Showing papers on "Magnetic field published in 1981"

Magnetic loop behind an interplanetary shock: Voyager, Helios and IMP-8 observations

Abstract: The flow behind an interplanetary shock was analyzed through the use of magnetic field and plasma data from five spacecraft, with emphasis on the magnetic cloud identified by a characteristic variation of the latitude angle of the magnetic field. The size of the cloud was found to be about 0.5 AU in radial extent and greater than 30 deg in azimuthal extent, with its front boundary almost normal to the radial direction. Because the field direction of the magnetic cloud as it moved past the spacecraft was observed to rotate nearly parallel to a plane, it is thought that the field configuration of the cloud was essentially two-dimensional. These results further suggest that the lines of force in the magnetic cloud formed loops, but it could not be determined whether these loops were open or closed.

Neoclassical transport of impurities in tokamak plasmas

Abstract: Tokamak plasmas are inherently comprised of multiple ion species. This is due to wall-bred impurities and, in future reactors, will result from fusion-born alpha particles. Relatively small densities nI, of highly charged non-hydrogenic impurities can strongly influence plasma transport properties whenever . The determination of the complete neoclassical Onsager matrix for a toroidally confined multispecies plasma, which provides the linear relation between the surface averaged radial fluxes and the thermodynamic forces (i.e. gradients of density and temperature, and the parallel electric field), is reviewed. A closed set of one-dimensional moment equations is presented for the time evolution of thermodynamic and magnetic field quantities which results from collisional transport of the plasma and two-dimensional motion of the magnetic flux surface geometry. The effects of neutral-beam injection on the equilibrium and transport properties of a toroidal plasma are consistently included.

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.

Schrödinger operators with magnetic fields. III. Atoms in homogeneous magnetic field

Abstract: We prove a large number of results about atoms in constant magnetic field including (i) Asymptotic formula for the ground state energy of Hydrogen in large field, (ii) Proof that the ground state of Hydrogen in an arbitrary constant field hasL z = 0 and of the monotonicity of the binding energy as a function ofB, (iii) Borel summability of Zeeman series in arbitrary atoms, (iv) Dilation analyticity for arbitrary atoms with infinite nuclear mass, and (v) Proof that every once negatively charged ion has infinitely many bound states in non-zero magnetic field with estimates of the binding energy for smallB and largeL z .

Modeling the Jovian current sheet and inner magnetosphere

Abstract: Voyager 1 and 2 magnetic field observations confirm and extend the earlier Pioneer 10 detection of the Jovian magnetodisc, a region of enhanced charged particles and plasma and reduced magnetic field intensity located near the magnetic equatorial plane. Modeling of the azimuthal current sheet by a finite thickness annulus of inner radius 5 Jovian radii, 5-Jovian radii thickness, and extending to about 50 Jovian radii provides detailed fits of the vector magnetic field perturbations observed in relation to the planetary field for distances less than 30 Jovian radii. Field line geometry is also investigated, and better insight into the phenomena of charged particle absorption by the Galilean satellites is obtained which provides improved explanations of observed effects due to Ganymede.

Plasma equilibrium with rational magnetic surfaces

Abstract: The self‐consistent classical plasma equilibrium with diffusion is studied in a toroidal geometry having a sheared magnetic field. Near each rational surface it is found that the pressure gradient is zero unless the Fourier component of 1/B2, which resonates with that surface, vanishes. Despite the resonances, the overall plasma confinement is, in practice, only slightly modified by the rational surfaces.

Anisotropic magnetohydrodynamic turbulence in a strong external magnetic field

Abstract: A strong external dc magnetic field introduces a basic anisotropy into incompressible magnetohydrodynamic turbulence. The modifications that this is likely to produce in the properties of the turbulence are explored for the high Reynolds number case. The conclusion is reached that the turbulent spectrum splits into two parts: an essentially two dimensional spectrum with both the velocity field and magnetic fluctuations perpendicular to the dc magnetic field, and a generally weaker and more nearly isotropic spectrum of Alfven waves. A minimal characterization of the spectral density tensors is given. Similarities to measurements from the Culham-Harwell Zeta pinch device and the UCLA Macrotor Tokamak are remarked upon, as are certain implications for the Belcher and Davis measurements of magnetohydrodynamic turbulence in the solar wind.

Confinement of high energy trapped particles in tokamaks

Abstract: The banana orbits of high energy trapped particles in tokamaks are found to diffuse rapidly in the radial direction if the toroidal ripple exceeds a low critical value. During this diffusion the energy, the magnetic moment, and the value of the magnetic field strength at the banana tips are conserved.

Wave propagation in a magnetically structured atmosphere

Abstract: The solar atmosphere, from the photosphere to the corona, is structured by the presence of magnetic fields. We consider the nature of such inhomogeneity and emphasis that the usual picture of hydromagnetic wave propagation in a uniform medium may be misleading if applied to a structured field. We investigate the occurrence of magnetoacoustic surface waves at a single magnetic interface and consider in detail the case where one side of the interface is field-free. For such an interface, a slow surface wave can always propagate. In addition, a fast surface wave may propagate if the field-free medium is warmer than the magnetic atmosphere.

Lower hybrid acceleration and ion evolution in the suprauroral region

Abstract: It is shown that ions can be accelerated perpendicularly to the magnetic field lines by resonant interactions with lower hybrid modes. Taking into account the effects of the magnetic field inhomogeneity, we demonstrate that the accelerated portion of the ions of ionospheric origin in the suprauroral region of the magnetosphere can evolve into conic distributions and propagate upwards along the field lines. Thus these ions can reach the region where they can be strongly energized along the field lines by the electrostatic shocks. We can argue that the ion distribution resulting from the combination of these accelerated ions with the background ions can depart significantly from a thermal distribution and lead to the excitation of electrostatic ion cyclotron modes.

Magnetic structure of small NiFe2O4 particles

Abstract: The magnetic structure of small NiFe2O4 particles has been investigated. Samples (in the few hundred angstrom size range and up) were prepared by chemical precipitation followed by a heat treatment at relatively low temperatures. Mossbauer spectra of the 57Fe nuclei, obtained with a longitudinal magnetic field applied, unambiguously establish that a non‐collinear structure exists. Further, the magnetic moment at low temperatures is appreciably lower than the value reported for bulk material. A model is proposed in which the NiFe2O4 particles consist of a core with the usual spin arrangement and a surface layer with atomic moments inclined to the direction of the net magnetization. The temperature dependence of this structure is also reported.

Calculation of force-free magnetic field with non-constant α

Abstract: A numerical method is developed for solving the force-free magnetic field equation, ▽ × B = α B, with spatially-varying α. The boundary conditions required are the distribution of B n (viz. normal component of the field on the photosphere) as well as the value of α in the region of positive (or negative) B n . Examples of calculations are presented for a simple model of a solar bipolar magnetic region. It is found that the field configuration and the energy stored in the field depend crucially on the distribution of α. The present method can be applied to a more complex configuration observed on the Sun by making use of actual magnetic field measurements.

Integral equation modeling of three-dimensional magnetotelluric response

Abstract: We have adapted a three‐dimensional (3-D) volume integral equation algorithm to magnetotelluric (MT) modeling. Incorporating an integro‐difference scheme increases accuracy somewhat. Utilizing the two symmetry planes of a buried prismatic body and a normally incident plane wave source greatly reduces required computation time and storage. Convergence checks and comparisons with one‐dimensional (1-D) and two‐dimensional (2-D) models indicate that our results are valid. We show theoretical surface anomalies due to a 3-D prismatic conductive body buried in a half‐space earth. Instead of studying the electric and magnetic fields, we have obtained impedance tensor and magnetic transfer functions by imposing two different source polarizations. Manipulation of the impedance tensor and magnetic transfer functions yields the following MT quantities: apparent resistivity and phase, impedance polar diagrams, tipper direction and magnitude, principal directions, skew, and ellipticity. With our preliminary analyses of...

Discrete aurora as the direct result of an inferred high‐altitude generating potential distribution

Abstract: In a previous paper it was shown that spatial changes in the high-altitude magnetospheric electric field E with ▽ · E <0 can result in the generation of large-scale ‘inverted V’ regions of auroral electron precipitation. In the present paper it is shown that smaller-scale precipitation regions, as are required to account for discrete aurora, result from the basic analysis in the previous paper if appropriate structure is introduced in the high-altitude electric field distribution. Using observations of electric fields and precipitating electrons from a rocket flight over discrete aurora the required structure in the electric field is inferred to exit at high altitudes along magnetic field lines connected to the aurora. By using this inferred high-altitude electric potential distribution the ionospheric current continuity equation is solved for the ionospheric potential. The analysis assumes that the field-aligned current is governed by single-particle motion along field lines. The solution gives ionospheric potentials and precipitating electron energy fluxes in good quantitative agreement with those observed throughout the auroral rocket flight. The results in this and the previous paper imply that the ‘inverted V’ scale size (order of 200 km in width) is a natural result of the current versus electric potential relations along auroral field lines and in the ionosphere. This scale size need not be imposed by structure in the high-altitude electric field distribution. Smaller-scale (tens of kilometers) discrete auroral precipitation regions require the same current versus electric potential relations, but the scale size is imposed by structure in the high-altitude electric potential distribution. The cause of this structure is not considered in the present analysis.

Magnetic fields in extragalactic radio sources.

Abstract: Distributions of total intensity and linear polarization have been calculated for synchrotron radiation from a variety of magnetic fields which may occur in extragalactic radio sources, assuming that the energy distributions of radiating electrons are uniform and isotropic. Analytical solutions are derived for helical and transverse self-similar fields of the type suggested for radio jets by Chan and Henriksen and compared with results for a field which has no component along the jet axis, but is otherwise random (as is expected to develop in an expanding jet containing a tangled field). A comparison with observation favors the last model, but is not conclusive; a search for variations of Faraday rotation across radio jets is suggested. The tangled-field model predicts that the fractional polarizations of jets are determined by their inclinations to the line of sight; it may therefore be possible to derive their three-dimensional structures. The polarization distributions of hot spots and extended radio lobes can be produced by magnetic fields which have been sheared also as to be tangential to the boundaries of the structures but which are otherwise random.

On mapping the magnetic field direction in molecular clouds by polarization measurements

Abstract: We predict that interstellar radio-frequency lines possess a few percent linear polarization, provided that (1) the radiative transition rate is at least comparable to the collision rate, (2) the optical depth is moderate. and anisotropic, and (3) the number of extrema of the velocity component along the line of sight through the source is small. If the Zeeman splitting exceeds both the collisional frequency and the radiative transition rate, then the polarization is aligned either perpendicular to or parallel to the projection of the magnetic field on the plane of the sky.

Biophysics of geomagnetic field detection

Abstract: In biology, the study of geomagnetic orientation has gained new momentum since the discovery of magnetic field detectors in aquatic organisms. Sharks and rays respond to dc and low frequency voltage gradients of 0.005 μV/cm. By moving through the earth's magnetic field, they induce electric fields well within the sensitivity range of their keen electric sense. As these fields depend on the direction in which the animal is heading, the induced voltage gradients may serve as the biophysical basis of an electromagnetic compass sense. The ability of sharks and rays to orient to the earth's magnetic field has been demonstrated in behavioral experiments. Also, various marine and freshwater mud bacteria are endowed with permanent magnetic dipole moments, directed parallel to the axis of motility. When separated from the sediments, these bacteria return to the mud by migrating downward along the earth's inclined magnetic field lines. Their orientation is largely determined by the principles of statistical mechanics and may be expressed in terms of the directive magnetic force, the randomizing effect of thermal agitation, and the cells' flagellar thrust. Observations on live bacteria yield individual dipole moments circa 15 × kT/G.

Augmented electric‐ and magnetic‐field integral equations

Abstract: Augmented electric- and magnetic-field integral equations, which preserve the basic simplicity, solution capability, and pure electric- and magnetic-field character of Maue's original integral equations, are introduced to eliminate the spurious resonances from the exterior solution of the original integral equations. The exact dependence of the original and augmented integral equations on the geometry of the principal area (self patch) which excludes the singularity of their kernels is also determined, and alternate forms for the integral equations are provided that avoid integrals dependent upon the geometry of the principal area. Numerical results obtained for scattering from the perfectly conducting cube, sphere, and infinite circular cylinder confirm the theoretically predicted result that the augmented integral equations eliminate the spurious resonances for all perfectly conducting scatterers except the sphere.

Impulsive phase of solar flares. I. Characteristics of high energy electrons

Abstract: The variation along a magnetic field line of the energy and pitch angle distribution of high energy electrons injected into a cold hydrogen plasma containing either an open or closed magnetic field structure was investigated. The problem is formulated as a time independent Fokker-Planck Equation for the electron number distribution as a function of the electron energy, electron pitch angle, and the structure of the global magnetic field. Simple analytic solution valid in the small pitch angle regime and for slowly varying magnetic field is presented. For the more general situation a numerical code for solving the Fokker-Planck Equation was used and it was found that the analytic expression agrees well with the numerical results to values of the pitch angle much larger than expected. For most practical applications, one many confidently use the analytic expression instead of having to resort to lengthy numerical computations. These results are useful in the study of the nonthermal models of the impulsive phase of solar flares.

Atomic hydrogen in an inhomogeneous magnetic field: Density profile and Bose-Einstein condensation

Abstract: The density profiles for an interacting gas of atomic hydrogen in an inhomogeneous axial magnetic field are determined by extending the Gross Pitaevskii theory to finite temperature. The profiles can be used as an identifying feature of Bose Einstein condensation for all T Radial inhomogeneities in the field are also treated.

Transport of runaway and thermal electrons due to magnetic microturbulence

Abstract: The ratio of the runaway electron confinement to thermal electron energy confinement is derived for tokamaks where both processes are determined by free streaming along stochastic magnetic field lines. The runaway electron confinement is enhanced at high runaway electron energies due to phase averaging over the magnetic perturbations when the runaway electron drift surfaces are displaced from the magnetic surfaces. Comparison with experimental data from LT‐3, Ormak, PLT, ST, and TM‐3 indicates that magnetic stochasticity may explain the relative transport rates of runaways and thermal electron energy.

Observational evidence for a boundary layer source of dayside region 1 field‐aligned currents

Abstract: With the addition of magnetic field information to the Atmosphere Explorer C data base it is now possible to make simultaneous observations of charged particles, magnetic field perturbations, and ion drift velocity. The most recently compiled observations from this data set are presented, and attention is given to the observed relationships between the convection reversal boundary, the high-latitude limit of plasma sheet particles, and the high-latitude limit of the region 1 current sheets in the low-altitude ionosphere, taking into account the association with similar boundaries and regions in the distant magnetosphere. The observed relationships between these boundaries are further discussed in light of the current-generating mechanisms considered.

Incoherent scattering of radar waves in the auroral ionosphere

Abstract: We have systematically studied the effects that non-Maxwellian ion velocity distributions produce on the ionic part of the spectrum of radar waves incoherently scattered from the disturbed high-latitude ionosphere in the upper E and lower F regions. For ion to electron temperature ratios smaller than or equal to unity and electric field strengths greater than about 70 mV/m the spectrum is seriously distorted from the shape that it would normally have if the ion velocity distribution were Maxwellian. In events where the magnetospheric convection electric field exceeds 70 mV/m the interpretation of the data is affected if the erroneous assumption of a Maxwellian distribution is used to analyze the data. The electron temperature is the property most seriously affected by the erroneous interpretation, as it can be underestimated by as much as a factor of 2. Finally, the spectra obtained under disturbed conditions are anisotropic, the least distortions from a Maxwellian-type spectrum being found along the magnetic field direction. The apparent ion temperature seen along the line of sight of the radar as it scans from parallel to the magnetic field to nearly perpendicular to the magnetic field also increases as the angle with the magnetic field increases, even for dc electric field strengths as small as 20 mV/m.

Nonthermal radiation from supernova remnants in the adiabatic stage of evolution.

Abstract: We develop a physically self-consistent model for nonthermal radiation from supernova remnants in the adiabatic blast-wave (Sedov) phase of evolution, assuming relativistic electrons are accelerated in the shock to an energy density proportional to the postshock pressure, and that the magnetic field is either compressed ambient field or turbulently amplified. We have compared the resulting synchrotron profiles with observations of Tycho's remnant and find the amplified magnetic field model gives an adequate fit if there is a small radially ordered component of the magnetic field at the shock wave. The model predicts that surface brightness of Tycho declines as (diameter)/sup -4.4/ and that the flux declines by 0.25% per year. We explain the featureless power-law X-ray spectrum of the SN 1006 remnant as the extension of the radio emission: the entire spectrum can be fitted when synchrotron losses are included. The model implies that while several percent of the shock energy goes into The magnetic field, only 2 x 10/sup -5/ of the shock energy goes into relativistic electrons.