External Field Source Research Articles

Induction of currents in long submarine cables by natural phenomena

The influence of induced currents on long submarine cables has been observed for nearly a century. The currents originate from time variations in the magnetic field of external origin (magnetospheric, atmospheric tides) and earth and ocean tides. Steady earth currents of internal origin have also been proposed as a possible source. The induced currents will depend upon the length, depth, and geomagnetic location of the cables. Present understanding of the phenomena is reviewed. Also discussed are potential scientific problems which might be addressed with data from such long cables and possible limitations on the information obtained. Such problems include studies of (1) the deep conductivity structure of the earth, (2) telluric currents flowing in mid‐oceanic ridges, (3) the spatial extent of external source fields, and (4) a possible role of electromagnetic eddy currents in fluctuations of the earth's spin rate.

A magnetovariational study of a geothermal anomaly

Summary. During 1980 September and October an array of 31 threecomponent magnetometers was operated over a known geothermal anomaly in the Rocky Mountains foothills of Alberta. The purpose of the array was to investigate the possibility of the existence of a geomagnetic induction anomaly coincident with the geothermal anomaly. Examples of magnetograms and Fourier transform anomaly maps are presented and show that although there is some indication of an anomaly this is to a great extent masked by the presence of large external source field gradients. A detailed transfer function analysis of the data is presented. Both single-station transfer functions and a solution of the induction matrix are calculated. The latter involves the representation of the normal fields by least-squares fitted planes and a justification of the validity of this method is given. Contour plots of the response of the anomalous vertical field to horizontal hypothetical events, and pseudosections of IZa/HnI for traverses across the array, are shown. These indicate the existence beneath the geothermal anomaly of a body of anomalously high electrical conductivity. By treating the observed values of the in-phase part of IZa/Ha I along a traverse across the anomaly as due to vertical dipoles, it is shown that the top of the conductive body probably lies at 10-20km depth. The pseudosections suggest that the body extends at least to lower crustal depths. The correlation of the conductivity and geothermal anomalies supports the inference that both are due to the presence of partial melt in the crust.

Model simulations of possible electromagnetic induction effects at MAGSAT activities

This report uses model simulations to consider two questions: first are magnetic field effects of induced currents flowing in the earth significant for near‐earth satellite observations; second, what are the effects at satellite altitudes of lateral differences in the gross conductivity structure of the earth. For distant sources in the magnetosphere, magnetic fields from electric currents induced in a spherical earth may contribute from 30 to 40% of the external source field amplitude. For near‐earth sources beneath the satellite, e.g. in the ionosphere, the external and internal fields tend to cancel, resulting in a much smaller total horizontal field than would be associated with sources above the satellite. On the other hand, when lateral differences in conductivity are associated with the contact between major geologic units in the lithosphere, local concentrations of induced current may generate unique magnetic field signatures at satellite altitudes. Although the absolute amplitude of the field components may be an order of magnitude smaller than for the case when the source is above the satellite, the magnetic field perturbation measured by the satellite as it crosses the geologic contact is significantly larger in relative terms for ionospheric sources.

The principle of minimal gravitational coupling

The principle of minimal gravitational coupling requires that the total Lagrangian for the field equations of general relativity consist of two additive parts, one part corresponding to the free gravitational Lagrangian and the other part to external source fields in curved spacetime. For source fields characterized by scalars, vectors and two-component spinors we prove that this additive decomposition of the total Lagrangian is an inevitable consequence of certain very general assumptions concerning the Euler-Lagrange expressions arising from the total Lagrangian.

An overview of the external magnetic field with regard to magnetic surveys

Abstract : This report sponsored by the Earth Physics Program of the Office of Naval Research, is an attempt to bring together in a coherent manner, for those involved in magnetic surveys, the relevant aspects of the external geomagnetic field. Although there are several excellant introductory texts on geomagnetism that treat much of this subject (most notably, Sugiura and Heppner, 1968; and Knecht, 1972) it was thought that there was a need to condense and present this and other supportive material in a form that would be more applicable to the objectives of magnetic surveys, as there is considerable interest in correcting magnetic surveys for the effects of the external field. Such efforts must start with a sufficiently detailed understanding of the geomagnetic environment in which magnetic survey data are collected and particularly of the various external field sources. Hopefully this report will serve to provide such an understanding. (Author)

Limits on the possible intrinsic magnetic field of Venus

Magnetic field measurements obtained by the Pioneer Venus orbiter at low altitudes in the solar wind wake region are examined to search for possible surface‐correlated features and any possible intrinsic magnetic moment. The observed field variations in the wake do not resemble our expectations for a solar wind interaction with even a weak intrinsic magnetic field. There is little orbit‐to‐orbit persistence of features in the magnetic records. The magnetic field measurements in the wake have been averaged in 10° × 10° bins to minimize the effects of external field sources. The average fields in these 37 bins appear to be randomly oriented and consistent with zero mean in the region mapped. Using these 37 averaged vector fields, we obtain a maximum intrinsic magnetic dipole moment of 4.3±2.0 × 1021 G cm³, approximately an order of magnitude less than previous estimates. A more conservative estimate of the probable error of the mean is 5.5 × 1021 G cm³. Thus the Pioneer Venus measurements are consistent with zero planetary moment. The present measurements are also far below estimates based on angular momentum, the so‐called ‘magnetic Bode's law’ and far below the dynamo scaling law of Busse.

The electromagnetic field of external sources for an observer situated close to a black hole

The electromagnetic field of external sources for an observer situated close to a black hole

Photon propagation in nuclear matter and the correction to the theoretical nuclear charge density

The polarization function is derived for a virtual photon propagating in uncorrelated nuclear matter. The modification of the photon propagator and the induced charge distribution in the presence of the external field source are expressed in terms of a finite effective photon mass which is dependent on the Fermi momentum. The application is made to a finite nucleus by subtracting the local field due to the surrounding infinite matter distribution. It is found that the correction to the potential due to the scattering of exchanged virtual photons by the nucleus results in a short range repulsive force on the orbiting muon in muonic atoms. The estimated energy shifts are -560 eV for the $2{p}_{\frac{1}{2}}\ensuremath{\rightarrow}1{s}_{\frac{1}{2}}$ transition, and -291 eV for the $3{d}_{\frac{5}{2}}\ensuremath{\rightarrow}2{p}_{\frac{3}{2}}$ transition in $^{208}\mathrm{Pb}$; and -58 eV and -2.5 eV, respectively, in $^{90}\mathrm{Zr}$. The effects are negligible for higher orbits.NUCLEAR STRUCTURE $^{90}\mathrm{Zr}$, $^{208}\mathrm{Pb}$; calculated induced charge density in muonic atoms. Nucleon Compton scattering effect on photon propagator.

Linear response and transport equations in interacting phonon systems

A derivation of a transport equation for phonons, in terms of the microscopic description of an anharmonic crystal, is given. The starting point is a complete set of equations for the phonon Green function, the self-energy, and the vertex part, as given by a functional method using real times. The role of the external source field for introducing nonequilibrium into the system is discussed in detail. The essential equivalence of some recent theories is shown, which have been proposed in connection with phonon transport and second sound. The integral equation for the vertex part is related to a Boltzmann equation for a position and time dependent phonon density. For this quantity an operator, namely the Wigner operator, can be extracted from the energy density. It is shown, however, that the energy density can only approximately be expressed in terms of this phonon density. With the aid of the Wigner operator the variable phonon density is represented by a Kubo formula as the linear response of the phonon quasiparticle gas to the external source field. Cubic and quartic anharmonicities are taken into account and their influence on the collision operator and on the drift term of the transport equation is discussed.

Radiative heat-transfer calculations for infinite shells with circular-arc sections, including effects of an external source field

Diffuse radiation from infinite shells with circular-arc cross-sections is analyzed for cases involving external source fields of either diffuse or parallel radiation. The governing integral equations are derived under conditions corresponding to a modified grey-body analysis, and the inversions given in closed form. Specific applications provide local heat transfer corresponding to constant shell temperature, local temperature corresponding to constant heat transfer, and predictions of equilibrium temperatures when the shell is a thermal shield. Comparisons are made with similar calculations for hemispherical shields.

Failure of the Local-Field Concept for Hysteresis Calculations

The ``local field'' is the electric or magnetic field of external sources and of all molecules or particles except the one under examination, at the position of that one. The concept is useful in the molecular theory of dielectrics, where the basic relations are linear. It has also been applied, however, to magnetically interacting single-domain particles in a ferromagnetic powder; here the relations not only are nonlinear but involve discontinuous irreversible jumps, for which instability conditions are decisive. The local-field concept is then unreliable. In a two-particle system, for example, the local-field method examines the stability of one moment with respect to rotation in a fixed field due to the other moment and to external sources; this procedure imposes a constraint, for in the actual system the two moments may rotate simulatneously and independently. The incorrectness of the local-field method is demonstrated here by calculations for two-and three-particle chains of spheres and for two- and three-part composite cylinders. The concept of an interaction field that merely displaces the hysteresis loops of the particles without changing their widths, as in a common interpretation of the Preisach model, is thus erroneous.