconductor is investigated. (2) Effect of flux confinement in narrow superconducting channels with zigzag-shaped banks: the flux motion is confined in the transverse (perpendicular) direction of a diamond-cell-shape channel. The matching effect for the magnetic flux is found in the system relevantless of boundary condition. We discuss the dynamics of vortices in the samples and vortex pattern formation in the channel. We show how the inclusion of higher-Tc superconductor into the sample can lead to enhanced properties of the system. By adding an external driving force, we study the vortex dynamics. The different dynamic regimes are discussed. They allowed an effective control of magnetic flux in superconductors.

Kapra, Andrey

74

Vortex avalanches and magnetic flux fragmentation in superconductors.  

PubMed

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. PMID:11497845

Aranson, I; Gurevich, A; Vinokur, V

2001-08-01

75

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

76

Magnetic Flux Concentration and Zonal Flows in Magnetorotational Instability Turbulence  

E-print Network

Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the magnetic diffusivity of the MRI turbulence is anisotropic. I...

Bai, Xue-Ning

2014-01-01

77

MAGNETIC FLUX PARADIGM FOR RADIO LOUDNESS OF ACTIVE GALACTIC NUCLEI  

SciTech Connect

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

Sikora, Marek [Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw (Poland); Begelman, Mitchell C., E-mail: sikora@camk.edu.pl, E-mail: mitch@jila.colorado.edu [JILA, University of Colorado and National Institute of Standards and Technology, 440 UCB, Boulder, CO 80309 (United States)

2013-02-20

78

Magnetic Flux Paradigm for Radio Loudness of Active Galactic Nuclei  

NASA Astrophysics Data System (ADS)

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

Sikora, Marek; Begelman, Mitchell C.

2013-02-01

79

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

80

Propagation of axisymmetric disturbances on a twisted magnetic flux tube  

NASA Technical Reports Server (NTRS)

The propagation of axisymmetric disturbances on a twisted magnetic flux tube in an inviscid, incompressible, unstratified atmosphere of infinite electrical conductivity is considered. Dispersion effects due to twist appear quickly only on highly twisted flux tubes for a superposition of the lowest radial eigenmodes and wavelengths comparable to the flux tube radius. These radial profiles become distorted and show focusing effects after propagating on the order of ten flux tube radii. This suggests twist effects on hydromagnetic wave propagation may be important where the flux tubes come up through the surface and expand into the tenuous atmosphere.

Bogdan, T. J.

1984-01-01

81

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

82

A correlative study of magnetic flux transfer in the magnetosphere  

NASA Technical Reports Server (NTRS)

The applied magnetic flux of the southward component of the IMF in GSM coordinates (Bz-) which impinges upon the sunward magnetopause is compared to the time integral of the auroral AL index during 56 intervals within a 3-month period in 1969 when interplanetary records from Heos 1 and Explorer 35 were available. The periods of magnetic activity were those for which Bz was greater than 0 and AL was less than 20gamma at the beginning and end of the interval. It was found that for these intervals, the time integral of the AL index was proportional to the applied magnetic flux with a correlation coefficient of 0.94. In addition, the empirical relationships between magnetic flux transfer, applied southward flux, and the time integral of AL arrived at by Holzer and Slavin (1978) on the basis of expansion and contractions of the forward magnetosphere observed with OGO 5 are reexamined and improved.

Holzer, R. E.; Slavin, J. A.

1979-01-01

83

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

84

Kink Wave Propagation in Thin Isothermal Magnetic Flux Tubes  

NASA Astrophysics Data System (ADS)

We investigated the propagation of kink waves in thin and isothermal expanding flux tubes in cylindrical geometry. By using the method of radial expansion for fluctuating variables we obtained a new kink wave equation. We show that including the radial component of the tube magnetic field leads to cutoff-free propagation of kink waves along thin flux tubes.

Lopin, I. P.; Nagorny, I. G.; Nippolainen, E.

2014-08-01

85

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

NASA Astrophysics Data System (ADS)

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

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

2010-03-01

86

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

87

Calculation of magnetic field noise from high-permeability magnetic shields and conducting objects with simple geometry  

E-print Network

evaluation of magnetic field noise, either from current or magnetization fluctuations, inside enclosures made is obtained from calculation of power loss incurred in the material by a driving magnetic field. Sidles et al., for ex- ample, presented a comprehensive analysis of the spectrum of magnetic field noise from magnetic

Romalis, Mike

88

Space Weather in Magnetic Observatory Noise  

NASA Astrophysics Data System (ADS)

Space weather impacts human activity by degrading satellite operation or disrupting electrical power grids. By exploiting small differences in the time stamp between magnetometer pairs to facilitate data filtering, we find that ground-based magnetic observatories are well suited to measure space weather phenomena, and in particular, high frequency fluctuations known as pulsations. Several of the world's consortium of INTERMAGNET observatories are used in the analyses. They show that pulsation amplitudes attain a maximum near local noon over diurnal periods. Long-term trends in pulsation amplitude correlate well with the solar cycle, with the greatest effect occurring during the waning part of the cycle when the derivative of the number of sunspots attains a maximum rate of decrease. Seasonal variability and total amplitude of the diurnal expression of pulsations depends on latitude. Our study highlights the utility of ground-based observatories to understand solar phenomena and suggests how INTERMAGNET data and protocol could be better tuned to monitor space weather.

Gilder, S. A.; Truong, F.

2012-12-01

89

Magnetic Flux Compression Generator as Future Military Pulsed Power Supply  

Microsoft Academic Search

As a kind of maneuverable pulsed power supply (PPS), the Magnetic Flux Compression Generator (MFCG) can be used in many military fields, but the MFCG concept has not been explored systematically. The definitions of MFCG were given in two ways: the electric circuit model and the magnetic field model, and the latter was indispensable to resolve many technical difficulties. According

Qing-Ao Lv; Bin Lei; Min Gao; Zhi-Yuan Li; Xiao-Ping Chi; He Li

2009-01-01

90

Exact static equilibrium of vertically oriented magnetic flux tubes  

Microsoft Academic Search

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

B. C. Low

1980-01-01

91

Multidimensional MHD Simulations of Waves in Solar Magnetic Flux Tubes  

Microsoft Academic Search

Results of the propagation of non-linear MHD waves in multi-dimensional magnetic flux tubes will be presented. Starting with a potential magnetic field whose strength decreases with height, and which spreads out to give the familiar ``canopy'' shape, the evolution of a photospheric impulse is examined. It will be shown that the wave fronts propagate upward with both speeds and degrees

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

1996-01-01

92

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

93

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

94

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

SciTech Connect

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

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

2010-09-01

95

Detecting magnetic flux distributions in superconductors with polarized x rays  

NASA Astrophysics Data System (ADS)

The magnetic flux distribution arising from a high-Tc superconductor is detected and visualized using polarized x rays. Therefore, we introduce a sensor layer, namely, an amorphous, soft-magnetic Co40Fe40B20 cover layer, providing a large x-ray magnetic circular dichroism (XMCD). Temperature-dependent XMCD spectroscopy on the magnetic layer has been performed. Exploiting the temperature dependence of the critical current density of the superconductor we find a quantitative correlation between the XMCD signal and the in-plane stray field of the superconductor. Magneto-optical Kerr effect experiments on the sensor layer can simulate the stray field of the superconductor and hence verify the correlation. We show that the XMCD contrast in the sensor layer corresponds to the in-plane magnetic flux distribution of the superconductor and can hence be used to image magnetic structures in superconductors.

Stahl, Claudia; Audehm, Patrick; Gräfe, Joachim; Ruoß, Stephen; Weigand, Markus; Schmidt, Mathias; Treiber, Sebastian; Bechtel, Michael; Goering, Eberhard; Schütz, Gisela; Albrecht, Joachim

2014-09-01

96

Structure of sunspot penumbrae - Fallen magnetic flux tubes  

NASA Technical Reports Server (NTRS)

A model is presented of a sunspot penumbra involving magnetic flux tubes that have fallen into the photosphere and float there. An upwelling at the inner end of a fallen tube continuously provides additional gas. This gas flows along and lengthens the tube and is observable as the Evershed flow. Fallen flux tubes may appear as bright streaks near the upwelling, but they become dark filaments further out. The model is corroborated by recent optical high-resolution magnetic data regarding the penumbral filaments, by the 12-micron magnetic measurements relevant to the height of the temperature minimum, and by photographs of the umbra/penumbra boundary.

Wentzel, Donat G.

1992-01-01

97

The Solar Internetwork. I. Contribution to the Network Magnetic Flux  

NASA Astrophysics Data System (ADS)

The magnetic network (NE) observed on the solar surface harbors a sizable fraction of the total quiet Sun flux. However, its origin and maintenance are not well known. Here we investigate the contribution of internetwork (IN) magnetic fields to the NE flux. IN fields permeate the interior of supergranular cells and show large emergence rates. We use long-duration sequences of magnetograms acquired by Hinode and an automatic feature tracking algorithm to follow the evolution of NE and IN flux elements. We find that 14% of the quiet Sun (QS) flux is in the form of IN fields with little temporal variations. IN elements interact with NE patches and modify the flux budget of the NE either by adding flux (through merging processes) or by removing it (through cancellation events). Mergings appear to be dominant, so the net flux contribution of the IN is positive. The observed rate of flux transfer to the NE is 1.5 × 1024 Mx day–1 over the entire solar surface. Thus, the IN supplies as much flux as is present in the NE in only 9-13 hr. Taking into account that not all the transferred flux is incorporated into the NE, we find that the IN would be able to replace the entire NE flux in approximately 18-24 hr. This renders the IN the most important contributor to the NE, challenging the view that ephemeral regions are the main source of flux in the QS. About 40% of the total IN flux eventually ends up in the NE.

Goši?, M.; Bellot Rubio, L. R.; Orozco Suárez, D.; Katsukawa, Y.; del Toro Iniesta, J. C.

2014-12-01

98

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

99

Scaling laws of free magnetic energy stored in a solar emerging flux region  

E-print Network

This Letter reports scaling laws of free magnetic energy stored in a solar emerging flux region which is a key to understanding the energetics of solar active phenomena such as solar flares and coronal mass ejections. By performing 3-dimensional magnetohydrodynamic simulations that reproduce several emerging flux regions of different magnetic configurations, we derive power law relationships among emerged magnetic flux, free magnetic energy and relative magnetic helicity in these emerging flux regions. Since magnetic flux is an observable quantity, the scaling law between magnetic flux and free magnetic energy may give a way to estimate invisible free magnetic energy responsible for solar active phenomena.

Magara, Tetsuya

2014-01-01

100

The origin of noise and magnetic hysteresis in crystalline permalloy ring-core fluxgate sensors  

NASA Astrophysics Data System (ADS)

6-81.3 Mo permalloy, developed in the 1960s for use in high performance ring-core fluxgate sensors, remains the state-of-the-art for permalloy-cored fluxgate magnetometers. The magnetic properties of 6-81.3, namely magnetocrystalline and magnetoelastic anisotropies and saturation induction are all optimum in the Fe-Ni-Mo system. In such polycrystalline permalloy fluxgate sensors a single phenomenon may cause both fluxgate noise and magnetic hysteresis, explain Barkhausen jumps, remanence and coercivity, and avoid domain denucleation. The phenomenon, domain wall reconnection, is presented as part of a theoretical model. In the unmagnetized state a coarse-grain high-quality permalloy foil ideally forms stripe domains, which present at the free surface as parallel, uniformly spaced domain walls that cross the entire thickness of the foil. Leakage flux "in" and "out" of alternating domains is a requirement of the random orientation, grain-by-grain, of magnetic easy axes' angles with respect to the foil free surface. Its magnetostatic energy together with domain wall energy determines an energy budget to be minimized. Throughout the magnetization cycle the free surface domain pattern remains essentially unchanged, due to the magnetostatic energy cost such a change would elicit. Thus domain walls are "pinned" to free surfaces. Driven to saturation, domain walls first bulge then reconnect via Barkhausen jumps to form a new domain configuration this author has called "channel domains", that are attached to free surfaces. The approach to saturation now continues as reversible channel domain compression. Driving the permalloy deeper into saturation compresses the channel domains to arbitrarily small thickness, but will not cause them to denucleate. Returning from saturation the channel domain structure will survive through zero H, thus explaining remanence. The Barkhausen jumps being irreversible exothermic events are sources of fluxgate noise, powered by the energy available from domain wall reconnection. A simplified domain energy model can then provide a predictive relation between ring core magnetic properties and fluxgate sensor noise power. Four properties are predicted to affect noise power, two of which, are well known: saturation total magnetic flux density and magnetic anisotropy. The two additional properties are easy axes alignment and foil thickness. Flux density and magnetic anisotropy are primary magnetic properties determined by an alloy's chemistry and crystalline lattice properties. Easy axes alignment and foil thickness are secondary, geometrical properties related to an alloy's polycrystalline fabric and manufacture. Improvements to fluxgate noise performance can in principle be achieved by optimizing any of these four properties in such a way as to minimize magnetostatic energy. Fluxgate signal power is proportional to B-H loop curvature (d2B/dH2). The degree to which Barkhausen jumps coincide with loop curvature is a measure of noise that accompanies fluxgate signal. B-H loops with significant curvature beyond the open hysteresis loop may be used to advantage to acquire fluxgate signal with reduced noise.

Narod, B. B.

2014-06-01

101

The origin of noise and magnetic hysteresis in crystalline permalloy ring-core fluxgate sensors  

NASA Astrophysics Data System (ADS)

Developed in the 1960s for use in high-performance ring-core fluxgate sensors, 6-81.3 Mo permalloy remains the state of the art for permalloy-cored fluxgate magnetometers. The magnetic properties of 6-81.3, namely magnetocrystalline and magnetoelastic anisotropies and saturation induction, are all optimum in the Fe-Ni-Mo system. In such polycrystalline permalloy fluxgate sensors, a single phenomenon may cause both fluxgate noise and magnetic hysteresis; explain Barkhausen jumps, remanence and coercivity; and avoid domain denucleation. This phenomenon, domain wall reconnection, is presented as part of a theoretical model. In the unmagnetized state a coarse-grain high-quality permalloy foil ideally forms stripe domains, which present at the free surface as parallel, uniformly spaced domain walls that cross the entire thickness of the foil. Leakage flux "in" and "out" of alternating domains is a requirement of the random orientation, grain by grain, of magnetic easy axes' angles with respect to the foil free surface. Its magnetostatic energy together with domain wall energy determines an energy budget to be minimized. Throughout the magnetization cycle the free-surface domain pattern remains essentially unchanged, due to the magnetostatic energy cost such a change would elicit. Thus domain walls are "pinned" to free surfaces. Driven to saturation, domain walls first bulge then reconnect via Barkhausen jumps to form a new domain configuration that I have called "channel domains", which are attached to free surfaces. The approach to saturation now continues as reversible channel domain compression. Driving the permalloy deeper into saturation compresses the channel domains to arbitrarily small thickness, but will not cause them to denucleate. Returning from saturation the channel domain structure will survive through zero H, thus explaining remanence. The Barkhausen jumps, being irreversible exothermic events, are sources of fluxgate noise powered by the energy available from domain wall reconnection. A simplified domain energy model can then provide a predictive relation between ring-core magnetic properties and fluxgate sensor noise power. Four properties are predicted to affect noise power, two of which are well known: saturation total magnetic flux density and magnetic anisotropy. The two additional properties are easy axes alignment and foil thickness. Flux density and magnetic anisotropy are primary magnetic properties determined by an alloy's chemistry and crystalline lattice properties. Easy axes alignment and foil thickness are secondary, geometrical properties related to an alloy's polycrystalline fabric and manufacture. Improvements to fluxgate noise performance can in principle be achieved by optimizing any of these four properties in such a way as to minimize magnetostatic energy. Fluxgate signal power is proportional to B - H loop curvature [d2B/dH2]. The degree to which Barkhausen jumps coincide with loop curvature is a measure of noise that accompanies the fluxgate signal. B - H loops with significant curvature beyond the open hysteresis loop may be used to advantage to acquire the fluxgate signal with reduced noise.

Narod, B. B.

2014-09-01

102

Strong limits on gut monopole fluxes in magnetic universes  

Microsoft Academic Search

We show that any realistic cosmological magnetic field (B ≲ 10-9 Gauss) may accelerate the heavy GUT magnetic monopoles to high velocities (beta ≲ 10-1) for a very small monopole density (n0 ≲ 10-32 cm-3) only, corresponding to an almost undetectable flux: F < 10-23 cm-2 s-1. Any larger monopole density will erode the strength of the magnetic field reducing

Daniele Fargion

1983-01-01

103

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

104

Temperature coefficient improvement for low noise magnetic measurements in LISA  

NASA Astrophysics Data System (ADS)

Previous research with Anisotropic Magnetoresistive sensors (AMR) have shown significant improvements for weak magnetic field applications using dedicated noise reduction techniques in the signal conditioning circuit. However, an important source of error that must be addressed is the thermal dependence of the sensor system, more significant in the AMR sensitivity. The external temperature fluctuations affect the output of the sensors due to the temperature coefficient of the magnetoresistors, which may cause an increase of the estimation of the noise spectral density at low frequencies. Ongoing research using a low noise/low temperature coefficient current source to supply the sensor's bridge enhances the thermal performance of the sensors at the lower end of the LISA bandwidth. Preliminary results are shown in this paper.

Mateos, I.; Diaz-Aguiló, M.; Gibert, F.; Lloro, I.; Lobo, A.; Nofrarias, M.; Ramos-Castro, J.

2012-06-01

105

Quasi-Periodicity in global solar radio flux at metric wavelengths during Noise Storms  

NASA Astrophysics Data System (ADS)

We present observational results from studying the quasi-periodicities in global solar radio flux during periods of enhanced noise storm activity, over durations of ~4 hr a day (`intra-day' variations), observed at 77.5 MHz with the newly commissioned log-periodic array tracking system at the Gauribidanur Radio Observatory. Positional information on the storm centers was obtained with the radio imaging data from the Nan\\c cay Radio Heliograph (NRH), while their active region counterparts on the photosphere (and the overlying chromosphere ) were located from the H? images of the Big Bear Solar Observatory. The quasi-periodicity in flux was found to be 110 min, with the fluctuation in flux being 3(+/-1.5) solar flux units (s.f.u.). The results of such pulsations are interpreted qualitatively as evidence for coronal seismology.

Sundaram, G. A. Shanmugha; Subramanian, K. R.

2004-08-01

106

The Solar Internetwork. I. Contribution to the Network Magnetic Flux  

E-print Network

The magnetic network observed on the solar surface harbors a sizable fraction of the total quiet Sun flux. However, its origin and maintenance are not well known. Here we investigate the contribution of internetwork magnetic fields to the network flux. Internetwork fields permeate the interior of supergranular cells and show large emergence rates. We use long-duration sequences of magnetograms acquired by Hinode and an automatic feature tracking algorithm to follow the evolution of network and internetwork flux elements. We find that 14% of the quiet Sun flux is in the form of internetwork fields, with little temporal variations. Internetwork elements interact with network patches and modify the flux budget of the network, either by adding flux (through merging processes) or by removing it (through cancellation events). Mergings appear to be dominant, so the net flux contribution of the internetwork is positive. The observed rate of flux transfer to the network is 1.5 x 10^24 Mx day^-1 over the entire solar s...

Goši?, Milan; Suárez, David Orozco; Katsukawa, Yukio; Iniesta, Jose Carlos Del Toro

2014-01-01

107

Magnetic clouds, helicity conservation, and intrinsic scale flux ropes  

NASA Technical Reports Server (NTRS)

An intrinsic-scale flux-rope model for interplanetary magnetic clouds, incorporating conservation of magnetic helicity, flux and mass is found to adequately explain clouds' average thermodynamic and magnetic properties. In spite their continuous expansion as they balloon into interplanetary space, magnetic clouds maintain high temperatures. This is shown to be due to magnetic energy dissipation. The temperature of an expanding cloud is shown to pass through a maximum above its starting temperature if the initial plasma beta in the cloud is less than 2/3. Excess magnetic pressure inside the cloud is not an important driver of the expansion as it is almost balanced by the tension in the helical field lines. It is conservation of magnetic helicity and flux that requires that clouds expand radially as they move away from the Sun. Comparison with published data shows good agreement between measured cloud properties and theory. Parameters determined from theoretical fits to the data, when extended back to the Sun, are consistent with the origin of interplanetary magnetic clouds in solar filament eruptions. A possible extension of the heating mechanism discussed here to heating of the solar corona is discussed.

Kumar, A.; Rust, D. M.

1995-01-01

108

The Evolution of Open Magnetic Flux Driven by Photospheric Dynamics  

NASA Technical Reports Server (NTRS)

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

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

2010-01-01

109

Magnetic Flux Ropes of Solar Origin and Helicity Conservation  

NASA Astrophysics Data System (ADS)

This dissertation contains a study of magnetic flux ropes of solar origin including those found on the Sun as well as those ejected from the Sun into the interplanetary space. Conservation of magnetic helicity in the flux ropes restricts the nature of their evolution and provides many insights into their behavior. In chapter 3 a filament is modeled as H? absorbing material sitting in the lower portions of the helical field lines of a flux rope that emerges fully formed from below the solar surface. Smaller flux ropes are forced to accumulate in neutral channels, in the inverse polarity configuration, where they bundle together to form the giant filament flux ropes. This scenario along with the observed hemispherical segregation of magnetic helicity, explains the difference between high latitude quiescent filaments and low latitude active region filaments. A filament oscillation model predicts time periods between 10 and 100 minutes in agreement with observations of Bashkirtsev and Mashnich (1984). In chapter 4 a flux rope model for interplanetary magnetic clouds, is developed that uses conservation of magnetic helicity, flux and mass to explain their evolution and heating. An expanding cloud continuously loses its magnetic energy. Even after accounting for the mechanical losses about 60% of the lost magnetic energy is available to dissipate into heat. The total plasma beta in a cloud approaches a constant value between 0.39 and 0.52. Derived expressions (scaling laws) for the magnetic field strength, radius, density, temperature, and the slope of the plasma velocity profile as functions of distance from the Sun, are in agreement with the cloud data obtained between 0.3 and 4 AU. In chapter 5 the case of solar wind consisting of numerous flux ropes of various sizes and strengths is considered to obtain scaling laws that are in agreement with the observations of Totten et al. (1995). In chapter 6 the evidence for kink instability in sigmoidal flux ropes seen in the X-ray pictures of the solar corona is discussed. The appendix contains an account of my work for the Flare Genesis Experiment.

Kumar, Ashok

1997-07-01

110

Analysis of neutron flux noise due to coolant parameter fluctuations in a VVÉR core  

Microsoft Academic Search

The results of investigations of noise appearing in the signals from direct-charge sensors of the in-reactor monitoring system\\u000a of VVÉR reactors as a result of coolant parameter fluctuations are presented. The calculations and experimental data are used\\u000a to analyze the dependence of the amplitude of the neutron flux oscillations (local intensity) at the locations of sensors\\u000a as a function of

Yu. M. Semchenkov; V. A. Mil’to; A. A. Pinegin; B. E. Shumskii

2007-01-01

111

The interchange instability of stellar magnetic flux tubes  

NASA Astrophysics Data System (ADS)

We investigate the stability of magnetic flux tubes in the surface layers of late-type stars, concentrating on the interchange (fluting) instability. Flux tubes on low gravity stars (log g ? 3.5) are generally unaffected by this instability. On stars with higher surface gravity, tubes are stable if their magnetic flux exceeds 1019 - 1021 Mx (the precise value depending on Teff and log g). Smaller structures on these stars may be stabilized by a sufficient external whirl flow. The magnitude of the required whirl flow shows only weak dependence on Teff but increases sharply with log g. In some stars, the whirl velocities needed for stabilization are larger than extant photo spheric velocities. In these cases, stable tubes are possibly separated into two regimes of permitted fluxes. Consequences for the field structure on the surfaces of late-type stars are discussed.

Bunte, M.; Saar, S. H.

1993-04-01

112

Tubes of Magnetic Flux and Electric Current in Space Physics  

NASA Astrophysics Data System (ADS)

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

Kleman, M.; Robbins, J. M.

2014-04-01

113

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

114

Three-dimensional Prominence-hosting Magnetic Configurations: Creating a Helical Magnetic Flux Rope  

NASA Astrophysics Data System (ADS)

The magnetic configuration hosting prominences and their surrounding coronal structure is a key research topic in solar physics. Recent theoretical and observational studies strongly suggest that a helical magnetic flux rope is an essential ingredient to fulfill most of the theoretical and observational requirements for hosting prominences. To understand flux rope formation details and obtain magnetic configurations suitable for future prominence formation studies, we here report on three-dimensional isothermal magnetohydrodynamic simulations including finite gas pressure and gravity. Starting from a magnetohydrostatic corona with a linear force-free bipolar magnetic field, we follow its evolution when introducing vortex flows around the main polarities and converging flows toward the polarity inversion line near the bottom of the corona. The converging flows bring the feet of different loops together at the polarity inversion line, where magnetic reconnection and flux cancellation happen. Inflow and outflow signatures of the magnetic reconnection process are identified, and thereby the newly formed helical loops wind around preexisting ones so that a complete flux rope grows and ascends. When a macroscopic flux rope is formed, we switch off the driving flows and find that the system relaxes to a stable state containing a helical magnetic flux rope embedded in an overlying arcade structure. A major part of the formed flux rope is threaded by dipped field lines that can stably support prominence matter, while the total mass of the flux rope is in the order of 4-5× 1014 g.

Xia, C.; Keppens, R.; Guo, Y.

2014-01-01

115

On the Dynamics of Emerging Toroidal Magnetic Flux Tubes  

NASA Astrophysics Data System (ADS)

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

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

1992-05-01

116

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

117

The magnetic flux in the quiet sun network  

NASA Technical Reports Server (NTRS)

Direct magnetic measurements are used to confirm that the Ca II K line emission from the quiet sun network does not vary with the 11 year cycle (White and Livingston, 1981). As the K emission intensity is correlated with magnetic field strength, magnetic flux measurements are valid for comparison. Data were taken from the full-disk Mount Wilson daily magnetograms, and the aperture was decreased from 17.5 to 12.5 during the interval 1970 through 1980. Measurements are restricted to the latitude zones centered on + or - 1.7 degrees. The mean total flux in the + or - 15.3 degree zones increased by a factor of 10 between activity minimum and maximum, and the quiet sun flux shows no variation, thus confirming White and Livingston's (1981) result. In order for the amplitude of the quiet sun magnetic flux to remain constant, a balance between increasing and decreasing factors must exist, and the primary factor in this balance is the rate of destruction of the quiet sun flux.

Labonte, B. J.; Howard, R.

1982-01-01

118

Simulations of Emerging Magnetic Flux. I. The Formation of Stable Coronal Flux Ropes  

NASA Astrophysics Data System (ADS)

We present results from three-dimensional visco-resistive magnetohydrodynamic simulations of the emergence of a convection zone magnetic flux tube into a solar atmosphere containing a pre-existing dipole coronal field, which is orientated to minimize reconnection with the emerging field. We observe that the emergence process is capable of producing a coronal flux rope by the transfer of twist from the convection zone, as found in previous simulations. We find that this flux rope is stable, with no evidence of a fast rise, and that its ultimate height in the corona is determined by the strength of the pre-existing dipole field. We also find that although the electric currents in the initial convection zone flux tube are almost perfectly neutralized, the resultant coronal flux rope carries a significant net current. These results suggest that flux tube emergence is capable of creating non-current-neutralized stable flux ropes in the corona, tethered by overlying potential fields, a magnetic configuration that is believed to be the source of coronal mass ejections.

Leake, James E.; Linton, Mark G.; Török, Tibor

2013-12-01

119

Sausage Mode Propagation in a Thick Magnetic Flux Tube  

E-print Network

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 waves.

Pardi, Anabele-Linda; Marcu, Alexandru; Orza, Beniamin

2013-01-01

120

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

121

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.

122

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

123

A magnetic flux leakage NDE system for CANDU feeder pipes  

NASA Astrophysics Data System (ADS)

This work examines the application of different magnetic flux leakage (MFL) inspection concepts to the non destructive evaluation (NDE) of residual (elastic) stresses in CANDURTM reactor feeder pipes. The stress sensitivity of three MFL inspection techniques was examined with flat plate samples, with stress-induced magnetic anisotropy (SMA) demonstrating the greatest stress sensitivity. A prototype SMA testing system was developed to apply magnetic NDE to feeders. The system consists of a flux controller that incorporates feedback from a wire coil and a Hall sensor (FCV2), and a magnetic anisotropy prototype (MAP) probe. The combination of FCV2 and the MAP probe was shown to provide SMA measurements on feeder pipe samples and predict stresses from SMA measurements with a mean accuracy of +/-38MPa.

Mak, Thomas Don

124

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

125

A flux-coupled ac/dc magnetizing device.  

PubMed

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

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

2013-06-01

126

3D simulation of prominence magnetic structure: a helical magnetic flux rope  

NASA Astrophysics Data System (ADS)

The magnetic configuration hosting prominences and their surrounding coronal structure is a key research topic in solar physics. Recent theoretical and observational studies strongly suggest that a helical magnetic flux rope is an essential ingredient to fulfill most of the theoretical and observational requirements for hosting prominences. To understand flux rope formation details and obtain magnetic configurations suitable for future prominence formation studies, we here report on three-dimensional isothermal magnetohydrodynamic simulations including finite gas pressure and gravity. Starting from a magnetohydrostatic corona with a linear force-free bipolar magnetic field, we follow its evolution when introducing vortex flows around the main polarities and converging flows towards the polarity inversion line near the bottom of the corona. The converging flows bring feet of different loops together at the polarity inversion line and magnetic reconnection and flux cancellation happens. Inflow and outflow signatures of the magnetic reconnection process are identified, and the thereby newly formed helical loops wind around pre-existing ones so that a complete flux rope grows and ascends. When a macroscopic flux rope is formed, we switch off the driving flows and find that the system relaxes to a stable state containing a helical magnetic flux rope embedded in an overlying arcade structure. A major part of the formed flux rope is threaded by dipped field lines which can stably support prominence matter.

Xia, Chun; Guo, Yang; Keppens, Rony

127

Isolated Warm Flux Tubes in the Inner Magnetosphere: Agents for Magnetic Flux Transport  

NASA Astrophysics Data System (ADS)

A surprising observation during the initial Saturn Insertion Orbit was the discovery of magnetic flux tubes, apparently isolated from similar plasma conditions, with depressed magnitudes in an otherwise quiet region of the E-ring plasma torus. These were not reported on previous missions (the flybys of Pioneer 11 and Voyagers 1 and 2), apparently as subsequent Cassini observations show, because these flux tubes are rare. At this writing they have been detected on only three passes through the E-ring. The tubes sometimes appear to be twisted, as would be appropriate if they were isolated tubes moving relative to other magnetospheric flux tubes. Also the tubes are inclined to neighboring flux tubes when observed away from the equator. The sense of the inclination is such that these tubes are less stretched and more dipolar than their neighbors. This suggests that these tubes are more buoyant than surrounding flux tubes and indeed electron density measurements with the plasma wave subsystem are greatly reduced. Thus these tubes appear to be the counterpart to the jovian flux tubes seen in the Io torus called depleted flux tubes. However, in the jovian case the flux tubes have an increased rather than decreased field strength. At both planets we interpret these tubes as agents for the return of `emptied' magnetic flux tubes, from which the mass loaded on the tubes has been removed, presumably by reconnection in the more distant magnetotail. The scarcity of these tubes may be a function of the infrequency of reconnection events (substorms?) together with their rapid return to the inner magnetosphere once formed.

Russell, C. T.; Leisner, J. S.; Khurana, K. K.; Dougherty, M. K.; Kurth, W. S.

2005-12-01

128

Isolated Warm Flux Tubes in the Inner Magnetosphere: Agents for Magnetic Flux Transport  

Microsoft Academic Search

A surprising observation during the initial Saturn Insertion Orbit was the discovery of magnetic flux tubes, apparently isolated from similar plasma conditions, with depressed magnitudes in an otherwise quiet region of the E-ring plasma torus. These were not reported on previous missions (the flybys of Pioneer 11 and Voyagers 1 and 2), apparently as subsequent Cassini observations show, because these

C. T. Russell; J. S. Leisner; K. K. Khurana; M. K. Dougherty; W. S. Kurth

2005-01-01

129

Alternative magnetic flux leakage modalities for pipeline inspection  

Microsoft Academic Search

Increasing quality consciousness is placing higher demands on the accuracy and reliability of inspection systems used in defect detection and characterization. Non-destructive 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

G. Katragadda; W. Lord; Y. S. Sun; S. Udpa; L. Udpa

1996-01-01

130

Self-field and magnetic-flux quantum mechanics  

E-print Network

Self-field and quantized magnetic-flux are employed to generate the quantum numbers n, m, and l of atomic physics. Wave-particle duality is shown to be a natural outcome of having a particle and its self-field.

Paul Harris

2005-04-06

131

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

132

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

133

Magnetic flux leakage inspection of tailor-welded blanks  

NASA Astrophysics Data System (ADS)

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

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

2002-02-01

134

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

135

Current evolution in a numerical emerging-magnetic-flux model  

NASA Technical Reports Server (NTRS)

The resistive-MHD equations are numerically solved in two-dimensions for an initial-boundary-value problem which models the emergence of magnetic flux from the photosphere into the corona. As the emergence begins a current sheet forms around the emerging region which separates the emerging region from the overlying coronal magnetic field. This current sheet is the source of the free-magnetic energy in the system, and in the limit of zero resistivity it is a simple tangential discontinuity. However, when the resistivity is finite, reconnection between the magnetic field in the emerging region and the overlying coronal magnetic field ensures, and the subsequent evolution of the enveloping current sheet becomes complex. The overall time history of the current evolution is suggestive of the expected current evolution for the pre-flare, impulsive, and main phases of flares.

Forbes, T.

1985-01-01

136

Solar Magnetic Flux and its Geoeffectiviness During 1700-2000  

NASA Astrophysics Data System (ADS)

Visual auroral observations provide perhaps the longest time series of geomagnetic activity; the oldest observations come from 1000 years ago. We show that the solar irradiance values deduced from C14 observations and auroral occurrence show considerable correlation over the past 1000 years, which indicates that the visual observations made at different locations do provide a quite reliable record of the level of auroral activity. In addition to the traditionally used sunspot number giving the level of solar activity, the model by Solanki et al. (2000) gives the open solar magnetic flux as computed from the sunspot data. These records extend to about 1700. We show results of comparisons of the solar magnetic flux with the variations of geomagnetic aa-index series (1844-2000) and auroral occurrence rate (1700-1980). Their characteristic secular variations as well as the 11-year solar periodicity follow closely the flux model variations. It is shown that during the solar cycle, the flux model implies a systematic delay (2.0 yr) in the flux maximum in respect to the sunspot maximum, which is similar to that detected in the aa-index and auroral occurrence variations.

Pulkkinen, T. I.

2001-05-01

137

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

138

MHD waves on solar magnetic flux tubes - Tutorial review  

NASA Astrophysics Data System (ADS)

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.

139

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

140

Low 1/f flux noise in sputtered YBa{sub 2}Cu{sub 3}O{sub 7{minus}x} bicrystal dc-superconducting quantum interference devices  

SciTech Connect

YBa{sub 2}Cu{sub 3}O{sub 7{minus}x} bicrystal dc-superconducting quantum interference devices (SQUIDs) and single layer magnetometers working at 77 K have been fabricated and analyzed. Samples have been made {ital in situ} by cylindrical magnetron dc sputtering on [001] SrTiO{sub 3} bicrystal substrates. Magnetic flux noise levels and magnetic field sensitivities by standard flux-locked-loop electronics have been measured out on dc-SQUIDs and magnetometers, respectively. Flux noise levels as low as a few {mu}{Phi}{sub 0}Hz{sup {minus}1/2} in the white noise regime have been measured in SQUIDs, whereas the magnetic field sensitivity of the magnetometers was influenced by external sources. A comparison of experimental data with analytical expressions modelling the behavior of high T{sub c} SQUIDs has been carried out, with special reference to the flux-to-voltage transfer parameter and noise characteristics. Finally, a reduction of 1/f noise of the SQUIDs has been achieved by a wide band bias reversal electronics. {copyright} {ital 1997 American Institute of Physics.}

Sarnelli, E.; Camerlingo, C.; Russo, M. [Istituto di Cibernetica del Consiglio Nazionale delle Ricerche, I-80072 Arco Felice (Napoli) (Italy)] [Istituto di Cibernetica del Consiglio Nazionale delle Ricerche, I-80072 Arco Felice (Napoli) (Italy); Torrioli, G.; Castellano, M.G. [Istituto di Elettronica dello Stato Solido del Consiglio Nazionale delle Ricerche, I-00156 Roma (Italy)] [Istituto di Elettronica dello Stato Solido del Consiglio Nazionale delle Ricerche, I-00156 Roma (Italy)

1997-10-01

141

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

142

Application of the flux noise reducing filter for CO2 inverse modelling  

NASA Astrophysics Data System (ADS)

Recent atmospheric remote sensing products from AIRS and GOSAT provide large volume of the observations but with larger errors and variance as compared to in-situ measurements, so efficient noise reduction techniques are required for inverse modeling of the surface fluxes. Inverse models of the atmospheric transport optimize regional or grid resolving surface CO2 fluxes to fit transport model simulation optimally to the observations. The optimization problem appears to be ill-posed so it is usually solved by applying regularization techniques. Most widely used regularization methods apply constraints on flux deviation from prior and/or from adjacent regions of same surface type (land-ocean, vegetation type), and from adjacent time periods. Convenient method for solving the problem of limited dimension is based on singular value decomposition (SVD) of the transport matrix, because it can decompose the solution space into a combination of the independent singular vectors. Introducing a simple constraint on fluxes limits amplitude of the corresponding singular vectors with larger reduction for smaller singular values. However this amplitude reduction is not sufficient in practice for inverse modeling of the regional CO2 fluxes, when we have large underconstrained regions in tropics. Alternatively other means of the amplitude reduction are also used, such as truncation, when all amplitudes below threshold singular value are set to zero. We apply a filter which is less abrupt is less abrupt compared to truncation but still suppressing strongly small singular value related vectors. Setting strength of a constraint is often done empirically. To decide a proper value of the cut-off singular value we suggest analyzing a dependence of the singular vector amplitude vs the singular value and set the cut-off value aiming at retaining most of useful information from observation. A graphical tool based on a plot of amplitude spectra is proposed. Advantage of the technique is demonstrated by applying it to optimization of the model-simulated CO2 seasonality at the Globalview observation network using Transcom-3 simulated transport matrixes. As compared to using a Transcom-3 constraint our filter produces smoother seasonal variations for fluxes with a minor impact on model fit with the observations. At the same time, retrieved flux variability at the regions with weak signal at observation sites is strongly suppressed. Applying a filter combined with iterative optimization procedure leads to faster convergence to the optimal flux solution compared to use of the SVD for larger size problems such as batch inversion of the interannually varying regional fluxes. The approach is extended to iterative optimization of the large dimension problems involving use of the adjoint operator of the atmospheric transport instead of precalculated transport matrix.

Maksyutov, Shamil; Yaremchuk, Alexey

2010-05-01

143

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

144

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

145

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

146

Quantifying the dynamic evolution of individual arched magnetic flux tubes  

NASA Astrophysics Data System (ADS)

Highly dynamic arched ‘loops’ of plasma were created in the laboratory with a magnetized plasma gun. The magnetic structure of the loops was found to be consistent with that of an expanding flux tube subject to a kink instability. High-speed flows were found to transport plasma along the loop axis, from both footpoints toward the apex of the arched loop. Two complementary MHD models were used to explain the expansion and axial flows, both of which scale in proportion to a ‘toroidal Alfven speed’.

Stenson, E. V.; Bellan, P. M.

2012-12-01

147

Buoyant magnetic flux tubes enhance radiation in Z pinches  

PubMed

In numerous experiments, magnetic energy coupled to strongly radiating Z-pinch plasmas exceeds the thermalized kinetic energy, sometimes by a factor of 2-3. We demonstrate that the enhanced energy coupling may be due to the buoyancy rise of toroidal magnetic flux tubes converging to the axis through the unstable pinch plasma. We derive an explicit formula for the enhanced dissipation rate and apply this formula to reconsider an old problem of power balance in a steady-state Z pinch, and then to analyze a new challenge of producing K-shell 3 to 10 keV radiation in long-pulse Z-pinch implosions. PMID:11019081

Rudakov; Velikovich; Davis; Thornhill; Giuliani; Deeney

2000-04-10

148

Laser-Driven Magnetic-Flux Compression: Theory and Experiments  

NASA Astrophysics Data System (ADS)

Laser-Driven Flux Compression (LDFC) is a technique used to compress the magnetic field in Inertial Confinement Fusion (ICF) targets driven by a laser. The compressed field in the ICF target is beneficial to the target performance. Embedding a magnetic field in a conventional ICF target reduces the heat loss if the central hot spot becomes magnetized. Higher hot spot temperatures lower the requirements on the implosion velocities, leading to larger shell masses and therefore higher energy gains. For a typical hot spot density of ˜ 10 g/cc, and temperature of ˜ 5 keV, a magnetic field B > 10 MG is required to magnetize the hot spot. Such a strong magnetic field is difficult to be externally generated. Instead of providing the strong magnetic field directly, a seed magnetic field much lower than the required field was provided and compressed by the imploding shell. The field needs to be compressed faster than its diffusion due to the finite resistivity of the fill gas and the shell. This requires the gas in the target being ionized by the shock so that the flux is frozen in the gas region and compressed by the imploding shell. In this thesis, theoretical models, numerical calculations, and basic experiments of flux compression in ICF targets are investigated. A measurable Lawson criterion, developed as a metric to assess the performance of an ICF target, is used to evaluate the benefits of suppressing the heat conductivity. A simple model is used to describe the process of field compression by shock waves during the shell implosion. The magnetohydrodynamics codes, LILAC-MHD and LILAC-MHD-SP, are used to simulate the field compression and the target performance. The Magneto-Inertial-Fusion-Electrical-Discharge-System (MIFEDS), the device providing the seed magnetic field, is described in detail. LDFC experiments using the OMEGA laser at the Laboratory for Laser Energetics are presented. The results include the first demonstration of ˜ 550-fold amplification of a 50 ˜ 60 kG seed field to a ˜ 30 MG compressed field using LDFC, and the first demonstration of 15% and 30 % ion temperature and neutron yield enhancement by compressed magnetic fields.

Chang, Po-Yu

149

Vacuum polarization by a magnetic flux of special rectangular form  

Microsoft Academic Search

We consider the ground state energy of a spinor field in the background of a\\u000asquare well shaped magnetic flux tube. We use the zeta- function regularization\\u000aand express the ground state energy as an integral involving the Jost function\\u000aof a two dimensional scattering problem. We perform the renormalization by\\u000asubtracting the contributions from first several heat kernel coefficients.

I. Drozdov

2002-01-01

150

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

151

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

152

The flux-dependent amplitude of broadband noise variability in X-ray binaries and active galaxies  

E-print Network

Standard shot-noise models, which seek to explain the broadband noise variability that characterises the X-ray lightcurves of X-ray binaries and active galaxies, predict that the power spectrum of the X-ray lightcurve is stationary (i.e. constant amplitude and shape) on short time-scales. We show that the broadband noise power spectra of the black hole candidate Cyg X-1 and the accreting millisecond pulsar SAX J1808.4-3658 are intrinsically non-stationary, in that RMS variability scales linearly with flux. Flux-selected power spectra confirm that this effect is due to changes in power-spectral amplitude and not shape. The lightcurves of three Seyfert galaxies are also consistent with a linear relationship between RMS variability and flux, suggesting that it is an intrinsic feature of the broadband noise variability in compact accreting systems over more than 6 decades of central object mass. The RMS variability responds to flux variations on all measured time-scales, raising fundamental difficulties for shot-noise models which seek to explain this result by invoking variations in the shot parameters. We suggest that models should be explored where the longest time-scale variations are fundamental and precede the variations on shorter time-scales. Possible models which can explain the linear RMS-flux relation include the fractal break-up of large coronal flares, or the propagation of fluctuations in mass accretion rate through the accretion disk. The linear relationship between RMS variability and flux in Cyg X-1 and SAX J1808.4-3658 is offset on the flux axis, suggesting the presence of a second, constant-flux component to the lightcurve which contributes 25% of the total flux. The spectrum of this constant component is similar to the total spectrum [abridged].

Philip Uttley; Ian M. McHardy

2001-03-22

153

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

154

Magnetic flux transport of decaying active regions and enhanced magnetic network. [of solar supergranulation  

NASA Technical Reports Server (NTRS)

Several series of coordinated observations on decaying active regions and enhanced magnetic network regions on the sun were carried out jointly at Big Bear Solar Observatory and at the Huairou Solar Observing Station of the Bejing Astronomical Observatory in China. The magnetic field evolution in several regions was followed closely for three to seven days. The magnetic flux transport from the remnants of decayed active regions was studied, along with the evolution and lifetime of the magnetic network which defines the boundaries of supergranules. The magnetic flux transport in an enhanced network region was studied in detail and found to be negative. Also briefly described are some properties of moving magnetic features around a sunspot. Results of all of the above studies are presented.

Wang, Haimin; Zirin, Harold; Ai, Guoxiang

1991-01-01

155

Magnetic Flux Circulation During Dawn-Dusk Oriented Interplanetary Magnetic Field  

NASA Technical Reports Server (NTRS)

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

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

2010-01-01

156

The effect of flux creep on the magnetization field in the SSC diopole magnets  

SciTech Connect

The sextuple fields of model SSC dipole magnets have been observed to change with time when the magnets are held at constant current under conditions similar to injection into the SSC accelerator. The changes in the sextupole component have close to a linear log time dependence, and is felt to be caused by flux creep decay of the magnetization currents in the superconductor filaments. Measurements of this decay have been made under various conditions. The conditions include various central field inductions and changes of field prior to when the decay was measured. The measured field decay in the dipole's sextupole is proportional to the magnitude and sign of the sextupole due to magnetization which was measured at the start of the decay. This suggests that the decay is a bulk superconductivity flux creep. Proximity coupling appears to play only a minor role in the flux creep according to recent LBL measurements with a stable power supply. 4 refs., 6 figs., 3 tabs.

Gilber, W.S.; Althaus, R.F.; Barale, P.J.; Benjegerdes, R.W.; Green, M.A.; Green, M.I.; Scanlan, R.M. (Lawrence Berkeley Lab., CA (USA))

1989-06-01

157

Dilation of force-free magnetic flux tubes. [solar magnetic field profiles  

NASA Technical Reports Server (NTRS)

A general study is presented of the mapping functions which relate the magnetic-field profiles across a force-free rope in segments subjected to various external pressures. The results reveal that if the external pressure falls below a certain critical level (dependent on the flux-current relation which defines the tube), the magnetic profile consists of an invariant core sheathed in a layer permeated by an azimuthal magnetic field.

Frankenthal, S.

1977-01-01

158

Nonlinear evolution of magnetic flux ropes. 2: Finite beta plasma  

NASA Technical Reports Server (NTRS)

In this second paper on the evolution of magnetic flux ropes we study the effects of gas pressure. We assume that the energy transport is described by a polytropic relationship and reduce the set of ideal MHD equations to a single, second-order, nonlinear, ordinary differential equation for the evolution function. For this conservative system we obtain a first integral of motion. To analyze the possible motions, we use a mechanical analogue -- a one-dimensional, nonlinear oscillator. We find that the effective potential for such an oscillator depends on two parameters: the polytropic index gamma and a dimensionless quantity kappa the latter being a function of the plasma beta, the strength of the azimuthal magnetic field relative to the axial field of the flux rope, and gamma. Through a study of this effective potential we classify all possible modes of evolution of the system. In the main body of the paper, we focus on magnetic flux ropes whose field and gas pressure increase steadily towards the symmetry axis. In this case, for gamma greater than 1 and all values of kappa, only oscillations are possible. For gamma less than 1, however, both oscillations and expansion are allowed. For gamma less than 1 and kappa below a critical value, the energy of the nonlinear oscillator determines whether the flux rope will oscillate or expand to infinity. For gamma less than 1 and kappa above critical, however, only expansion occurs. Thus by increasing kappa while keeping gamma fixed (less than 1), a phase transition occurs at kappa = kappa(sub critical) and the oscillatory mode disappears. We illustrate the above theoretical considerations by the example of a flux rope of constant field line twist evolving self-similarly. For this example, we present the full numerical MHD solution. In an appendix to the paper we catalogue all possible evolutions when (1) either the magnetic field or (2) the gas pressure decreases monotonically toward the axis. We find that in these cases critical conditions can occur for gamma greater than 1. While in most cases the flux rope collapses, there are notable exceptions when, for certain ranges of kappa and gamma, collapse may be averted.

Osherovich, V. A.; Farrugia, C. J.; Burlaga, L. F.

1995-01-01

159

The Helical Kink Instability of Isolated, Twisted Magnetic Flux Tubes  

NASA Astrophysics Data System (ADS)

To understand the dynamics of twisted active region flux tubes below the solar photosphere, we investigate the linear kink stability of isolated, twisted tubes of magnetic flux. We apply linearized equations of MHD to a cylindrical magnetic equilibrium (screw pinch), but with significant differences from earlier work. The magnetic field vanishes outside a radius r = R where it is confined by the higher pressure of the unmagnetized plasma. The outside boundary of the tube is free to move, displacing the unmagnetized plasma as it does so. We concentrate on equilibria where all field lines have the same helical pitch: B?/rBζ = q = const. The main results are as follows. 1. These equilibria are stable, provided that the field line pitch does not exceed a threshold; q ?qcr for stability. The threshold is qcr=(?)˝, where ? is the r2 coefficient in the series expansion of the equilibrium axial magnetic field (B?) about the tube axis (r = 0): B?(r) = BO(1 - ?r2 + ?). When this criterion is violated, there are unstable eigenmodes, ? ? e1(?+kz). The most unstable of these have a helical pitch k which is near (but not equal to) the field line pitch q. 2. For weakly twisted tubes (qR ? 1) we derive growth rates and unstable eigenfunctions analytically. For strongly twisted tubes (qR ?1), we find growth rates and unstable eigenfunctions numerically. 3. The maximum growth rate and range of unstable wavenumbers for a strongly twisted tube can be predicted qualitatively by using the analytical results from the weakly twisted case. The maximum growth rate in that case is given by ?max = ?AR(q2 - q2cr)/3.83, where ?A is the axial Alfvén speed. The range of unstable wavenumbers is (- q - ?k/2) < k <(- q + ?k/2), where ?k = 4qR(q2 -q2cr)˝/3.83. 4. The kink instability we find consists mainly of internal motions. Helical translations of the entire tube are stable. 5. We argue that an emerging, twisted magnetic flux loop will tend to have a uniform q along its length. The increase in the tube radius R as it rises results in a decreasing value of qcr. This means that the apex of the flux loop will become kink unstable before the rest of the tube. 6. Our results suggest that most twisted flux tubes rising through the convection zone will be stable to kinking. Those few tubes which are kink unstable, and which presumably become knotted or kinked active regions upon emergence, only become kink unstable some time after they have begun rising through the convection zone.

Linton, M. G.; Longcope, D. W.; Fisher, G. H.

1996-10-01

160

Magnetic environment and magnetic field standards at NPL for the calibration of low noise magnetometers and gradiometers for cleanliness studies  

Microsoft Academic Search

The magnetic field standards, facilities and capabilities available at NPL for the calibration of magnetometers and gradiometers and the measurement of the magnetic moment and relative magnetic permeability of materials will be presented. This work is performed in the NPL low magnetic field facility. The details of this facility will be explained, how the noise floor is being reduced and

M. J. Hall; S. A. C. Harmon; S. Turner

2012-01-01

161

Measurement of magnetic helicity flux into the solar corona  

NASA Astrophysics Data System (ADS)

We developed a new methodology which can determine magnetic helicity flux across the photosphere based on the magnetograph observation, In order to derive the helicity flux, first the velocity tangential to the solar surface is constructed by applying a correlation tracking technique on the magnetic observation, and secondly the velocity component across the photosphere is derived from the condition that the magnetic evolution must be consistent with the induction equation, Through this procedure, we can determine the helicity flow across the photosphere as a function of time and space, Based on this new method, we analyzed magnetic helicity of the active regions NOAA 9026 and 9077. using the data taken by SOHO/MDI and the vector magnetograph at NAOJ/Tokyo. As a result, it was revealed that positive and negative helicity is supplied in each region simultaneously. In particular, temporary activation of the helicity injection of the both signs was observed prior to X-class flare events in the GOES classification. The investigation on the helicity distribution suggests some relationship between flare onset and the increase of complexity in the helicity structure.

Kusano, K.; Maeshiro, T.; Yokoyama, T.; Sakurai, T.

162

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

163

Reconstruction of a magnetic flux rope from THEMIS observations  

NASA Astrophysics Data System (ADS)

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

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

2008-04-01

164

Acoustic waves in random ensembles of magnetic fluxes  

SciTech Connect

To analyze the observational data and provide the appropriate diagnostic procedure for photospheric manifestation of solar oscillations it is necessary to take into account strong inhomogeneity of solar atmosphere with respect to distribution of magnetic fields. We study the collective phenomena in the propagation of acoustic waves and unsteady wave-packets through quite regions, sunspots and plages, including time-dependent response of these regions to solar oscillations, the energy transfer mechanisms, frequency shift effects and reradiation of the acoustic waves in higher layers of atmosphere. We show that the dynamics of differently magnetized regions, their dispersion properties, and their response to the propagation of acoustic waves are completely different. We describe the effects caused by the specific distribution and randomness of magnetic flux tubes, which can be observed and which can provide the tools for diagnostic goals.

Ryutova, M.P.

1995-10-10

165

Magnetic flux compression for high voltage pulse applications  

NASA Astrophysics Data System (ADS)

Helical Magnetic Flux Compression Generators (MFCG) are the most promising energy sources with respect to their current amplification and compactness. However, the main concern is their intrinsic flux loss that limits severely their performance and which is not yet well understood. All flux losses have a differing degree of impact, depending on the generator's volume, current and energy amplification, size of the driven load, and angular frequency of armature-helix contact point. Although several computer models have been developed, none of them truly quantify, the ohmic and intrinsic flux losses. This dissertation describes a novel method that provides a separate calculation of intrinsic flux losses (flux that is left behind in the conductors and lost for compression) and ohmic losses. I also provided a second method that uses a simple flux quantification, making a mathematical connection between the intrinsic flux losses, quantified by the first method, and the intrinsic flux losses observed in the generators. This second method can also be used with the first method to a priori estimate the MFCG performance. Simple MFCG with a single helix produce high output energy only into low inductance loads, thus producing several 100kA of current at a voltage level of less than 10kV. Many pulsed power devices require less current but a considerably higher voltage level. For effectively driving a high inductance load of several muH, a multistage MFCG design has been successfully tested with a total length of 250mm, a helix inner diameter of 51mm, achieving an energy gain of ˜13 into a 3muH load. Typical load parameters of electron beam devices are several 100kV operating voltage with an impedance of a few tens of Ohms. Utilizing a multi-stage FCG as primary source for inductive energy storage with opening switch enables the production of voltages in excess of 100's kV. We built an exploding wire fuse with a length of 140mm and 100mm in diameter (including the storage inductor), conditioned to the MFCG described above. We achieved a voltage of ˜43kV directly across the 3muH inductor, and more than 120kV with the fuse opening switch operating into a ˜10Ohm load.

Hernandez Llambes, Juan Carlos

166

Collapse and Fragmentation of Rotating Magnetized Clouds. I. Magnetic Flux - Spin Relation  

E-print Network

We discuss evolution of the magnetic flux density and angular velocity in a molecular cloud core, on the basis of three-dimensional numerical simulations, in which a rotating magnetized cloud fragments and collapses to form a very dense optically thick core of > 5 times 10 ^10 cm^-3 . As the density increases towards the formation of the optically thick core, the magnetic flux density and angular velocity converge towards a single relationship between the two quantities. If the core is magnetically dominated its magnetic flux density approaches 1.5 (n/5 times 10^10 cm^-3)^1/2 mG, while if the core is rotationally dominated the angular velocity approaches 2.57 times 10^-3, (n/5 times 10^10 cm^-3)^1/2 yr^-1, where n is the density of the gas. We also find that the ratio of the angular velocity to the magnetic flux density remains nearly constant until the density exceeds 5 times 10^10 cm^-3. Fragmentation of the very dense core and emergence of outflows from fragments are shown in the subsequent paper.

Masahiro N Machida; Tomoaki Matsumoto; Kohji Tomisaka; Tomoyuki Hanawa

2005-06-19

167

Simulation of magnetic flux leakage: Application to tube inspection  

NASA Astrophysics Data System (ADS)

The detection of flaws in steel pipes using Magnetic Flux Leakage (MFL) consists in detecting magnetic flux leaks outside the pipe, either with a magnetic sensor or with an induction coil, while the pipe is rotating. In the Vallourec group, many NDT units use MFL for testing ferromagnetic pipes. In order to improve the performances of flaw detection, CEA LIST and the Vallourec Research Aulnoye (VRA) group are collaborating on MFL modelling. The aim is to be able to perform parametric studies thanks to a fast 3D numerical model dedicated to MFL systems. A simplified 2D geometry has already been derived for the development of first simulation tools. When considering the B-H curve of ferromagnetic materials, the non-linear magnetostatic problem can be solved with the generalized boundary element method (BEMG), which comes to the evaluation of two equivalent scalar potentials: the surface charge density and the volume charge density. When applying the Galerkin method for the discretization of integral equations, the particularity of this numerical model lies in the implementation of high order basis functions for the interpolation of the scalar unknowns. This paper presents some first numerical results for the numerical validation of the semi-analytical model.

Prémel, Denis; Fnaeich, E. A.; Djafa, S.; Pichon, L.; Trillon, A.; Bisiaux, B.

2012-05-01

168

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

169

Exchange-coupling-strength effect on the noise from soft magnetic underlayer  

NASA Astrophysics Data System (ADS)

The exchange-coupling-strength effect on the magnetic properties and noise of laminated [FeCo(tnm)/Ru(0.9nm)]nC(5nm) films as a magnetic soft underlayer (SUL) for perpendicular recording was investigated. It is found that the remanent magnetization decreases significantly with increasing the exchange-coupling strength, whereas, the coercivity remains almost unchanged. The spike noise is clearly seen in the single layer films without exchange coupling and its peak value is as high as 250 mV. The spike noise is significantly suppressed by introducing antiferromagnetic coupling and even eliminated in the films with strongest antiferromagnetic coupling. The noise peak value can be as low as 25 mV for the films with strongest coupling. The average noise decreases with increasing exchange-coupling strength, but increases linearly with increasing the Mrt of SUL. The noise-reduction mechanism is also discussed.

Zhou, T. J.; Chen, J. S.; Xu, Y. J.; Zhang, W.; Sun, C. J.; Liu, B.

2005-05-01

170

Magnetic Properties of Metric Noise Storms Associated with Coronal Mass Ejections  

NASA Astrophysics Data System (ADS)

Using Nançay Radioheliograph (NRH) imaging observations, combined with SOHO/Michelson Doppler Imager (MDI) magnetogram observations and coronal magnetic field extrapolation, we studied the magnetic nature of metric noise storms that are associated with coronal mass ejections (CMEs). Four events are selected: the events of 2000 July 14, 2001 April 26, 2002 August 16 and 2001 March 28. The identified noise storm sources cover or partially cover the active regions (ARs), but the centers of storm sources are offset from the ARs. Using extrapolated magnetic field lines, we find that the noise storm sources trace the boundary between the open and closed field lines. We demonstrate that the disappearance of noise storm source is followed by the appearance of the burst source. The burst sources spread on the solar disk and their distributions correspond to the extent of the CME in LASCO C2 field of view. All the SOHO/Extreme Ultraviolet Imaging Telescope (EIT) dimmings associated with noise storm sources are located at the periphery of noise storms where the magnetic lines of force were previously closed and low-lying. When the closed field becomes partially or fully open, the basic configurations of noise storm sources are changed, then the noise storm sources are no longer observed. These observations provide the information that the variations of noise storms manifest the restructuring or reconfiguring of the coronal magnetic field.

Wen, Ya-Yuan; Wang, Jing-Xiu; Zhang, Yu-Zong

2007-04-01

171

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

172

Plasma dynamics on current-carrying magnetic flux tubes  

NASA Technical Reports Server (NTRS)

A 1D numerical simulation is used to investigate the evolution of a plasma in a current-carrying magnetic flux tube of variable cross section. A large potential difference, parallel to the magnetic field, is applied across the domain. The result is that density minimum tends to deepen, primarily in the cathode end, and the entire potential drop becomes concentrated across the region of density minimum. The evolution of the simulation shows some sensitivity to particle boundary conditions, but the simulations inevitably evolve into a final state with a nearly stationary double layer near the cathode end. The simulation results are at sufficient variance with observations that it appears unlikely that auroral electrons can be explained by a simple process of acceleration through a field-aligned potential drop.

Swift, Daniel W.

1992-01-01

173

The heliospheric magnetic flux, solar wind proton flux, and cosmic ray intensity during the coming solar minimum  

NASA Astrophysics Data System (ADS)

Recent papers have linked the heliospheric magnetic flux to the sunspot cycle with good correlation observed between prediction and observation. Other papers have shown a strong correlation between magnetic flux and solar wind proton flux from coronal holes. We combine these efforts with an expectation that the sunspot activity of the approaching solar minimum will resemble the Dalton or Gleissberg Minimum and predict that the magnetic flux and solar wind proton flux over the coming decade will be lower than at any time during the space age. Using these predictions and established theory, we also predict record high galactic cosmic ray intensities over the same years. The analysis shown here is a prediction of global space climate change within which space weather operates. It predicts a new parameter regime for the transient space weather behavior that can be expected during the coming decade.

Smith, Charles W.; McCracken, K. G.; Schwadron, Nathan A.; Goelzer, Molly L.

2014-07-01

174

Quench characteristics of HTSC elements in series-connected flux-lock type SFCLs through magnetic flux-linkage  

Microsoft Academic Search

We investigated simultaneous quench characteristics of high-TC superconducting (HTSC) elements in the flux-lock type superconducting fault current limiters (SFCLs) connected in series through the magnetic flux-linkage. To apply the flux-lock type SFCL into power system, its current and voltage ratings are required to increase, which can be achieved by increasing the number of HTSC elements connected in series and in

S. H. Lim; H. S. Choi

2006-01-01

175

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

Microsoft Academic Search

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

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

2003-01-01

176

SENSITIVITY ENHANCEMENT OF MAGNETIC SENSORS BASED ON METGLAS/PVDF LAMINATES USING THE FLUX CONCENTRATION EFFECT  

E-print Network

magnetic field sensors. For comparison, both a custom-made voltage mode read-out IC and a charge mode readSENSITIVITY ENHANCEMENT OF MAGNETIC SENSORS BASED ON METGLAS/PVDF LAMINATES USING THE FLUX for Metglas/Polyvinylidene fluoride (PVDF) laminar composites. The magnetic flux density inside a Metglas

Yener, Aylin

177

Two dimensional electron transport in disordered and ordered distributions of magnetic flux vortices  

Microsoft Academic Search

We have considered the conductivity properties of a two dimensional electron gas (2DEG) in two different kinds of inhomogeneous magnetic fields, i.e.\\\\ a disordered distribution of magnetic flux vortices, and a periodic array of magnetic flux vortices. The work falls in two parts. In the first part we show how the phase shifts for an electron scattering on an isolated

Mads Nielsen

1994-01-01

178

Estimating Total Heliospheric Magnetic Flux from Single-Point in Situ Measurements  

NASA Technical Reports Server (NTRS)

A fraction of the total photospheric magnetic flux opens to the heliosphere to form the interplanetary magnetic field carried by the solar wind. While this open flux is critical to our understanding of the generation and evolution of the solar magnetic field, direct measurements are generally limited to single-point measurements taken in situ by heliospheric spacecraft. An observed latitude invariance in the radial component of the magnetic field suggests that extrapolation from such single-point measurements to total heliospheric magnetic flux is possible. In this study we test this assumption using estimates of total heliospheric flux from well-separated heliospheric spacecraft and conclude that single-point measurements are indeed adequate proxies for the total heliospheric magnetic flux, though care must be taken when comparing flux estimates from data collected at different heliocentric distances.

Owens, M. J.; Arge, C. N.; Crooker, N. U.; Schwardron, N. A.; Horbury, T. S.

2008-01-01

179

On the Magnetic Field Topology and Magnetic Flux Budget of CME-ICME Intercomparison  

NASA Astrophysics Data System (ADS)

Coronal Mass Ejections (CMEs) are explosive events that originate, propagate away from the Sun, and carry along solar material with embedded solar magnetic field. The entire process, especially for certain flare-associated CMEs, can be observed by multiple instrumentations on-board several on-going spacecraft missions. The interplanetary counterparts of CMEs (ICMEs) are often detected in-situ by spacecraft monitoring solar wind conditions, which provides both magnetic field and plasma measurements sampled along the spacecraft path across the ICME structure. All these remote-sensing and in-situ measurements make it possible to perform the intercomparison between the (I)CMEs and their source regions at the Sun. We will follow up a prior study of examining the magnetic field topology of flare-associated CME-ICME events by seeking a quantitative relationship characterized by the magnetic flux budget comparison. We will select recent events during the rising phase of enhanced solar activity, and utilize modern observations from the most recent spacecraft missions, such as the STEREO and SDO missions. Both observational analyses of solar source region flaring characteristics and the corresponding ICME structures will be carried out. Relevant physical quantities will be derived and inter-compared. In particular, we are seeking a quantitative correlation between the magnetic flux contained within an ICME flux rope and that injected during the origination process at the Sun to validate or possibly update our previous findings.

Hu, Q.; Qiu, J.; Fink, D. J.; Zheng, J.

2012-12-01

180

Micro-magnetic imprinting of high field gradient magnetic flux sources  

NASA Astrophysics Data System (ADS)

We report here on the fabrication of hard magnetic powder based micro-flux sources using micro-patterned hard magnetic films as templates or master structures. The micro-magnetic imprinting (?MI) process is simple and the constituent materials of the final structures, commercial hard magnetic powders and polymer, are inexpensive. The structures may be transparent, and either flexible or rigid, depending on the choice of polymer matrix used. The peak-to-peak intensity of the z-component of the stray magnetic field measured above a test ?MI structure made with spherical NdFeB particles of average particle size 16 ?m is in good agreement with simulated field values (150 mT at 5 ?m). Simulations indicate magnetic field gradients of up to 5 × 105 T/m at the surface of such ?MI structures. The trapping of cells functionalised with superparamagnetic beads by these structures has been demonstrated. The ?MI fabrication technique has much potential for the development of high field gradient magnetic flux sources for applications in biology and beyond.

Dempsey, N. M.; Le Roy, D.; Marelli-Mathevon, H.; Shaw, Gorky; Dias, A.; Kramer, R. B. G.; Viet Cuong, Le; Kustov, M.; Zanini, L. F.; Villard, C.; Hasselbach, K.; Tomba, C.; Dumas-Bouchiat, F.

2014-06-01

181

Signatures of secondary collisionless magnetic reconnection driven by kink instability of a flux rope  

NASA Astrophysics Data System (ADS)

The kinetic features of secondary magnetic reconnection in a single flux rope undergoing internal kink instability are studied by means of three-dimensional particle-in-cell simulations. Several signatures of secondary magnetic reconnection are identified in the plane perpendicular to the flux rope: a quadrupolar electron and ion density structure and a bipolar Hall magnetic field develop in proximity of the reconnection region. The most intense electric fields form perpendicularly to the local magnetic field, and a reconnection electric field is identified in the plane perpendicular to the flux rope. An electron current develops along the reconnection line, in the opposite direction of the electron current supporting the flux rope magnetic field structure. Along the reconnection line, several bipolar structures of the electric field parallel to the magnetic field occur, making the magnetic reconnection region turbulent. The reported signatures of secondary magnetic reconnection can help to localize magnetic reconnection events in space, astrophysical and fusion plasmas.

Markidis, S.; Lapenta, G.; Delzanno, G. L.; Henri, P.; Goldman, M. V.; Newman, D. L.; Intrator, T.; Laure, E.

2014-06-01

182

Signatures of Secondary Collisionless Magnetic Reconnection Driven by Kink Instability of a Flux Rope  

E-print Network

The kinetic features of secondary magnetic reconnection in a single flux rope undergoing internal kink instability are studied by means of three-dimensional Particle-in-Cell simulations. Several signatures of secondary magnetic reconnection are identified in the plane perpendicular to the flux rope: a quadrupolar electron and ion density structure and a bipolar Hall magnetic field develop in proximity of the reconnection region. The most intense electric fields form perpendicularly to the local magnetic field, and a reconnection electric field is identified in the plane perpendicular to the flux rope. An electron current develops along the reconnection line in the opposite direction of the electron current supporting the flux rope magnetic field structure. Along the reconnection line, several bipolar structures of the electric field parallel to the magnetic field occur making the magnetic reconnection region turbulent. The reported signatures of secondary magnetic reconnection can help to localize magnetic re...

Markidis, S; Delzanno, G L; Henri, P; Goldman, M V; Newman, D L; Intrator, T; Laure, E

2014-01-01

183

Magnetic flux density produced by finite-length twisted-wire pairs  

Microsoft Academic Search

Twisting a wire pair is often used to reduce the wire's low frequency magnetic flux density impinging upon spatially adjacent circuitry. Typically, predictions of the magnetic flux density due to the twisted-wire pair are based upon mathematical formulations for infinite-length wires. In the present work, expressions for predicting the quasistatic magnetic flux density near a finite-length, current-carrying twisted-wire pair are

G. R. Piper

1996-01-01

184

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

185

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

186

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

187

Elimination of residual signals and reduction of noise in a low-frequency magnetic fiber sensor  

Microsoft Academic Search

Low-frequency sensitivities of interferometric magnetic fiber optic sensors have been limited by the presence of a large residual signal at the dither frequency which limits the dynamic range and upconverts low-frequency noise into sideband noise around the carrier. We present an operating regime that enables us to eliminate residual signals associated with the ac carrier. By choosing the proper carrier

Dominique M. Dagenais; Frank Bucholtz; Kee P. Kooa

1988-01-01

188

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

189

Noise  

Microsoft Academic Search

The proliferation of DNA sequence data has generated a concern about the effects of “noise” on phylogeny reconstruction. This concern has led to various recommendations for weighting schemes and for separating data types prior to analysis. A new technique is explored to examine directly how noise influences the stability of parsimony reconstruction. By appending purely random characters onto a matrix

John W Wenzel; Mark E Siddall

1999-01-01

190

Reduction of 1/{ital f} noise in high-{ital T}{sub {ital c}} dc superconducting quantum interference devices cooled in an ambient magnetic field  

SciTech Connect

The spectral density {ital S}{sub {Phi}}({ital f}) of the low-frequency 1/{ital f} noise of high transition temperature dc superconducting quantum interference devices (SQUIDs) with narrow linewidths was independent of {ital B}{sub 0}, the magnetic field in which they were cooled, up to a threshold value, about 33 {mu}T in the best case. Above this threshold, which is associated with the entry of flux vortices into the film, the noise increased rapidly. By contrast, for large square washer SQUIDs, {ital S}{sub {Phi}}({ital f}) scaled linearly with {ital B}{sub 0}. Estimates indicate that the 1/{ital f} flux noise produced by the pickup loop of a directly coupled magnetometer is negligible. {copyright} {ital 1996 American Institute of Physics.}

Dantsker, E.; Tanaka, S.; Nilsson, P.; Kleiner, R.; Clarke, J. [Department of Physics, University of California, Berkeley, California 94720 (United States)] [Department of Physics, University of California, Berkeley, California 94720 (United States); [Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (United States)

1996-12-01

191

Testing of the Mark 101 magnetic flux compression generator  

SciTech Connect

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

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

1986-01-01

192

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

193

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

194

The self-similar, non-linear evolution of rotating magnetic flux ropes  

Microsoft Academic Search

We study, in the ideal MHD approximation, the non-linear evolution of cylindrical magnetic flux tubes differentially rotating about their symmetry axis. Our force balance consists of inertial terms, which include the centrifugal force, the gradient of the axial magnetic pressure, the magnetic pinch force and the gradient of the gas pressure. We employ the separable class of self-similar magnetic fields,

C. J. Farrugia; V. A. Osherovich; L. F. Burlaga

1995-01-01

195

The self-similar, non-linear evolution of rotating magnetic flux ropes  

Microsoft Academic Search

We study, in the ideal MHD approximation, the non-linear evolution of cylindrical magnetic flux tubes differentially rotating about their symmetry axis. Our force balance consists of inertial terms, which include the centrifugal force, the gradient of the axial magnetic pressure, the magnetic pinch force and the gradient of the gas pressure. We employ the “separable” class of self-similar magnetic fields,

C. J. Farrugia; V. A. Osherovich; L. F. Burlaga

1995-01-01

196

Implementation of the Generalized Complementary Flux Constraint for Low-Loss Active Magnetic  

E-print Network

magnetic bearing FWB flywheel battery CMG control moment gyroscope ESCMG energy storage control momentImplementation of the Generalized Complementary Flux Constraint for Low-Loss Active Magnetic on a spacecraft reaction wheel that is magnetically suspended by a low-loss active magnetic bearing (AMB

Tsiotras, Panagiotis

197

Apparatus for study of magnetic-flux discontinuities in ribbon superconductors  

Microsoft Academic Search

An external magnetic field is created by a two-section superconducting magnet, which simulates the magnetic-field configuration that is typical of pancake coils. The maximum radial component of the magnetic field is 2 T. The magnetic-flux discontinuities are recorded by inductive sensors and the magnetic field is recorded by a Hall sensor. The effect of a coating of normal metal and

A. V. Andrianov; Yu. A. Bashkirov; M. G. Kremlev

1987-01-01

198

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

199

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

200

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. Acuńa; N. F. Ness; T. Satoh

1998-01-01

201

DEPLETED MAGNETIC FLUX TUBES AS PROBES OF THE IO TORUS PLASMA  

E-print Network

DEPLETED MAGNETIC FLUX TUBES AS PROBES OF THE IO TORUS PLASMA C. T. Russell1 , M. G. Kivelson1 , W spacecraft detected thin tubes of magnetic flux that had stronger fields than their surroundings indicating that they were depleted in their energy content. These tubes have not been seen on every return to the Io torus

Russell, Christopher T.

202

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

E-print Network

- 1 - Investigation of Flux Linkage Profile Measurement Methods for Switched Reluctance Motors and Permanent Magnet Motors K. Y. Lu, P. O. Rasmussen, A. E. Ritchie Abstract ­ Knowledge of actual flux linkage pure AC flux linkage measurement method is discussed and evaluated, which does not require a search

Lu, Kaiyuan

203

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

204

Numerical simulations of three-dimensional magnetic swirls in a solar flux-tube  

NASA Astrophysics Data System (ADS)

We aim to numerically study evolution of Alfvén waves that accompany short-lasting swirl events in a solar magnetic flux-tube that can be a simple model of a magnetic pore or a sunspot. With the use of the FLASH code we numerically solve three-dimensional ideal magnetohydrodynamic equations to simulate twists which are implemented at the top of the photosphere in magnetic field lines of the flux-tube. Our numerical results exhibit swirl events and Alfvén waves with associated clockwise and counterclockwise rotation of magnetic lines, with the largest values of vorticity at the bottom of the chromosphere, and a certain amount of energy flux.

Chmielewski, Piotr; Murawski, Krzysztof; Solov'ev, Alexandr A.

2014-07-01

205

BUILDUP OF MAGNETIC SHEAR AND FREE ENERGY DURING FLUX EMERGENCE AND CANCELLATION  

SciTech Connect

We examine a simulation of flux emergence and cancellation, which shows a complex sequence of processes that accumulate free magnetic energy in the solar corona essential for the eruptive events such as coronal mass ejections, filament eruptions, and flares. The flow velocity at the surface and in the corona shows a consistent shearing pattern along the polarity inversion line (PIL), which together with the rotation of the magnetic polarities, builds up the magnetic shear. Tether-cutting reconnection above the PIL then produces longer sheared magnetic field lines that extend higher into the corona, where a sigmoidal structure forms. Most significantly, reconnection and upward-energy-flux transfer are found to occur even as magnetic flux is submerging and appears to cancel at the photosphere. A comparison of the simulated coronal field with the corresponding coronal potential field graphically shows the development of non-potential fields during the emergence of the magnetic flux and formation of sunspots.

Fang Fang; Manchester, Ward IV; Van der Holst, Bart [Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109 (United States); Abbett, William P. [Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States)

2012-07-20

206

Measurements and Phenomenological Modeling of Magnetic FluxBuildup in Spheromak Plasmas  

SciTech Connect

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

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

2007-12-14

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

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

NASA Astrophysics Data System (ADS)

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

Brady, Megan; Laramee, Craig

2013-03-01

209

Asymmetric planar gradiometer for rejection of uniform ambient magnetic noise  

SciTech Connect

An asymmetric planar gradiometer for use in making biomagnetic measurements. The gradiometer is formed from a magnetometer which is inductively-coupled to the smaller of two connected loops patterned in a superconducting film which form a flux transformer. The magnetometer is based on a SQUID formed from a high T.sub.c superconducting material. The flux transformer and magnetometer may be formed on separate substrates, allowing the baseline to be increased relative to presently available devices.

Dantsker, Eugene (Torrance, CA); Clarke, John (Berkeley, CA)

2000-01-01

210

Design of Flux-Switching Permanent Magnet Machine Considering the Limitation of Inverter and Flux-Weakening Capability  

Microsoft Academic Search

In this paper, a general design procedure for the flux-switching permanent magnet (FSPM) machine with different topologies is proposed. Firstly, a 3-phase 12-stator-tooth\\/10-rotor-pole topology is introduced and its operation principle is described. Then, the basic design method, including determination of the stator, rotor and magnet dimensions, is proposed under the constraints of some dimensions and electrical parameters. The winding turns

Wei Hua; Ming Cheng; Z. Q. Zhu; D. Howe

2006-01-01

211

Two-dimensional electron transport in the presence of magnetic flux vortices  

Microsoft Academic Search

We have considered the conductivity properties of a two-dimensional electron gas (2DEG) in two different kinds of inhomogeneous magnetic fields, i.e., a disordered distribution of magnetic flux vortices and a periodic array of magnetic flux vortices. The work falls in two parts. In the first part we show how the phase shifts for an electron scattering on an isolated vortex

Mads Nielsen; Per Hedegĺrd

1995-01-01

212

Computation of the magnetic flux in the finite elements method  

NASA Astrophysics Data System (ADS)

For designers, calculation of local fluxes can be very useful. In the vector potential formulation, the local fluxes can be easily deduced. In the scalar potential formulation, the determination of these fluxes presents some difficulties. In this paper, we present three methods to compute a flux through any surface in the scalar potential formulation. These are compared with the one used in the vector potential formulation for two application examples.

Henneron, T.; Clénet, S.; Piriou, F.

2007-08-01

213

Flux trapping in a ring-shaped YBCO bulk by pulsed field magnetization  

Microsoft Academic Search

A pulsed magnetic field can be applied using small coils to generate a strong magnetic field for the magnetization of the high-Tc superconductors (HTS) to be used as quasi-permanent magnets in flywheels and motors. The dynamic electromagnetic behavior of two melt-processed ring-shaped Y-Ba-Cu-O bulks using the pulsed field magnetization (PFM) process has been experimentally investigated and analyzed. The flux trapped

Massimo Fabbri; Pier Giuseppe Albano; Miguel Pretelli; Francesco Negrini; Hiroyuki Ohsaki

2001-01-01

214

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

SciTech Connect

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

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

2010-09-01

215

Magnetic sensor noise impact on the evaluation of magnetorelaxometry experiments  

E-print Network

as their dimensional characteristics. Two particular magnetometers, the fluxgate and the iGMR (improved Giant Magneto performance extrapolation from the characteristics of the magnetometer. Keywords--magnetorelaxometry, figure comparison of the performance of magnetometers taking into account their intrinsic noise as well

Paris-Sud XI, UniversitĂŠ de

216

Magnetic Field-line Twist in Interplanetary Flux Ropes and its Implications for Their Solar Sources  

NASA Astrophysics Data System (ADS)

Interplanetary flux ropes, embedded within interplanetary coronal mass ejections (ICMEs), are often detected in-situ by spacecraft ACE, Wind, and STEREO. Both magnetic field and plasma measurements sampled along the spacecraft path across the ICME structure are available for quantitative analysis. We apply the Grad-Shafranov reconstruction technique to examine the configuration of the flux ropes and to derive relevant physical quantities, such as magnetic flux content, relative magnetic helicity, and the field-line twist. We select recent events during the rising phase of enhanced solar activity, and utilize additional imaging observations from STEREO and SDO spacecraft. Both detailed analyses of solar source region characteristics including flaring and magnetic reconnection sequence, and the corresponding flux rope structures will be presented. In particular, we examine the distribution of magnetic field-line twist in flux ropes on nested cylindrical iso-surfaces of the magnetic flux function. We compare the in-situ characterization of these flux-rope structures with their corresponding solar source region properties. We discuss the implications of such comparison for the origination of flux ropes on the Sun.

Hu, Q.; Qiu, J.

2013-12-01

217

A high-speed induction motor making use of the third harmonic of the magnetic flux  

NASA Astrophysics Data System (ADS)

This paper indicates some possibilities of construction of high-speed induction motors taking advantage of the magnetic flux third harmonic due to a process of magnetization of nonlinear magnetic circuits. Configurations of magnetic frequency triplers, which can be used as basic stator structures and generate a distorted flux, are presented. The paper also describes an experimental high-speed induction motor supplied from a single-phase source via a magnetic frequency tripler that make possible to obtain a rotating field having a synchronous speed equal to 9000 rpm at a supply voltage angular frequency of 314 rd.

Goleman, Ryszard

1994-05-01

218

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

219

Flux tubes, surface magnetism, and the solar dynamo: constraints and open problems  

NASA Astrophysics Data System (ADS)

The flux-tube paradigm has proven to be a remarkably useful tool to understand the connection between the dynamo process in the solar interior and its observable manifestations at the surface. After a brief review of the justification of this approach and of its successes, we discuss in some detail its loose ends and the remaining open questions - and attempt to provide some tentative answers. This includes the origin of fields much stronger than the dynamical equipartition value, the structure of the stored magnetic flux (flux tubes versus a magnetic layer) and the importance of convective pumping, as well as processes connected with flux emergence and the subsequent development of the magnetic flux. It is argued that the observations of the surface field indicate a dynamical disconnection of the emerged flux from its roots in the deep convection zone. Based on the `explosion' of magnetic flux tubes, a scenario and an illustrative model for the disconnection process are suggested. In the last part of the paper, the significance of observed properties of the surface magnetism for constraining solar dynamo models a critically discussed. It is shown that some properties of the butterfly diagram do neither confirm nor refute specific dynamo models. Furthermore, the observed phase relationship between the average toroidal and poloidal magnetic field components is shown to result from the tilt angle of bipolar magnetic regions, so that it imposes no constraint for models of the deep-seated solar dynamo.

Schüssler, M.

2005-04-01

220

Reduced magnetization produced by increased methane flux at a gas hydrate vent  

Microsoft Academic Search

Magnetic susceptibility measurements on near-surface sediment cores from the North Cascadia accretionary sedimentary prism show that seismic blanking or wipe-out zones in the upper few hundred metres of sediments are associated with a prominent low magnetic susceptibility signature. Seismic blanking and low magnetization are both attributed to high upward methane flux within a vent zone, as evidenced by the presence

I. Novosel; G. D. Spence; R. D. Hyndman

2005-01-01

221

HIGH-SPEED AXIAL-FLUX PERMANENT MAGNET MICROMOTORS WITH ELECTROPLATED WINDINGS  

E-print Network

these devices. With regards to the research in permanent-magnet micro- motors, achieving high performanceHIGH-SPEED AXIAL-FLUX PERMANENT MAGNET MICROMOTORS WITH ELECTROPLATED WINDINGS Florian Herrault- plated multi-phase stator windings and their use in a high-speed permanent magnet micromotor

222

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

223

Asymmetry in the current sheet and secondary magnetic flux ropes during guide field magnetic reconnection  

NASA Astrophysics Data System (ADS)

A magnetic reconnection event with a moderate guide field encountered by Cluster in the near-Earth tail on 28 August 2002 is reported. The guide field points dawnward during this event. The quadrupolar structure of the Hall magnetic field within the ion diffusion region is distorted toward the northern hemisphere in the earthward part while toward the southern hemisphere tailward part of X-line. Observations of current density and electron pitch angle distribution indicate that the distorted quadrupolar structure is formed due to a deformed Hall electron current system. Cluster crossed the ion diffusion region from south to north earthward of the X-line. An electron density cavity is confirmed in the northern separatrix layer while a thin current layer (TCL) is measured in the southern separatrix layer. The TCL is formed due to electrons injected into the X-line along the magnetic field. These observations are different from simulation results where the cavity is produced associated with inflow electrons along the southern separatrix while the strong current sheet appears with the outflow electron beam along the northern separatrix. The energy of the inflowing electron in the separatrix layer could extend up to 10 keV. Energetic electron fluxes up to 50 keV have a clear peak in the TCL. The length of the separatrix layer is estimated to be at least 65 c/?pi. These observations suggest that electrons could be pre-accelerated before they are ejected into the X-line region along the separatrix. Multiple secondary flux ropes moving earthward are observed within the diffusion region. These secondary flux ropes are all identified earthward of the observed TCL. These observations further suggest there are numerous small scale structures within the ion diffusion region.

Wang, Rongsheng; Nakamura, Rumi; Lu, Quanming; Du, Aimin; Zhang, Tielong; Baumjohann, Wolfgang; Khotyaintsev, Yuri V.; Volwerk, Martin; André, Mats; Fujimoto, M.; Nakamura, T. K. M.; Fazakerley, Andrew N.; Du, Jian; Teh, Waileong; Panov, Evgeny V.; Zieger, Bertalan; Pan, Yongxin; Lu, San

2012-07-01

224

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

225

Analytical models of axisymmetric toroidal magnetic fields with non-circular flux surfaces  

NASA Astrophysics Data System (ADS)

Axisymmetric toroidal magnetic configurations relevant to current and future tokamak experiments are analytically modelled taking into account an arbitrary aspect ratio, noncircularity and up-down asymmetry of flux surface shapes. It is shown that equilibrium tokamak magnetic fields can be satisfactorily described using only four geometric parameters, which determine the shape and position of the flux surfaces. These are the Shafranov shift, the elongation, and the parameters of triangularity and of up-down asymmetry. To cover the variety of observed flux surface shapes, three different analytical expansions are provided. Comparison of the modelled with the equilibrium flux surfaces in NSTX (Princeton), JET (Abingdon) and TCV (Lausanne) demonstrates good coincidence.

Yavorskij, V. A.; Schoepf, K.; Andrushchenko, Zh N.; Cho, B. H.; Goloborod'ko, V. Ya; Reznyk, S. N.

2001-03-01

226

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

NASA Astrophysics Data System (ADS)

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

Fan, Y.; Gibson, S. E.

2004-05-01

227

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

228

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

229

Design and Calibration of a Compact Low-Noise Magnetic Gradiometer  

NASA Astrophysics Data System (ADS)

The paper describes the design, test and calibration of a compact, low-noise magnetic gradiometer developed under contract to ESA (Contract reference: AO/1- 6085/09/NL/AF) by the Science and Technology Facility Council, Rutherford Appleton Laboratory (RAL Space), Bartington Instruments and the National Physical Laboratory (NPL). The gradiometer is being developed as a technology pathfinder for a diagnostics payload to characterize magnetic disturbances around space instruments susceptible to magnetic fields and gradients such as the candidate Cosmic Vision mission LISA/NGO. The gradiometer is also to be used in the verification of the magnetic cleanliness of spacecraft and spacecraft subsystems.

Griffin, D. K.; Masseglia, O.; Hall, M.; Trougnou, L.; Hewitson, M.; Howe, C.; Olly Pountz-Wright, M.; Leopoldi, L. Ding.; Turner, S.; Harmon, S.

2012-05-01

230

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

E-print Network

To deepen our understanding of the role of small-scale magnetic fields in active regions (ARs) and in the quiet Sun (QS) on the solar irradiance, it is fundamental to investigate the physical processes underlying their continuum brightness. Previous results showed that magnetic elements in the QS reach larger continuum intensities than in ARs at disk center, but left this difference unexplained. We use Hinode/SP disk center data to study the influence of the local amount of magnetic flux on the vigour of the convective flows and the continuum intensity contrasts. The apparent (i.e. averaged over a pixel) longitudinal field strength and line-of-sight (LOS) plasma velocity were retrieved by means of Milne-Eddington inversions (VFISV code). We analyzed a series of boxes taken over AR plages and the QS, to determine how the continuum intensity contrast of magnetic elements, the amplitude of the vertical flows and the box-averaged contrast were affected by the mean longitudinal field strength in the box (which sca...

Kobel, P; Borrero, J M

2014-01-01

231

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

232

Comparison of an axial flux and a radial flux permanent magnet motor for solar race cars  

Microsoft Academic Search

This paper provides analyses, models and a new frame design for the Csiro electric drive for solar cars. The main objective is to determine whether the axial flux Csiro motor (CM) is more efficient than the radial flux Biel motor (BM). To do that, performance tests were conducted and both drives were compared. Results showed that the CM is more

R. Al Zaher; S. de Groot; H. Polinder; P. Wieringa

2010-01-01

233

Vertical magnetic noise in the voice frequency band within and above coal mines. Report of investigations\\/1983  

Microsoft Academic Search

Information on vertical magnetic noise in the voice frequency band, both within and above coal mines, is needed for the evaluation of through-the-earth baseband electromagnetic communications at mines where horizontal loop antennas are used. This report discusses the theory of the source of electromagnetic noise, the propagation of this noise to an observation point above a mine, and its interaction

Durkin

1983-01-01

234

Plasma fluxes to surfaces for an oblique magnetic field  

Microsoft Academic Search

The poloidal and toroidal spatial distributions of Dalpha, 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

C. S. Pitcher; P. C. Stangeby; M. G. Bell; J. D. Elder; S. J. Kilpatrick; D. M. Manos; S. S. Medley; D. K. Owens; A. T. Ramsey; M. Ulrickson

1992-01-01

235

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

236

Observation of full shot noise in CoFeB/MgO/CoFeB-based magnetic tunneling junctions  

NASA Astrophysics Data System (ADS)

The electron transport through the CoFeB/MgO/CoFeB-based magnetic tunneling junction (MTJ) was studied by the shot noise measurement. The obtained Fano factor to characterize the shot noise is very close to unity, indicating the full shot noise, namely, the shot noise in the Schottky limit, both in the parallel and antiparallel magnetization configurations. This means the Poissonian process of the electron tunneling and the absence of the electron-electron correlation in the low bias regime. The shot noise measurements will be a good guideline to make up tunneling criteria for designing MTJ-based spin devices.

Sekiguchi, K.; Arakawa, T.; Yamauchi, Y.; Chida, K.; Yamada, M.; Takahashi, H.; Chiba, D.; Kobayashi, K.; Ono, T.

2010-06-01

237

Modeling magnetic fields measured by surface probes embedded in a cylindrical flux conserver.  

PubMed

Calculating magnetic fields at the surface of a flux conserver, perfect conductor, for displaced plasma currents is useful for understanding modes of a Z-pinch. The magnetic fields measured at the flux conserver are a sum of the magnetic fields from the plasma current and the eddy currents which form in the walls to keep the flux constant. While the magnetic field at the wall from the plasma current alone is easily calculated using the Biot-Savart law, finding the eddy currents in the flux conserver which satisfy the boundary conditions can be a tedious process. A simple method of calculating the surface magnetic field for a given Z-pinch displacement off-axis is derived for a cylindrical flux conserver. This relationship does not require the explicit calculation of the eddy currents, saving time when analyzing surface magnetic probe measurements. Analytic expressions can be used to describe the surface magnetic field which increase the understanding of the magnetic probe measurements. PMID:17411182

Golingo, R P

2007-03-01

238

Defect profile estimation from magnetic flux leakage signal via efficient managing particle swarm optimization.  

PubMed

In this paper, efficient managing particle swarm optimization (EMPSO) for high dimension problem is proposed to estimate defect profile from magnetic flux leakage (MFL) signal. In the proposed EMPSO, in order to strengthen exchange of information among particles, particle pair model was built. For more efficient searching when facing different landscapes of problems, velocity updating scheme including three velocity updating models was also proposed. In addition, for more chances to search optimum solution out, automatic particle selection for re-initialization was implemented. The optimization results of six benchmark functions show EMPSO performs well when optimizing 100-D problems. The defect simulation results demonstrate that the inversing technique based on EMPSO outperforms the one based on self-learning particle swarm optimizer (SLPSO), and the estimated profiles are still close to the desired profiles with the presence of low noise in MFL signal. The results estimated from real MFL signal by EMPSO-based inversing technique also indicate that the algorithm is capable of providing an accurate solution of the defect profile with real signal. Both the simulation results and experiment results show the computing time of the EMPSO-based inversing technique is reduced by 20%-30% than that of the SLPSO-based inversing technique. PMID:24926693

Han, Wenhua; Xu, Jun; Wang, Ping; Tian, Guiyun

2014-01-01

239

Evaluation of leakage flux out of a dental magnetic attachment.  

PubMed

A dental magnetic attachment is a device to retain dental prostheses such as overdentures by magnetic attraction. As compared with mechanical attachments, the magnetic attachment has a superior retention properties due to less lateral pressure to its abutment tooth, and hence it has come to be widely used, particularly for retaining overdentures. Because the dental magnetic attachments are a device used in the mouth, the tissues in the mouth are exposed to the magnetic fields leaking out of the magnetic attachments for a long time. It may therefore be important to discuss biological effects of the leakage magnetic fields. It is required at first to evaluate the strength of the leakage magnetic fields. PMID:18002756

Nishida, M; Tegawa, Y; Kinouchi, Y

2007-01-01

240

Design and calibration of a compact low-noise magnetic gradiometer  

Microsoft Academic Search

The paper describes the design, test and calibration of a compact, low-noise magnetic gradiometer developed under contract to ESA (Contract reference: AO\\/1–6085\\/09\\/NL\\/AF) by the Science and Technology Facility Council, Rutherford Appleton Laboratory (RAL Space), Bartington Instruments and the National Physical Laboratory (NPL). The gradiometer is being developed as a technology pathfinder for a diagnostics payload to characterize magnetic disturbances around

D. K. Griffin; O. Masseglia; M. Hewitson; L. Trougnou; C. Howe; Olly Poyntz-Wright; M. Leopoldi; L Ding; S. Turner; S. Harmon

2012-01-01

241

Computer model simulation of null-flux magnetic suspension and guidance  

SciTech Connect

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

He, Jianliang; Rote, D.M.

1992-06-01

242

Computer model simulation of null-flux magnetic suspension and guidance  

SciTech Connect

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

He, Jianliang; Rote, D.M.

1992-01-01

243

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

Microsoft Academic Search

A strong (~few×105 G) and slightly twisted magnetic field (of flux 1023 Mx) can be stored in a dynamo flux tube within the overshoot region provided that the tube has a slightly lower temperature than the background gas, which has a stratification profile gamma(dlnrho\\/dlnP)-1>=B2\\/8piP, where P, rho, B and gamma are the gas pressure, density, magnetic field strength, and adiabatic

Tzihong Chiueh

2000-01-01

244

Photospheric Magnetic Field: Relationship Between North-South Asymmetry and Flux Imbalance  

NASA Astrophysics Data System (ADS)

Photospheric magnetic fields were studied using the Kitt Peak synoptic maps for 1976 - 2003. Only strong magnetic fields ( B>100 G) of the equatorial region were taken into account. The north-south asymmetry of the magnetic fluxes was considered as well as the imbalance between positive and negative fluxes. The north-south asymmetry displays a regular alternation of the dominant hemisphere during the solar cycle: the northern hemisphere dominated in the ascending phase, the southern one in the descending phase during Solar Cycles 21 - 23. The sign of the imbalance did not change during the 11 years from one polar-field reversal to the next and always coincided with the sign of the Sun's polar magnetic field in the northern hemisphere. The dominant sign of leading sunspots in one of the hemispheres determines the sign of the magnetic-flux imbalance. The sign of the north-south asymmetry of the magnetic fluxes and the sign of the imbalance of the positive and the negative fluxes are related to the quarter of the 22-year magnetic cycle where the magnetic configuration of the Sun remains constant (from the minimum where the sunspot sign changes according to Hale's law to the magnetic-field reversal and from the reversal to the minimum). The sign of the north-south asymmetry for the time interval considered was determined by the phase of the 11-year cycle (before or after the reversal); the sign of the imbalance of the positive and the negative fluxes depends on both the phase of the 11-year cycle and on the parity of the solar cycle. The results obtained demonstrate the connection of the magnetic fields in active regions with the Sun's polar magnetic field in the northern hemisphere.

Vernova, E. S.; Tyasto, M. I.; Baranov, D. G.

2014-08-01

245

Solar Magnetic Flux and Long-term Changes In The Space Weather, 1700-2000  

NASA Astrophysics Data System (ADS)

Some new results are presented about long-term (since the year 1700) behavior of the solar open field magnetic flux and its geoeffectiviness. The time dependent flux model introduced by Solanki et al. (2000) has been applied to the variations of geomagnetic aa-index series (1844-2000) and auroral occurrence rate (1700-1980). Their charac- teristic secular variations (Gleissberg cycle, about 90 yrs) as well as the 11-year solar periodicity follow closely to the flux model variations.

Nevanlinna, H.

246

Electron heat flux dropouts in the solar wind: evidence for interplanetary magnetic field reconnection  

Microsoft Academic Search

Electron heat flux dropout events have been observed in the solar wind using the ISEE 3 plasma electron data set. These events manifest themselves as dropouts of the solar wind halo electrons which are normally found streaming outward along the local magnetic field. These dropouts leave nearly isotropic distributions of solar wind halo electrons, and consequently, the heat flux in

D. J. McComas; J. T. Gosling; J. L. Phillips; S. J. Bame; J. G. Luhmann; E. J. Smith

1989-01-01

247

Perturbation Finite Element Method for Magnetic Model Refinement of Air Gaps and Leakage Fluxes  

E-print Network

ideal flux tubes [3] and thin-shell air-gap models [4]. The developments are performed for the magnetic finite element method based on a perturbation technique. An approximate problem considering ideal flux. The procedure simplifies both meshing and solving processes, and al- lows to quantify the gain given by each

Paris-Sud XI, UniversitĂŠ de

248

Novel Magnetic Geometries to Cure the Divertor Heat Flux Problem for Reactors  

Microsoft Academic Search

A novel magnetic divertor geometry with a second axi-symmetric x-point and flux expansion region along the separatrix is analysed. It can provide a stable, completely detached plasma state compatible with reactor operation; avoiding serious physics and engineering problems: 1) extreme divertor heat fluxes, 2) poor global confinement and high disruptivity due to low edge temperatures, 3) lack of access to

P. Valanju; M. Kotschenreuther; J. C. Wiley; M. Pekker

2004-01-01

249

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

250

Long-term drift of the coronal source magnetic flux and the total solar irradiance  

Microsoft Academic Search

We test the method of Lockwood et al. [1999] for deriving the coronal source flux from the geomagnetic aa index and show it to be accurate to within 12% for annual means and 4.5% for averages over a sunspot cycle. Using data from four solar constant monitors during 1981-1995, we find a linear relationship between this magnetic flux and the

M. Lockwood; R. Stamper

1999-01-01

251

Equations for the motion of an isolated thin magnetic flux tube  

NASA Astrophysics Data System (ADS)

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

Cheng, J.

1992-10-01

252

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

253

1/f noise in magnetic tunnel junctions with MgO tunnel barriers  

NASA Astrophysics Data System (ADS)

Electrical noise measurements are reported for magnetic tunnel junctions having magnesium oxide tunnel barriers. These junctions have resistance-area products (RAPs) of order 10-100 M? ?m2 and exhibit zero-bias tunneling magnetoresistance ratios (TMRs) as high as 120% at room temperature. The TMR is bias dependent and decreases to half its maximum value for biases near 300 mV. The dominant low-frequency electrical noise is due to resistance fluctuations having a 1/f-like power spectral dependence and a nonmagnetic origin. The normalized 1/f noise parameter, ?, is found to be of order 10-7 to 10-6 which compares favorably to magnetic tunnel junctions consisting of an aluminum oxide barrier with comparable RAPs but lower TMR. At high biases, ? is found to decrease which we attribute to defect-assisted tunneling mechanisms.

Gokce, Aisha; Nowak, E. R.; Yang, See Hun; Parkin, S. S. P.

2006-04-01

254

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

255

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

PubMed Central

This letter describes a data acquisition setup for recording, and processing, running speech from a person in a magnetic resonance imaging (MRI) scanner. The main focus is on ensuring synchronicity between image and audio acquisition, and in obtaining good signal to noise ratio to facilitate further speech analysis and modeling. A field-programmable gate array based hardware design for synchronizing the scanner image acquisition to other external data such as audio is described. The audio setup itself features two fiber optical microphones and a noise-canceling filter. Two noise cancellation methods are described including a novel approach using a pulse sequence specific model of the gradient noise of the MRI scanner. The setup is useful for scientific speech production studies. Sample results of speech and singing data acquired and processed using the proposed method are given. PMID:17069275

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

2007-01-01

256

Synchronized and noise-robust audio recordings during realtime magnetic resonance imaging scans.  

PubMed

This letter describes a data acquisition setup for recording, and processing, running speech from a person in a magnetic resonance imaging (MRI) scanner. The main focus is on ensuring synchronicity between image and audio acquisition, and in obtaining good signal to noise ratio to facilitate further speech analysis and modeling. A field-programmable gate array based hardware design for synchronizing the scanner image acquisition to other external data such as audio is described. The audio setup itself features two fiber optical microphones and a noise-canceling filter. Two noise cancellation methods are described including a novel approach using a pulse sequence specific model of the gradient noise of the MRI scanner. The setup is useful for scientific speech production studies. Sample results of speech and singing data acquired and processed using the proposed method are given. PMID:17069275

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

2006-10-01

257

Cogging torque and acoustic noise reduction in permanent magnet motors by teeth pairing  

Microsoft Academic Search

This paper investigates reduction of cogging torque and acoustic noise in permanent magnet motors with larger stator slot opening. Using an energy method with Fourier series expansion, cogging torque is analytically determined with airgap MMF function and airgap permeance function. It shows that there exist several tooth widths which minimize the cogging torque. It also proposes a teeth pairing with

Sang-Moon Hwang; Jae-Boo Eom; Geun-Bae Hwang; Weui-Bong Jeong; Yoong-Ho Jung

2000-01-01

258

Complex Inductance, Excess Noise, and Surface Magnetism in dc SQUIDs S. Sendelbach,1  

E-print Network

Complex Inductance, Excess Noise, and Surface Magnetism in dc SQUIDs S. Sendelbach,1 D. Hover,1 M (Received 12 May 2009; published 9 September 2009) We have characterized the complex inductance of dc SQUIDs cooled to millikelvin temperatures. The SQUID inductance displays a rich, history-dependent structure

Saffman, Mark

259

Noise Removal in Magnetic Resonance Diffusion Tensor and Edward W. Hsu1,2  

E-print Network

taken as the local fiber orientation. In 3D, the diffusion tensor is a rank-2, symmetrical 3 3 matrixNoise Removal in Magnetic Resonance Diffusion Tensor Imaging Bin Chen1 and Edward W. Hsu1,2 * Although promising for visualizing the structure of ordered tis- sues, MR diffusion tensor imaging (DTI

260

An Ultrasensitive CMOS Magnetic Biosensor Array with Correlated Double Counting Noise Suppression  

E-print Network

, Pasadena, California, 91125, USA Abstract -- This paper presents a scalable and ultrasensitive frequency magnetic particles with an SNR of 14.6dB for a single bead and an effective dynamic range of at least 74.5d integrated circuits, integrated circuit noise. I. INTRODUCTION Portable microarrays are promising for Point-of-Care

Hajimiri, Ali

261

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

262

AN ANALYSIS OF MAGNETOHYDRODYNAMIC INVARIANTS OF MAGNETIC FLUCTUATIONS WITHIN INTERPLANETARY FLUX ROPES  

SciTech Connect

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

Telloni, D. [National Institute for Astrophysics, Astrophysical Observatory of Torino, Via Osservatorio 20, I-10025 Pino Torinese (Italy); Perri, S.; Carbone, V. [Department of Physics, University of Calabria, Ponte P. Bucci Cubo 31C, I-87036 Rende (Italy); Bruno, R.; D Amicis, R. [National Institute for Astrophysics, Institute for Space Astrophysics and Planetology, Via del Fosso del Cavaliere 100, I-00133 Roma (Italy)

2013-10-10

263

Noise annoyance caused by magnetic levitation train passbys  

Microsoft Academic Search

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

Joos Vos

2001-01-01

264

Jc Measurements and the Observation of Flux Movements and Flux Jumps in Quench-and-Melt Growth YBCO by a Pulsed Magnetic Field  

NASA Astrophysics Data System (ADS)

An observation of the flux motion in a ring-shaped quench-and-melt growth (QMG) YBCO has been attempted under a pulsed magnetic field in a temperature range from Tc down to 25 K. The transport critical current density (Jc) is estimated from the magnetic field where the flux just enters into the central hole of the sample ring. At low temperatures, the flux jump is clearly observed and its interpretation is made in terms of the traditional stabilization theory. In a thin sample, a peculiar flux motion suggesting the undulation of flux lines is observed.

Takizawa, Takeo; Kosaka, Masashi; Kanbara, Kohji; Morita, Mitsuru; Sawano, Kiyoshi

1991-10-01

265

Electron heat flux dropouts in the solar wind - Evidence for interplanetary magnetic field reconnection?  

NASA Technical Reports Server (NTRS)

An examination of ISEE-3 data from 1978 reveal 25 electron heat flux dropout events ranging in duration from 20 min to over 11 hours. The heat flux dropouts are found to occur in association with high plasma densities, low plasma velocities, low ion and electron temperatures, and low magnetic field magnitudes. It is suggested that the heat flux dropout intervals may indicate that the spacecraft is sampling plasma regimes which are magnetically disconnected from the sun and instead are connected to the outer heliosphere at both ends.

Mccomas, D. J.; Gosling, J. T.; Phillips, J. L.; Bame, S. J.; Luhmann, J. G.; Smith, E. J.

1989-01-01

266

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

E-print Network

-scale corona. This suggests that unresolved observations of coronal loops should exhibit enhanced heating nearA FLUX-TUBE TECTONICS MODEL FOR SOLAR CORONAL HEATING DRIVEN BY THE MAGNETIC CARPET Eric R. Priest May 1 ABSTRACT We explore some of the consequences of the magnetic carpet for coronal heating

Priest, Eric

267

Analysis and Optimization of Back EMF Waveform of a Flux-Switching Permanent Magnet Motor  

Microsoft Academic Search

Flux-switching permanent magnet (FSPM) motors have a doubly salient structure, the magnets being housed in the stator and the stator winding comprising concentrated coils. They have attracted considerable interest due to their essentially sinusoidal phase back electromotive force (EMF) waveform. However, to date, the inherent nature of this desirable feature has not been investigated in detail. Thus, a typical three-phase

Wei Hua; Ming Cheng; Z. Q. Zhu; David Howe

2008-01-01

268

The linear spectrum of twisted magnetic flux tubes in viscous MHD  

E-print Network

The linear spectrum of twisted magnetic flux tubes in viscous MHD B. Pint'er, R. Erd'elyi and M (Received ) In the present paper we investigate the influence of tensorial viscosity on the linear MHD loops. Numerical simulations show that in ideal MHD a poloidal magnetic field can destabilize the mag

269

Design of a Lightweight Transverse Flux Permanent Magnet Machine for Direct-Drive Wind Turbines  

Microsoft Academic Search

The aim of this paper is to design a lightweight transverse flux permanent magnet (TFPM) machine for large scale direct-drive wind turbines. The requirements for suitable generator systems for wind turbines are discussed. Analytical modeling of TFPM machines is discussed by considering the machine parameters and the magnetic circuit. The proposed analytical model is verified by comparing it to results

Deok-Je Bang; Henk Polinder; Ghanshyam Shrestha; Jan Abraham Ferreira

2008-01-01

270

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

271

Exact scattering matrix of graphs in magnetic field and quantum noise  

E-print Network

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 Schr\\"odinger 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.

Vincent Caudrelier; Mihail Mintchev; Eric Ragoucy

2014-01-17

272

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

SciTech Connect

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

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

2012-09-20

273

Magnetic flux relaxation in YBa2Cu3)(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 is parallel to c was studied in the range 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 to 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, J. D.; Alterovitz, S. A.

1991-01-01

274

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

275

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

276

Empirical models of solar magnetic flux-tubes and their non-magnetic surroundings  

NASA Astrophysics Data System (ADS)

A powerful method for the analysis of the structure of small scale magnetic elements in the solar photosphere is the inversion of Stokes spectra. In previous papers based on such inversions \\cite{BellotRubio:etal:1997, BellotRubio:etal:1999} and \\cite{Frutiger:etal:1999} have argued in favor of models with rather different dynamic properties. In this paper we return to this debate and compare results returned by inversions based on new multi-component models applied to several Fe i, Fe ii and C i spectral line profiles obtained in active region plage with a Fourier Transform Spectrometer. These inversions differ from earlier ones by the fact that mass conservation is strictly imposed both inside the magnetic elements and on the surrounding external flow field. These flux-tube models are not only able to reproduce the characteristic Stokes V asymmetries and line-shifts observed in active regions plages or network elements, but also the Stokes I line profiles, including line bisectors. It is confirmed that from the quality of the fits alone it is not possible to distinguish between the steady flow proposed by \\cite{BellotRubio:etal:1997} and the oscillatory model of \\cite{Frutiger:Solanki:1998}. If, however, physical constraints are imposed (e.g. mass conservation or that the flow retains the same direction over height in the flux tube) then the oscillatory model is found to be superior. In addition, the current investigation also provides the first inversion-based model of abnormal granulation.

Frutiger, C.; Solanki, S. K.

2001-04-01

277

Amplitudes of random telegraph noise in HTSC thin films  

Microsoft Academic Search

Amplitudes of Random Telegraph Voltage Noise in YBaCuO and BiSrCaCuO thin films of different microstructures have been investigated. Telegraph voltage noise originates from thermally activated flux jumps converted into voltage signals by means of intrinsic Josephson junction cluster acting as a dc SQUID. Magnetic field dependencies of noise amplitudes are consistent with the proposed model.

M. Bonaldi; G. Jung; A. Vecchione; S. Vitale

1994-01-01

278

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

279

Solar Coronal Heating and the Magnetic Flux Content of the Network  

NASA Technical Reports Server (NTRS)

We investigate the heating of the quiet corona by measuring the increase of coronal luminosity with the amount of magnetic flux in the underlying network at solar minimum when there were no active regions on the face of the Sun. The coronal luminosity is measured from Fe IX/X-Fe XII pairs of coronal images from SOHO/EIT. The network magnetic flux content is measured from SOHO/MDI magnetograms. We find that the luminosity of the corona in our quiet regions increases roughly in proportion to the square root of the magnetic flux content of the network and roughly in proportion to the length of the perimeter of the network magnetic flux clumps. From (1) this result, (2) other observations of many fine-scale explosive events at the edges of network flux clumps, and (3) a demonstration that it is energetically feasible for the heating of the corona in quiet regions to be driven by explosions of granule-sized sheared-core magnetic bipoles embedded in the edges of network flux clumps, we infer that in quiet regions that are not influenced by active regions the corona is mainly heated by such magnetic activity in the edges of the network flux clumps. Our observational results together with our feasibility analysis allow us to predict that (1) at the edges of the network flux clumps there are many transient sheared-core bipoles of the size and lifetime of granules and having transverse field strengths greater than approximately - 100 G, (2) approximately 30 of these bipoles are present per supergranule, and (3) most spicules are produced by explosions of these bipoles.

Moore, R. L.; Falconer, D. A.; Porter, J. G.; Hathaway, D. H.

2003-01-01

280

Solar Coronal Heating and the Magnetic Flux Content of the Network  

NASA Technical Reports Server (NTRS)

We investigate the heating of the quiet corona by measuring the increase of coronal luminosity with the amount of magnetic flux in the underlying network at solar minimum when there were no active regions on the face of the Sun. The coronal luminosity is measured from Fe IX/X-Fe XII pairs of coronal images from SOHO/EIT. The network magnetic flux content is measured from SOHO/MDI magnetograms. We find that the luminosity of the corona in our quiet regions increases roughly in proportion to the square root of the magnetic flux content of the network and roughly in proportion to the length of the perimeter of the network magnetic flux clumps. From (1) this result, (2) other observations of many fine-scale explosive events at the edges of network flux clumps, and (3) a demonstration that it is energetically feasible for the heating of the corona in quiet regions to be driven by explosions of granule-sized sheared-core magnetic bipoles embedded in the edges of network flux clumps, we infer that in quiet regions that are not influenced by active regions the corona is mainly heated by such magnetic activity in the edges of the network flux clumps. Our observational results together with our feasibility analysis allow us to predict that (1) at the edges of the network flux clumps there are many transient sheared-core bipoles of the size and lifetime of granules and having transverse field strengths > approx. 100 G, (2) approx. 30 of these bipoles are present per supergranule, and (3) most spicules are produced by explosions of these bipoles.

Falconer, D. A.; Moore, R. L.; Porter, J. G.; Hathaway, D. H.

2003-01-01

281

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 active regions that locally have a potential bipolar-type magnetic field with a thin force-free flux rope embedded inside it. The flux rope has a circular-arc axis and circular cross-section in which the interior magnetic field is predominantly toroidal (axial). Its magnetic pressure is balanced outside by that of the poloidal (azimuthal) field created at the boundary by the electric current sheathing the flux rope. To facilitate the implementation of the method in our numerical magnetohydrodynamic (MHD) code, the entire solution is described in terms of the vector potential of the magnetic field. The parameters of the flux rope can be chosen so that a subsequent MHD relaxation of the constructed configuration under line-tied conditions at the boundary provides a numerically exact equilibrium. Such equilibria are an approximation for the magnetic configuration preceding solar eruptions, which can be triggered in our model by imposing suitable photospheric flows beneath the flux rope. The proposed method is a useful tool for constructing pre-eruption magnetic fields in data-driven simulations of solar active events. Research supported by NASA's Heliophysics Theory and LWS Programs, and NSF/SHINE and NSF/FESD.

Titov, V. S.; Torok, T.; Mikic, Z.; Linker, J.

2013-12-01

282

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

SciTech Connect

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

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

1998-01-01

283

Conditions for Transverse Waves Propagation along Thin Magnetic Flux Tubes on the Sun  

NASA Astrophysics Data System (ADS)

The propagation of kink waves in the thin gravity stratified flux tubes with a generalized magnetic field distribution model is considered in cylindrical geometry. The new kink wave equations for both wave variables are obtained. It is shown that the inclusion of the radial component of an unperturbed tube magnetic field sufficiently transforms the conditions for the propagation of transverse waves. It is demonstrated that, for the models of isothermal and polytropic atmosphere in the tube and its environment, the propagation of kink waves along thin magnetic flux tubes is cutoff-free.

Lopin, Igor; Nagorny, Ivan

2013-09-01

284

Spatial-temporal structure of dissipative magnetic fluxes in type II superconductors  

NASA Astrophysics Data System (ADS)

The nonlinear boundary-value problem of the penetration of a magnetic field b into a superconducting half space with resistivity ?f(b)˜b? is investigated under the condition that the field amplitude at the boundary increases in time by a power law (b(0,t)˜(1+t)m,m>0) or in the peaking regime (b(0,t)˜(1-t/t0)m,m<0,0magnetic flux penetration can occur. The velocity of the magnetic flux front is calculated in relation to the parameters m and ?.

Medvedev, Yu. V.; Krasnyuk, I. B.

2005-12-01

285

CONDITIONS FOR TRANSVERSE WAVES PROPAGATION ALONG THIN MAGNETIC FLUX TUBES ON THE SUN  

SciTech Connect

The propagation of kink waves in the thin gravity stratified flux tubes with a generalized magnetic field distribution model is considered in cylindrical geometry. The new kink wave equations for both wave variables are obtained. It is shown that the inclusion of the radial component of an unperturbed tube magnetic field sufficiently transforms the conditions for the propagation of transverse waves. It is demonstrated that, for the models of isothermal and polytropic atmosphere in the tube and its environment, the propagation of kink waves along thin magnetic flux tubes is cutoff-free.

Lopin, Igor [Ussuriisk Astrophysical Observatory, Russian Academy of Sciences, Ussuriisk (Russian Federation); Nagorny, Ivan, E-mail: lopin78@mail.ru [Institute of Automation and Control Processes FEB RAS, Vladivostok (Russian Federation)

2013-09-10

286

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

287

Energetic protons, alpha particles, and electrons in magnetic flux transfer events  

NASA Technical Reports Server (NTRS)

Energetic proton, alpha particle, and electron data are presented for two magnetopause crossings, which show magnetic field signatures characteristic of flux transfer events (FTEs). Energetic proton and alpha particles are observed streaming along the magnetic field within the magnetosheath in all events showing magnetic signatures characteristic of the FTEs. Flux ratios as high as about 180 parallel and antiparallel to the magnetic field are observed, which means that ions of about 30 keV per charge are at times streaming almost scatter-free from the magnetopause into the magnetosheath. Energetic ion bursts with signatures equal to those observed in FTEs are reduced by more than an order of magnitude as compared to the trapped particle flux.

Scholer, M.; Hovestadt, D.; Ipavich, F. M.; Gloeckler, G.

1982-01-01

288

Formation processes of flux ropes downstream from Martian crustal magnetic fields inferred from Grad-Shafranov reconstruction  

NASA Astrophysics Data System (ADS)

We applied the Grad-Shafranov reconstruction (GSR) technique to Martian magnetic flux ropes observed downstream from strong crustal magnetic fields in the southern hemisphere. The GSR technique can provide a two-dimensional axial magnetic field map as well as the axial orientation of flux ropes from single-spacecraft data under assumptions that the structure is magnetohydrostatic and time independent. The reconstructed structures, including their orientation, allowed us to evaluate possible formation processes for the flux ropes. We reconstructed 297 magnetic flux ropes observed by Mars Global Surveyor between April 1999 and November 2006. Based on characteristics of their geometrical axial orientation and transverse magnetic field topology, we found that they can be mainly distinguished according to whether draped interplanetary magnetic fields overlaying the crustal magnetic fields are involved or not. Approximately two thirds of the flux ropes can be formed by magnetic reconnection between neighboring crustal magnetic fields attached to the surface. The remaining events seem to require magnetic reconnection between crustal and overlaid draped magnetic fields. The latter scenario should allow planetary ions to be transferred from closed magnetic flux tube to flux tubes connected to interplanetary space, allowing atmospheric ions to escape from Mars. We quantitatively evaluate lower limits on potential ion escape rates from Mars owing to magnetic flux ropes.

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

2014-09-01

289

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

290

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

SciTech Connect

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

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

1989-01-01

291

Effects of constant frequency noise in magnetic resonance imaging with nonuniform k-space sampling.  

PubMed

Magnetic resonance images can be compromised by constant frequency (CF) noise, such as radio frequency (RF) noise. We investigate the effects of CF noise in four imaging methods with non-uniform k-space sampling: single-shot echo-planar imaging (EPI) with sinusoidal readout gradients, phase-encoded echo-planar spectroscopic imaging (EPSI) with sinusoidal readout gradients, projection-reconstruction imaging, and single-shot spiral imaging. The characteristics of the CF artifacts in each imaging method are studied with numerical simulations. CF noise is found to cause artifacts of nonclassic forms in the reconstructed images. Specifically, dashed-line, streak, and spiral patterns of CF noise appear in EPI/EPSI with sinusoidal readout gradients, projection-reconstruction imaging, and spiral imaging, respectively. The analytical expression for dashed-line artifacts is found to be a weighted sum of Bessel functions and is verified with in vivo experiments. The CF artifacts can be removed during post-processing by eliminating the noise spikes in the Fourier domain of the raw data. PMID:12201430

Du, Weiliang; Karczmar, Gregory S; Pan, Xiaochuan

2002-08-01

292

Magnetic helicity transported by flux emergence and shuffling motions  

NASA Astrophysics Data System (ADS)

Magnetic helicity can be transported from sub-photosphere into corona by the emergence of helical magnetic field lines and the shuffling motions of foot-points of pre-existing coronal field lines. Active region NOAA 10930 was observed by SP and NFI of SOT on board Hinode when it pass through the solar meridian. Based on these observations, we calculate magnetic helicity flow of both terms, by regarding Doppler velocity as normal velocity. The results are compared with which calculated by method proposed by Zhang et. al. (2012). Our results show that helicity injection maps calculated by both methods have similar distribution and the integration values have the same magnitude.

Zhang, Y.; Kitai, R.; Takizawa, K.

2013-07-01

293

Plasmas fluxes to surfaces for an oblique magnetic field  

Microsoft Academic Search

The poloidal and toroidal spatial distributions of D{sub Îą}, 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

C. S. Pitcher; P. C. Stangeby; J. D. Elder; M. G. Bell; S. J. Kilpatrick; D. M. Manos; S. S. Medley; D. K. Owens; A. T. Ramsey; M. Ulrickson

1992-01-01

294

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.

295

Relaxation of flux ropes and magnetic reconnection in the Reconnection Scaling Experiment at LANL  

NASA Astrophysics Data System (ADS)

Magnetic reconnection and plasma relaxation are studied in the Reconnection Scaling Experiment (RSX) with current carrying plasma columns (magnetic flux ropes). Using plasma guns, multiple flux ropes (B? ? 100 Gauss, L=90 cm, r?3 cm) are generated in a three-dimensional (3D) cylindrical geometry and are observed to evolve dynamically during the injection of magnetic helicity. Detailed evolution of electron density, temperature, plasma potential and magnetic field structures is reconstructed experimentally and visible light emission is captured with a fast-gated, intensified CCD camera to provide insight into the global flux rope dynamics. Experiments with two flux ropes in collisional plasmas and in a strong axial guide field (Bz / B? > 10) suggest that magnetic reconnection plays an important role in the initial stages of flux rope evolution. During the early stages of the applied current drive (t? 20 ? Alfv´ {e}n), the flux ropes are observed to twist, partially coalesce and form a thin current sheet with a scale size comparable to that of the ion sound gyro-radius. Here, non-ideal terms in a generalized Ohm's Law appear to play a significant role in the 3D reconnection process as shown by the presence of a strong axial pressure gradient in the current sheet. In addition, a density perturbation with a structure characteristic of a kinetic Alfvén wave is observed to propagate axially in the current layer, anti-parallel to the induced sheet current. Later in the evolution, when a sufficient amount of helicity is injected into the system, a critical threshold for the kink instability is exceeded and the helical twisting of each individual flux rope can dominate the dynamics of the system. This may prevent the complete coalescence of the flux ropes.

Furno, I.; Intrator, T.; Hemsing, E.; Hsu, S.; Lapenta, G.; Abbate, S.

2004-12-01

296

Relaxation of flux ropes and magnetic reconnection in the Reconnection Scaling Experiment at LANL  

NASA Astrophysics Data System (ADS)

Magnetic reconnection and plasma relaxation are studied in the Reconnection Scaling Experiment (RSX) with current carrying plasma columns (magnetic flux ropes). Using plasma guns, multiple flux ropes (B_pol < 100 Gauss, L=90 cm, r < 3 cm) are generated in a three-dimensional (3D) cylindrical geometry and are observed to evolve dynamically during the injection of magnetic helicity. Detailed evolution of electron density, temperature, plasma potential and magnetic field structures is reconstructed experimentally and visible light emission is captured with a fast-gated, intensified CCD camera to provide insight into the global flux rope dynamics. Experiments with two flux ropes in collisional plasmas and in a strong axial guide field (Bz / B_pol > 10) suggest that magnetic reconnection plays an important role in the initial stages of flux rope evolution. During the early stages of the applied current drive (t < 20?_Alfven), the flux ropes are observed to twist, partially coalesce and form a thin current sheet with a scale size comparable to that of the ion sound gyro-radius. Here, non-ideal terms in a generalized Ohm's Law appear to play a significant role in the 3D reconnection process as shown by the presence of a strong axial pressure gradient in the current sheet. In addition, a density perturbation with a structure characteristic of a kinetic Alfvén wave is observed to propagate axially in the current layer, anti-parallel to the induced sheet current. Later in the evolution, when a sufficient amount of helicity is injected into the system, a critical threshold for the kink instability is exceeded and the helical twisting of each individual flux rope can dominate the dynamics of the system. This may prevent the complete coalescence of the flux ropes.

Furno, Ivo

2004-11-01

297

Magnetic Flux Penetration into MgB2  

NASA Astrophysics Data System (ADS)

We report magnetization and transport measurements on MgB2 in the form of powder, bulk ceramic, wire made by diffusion of Mg into B, and pulsed laser deposited thin films. Ceramic and wire forms show strong intergranular links, and we compare their properties with those of single crystals. The powder shows a magnetic moment versus temperature curve that scales with the moment at the lowest temperature, consistent with a distribution of grain sizes, on the scale of the London penetration depth. The ceramics shows anisotropic magnetization behavior, which is probably a consequence of the anisotropic compressional forces used in its manufacture. In both powder and ceramic, we have observed intriguing negative magnetic moments and steps therein upon changing temperature, well above the obvious superconducting transition. These could indicate small amounts of some higher Tc superconducting phases. However, magnetization loops measured in this regime show ferromagnetism, which we suggest is the origin of the magnetic properties above Tc. The wire shows a linear diamagnetic response up to an Hc1 of 236 Oe, has a normal state resistivity of 2.6 × 10-6 ?ˇcm just above the transition and a resistivity ratio of 21, which is also similar to those of single crystals. The thin films are composed of large crystalline platelets, have a Tc of 35 K, and are diamagnetic.

Mikheenko, P.; Chakalov, R.; Chakalova, R. I.; Colclough, M. S.; Muirhead, C. M.

298

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

E-print Network

solar rotation (27 days, one Car- rington rotation, from the vantage point of Earth). Total (unsignedHow 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

Lockwood, Mike

299

Twist and Writhe of the Magnetic Flux in the Super Active Region NOAA 11429  

NASA Astrophysics Data System (ADS)

We used full-disk line-of-sight magnetograms taken by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) to study the variation of coronal magnetic helicity in the Active Region (AR) NOAA 11429, where several GOES M- and X-class flares and coronal mass ejections (CMEs) occurred. The magnetic flux, total magnetic-helicity flux, and helicity accumulation over the period of interest, i.e. 6 to 11 March 2012, were measured and are discussed. We also evaluated the tilt-angle evolution within the standard polarity flux-weighted centroids approach. The AR displays a shearing motion of the magnetic structures along the polarity inversion line, reaching values of about 1.0 km s-1. The variations of magnetic helicity flux and the tilt-angle seem to be time-correlated, and both display three-phase evolutionary patterns. We also found that the flare/CME activity is higher during the first observation phase when the tilt-angle decreases and the negative magnetic helicity is accumulated. The main changes in the accumulated helicity curve are observed only after the onset of the two strongest flare/CME events. After the major event (GOES X5.4 class/CME of 7 March) there was a decrease in the occurrence of flares and CMEs. This phase is marked by a decrease of the flux of magnetic helicity from the convection zone to the corona and a change in the orientation of the tilt of the AR. This behavior suggests that the combination of these two quantities might be important in the description of the magnetic complexity accumulated by an AR during its lifetime.

Elmhamdi, A.; Romano, P.; Kordi, A. S.; Al-trabulsy, H. A.

2014-08-01

300

Noise annoyance caused by magnetic levitation train passbys  

NASA Astrophysics Data System (ADS)

In a laboratory study, the annoyance caused by the passby sounds from a magnetic levitation (maglev) train was investigated. The outdoor A-weighted sound exposure level (ASEL) of the maglev sounds varied from 65 to 90 dB. The driving speed of the maglev train varied from 100 to 400 km/h. Four important results were obtained. Provided that the outdoor ASELs were the same, (1) the annoyance was independent of the driving speed of the maglev train, (2) the annoyance caused by the maglev train was considerably higher than that caused by intercity trains, (3) the annoyance caused by the maglev train was hardly different from that caused by road traffic (passenger cars and trucks), and (4) the results (1)-(3) held true both for open or closed windows. On the basis of the present results, it might be expected that the sounds are equally annoying if the ASELs of the maglev-train passbys are at least 5 dB lower than those of the intercity train passbys. Consequently, the results of the present experiment do not support application of a railway bonus to the maglev-train sounds. Issues for future research, such as exploring further contributions of nonacoustic factors, will be discussed.

Vos, Joos

2001-05-01

301

White noise response of turbine blades subjected to heat flux and thermal gradient  

SciTech Connect

Design and production of the complex mechanical structures rarely result in an optimal solution. A typical example for this is a turbine blade design. Fatigue failures of the turbine blades is one of the most vexing problems of turbo-machine manufacturers, ever since the steam turbine became the main stay for power generating equipment and the gas turbines are increasingly used in air transport. Turbine blade failures due to fatigue are predominantly vibration related. The dynamic loads on the blading can arise from many different sources such as the high rotational speed, the high operating temperatures, the asymmetric aerofil tapered form of the turbine blade etc. Therefore, vibratory analysis is one of the most important stage in the designing of the turbine blades. In this study, the random response of the turbine blade to white noise excitation has been consistently calculated, including the internal damping mechanisms of the blade. Beside the damping effects, the rotational speed and the linear thermal gradient along the turbine blade are incorporated into the analysis. Pressure difference between the two surfaces of the turbine blades are modelled as white noise excitation along all over the turbine blade. The system dynamic equation of motion are derived and solved by using the combined Finite Element-Modal Analysis Method.

Karadag, V. [Technical Univ. of Istanbul (Turkey). Dept. of Mechanical Engineering; Aba, E. [Turkish Wagon Industry, Adapazari (Turkey). Dept. of Research and Development; Morguel, O.K. [Sakarya Univ., Adapazari (Turkey). Dept. of Mechanical Engineering

1997-07-01

302

Design of the Mark 101 magnetic flux compression generator  

SciTech Connect

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

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

1986-01-01

303

Modelling magnetic laminations under arbitrary starting state and flux waveform  

NASA Astrophysics Data System (ADS)

A numerical model able to predict the behaviour of a magnetic sheet under arbitrary supply conditions has been developed. The electromagnetic field problem is formulated in terms of an electric vector potential, which provides the magnetic field strength evolution. The hysteretic behaviour of the material is represented through the dynamic Preisach model where the activation law of the bi-state operators is modified in order to guarantee a smooth response. The problem has been solved through a time step procedure using the fixed Point technique for handling nonlinearity. The model has been validated by comparison with suitable experiments and it is applied to the investigation of the influence of the materials' starting state on the magnetic behaviour.

Bottauscio, Oriano; Chiampi, Mario; Ragusa, Carlo

2005-04-01

304

Fabrication issues in optimizing YBa{sub 2}Cu{sub 3}O{sub 7-x} flux transformers for low l/f noise  

SciTech Connect

We describe an improved interconnect technology for the fabrication of multiturn flux transformers from YBa{sub 2}Cu{sub 3}O{sub 7-x}-SrTiO{sub 3}-YBa{sub 2}Cu{sub 3}O{sub 7-x} multilayers. The essential improvements arc reductions in the thicknesses of the trilayer films, typically to 100 nm 250 nm and 250 mn respectively, and in the deposition rate, to 0.07 nm/laser pulse. This process yields crossovers in which the critical current density in the upper YBa{sub 2}Cu{sub 3}O{sub 7-x} film at 77K is (2-3) {times} 10{sup 6} A cm{sup {minus}2}. In situ trilayers exhibited 1/f flux noise levels at lHz below the measurement sensitivity of 15{mu}{Phi}{sub 0} Hz{sup {minus}1/2}, where {Phi}{sub 0} is the flux quantum. However, the flux noise of trilayers in which each layer had been patterned was significantly higher. The best flip-chip magnetometer had a white noise of 40 fT Hz{sup {minus}1/2} increasing to 340 fT Hz{sup {minus}1/2} at lHz; the corresponding flux noise levels were 9 {mu}{Phi}{sub 0} Hz{sup {minus}l/2} and 75 {mu}{Phi}{sub 0} Hz{sup {minus}l/2}, respectively.

Ludwig, F.; Dantsker, E.; Nemeth, D.T.; Koelle, D.; Miklich, A.H.; Clarke, J. [Lawrence Berkeley Lab., CA (United States); Knappe, S.; Koch, H. [Physikalisch-Technische Bundesanstalt - Inst. Berlin (Germany); Thomson, R.E. [National Inst. of Standards and Technology, Boulder, CO (United States)

1993-11-01

305

Plasma ? Scaling of Anisotropic Magnetic Field Fluctuations in the Solar Wind Flux Tube  

NASA Astrophysics Data System (ADS)

Based on various observations, it has been suggested that at 1 AU, solar wind consists of "spaghetti"-like magnetic field structures that have the magnetic topology of flux tubes. It is also observed that the plasma fluctuation spectra at 1 AU show a plasma ? dependence. Reconciling these two sets of observations and using the Invariance Principle, Bhattacharjee et al. suggested that the plasma inside every flux tube may become unstable with respect to pressure-driven instabilities and gives rise to fluctuation spectra that depend on the local plasma ?. The present work is the first direct numerical simulation of such a flux tube. We solve the full magnetohydrodynamic equations using the DEBS code and show that if the plasma inside the flux tube is driven unstable by spatial inhomogeneities in the background plasma pressure, the observed nature of the fluctuating power spectra agrees reasonably well with observations, as well as the analytical prediction of Bhattacharjee et al.

Sarkar, Aveek; Bhattacharjee, Amitava; Ebrahimi, Fatima

2014-03-01

306

The magnetic topology of the plasmoid flux rope in a MHD-simulation of magnetotail reconnection  

NASA Technical Reports Server (NTRS)

On the basis of a 3D MHD simulation, the magnetic topology of a plasmoid that forms by a localized reconnection process in a magnetotail configuration (including a net dawn-dusk magnetic field component B sub y N is discussed. As a consequence of B sub y N not equalling 0, the plasmoid assumes a helical flux rope structure rather than an isolated island or bubble structure. Initially all field lines of the plasmoid flux rope remain connected with the earth, while at later times a gradually increasing amount of flux tubes becomes separated, connecting to either the distant boundary or to the flank boundaries. In this stage, topologically different flux tubes become tangled and wrapped around each other, consistent with predictions on the basis of an ad hoc plasmoid model.

Birn, J.; Hesse, M.

1990-01-01

307

The magnetic topology of the plasmoid flux rope in a MHD simulation of magnetotail reconnection  

SciTech Connect

On the basis of a three-dimensional MHD simulation we discuss the magnetic topology of a plasmoid that forms by a localized reconnection process in a magnetotail configuration including a net dawn-dusk magnetic field component B/sub yN/. As a consequence of b/sub yN/ /ne/ 0 the plasmid gets a helical flux rope structure rather than an isolated island or bubble structure. Initially all field lines of the plasmid flux rope remain connected with the Earth, while at later times a gradually increasing amount of flux tubes becomes separated, connecting to either the distant boundary or to the flank boundaries. In this stage topologically different flux tubes become tangled and wrapped around each other, consistent with predictions on the basis of ad-hoc plasmid models. 10 refs., 8 figs.

Birn, J.; Hesse, M.

1989-01-01

308

Analysis of electromagnetic performance of flux-switching permanent-magnet Machines by nonlinear adaptive lumped parameter magnetic circuit model  

Microsoft Academic Search

A nonlinear adaptive lumped parameter magnetic circuit model is developed to predict the electromagnetic performance of a flux-switching permanent-magnet machine. It enables the air-gap field distribution, the back-electromotive force (back-EMF) waveform, the winding inductances, and the electromagnetic torque to be calculated. Results from the model are compared with finite-element predictions and validated experimentally. The influence of end effects is also

Z. Q. Zhu; Y. Pang; D. Howe; S. Iwasaki; R. Deodhar; A. Pride

2005-01-01

309

Mass Flux and Terminal Velocities of Magnetically Driven Jets from Accretion Disks  

NASA Astrophysics Data System (ADS)

In order to investigate astrophysical jets from accretion disks, we solve 1.5-dimensional steady MHD equations for a wide range of parameters, assuming the shape of poloidal magnetic field lines. We include a thermal effect to obtain the relation between the mass flux of the jet and the magnetic energy at the disk, although the jet is mainly accelerated by the magnetic force. It is found that the mass flux of the jets ( M dot ) is dependent on the magnetic energy at the disk surface, i.e., M dot ~ (rho Aa|Bp/B|)_{{slow}} ~ (rho Aa|Bp/Bphi|)_{{slow}} ~ Ealpha_{{mg}} [where rho is the density, a is the sound velocity, A is the cross section of the magnetic flux, B = (B2p + B2phi)^{1/2} , Bp and B phi are the poloidal and toroidal magnetic field strength, respectively, Emg is the magnetic energy in unit of the gravitational energy at the disk surface, and the suffix "slow" denotes the value at a slow point], when the magnetic energy is not too large. The parameter alpha increases from 0 to 0.5 with decreasing magnetic energy. Since the scaling law of Michel's minimum energy solution nearly holds in the magnetically driven flows, the dependence of the terminal velocity on the magnetic energy becomes weaker than had been expected, i.e., v_? ~ E^{(1-alpha)/3}_{{mg}} . It is shown that the terminal velocity of the jet is an order of Keplerian velocity at the footpoint of the jets for a wide range of values of Emg expected for accretion disks in star-forming regions and active galactic nuclei. We argue that the mass-loss rates observed in the star-forming regions would constrain the magnetic energies at the disk surfaces.

Kudoh, Takahiro; Shibata, Kazunari

1995-10-01

310

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

311

Magnetar Giant Flares in Multipolar Magnetic Fields --- I. Fully and Partially Open Eruptions of Flux Ropes  

E-print Network

We propose a catastrophic eruption model for magnetar's enormous energy release during giant flares, in which a toroidal and helically twisted flux rope is embedded within a force-free magnetosphere. The flux rope stays in stable equilibrium states initially and evolves quasi-statically. Upon the loss of equilibrium point is reached, the flux rope cannot sustain the stable equilibrium states and erupts catastrophically. During the process, the magnetic energy stored in the magnetosphere is rapidly released as the result of destabilization of global magnetic topology. The magnetospheric energy that could be accumulated is of vital importance for the outbursts of magnetars. We carefully establish the fully open fields and partially open fields for various boundary conditions at the magnetar surface and study the relevant energy thresholds. By investigating the magnetic energy accumulated at the critical catastrophic point, we find that it is possible to drive fully open eruptions for dipole dominated background...

Huang, Lei

2014-01-01

312

Damping rates of p-modes by an ensemble of randomly distributed thin magnetic flux tubes  

NASA Astrophysics Data System (ADS)

The magnetohydrodynamic (MHD) sausage tube waves are excited in the magnetic flux tubes by p-mode forcing. These tube waves thus carry energy away from the p-mode cavity which results in the deficit of incident p-mode energy. We calculate the loss of incident p-mode energy as a damping rate of f- and p-modes. We calculate the damping rates of f- and p-modes by a model Sun consisting of an ensemble of many thin magnetic flux tubes with varying plasma properties and distributions. Each magnetic flux tube is modelled as axisymmetric, vertically oriented and untwisted. We find that the magnitude and the form of the damping rates are sensitive to the plasma-? of the tubes and the upper boundary condition used.

Gascoyne, Andrew; Jain, Rekha

2011-08-01

313

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

314

Characterization of Dual-Phase Steels Using Magnetic Barkhausen Noise Technique  

Microsoft Academic Search

The aim of this work is to nondestructively characterize the dual phase steels using the Magnetic Barkhausen Noise (MBN) method.\\u000a By quenching of AISI 8620 steel specimens having two different starting microstructures, from various intercritical annealing\\u000a temperatures (ICAT) in the ferrite-austenite region, the microstructures consisting of different volume fractions of martensite\\u000a with morphological variations have been obtained. The microstructures were

M. Kaplan; C. H. Gür; M. Erdogan

2007-01-01

315

Coil Designs for Novel Magnetic Geometries to Cure the Divertor Heat Flux Problem for Reactors  

Microsoft Academic Search

Coil designs are developed for novel magnetic divertor geometries with a second axi-symmetric x-point and flux expansion region along the separatrix. Adjacent posters describe how these lead to spreading of heat flux and the possibility of stable, complete detachment to overcome serious physics and engineering problems in reactors. The principal feasibility issue is creating, with simple coils, additional X-points on

M. Pekker; P. Valanju; M. Kotschenreuther; J. C. Wiley; D. Strickler

2004-01-01

316

Asymmetric scattering and diffraction of two-dimensional electrons at quantized tubes of magnetic flux  

Microsoft Academic Search

The Hall conductivity of a high-mobility two-dimensional electron gas (2DEG) has been investigated in a distribution of quantized magnetic flux tubes (vortices) formed at a type-II superconducting ``gate'' layer. A pronounced suppression of the Hall effect was observed for long-Fermi wavelengths (as compared to the submicron vortex size) indicating a situation where electrons are diffracted by the flux quanta. In

A. K. Geim; S. J. Bending; I. V. Grigorieva

1992-01-01

317

Expansion of operating limits for permanent magnet motor by optimum flux-weakening  

Microsoft Academic Search

The optimum flux-weakening control method of the permanent magnet (PM) motor is examined in order to expand the operating limits. The effects of motor parameters, such as d- and q-axis inductances, PM flux-linkage, and so on, are examined by computer simulation. Furthermore, the control method and the output characteristics are examined considering the PM demagnetization due to the direct axis

Shigeo Morimoto; Yoji Takeda; Takao Hirasa; Katsunori Taniguchi

1989-01-01

318

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

319

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

SciTech Connect

We present the new results of the two-dimensional numerical experiments on the cross-sectional evolution of a twisted magnetic flux tube rising from the deeper solar convection zone (-20,000 km) to the corona through the surface. The initial depth is 10 times deeper than most of the previous calculations focusing on the flux emergence from the uppermost convection zone. We find that the evolution is illustrated by the following two-step process. The initial tube rises due to its buoyancy, subject to aerodynamic drag due to the external flow. Because of the azimuthal component of the magnetic field, the tube maintains its coherency and does not deform to become a vortex roll pair. When the flux tube approaches the photosphere and expands sufficiently, the plasma on the rising tube accumulates to suppress the tube's emergence. Therefore, the flux decelerates and extends horizontally beneath the surface. This new finding owes to our large-scale simulation, which simultaneously calculates the dynamics within the interior as well as above the surface. As the magnetic pressure gradient increases around the surface, magnetic buoyancy instability is triggered locally and, as a result, the flux rises further into the solar corona. We also find that the deceleration occurs at a higher altitude than assumed in our previous experiment using magnetic flux sheets. By conducting parametric studies, we investigate the conditions for the two-step emergence of the rising flux tube: field strength {approx}> 1.5 x 10{sup 4} G and the twist {approx}> 5.0 x 10{sup -4} km{sup -1} at -20,000 km depth.

Toriumi, S.; Yokoyama, T., E-mail: toriumi@eps.s.u-tokyo.ac.jp [Department of Earth and Planetary Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)

2011-07-10

320

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

NASA Technical Reports Server (NTRS)

Ten transient magnetic structures in Earth's magnetotail, as observed in GEOTAIL measurements, selected for early 1993 (at (-) X(sub GSM) = 90 - 130 Earth radii), 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 approximately = 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 are seen to be typically aligned with the Y(sub GSM) axis and the average diameter of these structures is approximately = 15 Earth radii.

Lepping, R. P.; Fairfield, D. H.; Jones, J.; Frank, L. A.; Paterson, W. R.; Kokubun, S.; Yamamoto, T.

1995-01-01

321

Nonlinear wave propagation along a magnetic flux tube  

Microsoft Academic Search

The weakly nonlinear wave propagation of a slow sausage surface wave traveling along a magnetized slab with a thin nonuniform boundary layer is considered. The ideal incompressible MHD equations are used and the nonlinearities are assumed to be due to second harmonic generation. A nonlinear dispersion relation and the related nonlinear Schrödinger equation is derived. The existence of a continuous

Jonas Lundberg

1994-01-01

322

Magnetic Configurations Related to the Coronal Heating and Solar Wind Generation I. Twist and Expansion Profiles of Magnetic Loops Produced by Flux Emergence  

E-print Network

The generation of outflows from the Sun known as solar winds is coupled with the heating of the solar corona, and both processes are operated in magnetic structures formed on the Sun. To study the magnetic configuration responsible for these processes, we use three-dimensional magnetohydrodynamic simulations to reproduce magnetic structures via flux emergence and investigate their configurations. We focus on two key quantities characterizing a magnetic configuration: the force-free parameter alpha and the flux expansion rate fex, the former of which represents how much a magnetic field is twisted while the latter represents how sharply a magnetic field expands. We derive distributions of these quantities in an emerging flux region. Our result shows that an emerging flux region consists of outer part where a magnetic loop takes a large flux expansion rate but a small value of alpha at their photospheric footpoints, and inner part occupied by those loops where a strong electric current flows. We also investigat...

Lee, Hwanhee

2014-01-01

323

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

324

Magnetic flux leakage: Experimental and numerical study of the influence of induced currents  

NASA Astrophysics Data System (ADS)

Eddy currents effects are important in many practical applications of the magnetic flux leakage technique. In particular, it is common to have relative velocities between the inspected part and the excitation yoke of several meters per second. The overall situation is very complex, because of nonlinearity, hysteresis, etc. In this work both experiments and computational results are shown, oriented to provide a better understanding of the role played by induced currents in the detection of internal defects in the magnetic flux inspection of steel pipes, and to identify the issues that must be solved for a meaningful comparison between theory and experiment.

Etcheverry, J. I.; Ziella, D. H.; Sánchez, G. A.

2013-01-01

325

Parallel heat flux from low to high parallel temperature along a magnetic field line.  

PubMed

In a long mean-free-path plasma where temperature anisotropy can be sustained, the parallel heat flux has two components with one associated with the parallel thermal energy and the other with the perpendicular thermal energy. In a kinetic simulation with magnetic flux expansion toward an absorbing boundary, the parallel heat flux of the parallel thermal energy is found to flow from a low to high parallel temperature region. This unusual behavior is understood with the help of an analytical calculation of the drift-kinetic model using the same upstream source in the simulation. PMID:22680728

Guo, Zehua; Tang, Xian-Zhu

2012-04-20

326

High Frequency Magnetization Dynamics and Popcorn Noise in Thin Film Heads.  

NASA Astrophysics Data System (ADS)

As the areal density and data rates in magnetic recording continue to increase, it is important to limit the occurrences of popcorn noise in thin film inductive recording heads, especially in disk drives utilizing sector servo schemes. In this dissertation, the mechanisms of popcorn noise are described. In particular, the effects of thermal transients after write, changes in stress-magnetostriction products, dynamic domain structures and dynamic domain instabilities on popcorn noise probabilities in thin film heads are discussed. A new and simple inductance fluctuation measurement technique using DC bias current to the head is shown to be useful in quickly detecting heads that are susceptible to popcorn noise. Finally, based upon the improved understanding of popcorn noise mechanisms, various criteria for head design, processing, and operating conditions are suggested to ensure low probabilities of popcorn noise in thin film heads. Delayed-relaxation Barkhausen wall jumps which cause popcorn noise have been modelled by Klaassen and van Peppen (IEEE Trans. Magn. MAG-5, 3212 (1989)) as Poisson processes. However, it has been found that additional factors, such as external stress gradients, dynamic domain structures and domain instabilities, also can strongly affect the occurrences of popcorn noise in thin film heads. In the cases where the stress-magnetostriction products in the top yokes are negative, popcorn noise probabilities were observed to increase as functions of applied stress gradients in micro Flexhead^{rm TM} components. On the other hand, applied stress gradients have negligible effect when the resultant stress-magnetostriction products in the top yokes are positive. This sign-dependent effect has been explained to be caused by negative-magnetoelastic-energy induced Bloch domain wall instabilities. Thin film heads with unstable domain structures near the backgap closure of top yokes, either in the configuration of edge closure domains of spike-like domains, have also been found to be particular noisy. Using a 0.5-nsec exposure time wide-field Kerr effect microscope, occurrences of delayed-relaxation Barkhausen wall jumps were actually imaged after specific write current excitations in noisy heads. A quantitative correlation between the probabilities of popcorn noise and dynamic domain instabilities was also observed in a noisy head. In addition to the improved understanding of popcorn noise mechanisms, it has also been found that the susceptibilities of thin film heads to Barkhausen wall jumps, thus to popcorn noise, can be measured electrically as inductance fluctuations under DC bias current. A quantitative correlation was found between the peak popcorn noise probabilities and peak inductance fluctuations of 12 different thin film heads. This inductance characterization technique was thus shown to be useful in quickly detecting heads that are susceptible to popcorn noise. Finally, several criteria in head design, processing and operating conditions were proposed to ensure low probabilities of popcorn noise in thin film heads.

Liu, Francis H.

327

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

328

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

329

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

330

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

331

Numerical Simulation of Magnetic Flux Compression in Helical-Cone Magnetoexplosive Generators  

NASA Astrophysics Data System (ADS)

We present the results of calculations of the physical processes of magnetic flux compression in a magnetocumulative generator with a large diameter spiral. The generator considered is a modification of the one developed for the multimegajoule energy source and intended for the PIRIT-EMG stationary electrophysical facility, pumping a pulsed energy of 80 MJ. The development of the magnetocumulative generator required calculating its output parameters and optimizing the generator dimensions, choosing the form and calculating the shape and thickness, insulation type and electric strength of the spiral wire. The authors developed a program package to simulate the helical-cone generator operation and numerically investigate the physical processes occurring at magnetic flux compression. To calculate the liner scatter dynamics, Eulerian equations were solved for counter-running sliding detonation waves. The system of equations is integrated using a finite-difference method for 2-D stationary grids adapting to the peculiarities of the flow. The liner collision with spiral coils as well as the destruction of the insulation is considered in 2-D through a model of nonviscous gas without heat conductivity. The magnetic flux compression is calculated using the analytical solutions of dynamic tasks and a 1-D non-linear diffusion of the magnetic field in conductors. Moreover, using a sufficiently simple algorithm, we managed to account for the basic losses of the magnetic flux related to diffusion, cuts-off at section and wire joints, and the losses related to spiral and liner misalignment.

Deryugin, Yu. N.; Korolev, P. V.; Kargin, V. I.; Pikar, A. S.; Popkov, N. F.; Ryaslov, E. A.

2004-11-01

332

Reduction of Thermal Loss in HTS Windings by Using Magnetic Flux Deflection  

NASA Astrophysics Data System (ADS)

Efforts on the generation of intensified magnetic flux have been made for the optimized shape of HTS winding applications. This contributes to the high efficiency of the rotating machines using HTS windings. Heat generation from the HTS windings requires to be suppressed as much as possible, when those coils are under operation with either direct or alternative currents. Presently, the reduction of such thermal loss generated by the applied currents on the HTS coils is reported with a magnetic flux deflection system. The HTS coils are fixed together with flattened magnetic materials to realize a kind of redirection of the flux pathway. Eventually, the magnetic flux density perpendicular to the tape surface (equivalent to the a-b plane) of the HTS tape materials is reduced to the proximity of the HTS coil. To verify the new geometry of the surroundings of the HTS coils with magnetic materials, a comparative study of the DC coil voltage was done for different applied currents in prototype field-pole coils of a ship propulsion motor.

Tsuzuki, K.; Miki, M.; Felder, B.; Koshiba, Y.; Izumi, M.; Umemoto, K.; Aizawa, K.; Yanamoto, T.

333

Flux dynamic behavior inside HTS bulks under rotating magnetic field  

Microsoft Academic Search

Melt texturing large-grain HTS materials has evolved in the last few years into a technology capable of producing large quantities of high-performance superconducting bulk materials. Such materials are used for developing novel engineering devices, such as high-speed bearing, flywheel system and motor. In these systems, however, it is very possible to generate rotating magnetic fields with the frequency higher than

M. Qiu; L. Z. Lin; G. M. Zhang; Y. S. Wang; L. Y. Xiao

2002-01-01

334

Magneto-optical study of magnetic-flux penetration into a current-carrying high-temperature-superconductor strip  

E-print Network

of magnetic-flux penetration into high-temperature superconductor HTSC films have been extensively performed and magneto-optical MO techniques, allow local magnetic-field distributions in various HTSC structures

Johansen, Tom Henning

335

A comparative study of 3D and axisymmetric magnetizer assemblies used in magnetic flux leakage inspection of pipelines  

SciTech Connect

Axisymmetric numerical codes are often used in modeling nondestructive testing phenomena, to approximate geometries and defects which are not truly axisymmetric in nature. This is especially true in cases where large computational resources are needed to model the exact geometry. However, there has been no study to confirm the nature and extent of errors introduced by an axisymmetric approximation. The gas pipeline industry makes use of both axisymmetric and non-axisymmetric magnetizers for the magnetic flux leakage inspection of pipelines. The authors utilize this platform to perform a comparative analysis of axisymmetric and non-axisymmetric magnetizers from a design perspective and also study the differences in respective electromagnetic behavior.

Katragadda, G.; Si, J.T.; 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

336

Diagnostic of the ionospheric parameters basing on measurements of ULF magnetic noise characteristics  

NASA Astrophysics Data System (ADS)

The analysis of possible influence of the horizontal inhomogeneity of the ionosphere on spectra of ULF noise is made basing on the calculations of the tangential magnetic fields propagating in a spherical horizontally non-uniform Earth -ionosphere wave guide. It is shown that this influence, as a rule, does not change essentially the character of spectra of background noise which is defined by properties of a local ionosphere. It confirms the possibility of development the diagnostic methods of the ionosphere basing on measurements of natural ULF magnetic fields. Numerical calculations of the magnetic components of background noise in frequency range of 0.1-15 Hz for homogeneous wave guide for the receiving points located at various geographical latitudes are executed. It was used for calculations the solution of a problem on excitation of the Earth -vertically non-uniform anisotropic ionosphere waveguide by a source like vertical electric dipole [1]. Calculations were carried out for various numerical and analytical models of ionospheric parameter profiles [2]. Comparison of results of modeling and experimental researches of spectrum resonant structure at stations: Crete (35.15N, 25,20E), New Life (the Nizhniy Novgorod region, 55.97N, 45.74 E), Lovozero (68N, 35E) and Barentsburg (78.09N, 14.12E) was made. The degree of conformity of the various modeling profiles to the really existing was displayed due comparison of theo-retical and experimental spectra. The investigations are also complemented by the analysis of calculated and experimental spectra of the polarization parameter of magnetic noise. It is shown that the analysis of spectra of polarization parameters is most convenient for ionosphere diagnostics at heights of 80-300 km. Work is executed under financial support of the RFBR, the grant 09-02-97035. [1] Sobchakov, L. A., Polyakov, S. V., and Astakhova, N. L., Excitation of electromagnetic waves in a plane waveguide with anisotropic upper wall, Radiophysics and Quantum Electron., 2003, V.46 (N12), p.918-925. [2] Ermakova, E. N., Kotik, D. S., and Polyakov, S. V.: Studying specific features of resonant structure of the background noise spectrum with allowance for the slope of the Earth magnetic field, Radiophysics and Quantum Electron., 2008, V. 51 (N7), p. 519-527.

Kotik, Dmitry; Ermakova, Elena

337

Factors Affecting Magnetic Flux Leakage Inspection of Tailor-Welded Blanks  

Microsoft Academic Search

The development of a laboratory-based tailor-welded blank (TWB) inspection system using the principles of magnetic flux leakage (MFL) is presented. The effects of variations in inspection system operating parameters are quantified to allow for optimized system performance. The parameters examined included the applied magnetic field strength, inspection scanning velocity, spatial resolution of acquired signals, specimen end effects, and pole-piece lift-off.

A. Montgomery; P. Wild; L. Clapham

2006-01-01

338

Sensing magnetic flux density of artificial neurons with a MEMS device.  

PubMed

We describe a simple procedure to characterize a magnetic field sensor based on microelectromechanical systems (MEMS) technology, which exploits the Lorentz force principle. This sensor is designed to detect, in future applications, the spiking activity of neurons or muscle cells. This procedure is based on the well-known capability that a magnetic MEMS device can be used to sense a small magnetic flux density. In this work, an electronic neuron (FitzHugh-Nagumo) is used to generate controlled spike-like magnetic fields. We show that the magnetic flux density generated by the hardware of this neuron can be detected with a new MEMS magnetic field sensor. This microdevice has a compact resonant structure (700?×?600?×?5 ?m) integrated by an array of silicon beams and p-type piezoresistive sensing elements, which need an easy fabrication process. The proposed microsensor has a resolution of 80 nT, a sensitivity of 1.2 V.T(-1), a resonant frequency of 13.87 kHz, low power consumption (2.05 mW), quality factor of 93 at atmospheric pressure, and requires a simple signal processing circuit. The importance of our study is twofold. First, because the artificial neuron can generate well-controlled magnetic flux density, we suggest it could be used to analyze the resolution and performance of different magnetic field sensors intended for neurobiological applications. Second, the introduced MEMS magnetic field sensor may be used as a prototype to develop new high-resolution biomedical microdevices to sense magnetic fields from cardiac tissue, nerves, spinal cord, or the brain. PMID:21113665

Tapia, Jesus A; Herrera-May, Agustin L; García-Ramírez, Pedro J; Martinez-Castillo, Jaime; Figueras, Eduard; Flores, Amira; Manjarrez, Elías

2011-04-01

339

Plasma PhysicsUnivMaryland 19-21Apr2004 Intrator 3D flux tubes 1 Magnetic reconnection, merging flux ropes,  

E-print Network

Plasma PhysicsUnivMaryland 19-21Apr2004 Intrator 3D flux tubes 1 Magnetic reconnection, merging flux ropes, 3D effects in RSX T. Intrator P-24 Plasma Physics I. Furno, E. Hemsing, S. Hsu, + many students G.Lapenta, P.Ricci T-15 Plasma Theory Second Workshop on Thin Current Sheets University

Sitnov, Mikhail I.

340

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

341

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 Ĺ and 94 Ĺ 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

342

Assessing the Impact of Magnetic Flux Erosion on Coronal Mass Ejection Geo-effectiveness  

NASA Astrophysics Data System (ADS)

We investigate the magnetic flux erosion that occurred due to magnetic reconnection at the front of several magnetic clouds (MC) observed near Earth. For some MCs the erosion process may be enhanced thanks to a significant magnetic shear at the front of the MC, in particular if the MC has a south-north magnetic polarity while the slow solar wind ahead of it has a predominantly northward orientation. Using a standard MC model, a semi-empirical model of the Dst index and an observational estimate of the magnetic flux erosion we find that the strength of the ensuing storm for these MCs are significantly lower than if no erosion had occurred. We further study and discuss the competitive interplay between adiabatic compression and magnetic erosion at the front of the MC. We conclude that the most geo-effective configuration for a south-north polarity MC is to be preceded by a slow solar wind with southward IMF. However, this stems not only from the formation of a geo-effective sheath region with southward IMF ahead of it, but also from the fact that adiabatic compression and reduced (or lack thereof) magnetic erosion constructively conspire for the structure to be more geo-effective.

Lavraud, B.; Ruffenach, A.

2012-12-01

343

TWO-STEP EMERGENCE OF THE MAGNETIC FLUX SHEET FROM THE SOLAR CONVECTION ZONE  

SciTech Connect

We perform two-dimensional magnetodydrodynamic simulations of the flux emergence from the solar convection zone to the corona. The flux sheet is initially located moderately deep in the adiabatically stratified convection zone (-20,000 km) and is perturbed to trigger the Parker instability. The flux rises through the solar interior due to the magnetic buoyancy, but suffers a gradual deceleration and a flattening in the middle of the way to the surface since the plasma piled on the emerging loop cannot pass through the convectively stable photosphere. As the magnetic pressure gradient enhances, the flux becomes locally unstable to the Parker instability so that the further evolution to the corona occurs. The second-step nonlinear emergence is well described by the expansion law by Shibata et al. To investigate the condition for this 'two-step emergence' model, we vary the initial field strength and the total flux. When the initial field is too strong, the flux exhibits the emergence to the corona without a deceleration at the surface and reveals an unrealistically strong flux density at each footpoint of the coronal loop, while the flux either fragments within the convection zone or cannot pass through the surface when the initial field is too weak. The condition for the 'two-step emergence' is found to be 10{sup 21}-10{sup 22} Mx with 10{sup 4} G at z = -20,000 km. We present some discussions in connection with recent observations and the results of the thin-flux-tube model.

Toriumi, S.; Yokoyama, T., E-mail: toriumi@eps.s.u-tokyo.ac.j [Department of Earth and Planetary Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033 (Japan)

2010-05-01

344

The effect of magnetic resonance imaging noise on cochlear function in dogs.  

PubMed

Noise produced by magnetic resonance imaging (MRI) scanners (which can peak at a sound pressure level of 131?dB) has been shown to cause noise-induced cochlear dysfunction in people. The aim of this study was to investigate whether noise produced during MRI had a deleterious effect on cochlear function in dogs, using distortion product otoacoustic emission (DPOAE) testing, which allows frequency specific, non-invasive assessment of cochlear function. DPOAE testing was performed before and after MRI in one or both ears under general anaesthesia at 14 frequency pairs (f2 frequency ranging from 0.84?kHz to 8.0?kHz). A control group comprised dogs undergoing anaesthesia of a similar duration for quiet procedures. Thirty-six dogs (66 ears) and 17 dogs (28 ears) were included in the MRI and control groups respectively. There was a reduction in DPOAE at all frequencies tested in the MRI group; a similar effect was not evident in the control group. This reduction in the MRI group was statistically significant in five of the 14 frequencies assessed (P?noise results in a significant reduction in frequency-specific cochlear function in dogs, although it is not known whether this is reversible or permanent. This suggests that all dogs undergoing MRI studies should be provided with ear protection as a routine precautionary measure. PMID:25155216

Venn, R E; McBrearty, A R; McKeegan, D; Penderis, J

2014-10-01

345

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

Xiaojun Duan; Zhengguo Liu; Xiang Li; Ying Wang; Zhiyuan Li

2005-01-01

346

Plasmonic Aharonov-Bohm effect: Optical spin as the magnetic flux parameter  

Microsoft Academic Search

A wave-front phase dislocation due to the scattering of surface plasmons from a topological defect is directly measured in the near field by means of interference. The dislocation strength is shown to be equal to the incident optical spin with analogy to the magnetic flux parameter in the Aharonov-Bohm effect.

Yuri Gorodetski; Sergey Nechayev; Vladimir Kleiner; Erez Hasman

2010-01-01

347

Plasmonic Aharonov-Bohm effect: Optical spin as the magnetic flux parameter  

NASA Astrophysics Data System (ADS)

A wave-front phase dislocation due to the scattering of surface plasmons from a topological defect is directly measured in the near field by means of interference. The dislocation strength is shown to be equal to the incident optical spin with analogy to the magnetic flux parameter in the Aharonov-Bohm effect.

Gorodetski, Yuri; Nechayev, Sergey; Kleiner, Vladimir; Hasman, Erez

2010-09-01

348

DYNAMIC COUPLING OF CONVECTIVE FLOWS AND MAGNETIC FIELD DURING FLUX EMERGENCE  

SciTech Connect

We simulate the buoyant rise of a magnetic flux rope from the solar convection zone into the corona to better understand the energetic coupling of the solar interior to the corona. The magnetohydrodynamic model addresses the physics of radiative cooling, coronal heating, and ionization, which allow us to produce a more realistic model of the solar atmosphere. The simulation illustrates the process by which magnetic flux emerges at the photosphere and coalesces to form two large concentrations of opposite polarities. We find that the large-scale convective motion in the convection zone is critical to form and maintain sunspots, while the horizontal converging flows in the near-surface layer prevent the concentrated polarities from separating. The footpoints of the sunspots in the convection zone exhibit a coherent rotation motion, resulting in the increasing helicity of the coronal field. Here, the local configuration of the convection causes the convergence of opposite polarities of magnetic flux with a shearing flow along the polarity inversion line. During the rising of the flux rope, the magnetic energy is first injected through the photosphere by the emergence, followed by energy transport by horizontal flows, after which the energy is subducted back to the convection zone by the submerging flows.

Fang Fang; Manchester IV, Ward; Van der Holst, Bart [Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109 (United States); Abbett, William P. [Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States)

2012-01-20

349

Magnetostatic-wave amplification by a flux of magnetic vortices in a ferrite-superconductor structure  

Microsoft Academic Search

The paper analyzes a possible mode of magnetostatic-wave amplification in ferrite-superconductor structures connected with the motion of a magnetic flux formed by a lattice of Abrikosov vortices in the superconductor under the effect of a transporting current. The results suggest that a large gain can be obtained due to a strong coupling between the vortex lattice and the spin wave

A. F. Popkov

1989-01-01

350

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

351

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

352

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

353

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

354

Dynamic Motor Parameter Identification for High Speed Flux Weakening Operation of Brushless Permanent Magnet Synchronous Machines  

E-print Network

Permanent Magnet Synchronous Machines Abstract: An experimental investigation is conducted to determine the behaviour of brushless PM synchronous machine parameters in the high speed flux weakening operating range synchronous machines. Special computer assisted measuring techniques are employed using an experimental vector

Szabados, Barna

355

The non-linear evolution of magnetic flux ropes: 3. effects of dissipation  

Microsoft Academic Search

We study the evolution (expansion or oscillation) of cylindrically symmetric magnetic flux ropes when the energy dissipation is due to a drag force proportional to the product of the plasma density and the radial speed of expansion. The problem is reduced to a single, second-order, ordinary differential equation for a damped, non-linear oscillator. Motivated by recent work on the interplanetary

C. J. Farrugia; V. A. Osherovich; L. F. Burlaga

1997-01-01

356

The non-linear evolution of magnetic flux ropes: 3. eects of dissipation  

Microsoft Academic Search

We study the evolution (expansion or oscilla- tion) of cylindrically symmetric magnetic flux ropes when the energy dissipation is due to a drag force proportional to the product of the plasma density and the radial speed of expansion. The problem is reduced to a single, second-order, ordinary dierential equation for a damped, non-linear oscillator. Motivated by recent work on the

C. J. Farrugia; V. A. Osherovich; L. F. Burlaga

357

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

358

Three-dimensional observations of magnetic flux density around fatigue crack tips of bearing steels  

NASA Astrophysics Data System (ADS)

Fatigue failure of steel occurs when small cracks form in a component and then continue to grow to a size large enough to cause failure. In order to understand the strength of steel components it is important to find these cracks. However, at present, it is not easy to distinguish the cracks that will grow fast and cause failure. We developed a three-dimensional scanning Hall probe microscope (3D-SHPM) and observed fatigue cracks at room temperature while they were growing. Four-point-bending fatigue tests were carried out using pre-cracked specimens (JIS-SUJ2, bearing steel). We observed the two-dimensional magnetic flux density distributions around the crack tips and found that there is a strong correlation between the changes in the magnetic flux densities and the crack growth. In order to understand this, we looked into all the three components of the magnetic flux densities, and found that they shape an arched bridge around a crack. We also found that the magnetic flux density moves in front of the crack tip along the crack growth direction.

Kida, Katsuyuki; Santos, Edson C.; Honda, Takashi; Tanabe, Hirotaka

2010-03-01

359

Three-dimensional observations of magnetic flux density around fatigue crack tips of bearing steels  

NASA Astrophysics Data System (ADS)

Fatigue failure of steel occurs when small cracks form in a component and then continue to grow to a size large enough to cause failure. In order to understand the strength of steel components it is important to find these cracks. However, at present, it is not easy to distinguish the cracks that will grow fast and cause failure. We developed a three-dimensional scanning Hall probe microscope (3D-SHPM) and observed fatigue cracks at room temperature while they were growing. Four-point-bending fatigue tests were carried out using pre-cracked specimens (JIS-SUJ2, bearing steel). We observed the two-dimensional magnetic flux density distributions around the crack tips and found that there is a strong correlation between the changes in the magnetic flux densities and the crack growth. In order to understand this, we looked into all the three components of the magnetic flux densities, and found that they shape an arched bridge around a crack. We also found that the magnetic flux density moves in front of the crack tip along the crack growth direction.

Kida, Katsuyuki; Santos, Edson C.; Honda, Takashi; Tanabe, Hirotaka

2009-12-01

360

Depleted magnetic flux tubes as probes of the Io torus plasma  

Microsoft Academic Search

On the initial pass by Io the Galileo spacecraft detected thin tubes of magnetic flux that had stronger fields than their surroundings indicating that they were depleted in their energy content. These tubes have not been seen on every return to the Io torus, only on the passes with the longest stretches of data. They are also not observed much

C.T. Russell; M. G. Kivelson; W. S. Kurth; D. A. Gurnett

2001-01-01

361

Distribution of reconnection geometry in flux transfer events using energetic ion, plasma and magnetic data  

Microsoft Academic Search

A survey of the locations of flux transfer events at the dayside magnetopause has been undertaken with emphasis on the direction of the energetic ion anisotropy along the magnetic field. The instrument used is the medium energy particle spectrometer on board ISEE 2. The events studied are from 1977 and 1978, giving coverge over the morning and afternoon sides within

P. W. Daly; M. A. Saunders; R. P. Rijnbeek; N. Sckopke; C.T. Russell

1984-01-01

362

Investigations of Flux Compression Energy Sources and Ultra-High Magnetic Field Generators in Vniief  

NASA Astrophysics Data System (ADS)

Results of VNIIEF investigations in the creation of flux compression energy generators (FCG) with different geometry are considered. Calculations and experimental data are analyzed on application of these devices for pulsed formation of high voltages, generation of high-power electron beams and of ultra-high magnetic fields. Perspectives on the development of explosive generators are connected with the creation of the "Sprut" energy system and complex EMIR, intended for investigations in high-energy density physics. Experiments using ultra-high magnetic field generators created, for the first time in the world, a magnetic field of 28 MG.

Selemir, V. D.; Demidov, V. A.

2004-11-01

363

DOI 10.1007/s11207-013-0257-0 Dynamics of an Erupting Arched Magnetic Flux Rope  

E-print Network

arched magnetic flux ropes. Keywords Coronal mass ejections, initiation and propagation ¡ Flares an important role in constituting the solar atmosphere and affecting its energetics (Lang, 2001). Solar of an electrical current in solar magnetic flux ropes has been implied in vector magnetograph measurements, which

California at Los Angles, University of

364

Potential Gradient and Flux Density Their Measurement by an Improved Method in Irregular Electrostatic and Magnetic Fields  

Microsoft Academic Search

The safety of electric power cables is dependent in part upon the maximum electrostatic flux density in the dielectric. The core loss in revolving machinery is, in part, a pole-face loss, dependent on variations in the magnetic flux density. Alternators work best in parallel, and give less interference with communication circuits, if of a good wave form, of flux density.

J. F. H. Douglas; E. W. Kane

1924-01-01

365

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

366

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.

W. Liu

2009-02-20

367

Direct Evidence for a Three-Dimensional Magnetic Flux Rope Flanked by Two Active Magnetic Reconnection X Lines at Earth's Magnetopause  

E-print Network

ropes involving two X lines is the presence of converging bidirectional plasma jets toward the center by three THEMIS spacecraft of a magnetic flux rope flanked by two active X lines producing colliding plasma jets near the center of the flux rope. The observed density depletion and open magnetic field topology

Shay, Michael

368

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

369

Black-hole jets without large-scale net magnetic flux  

E-print Network

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 timescales ~ 10-10^3 r_ g/c, where r_g 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; Beloborodov, Andrei M

2014-01-01

370

Modelling the Initiation of Coronal Mass Ejections by Magnetic Flux Emergence  

NASA Astrophysics Data System (ADS)

The possible role of magnetic flux emergence as triggering mechanism for the initiation of Coronal Mass Ejections (CMEs) is studied in the framework of the ideal magnetohydrodynamics (MHD) model. The full MHD equations are solved numerically on a spherical, axisymmetric (2.5D) domain. All simulations are performed with a modified version of the Versatile Advection Code (VAC) (Toth 1996). The magnetic field of the solution is maintained divergence-free at machine precision by exploiting an approach similar to that of Balsara and Spicer (1999): instead of storing the magnetic field components on a staggered mesh, we use the vector potential components in the nodes. In order to get satisfactorily solar wind properties, the Manchester et al. (2004) source term is implemented in the energy equation and gravity is taken into account as well in the model. Finally, a magnetic vector potential is superimposed at the inlet boundary of the Parker wind solution so that, when the steady state is reached, the Antiochos et al. (1999) triple arcade 'break out' magnetic field configuration (symmetric with respect to the equator) of a helmet streamers is obtained. When the steady state has been reached, we impose a magnetic flux emergence at the inlet boundary that is linearly growing in time during a time interval of ? t = 24 hours. After this time the vector potential at the solar base is again fixed. Due to the magnetic flux emergence at the solar base, extra radial magnetic field, is built up near the neutral line of the central arcade that expands outward. This generates an extra upward magnetic pressure force. As a consequence, the central flux system expands outward. Also the overlying field expands and, therefore, the downward magnetic tension increases. As a result, the X-point is flattened. When the distance between the central expanding arcade field and the overlying streamer field is of the order of the grid resolution, the (numerical) reconnection between these fields sets in. A flux rope is formed and, later, accelerated. Height-time and velocity-height plots of the ejected material are produced. The obtained eruption corresponds to a slow CME. The time evolution of the magnetic energy, kinetic energy and internal energy in the entire domain shows that magnetic energy is converted into kinetic energy, as expected. The energy evolution plots show, however, that only a small amount of magnetic energy is released in the system, so that the system evolves to a higher energy state. We think that the explanation of this behavior lies in the role of the magnetic helicity, which we neglected by only emerging radial magnetic field. In conclusion, we stress that by imposing a reasonable (Romano et al. (2007)) flux emergence rate, in a large but realistic active region (with, of course, model dimensionality limitations), quite realistic velocity profiles and energetics of slow CMEs are obtained.

Zuccarello, F. P.; Soenen, A.; Poedts, S.

2008-09-01

371

Tidal disruption and magnetic flux capture: powering a jet from a quiescent black hole  

NASA Astrophysics Data System (ADS)

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 of fossil disc magnetic flux into the infalling stellar debris to power the jet. We argue that the interaction with the fossil disc can have a pronounced effect on the structure and dynamics of mass fallback and likely the resulting transient. Finally, we describe possible ramifications of these interactions on unresolved problems in tidal disruption dynamics, in particular, the efficiency of debris circularization, and effects of the disruption on the pre-existing BH system.

Kelley, Luke Zoltan; Tchekhovskoy, Alexander; Narayan, Ramesh

2014-12-01

372

Permanent magnet online magnetization performance analysis of a flux mnemonic double salient motor using an improved hysteresis model  

NASA Astrophysics Data System (ADS)

The concept of the memory motor is based on the fact that the magnetization level of the AlNiCo permanent magnet in the motor can be regulated by a temporary current pulse and memorized automatically. In this paper, a new type of memory motor is proposed, namely a flux mnemonic double salient motor drive, which is particularly attractive for electric vehicles. To accurately analyze the motor, an improved hysteresis model is employed in the time-stepping finite element method. Both simulation and experimental results are given to verify the validity of the new method.

Zhu, Xiaoyong; Quan, Li; Chen, Yunyun; Liu, Guohai; Shen, Yue; Liu, Hui

2012-04-01

373

Heliospheric evolution of solar wind small-scale magnetic flux ropes  

NASA Astrophysics Data System (ADS)

We present results from the first comprehensive small-scale flux rope survey between 0.3 and 5.5 AU using the Helios 1, Helios 2, IMP 8, Wind, ACE, and Ulysses spacecrafts to examine their occurrence rate, properties, and evolution. Small-scale flux ropes are similar to magnetic clouds and can be modeled as a constant-alpha, force-free, cylindrically symmetric flux rope. They differ from magnetic clouds in that they have durations on the order of tens of minutes up to a few hours, they lack an expansion signature at 1 AU, and they do not have a depressed proton temperature compared to the surrounding solar wind plasma. The occurrence rate of small-scale flux ropes is slightly higher in the inner heliosphere than the outer heliosphere and has a weak dependence on the phase of the solar cycle. The duration of the events as a function of radial distance indicates there is a large, rapid expansion within 1 AU and it becomes constant in the outer heliosphere. This behavior implies small-scale flux ropes are created and nearly complete their evolution within 1 AU.

Cartwright, M. L.; Moldwin, M. B.

2010-08-01

374

Two Types of Magnetic Flux Cancellation in the Solar Eruption of 2007 May 20  

NASA Technical Reports Server (NTRS)

We study a solar eruption of 2007 May 20, in an effort to understand the cause of the eruption's onset. The event produced a GOES class B6.7 flare peaking at 05:56 UT, while ejecting a surge/filament and producing a coronal mass ejection (CME). We examine several data sets, including H-alpha images from the Solar Optical Telescope (SOT) on Hinode, EUV images from TRACE, and line-of-sight magnetograms from SOHO/MDI. Flux cancelation occurs among two different sets of flux elements inside of the erupting active region: First, for several days prior to eruption, opposite-polarity sunspot groups inside the region move toward each other, leading to the cancelation of approximately 10^{21} Mx of flux over three days. Second, within hours prior to the eruption, positive-polarity moving magnetic features (MMFs) flowing out of the positive-flux spots at approximately 1 kilometer per second repeatedly cancel with field inside a patch of negative-polarity flux located north of the sunspots. The filament erupts as a surge whose base is rooted in the location where the MMF cancelation occurs, while during the eruption that filament flows out along the polarity inversion line between the converging spot groups. We conclude that a plausible scenario is that the converging spot fields brought the magnetic region to the brink of instability, and the MMF cancelation pushed the system "over the edge." triggering the eruption.

Sterlin, Alphonse C.; Moore, Ronald L.; Mason, Helen

2010-01-01

375

Diffusion of magnetic field and removal of magnetic flux from clouds via turbulent reconnection  

Microsoft Academic Search

The diffusion of astrophysical magnetic fields in conducting fluids in the\\u000apresence of turbulence depends on whether magnetic fields can change their\\u000atopology via reconnection in highly conducting media. Recent progress in\\u000aunderstanding fast magnetic reconnection in the presence of turbulence is\\u000areassuring that the magnetic field behavior in computer simulations and\\u000aturbulent astrophysical environments is similar, as far as

R. Santos-Lima; A. Lazarian; J. Cho

2009-01-01

376

Magnetar Giant Flares in Multipolar Magnetic Fields. II. Flux Rope Eruptions with Current Sheets  

NASA Astrophysics Data System (ADS)

We propose a physical mechanism to explain giant flares and radio afterglows in terms of a magnetospheric model containing both a helically twisted flux rope and a current sheet (CS). With the appearance of a CS, we solve a mixed boundary value problem to get the magnetospheric field based on a domain decomposition method. We investigate properties of the equilibrium curve of the flux rope when the CS is present in background multipolar fields. In response to the variations at the magnetar surface, it quasi-statically evolves in stable equilibrium states. The loss of equilibrium occurs at a critical point and, beyond that point, it erupts catastrophically. New features show up when the CS is considered. In particular, we find two kinds of physical behaviors, i.e., catastrophic state transition and catastrophic escape. Magnetic energy would be released during state transitions. This released magnetic energy is sufficient to drive giant flares, and the flux rope would, therefore, go away from the magnetar quasi-statically, which is inconsistent with the radio afterglow. Fortunately, in the latter case, i.e., the catastrophic escape, the flux rope could escape the magnetar and go to infinity in a dynamical way. This is more consistent with radio afterglow observations of giant flares. We find that the minor radius of the flux rope has important implications for its eruption. Flux ropes with larger minor radii are more prone to erupt. We stress that the CS provides an ideal place for magnetic reconnection, which would further enhance the energy release during eruptions.

Huang, Lei; Yu, Cong

2014-11-01

377

Confined Partial Filament Eruption and its Reformation within a Stable Magnetic Flux Rope  

NASA Astrophysics Data System (ADS)

We present observations of a confined partial eruption of a filament on 2012 August 4, which restores its initial shape within ?2 hr after eruption. From the Global Oscillation Network Group H? observations, we find that the filament plasma turns into dynamic motion at around 11:20 UT from the middle part of the filament toward the northwest direction with an average speed of ?105 km s-1. A little brightening underneath the filament possibly shows the signature of low-altitude reconnection below the filament eruptive part. In Solar Dynamics Observatory/Atmospheric Imaging Assembly 171 Ĺ images, we observe an activation of right-handed helically twisted magnetic flux rope that contains the filament material and confines it during its dynamical motion. The motion of cool filament plasma stops after traveling a distance of ?215 Mm toward the northwest from the point of eruption. The plasma moves partly toward the right foot point of the flux rope, while most of the plasma returns after 12:20 UT toward the left foot point with an average speed of ?60 km s-1 to reform the filament within the same stable magnetic structure. On the basis of the filament internal fine structure and its position relative to the photospheric magnetic fields, we find filament chirality to be sinistral, while the activated enveloping flux rope shows a clear right-handed twist. Thus, this dynamic event is an apparent example of one-to-one correspondence between the filament chirality (sinistral) and the enveloping flux rope helicity (positive). From the coronal magnetic field decay index, n, calculation near the flux rope axis, it is evident that the whole filament axis lies within the domain of stability (i.e., n < 1), which provides the filament stability despite strong disturbances at its eastern foot point.

Joshi, Navin Chandra; Srivastava, Abhishek K.; Filippov, Boris; Kayshap, Pradeep; Uddin, Wahab; Chandra, Ramesh; Prasad Choudhary, Debi; Dwivedi, B. N.

2014-05-01

378

FULLY RESOLVED QUIET-SUN MAGNETIC FLUX TUBE OBSERVED WITH THE SUNRISE/IMAX INSTRUMENT  

SciTech Connect

Until today, the small size of magnetic elements in quiet-Sun areas has required the application of indirect methods, such as the line-ratio technique or multi-component inversions, to infer their physical properties. A consistent match to the observed Stokes profiles could only be obtained by introducing a magnetic filling factor that specifies the fraction of the observed pixel filled with magnetic field. Here, we investigate the properties of a small magnetic patch in the quiet Sun observed with the IMaX magnetograph on board the balloon-borne telescope SUNRISE with unprecedented spatial resolution and low instrumental stray light. We apply an inversion technique based on the numerical solution of the radiative transfer equation to retrieve the temperature stratification and the field strength in the magnetic patch. The observations can be well reproduced with a one-component, fully magnetized atmosphere with a field strength exceeding 1 kG and a significantly enhanced temperature in the mid to upper photosphere with respect to its surroundings, consistent with semi-empirical flux tube models for plage regions. We therefore conclude that, within the framework of a simple atmospheric model, the IMaX measurements resolve the observed quiet-Sun flux tube.

Lagg, A.; Solanki, S. K.; Riethmueller, T. L.; Schuessler, M.; Hirzberger, J.; Feller, A.; Borrero, J. M.; Barthol, P.; Gandorfer, A. [Max-Planck-Institut fuer Sonnensystemforschung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau (Germany); MartInez Pillet, V.; Bonet, J. A. [Instituto de Astrofisica de Canarias, C/Via Lactea s/n, 38200 La Laguna, Tenerife (Spain); Schmidt, W.; Berkefeld, T. [Kiepenheuer-Institut fuer Sonnenphysik, Schoeneckstrasse 6, 79104 Freiburg (Germany); Del Toro Iniesta, J. C. [Instituto de Astrofisica de AndalucIa (CSIC), Apartado de Correos 3004, 18080 Granada (Spain); Domingo, V. [Grupo de AstronomIa y Ciencias del Espacio, Universidad de Valencia, 46980 Paterna, Valencia (Spain); Knoelker, M. [High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000 (United States); Title, A. M., E-mail: lagg@mps.mpg.d [Lockheed Martin Solar and Astrophysics Laboratory, Bldg. 252, 3251 Hanover Street, Palo Alto, CA 94304 (United States)

2010-11-10

379

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

380

Catastrophe versus Instability for the Eruption of a Toroidal Solar Magnetic Flux Rope  

NASA Astrophysics Data System (ADS)

The onset of a solar eruption is formulated here as either a magnetic catastrophe or as an instability. Both start with the same equation of force balance governing the underlying equilibria. Using a toroidal flux rope in an external bipolar or quadrupolar field as a model for the current-carrying flux, we demonstrate the occurrence of a fold catastrophe by loss of equilibrium for several representative evolutionary sequences in the stable domain of parameter space. We verify that this catastrophe and the torus instability occur at the same point; they are thus equivalent descriptions for the onset condition of solar eruptions.

Kliem, B.; Lin, J.; Forbes, T. G.; Priest, E. R.; Török, T.

2014-07-01

381

Elucidation of the Heat-Flux Limit from Magnetic-Island Heating  

SciTech Connect

Recent experiments on heating of magnetic islands in a tokamak are analyzed to assess plasma transport characteristics. By comparing with the experimental data, both perpendicular and parallel components of the electron heat conductivity in the island are determined. As a consequence, the so-called heat-flux limit factor {xi}, the ratio of the parallel heat conduction flux in a collisionless plasma to that transferred by free-streaming electrons, can be estimated. The found factor {xi} does not contradict that established earlier by interpreting laser plasma experiments.

Tokar, M. Z.; Gupta, A. [Institut fuer Energieforschung-Plasmaphysik, Forschungszentrum Juelich, Association EURATOM-FZJ, Trilateral Euregio Cluster (Germany)

2007-11-30

382

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 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 the Solar Cycle 23 (Chertok et al., 2013, Solar Phys. 282, 175) revealed that the summarized unsigned magnetic flux in the arcades and dimming regions at the photospheric level, $\\Phi$, is significantly related to the intensity (Dst index) of geomagnetic storms. This provides the basis for the earliest diagnosis of geoefficiency of solar eruptions. In the present article, using the same data set, we find that a noticeable correlation exists also between the eruptive magnetic flux, $\\Phi$, and another geomagnetic index, Ap. As the magnetic flux increases from tens to $\\approx 500$ (in units of $10^{20}$ Mx), the geomagnetic storm intensity measured by the 3-hour Ap index, enhances in average from Ap $\\approx 50$ to a formally maximum value of 400 (in units of 2 nT). The established relationship shows that in fact 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.

2014-11-01

383

The total solar irradiance, UV emission and magnetic flux during the last solar cycle minimum  

E-print Network

We have analyzed the total solar irradiance (TSI) and the spectral solar irradiance as ultraviolet emission (UV) in the wavelength range 115-180 nm, observed with the instruments TIM and SOLSTICE within the framework of SORCE (The Solar Radiation and Climate Experiment) during the long solar minimum between the 23rd and 24th cycles. The wavelet analysis reveals an increase in the magnetic flux in the latitudinal zone of the sunspot activity, accompanied with an increase in the TSI and UV on the surface rotation timescales of solar activity complexes. In-phase coherent structures between the mid-latitude magnetic flux and TSI/UV appear when the long-lived complexes of the solar activity are present. These complexes, which are related to long- lived sources of magnetic fields under the photosphere, are maintained by magnetic fluxes reappearing in the same longitudinal regions. During the deep solar minimum (the period of the absence of sunspots) a coherent structure has been found, in which the phase between th...

Benevolenskaya, E E

2013-01-01

384

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

385

Magnetic flux creep in HTSC and Anderson-Kim theory (Review Article)  

NASA Astrophysics Data System (ADS)

Theoretical results and experimental data on flux creep in high-temperature superconductors (HTSC) are analyzed in this review paper. When reviewing experimental work, the main attention is paid to the most striking experimental results which have had a major impact on the study of flux creep in HTSC. On the other hand, the analysis of theoretical results is focused on the studies which explain the features of flux creep by introducing modifications to the Anderson-Kim (AK) theory, i.e., on the studies that have not received sufficient attention earlier. However, it turned out that the modified AK theory could explain a number of features of flux creep in HTSC: the scaling behavior of current-voltage curves in HTSC, the finite rate of flux creep at ultralow temperatures, the logarithmic dependence of the effective pinning potential on the transport current and its decrease with temperature. The harmonic potential field which is used in this approach makes it possible to solve accurately both the problem of viscous vortex motion and the problem of thermally activated flux creep in this magnetic field. Moreover, the energy distribution of pinning potential and the interaction of vortices with each other are also taken into account in the approach. Thus, the modification of the AK theory consists, essentially, in its refinement and achieving a more realistic approximation.

Lykov, A. N.

2014-09-01

386

Single-sided mobile NMR apparatus using the transverse flux of a single permanent magnet.  

PubMed

This study presents a simple design for a mobile, single-sided nuclear magnetic resonance (NMR) apparatus which uses the magnetic flux parallel to the magnetization direction of a single, disc-shaped permanent magnet polarized in radial direction. The stray magnetic field above the magnet is approximately parallel to the magnetization direction of the magnet and is utilized as the B(0) magnetic field of the apparatus. The apparatus weighs 1.8 kg, has a compact structure and can be held in one's palm. The apparatus generates a B(0) field strength of about 0.279 T at the center of apparatus surface and can acquire a clear Hahn echo signal of a pencil eraser block lying on the RF coil in one shot. Moreover, a strong static magnetic field gradient exists in the direction perpendicular to the apparatus surface. The strength of the static magnetic field gradient near the center of the apparatus surface is about 10.2 T/m; one-dimensional imaging of thin objects and liquid self-diffusion coefficient measurements can be performed therein. The available spatial resolution of the one-dimensional imaging experiments using a 5 x 5 mm horizontal sample area is about 200 mum. Several nondestructive inspection applications of the apparatus, including distinguishing between polyethylene grains of different densities, characterizing epoxy putties of distinct set times and evaluating the fat content percentages of milk powders, are also demonstrated. Compared with many previously published designs, the proposed design bears a simple structure and generates a B(0) magnetic field parallel to the apparatus surface, simplifying apparatus construction and simultaneously rendering the selection of the radiofrequency coil relatively flexible. PMID:19577401

Chang, Wei-Hao; Chen, Jyh-Horng; Hwang, Lian-Pin

2010-01-01

387

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

388

Study of the termination phase of plasma production and the formation of magnetic flux breakthroughs during wire array implosion  

NASA Astrophysics Data System (ADS)

The phenomenon of magnetic flux breakthrough into a wire array during its implosion was studied experimentally at the Angara-5-1 facility. It is shown that breakthroughs develop in the final stage of plasma production from the wire material and occur near the initial wire position. The spatial distributions of the azimuthal magnetic field within tungsten, molybdenum, copper, and aluminum wire arrays were studied using magnetic probes. The distributions of the azimuthal magnetic field B ?( z, t) along the array height in different stages of implosion were measured, and the characteristic dimensions of regions with a nonuniform magnetic field that appear during magnetic flux breakthroughs at the outer boundary of the wire array plasma were determined. The dimensions of these regions are compared with those of the regions with depressed plasma radiation observed in frame and time-integrated X-ray images. The dynamics of the distribution B ?( z, t) in regions with a nonuniform magnetic field during breakthroughs of the azimuthal magnetic flux is compared with that of the spatial distribution of pinch radiation in the frame X-ray images in different stages of implosion. The experimental data on the characteristics of spatially nonuniform breakthroughs of the magnetic flux into the wire array are analyzed using the plasma rainstorm model proposed by V.V. Aleksandrov et al. (JETP 97, 745 (2003)). The plasma density in the region of magnetic flux breakthrough is estimated.

Mitrofanov, K. N.; Aleksandrov, V. V.; Grabovski, E. V.; Ptichkina, E. A.; Gritsuk, A. N.; Frolov, I. N.; Laukhin, Ya. N.

2014-09-01

389

Quantized magnetic flux through the orbits of hydrogen-like atoms within the atomic model of Sommerfeld  

E-print Network

Within the Sommerfeld atomic model the quantization of magnetic flux through the electronic orbits is investigated together with its dependency on additional sources of magnetic fields. These sources alter the magnetic flux through the atomic orbits and in consequence are causing energy shifts. This effect is investigated for the cases, where the source is an external magnetic field, the magnetic moment of the nucleus or the magnetic moment of the electron. The energy shifts due to external magnetic fields, the magnetic dipole contribution of the hyperfine splitting and the spin-orbit coupling can be reproduced very well. The meaning of 'spin', however, changes within this approach drastically. The unusual Land\\'e g-factor of 2 for the electron is a result of the orbital motion and the magnetic moment of the electron rather than it is an intrinsic property of the electron.

W. -D. R. Stein

2013-02-25

390

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

391

3D Numerical Simulations of f-Mode Propagation Through Magnetic Flux Tubes  

E-print Network

Three-dimensional numerical simulations have been used to study the scattering of a surface-gravity wave packet by vertical magnetic flux tubes, with radii from 200 km to 3 Mm, embedded in stratified polytropic atmosphere. The scattered wave was found to consist primarily of m=0 (axisymmetric) and m=1 modes. It was found that the ratio of the amplitude of these two modes is strongly dependant on the radius of the flux tube: The kink mode is the dominant mode excited in tubes with a small radius while the sausage mode is dominant for large tubes. Simulations of this type provide a simple, efficient and robust way to start understanding the seismic signature of flux tubes, which have recently began to be observed.

Daiffallah, K; Bendib, A; Cameron, R; Gizon, L

2010-01-01

392

Analysis of flux distribution and core losses in interior permanent magnet motor  

SciTech Connect

The interior permanent magnet (IPM) motor with its robust rotor construction, hybrid torque production nature and flux-weakening capability is suitable for electric vehicle applications where wide speed and torque range is required. At high-speed operations, core losses become an important issue because they affect efficiency and raise operating temperatures. This paper discusses the results of two-dimensional finite element analysis into the relationship between flux distribution and core losses in the IPM motor. The analysis is further supported by flux measurements using search coils installed in an experimental motor. Three methods of predicting the core losses in IPM motor are also investigated. These methods are the empirical formula method, finite element computed waveform method and the search coil induced voltage method.

Tseng, K.J.; Wee, S.B.

1999-12-01

393

Noise correlation in fundamental mode orthogonal fluxgate  

NASA Astrophysics Data System (ADS)

One of the key issues to reduce noise in a flux gate is the knowledge of how the noise is originated within the sensor. In this paper we measured the noise of an orthogonal flux gate operated in the fundamental mode in narrow regions of the magnetic wire used as a core, and we show how the correlation between the noise measured in two different sections drops when the distance between the sections is increased. This indicates that the noise is originated locally in the magnetic wire and then spread thought the wire due to its high permeability. This was confirmed by the measurement with a longer wire that yields higher correlation than those observed with shorter ones. Therefore, uncorrelated noise generated in sections of the wire far enough can be compensated. Finally, we show how the noise from two different wires is totally uncorrelated even if excited by the same current source, confirming that the noise is originated within the magnetic core. Such results can be used to design sensors with lower noise.

Butta, M.; Sasada, I.

2012-04-01

394

Vacuum polarization for compactified $QED_{4+1}$ in a magnetic flux background  

E-print Network

We evaluate one-loop effects for $QED_{4+1}$ compactified to ${\\bf R}^4 \\times S^1$, in a non-trivial vacuum for the gauge field, such that a non-vanishing magnetic flux is encircled along the extra dimension. We obtain the vacuum polarization tensor and evaluate the exact parity breaking term, presenting the results from the point of view of the effective 3+1 dimensional theory.

Tira, C Ccapa; Malbouisson, A P C; Roditi, I

2010-01-01

395

Vacuum polarization for compactified QED{sub 4+1} in a magnetic flux background  

Microsoft Academic Search

We evaluate one-loop effects for QED{sub 4+1} compactified to R⁴xSš in a nontrivial vacuum for the gauge field such that a nonvanishing magnetic flux is encircled along the extra dimension. We obtain the vacuum polarization tensor and evaluate the exact parity-breaking term, presenting the results from the point of view of the effective (3+1)-dimensional theory.

C. Ccapa Ttira; C. D. Fosco; A. P. C. Malbouisson; I. Roditi

2010-01-01

396

Vacuum polarization for compactified QED4+1 in a magnetic flux background  

NASA Astrophysics Data System (ADS)

We evaluate one-loop effects for QED4+1 compactified to R4×S1 in a nontrivial vacuum for the gauge field such that a nonvanishing magnetic flux is encircled along the extra dimension. We obtain the vacuum polarization tensor and evaluate the exact parity-breaking term, presenting the results from the point of view of the effective (3+1)-dimensional theory.

Ccapa Ttira, C.; Fosco, C. D.; Malbouisson, A. P. C.; Roditi, I.

2010-03-01

397

Vacuum polarization for compactified QED{sub 4+1} in a magnetic flux background  

SciTech Connect

We evaluate one-loop effects for QED{sub 4+1} compactified to R{sup 4}xS{sup 1} in a nontrivial vacuum for the gauge field such that a nonvanishing magnetic flux is encircled along the extra dimension. We obtain the vacuum polarization tensor and evaluate the exact parity-breaking term, presenting the results from the point of view of the effective (3+1)-dimensional theory.

Ccapa Ttira, C.; Fosco, C. D.; Malbouisson, A. P. C.; Roditi, I. [Centro Atomico Bariloche and Instituto Balseiro, Comision Nacional de Energia Atomica, 8400 Bariloche (Argentina); Centro Brasileiro de Pesquisas Fisicas/MCT, 22290-180, Rio de Janeiro, RJ (Brazil)

2010-03-15

398

Manipulation of the magnetic flux in superconductor by the ferromagnetic domains in SC\\/FM hybrid  

Microsoft Academic Search

We studied magneto-optically the magnetic flux entry and exit in SC\\/FM hybrid of a ferromagnetic permalloy film sputtered on the superconducting NbSe2 single crystal. The FM film had growth induced perpendicular anisotropy and the labyrinth equilibrium domain structure. However, we could align the domain walls in a desired direction by application of a strong enough in-plane field. Thus formed stripe

Vitalii Vlasko-Vlasov; Ulrich Welp; Goran Karapetrov; Valentin Novosad; Andrei Belkin; Daniel Rosenmann; Wai Kwok

2007-01-01

399

Analysis of flux distribution and core losses in interior permanent magnet motor  

Microsoft Academic Search

The interior permanent magnet (IPM) motor with its robust rotor construction, hybrid torque production nature and flux-weakening capability is suitable for electric vehicle applications when wide speed and torque range is required. At high-speed operations, core losses become an important issue because they affect efficiency and raise operating temperatures. This paper discusses the results of two-dimensional finite element analysis into

K. J. Tseng; S. B. Wee

1999-01-01

400

Elementary Heating Events - Magnetic Interactions Between Two Flux Sources. III Energy Considerations  

E-print Network

The magnetic field plays a crucial role in heating the solar corona, but the exact energy release mechanism(s) is(are) still unknown. Here, we investigate in detail, the process of magnetic energy release in a situation where two initially independent flux systems are forced into each other. Work done by the foot point motions goes in to building a current sheet in which magnetic reconnection takes place. The scaling relations of the energy input and output are determined as functions of the driving velocity and the strength of fluxes in the independent flux systems. In particular, it is found that the energy injected into the system is proportional to the distance travelled not the rate of travel. Similarly, the rate of Joule dissipation is related to the distance travelled. Hence, rapidly driven foot points lead to bright, intense, but short-lived events, whilst slowly driven foot points produce weaker, but longer-lived brightenings. Integrated over the lifetime of the events both would produce the same heating if all other factors were the same. A strong overlying field has the affect of creating compact flux lobes from the sources. These appear to lead to a more rapid injection of energy, as well as a more rapid release of energy. Thus, the stronger the overlying field the more compact and more intense the heating. This means observers must know the rate of movement of the magnetic fragments involved in an events, as well as determine the strength and orientation of the surrounding field to be able to predict anything about the energy dissipated.

K. Galsgaard; C. E. Parnell

2005-01-27

401

The Transport of Plasma and Magnetic Flux in Giant Planet Magnetospheres  

NASA Astrophysics Data System (ADS)

Both Jupiter and Saturn have moons that add significant quantities of neutrals and/or dust beyond geosynchronous orbit. This material becomes charged and interacts with the planetary plasma that is "orbiting" the planets at near corotational speeds, driven by the planetary ionospheres. Since this speed is greater than the keplerian orbital speed at these distances, the net force on the newly added charged mass is outward. The charged material is held in place by the magnetic field which stretches to the amount needed to balance centripetal and centrifugal forces. The currents involved in this process close in the ionosphere which is an imperfect conductor and the feet of the field lines hence slip poleward and the material near the equator moves outward. This motion allows the magnetosphere to divest itself of the added mass by transferring it to the magnetotail. The magnetotail in turn can rid itself of the newly added mass by the process of reconnection, interior to the region of added mass, freeing an island of magnetized plasma which then moves down the magnetotail no longer connected to the magnetosphere. This maintains a quasi-stationary conservation of mass in the magnetosphere with roughly constant mass and "periodic" disturbances. However, there is one other steady state the magnetosphere needs to maintain. It needs to replace the mass loaded flux tubes with emptied flux tubes. Thus the "emptied" flux tubes in the tail must move inward against the outgoing mass-loaded flux tubes. That they are buoyant is a help in this regard but it appears also to be helpful if the returning flux separates into thin flux tubes, just like air bubbles rising in a container with a leak in the bottom. In this way the magnetospheres of Jupiter and Saturn maintain their dynamic, steady-state convection patterns.

Russell, C. T.

2013-05-01

402

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

E-print Network

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 October 2013; the northern and southern polar fields (mean above 60$^\\circ$ latitude) reversed in November 2012 and March 2014, 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 ...

Sun, Xudong; Liu, Yang; Zhao, Junwei

2014-01-01

403

Magnetohydrostatic equilibrium - I. Three-dimensional open magnetic flux tube in the stratified solar atmosphere  

NASA Astrophysics Data System (ADS)

A single open magnetic flux tube spanning the solar photosphere (solar radius ? R?) and the lower corona (R? + 10 Mm) is modelled in magnetohydrostatic equilibrium within a realistic stratified atmosphere subject to solar gravity. Such flux tubes are observed to remain relatively stable for up to a day or more, and it is our aim to apply the model as the background condition for numerical studies of energy transport mechanisms from the surface to the corona. We solve analytically an axially symmetric 3D structure for the model, with magnetic field strength, plasma density, pressure and temperature all consistent with observational and theoretical estimates. The self-similar construction ensures the magnetic field is divergence free. The equation of pressure balance for this particular set of flux tubes can be integrated analytically to find the pressure and density corrections required to preserve the magnetohydrostatic equilibrium. The model includes a number of free parameters, which makes the solution applicable to a variety of other physical problems and it may therefore be of more general interest.

Gent, F. A.; Fedun, V.; Mumford, S. J.; Erdélyi, R.

2013-10-01

404

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

E-print Network

Solar coronal mass ejections (CMEs) are 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 the Solar Cycle 23 (Chertok et al., 2013, Solar Phys. 282, 175) revealed that the summarized unsigned magnetic flux in the arcades and dimming regions at the photospheric level, Phi, is significantly related to the intensity (Dst index) of geomagnetic storms. This provides the basis for the earliest diagnosis of geoefficiency of solar eruptions. In the present article, using the same data set, we find that a noticeable correlation exists also between the eruptive magnetic flux, Phi, and another geomagnetic index, Ap. As the magnetic flux increases from tens to approx. 500 (in units of 10^{20} Mx), the geomagnetic storm intensity measured by the 3-hour Ap index, enhances in average from Ap approx. 50 to a formally maximum value of 400 (in units of 2 nT). The established rela...

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

2014-01-01

405

Magnetic rotor flux observer of induction motors with fast convergence and less transient oscillation  

NASA Astrophysics Data System (ADS)

This paper presents an observer design for the estimation of magnetic rotor flux of induction motors. We characterize the class of MIMO induction motor systems that consists of the linear observable and the nonlinear part with a block triangular structure. The similarity transformation that plays an important role in proving the convergence of the proposed observer is generalized to the systems. Since the gain of the proposed observer minimizes a nonlinear part of the system to suppress for the stability of the error dynamics, it improves the transient performance of the high gain observer. Moreover, by using the generalized similarity transformation, it is shown that under some observability and boundedness conditions, the proposed observer guarantees the global exponential convergence to zero o