Showing papers on "Magnetic field published in 1982"

Interplanetary magnetic clouds At 1 AU

Abstract: Magnetic clouds are defined as regions;with a radial dimension roughly-equal0.25 AU (at 1 AU) in which the magnetic field strength is high and the magnetic field direction changes appreciably by means of rotation of one component of B> nearly parallel to a plane. The magnetic field geometry in such a magnetic cloud is consistent with that of a magnetic loop, but it cannot be determined uniquely. Forty-five clouds were identified in interplanetary data obtained near earth between 1967 and 1978; at least one cloud passed the earth every 3 months. Three classes of clouds were identified, corresponding to the association of a cloud with a shock, a stream interface, or a CME. There are approximately equal numbers of clouds in each class, and the field and plasma parameters in each class are similar, suggesting that the three types of clouds might be different manifestations of a single phenomenon (e.g., a coronal transient). Interface-associated clouds may have been swept up by corotating streams. Shock-associated clouds move faster than the other two types, which are basically slow flows. The magnetic pressure inside the clouds is higher than the ion pressure, and the sum is higher than the pressure of the material outsidemore » of the cloud. This implies that the magnetic clouds were expanding even at 1 AU, and the average expansion speed is estimated to be of the order of half the ambient Alfven speed.« less

Nonlinear gyrokinetic equations for low-frequency electromagnetic waves in general plasma equilibria

Abstract: A nonlinear gyrokinetic formalism for low‐frequency (less than the cyclotron frequency) microscopic electromagnetic perturbations in general magnetic field configurations is developed The nonlinear equations thus derived are valid in the strong‐turbulence regime and contain effects due to finite Larmor radius, plasma inhomogeneities, and magnetic field geometries The specific case of axisymmetric tokamaks is then considered and a model nonlinear equation is derived for electrostatic drift waves Also, applying the formalism to the shear Alfven wave heating scheme, it is found that nonlinear ion Landau damping of kinetic shear‐Alfven waves is modified, both qualitatively and quantitatively, by the diamagnetic drift effects In particular, wave energy is found to cascade in wavenumber instead of frequency

Observations of Double Layers and Solitary Waves in the Auroral Plasma

Abstract: Small-amplitude double layers and solitary waves containing magnetic-field-aligned electric field components have been observed for the first time in the auroral plasma between altitudes of 6000 and 8000 km in association with electron and ion velocity distributions that indicate the presence of electric fields parallel to the magnetic field. The double layers may account for a large portion of the parallel potential drop that accelerates auroral particles.

Plasma–wall transition in an oblique magnetic field

Abstract: The effect of a magnetic field on the transition layer between a plasma and an absorbing wall is studied. A numerical model is used which simulates the motion of plasma particles in the electric and magnetic fields for a prescribed particle influx at the plasma boundary. Bohm’s condition for the existence of a monotonic profile of the layer is generalized. The transition layer proves to have a double structure comprising a quasineutral magnetic presheath preceding the electrostatic Debye sheath. The magnetic presheath scales with the ion gyroradius at the sound speed and with the angle of the magnetic field. The total electric potential drop between plasma and wall proves to be fairly insensitive to the magnitude and angle of the magnetic field.

Nonlocal stability analysis of the MHD Kelvin‐Helmholtz instability in a compressible plasma

Abstract: A general stability analysis is given of the Kevin-Helmholtz instability, for the case of sheared MHD flow of finite thickness in a compressible plasma which allows for the arbitrary orientation of the magnetic field, velocity flow, and wave vector in the plane perpendicular to the velocity gradient. The stability problem is reduced to the solution of a single second-order differential equation including a gravitational term to represent the coupling between the Kelvin-Helmholtz mode and the interchange mode. Compressibility and a magnetic field component parallel to the flow are found to be stabilizing effects, with destabilization of only the fast magnetosonic mode in the transverse case, and the presence of both Alfven and slow magnetosonic components in the parallel case. Analysis results are used in a discussion of the stability of sheared plasma flow at the magnetopause boundary and in the solar wind.

Why, how, and when, MHD turbulence becomes two-dimensional

Abstract: A description of MHD turbulence at low magnetic Reynolds number and large interaction parameter is proposed, in which attention is focussed on the role of insulating walls perpendicular to a uniform applied magnetic field. The flow is divided in two regions: the thin Hartmann layers near the walls, and the bulk of the flow. In the latter region, a kind of electromagnetic diffusion along the magnetic field lines (a degenerate form of Alfv6n waves) is displayed, which elongates the turbulent eddies in the field direction, but is not sufficient to generate a two-dimensional dynamics. However the normal derivative of velocity must be zero (to leading order) at the boundaries of the bulk region (as at a free surface), so that when the length scale 1, perpendicular to the magnetic field is large enough, the corresponding eddies are necessarily two-dimensional. Furthermore, if I, is not larger than a second limit, the Hartmann braking effect is negligible and the dynamics of these eddies is described by the ordinary Navier-Stokes equations without electromagnetic forces. MHD then appears to offer a means of achieving experiments on two-dimensional turbulence, and of deducing velocity and vorticity from measurements of electric field.

Stability of a thick two‐dimensional quasineutral sheet

Abstract: The stability of a two‐dimensional magnetic model of a thick quasineutral sheet including a small but finite normal magnetic field component is studied in a collisionless plasma. Such a magnetic configuration allows an adiabatic motion for electrons which are trapped in good agreement with the thick sheet model. A detailed kinetic study of the energy balance shows that the ion linear tearing mode instability is suppressed by the interaction between the perturbed electromagnetic field and the adiabatic electron motion; this interaction leads to a ‘‘compressibility’’ effect which prevents the instability onset. The connection with the magnetohydrodynamic energy principle for stability is demonstrated. A general formalism is developed for any magnetic configuration and applied to a parabolic and a magnetic island topology. Some conclusions concerning the nonlinear tearing mode behavior (explosive phase) are drawn.

A geomagnetic field spectrum

Abstract: A spherical harmonic model of the earth's internal magnetic field of degree and order 23 is derived from selected Magsat data, and its power spectrum, computed with terms developed by Mauersberger (1956) and Lowes (1974), is found to exhibit a change of a slope at n = 14 which is interpreted as an indication that the core field dominates at values lower than 13 while the crust field dominates above a value of 15. The representations of the two portions of the spectrum obtained can be used to establish order-of-magnitude inaccuracies due to both crustal fields and the inability to observe core field wavelengths beyond n = 13, at which point they are obscured by the crustal field, in core field models.

A self-consistent field theory for gyrotron oscillators: application to a low Q gyromonotron

Abstract: This paper presents an application of a self-consistent field theory for gyrotron oscillators to a low Q TE01 mode gyromonotron. In this model, the RF field profile function satisfies a wave equation in which the AC beam current appears as a source. The wave equation is solved simultaneously with the electron equations of motion. The results of calculations are compared with experimental data for the efficiency, starting current, and operating frequency. To facilitate a detailed comparison between theory and experiment, data have been taken over a wide range of beam currents and axial magnetic fields. The gyromonotron studied in this work has a tapered cavity structure to enhance efficiency, and a low Q factor to enhance output power. A strong self-consistent field effect is observed in the behaviour of the starting current.

Agglomerate formation in a magnetic fluid

Abstract: Monte Carlo techniques have been used to investigate the effects of magnetostatic and repulsive particle interactions on the formation of agglomerates in a magnetic fluid The dependence on particle size and applied field of the form of the agglomerates was studied using a spatial distribution function which allows a quantitative distinction to be made between clusters and anisotropic chain structures Magnetization curves have been calculated for magnetic particle sizes varying from 5 to 15 nm with and without magnetostatic interactions For the larger particle sizes, it was found that the initial susceptibility is reduced in the presence of interactions This is associated with the presence of pronounced agglomeration in zero field, where open clusters are formed As the applied field is increased the clusters break up to form long chains aligned in the field direction At intermediate particle sizes, there is evidence of magnetic field induced agglomeration leading to the formation of dimers and trimers preferentially aligned in the field direction The smallest particle size showed little evidence of ordering even in strong applied fields, since thermal disordering dominates the situation

Green's Function Methods for Potential Magnetic Fields

Abstract: The Green's function method to calculate potential magnetic field on the Sun, which was first established by Schmidt (1964) in the case that the field component normal to a flat boundary plane is specified, is extended to the following three cases: (a) The field component along the line of sight, which is not generally normal to the flat boundary plane, is specified; (b) the line of sight component on a spherical boundary surface is specified; (c) the normal component on a spherical surface is specified, together with the condition that the field becomes approximately radial on an outer spherical surface (the so-called source surface). Properties of these Green's functions are examined, and the applicability of these methods to solar magnetic data is discussed.

Toroidal and scattering effects on lower‐hybrid wave propagation

Abstract: The effects of toricity and density fluctuations on the propagation of lower‐hybrid waves are studied. These effects result mainly from changes in k∥ as the wave propagates (k∥ is the wavenumber parallel to the applied magnetic field). With respect to toroidal effects, k∥ changes due to the nonconstancy of the poloidal mode number in toroidal geometry. Using ray tracing techniques, an assessment of this toroidal effect has been made. It is found that waves, which on the basis of the cylindrical geometry theory would be inaccessible, may actually propagate to the plasma center and damp. The result is related to the onset of ray ergodicity. Further, it is shown that for accessible waves the optimal poloidal launch point is near the top or bottom of the torus. In order to include the effects of density fluctuations, an electromagnetic wave kinetic equation is derived and solved by a Monte Carlo technique. It is found that scattering can further enhance accessibility.

Antiferromagnetic long-range order in the anisotropic quantum spin chain

Abstract: This is a study of the ground-state properties of the one-dimensional spin-s (12⩽s<∞) anisotropic XYZ antiferromagnet in a magnetic field of arbitrary direction. It provides the first rigorous results for the general case of this model in non-zero field. By exact calculations we find the existence of an ellipsoidal surface h = hN in field space where the ground state is of the classical two-sublattice Neel type with non-zero antiferromagnetic long-range order. At hN there are no correlated quantum fluctuations. We give upper and lower bounds for the critical field hc where antiferromagnetic long-range order is suppressed by the field. The zero-temperature phase diagrams are discussed for a few representative cases.

Evaluation of magnetic helicity in homogeneous turbulence

Abstract: A technique for the measurement of magnetic helicity from values of the two point magnetic field correlation matrix under the assumption of spatial homogeneity is presented. Knowledge of a single scalar function of space, derivable from the correlation matrix, suffices to determine the magnetic helicity. The technique is illustrated by reporting the first measurement of the magnetic helicity of the solar wind.

Zonal harmonic model of Saturn's magnetic field from Voyager 1 and 2 observations

Abstract: An analysis of the magnetic field of Saturn is presented which takes into account both the Voyager 1 and 2 vector magnetic field observations. The analysis is based on the traditional spherical harmonic expansion of a scale potential to derive the magnetic field within 8 Saturn radii. A third-order zonal harmonic model fitted to Voyager 1 and 2 observations is found to be capable of predicting the magnetic field characteristics at one encounter based on those observed at another, unlike models including dipole and quadrupole terms only. The third-order model is noted to lead to significantly enhanced polar surface field intensities with respect to dipole models, and probably represents the axisymmetric part of a complex dynamo field.

A search for magnetic fields in normal upper-main-sequence stars.

Abstract: A search has been carried out for magnetic fields of the order for 10/sup 2/ gauss in 31 normal O9.5--F6 upper--main-sequence stars and 5 Am stars by measuring the circular polarization of the wings of single spectral lines of hydrogen, helium, or iron with a Cassegrain filter polarimeter or with a coude line-profile scanner polarimeter. The reduction formula used to infer longitudinal magnetic fields from such polarization measurements is discussed, and is shown in a simple analytical approximation to be essentially unaffected by a nonuniform magnetic field distribution and by instrumental broadening, although it is sensitive to rotational broading. Simple expressions are found for estimating the time required to obtain a magnetic field measurement of specified accuracy for a particular star with a particular Zeeman analyzer, and for estimating the measurement accuracy available from a photographic magnetic measurement on a plate of given dispersion and widening. These expression are used to select the best helium and metal lines for single-line magnetic measurements of upper--main-sequence stars. Measurements of the magnetic field of ..cap alpha../sup 2/CVn, 78 Vir, and ..beta.. CrB using lines of Fe/sup +/ are shown to be in resonable agreement with Balmer-line measurements in spite of a simplified measuringmore » procedure and approximate reduction scheme.« less

Conception of an air-gap element for the dynamic analysis of the electromagnetic field in electric machines

Abstract: In the design of electric machine working under unbalanced conditions or supplied by non sinusoidal current, a detailed knowledge of the magnetic field distribution is required to predict the machine performance. Under such conditions this field distribution must be calculated using a dynamic model for the machine air-gap: A particular element constituted by the uniform part of the air-gap (u.p.a.g.) allows such a dynamic model in the present paper. An analytical solution of the Laplace equation in the air-gap is done. The field distribution in a saturated machine can be obtained within this air-gap model, combined with the classical finite element solution. The results of field calculation obtained using such a method are more precise than those calculated when modelling the air-gap with classical elements. Examples are given in the paper, showing this fact for both the cases of static and dynamic calculations.

STARE and GEOS 2 observations of a storm time Pc 5 ULF pulsation

Abstract: A study is presented of a storm time Pc 5 pulsation, observed by GEOS 2 particle detectors and magnetometers at geocentric orbit and by the STARE auroral radar in the ionosphere. These measurements allow the comparison of phase relationships between the magnetic field components, energetic particle flux, and electric field in the ionosphere over a substantial region. They also permit measurements of the wavelength of the disturbance in the ionosphere. A theory of the coupling between a drift mirror wave and a standing Alfven wave is developed. The results are compared with this theory and are found to support it. In particular, it is found that the disturbance is of wavelike nature, propagating westward in the equatorial plane at 6.6 RE with a wavelength of about 1 RE. Its period is initially 213 s, lengthening to 300 s later in the event. In the equatorial plane the disturbance in the particle flux and the compressional magnetic field are out of phase as is consistent with a drift mirror wave. There is a large transverse magnetic field oscillation in the meridian plane as is consistent with a guided poloidal oscillation. In the ionosphere the oscillation is consistent with the existence of a guided poloidal Alfven wave and is entirely different from toroidal oscillations that have been observed at Pc 5 frequencies on other occasions. The observations are consistent with a particle driven drift mirror wave, couplied to a standing Alfven wave. The azimuthal wavelength is consistent with a drift mirror wave whose Doppler shifted frequency matches the natural frequency of the standing Alfven wave.

Effect of bias on trapping probes and bolometers for Tokamak edge diagnosis

Abstract: Analysis is presented for the particle and energy flow to an electrically biasable probe immersed in the magnetic field of the boundary plasma of a Tokamak or other magnetic confinement device. The analysis is appropriate to the operation of the probe in the voltage-current (Langmuir) probe mode, the thermal power (bolometer) mode, hydrogenic ion trapping mode or impurity deposition mode. The formulation of the analysis permits the use of any cross-field diffusion model, i.e. classical, semi-classical, empirical. The principal conclusion of the analysis is that a probe should be operated simultaneously in at least two modes, e.g. Langmuir and bolometer modes. In this way it is possible to measure plasma density and electron and ion temperatures separately with several consistency checks also available. Under some circumstances it is also possible to measure the hydrogenic cross-field diffusion coefficient and impurity ion density, temperature and charge state. It is necessary to operate a bolometer in a biased mode in order to infer heat loads to non-floating surfaces, e.g. limiters and divertor targets, since heat flux is a strong function of the potential difference between the plasma and surface.

Initial results on the correlation between the magnetic and electric fields observed from the DE-2 satellite in the field-aligned current regions

Abstract: Initial results of the electric and magnetic field observations from the DE-2 satellite show a remarkably good correlation between the north-south component of the electric field and the east-west component of the magnetic field in many passes of the field-aligned current regions. For a dayside cusp pass on August 15, 1981 the coefficient of correlation between these components was 0.996. A preliminary inspection of the available data from the first 6 months of the DE operation indicates that the similarity between the electric and magnetic field signatures of the field-aligned currents is a commonly observed feature at all local times. This high correlation is interpreted to be an indication that the closure of the field-aligned current is essentially meridional. When the correlation between these components is not good, the closure current is likely to be flowing along the auroral belt. When the correlation between the electric and magnetic fields is high, it is possible to estimate the height-integrated Pedersen conductivity from the observed field components.

Possible evidence for coronal Alfvén waves

Abstract: The 229 GHz S band carrier signals of the two Helios spacecraft are used to probe the magnetic and density structures of the solar corona inside 005 AU In this paper we analyze the observed fluctuations of the electron content and Faraday rotation A simple statistical ray analysis is employed We conclude that (1) the observed Faraday rotation fluctuations cannot be solely due to electron density fluctuations in the corona unless the coronal magnetic field is some 5 times stronger than suggested by current estimates, and (2) the observed Faraday rotation fluctuations are consistent with the hypothesis that the sun radiates Alfven waves with sufficient energies to heat and accelerate high-speed solar wind streams

The steady global corona

Abstract: The formation of the steady coronal structure which consists of coronal streamers and holes is seen in terms of a model employing numerical solutions of time-dependent, dissipationless, magnetohydrodynamic equations of motion applicable to the meridional plane. A coronal streamer consists of closed magnetic field lines near the solar surface with overlying and adjacent open field lines, and the atmosphere, which is stationary in the closed region, flows outward in the open region, or coronal hole. The steady coronal structure is obtained by starting the numerical calculation with a state comprising a polytropic, hydrodynamical solution to the steady-state radial equation of motion coupled with a dipole magnetic field. Global coronal configurations are calculated for values of the plasma beta which vary from 0.1 to 100. It is found that the height and lateral extension of the closed region are only weakly dependent on the data.

Electron spectroscopy for atoms, molecules, and condensed matter

Abstract: In my thesis [1], which was presented in 1944, I described some work which I had done to study β decay and internal conversion in radioactive decay by means of two different principles. One of these was based on the semi-circular focusing of electrons in a homogeneous magnetic field, while the other used a big magnetic lens. The first principle could give good resolution but low intensity, and the other just the reverse. I was then looking for a possibility of combining the two good properties into one instrument. The idea was to shape the previously homogeneous magnetic field in such a way that focusing should occur in two directions, instead of only one as in the semi-circular case. It was known that in betatrons the electrons performed oscillatory motions both in the radial and in the axial directions. By putting the angles of period equal for the two oscillations Nils Svartholm and I [2, 3] found a simple condition for the magnetic field form required to give a real electron optical image i.e. we established the two-directional or double focusing principle. It turned out that

Magnetoplasmadynamic apparatus and process for the separation and deposition of materials

Abstract: A plasma arc discharge method for deposition of metallic and semiconductor layers on a substrate for the purpose of producing semiconductor grade materials such as silicon at a reduced cost is disclosed. Magnetic fields are used so that silicon ions and electrons can be directed toward a target area where they are deposited. The ions and electrons are preferably injected as a compound in gaseous or liquid form but may also be injected in liquid elemental form or vaporized from a thermionic cathode. The magnetic fields include an accelerating magnetic field and a focusing magnetic field. The accelerating magnetic field is adjusted to support a desired high ion flux rate and the focusing magnet can control the plasma beam direction and divergence. The silicon provided in a compound form or in the form of metallurigical silicon is purified during the deposition process by a carrier substance which may be a part of the compound or separately injected. Chemical purification is accomplished by separation of the silicon due to ionization potential differences between silicon and other elements. The magnetic acceleration technique allows the use of pressures under 10 -1 torr thereby facilitating plasma formation and allowing the materials to be deposited with a desired high purity.

Statistical properties of low‐frequency magnetic field fluctuations in the solar wind from 0.29 to 1.0 AU during solar minimum conditions: HELIOS 1 and HELIOS 2

Abstract: To obtain information on the temporal and spatial evolution of MHD waves and discontinuities in the solar wind, we studied by means of statistical methods magnetic field fluctuations measured by the two HELIOS spacecraft in the frequency range between 2.4×10−5 Hz and 1.3×10−2 Hz at distances from the sun between 0.29 AU and 1.0 AU. Some statistical properties as magnetic field variances show the fluctuations remaining very similar on their way out from the sun. A different picture of the fluctuations emerges from analyzing the spectral properties of the magnetic field: The slope of the power spectral densities (both the vector components and the magnitude of the magnetic field) as a function of frequency becomes steeper with increasing heliographic distance—the solar wind seems to act as a low-pass filter. The major changes occur within a heliographic distance up to about 0.4 AU. Field magnitude fluctuations fall off less rapidly with increasing radius than do fluctuations in the vector components. Cross spectral analysis between magnetic field components reveal the fluctuations as generally being field magnitude conserving.