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  • Inorganic Chemistry  (602)
  • Geophysics
  • 1995-1999  (759)
  • 1996  (759)
  • 1
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    Studies of Westward Electrojets and Field-Aligned Currents in the Magnetotail during Substorms: Implications for Magnetic Field Models (1996)
    In:  CASI
    Details
    Publication Date: 2019-08-28
    Description: Our studies elucidated the relationship between the auroral arcs and magnetotail phenomena. One paper examined particle energization in the source region of the field-aligned currents that intensify at substorm onset when the arc brightens to form the westward electrojet. A second paper examined the relationship between the precipitating particles in the arcs, the location of the westward electrojet, and magnetospheric source regions. Two earlier papers also investigated the roles that field aligned currents and particle acceleration have during substorms.
    Keywords: Geophysics
    Type: NASA/CR-97-205872 , NAS 1.26:205872
    Format: application/pdf
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  • 2
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    Stratospheric Horizontal Wavenumber Spectra of Winds, Potential Temperature, and Atmospheric Tracers Observed by High-Altitude Aircraft (1996)
    In:  Other Sources
    Details
    Publication Date: 2019-08-27
    Description: Horizontal wavenumber power spectra of vertical and horizontal wind velocities, potential temperatures, and ozone and N(2)O mixing ratios, as measured in the mid-stratosphere during 73 ER-2 flights (altitude approx. 20km) are presented. The velocity and potential temperature spectra in the 100 to 1-km wavelength range deviate significantly from the uniform -5/3 power law expected for the inverse energy-cascade regime of two-dimensional turbulence and also for inertial-range, three-dimensional turbulence. Instead, steeper spectra approximately consistent with a -3 power law are observed at horizontal scales smaller than 3 km for all velocity components as well as potential temperature. Shallower spectra are observed at scales longer than 6 km. For horizontal velocity and potential temperature the spectral indices at longer scales are between -1.5 and -2.0. For vertical velocity the spectrum at longer scales become flat. It is argued that the observed velocity and potential temperature spectra are consistent with gravity waves. At smaller scales, the shapes are also superficially consistent with a Lumley-Shur-Weinstock buoyant subrange of turbulence and/or nonlinear gravity waves. Contemporaneous spectra of ozone and N(sub 2)O mixing ratio in the 100 to 1-km wavelength range do conform to an approximately uniform -5/3 power law. It is argued that this may reflect interactions between gravity wave air-parcel displacements and laminar or filamentary structures in the trace gas mixing ratio field produced by enstropy-cascading two-dimensional turbulence.
    Keywords: Geophysics
    Type: AD-A530959 , Paper 95JD03835 , Journal of Geophysical Research (ISSN 0148-0227); 101; D5; 9441-9470
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  • 3
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    Mineralogy (1996)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: The power of mineralogical analysis as a descriptive or predictive technique stems from the fact that only a few thousand minerals are known to occur in nature as compared to several hundred thousand inorganic compounds. Further, all of the known minerals have specific stability ranges in pressure, temperature, an composition. A specific knowledge of the mineralogy of a planets surface or interior therefore allows one to characterize the present or past conditions under which the minerals were formed or have existed. For the purposes of this paper, a slightly broader definition of mineralogy was adopted by including not only crystalline materials found on planetary surfaces, but also ices and classes that can benefit from in situ types of analyses. Both visual examination and the various spectroscopies available for robotic probes to planetary surfaces are discussed.
    Keywords: Geophysics
    Type: Planetary Surface Instruments Workshop; 65-83; NASA-CR-202215
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  • 4
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    Resolving Magnetic Flux Patches at the Surface of the Core (1996)
    In:  Other Sources
    Details
    Publication Date: 2019-08-17
    Description: The geomagnetic field at a given epoch can be used to partition the surface of the liquid outer core into a finite number of contiguous regions in which the radial component of the magnetic flux density, B (sub r), is of one sign. These flux patches are instrumental in providing detail to surface fluid flows inferred from the changing geomagnetic field and in evaluating the validity of the frozen-flux approximation on which such inferences rely. Most of the flux patches in models of the modem field are small and enclose little flux compared to the total unsigned flux emanating from the core. To demonstrate that such patches are not required to explain the most spatially complete and accurate data presently available, those from the Magsat mission, I have constructed a smooth core field model that fits the Magsat data but does not possess small flux patches. I conclude that our present knowledge of the geomagnetic field does not allow us to resolve these features reliably at the core-mantle boundary; thus we possess less information about core flow than previously believed.
    Keywords: Geophysics
    Type: Paper-96GL03028 , Geophysical Research Letters (ISSN 0094-8534); 23; 22; 3071-3074
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  • 5
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    Sprites as Luminous Columns of Ionization Produced by Quasi-Electrostatic Thundercloud Fields (1996)
    In:  Other Sources
    Details
    Publication Date: 2019-08-17
    Description: Quasi-electrostatic (QE) fields which exist above thunderclouds after lightning discharges can lead to the formation of columnar channels of breakdown ionization and carrot-like vertical luminous structures with typical transverse dimension approximately 5-10 km spaning an altitude range from approximately 80 km to well below approximately 50 km. The carrot-like forms closely resemble those observed in sprites. Results indicate that the appearance of optical emissions can be significantly delayed in time (approx. 1-20 ms) with respect to the causative lightning discharge.
    Keywords: Geophysics
    Type: Paper 96GL00473 , Geophysical Research Letters (ISSN 0094-8534); 23; 6; 649-652
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  • 6
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    Transient Creep of a Composite Lower Crust (1996)
    In:  Other Sources
    Details
    Publication Date: 2019-08-17
    Description: A composite model is proposed to describe the time-dependent response of the Earth's lower crust. The motivation for such it model is twofold: First, new observations of widespread postseismic deformation indicate that the deep continental crust responds viscoelastically, having both long-and short-term decay times. Second, by any number of observationally based rationales, the lower crust is compositionally and structurally heterogeneous over many length scales. For heterogeneities that have much smaller characteristic lengths than the minimum deformation wavelength of interest, the aggregate rheology can be described by composite media theory. For wavelengths of the order of the thickness of the lower crust (approx. = 25-40 km) and larger, composite theory may be applied to heterogeneities that are smaller than about several hundred meters, or equivalent to the vertical extent of a thick lower crustal mylonitic shear zone. The composite media theory developed here is constructed using both Eshelhy-Mori-Tanaka theory for aligned generalized spheroidal inclusions and a generalized self-consistent method. The inclusions and matrix are considered to be Maxwellian viscoelastic: a rheology that is consistent with past homogeneous models of postseismic stress relaxation. The composite theory presented here introduces a transient response to a suddenly imposed stress field which does not appear in homogeneous Maxwell models. Analytic expressions for the amplitude and duration of the transient and for the effective long-and short-term viscosities of the composite are given which describe the sensitivity to inclusion concentration (phi), to shape, and to ratio of inclusion-to-matrix viscosity (R).
    Keywords: Geophysics
    Type: Paper-96JB02847 , Journal of Geophysical Research (ISSN 0148-0227); 101; B12; 27,981-28,004
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  • 7
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    A Demonstration of Precise Calibration of Tropospheric Delay Fluctuations with Water Vapor Radiometers (1996)
    In:  Other Sources
    Details
    Publication Date: 2019-08-17
    Description: The ability of water vapor radiometers (WVRs) to calibrate changes in tropospheric delay was demonstrated during very long baseline radio interferometer (VLBI) observations at Goldstone, California. WVR measurements reduced the observed VLBI delay variations over a 13 hr period by a factor of approx. = 2.5. When applied to shorter time scales, a approx. = 50% reduction in 100-700 s delay variations was achieved during conditions of high tropospheric activity. Thermal WVR noise precluded calibration of short time scale delay fluctuations during quiet tropospheric conditions.
    Keywords: Geophysics
    Type: Paper-96GL03309 , Geophysical Research Letters (ISSN 0094-8534); 23; 25; 3719-3722
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  • 8
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    Propagation and Breaking at High Altitudes of Gravity Waves Excited by Tropospheric Forcing (1996)
    In:  Other Sources
    Details
    Publication Date: 2019-08-17
    Description: An anelastic approximation is used with a time-variable coordinate transformation to formulate a two-dimensional numerical model that describes the evolution of gravity waves. The model is solved using a semi-Lagrangian method with monotone (nonoscillatory) interpolation of all advected fields. The time-variable transformation is used to generate disturbances at the lower boundary that approximate the effect of a traveling line of thunderstorms (a squall line) or of flow over a broad topographic obstacle. The vertical propagation and breaking of the gravity wave field (under conditions typical of summer solstice) is illustrated for each of these cases. It is shown that the wave field at high altitudes is dominated by a single horizontal wavelength; which is not always related simply to the horizontal dimension of the source. The morphology of wave breaking depends on the horizontal wavelength; for sufficiently short waves, breaking involves roughly one half of the wavelength. In common with other studies, it is found that the breaking waves undergo "self-acceleration," such that the zonal-mean intrinsic frequency remains approximately constant in spite of large changes in the background wind. It is also shown that many of the features obtained in the calculations can be understood in terms of linear wave theory. In particular, linear theory provides insights into the wavelength of the waves that break at high altitudes, the onset and evolution of breaking. the horizontal extent of the breaking region and its position relative to the forcing, and the minimum and maximum altitudes where breaking occurs. Wave breaking ceases at the altitude where the background dissipation rate (which in our model is a proxy for molecular diffusion) becomes greater than the rate of dissipation due to wave breaking, This altitude, in effect, the model turbopause, is shown to depend on a relatively small number of parameters that characterize the waves and the background state.
    Keywords: Geophysics
    Type: Journal of the Atmospheric Sciences; 53; 15; 2186-2216
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  • 9
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    A Model of the Chicxulub Impact Basin Based on Evaluation of Geophysical Data, Well Logs, and Drill Core Samples (1996)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: Abundant evidence now shows that the buried Chicxulub structure in northern Yucatan, Mexico, is indeed the intensely sought-after source of the ejecta found world-wide at the Cretaceous-Tertiary (K/T) boundary. In addition to large-scale concentric patterns in gravity and magnetic data over the structure, recent analyses of drill-core samples reveal a lithological assemblage similar to that observed at other terrestrial craters. This assemblage comprises suevite breccias, ejecta deposit breccias (Bunte Breccia equivalents), fine-grained impact melt rocks, and melt-matrix breccias. All these impact-produced lithologies contain diagnostic evidence of shock metamorphism, including planar deformation features in quartz, feldspar, and zircons; diaplectic glasses of quartz and feldspar; and fused mineral melts and whole-rock melts. In addition, elevated concentrations of Ir, Re, and Os, in meteoritic relative proportions, have been detected in some melt-rock samples from the center of the structure. Isotopic analyses, magnetization of melt-rock samples, and local stratigraphic constraints identify this crater as the source of K/T boundary deposits.
    Keywords: Geophysics
    Type: NASA-CR-204186 , NAS 1.26:204186 , LPI-Contrib-873 , Special Paper-307
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  • 10
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    Research on Spectroscopy, Opacity, and Atmospheres (1996)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: I discuss errors in theory and in interpreting observations that are produced by the failure to consider resolution in space, time, and energy. I discuss convection in stellar model atmospheres and in stars. Large errors in abundances are possible such as the factor of ten error in the Li abundance for extreme Population II stars. Finally I discuss the variation of microturbulent velocity with depth, effective temperature, gravity and abundance. These variations must be dealt with in computing models and grids and in any type of photometric calibration.
    Keywords: Geophysics
    Type: NASA-CR-203108 , NAS 1.26:203108 , Workshop on Model Atmospheres and Spectrum Synthesis; Jul 06, 1995 - Jul 11, 1995; Vienna; Austria
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