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At least 235 records · Page 13

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NASA NTRS1986

The effect of CO chemisorption on the metal surface CO/Ni(100)

The chemisorption of CO on Ni(100) is studied with a cluster model. The calculations suggest that chemisorption changes the nature of the Ni 3d orbitals. The open shell Ni 3d accepts charge from the Ni valence 4s4p orbitals, reducing the open shell 3d character on the Ni. The closed shell Ni 3d orbitals also mix with the Ni valence 4s4p orbitals and donate charge to the CO; thus helping to maintain the Ni 3d population near 9. These changes should affect the density of 3d states near the Fermi level and the surface magnetic moment. The variation in the bonding with the size of cluster is also discussed.

Bauschlicher, Charles, W., jr.↗

NASA NTRS1987

Pulsar activation by the interstellar medium?

Recent work suggests that rotating magnetized neutron stars (i.e., pulsar models) trap plasma instead of emitting it. The trapping arises because nonneutral plasma can be stably trapped within such a magnetosphere provided the overall system charge is nonzero. It has been argued that particles from the interstellar medium would discharge this system, thereby presumably reactivating the system as a pulsar. However, radiation pressure either precludes such discharging (requiring an alternative source of ionization) or pulsar magnetic moments must be almost perfectly aligned with the spin axis (a revolutionary alternative). Indeed, the pulsars for which particles could reach the neutron star are those with periods of at least 3 sec. But those periods are where pulsars become inactive, not active. Conceivably, nulling might represent intermittent accretion of the interstellar medium.

Michel, F. Curtis↗

NASA NTRS1987

On the generation of magnetosheath lion roars

A theoretical model is proposed to discuss the electron dynamics associated with the mirror waves and their effects on the generation of the observed lion roars in the magnetosheath. It is pointed out that the usual double-adiabatic theory of hydromagnetics is not applicable to the electrons in mirror waves. Although the electron magnetic moment is conserved, the energy of each electron in the mirror waves is expected to be constant. Assuming an initial electron temperature anisotropy, it can be shown that in the low field region the electron temperature and thermal anisotropy are higher than the initial values, whereas in the high field region the electron temperature and anisotropy are lower. This point can lead to a theoretical explanation of the important features of the observed lion roars. Then present discussion complements the existing theories in the literature.

Lee, L. C.↗

NASA NTRS1987

Electrostatic waves in the magnetosphere of Uranus

Observations of electrostatic waves in the magnetosphere of Uranus are discussed with attention focused on the Bernstein emissions. The most intense Bernstein waves were observed near the magnetic equator of the planet, even though the tilt of the Uranian magnetic moment is very large relative to the rotational axis. In addition to the Bernstein modes, a number of highly sporadic emissions were detected in the vicinity of the Miranda L shell.

Kurth, W. S↗

NASA NTRS1988

Generalized adiabatic theory applied to the magnetotail current sheet

The generalized first adiabatic invariant, an extension of the magnetic moment for regions of large field gradients, has been applied to the study of particles in the magnetotail current sheet. Boundaries in phase space are obtained which form a generalized loss cone and separate particles drifting into and out of the layer from particles trapped within the layer, and the boundaries can be employed in the moment integrals for densities and currents when the drifting particles differ in temperature. It is shown that the parallel pressure of the drifting particles must exceed the transverse pressure for self-consistent solutions to exist. Fully self-consistent solutions are obtained using bi-Maxwellian ion and Maxwellian electron distributions.

Rogers, Shelley H.↗

NASA NTRS1990

Magnetic correlations in La(2-x)Sr(x)CuO4 from NQR relaxation and specific heat

La-139 and Cu-63 Nuclear Quadrupole Resonance (NQR) relaxation measurements in La(2-x)Sr(x)CuO4 for O = to or less than 0.3 and in the temperature range 1.6 + 450 K are analyzed in terms of Cu(++) magnetic correlations and dynamics. It is described how the magnetic correlations that would result from Cu-Cu exchange are reduced by mobile charge defects related to x-doping. A comprehensive picture is given which explains satisfactorily the x and T dependence of the correlation time, of the correlation length and of the Neel temperature T(sub n)(x) as well as being consistent with known electrical resistivity and magnetic susceptibility measurements. It is discussed how, in the superconducting samples, the mobile defects also cause the decrease, for T yields T(sub c)(+) of the hyperfine Cu electron-nucleus effective interaction, leading to the coexistence of quasi-localized, reduced magnetic moments from 3d Cu electrons and mobile oxygen p-hole carriers. The temperature dependence of the effective hyperfine field around the superconducting transition yields an activation energy which could be related to the pairing energy. New specific heat measurements are also presented and discussed in terms of the above picture.

Borsa, F.↗

NASA NTRS1990

Localization effects in radiationally disordered high-temperature superconductors: Theoretical interpretation

Theoretical interpretation of recent experiments on radiationally disordered high-temperature superconductors is presented, based on the concepts of mutual interplay of Anderson localization and superconductivity. Microscopic derivation of Ginzburg-Landau coefficients for the quasi-two-dimensional system in the vicinity of localization transition is given in the framework of the self-consistent theory of localization. The 'minimal metallic conductivity' for the quasi-two-dimensional case is enhanced due to a small overlap of electronic states on the nearest neighbor conducting planes. This leads to a stronger influence of localization effects than in ordinary (three-dimensional) superconductors. From this point of view even the initial samples of high-temperature superconductors are already very close to Anderson transition. Anomalies of H(c2) are also analyzed, explaining the upward curvature of H(c2)(T) and apparent independence of dH(c2)/dT (T = T(sub c)) on the degree of disorder as due to localization effects. Researchers discuss the possible reasons of fast T(sub c) degradation due to the enhanced Coulomb effects caused by the disorder induced decrease of localization length. The appearance and growth of localized magnetic moments is also discussed. The disorder dependence of localization length calculated from the experimental data on conductivity correlates reasonably with the theoretical criterion for suppression of superconductivity in the system with localized electronic states.

Goshchitskii, B. N.↗

NASA NTRS1989

The ion energy spectra in the ring current during the geomagnetic storm of February 1986

The largest geomagnetic storm in several decades occurred in February 1986, with a minimum Dst of -312 nT. In this experiment, the CHEM instrument on the AMPTE/CCE spacecraft has been used to study the development of this storm in more detail. By comparing measurements of phase space density (f) vs magnetic moment (mu) for four ion species H(+), O(+), He (+), and He(++) during consecutive passes through the ring current region, a distinction can be made between flux increases that require the injection of a new population and those that are consistent with the adiabatic acceleration of a pre-existing population. It is shown that the smaller flux observed during the outbound pass compared to the preceding inbound pass at the time of minimum Dst is due to the difference in local times of the two measurements. This local time difference is consistent with a greatly enhanced convection electric field, which brings a new population from the magnetotail to the post-noon, but not to pre-noon local time sector.

Kistler, L. M.↗

NASA NTRS1988

Theory of magnetic cataclysmic binary X-ray sources

The theory of magnetic cataclysmic binary X-ray sources is reviewed. The physics of the accretion torque for disk and for stream accretion is described, and the magnetic field strengths of DQ Her stars inferred from their spin behavior and of AM Her stars from direct measurement are discussed. The implications of disk and stream accretion for the geometry of the emission region and for the X-ray pulse profiles are considered. The physicl properties of the X-ray emission region and the expected infrared, optical, soft X-ray, and hard X-ray spectra are described. The orientations of the magnetic moment in AM Her stars inferred from the circular and linear polarization of the optical light and the optical light curve are commented on.

Lamb, Don Q.↗

NASA NTRS1989

Electron acceleration at nearly perpendicular collisionless shocks. I - One-dimensional simulations without electron scale fluctuations

Under certain conditions electrons can be reflected and effectively energized at quasi-perpendicular shocks. This process is most prominent close to the point where the upstream magnetic field is tangent to the curved shock. A theoretical explanation of the underlying physical mechanism has been proposed which assumes conservation of magnetic moment and a static, simplified shock profile are performed. Test particle calculations of the electron reflection process in order to examine the results of the theoretical analysis without imposing these restrictive conditions. A one-dimensional hybrid simulation code generates the characteristic field variations across the shock. Special emphasis is placed on the spatial and temporal length scales involved in the mirroring process. The simulation results agree generally well with the predictions from adiabatic theory. The effects of the cross-shock potential and unsteadiness are quantified, and the influence of field fluctuations on the reflection process is discussed.

Krauss-Varban, D.↗

NASA NTRS1989

Fast Fermi and gradient drift acceleration of electrons at nearly perpendicular collisionless shocks

Electrons can be reflected and effectively energized at quasi-perpendicular shocks. At a curved shock this process is most prominent where the upstream magnetic field and the shock surface are tangent. A theoretical explanation of the underlying physical mechanism in terms of fast Fermi acceleration has been proposed in the literature. The theory uses properties of the de Hoffmann-Teller frame and assumes conservation of magnetic moment in a static, simplified shock profile. Here, the discussion is extended in order to clarify certain aspects of the reflection process and to pinpoint the physical mechanisms that are operative and dominant from the viewpoint of the normal incidence frame. By reducing the analysis to the essential physical content and solving the pertinent energy equation, the equivalence of fast Fermi and gradient drift acceleration is shown.

Krauss-Varban, D.↗

NASA NTRS1990

High Tc composite silver/oxide superconductors

Factors involved in the strong flux pinning effect of high-Tc YBa2Cu3O7/AgO (Y-123/AgO) composite and other REE-123/AgO composites were investigated. Samples of superconducting REE-123 and REE-123/AgO (where REE was Nd, Sm, Eu, Gd, Dy, Ho, Er, or Y) were prepared and used to obtain magnetic moments, critical field, and microstructure data. The optimum heat treatment conditions for the formation of strong flux-pinning REE-123/AgO composites were found to be different for different REE-123 compounds. It was found that the annealing temperature depends on the ionic size of the REE, with larger rare-earth ions requiring higher temperature. It was also found that strong flux-pinning REE-123/AgO composites form only in a narrow annealing temperature range.

Curreri, P. A.↗

NASA NTRS1990

Polarization of the binary radio pulsar 1913 + 16 - Constraints on geodetic precession

Using polarization data, the relative orientations of the line of sight and the pulsar's spin axis and magnetic moment are determined. The data are sensitive enough to allow detection of spin-orbit geodetic precession in a few years if the spin axis is misaligned from the orbital angular momentum vector by more than a few degrees. From the secular pulse shape changes measured by Weisberg, et al. (1989) it is concluded that the misalignment angle is no more than 15 deg but is probably more than 1 deg. A specific beaming model is proposed to account for the pulse shape changes.

Cordes, J. M.↗

NASA NTRS1990

Shock-drift particle acceleration in superluminal shocks - A model for hot spots in extragalactic radio sources

Shock-drift acceleration at relativistic shock fronts is investigated using a fully relativistic treatment of both the microphysics of the shock-drift acceleration and the macrophysics of the shock front. By explicitly tracing particle trajectories across shocks, it is shown how the adiabatic invariance of a particle's magnetic moment breaks down as the upstream shock speed becomes relativistic, and is recovered at subrelativistic velocities. These calculations enable the mean increase in energy of a particle which encounters the shock with a given pitch angle to be calculated. The results are used to construct the downstream electron distribution function in terms of the incident distribution function and the bulk properties of the shock. The synchrotron emissivity of the transmitted distribution is calculated, and it is demonstrated that amplification factors are easily obtained which are more than adequate to explain the observed constrasts in surface brightness between jets and hot spots.

Begelman, Mitchell C.↗

NASA NTRS1989

Hyperfine magnetic field on Cd-111 in Heusler alloys Co2MnZ (Z = Si, Ga, Ge, Sn)

The time differential perturbed angular correlation method has been used to measure, as a function of temperature, the hyperfine magnetic field at Cd sites in the Heusler alloys Co2MnZ (Z = Si, Ga, Ge, Sn). The hyperfine fields, normalized to the total magnetic moment per formula unit, show an approximately linear trend toward more positive values with increasing lattice parameter.

Jha, S.↗

NASA NTRS1990

Beam-generated upper hybrid noise in Jupiter's outer magnetosphere

A model for generation of upper hybrid waves in Jupiter's outer magnetosphere is presented. Energetic electrons accelerated on high-latitude auroral field lines at 1 R(J) altitudes are assumed to stream outwards to the distant magnetosphere. The large decrease in the local magnetic field strength results in a highly collimated, field-aligned electron beam due to conservation of the particle's magnetic moment. As the beam travels outward it passes through the plasma sheet boundary layer region located on the edges of the plasma sheet. Because of the large field-aligned anisotropy of the beam, the distribution is unstable to the excitation of the upper hybrid waves which amplify and then undergo mode conversion at the f(p) layer to generate the Jovian continuum radiation. A novel feature of the model is the beam-anisotropic heat flux instability which drives UH waves unstable without the need for any positive slope on the electron distribution function.

Barbosa, D. D.↗

NASA NTRS1991

Interplanetary magnetic field control of the Mars bow shock - Evidence for Venuslike interaction

The Mars bow shock location and shape have been determined by examining the Phobos spacecraft magnetometer data. Observations show that the position of the terminator bow shock varies with interplanetary magnetic field orientation in the same way as at Venus. The shock is farthest from Mars in the direction of the interplanetary electric field, consistent with the idea that mass loading plays an important role in the solar wind interaction with Mars. The shock cross section at the terminator plane is asymmetric and is controlled by the interplanetary magnetic field. The shock is farther from Mars during solar maximum. Thus the solar wind interaction with Mars appears to be Venuslike, with a magnetic moment too small to affect significantly the solar wind interaction.

Zhang, T. L.↗

NASA NTRS1991

Gyro-phase effects near the storm-time boundary of energetic plasma

The nonadiabatic acceleration of plasma sheet ions during the expansion phase of substorms is examined by means of single-particle codes. It is shown that, in the near-earth plasma sheet, the gyration phase at substorm onset controls the net ion energization, as the gyro-period is locally comparable to the field variation time scale. This can yield a particularly significant decrease of the particle magnetic moment. It is accordingly argued that, via adiabatic invariant violation, the dipolarization of magnetospheric field lines can induce a 'de-trapping' of inner plasma sheet populations and, hence, give rise to short-lived precipitation enhancements over the auroral zone. As this effect depends upon cyclotron frequency, it is expected to occur at distinct latitudes for different ion species. This contrasts with nonadiabatic ion behavior further out into the geotail, which is characterized by intense perpendicular heating and a likely collective trapping.

Delcourt, D. C.↗