Showing papers on "Magnetic field published in 1968"
TL;DR: Weak alternating magnetic fields outside the human scalp, produced by alpha-rhythm currents, are demonstrated and a course distribution shows left-right symmetry for the particular averaging technique used here.
Abstract: Weak alternating magnetic fields outside the human scalp, produced by alpha-rhythm currents, are demonstrated. Subject ard magnetic detector were housed in a multilayer magnetically shielded chamber. Background magnetic noise was reduced by signal-averaging. The fields near the scalp are about 1 x 10-9 gauss (peak to peak). A course distribution shows left-right symmetry for the particular averaging technique used here.
683 citations
TL;DR: The observed critical parameters indicate that a laminar flow in the core, due to the earth's precession, would have weak hydrodynamic instabilities at most, but that finite-amplitude hydromagnetic instability could lead to fully turbulent flow.
Abstract: I have proposed that the precessional torques acting on the earth can sustain a turbulent hydromagnetic flow in the molten core. A gross balance of the Coriolis force, the Lorentz force, and the precessional force in the core fluid provided estimates of the fluid velocity and the interior magnetic field characteristic of such flow. Then these numbers and a balance of the processes responsible for the decay and regeneration of the magnetic field provided an estimate of the magnetic field external to the core. This external field is in keeping with the observations, but its value is dependent upon the speculative value for the electrical conductivity of core material. The proposal that turbulent flow due to precession can occur in the core was tested in a study of nonmagnetic laboratory flows induced by the steady precession of fluid-filled rotating spheroids. It was found that these flows exhibit both small wavelike instabilities and violent finite-amplitude instability to turbulent motion above critical values of the precession rate. The observed critical parameters indicate that a laminar flow in the core, due to the earth9s precession, would have weak hydrodynamic instabilities at most, but that finite-amplitude hydromagnetic instability could lead to fully turbulent flow.
442 citations
TL;DR: In this article, the Mossbauer line shape in the presence of time-dependent electric field gradients and magnetic fields is considered, and two specific soluble stochastic models are treated: (1) a static electric field gradient with a randomly fluctuating magnetic field which takes on values $+h$ and $\ensuremath{-}h$, each directed along the axis of the field gradient, and (2) as in (1), but with the fluctuating magnet field perpendicular to the orientation of field gradient.
Abstract: The M\"ossbauer line shape in the presence of time-dependent electric field gradients and magnetic fields is considered. Two specific soluble stochastic models are treated: (1) a static electric field gradient with a randomly fluctuating magnetic field which takes on values $+h$ and $\ensuremath{-}h$, each directed along the axis of the field gradient, and (2) as in (1), but with the fluctuating magnetic field perpendicular to the axis of the field gradient. Example (2) is more complex than (1), since the fluctuating field is in this case capable of inducing transitions between the nuclear levels, while in (1) this is not possible. Specific calculations for the two cases illustrate the differences between them.
432 citations
TL;DR: In this article, a theory of low-frequency drift instabilities in a nonuniform collisionless plasma is developed for general magnetic field configurations including trapped particle effects, rather than the plane geometry which has previously received most attention.
Abstract: A theory of low‐frequency drift (universal) instabilities in a nonuniform collisionless plasma is developed for general magnetic field configurations including trapped particle effects, rather than the plane geometry which has previously received most attention. A type of energy principle shows that the special equilibrium distribution F(∈, μ), of interest in minimum‐B mirror configurations, is absolutely stable to these modes provided ∂F/∂∈ < 0 together with a second condition on ∂F/∂μ. For equilibrium distributions not of this special form, in particular for a Maxwell distribution with a density gradient, the case of axisymmetric toroidal configurations with closed poloidal field lines is considered in detail. Three unstable drift modes are found, a flute‐like mode, a drift‐ballooning mode local to the region of unfavorable curvature, and a drift‐universal mode. Stability criteria and growth rates for the modes are given. The equations also describe a recently discussed low‐frequency trapped‐particle in...
312 citations
TL;DR: In this paper, a general method is presented for calculating the structural changes resulting from electric or magnetic fields acting on anisotropy of the anisotropic susceptibility of a liquid crystal.
Abstract: Electric or magnetic fields acting on the anisotropy of the electric or magnetic susceptibility exert torques within a liquid crystal which may compete with the elastic torques determining its internal structure. A general method is presented for calculating the structural changes thus produced. Two examples are given, and the field strength for the effects is estimated to be 105 V/cm or 105 G. Two experiments are suggested to test the theory.
292 citations
TL;DR: In this paper, a gravimeter is constructed which uses the near perfect stability of superconducting persistent currents to make it a device with exceptional stability, and it is used to measure changes in the vertical position of the sphere or as changes in a feedback force, independent of the primary supporting force, which holds the sphere in a fixed position.
Abstract: A gravimeter is constructed which uses the near perfect stability of superconducting persistent currents to make it a device with exceptional stability. A superconducting sphere is levitated in the magnetic field generated by two superconducting coils. Changes in gravitational or inertial forces in the vertical direction are measured as changes in the vertical position of the sphere or as changes in a feedback force, independent of the primary supporting force, which holds the sphere in a fixed position. In its present state of development the instrument produces data which, when compared to calculated earth tides, set an upper limit on the drift rate of ±6 parts in 109 of g/day. Several possible sources of drift are discussed.
266 citations
TL;DR: In this paper, a detailed theory of electronic surface quantum states in a low magnetic field, as well as their effects on the microwave surface impedance is presented, and a considerable amount of quantitative information can be extracted from the experimental data.
Abstract: We present here a detailed theory of electronic surface quantum states in a low magnetic field, as well as of their effects on the microwave surface impedance. A marked oscillatory structure in the microwave absorption as a function of magnetic field has been carefully observed by Khaikin and by Koch et al. The quantized magnetic surface levels are bound states of electrons trapped against the surface by the magnetic field. Even though these levels are somewhat analogous to Landau levels, they have considerably different properties. Resonant transitions between these levels give rise to a series of spectral lines in the surface impedance, just as cyclotron resonance is a result of transitions between Landau levels. The present effect is essentially quantum in nature, however. A considerable amount of quantitative information can be extracted from the experimental data. The Fermi velocity, radius of curvature of the Fermi surface, and mean free time at certain points on the Fermi surface can be obtained. Most novel, however, is the fact that one can extract information on the scattering of electrons by the surface, as a function of impact angle.
239 citations
223 citations
TL;DR: In this paper, a detailed study of the velocity, intensity and magnetic-field fine structure in and around a sunspot is described, where the magnetic field in these knots causes a striking decrease of the line depth.
Abstract: The observational set-up for a detailed study of the velocity, intensity and magnetic-field fine structure in and around a sunspot is described. On highly resolved spectra we detected in the vicinity of a sunspot a large number of points with strong magnetic fields (magnetic knots). The magnetic field in these knots causes a striking decrease of the line depth (or a ‘line gap’ after Sheeley, 1967). The properties of the magnetic knots are: (1) magnetic fields up to 1400 gauss; (2) diameter ≈ 1100 km; (3) coincidence with dark intergranular spaces; (4) generally downward material motion; (5) lifetime>30min; (6) estimated total number around an unipolar spot ⩾ 2000; (7) combined magnetic flux comparable to the sunspot flux; (8) coincidence with Ca+ plages. For the smallest sunspots (pores) we obtained magnetic fields >1500 gauss. Hence a magnetic field of about 1400–1500 gauss appears to be a rather critical level for pore and spot formation. We found a large number of small areas producing line gaps without measurable magnetic field. These ‘non-magnetic gap-regions’ coincide with bright continuum structures. Some aspects arising from the occurrence of hundreds of magnetic knots in an active region are discussed in the last section.
222 citations
TL;DR: Magnetic field lines of force random walk and astrophysical magnetic fields random fluctuations, noting interplanetary magnetic field and solar photosphere turbulence are noted in this article, where the authors discuss the effects of magnetic field line of forces random walk on magnetic field random fluctuations.
Abstract: Magnetic field lines of force random walk and astrophysical magnetic fields random fluctuations, noting interplanetary magnetic field and solar photosphere turbulence
202 citations
TL;DR: Magnetic field configuration in outer magnetosphere from data obtained by Interplanetary Monitoring Platform /IMP/ was analyzed by as discussed by the authors, where the magnetic field configuration was determined from data collected from the outer magnetosphere of the Earth's magnetosphere.
Abstract: Magnetic field configuration in outer magnetosphere from data obtained by Interplanetary Monitoring Platform /IMP/
TL;DR: In this article, the authors consider the effects of heating when the applied field is changed suddenly and magnetic flux enters adiabatically into the bulk of a semi-infinite slab of superconductor.
Abstract: The isothermal critical‐state model of hard superconductors is extended to include the effects of heating when the applied field is changed suddenly and magnetic flux enters adiabatically into the bulk. We consider the following specific situation. A semi‐infinite slab of superconductor is cooled in a magnetic field lying in its surface plane. Next, the external field is raised isothermally by an amount Hs. This excess field decreases linearly to a depth δ= 10Hs/4πJc from the surface. Finally, the field is raised by an infinitesimal amount ΔH in a time short compared to the thermal diffusion time and long compared to the electromagnetic diffusion time. Each element of volume exposed to the changing field receives a thermal impulse proportional to the local‐flux‐change times Jc. This thermal impulse, in turn, lowers the critical current and allows more flux to penetrate. We find that if Hs exceeds some critical value Hfj, then the isothermal critical state is not the only allowed state of the superconductor. This instability field is given in terms of the critical current density Jc, derivative of the critical current density with temperature, ∂Jc/∂T, and the volume specific heat C by the formula Hfj= [−π3CJc/(∂Jc/∂T)]1/2. The application of the incremental field ΔH can initiate an avalanching process, or a flux jump, that terminates in an adiabatic critical state. Immediately following the flux jump the internal field, the induced supercurrent, and the temperature rise at each position are associated in a self‐consistent way with the avalanche of flux that has entered the superconductor. In this framework a flux jump is viewed as a switching from the isothermal critical state to an adiabatic critical state. The magnitude of the jump is related to Js and is calculated.
TL;DR: In this paper, the spin wave contribution to low temperature magnetization is considered in the limits of strong and very weak ferromagnetism, and a simple equation for the magnetic isotherms is obtained in this limit and shown to be valid over a wide temperature range.
Abstract: Calculations are given for the dependence on temperature and magnetic field strength H of the magnetization M of ferromagnetic metals treated on the basis of the itinerant electron model. The spin wave contribution to the low temperature magnetization is considered in the limits of strong and very weak ferromagnetism. For the first limit, problems related to the well-known divergence of the zero field differential susceptibility are briefly discussed. The main part of the paper is concerned with the single particle contributions to the magnetization and associated differential susceptibility for very weak ferromagnetism. A simple equation for the magnetic isotherms is obtained in this limit and shown to be valid over a wide temperature range including 0 °K and the Curie temperature. This equation implies that plots of M 2 against H / M at various temperatures in this range give a series of parallel straight lines. Recently measured isotherms for the material ZrZn 2 are analysed on the basis of the theory, and several characteristic properties of this material are obtained from the analysis.
TL;DR: In this article, the propagation of cosmic rays in the regular and random interplanetary magnetic fields transferred by the clouds of the inter-planetary plasma is considered and the equation for the distribution function is obtained.
TL;DR: In this paper, a densitygradient-driven collisional drift wave is identified by the dependences of ω and k on density, temperature, magnetic field, and ion mass, and by comparison with a linear theory which includes resistivity and viscosity.
Abstract: Density‐gradient‐driven collisional drift waves are identified by the dependences of ω and k on density, temperature, magnetic field, and ion mass, and by comparisons with a linear theory which includes resistivity and viscosity. Abrupt stabilization of azimuthal modes is observed when the stabilizing ion diffusion over the transverse wavelength due to the combined effects of ion Larmor radius and ion‐ion collisions (viscosity) balances the destabilizing electron‐fluid expansion over the parallel wavelength, determined by electron‐ion collisions (resistivity). The finite‐amplitude (ũ/n0 ≃ 10%) coherent oscillation, involving the entire plasma body, shows a phase difference between density and potential waves (which is predicted by linear theory for growing perturbations). The wave‐induced radial transport exceeds classical diffusion, but is below the Bohm value by an order of magnitude. Although observations have been extended to magnetic fields three times those for drift‐wave onset, turbulence has not b...
Journal Article•
TL;DR: In this article, a method for removing impurities from a copper electrolyte of having a free sulfuric acid concentration of less than 700g/l was proposed, and containing at least arsenic among arsenic, antimony and bismuth as impurities.
Abstract: A method for removing As, or As and Sb and/or Bi wherein acidic solution of sulfuric acid such as a copper electrolyte of having a free sulfuric acid concentration of less than 700g./l. and containing at least arsenic among arsenic, antimony and bismuth as impurities is warmed to be above 50 DEG C. or preferably above 60 DEG C. with the addition of H3AsO4 in advance as required, one or a mixture of both of Sb2O3 and Bi2O3 preferably kneaded with water or the added solution is added to and mixed with the solution and the produced precipitate is separated to discharge the impurities out of the solution.
TL;DR: Relativistic electron gas in strong magnetic field, obtaining equation of state with quantum theory as discussed by the authors, was used to obtain the Eq. 1.1.0.0
Abstract: Relativistic electron gas in strong magnetic field, obtaining equation of state with quantum theory
TL;DR: In this paper, it was shown that the electron energization was caused by the electric fields associated with some static 4-kev potential difference in the magnetosphere, which is known to exist and have been measured directly.
Abstract: Rocket-borne observations were made of a nearly monoenergetic beam of electrons in association with a prebreakup auroral display. The characteristic energy of this beam was about 4 kev and was very stable over a 150-second period of time. The nature of this energy spectra strongly suggests that the electron energization was by the electric fields associated with some static 4-kev potential difference in the magnetosphere. A model in which this potential was established directly along a magnetic field line cannot be uniquely excluded by these observations but is discounted on other grounds. The alternative is a model in which the required electric field is transverse to the magnetic field lines. Such electric fields are known to exist and have been measured directly. Energization of low-energy electrons by an electric field of this geometry, would appear to require that these electrons be stably trapped on closed lines of magnetic force, the energization occurring by virtue of gradient and line curvature drift across equipotential lines. It is this process, which is similar to that proposed by Taylor and Hones (1965), that is believed to have produced the auroral particles observed in this aurora. The electrical potential differences known to be available in the magnetosphere although sufficient to produce these 4-kev electrons are inadequate to produce the 100-kev electrons seen in association with the aurora, thus implying the existence of more than one acceleration mechanism.
TL;DR: In this paper, thermal anisotropy of electrons and instability of circularly polarized electromagnetic waves propagating along magnetic field in solar wind were investigated. But the results were limited to the case of solar wind.
Abstract: Thermal anisotropy of electrons and instability of circularly polarized electromagnetic waves propagating along magnetic field in solar wind
TL;DR: It was shown theoretically that nematic liquid crystals in magnetic fields may display alignment inversion walls partially similar to Bloch and Neely walls in ferromagnetics as mentioned in this paper.
Abstract: It is shown theoretically that nematic liquid crystals in magnetic fields may display alignment inversion walls partially similar to Bloch and N\'eel walls in ferromagnetics.
TL;DR: In this article, an adiabatic approach is used to describe the quasilinear behavior of a collisionless, relativistic plasma immersed in a uniform, homogeneous magnetic field.
Abstract: Equations are set up to describe the quasilinear behavior of a collisionless, relativistic plasma immersed in a uniform, homogeneous magnetic field. Use of the adiabatic assumption, together with an approximation to resonant denominators, enables an estimate to be given of the change of the distribution function due to the turbulent waves. In the particular cases of pure electrostatic waves, magneto‐sonic waves, and cyclotron waves, estimates are made of the rate of energy change of resonant particles. It is shown that some modes exhibit resonant diffusion only when relativistic particles are present and for which there is no nonrelativistic counterpart. It is believed that this calculation may be of some interest in situations where relativistic particles play, or are suspected of playing, a major role; for example, the galactic cosmic ray gas and nonthermal radio sources.
TL;DR: In this article, a systematic survey has been made with epitaxial films of n-type GaAs of the dependence of the position and amplitude of magnetophonon resistance oscillations on the experimental parameters of magnetic field, temperature and sample mobility.
Abstract: A systematic survey has been made with epitaxial films of n-type GaAs of the dependence of the position and amplitude of magnetophonon resistance oscillations on the experimental parameters of magnetic field, temperature and sample mobility. A number of corroborative experiments have also been performed with n-type InSb and InAs. With the magnetic field applied perpendicular to the current direction, a dependence of magnetophonon peak position on both temperature and mobility was found. A change in the curvature of the band edge can be deduced from the temperature dependence, and the low-frequency effective mass at the bottom of the conduction band of GaAs is found to be 0?0653m at 280 ?K and 0?0675m at 70 ?K. With InSb the band-edge mass of the electrons is found to be 0?0127m at 160 ?K and 0?0134m at 60 ?K. The dependence of the peak position on mobility is thought to be related to an expression describing the magnetic field dependence of the amplitude of the oscillatory terms. The amplitudes of the oscillatory terms do not depend on the mobility in a simple manner and appear to be a function of the number of ionized impurity sites rather than of the actual mobility of the sample. When the magnetic field is applied parallel to the direction of current flow, resistance minima are always observed at magnetic fields close to the fields at which maxima are observed in the transverse orientation, but these minima cannot be used to determine band parameters as they are shifted with respect to the transverse maxima by amounts which can be as high as 16% in the case of InAs. Additional series of minima were observed at high temperatures with both GaAs and InAs in the longitudinal configuration. These extra series are attributed to the presence of indirect scattering processes involving two optical phonons.
TL;DR: In this paper, the authors detect electric field turbulence in earth bow shock, noting wave amplitude correlation with magnetic field structure, and conclude that the electric field is associated with the magnetic field.
Abstract: Detection of electric field turbulence in earth bow shock, noting wave amplitude correlation with magnetic field structure
TL;DR: In this paper, the problem of the accumulation and storage of the energy released in solar flares is discussed; it is proposed that convective energy of the photosphere is transformed into magnetic energy of chromosphere and corona and the consequences of a large ratio of magnetic pressure to gas pressure are investigated.
Abstract: The problem of the accumulation and storage of the energy released in solar flares is discussed; it is proposed that convective energy of the photosphere is transformed into magnetic energy of the chromosphere and corona. The consequences of a large ratio of magnetic pressure to gas pressure are investigated. In this case the field must be approximately force-free. The only suitable force-free fields which allow an analytical treatment are those of cylindrical symmetry. The stability of these fields is studied with the energy principle. It is shown that they are always unstable due to kink type instabilities. The shape of the unstable perturbations is described in detail and an upper limit for their amplitude is estimated. The consequences for the proposed mechanism of energy storage are briefly discussed.
TL;DR: In this paper, the anomalous high-temperature increase in electrical conductivity of pure and vanadium-doped single crystals of Ti2O3 has been measured as a function of applied magnetic field strength and temperature, and it is concluded that the existence of antiferromagnetic order is in doubt and that the resistivity behavior can be readily interpreted without recourse to a magnetic ordering mechanism.
Abstract: Electrical resistivities of pure and vanadium‐doped single crystals of Ti2O3 have been measured as a function of applied magnetic field strength and temperature. The anomalous high‐temperature increase in electrical conductivity has been confirmed. It is argued that the symmetry of Ti2O3 leads naturally to a two‐band model having bandwidths and an energy separation between band centers that are sensitive to the crystallographic axial ratio c/a. With this model, the change in conductivity with temperature can be adequately described on the basis of known variations in the c/a ratio with temperature. The vanadium‐doped samples are good p‐type conductors and exhibit a negative magnetoresistance at low temperatures. With a model of inelastic scattering at trace impurities carrying a localized magnetic moment, it is possible to describe the negative magnetoresistance of vanadium‐doped samples provided the total, local (applied plus intrinsic) magnetic fields are within a few gauss of the applied field. It is concluded that the existence of antiferromagnetic order in Ti2O3 is in doubt and that the resistivity behavior can be readily interpreted without recourse to a magnetic‐ordering mechanism.
TL;DR: In this paper, a system of hydrodynamic equations for the description of a collisionless plasma is derived, which is an extension of those of CHEW et al. (1956).
Abstract: A system of hydrodynamic equations for the description of a collisionless plasma is derived. The equations for motion perpendicular to the magnetic field are an extension of those of CHEW et al. (1956). Hydrodynamic description of motion along the magnetic field is possible if v||t0/L|| is a small parameter (v|| is the thermal velocity parallel to the magnetic field, t0 the characteristic time, L|| the characteristic length scale for variation of macroscopic quantities along the magnetic field). The method of deriving the hydrodynamic equations is an extension of a method used by GRAD (1949). This extension is necessary because in collisionless plasma thermal energy is not equally distributed over all degrees of freedom. Therefore one has to consider two heat-flux vectors for the transport in thermal energy of parallel and perpendicular degrees of freedom separately. These independent heat-flux vectors are caused by gradients of the parallel and perpendicular temperature respectively. Thus one arrives at a system of equations for 16 moments of the distribution function.
TL;DR: In this article, a system of spins on a lattice with magnetic dipole-dipole interactions, exchange, and anisotropy forces is shown to possess a well-defined bulk free energy, independent of sample shape, in the thermodynamic limit, provided there is no external magnetic field.
Abstract: A system of spins on a lattice with magnetic dipole-dipole interactions, exchange, and anisotropy forces is shown to possess a well-defined bulk free energy, independent of sample shape, in the thermodynamic limit, provided there is no external magnetic field. The proof applies to classical or quantum spin systems with arbitrary $g$ tensors, and is independent of any assumption of order or disorder in the magnetic phase, convergence of perturbation series, etc.