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At least 577 records · Page 32

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NASA NTRS2005

The Physical Character of Small-Scale Interstellar Structures

The primary objective of this program was to obtain FUSE observations of the multiple interstellar absorption lines of H2 toward the members of 3 resolvable binary/multiple star systems to explore the physical conditions in known interstellar small-scale structures. Each of the selected systems was meant to address a different aspect of the models for the origin of these structures: 1) The stars HD 32039/40 were meant to probe a temporally varying component which probed a cloud with an inferred size of tens to a few hundreds of AU. The goal was to see if there was any significant H2 associated with this component; 2) The star HD 36408B and its companion HD 36408A (observed as part of FUSE GTO program P119) show significant spatial and temporal (proper motion induced) Na I column variations in a strong, relatively isolated component, as well as a relatively simple component structure. The key goal here was to identify any differences in H2 or C I excitation between the sightlines, and to measure the physical conditions (primarily density and temperature) in the temporally varying component; 3) The stars HD 206267C and HD 206267D are highly reddened sightlines which showed significant variations in K I and molecular absorption lines in multiple velocity components. Coupled with FUSE GTO observations of HD 206267A (program P116), the goal was to study the variations in H2 along sightlines which are significantly more distant, with larger separations, and with greater extinctions than the other selected binary systems.

Lauroesch, James T.↗

NASA NTRS2008

Test Population Selection from Weibull-Based, Monte Carlo Simulations of Fatigue Life

Fatigue life is probabilistic and not deterministic. Experimentally establishing the fatigue life of materials, components, and systems is both time consuming and costly. As a result, conclusions regarding fatigue life are often inferred from a statistically insufficient number of physical tests. A proposed methodology for comparing life results as a function of variability due to Weibull parameters, variability between successive trials, and variability due to size of the experimental population is presented. Using Monte Carlo simulation of randomly selected lives from a large Weibull distribution, the variation in the L10 fatigue life of aluminum alloy AL6061 rotating rod fatigue tests was determined as a function of population size. These results were compared to the L10 fatigue lives of small (10 each) populations from AL2024, AL7075 and AL6061. For aluminum alloy AL6061, a simple algebraic relationship was established for the upper and lower L10 fatigue life limits as a function of the number of specimens failed. For most engineering applications where less than 30 percent variability can be tolerated in the maximum and minimum values, at least 30 to 35 test samples are necessary. The variability of test results based on small sample sizes can be greater than actual differences, if any, that exists between materials and can result in erroneous conclusions. The fatigue life of AL2024 is statistically longer than AL6061 and AL7075. However, there is no statistical difference between the fatigue lives of AL6061 and AL7075 even though AL7075 had a fatigue life 30 percent greater than AL6061.

Vlcek, Brian L.↗

NASA NTRS2007

Space Technology 5 Multi-point Measurements of Near-Earth Magnetic Fields: Initial Results

The Space Technology 5 (ST-5) mission successfully placed three micro-satellites in a 300 x 4500 km dawn-dusk orbit on 22 March 2006. Each spacecraft carried a boom-mounted vector fluxgate magnetometer that returned highly sensitive and accurate measurements of the geomagnetic field. These data allow, for the first time, the separation of temporal and spatial variations in field-aligned current (FAC) perturbations measured in low-Earth orbit on time scales of approximately 10 sec to 10 min. The constellation measurements are used to directly determine field-aligned current sheet motion, thickness and current density. In doing so, we demonstrate two multi-point methods for the inference of FAC current density that have not previously been possible in low-Earth orbit; 1) the "standard method," based upon s/c velocity, but corrected for FAC current sheet motion, and 2) the "gradiometer method" which uses simultaneous magnetic field measurements at two points with known separation. Future studies will apply these methods to the entire ST-5 data set and expand to include geomagnetic field gradient analyses as well as field-aligned and ionospheric currents.

Slavin, James A.↗

NASA NTRS2011

SO2 Emissions and Lifetimes: Estimates from Inverse Modeling Using In Situ and Global, Space-Based (SCIAMACHY and OMI) Observations

Top-down constraints on global sulfur dioxide (SO2) emissions are inferred through inverse modeling using SO2 column observations from two satellite instruments (SCIAMACHY and OMI). We first evaluated the S02 column observations with surface SO2 measurements by applying local scaling factors from a global chemical transport model (GEOS-Chem) to SO2 columns retrieved from the satellite instruments. The resulting annual mean surface SO2 mixing ratios for 2006 exhibit a significant spatial correlation (r=0.86, slope=0.91 for SCIAMACHY and r=0.80, slope = 0.79 for OMI) with coincident in situ measurements from monitoring networks throughout the United States and Canada. We evaluate the GEOS-Chem simulation of the SO2 lifetime with that inferred from in situ measurements to verity the applicability of GEOS-Chem for inversion of SO2 columns to emissions. The seasonal mean SO2 lifetime calculated with the GEOS-Chem model over the eastern United States is 13 h in summer and 48 h in winter, compared to lifetimes inferred from in situ measurements of 19 +/- 7 h in summer and 58 +/- 20 h in winter. We apply SO2 columns from SCIAMACHY and OMI to derive a top-down anthropogenic SO2 emission inventory over land by using the local GEOS-Chem relationship between SO2 columns and emissions. There is little seasonal variation in the top-down emissions (<15%) over most major industrial regions providing some confidence in the method. Our global estimate for annual land surface anthropogenic SO2 emissions (52.4 Tg S/yr from SCIAMACHY and 49.9 Tg S / yr from OMI) closely agrees with the bottom-up emissions (54.6 Tg S/yr) in the GEOS-Chem model and exhibits consistency in global distributions with the bottom-up emissions (r = 0.78 for SCIAMACHY, and r = 0.77 for OMI). However, there are significant regional differences.

Lee, Chulkyu↗

NASA NTRS2012

Test Population Selection from Weibull-Based, Monte Carlo Simulations of Fatigue Life

Vlcek, Brian L.↗

NASA NTRS2019

Using Chemcam Derived Geochemistry to Identify the Paleonet Sediment Transport Direction and Source Region Characteristics of the Stimson Formation in Gale Crater, Mars

The NASA Curiosity rover has encountered both ancient and modern dune deposits within Gale crater. The modern dunes are actively migrating across the surface within the Bagnold Dune field of which Curiosity conducted analysis campaigns at two different localities. Variations in mafic-felsic mineral abundances between these two sites have been related to the aeolian mineral sorting regime for basaltic environments identified on the Earth which become preferentially enriched in olivine relative to plagioclase feldspar with increasing distance from the source. This aeolian mineral sorting regime for basaltic minerals has also been inferred for Mars from orbital data. The aim of this study is to investigate whether this aeolian mafic-felsic mineral sorting trend has left a geochemical signature in the ancient dune deposits preserved within the Stimson formation. The Stimson formation unconformably overlies the Murray formation and consists of thickly laminated, cross-bedded sandstone. Stimson outcrops have a variable thickness up to 5 meters covering a total area of 17 square kilometers. A dry, aeolian origin was determined for this sandstone due to the high sphericity and roundness of the grains, uniform bimodal grain size distribution (250-710 microns), and 1-meter-thick cross-beds. Identifying the geochemical signature of mineral sorting can provide insights about the paleo-net sediment transport direction of the dunes and prevailing wind direction at the time of deposition.

Bedford, C. C.↗

NASA NTRS1978

Crustal evolution inferred from Apollo magnetic measurements

Magnetic field and solar wind plasma density measurements were analyzed to determine the scale size characteristics of remanent fields at the Apollo 12, 15, and 16 landing sites. Theoretical model calculations of the field-plasma interaction, involving diffusion of the remanent field into the solar plasma, were compared to the data. The information provided by all these experiments shows that remanent fields over most of the lunar surface are characterized by spatial variations as small as a few kilometers. Large regions (50 to 100 km) of the lunar crust were probably uniformly magnetized during early crustal evolution. Bombardment and subsequent gardening of the upper layers of these magnetized regions left randomly oriented, smaller scale (5 to 10 km) magnetic sources close to the surface. The larger scale size fields of magnitude approximately 0.1 gammas are measured by the orbiting subsatellite experiments and the small scale sized remanent fields of magnitude approximately 100 gammas are measured by the surface experiments.

Dyal, P.↗

NASA NTRS2017

Multiple Igneous Bodies for Nakhlites and Chassignites as Inferred from Olivine Cooling Rates using Calcium Zoning

Nakhlites and chassignites are ultramafic cumulate rocks of clinopyroxene and olivine, respec-tively, considered to have been formed in a thick lava flow or shallow intrusion near the Martian surface [e.g., 1,2]. Although more than 100 Martian meteorites have been found so far, most of them are shergottites and only nine nakhlites and three chassignites are known (considering paired samples) [3]. In contrast to shergottites which show large variations in both mineralogy and ages, nakhlites and chassignites are suggested to have been petrogenetically related, crystallized at about the same time and been ejected by the same impact event because of their identical crystallization (approximately 1.3 Ga) and cosmic-ray exposure (10-11 My) ages [e.g., 1]. In this study we discuss the possibility of a common igneous body for all samples belonging to these two groups as suggested by previous studies [e.g., 4]. To do this we estimated cooling rates of olivine using Ca zoning profiles, especially by paying attention to the newest samples of each group (NWA 10720 nakhlite and NWA 8694 chassignite).

Mikouchi, T.↗

NASA NTRS1976

X-ray observation of 3U 1700-37

X-ray observations with Copernicus reveal three categories of flux variability in 3U 1700-37. High amplitude hourly variations are energy independent in the 3-11 keV range while a change in the low energy absorbing column causes variations in flux level on an orbital time scale. This absorption is most severe prior to eclipse ingress, suggesting that the distribution of absorbing material around the X-ray source is asymmetrical with respect to the line of centers of the binary system. The absorbing material may be identical with a high density region inferred from optical observations of HD 153919. In the third category, the maximum source intensity per binary cycle is variable by at least a factor of two between observations. Measurement of the eclipse duration on three occasions indicate that it is significantly less than when observed by Uhuru.

Mason, K. O.↗

NASA NTRS1977

Lunar gravity determinations and their implications

Mass distributions inferred from lunar gravity data are studied, with emphasis on mascons as mass excesses in topographic lows in all the near-side ringed basins, with best representation as near-surface disks with excess loads of 800 kg per sq cm. The gravity data are derived from earth-based radiometric data on speed variations of orbiting Apollo spacecraft observed from the earth. Mass deficits are found in the case of large 100 km craters (Langrenus, Theophilus, Copernicus), and are consistent with crater volume. Earlier states of the moon are hypothesized on the basis of the small gravitational anomaly associated with the Apennine mountains, and on the basis of lunar isostasy, plasticity, and maria flooding and filling.

Sjogren, W. L.↗

NASA NTRS1980

On the physics of a long decay X-ray event

The paper discusses the long decay X-ray event which appeared as an expanding loop system on the solar limb on 13-14 Aug. 1973 which was also observed temporally and spectrally. A one-dimensional hydrodynamic study was undertaken to investigate increasing and decreasing phases of the event; it was shown that the inferred temperature gradients along the loops during the heating phase are consistent with unrestricted dynamic and conductive flows along the magnetic field lines. It was concluded that it cannot be definitely stated that enhanced emission at the tops of the loops is due to pressure gradients along the field lines; also, the large emission measure variations in the 10 to the 5th to 10 to the 6th K plasma during the event's decline may be due to the temperature dependence of radiative decay within a multiloop configuration.

Krall, K. R.↗

NASA NTRS1988

Multipoint observations of the magnetopause - Results from ISEE and AMPTE

Multiple spacecraft observations of the magnetopause have made it possible to infer boundary motion and structure, and to compare these with theoretical expectations. the results of the ISEE mission indicate the magnetopause is thicker than expected, often ten magnetosheath ion gyroradii or more, and that its characteristic radial motion is usually sub-Alfvenic. With small spacecraft separations it is possible to infer considerable small-scale magnetic structure within the current layer, suggesting filamentary currents much smaller than an ion gyroradius. Such a microstructure may be important in particle diffusion and, hence, reconnection. There are also phenomena on much larger scales, the highly variable boundary layer, flux transfer events and surface waves due to dynamic pressure variations, the structure of which can be best determined when spacecraft separations are large.

Elphic, R. C.↗

NASA NTRS1994

Evidence from masers for a rapidly rotating disk at the nucleus of NGC 4258

We demonstrate how maser emission from a rapidly rotating, thin Keplerian disk viewed edge-on can reproduce the general features of the observed 22 GHz radiation from the nucleus of NGC 4258, including the high-velocity satellites that are Doppler shifted by nearly +/- 1000 km/s from the central emission. Newly reported data about variations in time and space for the Doppler shifts of the central emission features fit especially well into a disk interpretation. We point out that their ratio gives a velocity of 700 km/s for the rotation at the outer edge of the masing ring -- a value that is essentially the same as the rotational velocity inferred from the Doppler shifts of the high-velocity satellites. The radius of the masing ring is 0.1 pc and the central mass at the nucleus of NGC 4258 about which the disk rotates is then 10(exp 7) solar mass.

Watson, William D.↗

NASA NTRS1984

Modeling the directional reflectance from complete homogeneous vegetation canopies with various leaf-orientation distributions

The directional-reflectance distributions of radiant flux from homogeneous vegetation canopies with greater than 90 percent ground cover are analyzed with a radiative-transfer model. The model assumes that the leaves consist of small finite planes with Lambertian properties. Four theoretical canopies with different leaf-orientation distributions were studied: erectophile, spherical, planophile, and heliotropic canopies. The directional-reflectance distributions from the model closely resemble reflectance distributions measured in the field. The physical scattering mechanisms operating in the model explain the variations observed in the reflectance distributions as a function of leaf-orientation distribution, solar zenith angle, and leaf transmittance and reflectance. The simulated reflectance distribution show unique characteristics for each canopy. The basic understanding of the physical scattering properties of the different canopy geometries gained in this study provide a basis for developing techniques to infer leaf-orientation distributions of vegetation canopies from directional remote-sensing measurements.

Kimes, D. S.↗

NASA NTRS1991

Interstellar pickup protons at pressure-balanced structures

An improved version is presented of a recent investigation that attempted to measure the presence of interstellar pickup protons in the distant solar wind through their effects on the variations within pressure-balanced structures. The previous study's conclusion that the pickup protons, expected to be correlated with the solar wind density through the charge exchange ionization process, had become smeared in the radial direction is confirmed. It is proposed that quasi-linear diffusion of these energetic particles will produce the spatial uniformity inferred from the observations.

Isenberg, Philip A.↗

NASA NTRS1998

Envelope structure on 700 AU scales and the molecular outflows of low-mass young stellar objects

Aperture synthesis observations of HCO+ J = 1-0, 13CO 1-0, and C18O 1-0 obtained with the Owens Valley Millimeter Array are used to probe the small-scale (5" approximately 700 AU) structure of the molecular envelopes of a well-defined sample of nine embedded low-mass young stellar objects in Taurus. The interferometer results can be understood in terms of: (1) a core of radius approximately or less than 1000 AU surrounding the central star, possibly flattened and rotating; (2) condensations scattered throughout the envelope that may be left over from the inhomogeneous structure of the original cloud core or that may have grown during collapse; and (3) material within the outflow or along the walls of the outflow cavity. Masses of the central cores are 0.001-0.1 M (solar), and agree well with dust continuum measurements. Averaged over the central 20" (3000 AU) region, an HCO+ abundance of 4 x 10(-8) is inferred, with a spread of a factor of 3 between the different sources. Reanalysis of previously presented single-dish data yields an HCO+ abundance of (5.0 +/- 1.7) x 10(-9), which may indicate an average increase by a factor of a few on the smaller scales sampled by the interferometer. Part of this apparent abundance variation could be explained by contributions from extended cloud emission to the single-dish C18O lines, and uncertainties in the assumed excitation temperatures and opacities. The properties of the molecular envelopes and outflows are further investigated through single-dish observations of 12CO J = 6-5, 4-3, and 3-2, 13CO 6-5 and 3-2, and C18O 3-2 and 2-1, obtained with the James Clerk Maxwell and IRAM 30 m telescopes, along with the Caltech Submillimeter Observatory. Ratios of the mid-J CO lines are used to estimate the excitation temperature, with values of 25-80 K derived for the gas near line centre. The outflow wings show a similar range, although Tex is enhanced by a factor of 2-3 in at least two sources. In contrast to the well-studied L1551 IRS 5 outflow, which extends over 10' (0.4 pc), seven of the remaining eight sources are found to drive 12CO 3-2 outflows over < or = 1' (0.04 pc); only L1527 IRS has a well-developed outflow of some 3'(0.12 pc). Estimates are obtained for the outflow kinetic luminosity, Lkin, and the flow momentum rate, FCO, applying corrections for line opacity and source inclination. The flow force FCO correlates with the envelope mass and with the 2.7 mm flux of the circumstellar disk. Only a weak correlation is seen with Lbol, while none is found with the relative age of the object as measured by integral Tmb(HCO+ 3-2)dV/Lbol. These trends support the hypothesis that outflows are driven by accretion through a disk, with a global mass infall rate determined by the mass and density of the envelope. The association of compact HCO+ emission with the walls of the outflow cavities indicates that outflows in turn influence the appearance of the envelopes. It is not yet clear, however, whether they are actively involved in sweeping up envelope material, or merely provide a low-opacity pathway for heating radiation to reach into the envelope.

NASA Discipline Exobiology↗

NASA NTRS2015

Towards the 1 mm/y Stability of the Radial Orbit Error at Regional Scales

An estimated orbit error budget for the Jason-1 and Jason-2 GDR-D solutions is constructed, using several measures of orbit error. The focus is on the long-term stability of the orbit time series for mean sea level applications on a regional scale. We discuss various issues related to the assessment of radial orbit error trends; in particular this study reviews orbit errors dependent on the tracking technique, with an aim to monitoring the long-term stability of all available tracking systems operating on Jason-1 and Jason-2 (GPS, DORIS, SLR). The reference frame accuracy and its effect on Jason orbit is assessed. We also examine the impact of analysis method on the inference of Geographically Correlated Errors as well as the significance of estimated radial orbit error trends versus the time span of the analysis. Thus a long-term error budget of the 10-year Jason-1 and Envisat GDR-D orbit time series is provided for two time scales: interannual and decadal. As the temporal variations of the geopotential remain one of the primary limitations in the Precision Orbit Determination modeling, the overall accuracy of the Jason-1 and Jason-2 GDR-D solutions is evaluated through comparison with external orbits based on different time-variable gravity models. This contribution is limited to an East-West "order-1" pattern at the 2 mm/y level (secular) and 4 mm level (seasonal), over the Jason-2 lifetime. The possibility of achieving sub-mm/y radial orbit stability over interannual and decadal periods at regional scales and the challenge of evaluating such an improvement using in situ independent data is discussed.

Precision Orbit Determination (POD)↗

NASA NTRS1983

Simulating spatial and temporal variation of corn canopy temperature during an irrigation cycle

The canopy air temperature difference (delta T) which provides an index for scheduling irrigation was examined. The Monteith transpiration equation was combined with both uptake from a single layered root zone and change in internal storage of the plant and the continuity equation for water flux in the soil plant atmosphere system was solved. The model indicates that both daily total transpiration and soil induced depression of plant water potential may be inferred from mid-day delta T. It is suggested that for the soil plant weather data used in the simulation, either a mid day spatial variability of about 0.8K in canopy temperatures or a field averaged delta T of 2 to 4K may be a suitable criterion for irrigation scheduling.

Choudhury, B. J.↗