Showing papers on "Magnetic flux published in 1986"

Evidence for Aharonov-Bohm effect with magnetic field completely shielded from electron wave.

Abstract: Evidence for the Aharonov-Bohm effect was obtained with magnetic fields shielded from the electron wave. A toroidal ferromagnet was covered with a superconductor layer to confine the field, and further with a copper layer for complete shielding from the electron wave. The expected relative phase shift was detected with electron holography between two electron beams, one passing through the hole of the toroid, and the other passing outside. The experiment gave direct evidence for flux quantization also.

Leaky and non-leaky oscillations in magnetic flux tubes

Abstract: An extensive analysis, both analytic and numerical, of waves in flux tubes imbedded in (possibly) magnetic surroundings is given. It is shown that any wave confined to the tube and its neighbourhood can be put into one of seven categories. Simple criteria for deciding the existence of each type in any particular case are derived. Many other (leaky) modes are found which excite waves in the external medium and thereby lose energy to the surroundings. A number of asymptotic analyses allow much information to be gained about these without the need for numerical solution of the complicated equations involved. Three particular cases, pertaining to photospheric flux tubes, Hα fibrils, and coronal loops, are considered in detail.

Vertical magnetic recording arrangement

Abstract: The present device provides a downstream magnetic shield which is integrally formed with a write pole section having a tip to make up a magnetic recording head. The shield and the write pole tip are located with small gap therebetween, so that magnetic flux which fringes from the magnetic write pole tip is intercepted by the shield. Accordingly such fringing flux does not pass through the recording medium, and hence if a data bit has been written into the recording medium its dipole identity will not be weakened by the fringing flux of a subsequent data bit being written into the magnetic recording medium. In addition, the magnetic recording medium is made up of a first layer of material having perpendicular uniaxial anisotropy and an underlying layer made of a material which has a low magnetic flux reluctance characteristic. Accordingly, the magnetic flux passes vertically through the first layer, along the second layer and finally passes vertically from the second layer through the first layer into the shield. The face of the shield is designed to be many times as large as the face of the write pole section so that the density of the flux from the tip is sufficient to effect a vertical recording while the density of the flux passing into the downstream magnetic shield is low and hence a previously recorded pattern is not reversed.

Fractal structure of the interplanetary magnetic field

Abstract: Under some conditions, time series of the interplanetary magnetic field strength and components have the properties of fractal curves. Magnetic field measurements made near 8.5 AU by Voyager 2 from June 5 to August 24, 1981 were self-similar over time scales from approximately 20 sec to approximately 3 x 100,000 sec, and the fractal dimension of the time series of the strength and components of the magnetic field was D = 5/3, corresponding to a power spectrum P(f) approximately f sup -5/3. Since the Kolmogorov spectrum for homogeneous, isotropic, stationary turbulence is also f sup -5/3, the Voyager 2 measurements are consistent with the observation of an inertial range of turbulence extending over approximately four decades in frequency. Interaction regions probably contributed most of the power in this interval. As an example, one interaction region is discussed in which the magnetic field had a fractal dimension D = 5/3.

Elastic and plastic properties of the flux-line lattice in type-II superconductors.

Abstract: The present status is reviewed for the elastic and plastic properties of the flux-line lattice in type-II superconductors, which crucially enter the modern theory of collective pinning. In contrast to atomic lattices the flux-line lattice exhibits pronounced elastic nonlocality and the screw dislocation (oriented perpendicular to the flux lines) is strongly anisotropic and can move freely since there is no Peierls potential along the flux lines. General expressions for the interaction between straight or arbitrarily curved flux lines are presented which may be used to compute equilibrium arrangements of flux lines.

Magnetopause stability threshold for patchy reconnection

Abstract: This review is devoted to the problem of the internal fine structure of the Earth's magnetopause. A number of theoretical and experimental papers dealing with this subject is discussed from a unified viewpoint. The Vlasov kinetic approach is used to study the stability of magnetopause magnetic surfaces that can be destructed by the growth and overlapping of magnetic islands. The stochastic wandering of magnetic field lines between the destructed surfaces can result in magnetic percolation, i.e. the appearance of a topological connection of interplanetary and geomagnetic field lines. Such a process may be considered as a mechanism of the macroscopic (but spatially localized) reconnection. We discuss this in relation with the phenomena of spontaneous ‘patchy’ reconnection, recently observed at ISEE satellites and now known as flux transfer events. Drift tearing mode, which is responsible for the growth of magnetic islands can be stabilized due to its coupling with ion sound waves, and the process of percolation will be interrupted if even a thin region with smooth stable magnetic surfaces exists within the magnetopause. Accordingly, we obtain a magnetopause stability threshold for localized reconnection. It is represented in the form of dependence of marginal dimensionless thickness of the magnetopause on the angle of magnetic field rotation within it. Further, we discuss the possible role of lower hybrid turbulence permanently observed within the. magnetopause and speeding up the process of reconnection. Nonlinear calculations supporting the developed model are given in the appendices. We consider briefly the motion of reconnecting flux tubes and evaluate the time necessary for the accomplishment of percolation. The calculations show that the appearance of reconnection ‘patchies’ at the dayside magnetopause cannot occur too far from the stagnation region. The latter agrees with experimental indications on the most probable site of the formation of flux transfer events. In the concluding part of the review we discuss the necessary limitations on the theory, possible lines of its future advance and comparison with the experimental data.

Wave propagation in pulsar magnetospheres - Refraction of rays in the open flux zone

Abstract: The propagation of waves through a relativistically outflowing electron-positron plasma in a very strong dipolar magnetic field, conditions expected in pulsar magnetospheres, is investigated. Halmilton's equations is derived for the propagation of rays through a plasma which is inhomogeneous in density, magnetic field directions, and Lorentz factor. These equations are solved for rays propagating through the plasmas outflowing along the 'open' dipolar field lines in which the density decreases inversely as the radius cubed and in the case where gradients transverse to the radial direction exist. In the radial case, the effects of refraction on pulse profiles, spectrum, and polarization are examined, and the effects of a transverse gradient are indicated. Attention is given to models in which the observed broad bandwidth in the radio emission has its origin in a radius to frequency map. Models with broad-band emission at a single radius are also studied. These are compared to observations of pulse width and pulse component separation as a function of frequency. The origin of 'orthogonal modes' is discussed.

Pulse electro-magnetic field therapy device with auto bias circuit

Abstract: A method of treating animal tissue with a time-varying, magnetic field and a device for producing such a field wherein a substantially unipolar, rectangular electro-magnetic treatment signal having pulses of a predetermined frequency and amplitude is generated. The signal is transmitted to a coil wherein it induces a magnetic flux. The magnetic flux so produced is applied to a treatment site to promote healing of tissue. A biasing circuit is provided in the device to prevent the occurrence of a reverse polarity pulse upon the fall of the magnetic flux induced by the fall of the generated pulse and to diminish high frequency ringing at the beginning of a treatment signal.

Hall Effects on Magnetic Relaxation

Abstract: A generalized vorticity is introduced whose self-linkage (the hybrid helicity) and flux are invariants of ideal incompressible magnetohydrodynamics (MHD) when the Hall term is included. A model of magnetofluid relaxation is constructed for Hall magnetohydrodynamics by assuming that the energy seeks the minimum value compatible with constrained values of magnetic helicity, hybrid helicity, axial magnetic flux, and fluid vorticity flux. As a result of the coupling of magnetic field to fluid vorticity in the generalized vorticity, it is found that the relaxed magnetic-field configuration need not be force free. The presence of a nonvanishing fluid vorticity is shown to be necessary for the existence of relaxed magnetic-field configurations that confine a finite plasma pressure. The study has potential relevance to the dynamics and morphology of space and cosmic plasmas, as well as to pressure confinement and current drive in fusion plasmas.

ISEE-1 and 2 observations of magnetic flux ropes in the magnetotail: FTE's in the plasma sheet?

Abstract: Magnetic field observations on ISEE-1 and 2 in and near the neutral sheet about 20 Re down the near-earth magnetotail reveal the occurrence of structures resembling magnetic flux ropes. Both electric field and fast plasma data show that these structures convect across the spacecraft at speeds of 200 - 600 km/s, and that they have scale sizes of roughly 3 5 Re. The rope axis orientation is across the tail, approximately in the -Y GSM direction. Their magnetic structure is strikingly similar to magnetic flux ropes observed in the Venus ionosphere, and to flux transfer events observed at the dayside magnetopause. The total field-aligned current within these ropes may approach a million amps. These structures may arise because of patchy reconnection within the plasma sheet, or may be tearing islands formed when the plasma sheet magnetic field has a cross-tail component. Plasma sheet flux ropes are not a common feature at ISEE orbital altitudes; this suggests that near-earth neutral line formation within ISEE apogee (22 Re) may be equally rare.

Cross-Saturation in Smooth-Air-Gap Electrical Machines

Abstract: A detailed analysis of the phenomenon of cross-saturation is presented and it is shown that in a smooth-air-gap quadrature-phase machine this phenomenon is similar to the phenomenon of the demagnetizing effect of cross-magnetizing armature reaction in a d.c. machine. In a quadrature-phase smooth-air-gap machine the resultant air-gap flux density distribution is decreased over one half of the pole-pitch and is increased under the other half of the pole-pitch and due to saturation the decrease is greater than the increase. Thus under saturated conditions, if the resultant m.m.f. distribution is displaced from the magnetic axes of the windings, a change in the quadrature-axis magnetizing current will cause a change of flux linkage in the direct-axis winding and vice versa, a change in the direct-axis magnetizing current will cause a change of flux linkage in the quadrature-axis winding. It is an important result that cross-saturation can also exist if the m.m.f. distributions are assumed to be sinusoidal. A physical derivation of the cross-saturation coupling factor is presented and a test is described which shows the existence of cross-saturation. A theoretical analysis of the experiment gives results which are in good agreement with the experimental results.

The expulsion of magnetic flux from the Earth's core

Abstract: Summary. A simple 2-D model of the effect of an upwelling motion on the toroidal field in the Earth’s core is investigated. It is found that flux is readily expelled from the core, indicating that the frozen-flux approximation can fail, even when the magnetic Reynolds number in the core is high. This process provides a simple explanation of features in the magnetic field which have recently been identified at the core-mantle boundary beneath southern Africa.

Geomagnetic field analysis—IV. Testing the frozen-flux hypothesis

Abstract: Summary. We present a model of the magnetic field at the core–mantle boundary, for epoch 1959.5, based on a large set of observatory and survey measurements. Formal error estimates for the radial field at the core are 50 μT, compared with 30 and 40 μT for our previous MAGSAT (1980) and POGO (1970) models. Current work on the determination of the velocity of the core fluid relies on the assumption that the core behaves as a perfect conductor, so that the field lines remain frozen to the fluid at the core surface. This frozen-flux condition requires that the integrated flux over patches of the core surface bounded by contours of zero radial field remain constant in time. A new method is presented for constructing core fields that satisfy these frozen-flux constraints. The constraints are non-linear when applied to main field data, unlike the case of secular variation which was considered in an earlier paper. The method is applied to datasets from epochs 1969.5 and 1959.5 to produce fields with the same flux integrals as the 1980 model. The frozen-flux hypothesis is tested by comparing the changes in the flux integrals between 1980/1969.5, 1969.5/1959.5 and 1980/1959.5 with their errors. We find that the hypothesis can be rejected with 95 per cent confidence. The main evidence for flux diffusion is in the South Atlantic region, where a new null flux curve appears between 1960 and 1970, and continues to grow at a rapid rate from 1970 to 1980. However, the statistical result depends critically on our error estimates for the field at the core surface, which are difficult to assess with any certainty; indeed, doubling the error estimates negates the statistical argument. The conclusion is therefore, at this stage, tentative, and requires further evidence, either from older data, if good enough, or from future satellite measurements.

A Dynamic Relaxation Technique for Determining the Structure and Stability of Coronal Magnetic Fields

Abstract: A new approach to the problem of determining the structure and stability of solar and stellar coronal magnetic fields is presented. It is pointed out that the conventional approach which proceeds by examining the linear stability of simple, analytic, but generally unrealistic equilibria fails to address many important questions - for example, the influence of finite-amplitude disturbances, the existence of multiple equilibria, or the possibility of perturbed boundary conditions. In contrast, the present paper invokes a Lagrange variable technique to formulate the problem dynamically as a mixed initial-value-boundary-value problem in three space dimensions. Thus, the determination of magnetic field structure is reduced to simulating the dynamic relaxation of an arbitrary initial magnetic field configuration toward equilibrium. The feasibility of the technique is illustrated by investigating straight but nonuniform twisted flux tubes as well as the typical twisted arch topology of coronal loops. 25 references.

A Superconducting Vibrating Reed Applied to Flux-Line Pinning. I. Theory

Abstract: A theoretical treatment is given of a superconducting reed clamped at one end and performing flexural vibrations in a homogeneous longitudinal magnetic fieldB a. When the flux lines are rigidly pinned the reed behaves like an ideal diamagnet whose bending distorts the external field. This generates a magnetic restoring force (line tension) ∝B 2 which is independent of the reed thicknessd, whereas the mechanical restoring force (stiffness) is ∝d 3. Therefore, the resonance frequency ω/2π of a thin superconducting reed increases drastically when a fieldB a is applied, or for a givenB a, when the reed is cooled below its critical temperatureT c. With decreasing pinning strength (characterized by Labusch's parameter α) the resonance frequency decreases, ω2→ω2–ω pin 2 where ω pin 2 ∝α−1, and an attenuation Γ v ∝α−2 occurs due to the viscous motion of flux lines. For larger vibration amplitudes an additional, amplitude-dependent damping Γ h ∝α−3 occurs due to the hysteretic losses caused by elastic instabilities during flux motion.

Inhibition of convective collapse of solar magnetic flux tubes by radiative diffusion

Abstract: Interaction of convection with a magnetic field leads to an intermittent distribution of magnetic flux1. Such a process operating on the solar surface can lead to ‘equipartition’ fields of 700 G (ref. 2). These fields are further prone to a convective instability and eventually collapse to kilogauss intensity3–5. I show here that radiative diffusion can inhibit this collapse to a varying degree, depending on the field strength and the thickness of the flux elements. As a consequence, one would expect the field strength of the photospheric magnetic flux elements to depend on their sizes. It is shown that at one end of such a distribution there would be kilogauss tubes with small dispersion in field strength and large dispersion in size. At the other extreme of the spectrum would be thin tubes of fairly constant size but with a wide range in field strength, from kilogauss intensities to the equipartition values of 700 G. High-resolution observations from space-borne telescopes should reveal the existence of the latter variety of tubes.

Position detecting apparatus using a magnetic sensor and a closed magnetic circuit with non-uniform magnetic flux distribution

Abstract: A position detector having a magnetic sensor for measuring linear or angular displacement, or detecting a position of a body without contact thereto. The detector has a closed circuit including magnets and yokes. The magnetic leakage flux which emanates from the inside surfaces of the yokes is utilized to detect the position of the body. The magnetic sensor is transferred along the inside surfaces of the yokes by a transferring circuit, which senses the magnetic field intensity of the leakage flux distributed thereon. This enables the detector to have a longer measuring range than that of a prior art detectors. An output signal is stable due to the shielding effect of the yokes. The magnetic closed circuit has many magnetic and mechanical configurations. For example, for the magnet, a permanent magnet and an electromagnet can be used. Further, improvements are proposed for enhancing the linearity of the output signal voltage with respect to the transferring distance of the sensing element and for compensating the offset drift of an amplifier or changing the coercive force of permanent magnets.

Steady state plasma transport in a corotation-dominated magnetosphere

Abstract: We present a model for plasma trans- port in a rotat i on-dominated magnet os pher e (e.g., Jupiter) containing an internal plasma source (e.g., Io). We make a distinction between magnetic flux tubes filled with outward moving plasma and less dense flux tubes that move in to compensate. In the outer region where the heavier tubes are effectively iso- lated, their motion is described by a pair of coupled differential equations which have ana- lytic solutions in the small velocity approxima- tion. In a steady state, conservation of flux- tube plasma content requires that the fraction of mass-loaded flux tubes be a function of the radial outflow speed. Similar equations govern the motion in the inner region where heavy flux tubes predominate and lighter tubes are isolated. This raises the possibility of net outward radial transport even when the radial gradient of average flux-tube content is posi- tive. In the outer region where heavy flux tubes are isolated, the average steady-state flux shell content decreases outward as the inverse square of radial distance.

Drastic increases of frequency and damping of a superconducting vibrating reed in a longitudinal magnetic field.

Abstract: Drastic increases of resonance frequency (factor of 7) and damping (factor of >500) of a superconducting reed is observed when a longitudinal magnetic field is applied. The frequency change is much larger than expected from the pole effect (10/sup 5/ times) or from the flux-line-tilt modulus (20 times). We give a quantitative explanation with no adjustable parameter. Unpinning of flux lines leads to frequency corrections and to damping. This allows novel precision measurements of extremely weak pinning in amorphous alloys. First experiments demonstrate the feasibility of this method.

Cyclotron radiation in strong magnetic fields

Abstract: Radiative transitions of electrons between low-lying Landau levels and the ground state in high magnetic fields are investigated. The relevant transition rates are calculated in the framework of spinor electrodynamics. Analytical approximations for various cases are compared to numerical evaluations. Special attention is given to the X-ray emission and absorption lines in magnetic fields of the order of 400 MT. 9 references.

The physics of the cometary contact surface

Abstract: The contact surface, which separates outflowing cometary plasma from solar wind controlled cometary plasma, is explained in terms of a balance between the magnetic pressure gradient force and ion-neutral drag. Giotto data indicate that the plasma pressure inside the contact surface cannot balance the sum of the external plasma and magnetic pressures, and therefore a Venus-like ionopause is not present at the contact surface. An expression for the magnetic field strength as a function of cometocentric distance is derived from the momentum equation. The theoretical magnetic field profile agrees quite well with the profile measured by the Giotto magnetometer in the vicinity of the contact surface.

Flux diverting flow chamber for high gradient magnetic separation of particles from a liquid medium

Abstract: A system for the magnetic separation of fragile particles, such as intact biological cells, from a fluid medium. The system includes at least one high-gradient magnetic separator (10) having a flow chamber (12) housing an interstitial separation matrix (30) and associated magnetizing apparatus (18, 22) for coupling magnetic flux to the matrix (30). The magnetizing apparatus includes opposing North (18) and South (22) poles and field-guiding pole pieces (20, 24), external to the flow chamber. The flow chamber comprises a dual-position flux-coupler operative in a first position in the capture phase and in a second position in an elutriation phase. In the capture phase, the flux-coupler is positioned to permit the magnetic flux from one magnetic pole to pass through the matrix to the other magnetic pole. In the elutriation phase, the flux-coupler is positioned so that magnetic flux is diverted from the matrix.

Electromagnetic torque sensor for a rotary shaft

Abstract: An electromagnetic sensor for measuring the twist of a rotary shaft in response to the transmission of torque therethrough. Rotary magnetic sleeve means coupled to longitudinally displaced points on the shaft rotate relative to each other in proportion to the shaft deformation. Permanent magnets provide a source of magnetic flux, and a magnetic flux path is defined by a combination of the rotary sleeve means and a segmented stationary sleeve means. Relative rotation of the rotary sleeve means with deformation of the shaft progressively alters the density of the magnetic flux in the segmented stationary sleeve means, and a magnetic flux transducer positioned in an air gap between the stationary sleeve means segments detects the flux density therein to provide an indication of the amount of torque transmitted through the shaft.

On the magnetic field line topology in solar active regions

Abstract: The field lines of closed magnetic structures above the photosphere define a mapping from the photosphere to itself. This mapping is discontinuous, and the field line connectivity to the boundary can change discontinuously in response to continuous changes of field strength and direction, if field lines either end in a singular point of the field or are tangential to the photosphere at one end. Whereas the general existence of singular points is questionable, the field has typically a cell structure due to the presence of segments of the zero line of the photospheric longitudinal field on which the transversal field is directed from negative (pointing into the Sun) to positive fields. The cell boundaries are made up of field lines which all touch the photosphere on one of these line segments. Within each of the cells the field line mapping is continuous. When during a slow evolution a substantial part of a coronal loop or of an arcade has passed from one cell into another a fast dynamic instability may set in which was previously prevented by the anchoring of field lines in the dense photosphere.