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At least 1,171 records · Page 65

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NASA NTRS1992

Effects of Fe/C phase separation on the ages of white dwarfs

The energy release associated with the phase separation of Fe from C in a predominantly C white dwarf is calculated. The total gravitational-plus-internal energy differences between models of homogeneous compositions and those wth Fe-enriched cores are computed. In the unlikely case where the core is pure Fe, a substantial extension of the white dwarf cooling times is found, even with the small cosmic abundance of this element. For the more realistic core compositions that result if the Fe/C phase diagram is either of the spindle or of the azeotropic type, the energy release is still sufficient to prolong the cooling times by about 0.6 Gyr, comparable to that produced by C/O phase separation. Phase separation is found to produce an appreciable 'bump' in the luminosity function, although not one large enough to exceed the observational errors at low luminosities.

Xu, Z. W.↗

NASA NTRS1991

Correlation studies of passive and active microwave data in the marginal ice zone

The microwave radiative and backscatter characteristics of sea ice in an Arctic marginal ice zone have been studied using near-simultaneous passive and active synthetic aperture radar microwave data. Intermediate-resolution multichannel passive microwave data were registered and analyzed. Passive and active microwave data generally complement each other as the two sensors are especially sensitive to different physical properties of the sea ice. In the inner pack, undeformed first-year ice is observed to have low backscatter values but high brightness temperatures while multiyear ice has generally high backscatter values and low brightness temperatures. However, in the marginal ice zone, the signature and backscatter for multiyear ice are considerably different and closer to those of first-year ice. Some floes identified by photography as snow-covered thick ice have backscatter similar to that of new ice or open water while brash ice has backscatter similar to or higher than that of ridged ice.

Comiso, J. C.↗

NASA NTRS1992

The LDEF experience. Materials

An overview is provided of the LDEF (Long Duration Exposure Facility) mission and the preliminary findings, focusing on the materials related disciplines. These findings are based on observations of low-earth orbit environmental effects on materials made in space during LDEF retrieval and during LDEF tray deintegration, and on initial findings of laboratory analyses of LDEF materials. Included are effects of individual environmental parameters: atomic oxygen, ultraviolet radiation, meteoroid and debris impacts, thermal cycling, vacuum, and contamination, plus combined effects of these parameters. Materials considered include thermal control coatings and surface treatments, ceramics and glasses, metals, polymers, and polymer matrix composites. In general, preliminary LDEF data are remarkably consistent; LDEF will provide a benchmark for materials design data bases for satellites in low earth orbit. Some materials were identified to be encouragingly resistant to low earth orbit space environmental effects for 5.8 years; other space qualified materials displayed significant environmental degradation. Molecular contamination was widespread.

Stein, Bland A.↗

NASA NTRS1993

Investigation of a geodesy coexperiment to the Gravity Probe B relativity gyroscope program

Geodesy is the science of measuring the gravitational field of and positions on the Earth. Estimation of the gravitational field via gravitation gradiometry, the measurement of variations in the direction and magnitude of gravitation with respect to position, is this dissertation's focus. Gravity Probe B (GP-B) is a Stanford satellite experiment in gravitational physics. GP-B will measure the precession the rotating Earth causes on the space time around it by observing the precessions of four gyroscopes in a circular, polar, drag-free orbit at 650 km altitude. The gyroscopes are nearly perfect niobium-coated spheres of quartz, operating at 1.8 K to permit observations with extremely low thermal noise. The permissible gyroscope drift rate is miniscule, so the torques on the gyros must be tiny. A drag-free control system, by canceling accelerations caused by nongravitational forces, minimizes the support forces and hence torques. The GP-B system offers two main possibilities for geodesy. One is as a drag-free satellite to be used in trajectory-based estimates of the Earth's gravity field. We described calculations involving that approach in our previous reports, including comparison of laser only, GPS only, and combined tracking and a preliminary estimate of the possibility of estimating relativistic effects on the orbit. The second possibility is gradiometry. This technique has received a more cursory examination in previous reports, so we concentrate on it here. We explore the feasibility of using the residual suspension forces centering the GP-B gyros as gradiometer signals for geodesy. The objective of this work is a statistical prediction of the formal uncertainty in an estimate of the Earth's gravitation field using data from GP-B. We perform an instrument analysis and apply two mathematical techniques to predict uncertainty. One is an analytical approach using a flat-Earth approximation to predict geopotential information quality as a function of spatial wavelength. The second estimates the covariance matrix arising in a least-squares estimate of a spherical harmonic representation of the geopotential using GP-B gradiometer data. The results show that the GP-B data set can be used to create a consistent estimate of the geopotential up to spherical harmonic degree and order 60. The formal uncertainty of all coefficients between degrees 5 and 50 is reduced by factors of up to 30 over current satellite-only estimates and up to 7 over estimates which include surface data. The primary conclusion resulting from this study is that the gravitation gradiometer geodesy coexperiment to GP-B is both feasible and attractive.

Everitt, C. W. F.↗

NASA NTRS1992

The statistical cusp - A flux transfer event model

In this paper, we predict the precipitation signatures which are associated with transient magnetopause reconnection, following recent observations of the dependence of dayside ionospheric convection on the orientation of the IMF. We then employ a simple model of the longitudinal motion of flux-transfer-event signatures to show how such events can easily reproduce the local time distribution of cusp occurrence probabilities, as observed by low-altitude satellites. This is true even in the limit where the cusp is a series of discrete events. Furthermore, we investigate the existence of double cusp patches predicted by the simple model and show how these events may be identified in the data.

Smith, M. F.↗

NASA NTRS1991

Distributed-effect optical fiber sensors for trusses and plates

Modal domain optical fiber sensors, or distributed effect sensors, for active vibration suppression in flexible structures are considered. Preliminary modeling results indicate that these sensors can be used to sense vibrations in a flexible beam and the signal can be used to damp vibrations in the beam. Weighted distributed-effect sensors can be used to implement high order compensators with low order functional observers.

Reichard, Karl↗

NASA NTRS1993

Seasonal variation in the structure of red reflectance of leaves from yellow poplar, red oak, and red maple

The light scattered from leaves was measured as a function of view angle in the principal plane for yellow poplar, red oak, and red maple. The source was a parallel-polarized helium-neon laser. Yellow poplar leaves had the highest reflectance of the three species, which may have been due to its shorter palisade cells and more extensive spongy mesophyll. Prior to senescence, there was a significant decrease, but not total extinction, in the reflectance of the beam incident at 60 deg from nadir on the adaxial side of the leaves of all three species. Low-temperature SEM observations showed differences in the surface wax patterns among the three species but did not indicate a cause of the reflectance changes other than possibly the accumulation and aging of the wax.

Brakke, Thomas W.↗

NASA NTRS1993

Observations of the white dwarf in the U Geminorum system with the Hopkins Ultraviolet Telescope

The Hopkins Ultraviolet Telescope was used to obtain an FUV spectrum of U Gem about 10 d after the end of an outburst. Most of the FUV emission from U Gem appears to rise from the white dwarf (WD) in the system. Various possibilities to explain the hot component in the present spectrum are explored. It is most likely due to radiation from the boundary-layer region of the WD surface rather than from the accretion disk which exists in U Gem in quiescence. If the standard picture of boundary-layer emission is correct, then the hot component is not due to ongoing accretion onto the WD because the temperature observed is too low. Delayed emission from a viscously heated, differentially rotating atmosphere is argued to be a more valid explanation.

Long, Knox S.↗

NASA NTRS1993

Damped Lyman-alpha absorption in Q0957+561A, B

A series of archival IUE low-dispersion spectral observations of the double quasar Q0957+561A, B are analyzed. The components of this gravitationally lensed quasar are separated by about 6 arcsec. Damped Ly-alpha absorption at redshift z(abs) = 1.3911 with neutral hydrogen column density N(H I) about 7 x 10 exp 19/sq cm is present in the spectra of both the A and B components. Using an accepted model by Young et al. for the gravitational lens, the damped Ly-alpha absorber is inferred to have a minimum size of about 0.5/h50 kpc. The standard interpretation is that a damped Ly-alpha absorber represents a galactic or protogalactic H I disk. In the context of this standard model, the absorption-line properties and evidence for Faraday rotation are discussed.

Turnshek, David A.↗

NASA NTRS1993

Two-dimensional viscous accretion disk models. II - On viscous overstability

Accretion disks have been shown to be susceptible to pulsational overstability when the viscosity coefficient increases upon compression. In this study, the stability of an axisymmetric accretion disk with vertical structure is considered. 2D axisymmetric calculations were performed by solving the set of fully nonlinear hydrodynamic equations, including radiation transport. The time evolution of a representative section (ring) of a thin radiative accretion disk is studied. The existence of the overstability in 2D is confirmed. It is found that the presence of periodic oscillations near the outer boundary of a real disk might account for some of the low-amplitude flickering observed in dwarf novae.

Kley, W.↗

NASA NTRS1992

MEDSAT - A remote sensing satellite for malaria early warning and control

A remote sensing, medical satellite (MEDSAT) aids in the control of carrier (vector) borne disease. The prototype design is a light satellite to test for control of malaria. The design features a 340-kg satellite with visual/IR and SAR sensors in a low inclination orbit observing a number of worldwide test sites. The approach is to use four-band visual/IR and dual-polarized L-band SAR images obtained from MEDSAT in concert with in-situ data to estimate the temporal and spatial variations of malaria risk. This allows public health resources to focus on the most vulnerable areas at the appropriate time. It is concluded that a light-satellite design for a MEDSAT satellite with a Pegasus launch is feasible.

Vesecky, John↗

NASA NTRS1993

Least-squares finite element solutions for three-dimensional backward-facing step flow

Comprehensive numerical solutions of the steady state incompressible viscous flow over a three-dimensional backward-facing step up to Re equals 800 are presented. The results are obtained by the least-squares finite element method (LSFEM) which is based on the velocity-pressure-vorticity formulation. The computed model is of the same size as that of Armaly's experiment. Three-dimensional phenomena are observed even at low Reynolds number. The calculated values of the primary reattachment length are in good agreement with experimental results.

Jiang, Bo-Nan↗

NASA NTRS1993

Variable-permittivity linear inverse problem for the H(sub z)-polarized case

The H(sub z)-polarized inverse problem has rarely been studied before due to the complicated way in which the unknown permittivity appears in the wave equation. This problem is equivalent to the acoustic inverse problem with variable density. We have recently reported the solution to the nonlinear variable-permittivity H(sub z)-polarized inverse problem using the Born iterative method. Here, the linear inverse problem is solved for permittivity (epsilon) and permeability (mu) using a different approach which is an extension of the basic ideas of diffraction tomography (DT). The key to solving this problem is to utilize frequency diversity to obtain the required independent measurements. The receivers are assumed to be in the far field of the object, and plane wave incidence is also assumed. It is assumed that the scatterer is weak, so that the Born approximation can be used to arrive at a relationship between the measured pressure field and two terms related to the spatial Fourier transform of the two unknowns, epsilon and mu. The term involving permeability corresponds to monopole scattering and that for permittivity to dipole scattering. Measurements at several frequencies are used and a least squares problem is solved to reconstruct epsilon and mu. It is observed that the low spatial frequencies in the spectra of epsilon and mu produce inaccuracies in the results. Hence, a regularization method is devised to remove this problem. Several results are shown. Low contrast objects for which the above analysis holds are used to show that good reconstructions are obtained for both permittivity and permeability after regularization is applied.

Moghaddam, M.↗

NASA NTRS1994

Mars atmospheric loss and isotopic fractionation by pick-up-ion sputtering and photochemical escape

We examine the effects of loss of constituents of the Martian atmosphere due to sputtering by solar-wind pick-up ions and photochemical escape during the last 3.8 billion years. Sputtering is capable of efficiently removing species from the upper atmosphere to space, including the light noble gases; nitrogen and oxygen are removed by both sputtering ad photochemical processes. Due to diffusive separation (by mass) above the homopause, removal from the top of the atmosphere will fractionate the isotopes of each species, with the lighter isotope being preferentially lost. This allows current measurements of the isotopic ratios to be used as a measure of the atmospheric evolution as integrated over geologic time. For carbon and oxygen, isotopic fractionation is buffered by exchange of atmospheric species with non-atmospheric reservoirs of CO2 and H2O. This allows us to determine the size of the non-atmospheric reservoirs which are capable of mixing with the atmosphere; these reservoirs can be CO2 absorbed in the regolith and/or H2O in the polar ice caps. Such an exchangeable reservoir is required in order to keep the fractionation of the atmospheric gases as low as is observed.

Jakosky, B. M.↗

NASA NTRS1994

MHD simulations of the magnetosheath

Ideal magnetohydrodynamic (MHD) simulations of the dayside magnetosheath are reported. The calculations show a depletion layer near the magnetopause and a new stagnation flow structure. The stagnation flow appears to be a generalization of the stagnation line for ideal MHD. For Mach numbers typical of the solar wind at the Earth, there is no evidence of the slow mode waves recently observed, but for low Mach numbers prominent slow mode waves are seen in the magnetosheath.

Lyon, J. G.↗

NASA NTRS1993

Electron distributions in the inner Jovian magnetosphere: Voyager 1 observations

Using several improvements in the analysis of the observations of the Low Energy Charged Particle (LECP) experiment on Voyager 1, electron phase space densities in the inner Jovian magnetosphere (5 - 10 R(sub J) were first calculated at constant first and second invariants (represented by mu and K, respectively), based on the LECP measurements. The calculated electron phase space density profiles show that in the inner Jovian magnetosphere there exist evident time and longitude variations, energetic electron injections, and present radial transport and distributed losses. To study the radial and pitch angle diffusions of Jovian electrons, we have calculated the phase space densities in the K-L space. It is found that the electron population in the inner Jovian magnetosphere seems to consist of two components: electrons radially diffusing from a main external source and electrons generated from local sources. The radially diffusing electrons have a relatively time stationary and isotropic distribution, while the locally created electrons mainly concentrate around the equatorial plane and have relatively lower energies, in comparison with the inward diffusing electrons. Consequently, the sources of precipitation losses to the ionosphere must be primarily electrons transported from outer sources, and the major precipitations occur in the inner magnetosphere (L less than 7.5 R(sub J). In the inner Jovian magnetosphere (L = 5 to approximately 10 R(sub J)) it is estimated that for electrons with magnetic moment mu = 300 MeV/G, the diffusion coefficient D is roughly 10(exp -8) to approximately 10(exp -6) R(exp 2)(sub J)/s, and the lifetime against the diffusion losses is of the order of 10(exp 4) to approximately 10(exp 6) s.

Ye, G.↗

NASA NTRS1995

RE 0044+09: A new K dwarf rapid rotator with a white dwarf companion

We report the discovery of a new K dwarf rapid rotator with a potential white dwarf companion. The white dwarf accounts for over 90% of the observed extreme ultraviolet flux detected from this system. Analysis of ROSAT Wide Field Camera (WFC) and IUE data both suggest a white dwarf temperature of approximately 28,700 K. Optical photometry and the IUE long wavelength prime (LWP) spectrum (with the white dwarf contribution removed) imply that the late-type star has a spectral type of K1-3 V, and a distance of 55 +/- 5 pc. Using this distance, the observed IUE SWP flux, and the best-fit temperature results in a white dwarf radius of 0.0088 solar radius. The estimated white dwarf mass is then approximately 0.91 solar mass; somewhat over-massive compared to field white dwarfs. Optical photometry of the K star reveals a 'spot' modulation period of approximately 10 hr (now observed over 3 yr). However, radial velocity observations have revealed no significant variations. Spectroscopic observations place a low limit on the lithium abundance, but do show rapid rotation with a v sin i of 90 +/- 10 km/s. The K star was detected as a radio source at 3.6 cm (on two occasions) and 6 cm by the Very Large Array (VLA). The most likely evolutionary scenario is that the K star and hot white dwarf from either a wide binary or common proper motion pair with an age of 0.1-0.1 Gyr-consistent with the evolutionary timescale of the white dwarf and the rapid rotation of the K star. However, from the proper motion of the K star, this system does not seem to be associated with any of the known young stellar groups.

Kellett, Barry J.↗

NASA NTRS1996

A Fundamental Study of Nucleate Pool Boiling Under Microgravity

An experimental study of incipient boiling in short-term microgravity and with a/g = +/- 1 for pool boiling was performed. Calibrated thin gold films sputtered on a smoothly polished quartz surface were used simultaneously for thermal-resistance measurements and heating of the boiling surface. The gold films were used for both transient and quasi-steady heating surface temperature measurements. Two test vessels were constructed for precise measurement and control of fluid temperature and pressure: a laboratory pool boiling vessel for the a/g = +/- 1 experiments and a pool boiling vessel designed for the 131 m free-fall in the NASA Lewis Research Center Microgravity Research Facility for the microgravity tests. Measurements included the heater surface temperature, the pressure near the heating surface, the bulk liquid temperatures. High speed photography (up to 1,000 frames per second) was used in the experiments. With high quality microgravity and the measured initial temperature of the quiescent test fluid, R113, the temperature distribution in the liquid at the moment of boiling inception resulting from an imposed step in heat flux is known with a certainty not possible previously. The types of boiling propagation across the large flat heating surface, some observed here for the first time, are categorized; the conditions necessary for their occurrence are described. Explosive boiling propagation with a striking pattern of small scale protuberances over the entire vapor mass periphery not observed previously at low heat flux levels (on the order of 5 W/cm(exp 2)) is described. For the heater surface with a/g = -1, a step in the heater surface temperature of short duration was imposed. The resulting liquid temperature distribution at the moment of boiling inception was different from that obtained with a step in heat flux.

Ervin, Jamie S.↗