Showing papers on "Magnetic field published in 1979"

Accretion by rotating magnetic neutron stars. III. Accretion torques and period changes in pulsating X-ray sources.

Abstract: The solutions of the two-dimensional hydromagnetic equations are used to calculate the torque on a magnetic neutron star accreting from a Keplerian disk. It is found that the magnetic coupling between the star and the plasma in the outer transition zone is appreciable; that as a result, the spin-up torque on fast rotators is substantially less than that on slow rotators, and that for sufficiently high stellar angular velocities or sufficiently low mass accretion rates, the rotation of the star can be braked while accretion continues. These results are applied to pulsating X-ray sources, revealing that at high luminosities a star of given spin period rotating in the same direction as the disk can experience either spin-up or spin-down, depending on its luminosity. Also considered are the general problem of interpreting period changes in pulsating X-ray sources, and the dipole magnetic moments of nine pulsating X-ray sources are estimated by fitting the theoretical spin-up equation to estimates of the average luminosity and spin-up rate of each source.

Pinning in type II superconductors

Abstract: Large and randomly arranged pinning centers cause a strong deformation of a flux line lattice, so that each pinning center acts on the lattice with a maximum force. The average force for such single-particle pinning can be inferred from a simple summing procedure and has a domelike dependence on magnetic field. Pinning centers of average force, such as clusters of dislocations, strongly deform the flux line lattice only in weak fields and in fields close to the critical field, where there is a peak in the dependence of the critical current on magnetic field. In the range of intermediate fields there is a weak collective pinning. A large concentration of weak centers leads to collective pinning in all fields. In this case, near the critical field a critical current peak should be observed. To explain this peak and to define the boundaries between the regions of collective and single-particle pinning the possible break-off of the flux line lattice from the lines of magnetic force should be taken into consideration, which leads to extra softening of the lattice.

Random Magnetic Fields, Supersymmetry and Negative Dimensions

Abstract: We prove the equivalence, near the critical point, of a $D$-dimensional spin system in a random external magnetic field with a ($D\ensuremath{-}2$)-dimensional spin system in the absence of a magnetic field. This is due to the hidden supersymmetry of the associated stochastic differential equation. We identify a space with one anticommuting coordinate with a space having negative dimensions -2.

Structured coronae of accretion disks.

Abstract: A model for the fluctuating hard component of intense cosmic X-ray sources (such as Cyg X-1) is developed, based upon the amplification of magnetic fields by convective motions and differential rotation within a hot (T> or approx. =10/sup 6/K) accretion disk. Field reconnection within the inner portion of the disk is shown to be ineffective in limiting field amplification; magnetic fields may therefore attain strengths comparable to the equipartition value, leading to their emergence via buoyancy in the form of looplike structures and resulting in a very hot (T>10/sup 8/K) magnetically confined, structured corona analogous to the observed structure of the solar corona. The energy balance of these loop structures is examined, and it is shown that the disk soft X-ray luminosity determines the predominant energy loss mechanism in loops: at low disk luminosities, thermal bremstrahlung from these loops dominates and contributes a steady, shot-noise--like hard X-ray component. At high disk luminosities the emerging loops are Compton-cooled; the soft X-ray flux from the disk is Comptonized by the emerged loops, forming a transient, flarelike hard X-ray component.

Magnetosphere-ionosphere coupling

Abstract: A simple two-dimensional model of the magnetosphere-ionosphere system is discussed in which a localized electromotive force applied across a magnetic field line at t=0 is shown to propagate along the magnetic field with the Alfven velocity. The perpendicular electric field is assumed to reverse direction across the field line. Since the perpendicular electric field is limited in space, the propagation involves parallel electric fields whose magnitude depends on the characteristic scale length of the applied emf and the local plasma parameters. The electric field pulse associated with the ‘shock’ front is reflected at the ionosphere and propagates back to the source region. The finite Pedersen conductivity in the ionosphere damps the wave, and a steady state current system is established in the order of several hours. The parallel electric field can accelerate ions and electrons.

Liquid-Crystal Lens-Cells with Variable Focal Length

Abstract: Liquid-crystal cells shaped like a plano-convex lens or a plano-concave lens are prepared. The focal length can be varied from the value fe for an extraordinary ray, where the liquid-crystal cell takes a homogeneous alignment, to fo for an ordinary ray, where it takes a homeo-tropic alignment, by applying an electric field or a magnetic field across the lens-cell. The focal length in the lens-cell with the homogeneous alignment can also be switched between fe and fo. This is done by switching the direction of polarization of the incident light by applying a voltage across a TN cell sandwiched between the polarizer and the lens-cell. Since the thickness of the liquid-crystal layers increases in the lens-cells, the optical transmission properties are investigated as a function of the cell thickness.

High Mode Number Stability of an Axisymmetric Toroidal Plasma

Abstract: In the investigation of stability of a plasma confined by magnetic fields some of the most important modes of oscillation are those with long wavelength parallel to the magnetic field and short wavelength perpendicular to it. However, these characteristics conflict with the requirement of periodicity in a toroidal magnetic field with shear. This conflict can be resolved by transforming the calculation to one in an infinite domain without periodicity constraints. This transformation is the starting point for a full investigation of the magnetohydrodynamic stability of an axisymmetric plasma at large toroidal wave number n . (Small values of n can be studied by direct numerical computation but this fails when n is large.) For n > 1 there are two distinct length scales in the problem and a systematic approximation is developed around an eikonal representation, formally as an expansion in 1/ n . In lowest order the oscillations of each magnetic surface are decoupled and a local eigenvalue is obtained. However, the mode structure is not fully determined in this lowest order. In higher orders a second eigenvalue equation is obtained which completes the determination of the structure of the mode and relates the local eigenvalue of the lower order theory to the true eigenvalue for the problem. This higher order theory shows that unstable modes are localized in the vicinity of the surface with the smallest local eigenvalue, that the true eigenvalue is close to the lowest local eigenvalue and that the most unstable high n modes occur for n -> oo. Hence the local theory, which involves no more than the solution of an ordinary differential equation, is normally adequate for the determination of stability of any axisymmetric plasma to high mode number oscillations.

An explanation for anomalous equatorial ionospheric electric fields associated with a northward turning of the interplanetary magnetic field

Abstract: Anomalous reversals of the zonal equatorial electric field component have some- times been observed when the interplanetary mag- netic field turns northward from a steady south- erly direction. We suggest that this reversal is associated with a sudden change in the convection electric field in the magnetosphere and present measurements to support this explanation. Al- though slower variations in the convection field are shielded from the low latitude ionosphere by polarization charges at the inner edge of the ring current, these charges may require an hour or more to vary. A sudden decrease in the cross- tail electric field will thus be accompanied by a dusk-dawn perturbation electric field across the inner magnet o s phe re.

Ground state of a spin-½ charged particle in a two-dimensional magnetic field

Abstract: We prove that a spin-\textonehalf{} charged particle moving in a plane under the influence of a perpendicular magnetic field has ($N\ensuremath{-}1$) zero-energy states, where $N$ is the closest integer to the total flux in units of the flux quantum. The ($N\ensuremath{-}1$) independent wave functions are calculated explicitly. The result, which is extremely simple to prove, is an example of the Atiyah-Singer index theorem when applied to the Euclidean two-dimensional Dirac equation.

Random field effects in disordered anisotropic antiferromagnets

Abstract: A uniform magnetic field is shown to generate random local fields in uniaxially anisotropic antiferromagnets with random exchange interactions. This leads to a stronger divergence of the temperature derivative of the static susceptibility even at zero field, and to a drastic change in the critical exponents at the Ising-like antiferromagnetic transition as well as at the tricritical or bicritical points occurring at finite fields.

Polar cap electric field structures with a northward interplanetary magnetic field

Abstract: Polar cap electric fields patterns are presented from times when the S3-2 Satellite was near the dawn-dusk meridian and IMF data were available. With Bz ≥ 0.7 γ, two characteristic types of electric field patterns were measured in the polar cap. In the sunlit polar cap the convection pattern usually consisted of four cells. Two of the cells were confined to the polar cap with sunward convection in the central portion of the cap. The other pair of cells were marked by anti-sunward flow along the flanks of the polar cap and by sunward flow in the auroral oval. These observations are interpreted in terms of a model for magnetic merging at the poleward wall of the dayside polar cusp. The sunward flow in the auroral zone is not predicted by the magnetic model and may be due to a viscous interaction between the solar wind and magnetosphere. The second type, which was observed in some of the summer hemisphere passes and all of the winter ones, was characterized by an electric field pattern which was very turbulent, and may be related to inhomogeneous merging.

Equatorial electric fields during magnetically disturbed conditions 2. Implications of simultaneous auroral and equatorial measurements

Abstract: Simultaneous auroral and equatorial electric field data are used along with magnetic field data to study anomalous electric field patterns during disturbed times. During some substorms, accompanied by ring current activity, the worldwide equatorial zonal electric field component reverses from the normal pattern. This is interpreted as a partial closure of high latitude field aligned currents in the dayside, low latitude ionosphere. These currents flow westward across the dayside. In several cases the zonal equatorial electric field component was nearly identical in form to the zonal auroral component, indicating the close electrical coupling between these regions. Less certain, but equally intriguing, is the evidence presented for a close relationship between the zonal equatorial electric field and the time derivative of the ring current induced magnetic field. Another class of events seems related to rapid changes of magnetospheric convection and a temporary imbalance between the field external to the plasmasphere and the shielding charges in the Alfven layer. Examples of both rapid increases and decreases are presented. The latter seems oftern to be related to a northward turning of the interplanetary magnetic field.

Characterization of lightning return stroke electric and magnetic fields from simultaneous two‐station measurements

Abstract: The paper presents a characterization of Florida lightning return stroke electric and magnetic fields derived from simultaneous measurements of the fields at two separate stations, one station being within 15 km of the lightning, the other at either about 50 or 200 km from the lightning. Also presented are: (1) examples of correlated wave forms, (2) typical first and subsequent stroke wave forms over the distance range 1.0-200 km, and (3) the following statistical data from which the typical wave forms were derived: for electric field, rise time, initial peak value, ramp starting time, ramp slope, value at 170 microsec, ratio of value at 170 microsec to initial peak, zero-crossing time for 50 and 200 km wave forms; for magnetic fields, time of hump following initial peak, ratio of hump value to initial peak value, zero-crossing time for 50 and 200 km wave forms. Return stroke electric and magnetic field characteristics appear to be independent of location in Florida.

Association of low‐frequency waves with suprathermal ions in the upstream solar wind

Abstract: Observations obtained upstream of the earth's bowshock with the LASL/MPI plasma instruments and the UCLA magnetometers on ISEE-1 and 2 have revealed a striking relationship between the presence of low-frequency fluctuations in solar wind density and field strength and the different types of distribution functions of upstream ions. Waves are absent when the ions have the beamlike distribution of the 'reflected' ions. Large-amplitude waves are present only in conjunction with the 'diffuse' ions, which are characterized by flat energy spectra and broad angular distributions. The waves are largely compressive, showing very good correlation between oscillations in magnetic field strength and plasma density.

Magnetic field studies at Jupiter by Voyager 1 - Preliminary results

Abstract: Results obtained by the Goddard Space Flight Center magnetometers on Voyager 1 concerning the large scale configuration of the Jovian bow shock and magnetopause, and the magnetic field in both the inner and outer magnetosphere are highlighted. There is evidence that a magnetic tail extending away from the planet on the nightside is formed by the solar wind-Jovian field interaction. This is much like Earth's magnetosphere but is a new configuration for Jupiter's magnetosphere not previously considered from earlier Pioneer data. Magnetic field perturbations associated with intense electrical currents (approximately 5 x 10 to the 6th power amps) flowing near or in the magnetic flux tube linking Jupiter with the satellite Io and induced by the relative motion between Io and the co-rotating Jovian magnetosphere are analyzed and interpreted. These currents may be an important source of heating the ionosphere and interior of Io through Joule dissipation.

Remote position and orientation detection employing magnetic flux linkage

Abstract: The process and system for locating the position and orientation of an implement in space wherein a varying magnetic field responsive component (14) such as a coil is positioned upon the implement. A magnetic field source (12) excitable from an a.c. source derives alternating flux fields which are moved along or parallel to coordinate axes established at a reference plane. The coordinate position of the sensor is derived by determining the positions of minima and maxima flux linkage during magnetic field source movement.

Heating of ions to superthermal energies in the topside ionosphere by electrostatic ion cyclotron waves

Abstract: The soft particle spectrometer on the Isis 2 spacecraft occasionally observes fluxes of ions moving upward out of the ionosphere in the vicinity of the auroral oval. These ion fluxes are characterized by a sharp pitch angle distribution usually peaked at an angle somewhat greater than 90 deg, indicative of particles heated to a large transverse temperature in a narrow range below the spacecraft. The observations are interpreted in terms of electrostatic ion cyclotron waves, which heat the ions to superthermal energies transverse to the earth's magnetic field. When the transverse energy increases, the repulsive force of the earth's magnetic field, proportional to the particle magnetic moment, repels the particles away from the earth.

Relationship between mean radiated energy, mean squared radiated energy and spontaneous power spectrum in a power series expansion of the equations of motion in a free-electron laser

Abstract: To lowest order in the electric field and the inverse electron energy, the classical mean energy radiated by electrons in a free-electron laser with a symmetric magnet is equal to one-half the derivative, with respect to energy, of the classical mean squared radiated energy. The integral for the mean squared energy is also shown to be identical to the integral for the classical spontaneous power spectrum.

Magnetic diffusion and ionization fractions in dense molecular clouds - The role of charged grains

Abstract: The physics of magnetic diffusion in dense molecular clouds is examined, with particular attention given to the role of charged grains in controlling the process. The ionization fraction of dense molecular clouds in the presence of grains is determined from considerations of charge exchange, dissociative recombination, radiative recombination and collisions between grains and charged species, and it is found that the inclusion of grains tends to lower the ionization fraction for a given cosmic-ray ionization rate and metal depletion. The kinematics of grain motion is discussed and it is shown that at temperatures less than 30 K, each grain alternates rapidly in charge between -1 and 0 and thus executes periodic motion in a self-gravitating cloud containing a magnetic field. The full kinematics of magnetic diffusion including the motions of ions and electrons are then examined, taking into account the additional viscous force from charged grains, and numerical calculations of the diffusion time scales of uniform, magnetically supported clouds or cloud cores are presented.

The dc and ac electric field, plasma density, plasma temperature, and field‐aligned current experiments on the S3‐3 satellite

Abstract: Measurements of dc electric fields, field-aligned currents, the plasma density, and wave electric fields and density fluctuations have been made for the first time at auroral zone altitudes between 1000 and 8000 km on the S3-3 satellite. The design and operation of the various detectors that made these measurements is described, and some typical scientific results are presented. These include the observations that the high altitude auroral zone plasma density is typically 5–50 particles/cm³, that field-aligned currents exceeding the electrostatic ion cyclotron instability threshold are often observed in regions of ion cyclotron wave emission and accelerated upgoing ions, that the auroral zone electric field penetrates the plasmapause to cause deviations from corotation of the plasmasphere, and that the energetic electron high latitude trapping boundary is sometimes equatorward of the last closed magnetic field line, as determined by the reversal in the poleward electric field component.

Gaussons: Solitons of the Logarithmic Schrödinger Equation

Abstract: Properties of the Schrodinger equation with the logarithmic nonlinearity are briefly described. This equation possesses soliton-like solutions in any number of dimensions, called gaussons for their Gaussian shape. Excited, stationary states of gaussons of various symmetries, in two and three dimensions are found numerically. The motion of gaussons in uniform electric and magnetic fields is studied and explicit solutions describing linear and rotational internal oscillations are found and analyzed.

Fluidization: hydrodynamic stabilization with a magnetic field

Abstract: Fluidization of magnetizable particles by a gas stream in the presence of a uniform applied magnetic field oriented parallel to the flow prevents the hydrodynamic instability that otherwise leads to bubbles and turbulent motion within the medium. The fluidized emulsion expands uniformly in response to gas flow speeds in excess of that at the incipient fluidization point, with transition from the quiescent stable state to bubbling occurring suddenly at a characteristic increased rate of flow. Experimental data demonstrate the dependence of this transition velocity on the intensity of the applied magnetic field, length of the bed, and type of magnetic solids. Data illustrate the pressure distribution through the bed medium, the bedflow characteristics, and other phenomena.

Magnetic subband structure of electrons in hexagonal lattices

Abstract: The energy spectrum of an electron in the presence of a uniform magnetic field and a potential of hexagonal symmetry is analyzed. Two alternative approaches are used, one that takes as a basis set free-electron Landau functions, and a second one that treats an effective single-band Hamiltonian with the Peierls substitution. Both methods lead to consistent results. The energy spectrum is found to have recursive properties similar to those discussed by Hofstadter for the case of a square lattice. The density of states over each subband of the spectrum has the same structure as that for the original field-free band. The plot of integrated density of states versus field is also discussed.

Plasma wave turbulence at the magnetopause - Observations from ISEE 1 and 2

Abstract: An investigation of plasma wave electric and magnetic fields in the vicinity of the magnetopause using measurements from the ISEE 1 and 2 spacecraft is presented. Strong electric and magnetic field turbulence is often observed at the magnetopause; the electric field spectrum of this turbulence extends from less than a few hertz to over 100 kHz, and the magnetic field from a few hertz to about 1 kHz. Similar turbulence spectra are observed in association with flux transfer events and possible 'inclusions' of boundary layer plasma in the magnetosphere. Two possible plasma instabilities, the electrostatic ion-cyclotron and the lower-hybrid-drift instability, should explain the broad-band electric field turbulence; the narrow-band electrostatic emissions near the local electron plasma frequency are believed to be plasma oscillations or electrostatic waves near the upper-hybrid-resonance frequency.

Theory of ac losses in type‐II superconductors with a field‐dependent surface barrier

Abstract: The irreversible magnetic behavior of a type‐II superconductor in a periodically varying longitudinal applied magnetic field is theoretically examined. Bulk pinning is characterized by a critical current density Jc, and surface‐barrier effects are characterized by critical entry and exit fields Hen and Hex. Theoretical expressions for magnetic field and flux‐density profiles, voltage waveforms, hysteretic‐bulk and surface‐energy losses, and the ac permeability are derived for the arbitrary flux‐density (B) dependence of Jc, Hen, Hex, and the equilibrium magnetic field Heq. Numerical model calculations are performed to demonstrate the influence of the B dependence of the latter functions upon the measurable quantities.

Biological effects of magnetic fields: studies with microorganisms.

Abstract: Five bacteria and one yeast were grown in magnetic fields of 50–900 gauss with frequencies of 0–0.3 Hz and square, triangular, or sine waveform. Growth of these microorganisms could be stimulated o...

Density of states and magnetic properties of the rare-earth compounds RFe2, RCO2 and RNi2

Abstract: The density of states of yttrium compounds YM2 (M = Fe, Co, Ni) is calculated within a tight-binding scheme. Using the Stoner model, conclusions are drawn about their magnetic and electronic properties. In YCo2, we show that one has to introduce paramagnon effects in order to explain the electronic properties. The additional d-f interaction introduced by rare-earth atoms (R) in RM2 is treated with a Hartree-Fock approximation. Little change is obtained in iron and nickel compounds. Conversely, in RCo 2 compounds, the exchange field due to the rare-earth atoms can produce collective metamagnetism as described by Wohlfarth and Rhodes [1] and cobalt becomes magnetic. This effect stems from the position of the Fermi level of these compounds in a steep decrease of the density of states. Curie temperature, order of the transition and effect of an external magnetic field are studied following this method. Other consequences are drawn from the model.

Theory of electrokinetic‐magnetic anomalies in a faulted half‐space

Abstract: Fluid motion in the vicinity of a vertical fault separating regions of different streaming potential coefficient can produce an external magnetic field of observable magnitude. If tectonic stress changes along the fault produce fluid motion, the magnetic field changes would be indicative of stress changes, and might precede severe fault motion. The largest component of the magnetic field is oriented parallel to the strike of the fault. Magnetic field changes produced by this mechanism will have electric fields associated with them with similar time variation; this is in contrast to magnetic field changes produced by changes in susceptibility or remanent magnetization, which have no associated electric field. Experiments aimed at the detection of tectonomagnetic effects should include electric field sensors to help in determining the source of any observed variations. The hypothesis of Mizutani and Ishido (1976) that the magnetic field changes associated with the Matsushiro earthquake swarms were produced by electrokinetic currents seems reasonable.