Showing papers on "Magnetic field published in 1978"
TL;DR: The stationary point used by Sherrington and Kirkpatrick (1975) in their evaluation of the free energy of a spin glass by the method of steepest descent is examined carefully in this article, and it is found that although this point is a maximum of the integrand at high temperatures, it is not a maximum in the spin glass phase nor in the ferromagnetic phase at low temperatures.
Abstract: The stationary point used by Sherrington and Kirkpatrick (1975) in their evaluation of the free energy of a spin glass by the method of steepest descent is examined carefully. It is found that, although this point is a maximum of the integrand at high temperatures, it is not a maximum in the spin glass phase nor in the ferromagnetic phase at low temperatures. The instability persists in the presence of a magnetic field. Results are given for the limit of stability both for a partly ferromagnetic interaction in the absence of an external field and for a purely random interaction in the presence of a field.
1,654 citations
TL;DR: Magnetic Field Generation in Electrically-Conducting Fluids by H. K. Moffatt as mentioned in this paper is a seminal work in the field of magnetic field generation in electrically conducting fluids.
Abstract: Magnetic Field Generation in Electrically-Conducting Fluids. By H. K. Moffatt. Pp. 343. (Cambridge University Press: Cambridge and London, 1978.) £15.50.
1,529 citations
TL;DR: In this article, the toroidal electron currents, which sustain the poloidal magnetic field in tokamaks, may be generated by injecting waves with net parallel momentum into the plasma via phased waveguide arrays.
Abstract: Continuous toroidal electron currents, which sustain the poloidal magnetic field in tokamaks, may be generated by injecting waves with net parallel momentum into the plasma via phased waveguide arrays. Waves with high phase velocity can produce a current capable of confining a reactor plasma so that steady-state tokamak operation with acceptable power dissipation becomes possible.
549 citations
TL;DR: The results indicate that at NMR operating frequencies much greater than about 30 MHz, magnetic field amplitude and phase variations experienced by the nuclei may cause serious distortions in an image of a human torso.
Abstract: The magnetic field penetration, phase shift and power deposition in planar and cylindrical models of biological tissue exposed to a sinusoidal time-dependent magnetic field have been investigated theoretically over the frequency range 1 to 100 MHz. The results are based on measurements of the relative permittivity and resistivity dispersions of a variety of freshly excised rat tissue at 37 and 25 degrees C, and are analysed in terms of their implications for human body nuclear magnetic resonance (NMR) imaging. The results indicate that at NMR operating frequencies much greater than about 30 MHz, magnetic field amplitude and phase variations experienced by the nuclei may cause serious distortions in an image of a human torso. The maximum power deposition envisaged during an NMR imaging experiment on a human torso is likely to be comparable to existing long-term safe exposure levels, and will depend ultimately on the imaging technique and NMR frequency employed.
427 citations
335 citations
TL;DR: Honey bees orient to the earth's magnetic field and may be associated with a region of transversely oriented magnetic material in the front of the abdomen that develops in the pupal state and persists in the adults.
Abstract: Honey bees orient to the earth's magnetic field. This ability may be associated with a region of transversely oriented magnetic material in the front of the abdomen. The magnetic moment apparently develops in the pupal state and persists in the adults.
323 citations
TL;DR: In this article, it was shown that a system of particles in a homogeneous magnetic field, with translation invariant interaction, has a constant of motion analogous to the total momentum when B = 0.
279 citations
TL;DR: In this article, it was pointed out that the convective heat transport in the magnetic field swept into the downdrafts in the junctions of supergranule boundaries is strongly suppressed by magnetic field.
Abstract: The remarkable concentration of the general field of the Sun into isolated intense flux tubes at the visible surface must be a direct consequence of conditions immediately beneath the surface. It is pointed out that the convective heat transport in the magnetic field swept into the downdrafts in the junctions of supergranule boundaries is strongly suppressed by the magnetic field. The net heat transport is reduced to such a degree that the temperature of the downdraft within the field increases nearly adiabatically below the visible surface, and hence is significiantly cooler than the surrounding ambient gas. The reduced temperature enhances the downdraft within the field and permits the gravitational field to evacuate the flux tube. The magnetic field is then strongly compressed by the external gas pressure, leading to the extraordinary observed strengths of 1500 gauss or more. It is suggested that the magnetic knots found in active regions are formed wholly or partly by the same effect.The general occurrence of superadiabatic temperature gradients and convective heat transport in the dwarf and subdwarf main-sequence stars suggests that their magnetic fields are generally broken up and concentrated by this effect, much as in the Sun.
259 citations
TL;DR: In this article, the problem of magnetic field reconnection and the onset of instabilities resulting from it is analyzed for current carrying plasmas in high temperature regimes of thermonuclear interest.
250 citations
TL;DR: In this paper, the effect of radiative relaxation on the propagation of waves in an intense magnetic flux tube embedded in a uniform atmosphere was examined and it was shown that the radiative energy loss leads to wave damping.
Abstract: Radiative damping of waves is important in the upper photosphere. It is thus of interest to examine the effect of radiative relaxation on the propagation of waves in an intense magnetic flux tube embedded in a uniform atmosphere. Assuming Newton's law of cooling, it is shown that the radiative energy loss leads to wave damping. Both the ‘damping per wavelength’ and the ‘damping per period’ reach maximum value when the sound and radiative timescales are comparable. The stronger the magnetic field, the greater is the damping.
244 citations
TL;DR: A heuristic three-parameter model in which protein protons and solvent protons are considered as two separate thermodynamic systems that interact across the protein-solvent interface is introduced, finding that the magnetic field-dependence of the cross-relaxation contribution is much like that of the remaining solvent proton relaxation, i.e., about the same as the deuteron relaxation field dependence.
Abstract: The magnetic spin-lattice relaxation rates of solvent water nuclei are known to increase upon addition of diamagnetic solute protein. This enhancement of the relaxation rate is a function of magnetic field, and the orientational relaxation time of the protein molecules can be deduced from analysis of the field-dependent relaxation rates. Although the nature of the interactions that convey information about the dynamics of protein motion to the solvent molecules is not established, it is known that there is a contribution to the relaxation rates of solvent protons that plays no role in the relaxation of solvent deuterons and 17O nuclei. We show here that the additional interaction arises from a cross-relaxation process between solvent and solute protons. We introduce a heuristic three-parameter model in which protein protons and solvent protons are considered as two separate thermodynamic systems that interact across the protein-solvent interface. The three parameters are the intrinsic relaxation rates of each system and a cross-relaxation term. The sign of the latter term must always be positive, for all values of magnetic field, in order for magnetization energy to flow from the hotter to the cooler system. We find that the magnetic field-dependence of the cross-relaxation contribution is much like that of the remaining solvent proton relaxation, i.e., about the same as the deuteron relaxation field dependence. This finding is not compatible with the predictions of expressions for the cross-relaxation that have been used by other authors, but not applied to data over a wide range of magnetic field strength. The model predicts that the relaxation behavior of both the protein protons and the solvent protons is the sum of two exponentials, the relative contributions of which would vary with protein concentration and solvent isotopic composition in a fashion suggestive of the presence of two classes of protein protons, when there is in reality only one. This finding has immediate implications for the interpretation of published proton relaxation rates in complex systems such as tissues; these data should be reexamined with cross-relaxation taken into account.
TL;DR: In this article, a relativistic beam of electrons passing through a spatially periodic right-hand circularly polarized magnetic field was considered and the amplification was found to be due to a ponderomotive bunching of the electrons.
Abstract: Stimulated emission from a relativistic beam of electrons passing through a spatially periodic right-hand circularly polarized magnetic field is considered. The amplification is found to be due to a ponderomotive bunching of the electrons. The effect is completely classical and for an infinite interaction distance a dispersion relation, which takes into account space-charge effects, describing the scattered field is derived. Conditions on the pump field amplitude, beam density, and momentum spread of the beam for emission from individual electrons to occur or for emission from plasma oscillations to occur are examined. Also, emission from individual electrons over a finite interaction distance is considered and gain is determined for distances less than an $e$-folding length.
TL;DR: In this article, dissipative test electromagnetic fields in a black-hole background were studied and they were shown to satisfy Ohm's law with a surface resistivity of $4.377$ ohms.
Abstract: We study dissipative test electromagnetic fields in a black-hole background. Quantities such as surface velocity, tangential electric field, normal magnetic induction, total surface current, and conduction surface current are introduced and are shown to satisfy Ohm's law with a surface resistivity of $4\ensuremath{\pi}\ensuremath{\simeq}377$ ohms. Associated with these currents there exists a "Joule heating". These currents can exist when the black hole is inserted in an external electric circuit, but they can exist even in the absence of external currents. In particular, we study the eddy currents induced by the rotation of a black hole in an oblique uniform magnetic field, and we show how the computation of the ohmic losses allows a very simple derivation of the torque exerted on the hole.
TL;DR: In this paper, the damping experienced by standing Alfven waves in a dipole magnetic field was investigated and it was shown that typically the normalized damping rate, γ ω, is ∼0.1 for nightside values of conductivity and ∼ 0.01 for the dayside.
TL;DR: In this article, the authors investigated the nonlinear development of the feedback instability for a conjugate perturbation elongating in the east-west direction, having a width of several tens of kilometers at the ionospheric level, standing along the magnetic field line.
Abstract: Being fed by the large-scale magnetospheric convection, the coupled ionosphere-magnetosphere system is subject to a feedback instability for a conjugate perturbation elongating in the east-west direction, having a width of several tens of kilometers at the ionospheric level, standing along the magnetic field line. The growth time is as short as several minutes even when no hot electrons are involved in the field-aligned current. Nonlinear development of the feedback instability is numerically investigated, taking hot electrons into account. The following conclusions are obtained. (1) An auroral arc can develop within a few tens of seconds once hot electrons take part. (2) The induced potential associated with the arc can reach several hundred volts. This predicts that the upward field-aligned current may be provided by electrons with energies of the order of several hundred electron volts, provided anomalous resistivity or double layers develop. (3) The electron density enhancement is directly connected with the upward field-aligned current and hence the auroral arc. (4) The downward current is localized equatorward of the upward current, when the background electric field is westward. (5) The electrojet current can grow to a few thousand amperes, which can either be westward or eastward, depending on the direction of the electric field. (6) The induced electric field inside the arc is almost constant and different from outside. These results are consistent with the observational results.
TL;DR: An electrically conducting spherical body of gas rotates in the presence of an azimuthal (toroidal) magnetic field B and its own gravitational field andInstabilities of the system due to either differential rotation or meridional gradients of B are examined by means of a local analysis.
Abstract: An electrically conducting spherical body of gas rotates in the presence of an azimuthal (toroidal) magnetic field B and its own gravitational field. Instabilities of the system due to either differential rotation or meridional gradients of B are examined by means of a local analysis. Account is taken of viscous, ohmic and thermal diffusion, the diffusivities being denoted by $
u $, $\eta $ and $\kappa $ respectively. Attention is mainly focused on the 'rapidly rotating' case in which the magnetic energy of the system is only a small fraction ($\epsilon $) of the rotational energy. A discussion is given of some overlooked aspects of Goldreich-Schubert instability, which is usually said to occur if the angular momentum (per unit mass) decreases with distance r from the rotation axis or varies with distance z parallel to that axis. It is then shown that a toroidal magnetic field is not only less capable of suppressing the instability than has hitherto been supposed (when $
u \ll \eta $) but actually acts as a catalyst for another quite different differential rotation instability if $\eta $ is sufficiently small. This one is non-axisymmetric and substantially precedes that of Goldreich & Schubert by developing rapidly and with large azimuthal wavenumber if the angular velocity decreases more than a very small amount (O ($\epsilon $)) with r. When the gas is strongly thermally stratified this instability still occurs if $\eta $ is sufficiently small compared with $\kappa $. When the rotation is uniform, instability may still occur owing to the (r,z) distribution of the toroidal magnetic field itself. Its nature depends crucially on whether the region of interest is inside or outside a certain 'critical radius', the latter case being typically the more important astrophysically. Other geometrical effects of this kind complicate the issue, and though summarized at the end of the paper are difficult to report concisely here. The following results apply to a considerably simpler plane layer model previously investigated by Gilman (1970) and Roberts & Stewartson (1977). When the temperature gradient is almost adiabatic (as in a stellar convection zone) and rotation is absent, instability occurs (on the Alfvenic time scale) by Parker's mechanism of magnetic buoyancy if B decreases with height. Rapid uniform rotation, such that $\epsilon \ll $ 1, stabilizes some field distributions, but those which decrease with height faster than the density $\rho $ remain unstable (albeit with growth rates reduced by a factor of order $\epsilon ^{\frac{1}{2}}$) provided $\eta $ is sufficiently small. When the gas is strongly thermally stratified (as in a stellar radiative interior) these results still apply if the thermal diffusivity $\kappa $ is large enough to annul the effects of buoyancy, and this is the case if D$\_{\ast}\equiv \kappa $V$^{2}$/$\eta $N$^{2}$H$^{2}$ is large. Here V denotes the Alfven speed, H the scale height and N the (conventional) buoyancy frequency. In the rapidly rotating case the stability of the system behaves in a curious way as D$\_{\ast}$ is steadily decreased from an infinite value. The first significant effect of decreasing $\kappa $, or equivalently of increasing the stratification (!), is a destabilizing one, and only when D$_{\ast}$ drops below about unity does the stratification exert a significant stabilizing influence. The magnetic buoyancy instabilities above are all non-axisymmetric, but the possibility of axisymmetric instability, despite strong uniform rotation and stable stratification, is examined in an appendix. A somewhat novel instability, involving the simultaneous operation of two conceptually quite different doubly diffusive mechanisms, arises if $
u $/$\eta $ is sufficiently small and $\kappa $/$\eta $ is sufficiently large.
TL;DR: In this article, the authors introduced the transformation from the particle variables to the guiding centre variables and performed the guiding center gyrophase average before specifying the magnetic coordinates to be employed to obtain the unperturbed, gyro-averaged Vlasov operator which retains finite gyro effects.
Abstract: In preceding work on plasma gyro-kinetics magnetic coordinates were introduced prior to making the transformation to the guiding centre variables. It is the transformation from the particle variables to the guiding centre variables which permits finite gyroradius effects to be retained in lowest order. The present treatment avoids the substantial mathematical complications inherent in these prior treatments by introducing the transformation to the guiding centre variables and performing the guiding centre gyrophase average before specifying the magnetic coordinates to be employed. In this way the unperturbed, gyro-averaged Vlasov operator which retains finite gyro-effects is obtained in the most convenient manner for arbitrary unperturbed magnetic fields.
TL;DR: In this paper, it was shown that a magnetic field being force-free, i.e. satisfying ▽ × B = αB, with α = constant (α ≠ 0) in the whole exterior of the Sun cannot have a finite energy content and cannot be determined uniquely from only one magnetic field component given at the photosphere.
Abstract: At first it is shown that a magnetic field being force-free, i.e. satisfying ▽ × B = αB, with α = constant (α ≠ 0) in the whole exterior of the Sun cannot have a finite energy content and cannot be determined uniquely from only one magnetic field component given at the photosphere. Then the boundary value problem for a semi-infinite column of arbitrary cross section is solved by a Green's function method.
01 Jan 1978
TL;DR: The magnetic field strength within the polar caps of the sun is an important parameter for both the solar activity cycle and for our understanding of the interplanetary magnetic field as discussed by the authors, and measurements of the polar fields made at the Stanford Solar Observatory using the Fe I line at 525.02 nm.
Abstract: The magnetic field strength within the polar caps of the sun is an important parameter for both the solar activity cycle and for our understanding of the interplanetary magnetic field. Measurements of the line-of-sight component of the magnetic field generally yield 0.1 to 0.2 mT near times of sunspot minimum. This paper reports measurements of the polar fields made at the Stanford Solar Observatory using the Fe I line at 525.02 nm. It is found that the average flux density poleward of 55 deg latitude is about 0.6 mT peaking to more than 1 mT at the pole and decreasing to 0.2 mT at the polar cap boundary. The total open flux through either polar cap thus becomes about 3 x 10 to the 14th Wb. It is also shown that observed magnetic field strengths vary as the line-of-sight component of nearly radial fields.
TL;DR: In this article, it was shown that a surface electric potential anomaly will not be produced unless there is a boundary separating regions of differing streaming potential coefficient and there is an azimuthal component of the magnetic field inside the earth.
Abstract: According to the dilatancy-diffusion earthquake model, there will be fluid motion into a dilatant zone prior to an earthquake. One possible consequence of this fluid motion is the generation of an electric potential anomaly by means of electrokinetic processes. A surface electric potential anomaly will not be produced unless there is a boundary separating regions of differing streaming potential coefficient and there is a component of pressure gradient parallel to this boundary. The magnetic anomaly produced by the current flow is calculated. It is shown for the case of an n-layered half space that the surface magnetic field will be identically zero no matter what the pressure distribution is. There will, however, be an azimuthal component of the magnetic field inside the earth.
TL;DR: The regular arrangements of the peptide bonds in beta pleated sheet and collagen structures also produce substantial anisotropy, but less than for alpha helix.
Abstract: Magnetic anisotropy in proteins and polypeptides can be attributed to the diamagnetic anisotropy of the planar peptide bonds. The alpha helix in particular has large anisotropy due to the axial alignment of the peptide bonds. The regular arrangements of the peptide bonds in beta pleated sheet and collagen structures also produce substantial anisotropy, but less than for alpha helix. The anisotropy permits orientation of small structures of these types in magnetic fields of several kilogauss.
TL;DR: In this paper, the wave functions of a Bloch electron in a magnetic field were studied and the statistical weight of identifiable regions in an energy versus field plot was described. But all results are restricted to rational fields and depend heavily on the denominator of the representative fraction.
Abstract: There are some facts known about the wave functions of a Bloch electron in a magnetic field. A great deal more is known for a special model. All results are restricted to rational fields and depend heavily on the denominator of the representative fraction. A result is described below which is independent of rationality. It is the statistical weight of identifiable regions in an energy versus field plot. In fact if one goes further and plots instead of the energy the integrated density of state versus field a set of computable polygons results for these regions.
Es existieren einige Kenntnisse uber die Wellenfunktionen von Blochelektronen im Magnetfeld. Etwas mehr ist fur ein spezielles Modell bekannt. Alle Resultate sind auf rationale Felder beschrankt und hangen sehr stark vom Nenner des eingehenden Bruchs ab. Es wird ein Ergebnis beschrieben, das nicht von der Rationalitat des Feldes abhangt: das statistische Gewicht der identifizierbaren Gebiete in einer Darstellung, die die erlaubte Energie als Funktion des Feldes darstellt. Wenn man noch weiter geht und anstatt der Energie die integrierte Zustandsdichte auftragt, so bekommt man berechenbare Polygone fur diese Gebiete.
TL;DR: In this paper, a magnetic field was detected in the peculiar variable helium-rich B2 star sigma Ori E. The field varies between -2300 and +3100 gauss with the 1/sup d/19 period of the spectroscopic and light variations of the star.
Abstract: We have detected a magnetic field in the peculiar variable helium-rich B2 star sigma Ori E. The field varies between -2300 and +3100 gauss with the 1/sup d/.19 period of the spectroscopic and light variations of the star. We suggest that the detection of this field allows all the variable phenomena of sigma Ori E to be understood in terms of an oblique rotator model that has hot gas trapped in a magnetosphere above the magnetic equator, and atmospheric helium enhancement which has occurred preferentially in a zone around the magnetic equator.
TL;DR: In this paper, the flux flow critical current as a function of transverse magnetic field for an oxygen-doped Al film perforated with a two-dimensional triangular lattice pattern of holes shows pronounced structure at harmonically related values of applied magnetic field.
Abstract: The flux‐flow critical current as a function of transverse magnetic field for an oxygen‐doped Al film perforated with a two‐dimensional triangular lattice pattern of holes shows pronounced structure at harmonically related values of the applied magnetic field. Effects at thecharacteristic fields are interpreted in terms of the interaction between commensurate flux‐line sublattices, one of which is strongly pinned to the holes by the vortex‐hole coupling force and the other of which is located interstitially between the hole‐lattice sites. A model is proposed for the vortex‐hole coupling force which, in conjunction with a treatment of vortex‐vortex interactions, qualitatively accounts for the observed temperature and field dependence of these harmonically related critical currents.
TL;DR: In this paper, the existence of galaxies, galactic angular momenta, and galactic magnetic fields at recombination has been shown to be possible under the assumption of a disordered cosmic magnetic field, provided the field varies on a comoving scale corresponding to a typical galactic mass.
Abstract: We show that if we assume the existence of a disordered cosmic magnetic field at recombination, then, provided the field varies on a comoving scale corresponding to a typical galactic mass, we can account for the existence of galaxies, galactic angular momenta, and galactic magnetic fields. The theory predicts the existence of a present day intergalactic magnetic field B/sub I/G< or approx. =10/sup -9/ gauss varying on a characteristic length scale approx.1 Mpc.
TL;DR: In this article, the three-dimensional incompressible magnetohydrodynamic (MHD) equations for rectangular geometry and periodic boundary conditions are solved numerically using the spectral method of Orszag & Patterson (1972).
Abstract: The three-dimensional incompressible magnetohydrodynamic (MHD) equations for rectangular geometry and periodic boundary conditions are solved numerically using the spectral method of Orszag & Patterson (1972). The calculations are restricted to a magnetic Prandtl number of one and to Gaussian random initial conditions with zero mean magnetic and momentum fields. We permit non-mirror-symmetric (helical) flows. In all cases, there is a continuous transfer of energy from the momentum field to the magnetic field. A proposed mechanism for this transfer involves the cascading of energy from the large scales of the momentum field to the small scales, thence a redistribution of energy between the momentum and magnetic fields by Alfven waves, and, finally, an inverse cascade of energy from the small scales of the magnetic field to the large scales. This inverse cascade is found when magnetic helicity (〈a. b〉, where b = curl a is the magnetic induction) is present in the flow.
TL;DR: In this article, the authors deal with the properties of an ergodic magnetic limiter and present calculations concerning the reduction of the impurity rate in a tokamak by a boundary sheath with decreased confinement time.
TL;DR: The Mössbauer spectra of both oxidized and reduced ferredoxin from Bacillus stearothermophilus have been analysed using computer fits to theoretical spectra obtained from a spin Hamiltonian, interpreted in terms of a model for the active centre which is consistent with its electronic and magnetic properties in both redox states.
Abstract: The Mossbauer spectra of both oxidized and reduced ferredoxin from Bacillus stearothermophilus have been analysed using computer fits to theoretical spectra obtained from a spin Hamiltonian. A consistent set of parameters was obtained from fits to spectra obtained over a wide range of temperature and magnetic field.
These results are interpreted in terms of a model for the active centre which is consistent with its electronic and magnetic properties in both redox states. In the model for the oxidized centre all four iron atoms have essentially the same valence, intermediate between ferric and ferrous, with one pair spin-up and the other pair spin-down. On reduction the extra electron goes predominantly to one pair of iron atoms which become ferrous with the other pair remaining substantially unchanged.
Using this model it is possible to obtain relationships between the spin Hamiltonian parameters for individual iron atoms and those for the coupled centre. This can give further insight into the relation between the observed electron paramagnetic resonance and Mossbauer spectra.
TL;DR: In this article, an extensive study has been made of the large magnetic fields in the plasma near the focus of a high-irradiance Nd-laser beam, and the dependence of the magnetic field on many experimental conditions is noted.
Abstract: An extensive study has been made of the large magnetic fields in the plasma near the focus of a high-irradiance Nd-laser beam. The dependence of the magnetic field on many experimental conditions is noted---particularly timing and the presence of a preformed plasma. The data were obtained with a Raman-shifted, three-channel Faraday-rotation diagnostic system.
TL;DR: In this article, the effects of local magnetic anisotropy (RMA) on the magnetic properties of amorphous rare earth alloys have been examined with a view to assessing the effect of RMA on their magnetic properties.