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At least 1,693 records · Page 94

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NASA NTRS1981

Equivalent source modeling of the main field using Magsat data

Magsat dipole solution models were obtained with both 32 deg and 21 deg resolution based on a data set extending over 4 months. Time dependence was modeled using first time derivatives for the dipole magnetization vector components. This doubles the n total number of parameters in the solution. The solutions displayed a very slow convergence in time derivatives although at each iteration the conversion of the dipole parameters to spherical harmonic coefficients showed close agreement with the MGST (12/80) spherical harmonic model to degree 13 in the constant terms and 8 in the secular variation terms. The program error in the option to simultaneously estimate observatory anomaly biases was found and corrected. A 32 deg resolution dipole model was generated using the selected magnetic observatory data from 1960-1977 used in the GSFC (9/80) spherical harmonic model.

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NASA NTRS1984

Planetary Scale Interactions

Atmospheric teleconnections in the medium to long (10-90 days) time scales focusing on the interactions between extratropical circulation and tropical convection are studied. In a continuing effort to study short-term climate variability and atmospheric teleconnection as inferred from satellite observed outgoing longwave radiation, the low frequency variability (LFB) of tropical and extratropical cloud fluctuation over the Pacific was studied. It was found that during the Northern winter, the LFV of tropical cloud fluctuation is dominated by a 40-50 day dipole-like oscillation linking convection over Indonesia and the equatorial central Pacific. Eastward propagating signals appearing as outbursts of convective cloud clusters originating from the Indian Ocean appear to periodically feed energy into this dipole oscillation. It was also found that there are cloud features appearing over East Asia and subsequently over the eastern North Pacific which vary coherently with the tropical dipole anomaly. Based on analysis and an a priori phenomenological model, it is believed that the cloud fluctuations are associated with two space/time extended normal modes of tropical-extratropical interactions over the Pacific involving a coupling between the tropical dipole convective heating anomaly with cold surges over East Asia, and blocking over the eastern North Pacific respectively.

Lau, W. K. M.↗

NASA NTRS1986

Do large rate coefficients for ion-polar neutral reactions have a serious effect on chemical models of dense clouds?

In order to incorporate large ion-polar neutral rate coefficients into existing gas phase reaction networks, it is necessary to utilize simplified theoretical treatments because of the significant number of rate coefficients needed. The authors have used two simple theoretical treatments: the locked dipole approach of Moran and Hamill for linear polar neutrals and the trajectory scaling approach of Su and Chesnavich for nonlinear polar neutrals. The former approach is suitable for linear species because in the interstellar medium these are rotationally relaxed to a large extent and the incoming charged reactants can lock their dipoles into the lowest energy configuration. The latter approach is a better approximation for nonlinear neutral species, in which rotational relaxation is normally less severe and the incoming charged reactants are not as effective at locking the dipoles. The treatments are in reasonable agreement with more detailed long range theories and predict an inverse square root dependence on kinetic temperature for the rate coefficient. Compared with the locked dipole method, the trajectory scaling approach results in rate coefficients smaller by a factor of approximately 2.5.

Herbst, E.↗

NASA NTRS1987

The magnetosphere of Saturn

Pioneer 11 and Voyager 1 and 2 magnetic field measurements over the entire flyby of Saturn's magnetic field have been analyzed by fitting a magnetospheric dipole field (i.e., a dipole field plus the field due to currents in the magnetopause), higher moments of the internal field aligned with the dipole along the rotation axis, and the field due to an equatorial sheet current to the magnetic measurements. A dipole moment of 21,431 nT R(s) exp 3, a quadrupole moment of 2403 nT R(s) exp 4, an octopole moment of 2173 nT R(s) exp 5, and an equatorial sheet current of half thickness 2.0 R(s) from about 5 R(s) to the solar edge of the magnetopause, fit the measurements over the entire magnetosphere with an rms deviation of 3.2 nT where R(s) is the planet radius, 66,330 km. The primary feature of the present analysis is the explicit inclusion of the calculated magnetopause current field, which reduces the overall rms deviation over the entire flyby from sigma values of 4.7 and 5.9 nT, using previous models, to 3.2 nT using the present.

Beard, David B.↗

NASA NTRS1986

Statistics of the geomagnetic secular variation for the past 5Ma

A new statistical model is proposed for the geomagnetic secular variation over the past 5Ma. Unlike previous models, the model makes use of statistical characteristics of the present day geomagnetic field. The spatial power spectrum of the non-dipole field is consistent with a white source near the core-mantle boundary with Gaussian distribution. After a suitable scaling, the spherical harmonic coefficients may be regarded as statistical samples from a single giant Gaussian process; this is the model of the non-dipole field. The model can be combined with an arbitrary statistical description of the dipole and probability density functions and cumulative distribution functions can be computed for declination and inclination that would be observed at any site on Earth's surface. Global paleomagnetic data spanning the past 5Ma are used to constrain the statistics of the dipole part of the field. A simple model is found to be consistent with the available data. An advantage of specifying the model in terms of the spherical harmonic coefficients is that it is a complete statistical description of the geomagnetic field, enabling us to test specific properties for a general description. Both intensity and directional data distributions may be tested to see if they satisfy the expected model distributions.

Constable, C. G.↗

NASA NTRS1988

Cut-and-connect of two antiparallel vortex tubes

Motivated by an early conjecture that vortex cut-and-connect plays a key role in mixing and production of turbulence, helicity and aerodynamic noise, the cross-linking of two antiparallel viscous vortex tubes via direct numerical simulation is studied. The Navier-Stokes equations are solved by a dealiased pseudo-spectral method with 64 cubed grid points in a periodic domain for initial Reynolds numbers Re up to 1000. The vortex tubes are given an initial sinusoidal perturbation to induce a collision and keep the two tubes pressed against each other as annihilation continues. Cross-sectional and wire plots of various properties depict three stages of evolution: (1) Inviscid induction causing vortex cores to first approach and form a contact zone with a dipole cross-section, and then to flatten and stretch; (2) Vorticity annihilation in the contact zone accompanied by bridging between the two vortices at both ends of the contact zone due to a collection of cross-linked vortex lines, now orthogonal to the initial vortex tubes. The direction of dipole advection in the contact zone reverses; and (3) Threading of the remnants of the original vortices in between the bridges as they pull apart. The crucial stage 2 is shown to be a simple consequence of vorticity annihilation in the contact zone, link-up of the un-annihilated parts of vortex lines, and stretching and advection by the vortex tube swirl of the cross-linked lines, which accumulate at stagnation points in front of the annihilating vortex dipole. It is claimed that bridging is the essence of any vorticity cross-linking and that annihilation is sustained by stretching of the dipole by the bridges. Vortex reconnection details are found to be insensitive to asymmetry. Modeling of the reconnection process is briefly examined. The 3D spatial details of scalar transport (at unity Schmidt number), enstrophy production, dissipation and helicity are also examined.

Melander, Mogens V.↗

NASA NTRS1988

Statistics of the geomagnetic secular variation for the past 5 m.y

Constable, C. G.↗

NASA NTRS1989

The sunspot cycle variations of the neutral line on the source surface

The earlier presentation method of the sunspot cycle variations of the neutral line on the source surface by introducing the view longitude is defined. It is shown that the neutral line seen from the view longitude and the equivalent dipole (determined by Hoeksema, 1984) showed a trend of rotation throughout the sunspot cycle. However, the surface bounded by the neutral line is generally far from a circle. In fact, our combined presentation of both the equivalnet dipole and the nuetral line indicates graphically that such a simple description of the rotating (equivalent) dipole may be misleading. The concept of an inclined dipole with respect to the rotation axis may also be misleading.

Saito, T.↗

NASA NTRS1990

Charge transfer polarisation wave and carrier pairing in the high T(sub c) copper oxides

The High T(sub c) oxides are highly polarizable materials and are charge transfer insulators. The charge transfer polarization wave formalism is developed in these oxides. The dispersion relationships due to long range dipole-dipole interaction of a charge transfer dipole lattice are obtained in 3-D and 2-D. These are high frequency bosons and their coupling with carriers is weak and antiadiabatic in nature. As a result, the mass renormalization of the carriers is negligible in complete contrast to conventional electron-phonon interaction, that give polarons and bipolarons. Both bound and superconducting pairing is discussed for a model Hamiltonian valid in the antiadiabatic regime, both in 3-D and 2-D. The stability of the charge transfer dipole lattice has interesting consequences that are discussed.

Chakraverty, B. K.↗

NASA NTRS1989

The polarizabilities of neon

The static dipole polarizability, second hyperpolarizability, quadrupole polarizability and dipole-dipole-quadrupole hyperpolarizability for the Ne atom are computed, using large Gaussian basis sets and coupled cluster wavefunctions. Correlation effects are substantial, ranging from 15 percent for the dipole polarizability to 40 percent for the second hyperpolarizability. It is confirmed by explicit calculation earlier hypotheses that g-type and higher angular momentum functions and core correlation effects make almost no contribution to these properties in Ne. Triple excitations, on the other hand, are very important, accounting for as much as 25 percent of the correlation contribution to the second hyperpolarizability. The best estimate of the second hyperpolarizability is 119 + or - 4 au, in good agreement with the recent calculations of Maroulis and Thakkar (1989) and the latest experimental result of Shelton (1989).

Taylor, Peter R.↗

NASA NTRS1990

A color gradient in the soft X-ray diffuse background

It is shown that the deviations of the soft X-ray diffuse background B band to C band intensity ratio from a constant value can be described as a simple dipole-like variation across the sky. In terms of the observed Wisconsin B/C band intensity ratio, the mean value is 0.355, the dipole magnitude is 0.106, and the positive dipole axis points toward l = 168.7 deg, b = 11.2 deg, almost in the galactic anticenter direction. This gradient in the spectral hardness can be due to several causes; the simplest is a temperature gradient in the X-ray emitting plasma of the local cavity from about 10 exp 6.2 K toward the galactic center to about 10 exp 5.9 K in the anticenter direction. While the physical origin of such a temperature gradient is uncertain, the alignment of the dipole with the higher temperature (and absorbed) Loop I region may be significant.

Snowden, S. L.↗

NASA NTRS1991

Coercivity of domain wall motion in thin films of amorphous rare earth-transition metal alloys

Computer simulations of a two dimensional lattice of magnetic dipoles are performed on the Connection Machine. The lattice is a discrete model for thin films of amorphous rare-earth transition metal alloys, which have application as the storage media in erasable optical data storage systems. In these simulations, the dipoles follow the dynamic Landau-Lifshitz-Gilbert equation under the influence of an effective field arising from local anisotropy, near-neighbor exchange, classical dipole-dipole interactions, and an externally applied field. Various sources of coercivity, such as defects and/or inhomogeneities in the lattice, are introduced and the subsequent motion of domain walls in response to external fields is investigated.

Mansuripur, M.↗

NASA NTRS1991

The magnetic field and magnetospheric configuration of Uranus

A significant and unique planetary magnetic field discovered by Voyager 2 is presented. A large tilt of 58.6 deg of the magnetic-dipole axis from the rotation axis was found. Combined with a large offset of 0.3 RU of the magnetic dipole from the center of the planet, the moment of 0.23 gauss-RU3 leads to field magnitudes at the surface which vary widely between 0.1 and 1.0 gauss. A simple diagram illustrating the offset tilted dipole of Uranus and some field lines is shown. A more exact and accurate spherical-harmonic model of the planetary field, which includes both dipole and quadrupole moments, is derived. There exists a well-developed bipolar magnetic tail on the night side of the planet which rotates daily about the extended planet-sunline with Uranus because of the large obliquity of the Uranian rotation axis.

Ness, Norman F.↗

NASA NTRS1991

Millimeter-wave integrated-horn antennas. I - Theory. II - Experiment

Full-wave analysis is employed to determine the far-field pattern and input impedance of a dipole-fed horn antenna in a ground plane, and the theoretical results are compared with mm-wave and microwave data. The theoretical work exploits the Green's function corresponding to the horn structure and the method of moments. It is determined that the horn should have 70 sections/wavelength and 50 secondary modes for optimized accuracy, and certain dipole positions can reduce the resonance to zero. The experimentally derived impedance and radiation patterns agree with the constraints developed theoretically. The 70-degree flare-angle horn with selected dipole positions and horn apertures yields good radiation patterns, cross-polarization levels, and resonant dipole impedances. The conclusions are of interest to the development of the horn antennas etched in Si/GaAs for applications to zero-visibility tracking, radio astronomy, plasma diagnostics, and remote sensing.

Eleftheriades, George V.↗

NASA NTRS1992

Structure of the magnetopause current layer at the subsolar point

Attention is given to a 1D electromagnetic particle simulation model developed for the magnetopause current layer between the shocked solar wind and the dipole magnetic field at the subsolar point, which was extended to include the IMF in the solar wind. Interaction of the solar wind with the vacuum dipole field as well as the dipole field filled with a low-density magnetospheric plasma are investigated. The width and structure of the magnetopause current layer are found to differ markedly depending on the direction of the IMF. When the IMF is pointing southward, the current layer between the solar wind and the dipole field is narrow, and the magnetic field has a single ramp structure caused by the reflection of the solar wind at that point. The current layer becomes several times wider and the magnetic field develops a multiple ramp structure when the IMF is northward. Comparisons of these simulation results with the recent spacecraft data of the magnetopause crossing near the subsolar point are provided.

Okuda, H.↗

NASA NTRS1990

The all-sky extragalactic X-ray foreground

It is argued that the local gravitational dipole implied by the 50 brightest X-ray clusters of galaxies at z greater than 0.013 considered by Lahav et al. (1989) is relatively small compared with that inferred from the only three clusters at lower redshifts. Recent dipole analysis of the X-ray flux from bright AGN observed with HEAO-1A2 indicates that they are indeed strong tracers of this matter. The implications of this for the very pronounced large-scale foreground anisotropies to be measured via low-redshift AGN resolved in more sensitive all-sky surveys are explored. For the total extragalactic X-ray sky, i.e., including the relatively large Cosmic X-ray Background as well as the contribution of the resolved sources, the dipole moment is small compared to the monopole. Because of the large peculiar velocity of the Local Group, it is found that the currently estimated value for this total dipole moment can be used to set severe constraints on the volume emissivity arising from all X-ray sources within the present epoch.

Boldt, Elihu↗

NASA NTRS1994

Effect of dispersion forces on squeezing with Rydberg atoms

We report exact results concerning the effect of dipole-dipole interaction (dispersion forces) on dynamic and steady-state characteristics of squeezing in the emitted fluorescent field from two identical coherently driven two-level atoms. The atomic system is subjected to three different damping baths in particular the normal vacuum, a broad band thermal field and a broad band squeezed vacuum. The atomic model is the Dicke model, hence possible experiments are most likely to agree with theory when performed on systems of Rydberg atoms making microwave transitions. The presence of dipole-dipole interaction can enhance squeezing for realizable values of the various parameters involved.

Ng, S. K.↗

NASA NTRS1994

On the prediction of velocity fields from redshift space galaxy samples

We present a new method for recovering the underlying velocity field from an observed distribution of galaxies in redshift space. The method is based on a kinematic Zel'dovich relation between the velocity and density fields in redshift space. This relation is expressed in a differential equation slightly modified from the usual Poisson equation and which depends nontrivially on Beta identically equal to Omega(exp 0.6)/b. The linear equation can be readily solved by standard techniques of separation of variables by means of spherical harmonics. One can also include a term describing the 'rocket effect' discussed by Kaiser (1987). From this redshift space information alone, one can generate a prediction of the peculiar velocity field for each harmonic (l, m) as a function of distance. We note that for the quadrupole and higher order moments, the equation is a boundary value problem with solutions dependent on both the interior and exterior mass distribution. However, for a shell at distance r, the dipole, as well as the monopole, of the velocity field in the Local Group frame is fully determined by the interior mass distribution. This implies that the shear of the measured velocity field, when fitted to a dipole distortion, should be aligned and consistent with the gravity field inferred from the well determined local galaxy distribution. As a preliminary application we compute the velocity dipole of distant shells as predicted from the 1.2 Jy IRAS survey compared to the measured velocity dipole on shells, as inferred from a recent POTENT analysis. The coherence between the two fields is good, yielding a best estimate of Beta = 0.6 +/- 0.2.

Nusser, Adi↗