Showing papers on "Gravitational field published in 1990"

Galactic hydrostatic equilibrium with magnetic tension and cosmic-ray diffusion

Abstract: Three gravitational potentials differing in the content of dark matter in the Galactic plane are used to study the structure of the z-distribution of mass and pressure in the solar neighborhood. A P(0) of roughly (3.9 + or - 0.6) x 10 to the -12th dyn/sq cm is obtained, with roughly equal contributions from magnetic field, cosmic ray, and kinetic terms. This boundary condition restricts both the magnitude of gravity and the high z-pressure. It favors lower gravity and higher values for the cosmic ray, magnetic field, and probably the kinetic pressures than have been popular in the past. Inclusion of the warm H(+) distribution carries a significant mass component into the z about 1 kpc regime.

Oceanic tides from Geosat altimetry

Abstract: Direct tidal analysis of the altimetry from the Geosat Exact Repeat Mission's first year is used to derive estimates of the diurnal and semidiurnal oceanic tides. The geoid is removed by collinear differences at 34.1-day separation, and the orbit error is reduced by subtracting a slowly modulated 1-cycle/revolution term. A sequence of independent analyses at grid areas of 1 deg latitude x 1.5 deg longitude using 'orthotide' functions ensures complete definition of the diurnal and semidiurnal species. Global admittance maps for M2 and S2 within the latitudes 58 deg N and 59 deg S compare well with ground truth at 66 open-ocean sites. Maps of differences between Geosat and the Naval Surface Weapons Center model show important areas of the order of 10-15 cm.

Sling amplification and eccentric gravitational instabilities in gaseous disks

Abstract: An analytical description is presented of the modal mechanisms relevant to a recently discovered type of eccentric gravitational instability in nearly Keplerian disks. A quantum condition is derived which accurately predicts the pattern speeds for these modes. The growth rates for the modes are determined, and it is shown that the mode can grow when the disk is safely stable to axisymmetric disturbances. The case of marginal stability for the outside edge is discussed, and the implications of the results for the formation of binary companions and/or giant planets within disks associated with young stellar objects are considered.

Lovelock gravitational field equations in cosmology.

Abstract: We present a systematic study of cosmological solutions in the Lovelock theory of gravitation, including maximally symmetric space-times, Robertson-Walker universes, and product manifolds of symmetric subspaces.

Superpositions of time evolutions of a quantum system and a quantum time-translation machine.

Abstract: A method to obtain a superposition of time evolutions of a quantum system which correspond to different Hamiltonians as well as to different periods of time is derived. Its application to amplification of an effect due to the action of weak forces is considered. A quantum time-translation machine based on the same principle, utilizing the gravitational field, is also considered.

The GEM-T2 Gravitational Model

Abstract: The GEM-T2 is the latest in a series of Goddard Earth Models of the terrestrial field. It was designed to bring modeling capabilities one step closer towards ultimately determining the TOPEX/Poseidon satellite's radial position to an accuracy of 10-cm RMS (root mean square). It also improves models of the long wavelength geoid to support many oceanographic and geophysical applications. The GEM-T2 extends the spherical harmonic field to include more than 600 coefficients above degree 36 (which was the limit for its predecessor, GEM-T1). Like GEM-T1, it was produced entirely from satellite tracking data, but it now uses nearly twice as many satellites (31 vs. 17), contains four times the number of observations (2.4 million), has twice the number of data arcs (1132), and utilizes precise laser tracking from 11 satellites. The estimation technique for the solution has been augmented to include an optimum data weighting procedure with automatic error calibration for the gravitational parameters. Results for the GEM-T2 error calibration indicate significant improvement over previous satellite-only models. The error of commission in determining the geoid has been reduced from 155 cm in GEM-T1 to 105 cm for GEM-T2 for the 36 x 36 portion of the field, and 141 cm for the entire model. The orbital accuracies achieved using GEM-T2 are likewise improved. Also, the projected radial error on the TOPEX satellite orbit indicates 9.4 cm RMS for GEM-T2, compared to 24.1 cm for GEM-T1.

Multidimensional force sensor

Abstract: A multibeam structure measures displacement of one or more response elements to detect multiple components of applied force. The flexible beams are each coupled to a response element which may be displaced by a force arising from linear acceleration, angular acceleration, fluid flow, electric/magnetic/gravitational fields, and others sources. The displacement of the response element is detected with a variety of sensing methods including capacitive and piezoresistive sensing.

Physical effects in wormholes and time machines.

Abstract: Physical effects in a spacetime with a traversable wormhole are considered. It is shown that the interaction of a wormhole with the surrounding matter and with the external gravitational field almost inevitably transforms it into a time machine.

Multipole Moments in General Relativity —Static and Stationary Vacuum Solutions—

Abstract: In the first part of this paper, we review the most important properties of the axisymmetric and stationary solutions of Einstein's vacuum field equations, the solution generating techniques developed in the last few years the relativistic definitions of multipole moments, and their use for obtaining and investigating static and stationary axisymmetric vacuum solutions. In the second part, we derive the general static axisymmetric vacuum solution in prolate spheroidal coordinates. Finally, this solution with all mass multipole moments is generalized to include a further parameter related to the rotation of the mass. This stationary solution can be used in order to describe the exterior gravitational field of a rotating deformed mass.

Inhomogeneity and the onset of inflation.

Abstract: We study numerically the onset of inflation under inhomogeneous initial conditions for various inflationary scenarios and various initial parameters. We solve the full Einstein equations for the gravitational field coupled to scalar-field and radiation-field sources, under the assumptions of spherical symmetry and a closed-universe background. We find that a large initial inhomogeneity suppresses the onset of inflation, and that the scalar field must have values suitable for inflation over a region of several horizon sizes for the inflationary phase to begin.

Gravitational waves emitted from infinite strings

Abstract: Gravitational radiation from an infinite string with a helicoidal standing wave is studied in the weak-field approximation. The radiation power and the spectrum are calculated. The implications of the results for gravitational damping of small-scale structure on the long strings are briefly discussed.

Hamiltonian Dynamics of a Rigid Body in a Central Gravitational Field

Abstract: This paper concerns the dynamics of a rigid body of finite extent moving under the influence of a central gravitational field. A principal motivation behind this paper is to reveal the hamiltonian structure of the n-body problem for masses of finite extent and to understand the approximation inherent to modeling the system as the motion of point masses. To this end, explicit account is taken of effects arising because of the finite extent of the moving body. In the spirit of Arnold and Smale, exact models of spin-orbit coupling are formulated, with particular attention given to the underlying Lie group framework. Hamiltonian structures associated with such models are carefully constructed and shown to benon-canonical. Special motions, namely relative equilibria, are investigated in detail and the notion of anon-great circle relative equilibrium is introduced. Non-great circle motions cannot arise in the point mass model. In our analysis, a variational characterization of relative equilibria is found to be very useful.

Fermat Principle in Arbitrary Gravitational Fields

Abstract: The Fermat principle is reviewed and used to derive the zigzag path constructed for massive and massless particles in order to determine if these paths are a suitable approximation to the first order of the gravitational lens effect It is found that such paths are suitable for thin comoving gravitational lenses to the first order in the lens effect, and also if there is a nonstationary perturbation As an example, the Fermat principle is applied to a perturbation by gravitational waves, and the transverse velocity of the caustic motion is derived This velocity creates difficulty for the proposal by McBreen and Metcalfe (1988) that gamma-ray bursts come from small, hot BL Lac cores crossed by microcaustics 23 refs

Gravity anomaly separation by wiener filtering

Abstract: We introduce a gravity anomaly separation method based on frequency‐domain Wiener filtering. Gravity anomaly separation can be effected by such wavelength filtering when the gravity response from the geologic feature of interest (the signal) dominates one region (or spectral band) of the observed gravity field’s power spectrum. The Wiener filter is preferable to a conventional band‐pass filter because geologic information from the study area can be incorporated to a greater extent in specifying the filter’s transfer function. Our method differs from previous Wiener filtering schemes in that it provides, through direct modeling of known geology (e.g., outcrop and borehole data), a more objective estimate of the signal power spectrum required for defining the transfer function of the filter. We illustrate the technique first with synthetic data, and then with a field example from the southern Paradox basin. The Paradox basin example reveals the limitation inherent to all wavelength filtering which results f...

The response of a realistic earth to ocean tide loading

Abstract: SUMMARY A novel ocean tide loading model is developed which allows the earth to be self-gravitating, compressible, layered, anisotropic viscoelastic and rotating, with a solid inner core and fluid outer core. The equations of deformation of the earth are developed, following Lagrangean mechanics. The standard-linear-solid-type rheology, as well as the grain-boundary relaxation model for the dissipation mechanism within the earth are adopted in this study. The thermodynamic state of the earth is accounted for, through its absolute temperature, Gibbs free activation energy, viscosity and Q profiles. Complex load numbers h'n, k'n and l'n, are calculated up to degree 10000. It is shown that the rotation of the earth, as well as the anisotropy in the upper mantle affect the load numbers by a few per cent. There is a weak latitude dependence of the load numbers for n≤ 4. At fortnightly periods, viscoelastic load numbers are 1–2 per cent larger than their corresponding elastic values. Complex Green's functions are evaluated for displacements, gravity and tilt. Tests of the present model against accurately determined M2 gravity tide residuals show that the present model is very promising indeed.

Gravitational couplings of the inflaton in extended inflation

Abstract: We discuss a new extended-inflationary scenario evading the difficulties of the original model. Our model can thermalize the energy in the bubble walls by the necessary epoch, and establish a Robertson-Walker frame in the bubble clusters. The essential new ingredient in our model is the observation that the coupling of inflaton to the Jordan-Brans-Dicke field is expected to be different from that of visible matter.

An evaluation of some systematic error sources affecting terrestrial gravity anomalies

Abstract: Terrestrial free-air gravity anomalies form a most essential data source in the framework of gravity field determination. Gravity anomalies depend on the datums of the gravity, vertical, and horizontal networks as well as on the definition of a normal gravity field; thus gravity anomaly data are affected in a systematic way by inconsistencies of the local datums with respect to a global datum, by the use of a simplified free-air reduction procedure and of different kinds of height system. These systematic errors in free-air gravity anomaly data cause systematic effects in gravity field related quantities like e.g. absolute and relative geoidal heights or height anomalies calculated from gravity anomaly data.

Anisotropies in the microwave sky due to nonlinear structures

Abstract: The propagation of light in a nonstatic linear gravitational potential associated with nonlinear density fluctuations is studied. A potential approximation to Einstein's field equations makes it possible to derive simple expressions for the anisotropies induced in the temperature of the microwave background radiation, associated in particular with angular distortions induced by the time-varying gravitational potential along the line of sight to the surface of last scattering. These results are applied to two examples of interest: a compensated void in the thin-shell approximation and a compensated lump in the Swiss cheese approach, obtaining the same results, with regard to temperature profiles, as those obtained using a general-relativistic treatment. 20 refs.

Numerical simulation of 3‐D Bénard convection with gravitational modulation

Abstract: In this numerical study, randomly and sinusoidally modulated gravitational fields imposed on three‐dimensional Rayleigh–Benard convection are investigated in an effort to understand the effects of vibration (G‐Jitter) on fluid systems. The time‐dependent, Navier–Stokes equations and the energy equation with Boussinesq approximations are solved by a semi‐implicit, pseudospectral procedure. An analysis of energy balances indicates that with increasing modulation amplitude, transition from synchronous to relaxation oscillation goes through the subharmonic response. Random modulations are found to be less stabilizing than sinusoidal and are shown to impose local three‐dimensionality on the flow for some parameter ranges both at terrestrial and zero base gravity conditions.

The development of an isostatic gravitational model to degree 360 and its use in global gravity modelling

Abstract: Consideration is given to the possibility of combining low-degree satellite-derived geopotential models with the harmonic coefficients of the topographic-isostatic potential implied by the Airy/Heiskanen isostatic hypothesis. The compilation of a topographic database providing information pertaining to terrain type classification is discussed. The formulation for the determination of harmonic coefficients of the topographic-isostatic potential is extended beyond to cases discussed by Lachapelle (1976) to include various terrain types. This formulation and the series expansion approach of Rummel et al. (1988) are implemented for potential coefficient determinations complete to degree and order 360. The topographic-isostatic coefficients are used with satellite-derived geopotential models to estimate mean gravity anomalies. The results are compared with observations to evaluate the quality of different estimation procedures.

The gravitational field of plane symmetric thick domain walls

Abstract: Exact solutions of Einstein’s equations for a scalar field with a potential V(Φ) =V0 cos2(1−n) (Φ/f(n)) (0