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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Basic research for the geodynamics program

Further development of utility program software for analyzing final results of Earth rotation parameter determination from different space geodetic systems was completed. Main simulation experiments were performed. Results and conclusions were compiled. The utilization of range-difference observations in geodynamics is also examined. A method based on the Bayesian philosophy and entropy measure of information is given for the elucidation of time-dependent models of crustal motions as part of a proposed algorithm. The strategy of model discrimination and design of measurements is illustrated in an example for the case of crustal deformation models.

Source record↗
NASA NTRS1987

NORAD satellite tracking

NORAD routinely tracks about 6000 orbiting objects. During the last 30 days of orbital time, prior to reentry, special perturbations are used in the orbital update procedure. Besides routine orbit determination, NORAD does special tasks such as predicting satellite orbit conjunctions within 20 km, ephimerides of weather satellites, satellite decay predictions and other studies. Since their mission is operational, they do not store the data from their analyses. The ballistic coefficient is not known for most of the orbiting objects. If a ballistic coefficient were derived that was consistent with one density model, it might give erroneous results if used with a different density model. Given the ballistic coefficient, density values could, in principle, be obtained from their tracking data. The densities would represent an integrated mean over the orbital path near perigee. They would be model dependent and would not necessarily represent the real density. In summary, the primary need is for reliable forecasts of solar flux (F10.7) and geomagnetic activity (Ap) in the 1 to 4 week time scale. Forecasts over longer time spans would also be useful for special projects.

Liu, Joseph J. F.↗
NASA NTRS1983

Software development predictors, error analysis, reliability models and software metric analysis

The use of dynamic characteristics as predictors for software development was studied. It was found that there are some significant factors that could be useful as predictors. From a study on software errors and complexity, it was shown that meaningful results can be obtained which allow insight into software traits and the environment in which it is developed. Reliability models were studied. The research included the field of program testing because the validity of some reliability models depends on the answers to some unanswered questions about testing. In studying software metrics, data collected from seven software engineering laboratory (FORTRAN) projects were examined and three effort reporting accuracy checks were applied to demonstrate the need to validate a data base. Results are discussed.

Basili, Victor↗
NASA NTRS1986

Detection of OH from Comet Halley in the far-infrared

The 2 Pi 3/2 (J = 5/2 yields 3/2 + yields - parity) transition of OH was detected in comet Halley at 119.44 microns. The upper limit to the line intensity of the - yields + parity transition is 119.23 microns. The ratio of these lines is consistent with radiative pumping of the rotational levels through absorption of near ultraviolet solar photons which excite the low lying vibrational levels of OH. The far-infrared lines probe the inner regions of the coma where OH is produced through photodissociation of a parent molecule (presumably H2O). Hence, these lines complement the 18 cm radio measurements which are sensitive to the outer regions of the comet coma. The OH production rate is highly model dependent but is of the order 2 to 4 times 10 to the 29th power molecules/sec.

Stacey, Gordon J.↗
NASA NTRS1986

Origin of the moon from a circumterrestrial disk

The paper reveals the possibility that the moon could have formed from the long-term evolution of a circumterrestrial disk emplaced during the earth's final stages of formation. A model is presented which emphasizes silicate enrichment (or iron depletion) of lunar material within the disk. This model depends on the late-stage planetesimal population being dominated by small bodies.

Weidenschilling, S. J.↗
NASA NTRS1986

The removal of soluble species by warm stratiform clouds

The development of a one-dimensional, time-dependent model to study the removal of soluble gases from a warm, precipitating stratiform cloud is reported. The model calculates the distributions of water vapor and condensed water, in the form of cloud drops and raindrops, as well as the in-cloud concentration of a soluble species in the gas and aqueous phases for a specified profile of pressure and temperature and an assumed updraft velocity. Highly soluble gases are found to be rapidly dissolved into cloud droplets and then slowly incorporated into raindrops as cloudwater is converted to rainwater. The rainout rate for highly soluble species is found to be ultimately limited by the rate at which new gaseous material can be transferred from the cloud-free air into the cloud. The model calculations indicate that turbulence represents an important mechanism by which highly soluble gases are transported into stratiform clouds.

Qin, YU↗
NASA NTRS1987

Physical interpretation of variations in total solar irradiance

Radiometry from the Solar Maximum Mission and Nimbus 7 satellites has demonstrated that the solar constant varies at a peak-to-peak level of up to 0.2 percent on time scales of weeks. The rotation and evolution of dark spots and bright faculae across the sun's disk accounts for most of that variation. Reasonable explanations have been put forward to explain how the spot-blocked heat flow might be stored, and to explain the source of the intense radiation that gives rise to the increased irradiance produced by the bright magnetic faculae. Time-dependent models of the response of the solar convection zone to small perturbations also indicate that slower variations in total solar irradiance of comparable magnitude are likely. More precise observations of the total solar irradiance and radius over long time scales are required to demonstrate the existence of such climatologically relevant changes, and to test models that would enable the interpretation and prediction of these changes.

Foukal, P.↗
NASA NTRS1987

Response of middle atmosphere to short-term solar ultraviolet variations. I - Observations. II - Theory

The effects of solar UV variability on ozone and temperature are studied using Nimbus 7 stratospheric and mesospheric ozone, temperature, and 205-nm solar UV data, and Solar Mesosphere Explorer 1.27-micron ozone and 121.6-nm solar UV data. Consideration is given to the temperature/UV relation, the O3/UV relation with and without temperature feedback, and the responses of HNO3 and NO2 to solar UV variability. It is observed that the response times of temperature to solar UV variability are 6 days at 2 mbar and 1 day at 0.01 mbar. The stratospheric ozone response (with no correction for temperature effects) increases, while in the mesosphere a decrease is detected. The responses of ozone and temperature to solar variability are examined and calculated using a one-dimensional chemical-radiative time-dependent model. The comparison of the model results with the satellite-observed responses reveals generally good correlation between the data; however, the model predicts a larger time lag of the temperature response and a larger stratospheric HNO3 mixing ratio than the satellite data. It is noted that the relation between short-term variations in the solar UV radiation and stratospheric ozone is affected by the coupling between ozone and temperature.

Keating, G. M.↗
NASA NTRS1987

Thermosolutal convection during directional solidification. II - Flow transitions

The influence of thermosolutal convection on solute segregation in crystals grown by vertical directional solidification of binary metallic alloys or semiconductors is studied. Finite differences are used in a two-dimensional time-dependent model which assumes a planar crystal-melt interface to obtain numerical results. It is assumed that the configuration is periodic in the horizontal direction. Consideration is given to the possibility of multiple flow states sharing the same period. The results are represented in bifurcation diagrams of the nonlinear states associated with the critical points of linear theory. Variations of the solutal Rayleigh number can lead to the occurrence of multiple steady states, time-periodic states, and quasi-periodic states. This case is compared to that of thermosolutal convection with linear vertical gradients and stress-free boundaries.

Mcfadden, G. B.↗
NASA NTRS1987

Effects of thermospheric motions on the polar wind - A time-dependent numerical study

This paper presents the results of a numerical study investigating the effects of low-altitude short-duration plasma heating episodes (such as horizontal frictional heating) on transient heavy ion outflows from the polar ionosphere using a time-dependent model. In a previous set of calculations, where the effects of high-altitude transient heat sources were investigated, the low-altitude neutral atmosphere (z less than 800 km) acted as a very efficient heat sink, absorbing most of the energy conducted to this region and thus preventing large differences between the ion and neutral temperatures. One purpose of the present study was to investigate whether realistic, specified low-altitude frictional heating rates, based on published experimental data, can result in elevated ion temperatures and lift a part of the heavy ion population over the gravitational barrier before the extra energy is lost to the neutral atmosphere. In these model calculations the specified heating generated an upflowing O(+) disturbance (with a normalized peak flux of about 100 million/sq cm per sec) moving upward along the open magnetic field line with a velocity of about 2 km/s. The spatial extent of the disturbance was approximately equal to the disturbance velocity times the duration of the heat source. The model results provide a possible mechanism to explain recent European Incoherent Scatter observations of high-latitude O(+) upwellings.

Gombosi, T. I.↗
NASA NTRS1987

Ray tracing of Jovian low frequency radiation

The radio emission from Jupiter in the decametric range (2 to 40 MHz) has been studied for three decades. It is known that this emission is correlated with particular longitude regions on the planet and with the position of the satellite Io. The Planetary Radio Astronomy experiment on the Voyager 1 and 2 spacecraft observed this emission, obtaining valuable new information. The objectives of this study are as follows: (1) calculate raypaths for decametric wavelength radiation in Jupiter's magnetosphere; (2) compare the model-dependent raypaths with the Voyager observations; and (3) deduce characteristics of the source regions and the influence of propagation effects.

Six, N. Frank↗
NASA NTRS1988

Improved methods for the measurement and analysis of stellar magnetic fields

The paper presents several improved methods for the measurement of magnetic fields on cool stars which take into account simple radiative transfer effects and the exact Zeeman patterns. Using these methods, high-resolution, low-noise data can be fitted with theoretical line profiles to determine the mean magnetic field strength in stellar active regions and a model-dependent fraction of the stellar surface (filling factor) covered by these regions. Random errors in the derived field strength and filling factor are parameterized in terms of signal-to-noise ratio, wavelength, spectral resolution, stellar rotation rate, and the magnetic parameters themselves. Weak line blends, if left uncorrected, can have significant systematic effects on the derived magnetic parameters, and thus several methods are developed to compensate partially for them. The magnetic parameters determined by previous methods likely have systematic errors because of such line blends and because of line saturation effects. Other sources of systematic error are explored in detail. These sources of error currently make it difficult to determine the magnetic parameters of individual stars to better than about + or - 20 percent.

Saar, Steven H.↗
NASA NTRS1988

Can the high latitude ionosphere support large field-aligned ion drifts?

A three-dimensional time-dependent model of the ionosphere is used to examine recent results on vertical drift velocities and electron densities in the high latitude ionosphere. Upper limits for the downward ion velocity were found to be smaller than those obtained from previous measurements. The downward force in the model was arbitrarily increased to try to account for the narrow electron density profiles. It is noted that if the common volume measurement is made in a region of O(+) precipitation then the line profile would not be Doppler shifted when viewed off-zenith, and small field-aligned velocities and narrow profile widths would result.

Sica, R. J.↗
NASA NTRS1988

Io: IUE observations of its atmosphere and the plasma torus

Two of the main components of the atmosphere of Io, neutral oxygen and sulfur, were detected with the IUE. Four observations yield brightnesses that are similar, regardless of whether the upstream or the downstream sides of the torus plasma flow around Io is observed. A simple model requires the emissions to be produced by the interaction of O and S columns in the exospheric range with 2 eV electrons. Cooling of the 5 eV torus electrons is required prior to their interaction with the atmosphere of Io. Inconsistencies in the characteristics of the spectra that cannot be accounted for in this model require further analysis with improved atomic data. The Io plasma torus was monitored with the IUE. The long-term stability of the warm torus is established. The observed brightnesses were analyzed using a model of the torus, and variations of less than 30 percent in the composition are observed, the quantitative results being model dependent.

Ballester, G. E.↗
NASA NTRS1988

The discrete correlation function: A new method for analyzing unevenly sampled variability data

A method of measuring correlation functions without interpolating in the temporal domain, the discrete correlation function, is introduced. It provides an assumption-free representation of the correlation measured in the data, and allows meaningful error estimates. This method does not produce spurious correlations at zero lag due to correlated errors. It is shown that physical interpretation of active galactic nuclei cross-correlation functions requires knowledge of the input function's fluctuation power spectrum, involves model-dependence in the form of symmetry assumptions, and must take into account intrinsic scale bias. This technique was used to find a correlation in published IUE data for NGC 4151, which indicates that the broad C IV feature emanates from a shell 15 to 75 light-days in radius, assuming spherical symmetry.

Edelson, R. A.↗
NASA NTRS1989

An experimental study and prediction of a two-phase pressure drop in microgravity

Experimental two-phase pressure drop results obtained in normal gravity and in nearly zero-gravity aboard a NASA-JSC reduced-gravity KC-135 aircraft are used to evaluate several empirically based correlations and flow-regime dependent models. Pressure drops in reduced gravity and those in normal gravity are shown to be related to flow pattern models for each. Two annular flow models are developed which are found to accurately predict the reduced-gravity data.

Chen, I. Y.↗
NASA NTRS1988

Application of fracture mechanics to predict the strength of filament-wound composite specimens with surface damage

The strength of a thick surface notched filament wound graphite/epoxy tensile specimen was predicted using fracture mechanics analysis. The prediction model depended upon the depth of the surface notch and upon the type of failure. Specimens failed in two stages: first a sublaminate with thickness equal to the depth of the surface cut, and second, for increasing load, a sublaminate with thickness equal to the initial thickness minus the cut depth. Analytical predictions and experimental data were in good agreement.

Morris, D. H.↗
NASA NTRS1989

Spatial structure in the color of the dust coma of Comet P/Halley

Significant spatial variations are noted in the color of the dust in Comet Halley's dust coma, out to 156,000 km from the optocenter, in the present March 1-6, 1986 narrowband CCD images. In particular, the envelope of intermediate-mass particles formed around the nucleus by solar radiation pressure is redder than the rest of the dust coma, and a strong jet noted in the March 1 images is redder than both the dust envelope and the remainder of the dust coma. A wavelength-dependence model of the Comet Halley dust's color is developed on the basis of Mie theory.

Hoban, Susan↗