Showing papers on "Magnetic field published in 1984"

Oscillatory effects and the magnetic susceptibility of carriers in inversion layers

Abstract: Oscillatory effects in a strong magnetic field B and magnetic susceptibility are investigated, as applied to 2D systems, in which the twofold spin degeneracy is lifted by the spin-orbit-interaction Hamiltonian HSO= alpha ( sigma *k). nu . The term HSO is shown to change greatly the usual patterns of B-1-periodic oscillations; some oscillations are strongly suppressed due to the diminishing of the gaps between adjacent levels, and new oscillations appear due to intersections of levels.

Magnetic particles for use in separations

Abstract: A process is provided for the preparation of magnetic particles to which a wide variety of molecules may be coupled. The magnetic particles can be dispersed in aqueous media without rapid settling and conveniently reclaimed from media with a magnetic field. Preferred particles do not become magnetic after application of a magnetic field and can be redispersed and reused. The magnetic particles are useful in biological systems involving separations.

Hamiltonian guiding center drift orbit calculation for plasmas of arbitrary cross section

Abstract: A Hamiltonian guiding center drift orbit formalism is developed which permits the efficient calculation of particle trajectories in magnetic field configurations of arbitrary cross section with arbitrary plasma β. The magnetic field is assumed to be a small perturbation from a zero‐order ‘‘equilibrium’’ field possessing magnetic surfaces. The equilibrium field, possessing helical or toroidal symmetry, can be modeled analytically or obtained numerically from equilibrium codes. The formalism is used to study trapped particle precession. Finite banana width corrections to the toroidal precession rate are derived, and the bounce averaged trapped particle motion is expressed in Hamiltonian form. Particle drift‐pumping associated with the ‘‘fishbone’’ oscillation is investigated. A numerical code based on the formalism is used to study particle orbits in circular and bean‐shaped tokamak configurations.

Time-varying magnetic fields: effect on DNA synthesis

Abstract: Human fibroblasts have exhibited enhanced DNA synthesis when exposed to sinusoidally varying magnetic fields for a wide range of frequencies (15 hertz to 4 kilohertz) and amplitudes (2.3 X 10(-6) to 5.6 X 10(-4) tesla). This effect, which is at maximum during the middle of the S phase of the cell cycle, appears to be independent of the time derivative of the magnetic field, suggesting an underlying mechanism other than Faraday's law. The threshold is estimated to be between 0.5 X 10(-5) and 2.5 X 10(-5) tesla per second. These results bring into question the allegedly specific magnetic wave shapes now used in therapeutic devices for bone nonunion. The range of magnetic field amplitudes tested encompass the geomagnetic field, suggesting the possibility of mutagenic interactions directly arising from short-term changes in the earth's field.

On some properties of force-free magnetic fields in infinite regions of space

Abstract: Techniques for solving boundary value problems (BVP) for a force free magnetic field (FFF) in infinite space are presented. A priori inequalities are defined which must be satisfied by the force-free equations. It is shown that upper bounds may be calculated for the magnetic energy of the region provided the value of the magnetic normal component at the boundary of the region can be shown to decay sufficiently fast at infinity. The results are employed to prove a nonexistence theorem for the BVP for the FFF in the spatial region. The implications of the theory for modeling the origins of solar flares are discussed.

Fractional charge and zero modes for planar systems in a magnetic field

Abstract: Attention is drawn to a relation between the recently discovered three-dimensional chiral anomaly and fermion zero modes. Application to planar systems of electrons in an external magnetic field is suggested.

A quantitative study relating observed shear in photospheric magnetic fields to repeated flaring

Abstract: In this paper a quantitative evaluation of the shear in the magnetic field along the neutral line in an active region during an epoch of flare activity is presented. Shear is defined as the angular difference in the photosphere between the potential magnetic field, which fits the boundary conditions imposed by the observed line-of-sight field, and the observed magnetic field. For the active region studied, this angular difference (shear) is non-uniform along the neutral line with maxima occurring at the locations of repeated flare onsets. It is suggested that continued magnetic evolution causes the field's maximum shear to exceed a critical value of shear, resulting in a flare around the site of maximum shear. Evidently, the field at the site of the flare must relax to a state of shear somewhat below the critical value (but still far from potential), with subsequent evolution returning the field to the critical threshold. This inference is drawn because several flares occured at sites of maximum photospheric shear which were persistent in location.

Lower critical dimension of the random-field Ising model

Abstract: A new argument is given for a lower critical dimension ${d}_{l}=2$ for the Ising model in a random magnetic field. It forms the basis for a proof that the three-dimensional model exhibits long-range order at zero temperature and small disorder. This settles the controversy between the values ${d}_{l}=2$ and ${d}_{l}=3$.

The power spectrum of interplanetary Alfvénic fluctuations: Derivation of the governing equation and its solution

Abstract: A model is presented to explain the radial evolution of the power spectra of interplanetary Alfvenic fluctuations found by Bavassano et al. (1982a) based on the magnetic field data of Helios 1 and 2. It is assumed in this model that Alfvenic fluctuations represent an asymmetric state of MHD turbulence in which most of the fluctuations are in the Alfvenic wave mode propagating outward from the sun, with a small part of the fluctuations in the wave mode propagating inward. There is weak nonlinear interaction between the two modes. It is also assumed that the turbulence will not become dissociated from the sources that create the waves propagating inward and that the weak nonlinear interaction will remain between 0.3 AU and 1 AU. The nonlinear interaction results in an energy cascading process. Both the effects of the cascading process and the effects of the slow variation of the solar wind on the outward propagating waves determine the radial evolution of the power spectra. Starting with the magnetohydrodynamic equations and deriving the equations governing fluctuations and correlation moments, we finally get an equation which describes the power spectrum. We present an analytic solution. The radial evolution of the power spectra of the fluctuations given by the analytical solution, from 0.29 AU to 0.87 AU, is in agreement with the observation of Helios 1 and 2 in the following aspects: (1) The spectral slope increases (in absolute value) for the frequency range lower than 2.5×10−3 Hz while the slope remains almost unchanged for the frequency range higher than 10−1.5 Hz. (2) The radial gradient of the power spectrum densities increases with increasing frequency. The dissipation length remains nearly unchanged for the frequency range f ≥ 10−2 Hz. (3) The radial variation of 〈b²〉 is approximately r−3.56. (4) The ratio of 〈b²〉/Bo² is nearly a constant.

Correlated low‐frequency electric and magnetic noise along the auroral field lines

Abstract: Dynamics Explorer 1 measurements of intense low-frequency electric and magnetic noise observed at low altitudes over the auroral zone are described. The intensity of both the electric and magnetic fields decreases rapidly with increasing frequency. Most of the energy is at frequencies below the O(+) cyclotron frequency, and some evidence is found for a cutoff or change in spectral slope near that frequency. The magnetic to electric field ratio decreases rapidly with increasing radial distance and also decreases with increasing frequency. The polarization of the electric field in a plane perpendicular to the earth's magnetic field is essentially random. The transverse electric and magnetic fields are closely correlated, with the average Poynting flux directed toward the earth. The total electromagnetic power flow associated with the noise is substantial. Two general models are discussed to interpret these observations, one based on static electric and magnetic fields imbedded in the ionosphere and the other based on Alfven waves propagating along the auroral field lines.

Theoretical study of relaxation mechanism in magnetic field effects on chemical reactions.

Abstract: External magnetic field and magnetic isotope effects on the dynamic behavior of radical pairs in solutions have been studied theoretically, where the relaxation of their electron spins was taken into account. The decay observed with some radical pairs in micellar solutions under high magnetic fields is successfully interpreted in terms of the relaxation mechanism, and the magnetic field dependence of the relaxation rates is calculated for a model system.

Hydrogen atoms in arbitrary magnetic fields. I. Energy levels and wavefunctions

Abstract: The energy values of many low-lying states of the one-electron problem in the presence of a homogeneous magnetic field of arbitrary strength (0

Properties of a detrital remanence carried by haematite from study of modern river deposits and laboratory redeposition experiments

Abstract: Summary. Although detrital haematite is often observed in red sedimentary rocks, its contribution to the magnetization is usually a matter of debate. Part of the problem is that the properties of magnetic remanence carried by detrital haematite are not well known. Studies on both naturally and experimentally deposited modern river sediments whose remanence is carried by detrital haematite lead to the following observations: (1) The declinations of river-laid sediments deposited under known field conditions average to that of the Earth's field. (2) A substantial inclination error is observed in both river-laid and experimentally deposited sediments which varies as: tan (Io) =f. tan (If) where Io and If are the remanent and applied inclinations respectively and f is about 0.55 in these experiments. (3) The intensity of remanence is a function of both the magnitude and the orientation of the applied magnetic field, increasing with field strength and decreasing with field inclination. This observation is consistent with models involving contributions to the remanence by plates (constrained to lie nearly horizontally) and spheres (aligned with the applied field). (4) Sediments deposited in zero field and then subjected to an applied field acquired a p-DRM by grain rotation. The intensity of p-DRM increased with time according to a power law, P-DRM is acquired parallel to the applied field but, unless the sediment is disturbed, has an intensity an order of magnitude lower than the DRM acquired in the same field. (5) If generally valid, the inclination error for a haematite DRM presents the paradox that while both the age and the polarity of the DRM may be determined, the direction of the DRM magnetization will tend to underestimate palaeolatitude and give palaeopole positions that are far-sided.

Experimental Fine Tuning of Frustration: Two-Dimensional Superconducting Network in a Magnetic Field

Abstract: This Letter reports measurements of the magnetic field dependence of the resistive critical temperature ${T}_{c}$ of a regular square network of superconducting aluminum. We find new effects of flux quantization corresonding to both integral (1, 2, 3,...) and fractional (\textonequarter{}, $\frac{1}{3}$, $\frac{2}{5}$, \textonehalf{}, $\frac{3}{5}$, $\frac{2}{3}$, $\frac{3}{4}$) numbers of flux quanta per unit cell of the network. The fractal fine structure of the upper critical-field line is identified as the edge of the Landau-level spectrum for a tight-binding problem on a square lattice.

Exact image theory for the Sommerfeld half-space problem, part III: General formulation

Abstract: The general Sommerfeld problem with both \epsilon and \mu discontinuous and a source consisting of arbitrarily oriented electric and/or magnetic dipoles at the same location is considered in terms of image theory. The problem is formulated with electric and magnetic fields instead of potential quantities resulting in a vector transmission-line interpretation of the Fourier transformed problem. The image sources are seen to be located in complex space expressable in terms of a certain basic image current function, which was encountered in part II of this paper on the vertical electric dipole problem. The horizontal electric/magnetic dipole image is solved and found to consist of both vertical and horizontal current components. The image concept is generalized to the most general three-dimensional sources. As a check, the well-known reflection coefficient method is obtained as the far-field approximation of the present theory.

Current drive and helicity injection

Abstract: Magnetic configurations with currents parallel to the magnetic field need means for current drive. The concepts of transport of helicity are found useful for considering methods for current drive. The paper describes a formalism for considering transport of helicity, methods of injection of helicity, and specific examples of arrangements for current drive.

Theory of collective excitations in semiconductor superlattice structures

Abstract: In this paper electronic collective excitations of type-I and -II superlattices are examined in detail. Type-I superlattices consist of quasi-two-dimensional layers of electrons, while type-II superlattices consist of alternating quasi-two-dimensional layers of electrons and holes. We use a simple model of the electronic structure and linear-response theory to calculate the density response of the system to an external perturbation. From this, we obtain an expression for the dielectric tensor, the zeros of which yield the dispersion relations of the collective modes. The theory is such that one can take into account many-body effects (depolarization and excitonic shifts), magnetic fields, and electron-phonon coupling in a simple way. A rich spectrum of excitations is found: quasi-two-dimensional plasmons, intersubband plasmons, magnetoplasmons, phonon-plasmon modes, and so on. Some interesting features of the excitations are examined, and their relevance to experiment is discussed.

Auroral zone electric fields from DE-1 and -2 at magnetic conjunctions. Progress Report

Abstract: Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and 4,500 km during magnetic conjunctions. The measured electric fields are usually perpendicular to the magnetic field lines. The north-south meridional electric fields are projected to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields are measured by both DE-1 and DE-2 show that the large-scale electric field is the same at both altitudes, as expected. Superimposed on the large-scale fields, however, are small-scale features with wavelengths less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high and low altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small-scale electric field variations are associated with field-aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE-1.

Modulated, or long-periodic, magnetic structures of crystals

Abstract: The experimental results on modulated magnetic structures and the basic regularities of phase transitions between them are reviewed and are analyzed on the basis of the phenomenological theory of phase transitions with the use of the Ginzburg-Landau functionals for inhomogeneous distributions of the order parameter. Lists of presently known crystals, in which modulated magnetic structures have been observed, are presented and for many of them the form of these functionals, taking into account the crystalline anisotropy and the interaction with a magnetic field, is established. For systems admitting a Lifshitz invariant which is linear with respect to the gradient, a soliton picture of the structure of the incommensurate phase is established and the phase transition into the commensurate phase under the action of temperature or a magnetic field is analyzed. It is shown that this transition is accompanied by a "locking" of the wave vector to the commensurate value. For systems without Lifshitz invariants, which include most crystals with modulated structures, nonlinear equations for the distribution of the order parameter are investigated by asymptotic methods, and these solutions permit describing the entire complex of observed phenomena: the temperature and field dependence of the wave vector, the appearance of higher-order satellites in the neutron duffraction pattern, and the sequence of magnetic phases. Thus a systematic and complete exposition of the present experimental and theoretical status of long-periodic magnetic structures of crystals, such as the spiral structure, the longitudinal and transverse spin-wave structures, the fan structure, and others, is given in this review. The review is written so as to be accessible and of interest to a wide range of readers who are interested in both the theoretical and experimental aspects of the study of magnetic phase transitions in crystals.

Periodic and aperiodic dynamo waves

Abstract: In order to show that aperiodic magnetic cycles, with Maunder minima, can occur naturally in nonlinear hydromagnetic dynamos, we have investigated a simple nonlinear model of an oscillatory stellar dynamo. The parametrized mean field equations in plane geometry have a Hopf bifurcation when the dynamo number D=1, leading to Parker's dynamo waves. Including the nonlinear interaction between the magnetic field and the velocity shear results in a system of seven coupled nonlinear differential equations. For D>1 there is an exact nonlinear solution, corresponding to periodic dynamo waves. In the regime described by a fifth order system of equations this solution remains stable for all D and the velocity shear is progressively reduced by the Lorentz force. In a regime described by a sixth order system, the solution becomes unstable and successive transitions lead to chaotic behaviour. Oscillations are aperiodic and modulated to give episodes of reduced activity.

Reconnection rates of magnetic fields including the effects of viscosity

Abstract: The Sweet–Parker and Petschek scalings of the magnetic reconnection rate are modified to include the effect of the viscosity. The modified scalings show that the viscous effect can be important in high‐β plasmas. The theoretical reconnection scalings are compared with numerical simulation results in a tokamak geometry for three different cases: a forced reconnection driven by external coils, the nonlinear m=1 resistive internal kink, and the nonlinear m=2 tearing mode. In the first two cases, the numerical reconnection rate agrees well with the modified Sweet–Parker scaling when the viscosity is sufficiently large. When the viscosity is negligible, a steady state which was assumed in the derivation of the reconnection scalings is not reached and the current sheet in the reconnection layer either remains stable through sloshing motions of the plasma or breaks up to higher m modes. When the current sheet remains stable, a rough comparison with the Sweet–Parker scaling is obtained. In the nonlinear m=2 tearing mode case where the instability is purely resistive, the reconnection occurs on the slower dissipation time scale (ψs∼η). In addition, experimental data of the nonlinear m=1 resistive internal kink in tokamak discharges are analyzed and are found to give reasonable agreement with the modified Sweet–Parker scaling.

Relativity and Engineering

Abstract: 1. Kinematics in Inertial Axes.- 1.1 The "Aether" in the Nineteenth Century.- 1.2 Some Experimental Evidence.- 1.3 Einstein's Relativity Postulates.- 1.4 Time and Length Standards. Synchronization.- 1.5 The "Simple" Lorentz Transformation.- 1.6 More General Lorentz Transformations.- 1.7 Time Dilatation and Proper Time.- 1.8 Length Measurements.- 1.9 Volume and Surface Elements.- 1.10 Visual Perception of Objects in Motion.- 1.11 Transformation of Velocities and Accelerations.- 1.12 Four-Vectors.- 1.13 Kinematics in Four Dimensions.- Problems.- 2. Dynamics in Inertial Axes.- 2.1 Equation of Motion of a Point Mass.- 2.2 Mass and Energy.- 2.3 A Few Simple Trajectories.- 2.4 Transformation Equations for Force, Energy, and Momentum.- 2.5 Four-Dimensional Dynamics.- 2.6 Systems of Points.- 2.7 Elastic Collisions.- 2.8 Motion of a Point with Variable Rest Mass.- 2.9 Rocket Acceleration.- 2.10 Inelastic Collisions.- 2.11 Incoherent Matter.- 2.12 The Kinetic Energy-Momentum Tensor.- 2.13 The Total Energy-Momentum Tensor.- Problems.- 3. Vacuum Electrodynamics in Inertial Axes.- 3.1 Transformation Formulas for the Sources.- 3.2 Transformation Equations for the Fields.- 3.3 Force on a Charged Particle.- 3.4 Four-Currents.- 3.5 The Electromagnetic Tensors.- 3.6 Potentials.- 3.7 Transformation of a Plane Wave: The Doppler Effect.- 3.8 The Lienard-Wiechert Fields.- 3.9 Fields of a Charge in Uniform Motion.- 3.10 Fields of a Static Dipole in Uniform Motion.- 3.11 Radiation from an Antenna in Uniform Motion.- 3.12 Radiation from a Moving Oscillation Dipole.- 3.13 Doppler Spectrum from a Moving Source.- Problems.- 4. Fields in Media in Uniform Translation.- 4.1 Polarization Densities.- 4.2 Constitutive Equations.- 4.3 Some Useful Forms of Maxwell's Equations.- 4.4 Point Charge Moving Uniformly in a Dielectric Medium.- 4.5 The Cerenkov Effect.- 4.6 Waves in a Moving Dielectric. The Fresnel Dragging Coefficient.- 4.7 Green's Dyadic for a Moving Dielectric.- 4.8 Electric Dipole Radiating in a Moving Dielectric.- Problems.- 5. Boundary-Value Problems for Media in Uniform Translation.- 5.1 Boundary Conditions.- 5.2 Dielectric Slab Moving in Time-Independent Fields.- 5.3 The Wilsons' Experiment.- 5.4 Sliding Contacts. A Simple Problem.- 5.5 Material Bodies Moving at Low Velocities.- 5.6 Conductors Moving in a Pre-Existing Static Magnetic Field.- 5.7 Circuit Equations.- 5.8 Motional E.M.F..- 5.9 Normal Incidence of a Time-Harmonic Plane Wave on a Moving Mirror.- 5.10 Arbitrary Time-Dependence of the Incident Plane Wave.- 5.11 Oblique Incidence of a Time-Harmonic Plane Wave on a Moving Mirror.- 5.12 A Time-Harmonic Plane Wave Incident on a Dielectric Medium.- 5.13 Reflection of a Plane Wave on a Moving Medium of Finite Conductivity.- 5.14 Revisiting the Boundary Conditions at a Moving Interface.- 5.15 Scattering by a Cylinder Moving Longitudinally.- 5.16 Scattering by a Cylinder Moving Transversely.- 5.17 Three-Dimensional Scattering by Moving Bodies.- 5.18 The Quasistationary Method.- Problems.- 6. Electromagnetic Forces and Energy.- 6.1 Surface and Volume Forces in Vacuum.- 6.2 Maxwell's Stress Tensor.- 6.3 A Few Simple Force Calculations.- 6.4 Radiation Pressure on a Moving Mirror.- 6.5 Radiation Force on a Dielectric Cylinder.- 6.6 Static Electric Force on a Dielectric Body.- 6.7 Magnetic Levitation.- 6.8 Levitation on a Line Current.- 6.9 Electromagnetic Energy in an Inertial System.- 6.10 Four-Dimensional Formulation in Vacuum.- 6.11 The Electromagnetic Energy-Momentum Tensor in Material Media.- Problems.- 7. Accelerated Systems of Reference.- 7.1 Coordinate Transformations.- 7.2 The Metric Tensor.- 7.3 Examples of Transformations.- 7.4 Coordinates and Measurements.- 7.5 Time and Length.- 7.6 Four-Vectors and Tensors.- 7.7 Three-Vectors.- 7.8 Velocities and Volume Densities.- 7.9 Covariant Derivative.- Problems.- 8. Gravitation.- 8.1 Inertial and Gravitational Masses.- 8.2 The Principle of Equivalence.- 8.3 Curvature.- 8.4 Einstein's Equations.- 8.5 The Small-Field Approximation.- 8.6 Gravitational Frequency Shift.- 8.7 Time Measurement Problems.- 8.8 Some Important Solutions of Einstein's Equations.- 8.9 Point Dynamics.- 8.10 Motion in the Schwarzschild Metric.- 8.11 Motion of a Photon in the Schwarzschild Metric.- 8.12 Strongly Concentrated Masses.- 8.13 Static Cosmological Metrics.- 8.14 Nonstatic Cosmological Metrics.- 8.15 Recent Cosmological Observations.- Problems.- 9. Maxwell's Equations in a Gravitational Field.- 9.1 Field Tensors and Maxwell's Equations.- 9.2 Maxwell's Equations in Rotating Coordinates.- 9.3 Transformation Equations for Fields and Sources.- 9.4 Constitutive Equations in Vacuum.- 9.5 Constitutive Equations in a Time-Orthogonal Metric.- 9.6 Constitutive Equations in Material Media.- 9.7 The Co-Moving Frame Assumption.- 9.8 Boundary Conditions.- Problems.- 10. Electromagnetism of Accelerated Bodies.- 10.1 Conducting Body of Revolution Rotating in a Static Magnetic Field.- 10.2 Conducting Sphere Rotating in a Uniform Magnetic Field.- 10.3 Motional E.M.F.- 10.4 Generators with Contact Electrodes.- 10.5 Dielectric Body of Revolution Rotating in a Static Field.- 10.6 Rotating Permanent Magnets.- 10.7 Scattering by a Rotating Circular Dielectric Cylinder.- 10.8 Scattering by a Rotating Circular Conducting Cylinder.- 10.9 Scattering by a Rotating Dielectric Body of Revolution.- 10.10 Scattering by a Rotating Sphere.- 10.11 Reflection from a Mirror in Arbitrary Linear Motion.- 10.12 Reflection from an Oscillating Mirror, at Normal Incidence.- 10.13 Reflection from an Oscillating Mirror, at Oblique Incidence.- 10.14 Scattering by Other Moving Surfaces.- Problems.- 11. Field Problems in a Gravitational Field.- 11.1 Fields Associated with Rotating Charges.- 11.2 Schiff's Paradox.- 11.3 Kennard's Experiment.- 11.4 Optical Rotation Sensors.- 11.5 Scattering by a Rotating Body of Arbitrary Shape.- 11.6 Transformation of an Incident Wave to Rotating Coordinates.- 11.7 Scattered Field in Rotating Coordinates.- 11.8 Two Examples.- 11.9 Low Frequency Scattering by Rotating Cylinders.- 11.10 Quasistationary and Relativistic Fields.- 11.11 Axes in Hyperbolic Motion.- 11.12 The Induction Law.- 11.13 Maxwell's Equations in a Schwarzschild Metric.- 11.14 Light Deflection in a Gravitational Field.- Problems.- Appendix A. Complements of Kinematics and Dynamics.- A.1 Transformation Matrix for the "Parallel" Transformation.- A.2 Transformation with Rotation.- A.3 Transformation of Velocities.- A.4 Relationship Between Force and Acceleration.- A.5 Equations of Motion in Cylindrical Coordinates (r,?,z).- A.6 Equations of Motion in Spherical Coordinates (R,?,?).- Appendix B. Dyadics.- B.1 The Dyadic Notation.- B.2 Operators on Dyadics.- B.3 Green's Dyadic.- Appendix C. Basis Vectors.- Appendix D. Moving Open Circuits.- List of Symbols.- Some Useful Numerical Constants.- References.

Theory of plasma waves in the auroral E-region

Abstract: A general theory is developed for both electrojet waves and ion cyclotron and current convective waves observed above 120 km altitude. Previously defined electrojet instability theories are extended to encompass the effects of the magnetic field on ions and the presence of field-aligned currents. The ion-cyclotron (E) waves are assumed produced by the two-stream instability in regions dominated by ion magnetization effects. Field-aligned and cross-field currents drive the E waves, which have displayed threshold drift velocities (TDV) sensitive to conditions at altitudes with effective electron/ion and anomalous electron collision frequencies. The electron density gradients in the region affect the magnitude of the TDV for waves on scales of tens of meters. Recombinational damping increases the TDV for marginal damping of two-stream E waves and establishes a TDV for excitation of large-scale gradient drift waves which propagate nearly perpendicularly to the magnetic field and may have only 10-20 m wavelengths.

Kinematic dynamo problem in a linear velocity field

Abstract: A magnetic field is shown to be asymptotically (t → ∞) decaying in a flow of finite conductivity with v = Cr, where C = Cζ(t) is a random matrix. The decay is exponential, and its rate does not depend on the conductivity. However, the magnetic energy increases exponentially owing to growth of the domain occupied by the field. The spatial distribution of the magnetic field is a set of thin ropes and (or) layers.

MHD processes in the outer heliosphere

Abstract: The magnetic field measurements from Voyager and the magnetohydrodynamic (MHD) processes in the outer heliosphere are reviewed. A bibliography of the experimental and theoretical work concerning magnetic fields and plasmas observed in the outer heliosphere is given. Emphasis in this review is on basic concepts and dynamical processes involving the magnetic field. The theory that serves to explain and unify the interplanetary magnetic field and plasma observations is magnetohydrodynamics. Basic physical processes and observations that relate directly to solutions of the MHD equations are emphasized, but obtaining solutions of this complex system of equations involves various assumptions and approximations. The spatial and temporal complexity of the outer heliosphere and some approaches for dealing with this complexity are discussed.

Numerical simulation of reconnection in an emerging magnetic flux region

Abstract: The resistive MHD equations are numerically solved in two dimensions for an initial-boundary-value problem which simulates reconnection between an emerging magnetic flux region and an overlying coronal magnetic field The emerging region is modelled by a cylindrical flux tube with a poloidal magnetic field lying in the same plane as the external, coronal field The plasma betas of the emerging and coronal regions are 10 and 01, respectively, and the magnetic Reynolds number for the system is 2 × 103 At the beginning of the simulation the tube starts to emerge through the base of the rectangular computational domain, and, when the tube is halfway into the computational domain, its position is held fixed so that no more flux of plasma enters through the base Because the time-scale of the emergence is slower than the Alfven time-scale, but faster than the reconnection time-scale, a region of closed loops forms at the base These loops are gradually opened and reconnected with the overlying, external magnetic field as time proceeds The evolution of the plasma can be divided into four phases as follows: First, an initial, quasi-steady phase during which most of the emergence is completed During this phase, reconnection initially occurs at the slow rate predicted by the Sweet model of diffusive reconnection, but increases steadily until the fast rate predicted by the Petschek model of slow-shock reconnection is approached Second, an impulsive phase with large-scale, super-magnetosonic flows This phase appears to be triggered when the internal mechanical equilibrium inside the emerging flux tube is upset by reconnection acting on the outer layers of the flux tube During the impulsive phase most of the flux tube pinches off from the base to form a cylindrical magnetic island, and temporarily the reconnection rate exceeds the steady-state Petschek rate (At the time of the peak reconnection rate, the diffusion region at the X-line is not fully resolved, and so this may be a numerical artifact) Third, a second quasi-steady phase during which the magnetic island created in the impulsive phase is slowly dissipated by continuing, but low-level, reconnection And fourth, a static, non-evolving phase containing a potential, current-free field and virtually no flow During the short time in the impulsive phase when the reconnection rate exceeds the steady-state Petschek rate, a pile-up of magnetic flux at the neutral line occurs At the same time the existing Petschek-slow-mode shocks are shed and replaced by new ones; and, for a while, both new and old sets of slow shocks coexist

Quasistatic electric field measurements with spherical double probes on the GEOS and ISEE satellites

Abstract: Spherical double probes for measurements of electric fields on the GEOS-1, GEOS-2, and ISEE-1 satellites are described. An essential feature of these satellites is their conductive surfaces which eliminate errors due to differential charging and enable meaningful diagnostic experiments to be carried out. The result of these experiments is a good understanding of interactions between the plasma, the satellite and the probes, including photo-electron emission on satellite and probes. Electric field measurements are compared with measurements of plasma drift perpendicular to the magnetic field in the solar wind and the magnetosphere and the error bar for the absolute values of the electric field is found to be in the range ±(0.5–1.0) mV m-1 whereas relative variations can be determined with much better accuracy. A useful by-product from a spherical double probe system is the determination of satellite floating potential which is related to the plasma electron flux. This measurement allows high time resolution studies of boundary crossings. Examples of electric field measurements, which reflect the recent scientific results, are given for different regions of the magnetosphere from the bow shock, the inner magnetosphere and the tail. Several examples of simultaneous GEOS-ISEE observations are described.

Applications of NMR Imaging in Hyperthermia: An Evaluation of the Potential for Localized Tissue Heating and Noninvasive Temperature Monitoring

Abstract: Potential applications of three-dimensional nuclear magnetic resonance (NMR) imaging to noninvasive temperature monitoring as well as tissue heating are discussed. Although heat delivery using conventional NMR imaging is negligible, the physical basis for heat transfer is reviewed and the potential for increase is shown to depend on both the static magnetic field and the longitudinal relaxation time (T1)