Showing papers on "Magnetic field published in 1998"

Observational constraints on the dynamics of the interplanetary magnetic field dissipation range

Abstract: The dissipation range for interplanetary magnetic field fluctuations is formed by those fluctuations with spatial scales comparable to the gyroradius or ion inertial length of a thermal ion. It is reasonable to assume that the dissipation range represents the final fate of magnetic energy that is transferred from the largest spatial scales via nonlinear processes until kinetic coupling with the background plasma removes the energy from the spectrum and heats the background distribution. Typically, the dissipation range at 1 AU sets in at spacecraft frame frequencies of a few tenths of a hertz. It is characterized by a steepening of the power spectrum and often demonstrates a bias of the polarization or magnetic helicity spectrum. We examine Wind observations of inertial and dissipation range spectra in an attempt to better understand the processes that form the dissipation range and how these processes depend on the ambient solar wind parameters (interplanetary magnetic field intensity, ambient proton density and temperature, etc.). We focus on stationary intervals with well-defined inertial and dissipation range spectra. Our analysis shows that parallel-propagating waves, such as Alfven waves, are inconsistent with the data. MHD turbulence consisting of a partly slab and partly two-dimensional (2-D) composite geometry is consistent with the observations, while thermal paxticle interactions with the 2-D component may be responsible for the formation of the dissipation range. Kinetic Alfven waves propagating at large angles to the background magnetic field are also consistent with the observations and may form some portion of the 2-D turbulence component.

Concomitant gradient terms in phase contrast MR: Analysis and correction

Abstract: Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result This is a consequence of Maxwell's equations, ie, within the imaging volume the magnetic field must have zero divergence, and has negligible curl The concomitant, or Maxwell field has been described in the MRI literature for over 10 years In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms The concomitant gradient cross-terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy) The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated Since the lowest-order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields

Quantum Transport in Two-Dimensional Graphite System

Abstract: In a self-consistent Born approximation, the density of states and the conductivity are calculated in a two-dimensional graphite sheet in magnetic fields. Two different cases of scatterers are considered, the short-range case where the range is smaller than the lattice constant and the long-range case where it is comparable or slightly larger. The quantum theory provides results quite different from the results of Boltzmann transport theory even in the absence of a magnetic field. In high magnetic fields, the conductivity exhibits a series of peaks, whose values depend only on the natural constants and the Landau level index. The conductivity of undoped systems is always given by a universal conductivity \(e^{2}/\pi^{2}\hbar\) independent of a magnetic field.

Depolarization and Faraday effects in galaxies

Abstract: Faraday rotation and depolarization of synchrotron radio emission are considered in a consistent general approach, under conditions typical of spiral galaxies, i.e. when the magneto-ionic medium and relativistic electrons are non-uniformly distributed in a layer containing both regular and fluctuating components of magnetic field, thermal electron density and synchrotron emissivity. We demonstrate that non-uniformity of the magneto-ionic medium along the line of sight strongly affects the observable polarization patterns. The degree of polarization p and the observed Faraday rotation measure RM are very sensitive to whether or not the source is symmetric along the line of sight. The RM may change sign in a certain wavelength range in an asymmetric slab even when the line-of-sight magnetic field has no reversals. Faraday depolarization in a purely regular magnetic field can be much stronger than suggested by the low observed rotation measures. A twisted regular magnetic field may result in p increasing with λ— a behaviour detected in several galaxies. We derive expressions for statistical fluctuations in complex polarization and show that random fluctuations in the degree of polarization caused by Faraday dispersion are expected to become significantly larger than the mean value of p at λ ≳ 20 − 30 cm. We also discuss depolarization arising from a gradient of Faraday rotation measure across the beam, both in the source and in an external Faraday screen. We briefly discuss applications of the above results to radio polarization observations. We discuss how the degree of polarization is affected by the scaling of synchrotron emissivity ɛ with the total magnetic field strength B. We derive formulae for the complex polarization at λ 0 under the assumption that ɛ ∝ B2B2⊥, which may arise under energy equipartition or pressure balance between cosmic rays and magnetic fields. The resulting degree of polarization is systematically larger than for the usually adopted scaling ɛ ∝ B2⊥; the difference may reach a factor of 1.5.

New models of Jupiter's magnetic field constrained by the Io flux tube footprint

Abstract: Spherical harmonic models of the planetary magnetic field of Jupiter are obtained from in situ magnetic field measurements and remote observations of the position of the foot of the Io flux tube in Jupiter's ionosphere. The Io flux tube (IFT) footprint locates the ionospheric footprint of field lines traced from Io's orbital radial distance in the equator plane (5.9 Jovian radii). The IFT footprint is a valuable constraint on magnetic field models, providing “ground truth” information in a region close to the planet and thus far not sampled by spacecraft. The magnetic field is represented using a spherical harmonic expansion of degree and order 4 for the planetary (“internal”) field and an explicit model of the magnetodisc for the field (“external”) due to distributed currents. Models fitting Voyager 1 and Pioneer 11 magnetometer observations and the IFT footprint are obtained by partial solution of the underdetermined inverse problem using generalized inverse techniques. Dipole, quadrupole, octupole, and a subset of higher-degree and higher-order spherical harmonic coefficients are determined and compared with earlier models.

Structure of the dissipation region during collisionless magnetic reconnection

Abstract: Collisionless magnetic reconnection is studied using a 2 1/2-dimensional hybrid code including Hall dynamics and electron inertia. The simulations reveal that the dissipation region develops a two-scale structure: an inner electron region and an outer ion region. Close to the X line is a region with a scale of c/ωpe, the electron collisionless skin depth, where the electron flows completely dominate those of the ions and the frozen-in magnetic flux constraint is broken. Outside of this region and encompassing the rest of the dissipation region, which scales like c/ωpi, the ion inertial length, is the Hall region where the electrons are frozen-in to the magnetic field but the ions are not, allowing the two species to flow at different velocities. The decoupling of electron and ion motion in the dissipation region has important implications for the rate of magnetic reconnection in collisionless plasma: the ions are not constrained to flow through the very narrow region where the frozen-in constraint is broken so that ion flux into the dissipation region is large. For the simulations which have been completed to date, the resulting rate of reconnection is a substantial fraction of the Alfven velocity and is controlled by the ions, not the electrons. The dynamics of the ions is found to be inherently nonfluid-like, with multiple ion beams present both at the X line and at the downstream boundary between the inflow and outflow plasma. The reconnection rate is only slightly affected by the temperature of the inflowing ions and in particular the structure of the dissipation region is controlled by the ion inertial length c/ωpi and not the ion Larmor radius based on the incoming ion temperature.

Instability of Toroidal Magnetic Field in Jets and Plerions

Abstract: Astrophysical jets and pulsar-fed supernova remnants (plerions) are expected to develop highly organized magnetic structures dominated by concentric loops of toroidal field, B. It has been argued that such structures could explain the polarization properties of some jets and contribute to their lateral confinement through magnetic tension forces. A concentric toroidal field geometry is also central to the Rees-Gunn model for the Crab Nebula, the archetypal plerion, and leads to the deduction that the Crab pulsar's wind must have a weak magnetic field. Yet this kind of equilibrium between magnetic and gas pressure forces, the "equilibrium Z-pinch" of the controlled fusion literature, is well known to be susceptible to disruptive localized instabilities, even when the magnetic field is weak and/or boundary conditions (e.g., a dense external medium) slow or suppress global modes. Thus, the magnetic field structures imputed to the interiors of jets and plerions are unlikely to persist for very long. To determine the growth rates of Z-pinch instabilities under astrophysical conditions, I derive a dispersion relation that is valid for the relativistic fluids of which jets and plerions may be composed, in the ideal magnetohydrodynamics (MHD) limit. The dominant instabilities are kink (m = 1) and pinch (m = 0) modes. The former generally dominate, destroying the concentric field structure and probably driving the system toward a more chaotic state in which the mean field strength is independent of radius (and in which resistive dissipation of the field may be enhanced). I estimate the timescales over which the field structure is likely to be rearranged and relate these to distances along relativistic jets and radii from the central pulsar in a plerion. I conclude that the central tenet of the Rees-Gunn model for the Crab Nebula, the existence of a concentric toroidal field well outside the pulsar wind's termination shock, is physically unrealistic. With this assumption gone, there is no dynamical reason to conclude that the magnetic energy flux carried by the pulsar wind is much weaker than the kinetic energy flux. Abandoning the principal conclusion of Rees & Gunn would resolve a long-standing puzzle in pulsar wind theory.

POLAR observations of coherent electric field structures

Abstract: The POLAR plasma wave instrument often detects coherent electric field structures in the high altitude polar magnetosphere. The structures appear to be positively charged potentials which are found to move both up and down the ambient magnetic field. Typical estimated velocities and parallel scale sizes are the order of 1000 km/s and 100-1000 meters, respectively. We have observed the structures at radial distances of 2.02 to 8.5 Re and L shells of 6 - 12+, although the they are likely to occur over a broader range of space than suggested by this initial study. The structures are responsible for some of the the spectral features of broadband electrostatic noise, and are similar to recent GEOTAIL and FAST observations of solitary waves.

A soft magnetic CoNiFe film with high saturation magnetic flux density and low coercivity

Abstract: Magnetic materials are classed as ‘soft’ if they have a low coercivity (the critical field strength Hc required to flip the direction of magnetization). Soft magnetic materials are a central component of electromagnetic devices such as step motors, magnetic sensors, transformers and magnetic recording heads. Miniaturization of these devices requires materials that can develop higher saturation flux density, Bs, so that the necessary flux densities can be preserved on reducing device dimensions, while simultaneously achieving a low coercivity. Common high-Bs soft magnetic films currently in use are electroplated CoFe-based alloys1,2,3,4 electroplated CoNiFe alloys5,6,7 and sputtered Fe-based nanocrystalline8,9,10,11 and FeN films12,13,14. Sputtering is not suitable, however, for fabricating the thick films needed in some applications, for which electrochemical methods are preferred. Here we report the electrochemical preparation of a CoNiFe film with a very high value of Bs (2.0–2.1 T) and a low coercivity. The favourable properties are achieved by avoiding the need for organic additives in the deposition process, which are typically used to reduce internal stresses. Our films also undergo very small magnetostriction, which is essential to ensure that they are not stressed when an external magnetic field is applied (or conversely, that external stresses do not disrupt the magnetic properties). Our material should find applications in miniaturization of electromechanical devices and in high-density magnetic data storage.

Dynamics of a ferromagnetic domain wall: Avalanches, depinning transition, and the Barkhausen effect

Abstract: We study the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium. The avalanchelike motion of the domain walls between pinned configurations produces a noise known as the Barkhausen effect. We discuss experimental results on soft ferromagnetic materials, with reference to the domain structure and the sample geometry, and report Barkhausen noise measurements on Fe21Co64B15 amorphous alloy. We construct an equation of motion for a flexible domain wall, which displays a depinning transition as the field is increased. The long-range dipolar interactions are shown to set the upper critical dimension to dc53, which implies that mean-field exponents~with possible logarithmic correction! are expected to describe the Barkhausen effect. We introduce a mean-field infinite-range model and show that it is equivalent to a previously introduced single-degree-of-freedom model, known to reproduce several experimental results. We numerically simulate the equation in d53, confirming the theoretical predictions. We compute the avalanche distributions as a function of the field driving rate and the intensity of the demagnetizing field. The scaling exponents change linearly with the driving rate, while the cutoff of the distribution is determined by the demagnetizing field, in remarkable agreement with experiments.@S0163-1829~98!08833-X#

Resonant magnetic field control of elastic scattering of cold 85Rb

Abstract: A magnetic field dependent Feshbach resonance has been observed in the elastic scattering collision rate between atoms in the F = 2, M = -2 state of 85 Rb. Changing the magnetic field by several Gauss caused the collision rate to vary by a factor of 10,000, and the sign of the scattering length could be reserved. The resonance peak is at 155.2(4) G and its width is 11.6(5) G. From these results we extract much improved values for the three quantities that characterize the interaction porential: the van der Waals coefficient C6, the singlet scattering length, and the triplet scattering length.

An Analytic Solution for the Force Between Two Magnetic Dipoles

Abstract: An analytic equation describing the force between two magnetic dipoles is derived in this paper. We assumed that the dipole sizes are small compared to their separation. A Taylor expansion for the first non-zero term was performed in order to derive the magnetic force analytic expression. Vector differential and path integral derivation approaches are used, and the same result is achieved with both methods. This force decreases with the fourth power of the distance between the dipoles. Accuracy estimates due to the Taylor approximation are given.

The Magnetic Topology of Solar Eruptions

Abstract: We present an explanation for the well-known observation that complexity of the solar magnetic field is a necessary ingredient for strong activity such as large eruptive flares. Our model starts with the standard picture for the energy build up -- highly-sheared, newly-emerged magnetic field near the photospheric neutral line held down by overlying unsheared field. Previously, we proposed the key new idea that magnetic reconnection between the unsheared field and neighboring flux systems decreases the amount of overlying field and, thereby, allows the low-lying sheared flux to ``break out'' (Antiochos, DeVore and Klimchuk 1998, ApJ, submitted). In this paper we show that a bipolar active region does not have the necessary complexity for this process to occur, but a delta sunspot has the right topology for magnetic breakout. We discuss the implications of these results for observations from SOHO and TRACE.

Vortex Phase Diagram for Mesoscopic Superconducting Disks

Abstract: Solving numerically the 3D nonlinear Ginzburg-Landau (GL) equations, we study equilibrium and nonequilibrium phase transitions between different superconducting states of mesoscopic disks which are thinner than the coherence length and the penetration depth. We have found a smooth transition from a multivortex superconducting state to a giant vortex state by increasing both the disk thickness and the magnetic field. A vortex phase diagram is obtained which shows, as a function of the magnetic field, a reentrant behavior between the multivortex and the giant vortex state.

Debye-scale plasma structures associated with magnetic-field-aligned electric fields

Abstract: We report a new type of spatially coherent plasma structure that is associated with quasistatic, magnetic-field-aligned electric fields in space plasmas. The solitary structures form in a magnetized plasma, are multidimensional, and are highly supersonic. The size along ${\mathbf{B}}_{0}$ is a few ${\ensuremath{\lambda}}_{D}$ and increases with increasing amplitude, unlike a classical soliton. The perpendicular size appears to be influenced by ion motion. We show that the structures facilitate ion-electron momentum exchange and suggest that an aggregate of structures may play a role supporting large-scale, parallel electric fields.

The sub-Alfvénic interaction of the Galilean satellites with the Jovian magnetosphere

Abstract: Recent observations by the Galileo spacecraft and Earth-based techniques have motivated us to reconsider the sub-Alfvenic interaction between the Galilean satellites of Jupiter and the magnetosphere. (1) We show that the atomic processes causing the interaction between the magnetoplasma and a neutral atmosphere can be described by generalized collision frequencies with contributions from elastic collisions, ion pickup, etc. Thus there is no fundamental difference in the effect of these processes on the plasma dynamics claimed in the recent literature. For a magnetic field configuration including possible internal fields, we show that the sub-Alfvenic, low-beta interaction can be described by an anisotropically conducting atmosphere joined to an Alfven wing as one extreme case and the Jovian ionosphere as the other extreme case. (2) The addition of a small magnetic field of internal origin does not modify the general Alfven wing model qualitatively but only quantitatively. All magnetic moments discussed in the literature for In are small in this sense. For an aligned internal dipole and ambient Jovian magnetic field the interaction will be enhanced by focusing of the electric field. (3) A qualitative change occurs by the additional occurrence of closed magnetic field lines for larger internal magnetic fields as in the case of Ganymede. Here the focusing is even enhanced. (4) The first discussion of nonstationary plasma flows at the satellites shows that electromagnetically induced magnetic fields may play an important role if the satellite interiors are highly conducting. From the point of view of the external excitation, induction effects may be strong for Callisto, In, Europa, and Ganymede in order of decreasing importance. The magnetic field observations at the first Callisto encounter can be explained by these effects.

High average-power THz radiation from femtosecond laser-irradiated InAs in a magnetic field and its elliptical polarization characteristics

Abstract: The THz-radiation power from bulk InAs irradiated with femtosecond optical pulses is significantly enhanced and reaches 650 μW in a 1.7-T magnetic field with 1.5-W excitation power. The THz-radiation power is related almost quadratically both to the magnetic field and excitation laser power. We have also found that the power of the THz-radiation from an InAs sample in a magnetic field is over one order of magnitude higher than that from GaAs. Additionally, a dramatic change of ellipticity is observed, and the spectra of the horizontal and vertical polarization components are found to differ.

Particle Acceleration Zones above Pulsar Polar Caps: Electron and Positron Pair Formation Fronts

Abstract: We investigate self-consistent particle acceleration near a pulsar polar cap (PC) by the electrostatic field due to the effect of inertial frame dragging. Test particles gain energy from the electric field parallel to the open magnetic field lines and lose energy by both curvature radiation (CR) and resonant and nonresonant inverse Compton scattering (ICS) with soft thermal X-rays from the neutron star (NS) surface. Gamma rays radiated by electrons accelerated from the stellar surface produce pairs in the strong magnetic field, which screen the electric field beyond a pair formation front (PFF). Some of the created positrons can be accelerated back toward the surface and produce γ-rays and pairs that create another PFF above the surface. We find that ICS photons control PFF formation near the surface, but because of the different angles at which the electron and positron scatter the soft photons, positron-initiated cascades develop above the surface and may screen the accelerating electric field. Stable acceleration from the NS surface is therefore not possible in the presence of dominant ICS energy losses. However, we find that stable acceleration zones may occur at some distance above the surface, where CR dominates the electron and positron energy losses and there is up-down symmetry between the electron and positron PFFs. We examine the dependence of CR-controlled acceleration zone voltage, width, and height above the surface on parameters of the pulsar and its soft X-ray emission. For most pulsars, we find that acceleration will start at a height of 0.5-1 stellar radii above the NS surface.

Damping of cosmic magnetic fields

Abstract: We examine the evolution of magnetic fields in an expanding fluid composed of matter and radiation with particular interest in the evolution of cosmic magnetic fields. We derive the propagation velocities and damping rates for relativistic and non-relativistic fast and slow magnetosonic and Alfv{acute e}n waves in the presence of viscous and heat conducting processes. The analysis covers all magnetohydrodynamics modes in the radiation diffusion and the free-streaming regimes. When our results are applied to the evolution of magnetic fields in the early universe, we find that cosmic magnetic fields are damped from prior to the epoch of neutrino decoupling up to recombination. Similar to the case of sound waves propagating in a demagnetized plasma, fast magnetosonic waves are damped by radiation diffusion on all scales smaller than the radiation diffusion length. The characteristic damping scales are the horizon scale at neutrino decoupling (M{sub {nu}}{approx}10{sup {minus}4}M{sub {circle_dot}} in baryons) and the Silk mass at recombination (M{sub {gamma}}{approx}10{sup 13}M{sub {circle_dot}} in baryons). In contrast, the oscillations of slow magnetosonic and Alfv{acute e}n waves get overdamped in the radiation diffusion regime, resulting in frozen-in magnetic field perturbations. Further damping of these perturbations is possible only if before recombination the wave enters amore » regime in which radiation free-streams on the scale of the perturbation. The maximum damping scale of slow magnetosonic and Alfv{acute e}n modes is always smaller than or equal to the damping scale of fast magnetosonic waves, and depends on the magnetic field strength and its direction relative to the wave vector. Our findings have multifold implications for cosmology. The dissipation of magnetic field energy into heat during the epoch of neutrino decoupling ensures that most magnetic field configurations generated in the very early universe satisfy big bang nucleosynthesis constraints. Further dissipation before recombination constrains models in which primordial magnetic fields give rise to galactic magnetic fields or density perturbations. Finally, the survival of Alfv{acute e}n and slow magnetosonic modes on scales well below the Silk mass may be of significance for the formation of structure on small scales. {copyright} {ital 1998} {ital The American Physical Society}« less

Numerical modelings of superconducting wires for AC loss calculations

Abstract: Superconducting properties of superconducting wires as well as the influence of their composite structure and twisting should be taken into account for their numerical modeling for AC loss calculations. Furthermore, complicated electromagnetic conditions in electrical apparatuses under which superconducting wires are used influence their AC loss properties; superconducting wires carry their transport current and are exposed to the external magnetic field whose direction and magnitude vary spatially. A series of numerical models of superconducting tapes based on the finite element method has been developed. In each model, some of the above-mentioned factors that could influence the AC loss properties are taken into account. The models are formulated with the current vector potential and the scalar magnetic potential ( T – Ω method). Superconducting property is given by the E – J characteristic represented by a power law. The current distributions in non-twisted and twisted superconducting tapes carrying their transport current and/or exposed to the external magnetic field are calculated with these models to estimate their AC loss. The current distribution in a short piece of superconducting tape exposed to AC magnetic field is also calculated.

Asymmetric flux pinning in a regular array of magnetic dipoles

Abstract: The effect of a regular array of magnetic dipoles embedded in a superconducting film was investigated. A large asymmetry in critical currents was found between when the magnetic dipoles are aligned and antialigned with respect to an externally applied magnetic field. Enhanced pinning effects were observed when the flux lattice and the dipole lattice were commensurate. The data are used to infer pinning mechanisms, strengths, and sites.

Simulation of dispersionless injections and drift echoes of energetic electrons associated with substorms

Abstract: The term “dispersionless injection” refers to a class of events which show simultaneous enhancement (injection) of electrons and ions with different energies usually seen at or near geosynchronous orbit. We show that dispersionless injections can be understood as a consequence of changes in the electric and magnetic fields by modeling an electron injection event observed early on January 10, 1997 by means of a test-particle simulation. The model background magnetic field is a basic dipole field made asymmetrical by a compressed dayside and a weakened nightside. The transient fields are modeled with only one component of the electric field which is westward and a consistent magnetic field. These fields are used to model the major features of a dipolarization process during a substorm onset. We follow the electrons using a relativistic guiding center code. Our simulation results, with an initial kappa electron energy flux spectrum, reproduce the observed electron injection and subsequent drift echoes and show that the energization of injected electrons is mainly due to betatron acceleration of the preexisting electron population at larger radial distances in the magnetotail by transient fields.

Shape Transition of Magnetic Field Sensitive Polymer Gels

Abstract: Magnetic field sensitive gels, called ferrogels, are chemically cross-linked polymer networks swollen by a ferrofluid. The monodomain magnetic particles, with a typical size of about 10 nm, couple the shape of the polymer gel to the nonuniform external magnetic field. Shape distortion occurs instantaneously and disappears abruptly when an external magnetic field is applied or removed, respectively. This work provides a thermodynamic analysis of shape transition based on the free energy of the swollen network that includes the elasticity of network chains as well as magnetic interactions of finely dispersed solid particles with the external field. It is shown that noncontinuous shape transition is due to a shift of equilibrium state from one local minimum to another one, similar to a first-order phase transition. The discussions presented here may be useful for the design of magnetically active soft polymeric actuators.

Method for near field electromagnetic proximity determination for guidance of a borehole drill

Abstract: A method and apparatus for precise measurement of the distance and direction from a magnetic field sensor to a nearby magnetic field source includes an elongated iron core solenoid driven by a repetitive, nonsinusoidal current source. In the near field the solenoid has two spaced, temporally varying magnetic poles, and measurement of the distance and direction to this source includes analysis of field components which vary in synchronism with the current source.

Turbulence and Angular Momentum Transport in a Global Accretion Disk Simulation

Abstract: The global development of magnetohydrodynamic turbulence in an accretion disk is studied within a simplified disk model that omits vertical stratification. Starting with a weak vertical seed field, a saturated state is obtained after a few tens of orbits in which the energy in the predominantly toroidal magnetic field is still subthermal. The efficiency of angular momentum transport, parameterized by the Shakura-Sunyaev α-parameter, is of the order of 10−1. The dominant contribution to α comes from magnetic stresses, which are enhanced by the presence of weak net vertical fields. The power spectra of the magnetic fields are flat or decline only slowly toward the largest scales accessible in the calculation, suggesting that the viscosity arising from MHD turbulence may not be a locally determined quantity. I discuss how these results compare with observationally inferred values of α and possible implications for models of jet formation.

Thermal Conduction in a Tangled Magnetic Field

Abstract: The suppression of thermal conduction by a static stochastic magnetic field is calculated for different ratios of the field scale length to the collisional mean free path. The effects of magnetic trapping are determined through a two-scale analysis and Monte Carlo particle simulations. In galaxy-cluster cooling flows, thermal conductivity is reduced from the Spitzer value by a factor of order ${10}^{2}$ to ${10}^{3}$.

Far-Infrared and Submillimeter Polarization of OMC-1 Evidence for Magnetically Regulated Star Formation

Abstract: This paper presents large-scale polarization maps (8' × 8') of the Orion molecular cloud (OMC-1) at far-infrared (100 μm; 35'' resolution) and submillimeter (350 μm; 18'' resolution) wavelengths. The magnetic field shows a pinch at scales less than 0.5 pc with a centroid that is on the OMC-1 ridge, a site of high-mass star formation. We infer that gravitational collapse pulled the magnetic field into an hourglass shape. We estimate that the magnetic, kinetic, and gravitational energies are in equipartition on the ridge and that the magnetic energy dominates in the surrounding ambient envelope. We consider a model in which the ridge and thus high-mass stars gravitationally collapsed out of a cloud that was initially supported by the magnetic field. At flux peaks, there is a reduction in the percent polarization. This effect is discussed in relation to temperature, optical depth, and wavelength.

Spin-Polarized Vacuum Tunneling into the Exchange-Split Surface State of Gd(0001)

Abstract: Spin-polarized tunneling is demonstrated on Gd(0001) thin films using ferromagnetic probe tips in a low-temperature scanning tunneling microscope. The magnetic field dependent asymmetries in the differential tunneling conductivity are found at bias voltages which correspond to the energies of the spin components of the exchange-split Gd(0001) surface state. Maps of the spatial variation of the asymmetry reveal the magnetic structure of Gd(0001) thin films with a lateral resolution better than 20 nm. It is found that magnetic tip coatings thicker than 100 monolayers Fe may modify the sample domain structure due to the stray field of the tip.

Magnetic field calculation in permanent magnet motors with rotor eccentricity: without slotting effect

Abstract: An analytical technique for predicting the instantaneous magnetic field distribution in the air gap region of permanent magnet motors with rotor eccentricity is developed. The governing equations and associated boundary conditions are formulated and solved by a perturbation method. The predicted solutions are verified by the results of a corresponding finite element analysis. The results show the effectiveness of using the perturbation method for this eccentric field calculation. According to this study, the additional flux density due to rotor eccentricity is proportional to the amount of the eccentricity in a small range and has a quite different distribution from the normal flux density without rotor eccentricity. Accordingly, rotor eccentricity may cause undesirable effects on brushless permanent magnet motors.