Normal Component Of Magnetic Field Research Articles

Field distributions around arbitrary shape surface cracks in metals, induced by high-frequency alternating-current-carrying wires of arbitrary shape

We propose an efficient modeling technique for theoretical prediction of magnetic field distributions around an arbitrary shape surface-breaking crack in a metal, induced by an alternating-current-carrying wire of arbitrary shape. The finite-difference method is first used to solve the governing Laplace equation inside the crack region. The normal component of magnetic field at the crack opening is thus obtained to derive a closed-form solution in the Fourier domain for the regions outside the crack. To support the validity of the model, we compare experimental results with their theoretical counterparts

The “Newton Challenge”: Kinetic aspects of forced magnetic reconnection

Conventional magnetic reconnection flows are envisioned to be driven from outside the current sheet, typically owing to large‐scale MHD stresses. In the present work a two‐dimensional electromagnetic particle‐in‐cell code is used to study a standard reconnection problem (the “Newton Challenge”) involving the transition from a moderately thick current sheet to a thinner sheet induced by a temporally limited, spatially varying, inflow of magnetic flux. The external forcing produces a small electron temperature anisotropy T⊥/T∥ ≲ 1.1 and an embedded electron current layer on a scale below that of the ion inertia length. The reconnection involves a two‐stage process with an initial fast (cEy/vAB0 ∼ 0.1) phase followed by a second slow stage in which the O line structure of the magnetic island is established. For weak values of the flux inflow, these two processes occur on a common slow timescale. With an open geometry along the magnetic field line direction, the reconnection rate is increased and no island structure is formed; the plasma is simply expelled from the system. In the presence of an initial finite normal magnetic field component, the fast reconnection occurs only after the normal field is driven locally through zero and the electron stabilization effect is removed.

Cluster vision of the magnetotail current sheet on a macroscale

With Cluster 2001–2003 observations at ∼20 RE downtail we determine the cross‐tail current sheet configuration via mapping of the neutral sheet positions and direct measurements of the sheet inclination. The warping effect tilts both flanks of the sheet up to 40° in the YZ GSM plane, while the gradual hinging effect tilts the sheet up to 10–20° roughly in the XZ plane. These findings are in agreement with the recent neutral sheet model of Tsyganenko and Fairfield (2004). Nonuniform response of the neutral sheet By to IMF was also revealed. Typical IMF By penetration efficiency was 40–60%. However, efficiency for positive IMF By was below 20% at both flanks. Therefore at the flanks of the neutral sheet the normal magnetic field component (along the normal to the warped sheet) is generally larger than the shear component (along the current).

Modeling of the Jovian Magnetosphere

Abstract. This paper presents a global model of the Jovian magnetosphere which is valid not only in the equatorial plane and near the planet, as most of the existing models are, but also at high latitudes and in the outer regions of the magnetosphere. The model includes the Jovian dipole, magnetodisc, and tail current system. The tail currents are combined with the magnetopause closure currents. All inner magnetospheric magnetic field sources are screened by the magnetopause currents. It guarantees a zero normal magnetic field component for the inner magnetospheric field at the whole magnetopause surface. By changing magnetospheric scale (subsolar distance), the model gives a possibility to study the solar wind influence on the magnetospheric structure and auroral activity. A dependence of the magnetospheric size on the solar wind dynamic pressure psw (proportional to psw-0.23) is obtained. It is a stronger dependence than in the case of the Earth's magnetosphere (psw-1/6). The model of Jupiter's magnetospheric which is presented is a unique one, as it allows one to study the solar wind and interplanetary magnetic field (IMF) effects.

AC loss characteristics of YBCO conductors carrying transport currents in external AC magnetic fields

In this work we present dependence of AC losses in a YBCO tape conductor carrying an AC transport current in an external AC magnetic field on the angle between the direction of the external field and the tape face of the conductor. The losses were measured by a calorimetric method whose validity was proven by simultaneous electric and calorimetric measurements of the losses in the same part of the conductor. The experiment was conducted at 77 K. The measured data show that the AC losses are dominated by the normal component of the external magnetic field to the tape surface. It is also shown that the AC losses sharply drop when the external magnetic field is parallel to the tape surface, which is due to extremely high aspect ratio of the YBCO conductor.

Development of a Robust Quasi-Poloidal Compact Stellarator

A compact quasi-poloidally symmetric stellarator (QPS) plasma and coil configuration is described that has desirable physics properties and engineering feasibility with a very low aspect ratio plasma bounded by good magnetic flux surfaces both in vacuum and at β = 2%. The plasma is robust with respect to variations of pressure and the resulting bootstrap current, which leave the bounding flux surface approximately unchanged and thus reduce active positional control requirements. This configuration was developed by reconfiguring the QPS modular coils and applying a new computational method that maximizes the volume of good (integrable) vacuum flux surfaces as a measure of robustness. The stellarator plasma and coil design code STELLOPT is used to vary the coil geometry to determine the plasma geometry and profiles that optimize plasma performance with respect to neoclassical transport, infinite-n ballooning stability up to β = 2%, and coil engineering parameters. The normal component of the vacuum magnetic field is simultaneously minimized at the full-beta plasma boundary.

A model for magnetic flux leakage signal predictions

An analytical model was developed to determine the length of a surface-breaking defect along the direction of the applied field when using the magnetic flux leakage (MFL) non-destructive technique. The theoretical model fits the experimental MFL results from simulated defects. The extreme positions of the normal magnetic leakage field component were subsequently used for a quantitative evaluation of the defect length. Permeability variations were neglected by employing a flux density close to sample saturation. Four different defect geometries were experimentally investigated and the validity of the analytical model was verified. Good agreement between theoretical and experimental results suggests that this method can be used as an inverse MFL data interpretation technique.

SUPERBOLI fault-current limiters based on YBCO-coated stainless steel tapes

Tubular modules of fault-current limiter (FCL) have been developed, manufactured and industrially tested in the framework of the European SUPERPOLI Project. The modules of /spl phi/55 mm /spl times/ 500 mm are based on YBCO-coated stainless steel tapes exhibiting an excellent critical current of >150 A per cm-width at 77 K. A specific FCL architecture enables i) a soft current transfer from the YBCO film to the steel tape at over-critical currents, ii) double-sided cooling of the tape, and iii) protection against temperature shocks. The tubular configuration of the modules allows suppression of the normal component of the self magnetic field, and, as a result, gain of total critical current. Critical currents above 3000 A (78 K) have been realized in these modules in the course of industrial tests. A limitation of the fault current with an amplitude of 50 kA lead to a limited current of 3-4 kA, a voltage drop of 50 V (per module) and a power dissipation of about 100 kW. At sub-critical currents, the module introduces a negligible power loss. A quenching time of only 0.1-0.4 ms was observed.

The properties and causes of rippling in quasi-perpendicular collisionless shock fronts

Abstract. The overall structure of quasi-perpendicular, high Mach number collisionless shocks is controlled to a large extent by ion reflection at the shock ramp. Departure from a strictly one-dimensional structure is indicated by simulation results showing that the surface of such shocks is rippled, with variations in the density and all field components. We present a detailed analysis of these shock ripples, using results from a two-dimensional hybrid (particle ions, electron fluid) simulation. The process that generates the ripples is poorly understood, because the large gradients at the shock ramp make it difficult to identify instabilities. Our analysis reveals new features of the shock ripples, which suggest the presence of a surface wave mode dominating the shock normal magnetic field component of the ripples, as well as whistler waves excited by reflected ions.Key words. Space plasma physics (numerical simulation studies; shock waves; waves and instabilities)

Polarization phase relationships via multiple Morse wavelets. II. Data analysis

Three examples are provided of the use of multiple generalized Morse wavelets for polarization analysis of three–component recordings where phase relationships between components of the signal are transient. The example three–component signals are (i) synthetic, (ii) the radial, transverse and vertical components of an earthquake, and (iii) the radial, transverse and normal components of the solar magnetic field measured by the Ulysses spacecraft upstream of an interplanetary shock. Working in the scale–time plane ( a,b ), the multiple–wavelet approach detects coherent motion with elliptical polarization in all three cases. Our graphical presentations fully take into account restrictions imposed by two factors: the minimum usable scale, and wraparound effects caused by Fourier transform computations, and consequently a maximum usable scale.

Design of modular coils for a quasi-axisymmetric stellarator with a flexible control of the magnetic field configuration

A design of the modular coil system for CHS-qa, of which aspect ration is 3.2, has been made. The magnetic field strength and the major radius are 1.5 T and 1.5 m, respectively. The normal component of magnetic field produced by the modular coils is minimized on the plasma boundary to obtain the optimum coil design. In addition, the mirror component of magnetic field is evaluated to reproduce the configuration more accurately. Engineering constraints are imposed on the distance between adjacent modular coils and the radius of coil curvature. The dependence of the residual normal component of the field on these conditions is examined, and the realistic values for them are selected. Additional coils to control various properties of the magnetic field configuration, such as the rotational transform, the magnetic well depth, have been designed and flexibility of the magnetic field configuration is realized.

Magnetic field abnormality caused by welding residual stress

Residual stresses in ferromagnetic material affect the direction and structure of domains and generate the magnetic flux abnormality on the surface. Three-dimensional magnetic field on the surface of a circular-welding pipe specimen was inspected with 8mm lift-off before and after low-temperature annealing aimed for residual stress releasing. Small hole stress testing was also carried out with a contrast to the magnetic testing before annealing. The magnetic field characteristics and the residual stress distribution were compared and discussed. The normal component of magnetic field correlated with residual stress. The magnetic field abnormality on ferromagnetic material could be used for residual stress inspection.

Relationship between spatial resolution and noise in scanning superconducting quantum interference device microscopy

An inverse transformation based on the fast Fourier transform can convert a two-dimensional image of the normal component of magnetic field into a corresponding image of the two-dimensional source currents that generated the field. Applying such a transformation to a magnetic image from a scanning Superconducting Quantum Interference Device (SQUID) microscope reveals that the spatial resolution s in the current image can be over 20 times better than that found in the raw magnetic field image, and up to about 5 times smaller than the SQUID sample separation z. We describe a quantitative theory for the noise and spatial resolution found in such current density images. We find that s is proportional to z and logarithmically related to the magnetic field noise in the image, the current applied to the sample, and the pixel size. We discuss the unusual functional dependence of these parameters and compare our theory to experimental data obtained from a scanning SQUID microscope. Finally, we describe how selective filtering in Fourier space can reduce noise and other artifacts in the current density images.

Ion dynamics in the near‐Earth magnetotail: Magnetic turbulence versus normal component of the average magnetic field

The influence on ion motion of magnetic turbulence observed in the near‐Earth magnetotail is investigated by numerical simulation. The magnetotail current sheet is modeled as a magnetic field reversal with a normal magnetic field component Bn, plus a three‐dimensional spectrum of magnetic fluctuations δB, which represents the observed magnetic turbulence. A cross tail electric field Ey is included. A test particle simulation is performed assuming an anisotropic plasma source at the magnetospheric lobes, and using different values of Bn and of the fluctuation level δB/B0. In the relevant range of parameters, Bn and δB/B0 have opposite effects on the current structure and on the ion heating. In particular, for a substantial level of fluctuations, δB/B0 ≳ 0.2, the current splits into two sheets for Bn ∼ 0, but increasing Bn requires a higher δB/B0 in order to have the current splitting. In addition, ion heating increases with the increase of δB/B0 and with the decrease of Bn, as these changes favor motion perpendicular to the average magnetic field and along the potential drop. When the magnetic fluctuations are small, the diamagnetic current wings and the magnetic field overshoots at the boundary of the current sheet are recovered for weakly anisotropic plasma distributions. Implications for the substorm growth phase and onset are discussed.

Wind and ACE observations during the great flow of 1–4 May 1998: Relation to solar activity and implications for the magnetosphere

Using magnetic field, plasma, and energetic particle data from Wind and ACE, we analyze interplanetary features associated with the first strongly geoeffective interval in the rising phase of solar cycle 23, which affected Earth on 1–4 May 1998. As shown by Skoug et al. [1999], the configuration consisted of a compound stream made up of an interplanetary coronal mass ejection (ICME) containing a magnetic cloud and being trailed by a hot, faster flow. In addition, we find that the front boundary of the ICME is a rotational discontinuity and the leading edge of the fast stream has a zero normal magnetic field component and is followed by a magnetic field which is strongly enhanced (by a factor of ∼4) and whose fluctuations lie in a plane approximately parallel to the leading edge. Energetic particle and composition observations confirm that the field lines of the magnetic cloud were connected to at least two different flare sites in the same active region. We infer a lower limit for the size of the solar footprint of the connected flux tube of 0.02 Rs2, i.e., ∼1010 km2. A dramatic weakening of the halo electron distribution occurred during 3 May at the time when other experimenters have documented the presence of prominence material. We hypothesize that the solar wind halo population was scattered by enhanced frequency of Coulomb collisions in the dense and very cold plasma. We discuss our energetic particle observations in terms of local acceleration at interplanetary shocks and field discontinuities as well as in terms of acceleration in flares and CME‐driven shocks. We also compare, in a specific formulation, the power and energy input of the May 1998 configuration to the magnetosphere with other much studied geoeffective events. We find that the power input far exceeded that in all previous geoeffective events in our sample and attribute this to the fact that the 1–4 May 1998 event consisted of a compound stream structure with an unprecedented power input during a ∼3‐hour burst of high‐speed flow on 4 May. A statistical survey using the OMNI database for the 6‐year period 1995–2000 confirms these inferences and indicates further approximate saturation levels for energy and power input of 10 J m−2 and 0.3 mW m−2, only exceeded in exceptional events such as May 1998 and July 2000. The solar energetic particle event at the leading edge of the fast stream might be the only advance warning the Earth would receive of the approach of a configuration of such a concentrated geoeffective potential.

Interaction of mesoscopic magnetic textures with superconductors

Here we report a method to calculate the vortex and magnetization arrangement for a system of interacting superconductors and ferromagnets separated in space. The method is based on static London-Maxwell equations and the corresponding energy. Possible superconducting vortices are included in this system. Using this method we analyze screening currents in a superconducting film induced by magnetic textures in a thin magnetic film. We assume that the two films are parallel and positioned close to each to other, but interact exclusively via magnetic fields. We also consider possible vortices within this superconducting film and their interactions with magnetic texture. As an example of such magnetic texture we use a single magnetic dot with magnetization either perpendicular or parallel to the film. We derive a condition where spontaneous formation of one, two, or more vortices and antivortices is energetically favorable. We prove that, in the case of such a circular magnetic dot with perpendicular magnetization, when the vortex emerges in the superconducting film the normal component of magnetic field near the superconducting film changes sign outside of the dot range.

The role of spin polarization on the quantum Hall effect in 2DEG with periodically modulated filling factor

We report quantum magnetotransport experiments in novel 3 μm -period V-grooved GaAs/AlGaAs heterojunctions. In such structures a periodic spatial variation of the normal component of magnetic field is realized. We observe anomalous features in both weak and strong magnetic fields. The quantum Hall effect step-like Hall resistance is replaced by an oscillatory variation when the current is applied parallel to the grooves. The longitudinal resistance peaks attain unusually high values ⪢ h/ e 2 when the current is applied perpendicular to the grooves. Most of the features can be explained by a model of serial and parallel connection of stripes with different filling factors and different spin polarization at the adjacent stripes.

Stabilization of lower‐hybrid drift instability in the magnetotail by finite north‐south magnetic field component and destabilization by sheared cross‐field flow

The problem of nonlocal lower‐hybrid drift instability in the magnetotail‐like geometry characterized by a finite north‐south (normal) magnetic field component at the center of the neutral sheet is revisited. In a preliminary paper [Lui et al., 1995] it was shown that the one‐dimensional neutral sheet of the Harris type with constant cross‐field flow speed profile, v = v0 = const, is stable with respect to perturbations in the lower‐hybrid frequency range, if the normal field component is finite, Bn > 0. They then discussed a sheared flow speed profile, v(z) = v0/[1 + (z/Δ)2], which represents a much thinner current sheet configuration, and recovered the unstable mode. In this paper, the previous work is extended in two aspects. The effects of the variation of the normal field strength, Bn, on the transition from instability to stability in the absence of the shear (Δ = ∞) is examined systematically. In addition, the eigenmodes with odd symmetry, ϕ(−z) = − ϕ(z), are included. These two extensions show that indeed, the normal field component Bn has a strong stabilizing influence on the mode. As the shear parameter Δ is reduced, the threshold value of Bn for stabilization increases in proportion to the value of Δ−1, which leads to the conclusion that the shear in the cross‐field drift speed has a destabilizing influence on the mode. Comparison of the two symmetry modes shows that the odd symmetry eigenmodes generally possess higher growth rates than the even symmetry ones.

Magnetoresistance in a stripe-shaped two-dimensional electron gas

We measure large positive magnetoreistance in a nonplanar stripe-shaped two-dimensional electron gas (2DEG) in magnetic field up to 28T. If the applied magnetic field is quasi-parallel to the substrate, the effective magnetic field becomes essentially nonuniform and sign alternating, because the orbital motion of 2DEG is sensitive to only the normal component of magnetic field. We find that the shape of the magnetoresistance traces is dramatically changed, when magnetic field becomes exactly parallel to the substrate plane. We attribute such behaviour of the magnetoresistance to the formation of snake-like orbits which are channelling along the stripes.

Quantum magnetotransport in periodic V-grooved heterojunctions

We report quantum magnetotransport experiments in novel $3\ensuremath{-}\ensuremath{\mu}\mathrm{m}$-period V-grooved GaAs/AlGaAs heterojunctions. In such structures a periodic spatial variation of the normal component of magnetic field is realized. We observe anomalous features in both weak and strong magnetic fields. The quantum Hall-effect steplike Hall resistance is replaced by an oscillatory variation when the current is applied parallel to the grooves. The longitudinal resistance peaks attain unusually high values $\ensuremath{\gg}{h/e}^{2}$ when the current is applied perpendicular to the grooves. We propose a model that explains certain features of the observed behavior, however, some puzzling anomalies remain.