SEARCH · Search NASA

Results for “time-scale”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3

All sources NASA NTRS DOE OSTI

NASA NTRS1983

Time-resolved spectrophotometry of the emission lines in the galactic X-ray source H2252-035

The galactic X-ray source H2252-035 has been the object of a concentrated observational effort, since Griffiths et al. (1980) have identified it with a cataclysmic variable-like spectrum. Since the observed 859 s periodicity corresponds to the beat frequency between the X-ray and orbital periods, it has been interpreted as related to a reprocessing of the X-ray flux into optical emission in some region of the system which is rotating with the orbital period and heated by beamed X-ray emission from the compact star. In connection with the present investigation, H2252-035 has been observed with a 120 inch (3 m) telescope and image dissector scanner on 1980 September 9/10 and on 1981 September 20/21. At a time resolution of approximately 2 minutes, four bright emission lines showed short time-scale flickering and a pronounced orbital period variation in phase with the continuum modulation. A two-component model for the line emission region is proposed.

Clarke, J. T.↗

NASA NTRS1982

Charge neutralization as studied experimentally and theoretically

Problems regarding an isolated sphere which emits negative charge are considered. Such a sphere could charge up to large potentials which would inhibit the electron beam from leaving the vicinity of the body. In order to avoid charging to high potentials, a vehicle must attract a return current equal to the emitted current. The present investigation is concerned with theoretical models of some processes believed to be important for the vehicle neutralization problem under various conditions. Attention is given to general time-scale considerations, the low-altitude regime, the high-altitude regime, vehicle-induced discharge, and beam-plasma discharge. The general pattern which emeres as a result of measurements is that below altitudes of approximately 125 + or - 5 km the vehicle potential rarely rises more than several tens of volts.

Linson, L. M.↗

NASA NTRS1984

Aeolian processes on Venus

Many of the questions regarding aeolian processes on Venus and the subsequent implications for surface history involve understanding the physics of particle motion in the venusian environment. The surface environment of Venus is simulated as closely as practicable using the Venus Wind Tunnel and to determine threshold wind speeds, particle flux, particle velocities, and the characteristics of various aeolian bedforms. Despite the relatively low wind speeds on Venus, the flux of windblown material on Venus is potentially high. A high fraction of material is transported as surface creep by rolling, estimates yield rates up to 100 kg per cm lane width per year depending upon the availability of material and wind frequency, suggesting that the formation of lowland plains by aeolian processes and the burial of various landforms such as impact craters could occur on short geological time-scales. Wind tunnel simulations demonstrate that aeolian processes may be very effective in modifying the surface through erosion and deposited and may have an important influence on the composition of the atmosphere.

Greeley, R.↗

NASA NTRS1984

Lubricant Rheology in Concentrated Contacts

A revue of experimental investigations of lubricant behaviour in highly stressed situations shows that a Newtonian model for lubricant rheology is insufficient for explanation of traction behaviour. The oil film build-up, on the other hand, is fairly well predicted using a Newtonian lubricant model except at high slide to roll ratios and at very high loads, where the non-Newtonian behaviour starts to be important already outside the Hertzian contact area. Two main types of experiments are reported: static and dynamic. In static experiments the pressure is typically applied to the lubricant more than a million times longer than in an EHD contact. Depending on the pressure-temperature history of the experiment the lubricant will become a crystallized or amorphous solid at high pressures. In dynamic experiments, where the pressure is applied as short time as in real EHD contacts, the oil is in an amorphous solid state. Depending on the viscosity, time-scale, elasticity of the oil and the bearing surfaces, the oil film pressure, shear strain rate and the type of lubricant, different properties of the oil are important for prediction of shear stresses in the oil. This can be seen from the different proposed models for the lubricant, where it is described as being for instance a Newtonian liquid, an elastic liquid, a plastic liquid and an elastic-plastic solid.

Bo O Jacobson↗

NASA NTRS1983

Electrostatic Kelvin-Helmholtz instability in a radially injected plasma cloud

An elecrostatic finite-sized particle simulation model is used to study the early time-scale phenomena asociated with an impulsively injected plasma, expanding radially, normal to a strong ambient magnetic field. Results of the simulation show the early formation of a radial polarization electric field due to ion-electron charge separation which causes electrons to drift in the azimuthal direction. Velocity shear within this motion gives rise to a Kelvin-Helmholtz (dioxotron) instability, creating a radial fluted pattern as the plasma expands. In the nonlinear stage of instability, azimuthal variations in the electric field are observed which cause electrons to drift across the magnetic field, thereby reducing the space charge created by energetic ion expansion. The results of a linear stability analysis reveal that for a decreasing amount of charge separation in the plasma, the number of unstable azimuthal modes at maximum growth rate increases. Also, as the thickness of the electron ring increases, a fewer number of unstable modes develops for a given amount of charge separation. It is concluded that electrons become unstable at the very early time scale, within an ion gyroperiod, and that unstable electrons lead to modification of the ion dynamics.

Sydora, R. D.↗

NASA NTRS1984

X-ray observations of a large sample of cataclysmic variable stars using the Einstein Observatory

This paper presents the results of an X-ray survey of 31 known or suspected cataclysmic variables. Eighteen of these close binary systems are detected with inferred luminosities in the 0.1-4.0 keV band of between 10 to the 30th and 10 to the 32nd erg/sec. The majority have relatively hard X-ray spectra (kT greater than 2 keV) irrespective of luminosity state. Of seven dwarf novae observed during optical outbursts only U Gem exhibited enhanced ultrasoft X-ray emission (kT of about 10 eV) in addition to weak, hard X-ray emission. Variability of the X-ray flux is observed in many of these stars, on time-scales ranging from tens of seconds to hours. The contribution to the flux from extended X-ray emission is investigated for SU UMa and GK Per. Several possibilities for the origin of the hard X-rays are considered.

Cordova, F. A.↗

NASA NTRS1984

Variability of Neptune

Earth based observers of Neptune found that the planet varies in brightness at various wavelengths in ways that suggest that changes occur in the planet's atmosphere on several different time-scales. Global inhomogeneities in high altitude haze distribution that are stable for several days permit measurements of the planet's rotation period (about 18 hours), but this stability sometimes breaks down, obscuring the diurnal lightcurve. In addition, there is an apparent long term variability of the brightness of Neptune in anticorrelation with the cycle of solar activity. This slow variability of low amplitude may be punctuated by outbursts of high altitude condensation of particles in the atmosphere, whose decay time is several months.

Dale P Cruikshank↗

NASA NTRS1985

When did the average cosmic ray flux increase?

A new 129 to 129 Xe method to obtain cosmic ray exposure ages and to study the average cosmic ray flux on a 10 to the 7th power to 10 to the 8th power year time-scale was developed. The method is based on secondary neutron reactions on Te in troilite and the subsequent decay of 129I, the reaction product to stable 129 Xe. The first measurements of 129 I and 129 Xe in aliquot samples of a Cape York troilite sample are reported.

Nishiizumi, K.↗

NASA NTRS1984

Photometry of CPD - 48 deg 1577

U, B, and V photometry is presented for CPD - 48 deg 1577, the newly discovered cataclysmic variable with the greatest known apparent brightness. The light curves are dominated by flickering but do suggest an orbital period of nearly 4 hr. Power spectra of 5 nights of U-band observations failed to reveal the presence of any periodic or quasi-periodic oscillations. Autocorrelation functions suggest that two time-scales exist in the flickering activity, one of 77 s and the other 500 s. The implications for a shot noise model of flickering are discussed.

Williams, G. A.↗

NASA NTRS1983

Climatological studies of cloudiness

The influence of subresolution clouds on estimates of synoptic-scale averages and the effect of synoptic-scale clouds on estimates of planetary scale interannual variability are discussed. It is shown that climatic noise is strongly dependent on the season and geographic region, so that the largest anomaly is not always the most significant. The role of advection in determining the time-scales and degrees of freedom of synoptic scale cloudiness is empirically demonstrated and described in terms of a simple model of stochastic 'cloud patches'.

Cahalan, R. F.↗

NASA NTRS1985

Contribution to the CIRA Model from Ground-based Lidar

A new technique, the Rayleigh-Lidar Sounding, has been developed recently to probe the Middle Atmosphere and has demonstrated its ability to provide density and temperature profiles in the height range 30 to 90 km. It has been limited until now to ground-based observations. The set of data, as it exists to-day, has been obtained from a unique midlatitude site. Even with this limiting factor, it is thought to be a useful complement to a global model, mainly because of its spatial and temporal resolutions, usually not available from global data. Such resolutions give an insight into the atmospheric variability as a function of height and time, for time-scale ranging from minutes to years. The process is briefly described. A description of the analysis and its results is given.

Chanin, M. I.↗

NASA NTRS1986

Live 129I-129Xe dating

A technique of cosmic ray exposure age dating using cosmic ray produced I-129 and Xe-129 components is discussed. The live I-129 - Xe-129 method provides an ideal monitor for cosmic ray flux variations on the 10(7)y - 10(8)y time-scale. It is based on low-energy neutron reactions on Te, and these data, when coupled to those from other methods, may facilitate the detection of complex exposure histories.

Marti, K.↗

NASA NTRS1985

A comparison of two- and three-dimensional tracer transport within a stratospheric circulation model

Use of the residual circulation for stratospheric tracer transport has been compared to a fully three-dimensional calculation. The wind fields used in this study were obtained from a global, semispectral, primitive equation model, extending from 10 to 100 km in altitude. Comparisons were done with a passive tracer and an ozone-like substance over a two-month period corresponding to a Northern Hemisphere winter. It was found that the use of the residual circulation can lead to errors in the tracer concentrations of about a factor of 2. The error is made up of two components. One is fluctuating with a period of approximately one month and reflects directly the wave transience that occurs on that time-scale. The second part is increasing steadily over the integration period and results from an overestimate of the vertical transport by the residual circulation. Furthermore, the equatorward and upward mixing that occurs with transport by the three-dimensional circulation at low latitudes is not well reproduced when the residual circulation is used.

Schneider, H.-R.↗

NASA NTRS1985

Satellite observations of sea ice

An overview is presented of Antarctic and Arctic sea ice studies using data from the Nimbus-5 ESMR and the Nimbus-7 SMMR passive microwave radiometers. Four years (1973-1976) of ESMR data for the Antarctic Ocean define the characteristics of the seasonal cycle including regional contrasts and interannual variations. Major advances include the discovery of the Weddell polynya and the presence of substantial areas of open water in the Antarctic winter pack ice. Regional differences in sea ice extent on time-scales of about a month are shown to be associated with variations in surface-wind fields. In the Arctic, the computation of sea ice concentration is complicated by the presence of multiyear ice, but the amount of multiyear ice becomes an important measurable quantity with dual-polarized, multifrequency passive microwave sensors. Analysis of SMMR data demonstrates its advantage for studying the spatial and temporal variability of the Arctic ice cover. Large observed interannual variations in the distribution of the multiyear pack ice and the presence of significant divergent areas in the central Arctic during winter contrast markedly with the classical view of the Arctic pack ice.

Cavalieri, D. J.↗

NASA NTRS1985

Variability of Neptune

Cruikshank, D. P.↗

NASA NTRS1986

Rapid fluctuations in solar flares

Study of rapid fluctuations in the emission of radiation from solar flares provides a promising approach for probing the magneto-plasma structure and plasma processes that are responsible for a flare. It is proposed that elementary flare bursts in X-ray and microwave emission may be attributed to fine structure of the coronal magnetic field, related to the aggregation of photospheric magnetic field into magnetic knots. Fluctuations that occur on a subsecond time-scale may be due to magnetic islands that develop in current sheets during magnetic reconnection. The impulsive phase may sometimes represent the superposition of a large number of the elementary energy-release processes responsible for elementary flare bursts. If so, the challenge of trying to explain the properties of the impulsive phase in terms of the properties of the elementary processes must be faced. Magnetic field configurations that might produce solar flares are divided into a number of categories, depending on: whether or not there is a filament; whether there is no current sheet, a closed current sheet, or an open current sheet; and whether the filament erupts into the corona, or is ejected completely from the Sun's atmosphere. Analysis of the properties of these possible configurations is compared with different types of flares, and to Bai's subdivision of gamma-ray/proton events.

Sturrock, Peter A.↗

NASA NTRS1986

Calculations of the accretion and evolution of giant planets The effects of solid cores

The present calculation of giant planet evolution proceeds under the hypothesis that the solid cores formed by small particle accretion later attracted their gaseous envelopes from the solar nebula gravitationally. Evolutionary calculations are presented for the beginning of gaseous envelope formation around the core mass; the growth of core and envelope to a critical core mass; the derivation of high luminosity from the envelope mass due to gravitational contraction, and the termination of both core and envelope accretion on a time-scale of 5 Gyr.

Bodenheimer, P.↗

NASA NTRS1986

A simple diagnostic tool for the investigation of persistent phenomena with application to the summer 1980 U.S. heat wave

A simple and readily-computed index for diagnostic and numerical weather prediction studies of heat waves and other persistent anomalies has been developed. The index takes into account the two important factors of intensity and persistence and it is highly correlated to the surface air temperature on a variety of time-scales. The utilization of this index for investigating the severe, summer 1980 U.S. heat wave and drought results in an instructive description of the spatial and temporal development of the phenomenon. An assessment of the skill of the Goddard Laboratory for Atmospheres (GLA) model in forecasting the initiation and breakdown of the heat wave in terms of the index is also illustrated.

Wolfson, Noah↗