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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Density fluctuations in the interstellar medium: Evidence for anisotropic magnetogasdynamic turbulence. II - Stationary structures

A model of anisotropic, plasma-fluid variations was used to investigate the unknown origin of the power spectra of interstellar electron fluctuations inferred by Armstrong, Cordes, and Rickett (1981). The modeled electron variations are interpreted as density components of an anisotropic stationary mode of nonlinear magnetogasdynamics-tangential pressure balances. It is suggested that the wavenumber spectra of electron variations are identical to the spectra of the convecting velocity fields over a wide range of wavenumbers.

Higdon, J. C.↗
NASA NTRS1986

The Berkeley EUV/FUV Shuttle Telescope - Observations of dust reflection in the FUV

Observations by the Berkeley EUV/FUV Shuttle Telescope of the FUV reflection from dust clouds are reported. Spectra of the diffuse UV background from regions of low and high hydrogen column density have been taken. The intensity of the continuum correlates well with the column density of neutral hydrogen, and the slope of this correlation is interpreted with a simple model of optically thin scattering to obtain a measurement of a combination of the albedo and asymmetry parameter in the FUV.

Hurwitz, Mark↗
NASA NTRS1987

High-velocity emission in young supernova remnants: SN 1006 and SN 1572

The paper reports the discovery of broad H-alpha emission from the SN 1006 remnant with a FWHM velocity of 2600 + or - 100 km/s. This emission is similar to that seen in the remnant of SN 1572 which has a FWHM for H-alpha of 1800 km/s. The nonradiative model was used to interpret the line widths and the derived shock velocity was compared with proper motion measurements to derive distances of 1.4-2.1 kpc to SN 1006 and 2.0-2.8 kpc to SN 1572.

Kirshner, Robert↗
NASA NTRS1987

Propagating shocks in the corona

High resolution observations performed with the Decimeter spectrograph and the multichannel receiver at Nancay were analyzed in the range of 25-75 MHz. Sixty Type II bursts were selected. In this frequency range, type II events are generally associated with other radio emissions (such as storms of type III-U-I bursts); they are preceeded or followed by groups of U-bursts. One third of type II events show a nonuniform frequency drift, usually a steep decrease followed by an abrupt increase. This phenomenon can be explained by the propagation of an extended disturbance through the ambient corona when the density gradient is enhanced. An empirical coronal model is proposed to interpret these observations. The observations at fixed frequency of Type II bursts including fundamental and harmonic components are analyzed. It is shown that the spectrum of the intensity fluctuations differs with the fundamental and the harmonic components. The origin of these differences is discussed.

Leblanc, Yolanda↗
NASA NTRS1987

Evanescent pressure gradient response in the upper ocean to subinertial wind stress forcing of finite wavelength

A schematic model is used to interpret field observations related to the mixed layer response to wind stress at subinertial frequencies. It is shown that subinertial density and pressure fluctuations can arise locally from the finite wavelength character of the wind stress forcing as a fundamental part of the upper ocean transient, wind-driven response on time scales of 2-10 pendulum days. Evanescent vertical motions arise which alter the density field of the pycnocline, and hence the pressure field over the entire upper ocean. It is thus found that in the real ocean driven by wind stress, a transient geostrophic response exists which can be as large or larger than the transient Eckman response.

White, Warren B.↗
NASA NTRS1989

Modeling and characterization of the Earth Radiation Budget Experiment (ERBE) nonscanner and scanner sensors

The Earth Radiation Budget Experiment (ERBE) is making high-absolute-accuracy measurements of the reflected solar and Earth-emitted radiation as well as the incoming solar radiation from three satellites: ERBS, NOAA-9, and NOAA-10. Each satellite has four Earth-looking nonscanning radiometers and three scanning radiometers. A fifth nonscanner, the solar monitor, measures the incoming solar radiation. The development of the ERBE sensor characterization procedures are described using the calibration data for each of the Earth-looking nonscanners and scanners. Sensor models for the ERBE radiometers are developed including the radiative exchange, conductive heat flow, and electronics processing for transient and steady state conditions. The steady state models are used to interpret the sensor outputs, resulting in the data reduction algorithms for the ERBE instruments. Both ground calibration and flight calibration procedures are treated and analyzed. The ground and flight calibration coefficients for the data reduction algorithms are presented.

Halyo, Nesim↗
NASA NTRS1988

Ice sheet studies using synthetic aperture radar

The objective was to demonstrate the utility of synthetic aperture radar in ice sheet studies. The major advantage of SAR imagery over visible imagery is the all-weather capability of radar and the ability to specify look angle. Available digital SAR imagery over ice sheets was collected and examined both qualitatively and quantitatively using corroborative data, such as LANDSAT imagery, to confirm feature identification and interpretations. A simple scattering model will be developed to assess the relative importance of surface topography, composition, and subsurface layering to the intensity of radar backscatter. Recommendations of system parameters will be made for optimal SAR operation over ice sheets.

Bindschadler, R.↗
NASA NTRS1988

Gravity waves in the upper atmosphere of Venus

The transfer function model of Mayr et al. (1984, 1987) for a dissipative multiconstituent atmosphere on earth was modified to describe gravity waves in the thermosphere of Venus in terms of spherical and Fourier harmonics. This model is used to interpret density perturbations in CO2, O, and He which were observed with the orbital neutral mass spectrometer on Pioneer Venus. Results indicate that, in agreement with observations, for localized sources in the lower atmosphere and thermosphere of the same magnitude day and night, the computed wave amplitudes at satellite altitudes are about a factor of 3 to 5 larger during the night than during the day (due to the large night to day increase in the thermospheric temperature); waves excited in the lower atmosphere propagate more obliquely, and geometric attenuation and dissipation are less important than for the thermosphere. Due to wind-induced diffusion, the computed wave amplitudes for CO2, O, and He are of comparable magnitude (with the magnitude of CO2 being the largest).

Mayr, H. G.↗
NASA NTRS1988

Tropopause adjustment to tropical cyclones as inferred from satellite ozone observations

Data from the Total Ozone Mapping Spectrometer on the Nimbus-7 are used to map the climatological total ozone within and around tropical cyclones. A tropical cyclone numerical model is used to interpret total ozone patterns. The total ozone maps are used to diagnose storm intensity and to differentiate between intesifying and weakening tropical cyclones. The tropical cyclone intensity versus total ozone anomaly distribution is examined and examples of mapping the total ozone of tropical cyclones in the period 1979-1982 are presented.

Rodgers, Edward B.↗
NASA NTRS1990

Far-infrared spectroscopy of NGC 6946, IC 342, and Arp 299

Researchers investigated the physical conditions in the infrared bright galaxies NGC 6946, IC 342, and Arp 299 through measurements of far-infrared emission lines from Si II, O I, C II, and O III using the facility Cooled Grating Spectrometer on the Kuiper Airborne Observatory. These data are interpreted using our theoretical models for photodissociation regions and H II regions. For the central 45 inches of these galaxies, researchers determined that the dominant excitation mechanism for the far infrared radiation (FIR) lines is far ultraviolet radiation (FUR) radiation from young stars, and the authors derived the total mass, density, and temperature of the warm atomic gas and the typical sizes, number densities, and filling factors for the interstellar clouds.

Lord, Steven D.↗
NASA NTRS1991

The eclipsing AM Herculis variable H1907 + 690

The discovery is reported of an eclipsing cataclysmic variable that exhibits up to 10 percent circular polarization at optical wavelengths, securing its classification as an AM Herculis type binary. The object, H1907 + 609, was located with the guidance of X-ray positions from the HEAO 1 survey. Optical CCD photometry exhibits deep eclipses, from which is derived a precise orbital period of 1.743750 hr. The eclipse duration suggests an inclination angle about 80 deg for a main-sequence secondary star. The optical flux has been persistently faint during observations spanning 1987-1990, while the X-ray measurements suggest long-term X-ray variability. The polarization and photometric light curves can be interpreted with a geometric model in which most of the accretion is directed toward a single magnetic pole, with an accretion spot displaced about 17 deg in longitude from the projection of the secondary star on the white dwarf surface.

Remillard, R. A.↗
NASA NTRS1991

Remote sensing of the Martian surface

Researchers investigated the physical properties of the Martian surface as inferred from a combination of orbiting and earth-based remote sensing observations and in-situ observations. This approach provides the most detailed and self-consistent view of the global and regional nature of the surface. Results focus on the areas of modeling the diurnal variation of the surface temperature of Mars, incorporating the effects of atmospheric radiation, with implications for the interpretation of surface thermal inertia; modeling the thermal emission from particulate surfaces, with application to observations of the surfaces of the Earth, Moon, and Mars; modeling the reflectance spectrum of Mars in an effort to understand the role of particle size in the difference between the bright and dark regions; and determining the slope properties of different terrestrial surfaces and comparing them with planetary slopes derived from radar observations.

Jakosky, Bruce M.↗
NASA NTRS1991

Wind tunnel investigation of vortex flows on F/A-18 configuration at subsonic through transonic speed

A wind tunnel experiment was conducted in the David Taylor Research Center 7- by 10-Foot Transonic Tunnel of the wing leading-edge extension (LEX) and forebody vortex flows at subsonic and transonic speeds about a 0.06-scale model of the F/A-18. The primary goal was to improve the understanding and control of the vortical flows, including the phenomena of vortex breakdown and vortex interactions with the vertical tails. Laser vapor screen flow visualizations, LEX, and forebody surface static pressures, and six-component forces and moments were obtained at angles of attack of 10 to 50 degrees, free-stream Mach numbers of 0.20 to 0.90, and Reynolds numbers based on the wing mean aerodynamic chord of 0.96 x 10(exp 6) to 1.75 x 10(exp 6). The wind tunnel results were correlated with in-flight flow visualizations and handling qualities trends obtained by NASA using an F-18 High-Alpha Research Vehicle (HARV) and by the Navy and McDonnell Douglas on F-18 aircraft with LEX fences added to improve the vertical tail buffet environment. Key issues that were addressed include the sensitivity of the vortical flows to the Reynolds number and Mach number; the reduced vertical tail excitation, and the corresponding flow mechanism, in the presence of the LEX fence; the repeatability of data obtained during high angle-of-attack wind tunnel testing of F-18 models; the effects of particle seeding for flow visualization on the quantitative model measurements; and the interpretation of off-body flow visualizations obtained using different illumination and particle seeding techniques.

Erickson, Gary E.↗
NASA NTRS1993

Effects of Buoyancy on Laminar, Transitional, and Turbulent Gas Jet Diffusion Flames

Gas jet diffusion flames have been a subject of research for many years. However, a better understanding of the physical and chemical phenomena occurring in these flames is still needed, and, while the effects of gravity on the burning process have been observed, the basic mechanisms responsible for these changes have yet to be determined. The fundamental mechanisms that control the combustion process are in general coupled and quite complicated. These include mixing, radiation, kinetics, soot formation and disposition, inertia, diffusion, and viscous effects. In order to understand the mechanisms controlling a fire, laboratory-scale laminar and turbulent gas-jet diffusion flames have been extensively studied, which have provided important information in relation to the physico-chemical processes occurring in flames. However, turbulent flames are not fully understood and their understanding requires more fundamental studies of laminar diffusion flames in which the interplay of transport phenomena and chemical kinetics is more tractable. But even this basic, relatively simple flame is not completely characterized in relation to soot formation, radiation, diffusion, and kinetics. Therefore, gaining an understanding of laminar flames is essential to the understanding of turbulent flames, and particularly fires, in which the same basic phenomena occur. In order to improve and verify the theoretical models essential to the interpretation of data, the complexity and degree of coupling of the controlling mechanisms must be reduced. If gravity is isolated, the complication of buoyancy-induced convection would be removed from the problem. In addition, buoyant convection in normal gravity masks the effects of other controlling parameters on the flame. Therefore, the combination of normal-gravity and microgravity data would provide the information, both theoretical and experimental, to improve our understanding of diffusion flames in general, and the effects of gravity on the burning process in particular.

Bahadori, M. Yousef↗
NASA NTRS1993

Rb-Sr and Sm-Nd isotopic variations in dissected crustal xenoliths

The effect of magma-xenolith interaction on the Rb-Sr and Sm-Nd isotopic systematics was investigated by studying the Rb-Sr and Sm-Nd variations in dissected crustal xenoliths sampled from different localities across Scotland. The Nd isotopic compositions were found to be virtually uniform across each xenolith, but significant variations were found in Rb, Sr, and REE concentrations, as well as in Rb/Sr and Sm/Nd ratios and Sr isotopic composition. Most of these variations appear to be inherited from the protolith, but, in one case, they have been modified by melt infiltration from the host magma. The results lend confidence to the interpretation of the isotopic and chemical compositions of xenoliths transported in basaltic magmas as reflecting their source regions, but they also highlight the potential problems of interpreting Sm-Nd model ages from metamorphic rocks.

Lee, Der-Chuen↗
NASA NTRS1993

Rheology of water and ammonia-water ices

Creep experiments on fine-grained water and ammonia-water ices have been performed at one atmosphere and high confining pressure in order to develop constitutive relationships necessary to model tectonic processes and interpret surface features of icy moons of the outer solar system. The present series of experiments explores the effects of temperature, strain rate, grain size, and melt fraction on creep strength. In general, creep strength decreases with increasing temperature, decreasing strain rate, and increasing melt fraction. A transition from dislocation creep to diffusion creep occurs at finer grain sizes, higher temperatures, and lower strain rates.

Goldsby, D. L.↗
NASA NTRS1993

On reflection of Alfven waves in the solar wind

We have revisited the problem of propagation of toroidal and linear Alfven waves formulated by Heinemann and Olbert (1980) to compare WKB and non-WKB waves and their effects on the solar wind. They considered two solar wind models and showed that reflection is important for Alfven waves with periods of the order of one day and longer, and that non-WKB Alfven waves are no more effective in accelerating the solar wind than WKB waves. There are several recently published papers which seem to indicate that Alfven waves with periods of the order of several minutes should be treated as non-WKB waves and that these non-WKB waves exert a stronger acceleration force than WKB waves. The purpose of this paper is to study the origin of these discrepancies by performing parametric studies of the behavior of the waves under a variety of different conditions. In addition, we want to investigate two problems that have not been addressed by Heinemann and Olbert, namely, calculate the efficiency of Alfven wave reflection by using the reflection coefficient and identify the region of strongest wave reflection in different wind models. To achieve these goals, we investigated the influence of temperature, electron density distribution, wind velocity and magnetic field strength on the waves. The obtained results clearly demonstrate that Alfven wave reflection is strongly model dependent and that the strongest reflection can be expected in models with the base temperatures higher than 10(exp 6) K and with the base densities lower than 7 x 10(exp 7) cm(exp -3). In these models as well as in the models with lower temperatures and higher densities, Alfven waves with periods as short as several minutes have negligible reflection so that they can be treated as WKB waves; however, for Alfven waves with periods of the order of one hour or longer reflection is significant, requiring a non-WKB treatment. We also show that non-WKB, linear Alfven waves are always less effective in accelerating the plasma than WKB Alfven waves. Finally, it is evident from our results that the region of strongest wave reflection is usually located at the base of the models, and hence that interpretation of wave reflection based solely on the reflection coefficient can be misleading.

Krogulec, M.↗
NASA NTRS1994

High charge state carbon and oxygen ions in Earth's equatorial quasi-trapping region

Observations of energetic (1.5 - 300 keV/e) medium-to-high charge state (+3 less than or equal to Q less than or equal to +7) solar wind origin C and O ions made in the quasi-trapping region (QTR) of Earth's magnetosphere are compared to ion trajectories calculated in model equatorial magnetospheric magnetic and electric fields. These comparisons indicate that solar wind ions entering the QTR on the nightside as an energetic component of the plasma sheet exit the region on the dayside, experiencing little or no charge exchange on the way. Measurements made by the CHarge Energy Mass (CHEM) ion spectrometer on board the Active Magnetospheric Particle Tracer Explorer/Charge Composition Explorer (AMPTE/CCE) spacecraft at 7 less than L less than 9 from September 1984 to January 1989 are the source of the new results contained herein: quantitative long-term determination of number densities, average energies, energy spectra, local time distributions, and their variation with geomagnetic disturbance level as indexed by Kp. Solar wind primaries (ions with charge states unchanged) and their secondaries (ions with generally lower charge states produced from primaries in the magnetosphere via charge exchange)are observed throughout the QTR and have distinctly different local time variations that persist over the entire 4-year analysis interval. During Kp larger than or equal to 3 deg intervals, primary ion (e.g., O(+6)) densities exhibit a pronounced predawn maximum with average energy minimum and a broad near-local-noon density minimum with average energy maximum. Secondary ion (e.g., O(+5)) densities do not have an identifiable predawn peak, rather they have a broad dayside maximum peaked in local morning and a nightside minimum. During Kp less than or equal to 2(-) intervals, primary ion density peaks are less intense, broader in local time extent, and centered near midnight, while secondary ion density local time variations diminish. The long-time-interval baseline helps to refine and extend previous observations; for example, we show that ionospheric contribution to O(+3)) is negligible. Through comparison with model ion trajectories, we interpret the lack of pronounced secondary ion density peaks colocated with the primary density peaks to indicate that: (1) negligible charge exchange occurs at L greater than 7, that is, solar wind secondaries are produced at L less than 7, and (2) solar wind secondaries do not form a significant portion of the plasma sheet population injected into the QTR. We conclude that little of the energetic solar wind secondary ion population is recirculated through the magnetosphere.

Christon, S. P.↗