Showing papers on "Magnetic field published in 1974"

The motion of 180° domain walls in uniform dc magnetic fields

Abstract: The equations of motion of a 180° domain wall in an infinite uniaxially anisotropic medium which is exposed to an instantaneously applied uniform dc magnetic field H0 have been integrated numerically. Below the critical field Hc =2παM0 (α is the Gilbert loss parameter and M0 the saturation magnetization), where a steady‐state solution is known to exist, it is shown that the wall motion tends smoothly to this solution. Above Hc, the magnetization precesses about the field and a periodic component appears in the forward motion of the wall. Analytic solutions for the wall motion have been found based upon approximations suggested by the computed behavior; these reproduce the computer results very accurately.

First-Order Phase Transitions in Superconductors and Smectic- A Liquid Crystals

Abstract: The superconducting phase transition is predicted to be weakly first order, because of effects of the intrinsic fluctuating magnetic field, according to a Wilson-Fisher $\ensuremath{\epsilon}$-expansion analysis, as well as a generalized mean-field calculation appropriate to a type-I superconductor. Similar results hold for the phase transition from a smectic-$A$ to a nematic liquid crystal.

Plasma heating by spatial resonance of Alfven wave

Abstract: Heating of a collisionless plasma by utilizing the spatial resonance of shear Alfven waves is proposed and application to toroidal plasmas is discussed. The resonance exists due to the nonuniform Alfven speed. This heating scheme is analyzed in one dimension including the effects of a shear magnetic field and plasma compressibility. For plasmas with smooth nonuniformities (| k⊥l | ≫ 1, k⊥ is the wavenumber prependicular to the ambient magnetic field and the nonuniformity direction, and l is the scale length of the nonuniformity), the energy absorbed per unit surface area per driving cycle is [b02(μ0k⊥)−1]. Here, b0 is the flux density of the driving magnetic field evaluated at the resonant point. With sharp nonuniformities (| k⊥l | ≪ 1), absorption is large if the surface eigenmode is excited. The corresponding value is [b02(μ0k⊥)−1(k⊥l)−1]. Otherwise, it is [b02(μ0k⊥)−1(k⊥l)].

A theory of long‐period magnetic pulsations: 2. Impulse excitation of surface eigenmode

Abstract: A theory of long-period magnetic pulsations (Pc 3 to Pc 5) is presented as an initial value problem to explain impulse-excited pulsations. By using a one-dimensional model a wave equation that shows a coupling between a surface wave and a shear Alfven wave is derived. By solving this equation on the basis of initial value approach we conclude that there is a continuous spectrum with damping proportional to inverse power of time and that there are weakly damped discrete eigenmodes (surface eigenmodes) due to sharp variations in the plasma parameters. The frequency ωr and the damping rate γ of the surface eigenmode are given approximately by ωr = k∥[(BI² + BII²)/μ0(ρI + ρII)]1/2 and γ/ωr = |k⊥/▽(ln υA²)| respectively, where υA(=B/(μ0ρ)1/2) is the Alfen speed, k∥ and k⊥ are wave numbers parallel and perpendicular to the magnetic field, and subscripts I and II refer to quantities associated with each side of the surface. The result is used to explain recent observations of plasmapause-associated magnetic pulsations as well as magnetic pulsations excited by sudden commencements and sudden impulses near the magnetopause.

Incoherent scatter measurements of E region conductivities and currents in the auroral zone

Abstract: Data taken by incoherent scatter radar have been used to investigate ionospheric conductivities and electrical currents. During quiet days, the conductivities appear to vary in a way consistent with ionization arising from solar EUV radiation. In the evening hours, enhancements in the northward electric field are found to precede small increases in the conductivities. Strong enhancements of the Hall conductivity relative to the Pedersen conductivity occur during negative bays when the electric field is in a southwestward direction. The ionospheric currents calculated in the geomagnetic east-west direction are in good agreement with the H component measured by a nearby magnetometer; this result indicates that the current causing the ground level magnetic fluctuations is a broad horizontal sheet current. The north-south ionospheric current, however, consistently disagrees with the observed D component in a manner that cannot easily be explained unless currents parallel to the earth's magnetic field are present.

Theory of Quantum Transport in a Two-Dimensional Electron System under Magnetic Fields. IV. Oscillatory Conductivity

Abstract: The level broadening and the transverse conductivity under magnetic fields of arbitrary strength are studied in the simplest approximation free from the difficulty of divergence caused by the singular nature of the system. Scatterers are assumed to be of short-range. The peak value of the conductivity associated with each Landau level decreases slowly with decreasing magnetic field. Assumption of a phenomenological constant relaxation time is not sufficient and the self-consistency is important even under rather weak magnetic fields. The oscillation of the density of states and the conductivity are numerically calculated.

Nonlinear evolution of parallel‐propagating hydromagnetic waves

Abstract: The nonlinear evolution of plane hydromagnetic fluctuations propagating along the unperturbed magnetic field direction is considered. From an expansion of the ideal magnetohydrodynamic equations and the hydromagnetic shock jump conditions, it is shown that a wave in which the magnitude of the magnetic field is nonconstant steepens into a shock and subsequently evolves toward a purely Alfvenic fluctuations of lower mean energy density. Explicit expressions are derived for the asymptotic state and for the characteristic lines which describe the evolution toward that state. A class of fluctuations which includes linearly polarized waves is shown to evolve into rotational discontinuities. The results are applied to observations of hydromagnetic fluctuations in the solar wind.

Ionosphere-Magnetosphere Coupling

Abstract: A review is given of some aspects of the electric coupling between the ionosphere and magnetosphere. Topics discussed are the mapping of time-dependent electric fields, the effects of Birkeland currents on the magnetospheric plasma convection, the relation of Birkeland currents to ionospheric electric field structures such as field reversals, secondary Birkeland currents of ionospheric origin and triggering of auroral particle precipitation, double-layers causing voltage drops along the magnetic field lines, and an equivalent electric circuit for the substorm currents in the coupled ionosphere-magnetosphere system.

Theory of surface polaritons in anisotropic dielectric media with application to surface magnetoplasmons in semiconductors

Abstract: A theory is presented for surface polaritons associated with the planar surface of a semi-infinite anisotropic dielectric medium. Retardation is included. In general, two attenuating components with different attenuation constants must be superposed within the medium in order to satisfy the boundary conditions, and the macroscopic electric field vector does not lie in the sagittal plane. For special cases, however, only one attenuating component is required, and the electric vector does lie in the sagittal plane. The theory is applied to the specific case of surface magnetoplasmons in a semiconductor for magnetic fields either perpendicular or parallel to the surface. In the latter case, propagation directions parallel and perpendicular to the magnetic field are considered. Possibilities for the experimental observation of the effects predicted are discussed.

Magnetism and the interior of the Moon

Abstract: The application of lunar magnetic field measurements to the study of properties of the lunar crust and deep interior is reviewed. Following a brief description of lunar magnetometers and the lunar magnetic environment, measurements of lunar remanent fields and their interaction with the solar plasma are discussed. The magnetization induction mode is considered with reference to lunar magnetic permeability and iron abundance calculations. Finally, electrical conductivity and temperature calculations from analyses of poloidal induction, for data taken in both the solar wind and in the geomagnetic tail, are reviewed.

Cosmic-ray streaming perpendicular to the mean magnetic field

Abstract: Starting from a quasi-linear approximation for the ensemble-averaged particle distribution function in a random magnetic field, the complete diffusion tensor is derived. This is done by assuming a simple form for the ensemble-averaged distribution function, explicitly retaining all components of the streaming flux. This derivation obtains the antisymmetric terms in a natural manner. The necessary dropping of higher-order terms gives a criterion for the lower-energy limit of validity of the perpendicular and antisymmetric diffusion coefficients. The limit for the assumed distribution function is about 0.8 GV rigidity in the interplanetary field near 1 AU.

The average auroral zone electric field

Abstract: Four hundred and seventy-eight hours of electric field data obtained by the flight of 32 balloons between L = 5.4 and L = 8.2 have been averaged to determine mean properties of the auroral zone electric field. The results are understood in terms of a model in which the two-cell convection pattern is rotated about the sun-earth line by an amount that depends on the sign and magnitude of the y component of the interplanetary magnetic field. This model and the data suggest either that important parallel potential drops exist along auroral zone magnetic field lines or that simple conjugacy between the two hemispheres does not exist along such field lines.

High-latitude electric fields and the three-dimensional interaction between the interplanetary and terrestrial magnetic fields

Abstract: More than 200 hours of ionospheric electric field measurements taken on balloons flown from three polar cap sites have been analyzed to determine average properties of the large-scale polar cap electric field and its dependencies on the interplanetary magnetic field. The major component of this electric field was directed from dawn to dusk and produced an average polar cap potential drop of about 55 kV. The magnitude of this potential provides an upper limit of about 700 RE for the length of the magnetospheric tail and implies an energy input from the solar wind to the magnetosphere of about 5 × 1019 ergs/sec. The dawn to dusk component of the high-latitude polar cap electric field responds to Bz, the northward component of the interplanetary magnetic field, on a time scale ≲1 hour and with an average increase of about 3 mV/m for each 1 γ decrease of Bz. The hourly averages of the electric field data at each of the three sites are well described by a two-cell convection pattern whose location depends on the y component of the interplanetary magnetic field. When By is positive (negative), the two-cell convection pattern shifts toward dawn (dusk) in the northern hemisphere with the following consequences: the maximum intensity of the northern polar cap dawn to dusk electric field component occurs at local morning (evening), and the auroral zone return flow reaches higher latitudes in the evening (morning). Evidence of the vector nature of the interaction between interplanetary and terrestrial magnetic fields is provided by the observation that the above By dependent signatures are most evident when Bz is most negative.

Finite-Element Solution of the Eddy-Current Problem in Magnetic Structures

Abstract: Analysis of the eddy-currentproblem in magnetic structures by the method of Finite-elements is presented. The linear diffusion equation representing the appropriate energy functional is described. The field region is discretised by triangular Finite-elements and the solution to the field problem is obtained by minimizing the energy functional with respect to each of the vertex values of the vector potential. Expressions for the magnetic field, electric field and eddy-current losses are presented. The method is applied to a few cases of engineering interest and compared with results of classical analysis and tests.

Plasma oscillations of a two-dimensional electron gas in a strong magnetic field

Abstract: The quantum-mechanical response of a two-dimensional electron gas in the presence of a strong dc magnetic field is calculated in the random-phase approximation. The results are used to discuss the magnetoplasma oscillations of a two-dimensional electron gas. The general dispersion relation is derived, and numerical results are presented for both long and short wavelengths.

Theory of magnetic insulation

Abstract: An investigation is made of the behavior of a high voltage diode for the situation where the cathode‐anode gap is initially filled with a transverse magnetic field. An exact relativistic self‐consistent equilibrium is found for the electron sheath which is expected to form under the condition where the applied magnetic field is sufficiently strong to prevent electrons from flowing between the electrodes. The condition on the magnetic field for “insulation” is found to be (eBy0d/mc2)2 > 2|eV0/mc2| + (eV0/mc2)2, where By0 is the applied magnetic field, V0 is the voltage across the diode, d is the cathode‐anode separation in the x direction, and —e and m are the electron charge and rest mass, respectively.

A Convection-Driven Dynamo: I. The Weak Field Case

Abstract: A hydromagnetic dynamo model is considered. A Boussinesq, electrically conducting fluid is confined between two horizontal planes and is heated from below. The system rotates rapidly about the vertical axis with constant angular velocity. It is supposed that instability first sets in as stationary convection characterized by a small horizontal length scale. In preliminary calculations the Lorentz force is neglected so that the magnetic induction equation and the equation of motion are decoupled. The possibility that motions occurring at the onset of instability may sustain magnetic fields is thus reduced to a kinematic dynamo problem. Moreover, the existence of two length scales introduces simplifications which enable the problem to be studied by well-known techniques. The effect of the Lorentz force on the finite amplitude dynamics of the system is investigated also. Since only weak magnetic fields are considered the kinetic energy of the motion is fixed by other considerations and it is only the fine structure of the flow that is influenced by the magnetic field. A set of nonlinear equations, which govern the evolution of the hydromagnetic dynamo, are derived from an asymptotic analysis. The equations are investigated in detail both analytically and numerically. In spite of serious doubts concerning the existence of sufficiently complex stable motions, stable periodic dynamos are shown to exist. An interesting analytic solution of these equations, which may be pertinent to other problems arising from finite-amplitude Benard convection, is presented in the final section.

Relativistic electron dynamics in a cusped magnetic field

Abstract: Single‐particle motion of relativistic electrons in a cusped magnetic field has been studied both analytically and experimentally. The Lagrangian formulation is used to solve for the electron trajectories in some simple magnetic cusp configurations. A threshold energy for transmission through the cusp is found, and confirmed experimentally. Two different electron orbit off‐centering mechanisms are discussed; one arising from a nonzero radial component of particle velocity on the upstream side of the cusp transition, and another arising from the finite width of the cusp transition. Experiments are reported confirming the existence of these off‐centering mechanisms, and the results are compared with theoretical expectations.

Transport coefficients of InAs epilayers

Abstract: Heteroepitaxially grown single‐crystal InAs layers exhibit anomalies in the magnetic field dependence of their Hall coefficients, their magnetoresistance, and the temperature dependence of their electron mobilities. These are attributed to a degenerate surface accumulation of electrons and are interpreted in terms of a two‐layer model, one with bulklike and the other with surfacelike charge carrier transport coefficients.

The Fokker-Planck coefficient for pitch-angle scattering of cosmic rays

Abstract: For the case of homogeneous, isotropic magnetic field fluctuations, it is shown that most theories which are based on the quasi-linear and adiabatic approximation yield the same integral for the Fokker-Planck coefficient for the pitch angle scattering of cosmic rays. For example, despite apparent differences, the theories due to Jokipii and to Klimas and Sandri yield the same integral. It is also shown, however, that this integral in most cases has been evaluated incorrectly in the past. For large pitch angles these errors become significant, and for pitch angles of 90 deg the actual Fokker-Planck coefficient contains a delta function. The implications for these corrections relating cosmic ray diffusion coefficients to observed properties of the interplanetary magnetic field are discussed.

Magnetic field generation apparatus

Abstract: A device for generating an easily accessible, substantially uniform and parallel magnetic field A number of identical solenoid posts are arranged symmetrically about a central axis and secured by corresponding ends to a pair of end-plates of magnetic material, to form an open, cagelike structure Equal magnetomotive forces are applied by separate windings on each post, the resulting magnetic field in the space between the endplates being substantially uniform and parallel to the axis, any non-axial flux components within the structure tending to cancel each other

Magnetic field profiles in type-II superconductors with pinning using a new ac technique

Abstract: A new ac technique is described which allows the experimental determination of the magnetic field profile near the sample surface in type‐II superconductors with pinning. The technique, like a previous method of Campbell, depends only on the validity of the critical‐state model and can separate the position and magnetic field dependence of the flux‐pinning forces. The entire field profile is obtained from a measurement of the waveform of the response of the sample to a small ac magnetic field superimposed on the dc field. Profiles obtained in cold‐worked Nb and Nb–Ti alloys are reported and the results compared with previous measurements. We have found that the critical‐state model does not adequately describe the results obtained on the Nb–Ti alloys when the magnetic field is less than 0.5Hc2.