Showing papers on "Magnetic field published in 1975"

Possibility of an inverse cascade of magnetic helicity in magnetohydrodynamic turbulence

Abstract: Some of the consequences of the conservation of magnetic helicity for incompressible three-dimensional turbulent MHD flows are investigated. Absolute equilibrium spectra for inviscid infinitely conducting flows truncated at lower and upper wavenumbers kmin and kmax are obtained. When the total magnetic helicity approaches an upper limit given by the total energy (kinetic plus magnetic) divided by kmin, the spectra of magnetic energy and helicity are strongly peaked near kmin; in addition, when the cross-correlations between the velocity and magnetic fields are small, the magnetic energy density near kmin greatly exceeds the kinetic energy density. Several arguments are presented in favour of the existence of inverse cascades of magnetic helicity towards small wavenumbers leading to the generation of large-scale magnetic energy.

Observation of stimulated emission of radiation by relativistic electrons in a spatially periodic transverse magnetic field

Abstract: : Gain has been observed at 10.6 micrometers due to stimulated emission of radiation by relativistic electrons in a spatially periodic transverse magnetic field. The magnitude of the measured gain is close to the theoretical value for the stimulated emission process. The dependence of the gain on electron energy and current and on the polarization of the stimulating radiation are also consistent with the stimulated emission hypothesis. The results raise the possibility that this mechanism can be used in the development of a new class of tuneable high power lasers.

Three-Dimensional Induced Polarization and Electromagnetic Modeling

Abstract: The induced polarization (IP) and electromagnetic (EM) responses of a three‐dimensional body in the earth can be calculated using an integral equation solution. The problem is formulated by replacing the body by a volume of polarization or scattering current. The integral equation is reduced to a matrix equation, which is solved numerically for the electric field in the body. Then the electric and magnetic fields outside the inhomogeneity can be found by integrating the appropriate dyadic Green’s functions over the scattering current. Because half‐space Green’s functions are used, it is only necessary to solve for scattering currents in the body—not throughout the earth. Numerical results for a number of practical cases show, for example, that for moderate conductivity contrasts the dipole‐dipole IP response of a body five units in strike length approximates that of a two‐dimensional body. Moving an IP line off the center of a body produces an effect similar to that of increasing the depth. IP response va...

The motion of a proton in the equatorial magnetosphere

Abstract: A proton of low energy moving in the equatorial plane of the earth will experience drift motions due to both the magnetic field (magnetic gradient drift only, if the field is assumed to be that of a dipole) and the electric field The electric drift again separates into two parts - the drift due to the main electric field (or convection electric field) existing in the frame of the earth, and that due to the earth's rotation One result indicated by this work is that at distances of 4 - 6 earth radii, a transition from trapped proton orbits to open trajectories leading to the tail occurs at about 10 kev, the precise value depending upon local time Such a transition also seems to be indicated by particle observations using Explorer 45 The energy spectrum (at magnetically quiet times) of equatorial protons above this energy can be explained by charge exchange but increased flux observed below it seems to be related to the influx of particles on open orbits from the tail

A quantitative magnetospheric model derived from spacecraft magnetometer data

Abstract: Quantitative models of the external magnetospheric field were derived by making least-squares fits to magnetic field measurements from four IMP satellites The data were fit to a power series expansion in the solar magnetic coordinates and the solar wind-dipole tilt angle, and thus the models contain the effects of seasonal north-south asymmetries The expansions are divergence-free, but unlike the usual scalar potential expansions, the models contain a nonzero curl representing currents distributed within the magnetosphere Characteristics of four models are presented, representing different degrees of magnetic disturbance as determined by the range of Kp values The latitude at the earth separating open polar cap field lines from field lines closing on the dayside is about 5 deg lower than that determined by previous theoretically-derived models At times of high Kp, additional high latitude field lines are drawn back into the tail

The generation of magnetic fields in astrophysical bodies. X - Magnetic buoyancy and the solar dynamo

Abstract: The magnetic field appearing as bipolar magnetic regions at the surface of the sun represents the lines of force from a general azimuthal field of the order of 100 gauss somewhere beneath the surface. The amplification time, as a consequence of the nonuniform rotation, is of the order of 10 years. But magnetic buoyancy brings the azimuthal field up through much of the convective zone in a time rather less than 10 years, raising the question of where the azimuthal field can be retained long enough to be amplified. We show that magnetic fields can be retained for long periods of time in the stable radiative region beneath the convective zone, but unfortunately the solar dynamo cannot function there because turbulent diffusion is an essential part of its operation. The only possible conclusion appears to be that the dynamo operates principally in the very lowest levels of the convective zone at depths of 150,000 km or more, where the gas density is 0.1 g/cu cm, and the fields are limited to 50 gauss.

Observation of the Phase Shift of a Neutron Due to Precession in a Magnetic Field

Abstract: We have directly observed the sign reversal of the wave function of a fermion produced by its precession of 2π radians in a magnetic field using a neutron interferometer.

Faraday-Rotation Measurements of Megagauss Magnetic Fields in Laser-Produced Plasmas

Abstract: Magnetic fields in the megagauss range have been observed in the laser-produced plasma near the focus of a high-power laser pulse. Faraday-rotation measurements utilizing the light of a probing beam and the specularly reflected laser light both show the presence of these large fields.

Magnetic merging in a collisionless plasma

Abstract: This paper examines the problem of magnetic merging in a fully collisionless plasma, i.e., a plasma of noninteracting charged particles, explicitly avoiding the fluid (MHD) approximation. The Alfven self-consistency criterion that relates the plasma density to the electric and magnetic fields is shown to be equivalent, when suitably generalized, to the stress-balance requirement discussed by Rich, Vasyliunas, and Wolf. From this criterion the self-consistent electric field and the magnetic merging speed are obtained as functions of the magnetic field configuration and of the incident plasma parameters. The merging speed obtained for equal antiparallel fields is within a factor of 2 of the result of fluid theory if the incident plasma pressure is initially isotropic. The merging speed is respectively reduced or enhanced when the incident parallel plasma pressure P∥ is greater than or less than the incident transverse pressure P⊥. When P∥ - P⊥ = B²/µ0 (the marginal fire hose stability condition), merging ceases. For the more general field configuration wherein the opposing magnetic fields B1 and B2 have arbitrary magnitudes and directions the merging electric field is shown to have a maximum value when the fields are antiparallel and to decrease monotonically to zero as the angular separation of the fields decreases to arccos (B1/B2). The expected merging electric field at the day side magnetopause as a function of the strength and direction of the interplanetary magnetic field is presented in a form that can be compared directly with observations. Although a detailed comparison is not feasible, the general results of the analysis are compatible with available observations in the magnetosphere and solar wind.

Audio-frequency magnetotellurics with a grounded electric dipole source

Abstract: The conventional audio‐frequency magnetotelluric method has been useful in problems of shallow exploration to depths of a few kilometers. Because the natural sources are unpredictable in strength and direction, we have examined the possibility of doing magnetotelluric sounding utilizing the ratio of horizontal electric field to horizontal magnetic field from a controlled source. The source chosen for this study was a grounded electric dipole. The equations for the electric and magnetic fields around this antenna as a function of range and azimuth have been calculated for a half‐space and for a one‐layered earth. These calculations were checked for the half‐space case with an analog model and in the field at the Bonneville Salt Flats. In addition, layered cases were calculated, and a field example near Timmins, Ont., studied. Provided the distance of the observation point from the source is three skin depths (relative to the greatest resistivity in the section) or more, the conventional magnetotelluric int...

The macrodynamics of α-effect dynamos in rotating fluids

Abstract: Past study of the large-scale consequences of forced small-scale motions in electrically conducting fluids has led to the ‘α-effect’ dynamos. Various linear kinematic aspects of these dynamos have been explored, suggesting their value in the interpretation of observed planetary and stellar magnetic fields. However, large-scale magnetic fields with global boundary conditions can not be force free and in general will cause large-scale motions as they grow. I n this paper the finite amplitude behaviour of global magnetic fields and the large-scale flows induced by them in rotating systems is investigated. In general, viscous and ohmic dissipative mechanisms both play a role in determining the amplitude and structure of the flows and magnetic fields which evolve. In circumstances where ohmic loss is the principal dissipation, it is found that determination of a geo- strophic flow is an essential part of the solution of the basic stability problem. Nonlinear aspects of the theory include flow amplitudes which are independent of the rotation and a total magnetic energy which is directly proportional to the rotation. Constant a is the simplest example exhibiting the various dynamic balances of this stabilizing mechanism for planetary dynamos. A detailed analysis is made for this case to determine the initial equilibrium of fields and flows in a rotating sphere.

Theory of Cyclotron Resonance Lineshape in a Two-Dimensional Electron System

Abstract: The lineshape is studied in the simplest approximation free from the difficulty of divergence caused by a singular nature of the system. Characteristics of the linewidth are obtained for both short- and long-ranged scatterers. By assuming δ-potential scatterers, the lineshape is calculated explicitly as a function of the applied magnetic field in an n -channel inversion layer on Si (100) surface. At low temperatures, a Shubnikov-de Hass type oscillation appears in the lineshape. At high temperatures, it disappears and the lineshape becomes asymmetric around the resonance magnetic field.

Linear transformations of gravity and magnetic fields

Abstract: The spectral representation of gravity and magnetic fields shows that the mathematical expressions describing these fields are the result of convolution of factors which depend on the geometry of the causative body, the physical properties of the body and the type of field being observed. If a field is known, it is possible to remove or alter these factors to map other fields or physical parameters which are linearly related to the observed field. The transformations possible are: continuation, reduction to the pole, converting between gravity and magnetic fields, converting between components of measurement, calculation of derivatives, and mapping magnetization and density distribution, relief on interfaces, and vertical thicknesses of layers.

Electrostatic and electromagnetic turbulence associated with the earth's bow shock

Abstract: Simultaneous measurements were made of the electric and magnetic field spectral densities in the earth's bow shock by a plasma wave experiment on the Imp 6 spacecraft. The frequency range of the plasma wave detector was 20 Hz to 200 kHz. Electric fields were measured with high-sensitivity 100-m long dipole antennas and magnetic fields were measured with single-turn loop antennas. Two components are distinguished in the electric field spectrum in the bow shock: one component has a broad peak centered in the region 200-800 Hz, while the other component increases monotonically with decreasing frequency. The magnetic field spectrum has only one component that increases monotonically with decreasing frequency and has an upper cutoff frequency near the local electron gyrofrequency. This magnetic field turbulence is judged to be caused by whistler mode waves. The monotonic component of the electric field spectrum is thought to be the electric field spectrum of these whistler mode waves.

Tunnel diode oscillator for 0.001 ppm measurements at low temperatures

Abstract: The results of a systematic study of the design considerations of a low‐temperature tunnel diode oscillator are presented. The calculated circuit performance satisfactorily describes the measured frequency noise and dependence on bias voltage, magnetic field, and temperature. The overall performance allows measurement of changes in the resonant frequency of an LC circuit with a precision of 0.001 ppm. One can thereby detect extremely small changes in a number of material properties such as thermal expansion, surface impedance, and electric and magnetic susceptibilities.

New Theory of Coercive Force of Ferromagnetic Materials

Abstract: We have enlarged the theory of the coercive force to include the wave nature of the ferromagnetic domain walls. Specifically, we obtain an explicit equation for the magnetic field required to move a 180\ifmmode^\circ\else\textdegree\fi{} ferromagnetic domain wall across a planar defect such as a grain boundary. Using this expression, we have been able to classify successfully both low- and high-coercive-force materials.

Theory of the hydromagnetic generator

Abstract: It is shown that in a homogeneous medium, a magnetic field may generate helical motion in a cylinder with constant angular and axial velocities. The generation problem is solved exactly, and analytic expressions for the magnetic field are found. At high velocities the increment of field growth is maximal when the ratio of the velocities is of the order of unity. The maximum increment and frequency are of the order of the velocity to the two-thirds power. The field distribution has the form of a surface wave. The field decay decrement for departure from the cylinder surface is proportional to the square root of its increment.

Resistive MHD stagnation-point flows at a current sheet

Abstract: A family of exact solutions to the MHD equations is presented for steady incompressible two- and three-dimensional flow in the vicinity of the stagnation point, which forms in a current sheet separating two colliding plasma streams. The magnetic field in each plasma is strictly parallel to the current sheet, but can have different magnitudes and directions. Resistive and viscous effects are accounted for. These flows are of considerable interest in connexion with the magnetic field merging process. They represent the limit of resistive field annihilation with zero reconnexion.

Three‐dimensional equilibrium of the anisotropic, finite‐pressure guiding‐center plasma: Theory of the magnetic plasma

Abstract: Theoretical and numerical methods now give a complete solution to the problem of finite‐β plasma equilibrium in mirror magnetic wells and toroidal devices The equilibria can be made consistent on all of the progressively longer time scales of the guiding‐center fluid model, including the particle magnetic drifts and the Coulomb scattering equilibrium of a neutral injected plasma The theory of equilibrium in the guiding‐center fluid model of a finite‐β plasma with an arbitrary, anisotropic pressure tensor can be formulated as a classical magnetostatic system: ∇⋅B = 0, ∇×H = 0, B = H + 4πM(B) The plasma magnetization is found explicitly in terms of three physically distinct components related to the laws of conservation of magnetic moment, of longitudinal invariant, and of the sign of the velocity along B of particles that do not undergo mirror reflection A condition is derived upon the field geometry whereby a large class of special equilibria can be found in which all particles on a given line drift on the same surface, the omnigenous surface Such systems allow a specially simple connection between particle and fluid models in the guiding‐center fluid theory The usefulness of the theory is exemplified by application to the problem of a finite‐β plasma in a magnetic well Finally, a brief treatment of stability in terms of the energy principle is given The omnigenous equilibria have particularly simple stability criteria

Electrostatic noise bands associated with the electron gyrofrequency and plasma frequency in the outer magnetosphere

Abstract: Naturally occurring noise bands near the electron plasma frequency are frequently detected by the University of Iowa plasma wave experiment on the Imp 6 satellite in the region from just inside the plasmapause to radial distances of about 10 earth radii in the outer magnetosphere. The electric field strength of these noise bands is usually small with typical broad band electric field strengths of about 2 microvolts per m. A magnetic field has been detected only in a few unusually intense cases, and in these cases the magnetic field energy density is several orders of magnitude smaller than the electric field energy density. The bands are observed at all magnetic latitudes covered by the Imp 6 orbit and appear to be a permanent feature of the outer magnetosphere. They are found at all local times and occur least frequently in the quadrant from 18 to 24 hours. The bands appear to consist of two distinct spectral types called diffuse and narrow band. In both types the center frequency of the noise band is bounded by consecutive harmonics of the electron gyrofrequency, and these noise bands occur most often between harmonics that are near the local electron plasma frequency.

The Dielectric Properties of Nematic 44′ n-pentylcyanobiphenyI

Abstract: The electric permittivity of nematic 44′ n-pentylcyanobiphenyl has been measured in the presence of electric and magnetic fields. Values of the permittivity components for the aligned state have been obtained and analysed in terms of the statistical theory of Maier and Meier. The behaviour of bulk samples of the material in electric and magnetic fields has been investigated, and yielded a mean permittivity for the non-aligned state. The macroscopic behaviour is discussed in terms of the continuum theory. Variable frequency measurements have shown the presence of a dispersion region in e∥ at about 5 MHz.

Magnetic viscosity in relativistic accretion disks

Abstract: We formulate a detailed, self-consistent model of magnetic viscosity in an accretion disk around a rotating, Kerr black hole, in order to help clarify the nature of viscosity in the theory of accretion disks. The magnetic field is amplified by shear and dissipated by reconnection; in steady state a stable balance occurs, leading to a chaotic field configuration consisting of magnetic cells that reconnect with each other about once per orbit. We give detailed models of stationary disks based on the viscosity of this field. The observable properties of these models are similar to those of previous models based on less detailed models of viscosity. We include the relativistic equations governing the time-evolution of an accretion disk around a Kerr black hole, and some criteria for the existence of a radiation-pressure-dominated region or an optically thin region in a stationary disk.

Observations of Mercury's magnetic field

Abstract: Magnetic field data obtained by Mariner 10 during the third and final encounter with the planet Mercury on 16 March 1975 were studied. A well developed bow shock and modest magnetosphere, previously observed at first encounter on 29 March 1974, were again observed. In addition, a much stronger magnetic field near closest approach, 400 gamma versus 98 gamma, was observed at an altitude of 327 km and approximately 70 deg north Mercurian latitude. Spherical harmonic analysis of the data provide an estimate of the centered planetary magnetic dipole of 4.7 x 10 to the 22nd power Gauss/cu cm with the axis tilted 12 deg to the rotation axis and in the same sense as Earth's. The interplanetary field was sufficiently different between first and third encounters that in addition to the very large field magnitude observed, it argues strongly against a complex induction process generating the observed planetary field. While a possibility exists that Mercury possesses a remanent field due to magnetization early in its formation, a present day active dynamo seems to be a more likely candidate for its origin.

Linear theory of a cold relativistic beam propagating along an external magnetic field

Abstract: The linear theory of a cold relativistic electron beam propagating parallel to an external magnetic field and through a cold, homogeneous plasma is investigated. The electromagnetic dispersion relation is solved numerically and compared with analytical predictions based on the electrostatic approximation. It is found that electromagnetic effects are important for determining the entire unstable spectrum. However, except for the strong magnetic field regime, the maximum growth rates and corresponding frequencies are in agreement with those predicted by the electrostatic approximation. In the strong magnetic field regime the two−stream spectrum is found to be much narrower in angle than predicted by the electrostatic approximation. In the moderate and strong magnetic field regime the growth rate of waves propagating at large angles with respect to the beam are independent of beam energy.