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At least 343 records · Page 19

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NASA NTRS1991

Chromospheric activity in Delta Scuti stars - The suspected variable Tau Cygni

High-resolution IUE spectra of the suspected variable Tau Cyg were obtained to search for a possible variability of the Mg II h, k double-peaked emission. The observations, spanning an interval of about 6.3 h, have shown flux excursions within or just near 15 percent, a value suggested as the detection limit of actual variations with IUE spectra. A variability, difficult to explain, could be present in the ratios Fk2v/Fk2r. The emission fluxes seem to be higher than those of the Delta Scuti variables Rho Pup and Beta Cas. This comparison could give some insights on the possible role of the convection on the pulsational and chromospheric activities of Tau Cyg. A positive correlation between the total emission fluxes and the rotational velocities of these stars was found.

Fracassini, M.↗

NASA NTRS1992

Optical and radio variability in blazars

An analysis is presented of the optical (0.44 micron) and radio flux density variations (4.8, 8.0, and 14.5 GHz) of the five best observed variable AGN, of which three are BL Lac objects and two are optically violent variable (OVV) quasars. For each object the nature of the variability at individual wavelengths is determined by using a structure function analysis and the autocorrelation function as calculated from the discrete correlation technique. Both techniques yield similar results, showing that the optical variations are similar for all objects and are a combination of flicker and shot noise. The radio variability is distinct, being described by shot noise (less variation at short times), with the two OVVs exhibiting the most extreme behavior. The time lag between the variations at the three radio frequencies and the optical flux is studied with the discrete cross-correlation function and the mean variance methods. It is concluded that the optical and radio emitting regions are physically related, exist on distinct time scales, and excite their synchrotron emitting plasmas differently.

Hufnagel, Beth R.↗

NASA NTRS1992

The near-infrared counterpart of a variable galactic plane radio source

A near-infrared counterpart to the highly variable, unresolved galactic plane radio source GT 0116 + 622 is identified. This source is of particular interest, as it has been previously suggested to be the counterpart of the gamma-ray source Cas gamma-l. The present NIR and red images detect a faint, spatially extended (3 arcsec FWHM), very red object coincident with the radio position. There is complex spatial structure which may be due in part to an unrelated superposed foreground object. Observations on multiple nights show no evidence for flux variability, despite the high amplitude variability on a time-scale of days reported for the radio source. The data are consistent with an interpretation of GT 0116 + 622 as an unusually variable, obscured active galaxy at a distance of several hundred megaparsecs, although more exotic, and in particular galactic, interpretations cannot yet be ruled out. If the object is extragalactic, the previously suggested identification with the gamma-ray source would seem unlikely.

Margon, Bruce↗

NASA NTRS1991

Correspondence of surface temperatures and terrain variables over a tallgrass prairie

The time-dependent correspondence between maps of surface brightness temperature derived from Landsat Thematic Mapper data and mapped terrain variables over a tallgrass prairie in northeastern Kansas is examined. Individual terrain variables including burning treatment, vegetation cover type (agriculture, prairie, woody vegetation), hillslope position, and greenness exhibit varying degrees of association with surface temperature. Burning treatment is most strongly associated with mid-morning surface temperature. Examination of terrain strata based on combinations of terrain variables, notably burning treatment and hillslope position, suggest that terrain variables interact in affecting surface temperature. Interaction between hillslope position, burning treatment, and surface temperature is more important in August than in May.

Friedl, Mark A.↗

NASA NTRS1991

Sun-as-a-star spectrum variability

The sun is observed as a start in order to determine luminosity change, detect minute variability in average granulation and focular signals, and to use as a standard against which other stars might be compared. In this regard, topics discussed include: total irradiance variability as measured from space by the Activity Cavity Radiometer Irradiance Monitor and Earth Radiation Budget radiometers; Fraunhofer line heights of formation and examples of their variability in visible wavelenghts; ultraviolet and extreme ultraviolet irradiance variability as observed from space; the magnetic origin of irradiance change; and the observed mean magnetic field of the sun.

Livingston, William↗

NASA NTRS1992

A new explicit variable time-integration self-starting methodology for computational structural dynamics

A new explicit variable time-integration methodology and architecture which possesses self-starting attributes, eliminates the need to involve acceleration computations, and which has improved accuracy characteristics in comparison to the traditional central-difference-type formulations customarily advocated is described for applicability to computational structural dynamics. To sharpen the focus of the present study, an explicit variable time-integration architecture which is relatively simple, yet effective, is described. Unlike variable explicit time-integration formulations adopted in the past, the present self-starting variable time-integration architecture and implementation aspects facilitate a simplified representation and a straightforward and effective approach for combining finite element meshes requiring different time steps in a single analysis. Numerical test cases are provided which demonstrate the applicability of the proposed formulations.

Tamma, Kumar K.↗

NASA NTRS1991

Ocean transport and variability studies of the South Pacific, Southern, and Indian Oceans

The objectives of this study are to analyze ocean dynamics in the western South Pacific and the adjacent Southern Ocean and the eastern Indian Ocean. Specifically, our objectives for these three regions are, for the South Pacific Ocean: (1) To estimate the volume transport of the east Australian Current (EAC) along the Australian coast and in the Tasman Front, and to estimate the time variability (on seasonal and interannual time scales) of this transport. (2) To contribute to estimating the meridional heat and freshwater fluxes (and their variability) at about 30 deg S. Good estimates of the transport in the western boundary current are essential for accurate estimates of these fluxes. (3) To determine how the EAC transport (and its extension, the Tasman Front and the East Auckland Current) closes the subtropical gyre of the South Pacific and to better determine the structure at the confluence of this current and the Antarctic Circumpolar Current. (4) To examine the structure and time variability of the circulation in the western South Pacific and the adjacent Southern Ocean, particularly at the Tasman Front. For the Indian Ocean: (5) To study the seasonal interannual variations in the strength of the Leeuwin Current. (6) To monitor the Pacific-Indian Ocean throughflow and the South Equatorial and the South Java Currents between northwest Australia and Indonesia. (7) To study the processes that form the water of the permanent oceanic thermocline and, in particular, the way in which new thermocline water enters the permanent thermocline in late winter and early spring as the mixed layer restratifies. For the Southern Ocean: (8) To study the mesoscale and meridional structure of the Southern Ocean between 150 deg E and 170 deg E; in particular, to describe the Antarctic frontal system south of Tasmania and determine its interannual variability; to estimate the exchanges of heat, salt, and other properties between the Indian and Pacific Oceans; and to investigate the annual ventilation of the Antarctic Intermediate Water and Subantarctic Mode Water Masses.

Church, John A.↗

NASA NTRS1992

The broad-band x ray spectral variability of Mkn 841

The results of a detailed spectral analysis of four X-ray observations of the luminous Seyfert 1.5 galaxy Mkn 841 performed using the EXOSAT and Ginga satellites over the period June 1984 to July 1990 are reported. Preliminary results from a short ROSAT PSPC observation of Mkn 841 in July 1990 are also presented. Variability is apparent in both the soft (0.1-1.0 keV) and medium (1-20 keV) energy bands. Above 1 keV, the spectra are adequately modelled by a power-law with a strong emission line of equivalent width approximately 450 eV. The energy of the line (approximately 6.4 keV) is indicative of K-shell fluorescence from neutral iron, leading to the interpretation that the line arises via X-ray illumination of cold material surrounding the source. In addition to the flux variability, the continuum shape also changes in a dramatic fashion, with variations in the apparent photon index Delta(Gamma) approximately 0.6. The large equivalent width of the emission line clearly indicates a strongly enhanced reflection component in the source, compared to other Seyferts observed with Ginga. The spectral changes are interpreted in terms of a variable power-law continuum superimposed on a flatter reflection component. For one Ginga observation, the reflected flux appears to dominate the medium energy X-ray emission, resulting in an unusually flat slope (Gamma approximately 1.0). The soft X-ray excess is found to be highly variable by a factor approximately 10. These variations are not correlated with the hard flux, but it seems likely that the soft component arises via reprocessing of the hard X-rays. We find no evidence for intrinsic absorption, with the equivalent hydrogen column density constrained to be less than or equal to few x 10(exp 20) cm(exp -2). The implications of these results for physical models for the emission regions in this and other X-ray bright Seyferts are briefly discussed.

George, I. M.↗

NASA NTRS1992

The x ray variability of NGC6814: Power spectra

Simulation techniques are used to obtain the X-ray variability power spectrum of unevenly sampled GINGA data from NGC6814. A simple power law is not an adequate description of the power spectrum, with the residuals showing excess power on timescales consistent with the periodicity seen in EXOSAT observations of this object. However the shape of the folded lightcurve is very different, with 3 main peaks, two of which are separated by an extremely sharp dip instead of the single peak and small harmonic structure observed by EXOSAT. Using the dip as a fiducial mark, a second GINGA observation of this source taken one year later is found to be consistent with being completely periodic and phase coherent with this first GINGA observation. Thus the period is consistent with being constant over a period of 6 years, but phase coherence is only maintained on timescales of approximately 1 year. Over 75 percent of the total source variability is due to the periodic component (r.m.s. amplitude of 36 percent). The residual variability can be described as the more usual 'flicker noise' f(exp -1.1) powerlaw. This shows no apparent high frequency break on timescales greater than 300 seconds. Subtle differences in the shape of the folded light curve with energy, and the very large amount of power in the periodic component suggest occultation as its origin, though amplification of variability from an X-ray emitting 'hot spot' at the disk inner radius through gravitational lensing is also possible. The former suffers from the very arbitrary nature of the periodic timescale, while the latter is unattractive as it cannot simply explain the lack of high frequency break in the residual power. That these models probably fail to provide an adequate explanation may be due to the added complexity of anisotropy of the X-ray emission, suggested by the discrepancy between the lack of soft photons implied by the flat spectrum and the copious source of soft photons available from reprocessing in the iron line producing material.

Done, C.↗

NASA NTRS1993

Multidisciplinary optimization of a controlled space structure using 150 design variables

A controls-structures interaction design method is presented. The method coordinates standard finite-element structural analysis, multivariable controls, and nonlinear programming codes and allows simultaneous optimization of the structure and control system of a spacecraft. Global sensitivity equations are used to account for coupling between the disciplines. Use of global sensitivity equations helps solve optimization problems that have a large number of design variables and a high degree of coupling between disciplines. The preliminary design of a generic geostationary platform is used to demonstrate the multidisciplinary optimization method. Design problems using 15, 63, and 150 design variables to optimize truss member sizes and feedback gain values are solved and the results are presented. The goal is to reduce the total mass of the structure and the vibration control system while satisfying constraints on vibration decay rate. Incorporation of the nonnegligible mass of actuators causes an essential coupling between structural design variables and control design variables.

James, Benjamin B.↗

NASA NTRS1993

Control design variable linking for optimization of structural/control systems

A method is presented to integrate the design space of structural/control system optimization problems in the case of linear state feedback control. Conventional structural sizing variables and elements of the feedback gain matrix are both treated as strictly independent design variables in optimization by extending design variable linking concepts to the control gains. Several approximation concepts including new control design variable linking schemes are used to formulate the integrated structural/control optimization problem as a sequence of explicit nonlinear mathematical programming problems. Examples which involve a variety of behavior constraints, including constraints on dynamic stability, damped frequencies, control effort, peak transient displacement, acceleration, and control force limits, are effectively solved by using the method presented.

Jin, Ik Min↗

NASA NTRS1993

Decision theory for computing variable and value ordering decisions for scheduling problems

Heuristics that guide search are critical when solving large planning and scheduling problems, but most variable and value ordering heuristics are sensitive to only one feature of the search state. One wants to combine evidence from all features of the search state into a subjective probability that a value choice is best, but there has been no solid semantics for merging evidence when it is conceived in these terms. Instead, variable and value ordering decisions should be viewed as problems in decision theory. This led to two key insights: (1) The fundamental concept that allows heuristic evidence to be merged is the net incremental utility that will be achieved by assigning a value to a variable. Probability distributions about net incremental utility can merge evidence from the utility function, binary constraints, resource constraints, and other problem features. The subjective probability that a value is the best choice is then derived from probability distributions about net incremental utility. (2) The methods used for rumor control in Bayesian Networks are the primary way to prevent cycling in the computation of probable net incremental utility. These insights lead to semantically justifiable ways to compute heuristic variable and value ordering decisions that merge evidence from all available features of the search state.

Linden, Theodore A.↗

NASA NTRS1992

Studying cataclysmic variables with the Hubble Space Telescope

The Hubble Space Telescope with its UV and imaging capabilities will allow new methods of investigating cataclysmic variables to be used. It provides the first opportunity to obtain high-time-resolution UV observations of cataclysmic variables, both spectroscopically and photometrically. Phase-resolved observations of eclipses can now be obtained which will allow the UV surface-brightness distributions in accretion disks to be studied. Eclipses of white dwarfs that were not visible in optical light will be detected in some systems. Other variations such as dips (in dwarf novae, intermediate polars, and polars), flickering, and oscillations can also be resolved for the first time in the UV. High-spatial-resolution, long-slit spectroscopy, and imaging with the FOC and WF/PC can be used to study the physical condition and morphology of ejected material around novae, recurrent novae, and precataclysmic variables. The UV and H-alpha imaging capabilities will also enable searches for cataclysmic variables in globular clusters to be carried out further into their cores than has been possible before.

Wood, Janet H.↗

NASA NTRS1992

The prediction of nonlinear dynamic loads on helicopters from flight variables using artificial neural networks

A method of indirectly monitoring component loads through common flight variables is proposed which requires an accurate model of the underlying nonlinear relationships. An artificial neural network (ANN) model learns relationships through exposure to a database of flight variable records and corresponding load histories from an instrumented military helicopter undergoing standard maneuvers. The ANN model, utilizing eight standard flight variables as inputs, is trained to predict normalized time-varying mean and oscillatory loads on two critical components over a range of seven maneuvers. Both interpolative and extrapolative capabilities are demonstrated with agreement between predicted and measured loads on the order of 90 percent to 95 percent. This work justifies pursuing the ANN method of predicting loads from flight variables.

Cook, A. B.↗

NASA NTRS1992

A survey of ultraviolet variability in blazars

This is the first comprehensive, consistent survey of the ultraviolet variability properties of a large sample of blazars. All of the 14 blazars with five or more observations in a single IUE camera showed significant variability. The strength of the ultraviolet variability is positively correlated with both the apparent ultraviolet luminosity and the degree of optical polarization of the source. This is the first clear indication that the ultraviolet continua of blazars are beamed. A search for rapid variability yielded nine 'events' with doubling time scales of 0.5-7 days. These rapid flux variations and correlations argue for the standard synchrotron self-Compton model, and against the accretion disk model, which would predict slower variations and the opposite sense of correlation.

Edelson, Rick↗

NASA NTRS1993

Mars dust storms - Interannual variability and chaos

The hypothesis is that the global climate system, consisting of atmospheric dust interacting with the circulation, produces its own interannual variability when forced at the annual frequency. The model has two time-dependent variables representing the amount of atmospheric dust in the northern and southern hemispheres, respectively. Absorption of sunlight by the dust drives a cross-equatorial Hadley cell that brings more dust into the heated hemisphere. The circulation decays when the dust storm covers the globe. Interannual variability manifests itself either as a periodic solution in which the period is a multiple of the Martian year, or as an aperiodic (chaotic) solution that never repeats. Both kinds of solution are found in the model, lending support to the idea that interannual variability is an intrinsic property of the global climate system. The next step is to develop a hierarchy of dust-circulation models capable of being integrated for many years.

Ingersoll, Andrew P.↗

NASA NTRS1920

Variable Pitch Propellers

In this report are described four different types of propellers which appeared at widely separated dates, but which were exhibited together at the last Salon de l'Aeronautique. The four propellers are the Chaviere variable pitch propeller, the variable pitch propeller used on the Clement Bayard dirigible, the variable pitch propeller used on Italian dirigibles, and the Levasseur variable pitch propeller.

Source record↗

NASA NTRS1993

Statistical analysis of environmental variability within the CELSS breadboard project's biomass production chamber

Variability in the aerial and root environments of NASA's Breadboard Project's Biomass Production Chamber (BPC) was determined. Data from two lettuce and two potato growouts were utilized. One growout of each crop was conducted prior to separating the upper and lower chambers; the other was subsequent to separation. There were little or no differences in pH, EC, or solution temperature between the upper and lower chamber or within a chamber. Variation in the aerial environment within a chamber was two to three times greater than variation between chambers for air temperature, relative humidity, and PPF. High variability in air velocity, relative to tray position, was observed. Separating the BPC had no effect on PPF, air velocity, solution temperature, pH, or EC. Separation reduced the gradient in air temperature and relative humidity between the upper and lower chambers, but increased the variability within a chamber. Variation between upper and lower chambers was within 5 percent of environmental set-points and of little or no physiological significance. In contrast, the variability within a chamber limits the capability of the BPC to generate statistically reliable data from individual tray treatments at this time.

Stutte, G. W.↗