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At least 343 records · Page 19
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NASA NTRS1994

The Mg 2 lines as diagnostic of PMS nature in Herbig Ae/Be stars

The main objective of this program was to observe some newly discovered and previously unobserved Herbig Ae/Be (HAeBe) stars with IUE. A previous archival study suggested that there was a full range of profiles in the Mg II lines for these objects; ranging from the double peak emission to the narrow absorption lines. It has been found that by studying the IUE data of main sequence B and classical Be stars that the P Cygni profiles in the Mg II lines in HAeBe stars are truly unique and are the signature of accelerating winds in their extended circumstellar material. The arguement, for the purposes of this proposal, that the structure of the Mg II lines was a superior diagnostic of the dynamical activity and ultimately the PMS nature of these objects. This program was a follow up study of several interesting intermediate-mass stars with the goal of classifying these objects based on the shape of their MG II lines. A list of publications resulting from this research is included.

Source record↗
NASA NTRS1994

Does human cognition allow Human Factors (HF) certification of advanced aircrew systems?

This paper has examined the requirements of HF specification and certification within advanced or complex aircrew systems. It suggests reasons for current inadequacies in the use of HF in the design process, giving some examples in support, and suggesting an avenue towards the improvement of the HF certification process. The importance of human cognition to the operation and performance of advanced aircrew systems has been stressed. Many of the shortfalls of advanced aircrew systems must be attributed to over automated designs that show little consideration on either the mental limits or the cognitive capabilities of the human system component. Traditional approaches to system design and HF certification are set within an over physicalistic foundation. Also, traditionally it was assumed that physicalistic system functions could be attributed to either the human or the machine on a one to one basis. Moreover, any problems associated with the parallel needs, or promoting human understanding alongside system operation and direction, were generally equated in reality by the natural flexibility and adaptability of human skills. The consideration of the human component of a complex system is seen as being primarily based on manifestations of human behavior to the almost total exclusion of any appreciation of unobservable human mental and cognitive processes. The argument of this paper is that the considered functionality of any complex human-machine system must contain functions that are purely human and purely cognitive. Human-machine system reliability ultimately depends on human reliability and dependability and, therefore, on the form and frequency of cognitive processes that have to be conducted to support system performance. The greater the demand placed by an advanced aircraft system on the human component's basic knowledge processes or cognition, rather than on skill, the more insiduous the effects the human may have on that system. This paper discusses one example of an attempt to devise an improved method of specificaiton and certification with relation to the advanced aircrew system, that of the RN Merlin helicopter. The method is realized to have limitations in practice, these mainly associated with the late production of the system specification in relation to the system development process. The need for a careful appreciation of the capabilities and support needs of human cognition within the design process of a complex man machine system has been argued, especially with relation to the concept of system functionality. Unlike the physicalistic Fitts list, a new classification of system functionality is proposed, namely: (1) equipment - system equipment related; (2) cognitive - human cognition related; and (3) associated - necessary combinatin of equipment and cognitive. This paper has not proposed a method for a fuller consideration of cognition within systems design, but has suggested the need for such a method and indicated an avenue towards its development. Finally, the HF certification of advanced aircrew systems is seen as only being possible in a qualified sense until the important functions of human cognition are considered within the system design process. (This paper contains the opinions of its authors and does not necessarily refledt the standpoint of their respective organizations).

Macleod, Iain S.↗
NASA NTRS1994

The global resurfacing of Venus

The impact cratering record on Venus is unique among the terrestrial planets. Fully 84% of the craters are in pristine condition, and only 12% are fractured. Remarkably, only 2.5% of the craters and crater-related features are embayed by lava, although intense volcanism and tectonism have affected the entire planet. Furthermore, the spatial and hypsometric distribution of the craters is consistent with a completely random one, including stochastic variations. Monte Carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed, or unobserved nonrandom crater distributions for resurfacing areas between 0.03% and 100% of the planet's surface. These models also are not consistent with the number and nonrandom distribution of volcanoes, and the nonrandom distribution of embayed and heavily fractured craters. The constraints imposed by the cratering record strongly indicate that Venus experienced a global resurfacing event about 300 m.y. ago followed by a dramatic reduction of volcanism and tectonism. This global resurfacing event ended abruptly (less than 10 m.y.). The present crater population has accumulated since then and remains largely intact. Thermal history models suggest that similar global resurfacing events probably occured episodically in the past. We show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record. The effects of recent geologic activity are much less than those of the earlier global resurfacing event, when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity. Episodic regional resurfacing events that had global effects also occurred on Earth (e.g., the mid-Cretaceous superplume) and probably on Mars. On Mars they may have triggered the catastrophic releases of water that formed the outflow channels.

Strom, Robert G.↗
NASA NTRS1994

A survey of nebulae around Galactic Wolf-Rayet stars in the southern sky, 1

Images are presented from the first half of a survey of all Galactic Wolf-Rayet stars in the catalog of van der Hucht et al. (1981) residing in the southern skies. Previous surveys used only existing broad-band photographic plates. Encouraged by successes using CCD imaging with interference filters of the LMC and northern Galaxy (Miller & Chu 1993), we have expanded the survey to the southern hemisphere. In the first half of our southern survey, H alpha and (O III) narrow-band CCD images of fields centered on known Wolf-Rayet stars have indicated the existence of six new ring nebulae as well as revealing previously unobserved morphological features in the known ring nebulae. An example of this is an almost perfect ring of (O III) emission residing interior to the previously observed H alpha filaments of the Wolf-Rayet ring nebulae RCW 104. Our surveys to date indicate that 21% of all Wolf-Rayet stars have ring nebulae, with WN-type Wolf-Rayet stars having a greater likelihood for an associated ring.

Marston, A. P.↗
NASA NTRS1995

The 1991 October 24 flare: A challenge for standard models

The M9.8 solar flare of 1991 October 24 22:30 UT presents several interesting characteristics: (1) energy release starts high in the corona; (2) the primary chromospheric ribbons are initially well separated and do not move apart at an observable rate; (3) no evidence is found for an erupting filament or other driver. To explain this flare, we consider several canonical flare models, including a filament eruption, a confined filament eruption, current interruption, and interacting loops. We conclude that none of these scenarios unequivocally explains this flare. Two possibilities which cannot be ruled out are (1) the eruption of a filament unobservable in H-alpha which starts high in the corona and produces no ribbon motions smaller than our detection threshold and no perceptible expansion of the coronal X-ray source, and (2) energy release due to spontaneous, propagating reconnection which allows the system to essentially brighten in place.

Beaujardiere, J.-F. De LA↗
NASA NTRS1994

Drift shells and aurora computed using the O8 magnetic field model for Neptune

Charged particle drift shells are calculated using the O8 magnetic field model for Neptune. Inner drift shell morphologies differ significantly from dipolar drift shells for the parts of drift shells inward of r = 2 R(sub N). Outer drift shells (L approx. greater than 10), when traced down to Neptune's surface following magnetic field lines, are simple closed loops around magnetic poles. Inner drift shells (L approx. less than 4), on the other hand, when traced to the surface, are also single loops but stretched in a previously unknown way: sometimes with a cusp and sometimes into two joined loops. Inner drift shell footprints on R = 1 provide the basis for identifying precipitation L shells, interpreting observed aurora, and predicting additional emissions on that part of Neptune's surface unobserved by the Ultraviolet Spectrometer (UVS). Precipitation in a global magnetic anomaly, `ordinary' auroral precipitation near the south magnetic pole, and precipitation from field lines with no magnetic field minimum above Neptune's exobase collectively appear to account for all of the observed auroral emission regions at Neptune. To the extent that aurora can be understood in this model, it is suggested O8 may be reasonably accurate.

Paranicas, C.↗
NASA NTRS1994

Neptune's inner magnetosphere and aurora: Energetic particle constraints

A dramatic and peculiar dropout of greater than 500-keV ions (but not electrons) was observed within Neptune's inner magnetosphere near 2 R(sub N) as the Voyager 2 spacecraft approached the planet. Unlike a number of other energetic particle features this feature could not be accounted for by known material bodies in the context of the most utilized magnetic field models (neither the offset tilted dipole models nor the spehrical harmonic model 'O8'). However, the configuration of Neptune's inner magnetosphere is highly uncertain. By applying a novel technique, utilizing energetic particle measurements, to constrain the magnetic field configuration of the inner regions, we show that appeals to unobserved materials within Neptune's system are unnecessary, and that the ion dropout feature was, in all likelihood, the result of ion interactions with maximum L excursions of the ring 1989N1R. The constraints also favor the se of the M2 magnetic field model (Selesnick, 1992) over the previous models. An electron feature was probably absent because the electron interactions with the ring occurred substantially before the ion interactions (about 2 hours for the electrons versus a few minutes for the ions). Pitch-angle scattering apparently eliminated the electron signature. Minimum scattering rates determined based on this premise yield enough electron precipitation power to explain the brightest component of Neptune's aurora. We propose that this bright component is analogous to the Earth's diffuse aurora.

Mauk, B. H.↗
NASA NTRS1995

Outer layers of a carbon star: The view from the Hubble Space Telescope

To advance our understanding of the relationship between stellar chromospheres and mass loss, which is a common property of carbon stars and other asymptotic giant branch stars, we have obtained ultraviolet spectra of the nearby N-type carbon star UU Aur using the Hubble Space Telescope (HST). In this paper we describe the HST observations, identify spectral features in both absorption and emission, and attempt to infer the velocity field in the chromosphere, upper troposphere, and circumstellar envelope from spectral line shifts. A mechanism for producing fluoresced emission to explain a previously unobserved emission line is proposed. Some related ground-based observations are also described.

Johnson, H. R.↗
NASA NTRS1995

Surface temperature of a magnetized neutron star and interpretation of the ROSAT data. 1: Dipole fields

We model the temperature distribution at the surface of a magnetized neutron star and study the effects on the observed X-ray spectra and light curves. Generalrelativistic effects, i.e., redshift and lensing, are fully taken into account. Atmospheric effects on the emitted spectral flux are not included: we consider only blackbody emission at the local effective temperature. In this first paper we restrict ourselves to dipole fields. General features are studied and compared with the ROSAT data from the pulsars 0833 - 45 (Vela), 0656 + 14, 0630 + 178 (Geminga), and 1055 - 52, the four cases for which there is strong evidence that thermal radiation from the stellar surface is detected. The composite spectra we obtain are not very different from a blackbody spectrum at the star's effective temperature. We conclude that, as far as blackbody spectra are considered, temperature estimates using single-temperature models give results practically identical to our composite models. The change of the (composite blackbody) spectrum with the star's rotational phase is also not very large and may be unobservable inmost cases. Gravitational lensing strongly suppresses the light curve pulsations. If a dipole field is assumed, pulsed fractions comparable to the observed ones can be obtained only with stellar radii larger than those which are predicted by current models of neutron star struture, or with low stellar masses. Moreover, the shapes of the theoretical light curves with dipole fields do not correspond to the observations. The use of magnetic spectra may raise the pulsed fraction sufficiently but will certainly make the discrepancy with the light curve shapes worse: dipole fields are not sufficient to interpret the data. Many neutron star models with a meson condensate or hypersons predict very small radii, and hence very strong lensing, which will require highly nondipolar fields to be able to reproduce the observed pulsed fractions, if possible at all: this may be a new tool to constrain the size of neutron stars. The pulsed fractions obtained in all our models increase with photon energy: the strong decrease observed in Geminga at energies 0.3-0.5 keV is definitely a genuine effect of the magnetic field on the spectrum in contradistinction to the magnetic effects on the surface temperature considered her. Thus, a detailed analysis of thermal emission from the four pulsars we consider will require both complex surface field configurations and the inclusion of magnetic effects in the atmosphere (i.e., on the emitted spectrum).

Page, Dany↗
NASA NTRS1995

Hubble Space Telescope observations of cool white dwarf stars: Detection of new species of heavy elements

Observations of cool white dwarf stars with the Hubble Space Telescope (HST) has uncovered a number of spectral features from previouslly unobserved species. In this paper we present the data on four cool white dwarfs. We present identifications, equivalent width measurements, and brief summaries of the significance of our findings. The four stars observed are GD 40 (DBZ3, G 74-7 (DAZ), L 745-46A (DZ), and LDS 749B (DBA). Many additional species of heavey elements were detected in GD 40 and G 74-7. In L 745-46A, while the detections are limited to Fe 1, Fe II, and Mg II, the quality of the Mg II h and K line profiles should permit a test of the line broadening theories, which are so crucial to abundance determinations. The clear detection of Mg II h and k in LDS 749 B should, once an abundance determination is made, provide a clear test of the hypothesis that the DBA stars are the result of accretion from the interstellar medium. This star contains no other clear features other than a tantalizing hint of C II 1335 with a P Cygni profile, and some expected He 1 lines.

Shipman, Harry↗
NASA NTRS1995

Clouds and the Earth's Radiant Energy System (CERES) algorithm theoretical basis document: Overviews (subsystem 0) - Volume 1

The theoretical bases for the Release 1 algorithms that will be used to process satellite data for investigation of the Clouds and the Earth's Radiant Energy System (CERES) are described. The architecture for software implementation of the methodologies is outlined. Volume 1 provides both summarized and detailed overviews of the CERES Release 1 data analysis system. CERES will produce global top-of-the-atmosphere shortwave and longwave radiative fluxes at the top of the atmosphere, at the surface, and within the atmosphere by using the combination of a large variety of measurements and models. The CERES processing system includes radiance observations from CERES scanning radiometers, cloud properties derived from coincident satellite imaging radiometers, temperature and humidity fields from meteorological analysis models, and high-temporal-resolution geostationary satellite radiances to account for unobserved times. CERES will provide a continuation of the ERBE record and the lowest error climatology of consistent cloud properties and radiation fields. CERES will also substantially improve our knowledge of the Earth's surface radiation budget.

Wielicki, Bruce A.↗
NASA NTRS1996

Proportional and scale change models to project failures of mechanical components with applications to space station

In this paper we develop the mathematical theory of proportional and scale change models to perform reliability analysis. The results obtained will be applied for the Reaction Control System (RCS) thruster valves on an orbiter. With the advent of extended EVA's associated with PROX OPS (ISSA & MIR), and docking, the loss of a thruster valve now takes on an expanded safety significance. Previous studies assume a homogeneous population of components with each component having the same failure rate. However, as various components experience different stresses and are exposed to different environments, their failure rates change with time. In this paper we model the reliability of a thruster valves by treating these valves as a censored repairable system. The model for each valve will take the form of a nonhomogeneous process with the intensity function that is either treated as a proportional hazard model, or a scale change random effects hazard model. Each component has an associated z, an independent realization of the random variable Z from a distribution G(z). This unobserved quantity z can be used to describe heterogeneity systematically. For various models methods for estimating the model parameters using censored data will be developed. Available field data (from previously flown flights) is from non-renewable systems. The estimated failure rate using such data will need to be modified for renewable systems such as thruster valve.

Taneja, Vidya S.↗
NASA NTRS1996

ATMOS Measurements of H2O + 2CH4 and Total Reactive Nitrogen in the November 1994 Antarctic Stratosphere: Dehydration and Denitrification in the Vortex

Simultaneous stratospheric volume mixing ratios (VMR's) measured inside and outside the Antarctic vortex by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument in November 1994 reveal previously unobserved features in the distributions of total reactive nitrogen (NO(y)) and total hydrogen (H2O + 2CH4). Maximum removal of NO(y) due to sedimentation of polar stratospheric clouds (PSC's) inside the vortex occurred at a potential temperature (Theta) of 500-525 K (approximately 20 km), where values were 5 times smaller than measurements outside. Maximum loss of H2O + 2CH4 due to PSC's occurred in the vortex at 425-450 K, approximately 3 km lower than the peak NO(y) loss. At that level, H2O + 2CH4 VMR's inside the vortex were approximately 70% of corresponding values outside. The Antarctic and April 1993 Arctic measurements by ATMOS show no significant differences in H2O + 2CH4 VMR's outside the vortices in the two hemispheres. Elevated NO(y) VMRs were measured inside the vortex near 700 K. Recent model calculations indicate that this feature results from downward transport of elevated NO(y) produced in the thermosphere and mesosphere.

Rinsland, C. P.↗
NASA NTRS1997

Surface-Based Observations of Contrail Occurrence Over the US, Apr. 1993 to Apr. 1994

Surface observers stationed at 19 U.S. Air Force Bases and Army Air Stations recorded the daytime occurrence of contrails and cloud fraction on an hourly basis for the period April 1993 through April 1994. Each observation uses one of four main categories to report contrails as unobserved, non-persistent, persistent, and indeterminate. Additional classification includes the co-occurrence of cirrus with each report. The data cover much of the continental U.S. including locations near major commercial air routes. The mean annual frequency of occurrence in unobstructed viewing conditions is 13 percent for these sites. Contrail occurrence varied substantially with location and season. Most contrails occurred during the winter months and least during the summer with a pronounced minimum during July. Although nocturnal observations are not available, it appears that the contrails have a diurnal variation that peaks during mid morning over most areas. Contrails were most often observed in areas near major commercial air corridors and least often over areas far removed from the heaviest air traffic. A significant correlation exists between mean contrail frequency and aircraft fuel usage above 7 km suggesting predictive potential for assessing future contrail effects on climate.

Minnis, Patrick↗
NASA NTRS1998

Incorporating Parallel Computing into the Goddard Earth Observing System Data Assimilation System (GEOS DAS)

Atmospheric data assimilation is a method of combining actual observations with model forecasts to produce a more accurate description of the earth system than the observations or forecast alone can provide. The output of data assimilation, sometimes called the analysis, are regular, gridded datasets of observed and unobserved variables. Analysis plays a key role in numerical weather prediction and is becoming increasingly important for climate research. These applications, and the need for timely validation of scientific enhancements to the data assimilation system pose computational demands that are best met by distributed parallel software. The mission of the NASA Data Assimilation Office (DAO) is to provide datasets for climate research and to support NASA satellite and aircraft missions. The system used to create these datasets is the Goddard Earth Observing System Data Assimilation System (GEOS DAS). The core components of the the GEOS DAS are: the GEOS General Circulation Model (GCM), the Physical-space Statistical Analysis System (PSAS), the Observer, the on-line Quality Control (QC) system, the Coupler (which feeds analysis increments back to the GCM), and an I/O package for processing the large amounts of data the system produces (which will be described in another presentation in this session). The discussion will center on the following issues: the computational complexity for the whole GEOS DAS, assessment of the performance of the individual elements of GEOS DAS, and parallelization strategy for some of the components of the system.

Larson, Jay W.↗
NASA NTRS1995

A Game Theoretic Fault Detection Filter

The fault detection process is modelled as a disturbance attenuation problem. The solution to this problem is found via differential game theory, leading to an H(sub infinity) filter which bounds the transmission of all exogenous signals save the fault to be detected. For a general class of linear systems which includes some time-varying systems, it is shown that this transmission bound can be taken to zero by simultaneously bringing the sensor noise weighting to zero. Thus, in the limit, a complete transmission block can he achieved, making the game filter into a fault detection filter. When we specialize this result to time-invariant system, it is found that the detection filter attained in the limit is identical to the well known Beard-Jones Fault Detection Filter. That is, all fault inputs other than the one to be detected (the "nuisance faults") are restricted to an invariant subspace which is unobservable to a projection on the output. For time-invariant systems, it is also shown that in the limit, the order of the state-space and the game filter can be reduced by factoring out the invariant subspace. The result is a lower dimensional filter which can observe only the fault to be detected. A reduced-order filter can also he generated for time-varying systems, though the computational overhead may be intensive. An example given at the end of the paper demonstrates the effectiveness of the filter as a tool for fault detection and identification.

Chung, Walter H.↗
NASA NTRS1996

Noise and Dynamical Pattern Selection

In pattern-forming systems, such as Rayleigh-Benard convection or directional solidification, a large number of linearly stable, patterned steady states exist when the basic, simple steady state is unstable. Which of these steady states will be realized in a given experiment appears to depend on unobservable details of the system's initial conditions. We show, however, that weak, Gaussian white noise drives such a system toward a preferred wave number which depends only on the system parameters and is independent of initial conditions. We give a prescription for calculating this wave number, analytically near the onset of instability and numerically otherwise.

Kurtze, Douglas A.↗
NASA NTRS1999

The Kalman Filter and High Performance Computing at NASA's Data Assimilation Office (DAO)

Atmospheric data assimilation is a method of combining actual observations with model simulations to produce a more accurate description of the earth system than the observations alone provide. The output of data assimilation, sometimes called "the analysis", are accurate regular, gridded datasets of observed and unobserved variables. This is used not only for weather forecasting but is becoming increasingly important for climate research. For example, these datasets may be used to assess retrospectively energy budgets or the effects of trace gases such as ozone. This allows researchers to understand processes driving weather and climate, which have important scientific and policy implications. The primary goal of the NASA's Data Assimilation Office (DAO) is to provide datasets for climate research and to support NASA satellite and aircraft missions. This presentation will: (1) describe ongoing work on the advanced Kalman/Lagrangian filter parallel algorithm for the assimilation of trace gases in the stratosphere; and (2) discuss the Kalman filter in relation to other presentations from the DAO on Four Dimensional Data Assimilation at this meeting. Although the designation "Kalman filter" is often used to describe the overarching work, the series of talks will show that the scientific software and the kind of parallelization techniques that are being developed at the DAO are very different depending on the type of problem being considered, the extent to which the problem is mission critical, and the degree of Software Engineering that has to be applied.

Lyster, Peter M.↗