Showing papers on "Magnetic field published in 1970"
TL;DR: In this paper, an electron monochromator for use in an axial magnetic field is described, where the electrons are injected parallel to the magnetic field and an electric field is applied in a perpendicular direction.
Abstract: An electron monochromator for use in an axial magnetic field is described. Electrons are injected parallel to the magnetic field and an electric field is applied in a perpendicular direction. The electrons thus describe trochoids and drift in a direction perpendicular to both the electric and magnetic fields and disperse according to their initial velocities. An electron energy width at half‐maximum of 0.020 eV can be obtained, with a transmitted current of about 10−9 A.
338 citations
TL;DR: Magnetospheric VLF electric field emissions above electron cyclotron frequency from OGO 5 observation at magnetic equator were reported in this paper, showing that the magnetic field is magnetic at the equator.
Abstract: Magnetospheric VLF electric field emissions above electron cyclotron frequency from OGO 5 observation at magnetic equator
335 citations
TL;DR: In this paper, the effective nonlinear Lagrangian derived by Heisenberg and Euler is used to describe the propagation of photons in slowly varying but otherwise arbitrary electromagnetic fields.
Abstract: The effective nonlinear Lagrangian derived by Heisenberg and Euler is used to describe the propagation of photons in slowly varying but otherwise arbitrary electromagnetic fields. The group and the phase velocities for both propagation modes are calculated, and it is shown that the propagation is always causal. The photon splitting processes are also studied, and it is shown that they do not play any significant role even in very strong magnetic fields surrounding neutron stars.
333 citations
TL;DR: In this article, a theoretical model for the process of re-connexion of frozen-in magnetic field lines at an X-type null point in the field is proposed, which involves a diffusion region, immediately adjacent to the null point and an outer wave dominated region.
Abstract: A theoretical model is proposed for the process of re-connexion of frozen-in magnetic-field lines at an X-type null point in the field. The model involves a diffusion region, immediately adjacent to the null point and an outer wave- dominated region. For the latter an exact solution to the magneto-hydrodynamic equations is obtained; for the former an approximate relationship is derived between the field-re-connexion rate, measured by the Alfven number of the incident flow, and the magnetic Reynolds number, based on the width of the diffusion region. The maximum field-re-connexion rate is determined entirely by conditions near the null point and may under suitable conditions become large. A condition for maximum conversion of magnetic energy is derived.
289 citations
TL;DR: In this article, an analogy between the laser threshold region and a second-order phase transition is presented, where the electric field acts as the order parameter for the laser while the atomic population inversion plays the role of a temperature.
Abstract: An analogy between the laser threshold region and a second-order phase transition is presented. The electric field acts as the order parameter for the laser while the atomic-population inversion plays the role of a temperature. The quantum-statistical theory of a laser including a symmetry-breaking external signal is discussed in order to establish a correspondence with the problem of a ferromagnet (treated in the molecular-field approximation) situated in an externally applied magnetic field. It is demonstrated that the results of the laser analysis may be discussed in terms of an energy function which is similar to the free energy of a thermodynamic system.
287 citations
TL;DR: In this paper, particle-in-cell simulation methods for doing full electromagnetic simulations of collisionless plasma phenomena are described and applied to the Weibel instability in one and two dimensions, and magnetic particle trapping and subsequent mode coalescing are seen.
Abstract: Particle‐in‐cell simulation methods for doing full electromagnetic simulations of collisionless plasma phenomena are described and applied to the Weibel instability in one and two dimensions. Magnetic particle trapping and subsequent mode coalescing are seen. Magnetic field energy is seen to reach 10% of the total particle energy. The different electromagnetic simulation methods are compared.
272 citations
TL;DR: In this paper, the Boltzmann equation in the local approximation of the electromagnetic wave equation in a tensor medium yields two normal modes, whose complex propagation constants and polarizations are determined by the microscopic medium parameters as well as the angle θ between wave vector q and applied magnetic field B0.
Abstract: The interaction of electromagnetic waves with free-carrier plasmas in semiconductors and semimetals is analysed, with particular emphasis on microwave and infrared effects arising in the presence of external magnetic fields. The general frequency- and field-dependent dielectric tensor is initially developed for a single isotropic band via the Boltzmann equation in the local approximation. Solution of the electromagnetic wave equation in the tensor medium yields two normal modes, whose complex propagation constants and polarizations are determined by the microscopic medium parameters as well as the angle θ between wave vector q and applied magnetic field B0. We first consider wave propagation in the lossless (collisionless) limit, and concentrate on the properties of normal modes in the Faraday (q parallel B0) and Voigt (q perpendicular B0) geometries. Essential features of the wave interaction with the medium are conveniently set forth in `contour maps' which display zeroes and infinities of the lossless dielectric constants, indicating resonances, dielectric anomalies and black-out regions for each mode of propagation over a wide range of frequencies and fields. Losses are then explicitly incorporated into the dielectric tensor, and their effect discussed in some detail. A number of experimental examples associated with the single band model are presented, including free-carrier absorption, cyclotron resonance, Faraday and Voigt effects, helicon waves, magnetoplasma reflection and small-particle effects. The presentation is subsequently generalized to more complicated systems. The dielectric tensor is developed for a multiple-carrier plasma and for anisotropic bands. A variety of new effects is seen to arise, including hybrid resonances, tilted-orbit resonances and Alfven wave propagation. `Contour maps' of appropriate lossless dielectric constants are again found convenient in discussing these new features. Effects of spatial dispersion (nonlocal phenomena) and of orbital quantization on the high-frequency response of a free-carrier system are then considered. The contribution of the polar lattice, which gives rise to a vast array of coupled magnetoplasma-phonon modes, is finally examined. A further elaboration of certain topics arising in this review is presented in an appendix, including a detailed discussion of electron dynamics, the `polariton' formalism, fundamentals of Kramers-Kronig analysis and magneto-optics of birefringent media.
259 citations
TL;DR: In this article, the exchange of protons between sites with different chemical shift is studied by means of N.M. relaxation time measurements (T 1ρ ) in the presence of an r.f. magnetic field.
Abstract: The exchange of protons between sites with different chemical shift is studied by means of N.M.R. relaxation time measurements (T 1ρ ) in the presence of an r.f. magnetic field. The system studied is chair-to-chair isomerization in cyclohexane. The exchange rate and chemical shift are measured in the temperature range 215°k to 250°k. The method is compared with other N.M.R. techniques for studying chemical exchange and is found to have advantages particularly at high exchange rates.
248 citations
TL;DR: In this article, a model of auroral arcs is presented, where the magnetic field lines into the auroral oval are loaded with kev electrons, and precipitating auroral electrons locally constitute a net field-aligned current, and currents close in the outer magnetosphere by polarization currents.
Abstract: A model of auroral arcs is presented. It is assumed that the magnetic field lines into the auroral oval are loaded with kev electrons, that precipitating auroral electrons locally constitute a net field-aligned current, and that currents close in the outer magnetosphere by polarization currents. The model predicts that a flux tube convecting through the oval will undergo a highly nonlinear oscillation that produces standing waves, the auroral arcs. The conditions to be satisfied are (1) Ohm's law in the ionosphere, (2) electron number conservation in the ionosphere, (3) electric current continuity, (4) the tendency for electric fields to map between ionosphere and magnetosphere. The physics of condition 4 is not understood, but it is probable that arc-like solutions exist for several models. A specific model, including perfect mapping of electric fields, is studied. The solutions to this model closely resemble the arcs. Predicted in agreement with observation are arc thickness, inter-arc spacing, and the electric field behavior. The thickness and the spacing are functions of both the amplitude of the oscillation and the physical properties of the system. However, thickness times spacing is a function of the physical properties only, and the thickness divided by the spacing is a function of amplitude only. Presumably these and other quantitative relationships can be checked by observations.
247 citations
TL;DR: Theoretical self-consistent relativistic electron beam models are developed in this article, which allow the propagation in excess of the Alfven-Lawson critical current limit for a fully neutralized beam.
Abstract: Theoretical self‐consistent relativistic electron beam models are developed which allow the propagation of relativistic electron fluxes in excess of the Alfven–Lawson critical‐current limit for a fully neutralized beam. Development of a simple, fully relativistic, self‐consistent equilibrium is described which can carry arbitrarily large currents at or near complete electrostatic neutralization. A discussion of a model for magnetic neutralization is presented wherein it is shown that large numbers of electrons from a background plasma are counterstreaming slowly within the beam so that the net current density in the system, and therefore, the magnetic field, is nearly zero. A solution of an initial‐value problem for a beam–plasma system is given which indicates that magnetic neutralization can be expected to occur for plasma densities that are large compared with beam densities. It is found that the application of a strong axial magnetic field to a uniform beam allows propagation regardless of the magnitude of the beam current. Some comparisons are made with recent experimental data.
236 citations
TL;DR: Magnetic field splitting of the quasiparticle energy states in superconducting aluminum films has been observed in a tunneling experiment as discussed by the authors, and the magnitude of the splitting was found to be $2.
Abstract: Magnetic field splitting of the quasiparticle energy states in superconducting aluminum films has been observed in a tunneling experiment. The magnitude of the splitting was found to be $2\ensuremath{\mu}H$, and is attributed to the magnetic moment of the quasiparticles. The observed tunneling conductance is in qualitative agreement with theory.
TL;DR: In this paper, the high frequency permeability of partially magnetized ferrites is calculated for some simple domain configurations, comprising only ''up'' and ''down''‐domains, and the method used is based upon the magnetostatic approximation and neglects exchange effects.
Abstract: The high‐frequency permeability of partially magnetized ferrites is calculated for some simple domain configurations, comprising only ``up''‐ and ``down''‐domains. The method used is based upon the magnetostatic approximation and neglects exchange effects, but is otherwise substantially rigorous. The components of the effective permeability tensor (ratio of average induction to average magnetic field) in general depends upon details of the domain configuration in addition to the net dc magnetization. When the dc magnetization is cycled between the two states of complete magnetization the high frequency permeability, considered as a function of the dc magnetization, in general shows hysteresis. Detailed calculations of the high‐frequency permeability have been carried out for the case in which the domain configuration is cylindrically symmetric, i.e., invariant under rotation around the direction of magnetization. For any such domain configuration the two relevant components μeff and κeff of the effective ...
TL;DR: Magnetic field lines reconnection in steady incompressible hydromagnetic two dimensional flow, formulating governing equations with cylindrical polar coordinates as mentioned in this paper, is a classic example.
Abstract: Magnetic field lines reconnection in steady incompressible hydromagnetic two dimensional flow, formulating governing equations with cylindrical polar coordinates
TL;DR: In this article, the effect of turbulence on a magnetic field whose length-scale L is initially large compared with the scale l of the turbulence is considered, and it is shown that turbulence that lacks reflexional symmetry is capable of amplifying Fourier components of the field on length scales of order Rm−2l and greater.
Abstract: The effect of turbulence on a magnetic field whose length-scale L is initially large compared with the scale l of the turbulence is considered. There are no external sources for the field, and in the absence of turbulence it decays by ohmic dissipation. It is assumed that the magnetic Reynolds number Rm = u0l/λ (where u0 is the root-mean-square velocity and λ the magnetic diffusivity) is small. It is shown that to lowest order in the small quantities l/L and Rm, isotropic turbulence has no effect on the large-scale field; but that turbulence that lacks reflexional symmetry is capable of amplifying Fourier components of the field on length scales of order Rm−2l and greater. In the case of turbulence whose statistical properties are invariant under rotation of the axes of reference, but not under reflexions in a point, it is shown that the magnetic energy density of a magnetic field which is initially a homogeneous random function of position with a particularly simple spectrum ultimately increases as t−½exp (α2t/2λ3) where α(= O(u02l)) is a certain linear functional of the spectrum tensor of the turbulence. An analogous result is obtained for an initially localized field.
TL;DR: In this paper, the relationship between the components of electric field perpendicular to the magnetic field line in the ionosphere and at the equator is computed for dipole and distorted magnetic field models under the assumption that magnetic field lines are electric equipotentials.
TL;DR: In this article, it was shown that a random superposition of inertial waves in a rotating conducting fluid can act as a dynamo, i.e. can systematically transfer energy to a magnetic field which has no source other than electric currents within the fluid.
Abstract: It is shown that a random superposition of inertial waves in a rotating conducting fluid can act as a dynamo, i.e. can systematically transfer energy to a magnetic field which has no source other than electric currents within the fluid. Dynamo action occurs provided the statistical properties of the velocity field lack reflexional symmetry, and this occurs when conditions are such that there is a net energy flux (positive or negative) in the direction of the rotation vector Ω.If the magnetic field grows from an infinitesimal level, then the mode of maximum growth rate dominates before the back-reaction associated with the Lorentz force becomes significant. This mode is first determined, and then the back-reaction associated with it alone is analysed. It is shown that the magnetic energy grows exponentially during the stage when the Lorentz forces are negligible, then reaches a maximum depending on the values of the parameters
\[
R_m = u_0 l/\lambda,\quad Q = \Omega l^2/\lambda,
\]
(u0 = initial r.m.s. velocity, l = length scale characteristic of the velocity field, λ = magnetic diffusivity) and ultimately decays as t−1 (equation (5.15)). This decay is coupled with a decay of the velocity field due to ohmic dissipation, and it occurs because there is no external source of energy for the fluid motion.
Journal Article•
TL;DR: In this paper, a simple model with constant convection electric field and a dipole magnetic field was used to calculate the plasma flow patterns with the convection fields as scale factors, which indicated that protons of a few hundred electron volts convected in from the tail to L = 3-4 could be responsible for the storm-time ring currents.
Abstract: By using a simple model with constant convection electric field and a dipole magnetic field, plasma flow patterns are calculated with the convection fields as scale factors. Unlike other particle trajectories, the flow patterns for protons with certain relative magnetic moments show double forbidden regions: one is composed of orbits that circle the earth; the other is composed of orbits that do not circle the earth. These protons can penetrate very close to the earth through the space between the two forbidden regions. The calculations based on the model of constant electric field with charge exchange as a loss mechanism indicate that protons of a few hundred electron volts convected in from the tail to L = 3–4 could be responsible for the storm-time ring currents.
TL;DR: In this paper, it was shown that the broadening of wave-particle resonances by the random motion of particles in a turbulent electric field may determine the saturation level of a variety of high-frequency instabilities.
Abstract: It is shown that the broadening of wave‐particle resonances by the random motion of particles in a turbulent electric field may determine the saturation level of a variety of high‐frequency instabilities. Secular changes of the guiding center positions, cyclotron radii, and phase angles give rise to resonance broadening and diffusion, similar to that produced by collisional scattering. The field dependent broadening is expressed in terms of resonance functions which replace the familiar resonant denominators of the linear theory. Resonance functions are derived in a simple manner from the solution of a Brownian motion problem, leading to an expression in terms of diffusion coefficients. The close resemblance of the theory to quasilinear theory and the linear theory including collisions allows one to start from a linear stability analysis and then assess the importance of nonlinear effects. This method is illustrated by the determination of the saturation level of cyclotron instabilities from the condition of vanishing nonlinear growth rate.
TL;DR: In this paper, the magnetic susceptibility of conduction electrons in the presence of spin-orbit interaction is calculated and the model is that due to Wolff which assumes two bands separated by a small energy gap.
Abstract: The magnetic susceptibility of conduction electrons in the presence of spin-orbit interaction is calculated. The model is that due to Wolff which assumes two bands separated by a small energy gap. Our calculation shows that due to the interband effect of the magnetic field the susceptibility has a large contribution to diamagnetism when the Fermi energy lies in the band gap region. This result is applied to bismuth, and possible interpretations are proposed for the dependence of its magnetic property on the effective valence number and the cause of the anisotropy.
TL;DR: Vector magnetic field and energetic particle flux profiles, indicating geomagnetically aligned currents associated with visible auroral arc as discussed by the authors, were used to identify geomagnetic magnetic fields.
Abstract: Vector magnetic field and energetic particle flux profiles, indicating geomagnetically aligned currents associated with visible auroral arc
TL;DR: In this article, the absorption coefficient and polarization selection rules for photon splitting in a strong magnetic field were determined, and the possible application of their results to pulsars was discussed. But the authors did not consider the effect of the magnetic field on photon splitting.
Abstract: We determine the absorption coefficient and polarization selection rules for photon splitting in a strong magnetic field, and describe the possible application of our results to pulsars.
TL;DR: In this paper, a surface bound state of electrons localized in a narrow accumulation layer at the InAs-oxide interface was observed in a magnetic field of up to 85 kG.
Abstract: Using electron tunneling, we have made direct observation of a surface bound state of electrons localized in a narrow accumulation layer at the InAs-oxide interface. The Landau-level structure of the two-dimensional energy band, associated with the bound state, has been studied in a magnetic field up to 85 kG.
TL;DR: In this article, the problem of correcting the observed chemical shift for the difference in the bulk susceptibilities of the sample and the reference compound was treated for the Varian A-60 or HA-100 spectrometer.
Abstract: WHEN A NUCLEAR magnetic resonance (NMR) spectrum is referenced with an external standard contained either in a capillary inserted coaxially in a cylindrical sample or in the annular region of a coaxial sample, the observed chemical shift must be corrected for the difference in the bulk susceptibilities of the sample and the reference compound ( I ) . This problem has been treated at length for the conventional magnet/sample configuration, such as that of the Varian A-60 or HA-100 spectrometer, where the applied polarizing magnetic field is transverse to the long axis of the cylindrical sample (2). For this magnet/sample geometry, the bulk susceptibility correction to the observed chemical shift has been shown to be
TL;DR: In this paper, a model current system, in which magnetospheric and ionospheric sections are connected by currents flowing along the geomagnetic field lines, is proposed to represent the current system responsible for polar magnetic substorms.
Abstract: A model current system, in which magnetospheric and ionospheric sections are connected by currents flowing along the geomagnetic field lines, is proposed to represent the current system responsible for polar magnetic substorms. The magnetic perturbations from model current systems of this type are studied in terms of elementary loops, whose magnetic effects are evaluated numerically. The influence of currents induced in the ground is studied for a time-independent case, but it is found that such effects do not change the gross character of the perturbation pattern. Using a north-south chain of stations and an appropriate coordinate system, the model predictions are compared with the magnetic variations observed during some substorms. It is found that the model is capable of explaining the gross features of the magnetic perturbation pattern observed. However, at times there are minor deviations from the predictions, which suggest the presence of additional ionospheric and magnetospheric current systems.
TL;DR: In this article, saddle-shaped coils for generating a field perpendicular to the axis of a cylindrical shell to which the coils are confined can be designed in a compact and easily constructed form.
Abstract: Saddle shaped coils for generating a field perpendicular to the axis of a cylindrical shell to which the coils are confined can be designed in a compact and easily constructed form. The central magnetic field of such a system and its second derivatives with respect to displacements from the center are given as functions of the coil dimensions, and conditions for minimizing these derivatives are discussed. A coil pair with a length‐to‐diameter ratio of 2 and circular arcs of 120° will have no second order central field derivatives in any direction.
TL;DR: In this article, an asymptotic analysis of the Langmuir probe problem in a quiescent, fully ionized plasma in a strong magnetic field is performed, for electron cyclotron radius and Debye length much smaller than probe radius, and this not larger than either ion cycloton radius or mean free path, and it is found that the electric potential, which is not confined to a sheath, controls the diffusion far from the probe; inside the magnetic tube bounded by the probe cross section the potential overshoots to a large value before decaying to its value
Abstract: An asymptotic analysis of the Langmuir‐probe problem in a quiescent, fully ionized plasma in a strong magnetic field is performed, for electron cyclotron radius and Debye length much smaller than probe radius, and this not larger than either ion cyclotron radius or mean free path. It is found that the electric potential, which is not confined to a sheath, controls the diffusion far from the probe; inside the magnetic tube bounded by the probe cross section the potential overshoots to a large value before decaying to its value in the body of the plasma. The electron current is independent of the shape of the body along the field and increases with ion temperature; due to the overshoot in the potential, (1) the current at negative voltages does not vary exponentially, (2) its magnitude is strongly reduced by the field, and (3) the usual sharp knee at space potential, disappears. In the regions of the C‐V diagram studied the ion current is negligible or unaffected by the field. Some numerical results are presented. The theory, which fails beyond certain positive voltage, yields useful results for weak fields, too.
TL;DR: In this paper, the authors compared the theory proposed by Brown in 1949, in which the stress is replaced by an equivalent field, which has been found to be valid at the smaller stresses.
Abstract: Measurements have been made of magnetization curves obtained by applying stresses to various polycrystalline magnetic materials in the presence of a small, constant, magnetic field. The materials examined were nickel, mild steel, and both isotropic and cube textured silicon iron. Residual stray fields were carefully compensated. Both compressive and tensile stresses were applied in the range 0 to 10 kg mm−2. The curves are compared with the theory proposed by Brown in 1949, in which the stress is replaced by an equivalent field, which has been found to be valid at the smaller stresses. There are many features which cannot be reconciled with Brown's analysis, in particular the difference between tension and compression. A tentative explanation is put forward based on discontinuous changes in domain structure.
TL;DR: In this article, the effects of exchange on the magnetostatic surface and bulk spin waves of a ferromagnetic film in the regime where the dipole and exchange fields are both important are determined from the solutions of a sixth-order differential equation.
Abstract: The effects of exchange on the magnetostatic surface and bulk spin waves of a ferromagnetic film in the regime where the dipole and exchange fields are both important are determined from the solutions of a sixth‐order differential equation. The eigenstates are admixtures of bulk and surface waves. With an applied magnetic field parallel to the film surface and for small values of the wave vector parallel to the surface, k, the Damon and Eshbach surface state is split into segments which join adjacent bulk branches. No sharp cutoff of the surface branch occurs as the angle between k and the applied field is increased, instead the character of the branch changes continuously from surface‐like to bulk‐like. For values of | k |≳104 a number of branches have significant surface character so that no single branch can be identified with the Damon‐Eshbach surface state. When the applied field is perpendicular to the surface a new type of surface spin wave occurs below the bulk manifold and is characterized by a c...