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At least 1,135 records · Page 63
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NASA NTRS1987

Large-scale motions on the sun - An overview

The history and present status of observations of large-scale velocity fields in the solar atmosphere are reviewed. Observations of the torsional oscillation and of mean meridional circulation suggest a connection of large-scale dynamics with the solar cycle. Significant problems must be solved before Doppler observations can match the precision of tracer measurements, particularly allowing for the effects of changes in line-profile asymmetries and for scattered light. Coordinated observations would establish the reliability of Doppler techniques, but Doppler measurements with precision of order 1 m/s made in a proper spatial-temporal window appear necessary for the identification of sub-global velocity fields varying with time-scales less than that of the solar cycle.

Bogart, Richard S.↗
NASA NTRS1987

The unusual dust-scattering properties in the reflection nebula CED 201

Spectrophotometric data available from ground-based and IUE observations of the reflection nebula CED 201 and its illuminating star BD + 69.1231 deg are analyzed to deduce the most likely star-nebula geometry and the wavelength dependence of the scattering characteristics of the nebular grains. The dust albedo is found to decline from 0.72 at V to probably less than 0.2 at lambda less than 2000 A, explaining the unusual red color of this reflection nebula. The wavelength dependence of the optical depth and of the phase function asymmetry in the visible suggests the existence of a narrow size distribution of grains, skewed toward larger than normal particle sizes. CED 201 is one of two reflection nebulae with positive evidence for enhanced scattering in the long-wavelength wing of the 2175 A dust extinction feature, also pointing toward larger than average grains.

Witt, A. N.↗
NASA NTRS1987

Planetary satellites

Spectrophotometric observations of the atmosphereless planetary satellites obtained with the IUE since its launch in 1978 are reviewed. The value of IUE data in complementing the images obtained by planetary probes is indicated; the techniques used to interpret IUE images of the satellites and extract textural information are discussed; and data on the Gallilean satellites of Jupiter and the satellites of Saturn are presented in extensive tables and graphs and characterized in detail. Consideration is given to the broadband geometric albedos, the orbital phase curves, hemispherical spectral asymmetries, spectral orbital phase variations, and the prospects for further observations with the Space Telescope and other earth-orbiting platforms.

Nelson, Robert M.↗
NASA NTRS1987

Kinematic properties of the neutral gas outflow from comet P/Halley

High-resolution infrared spectra of H2O in Comet Halley, obtained by the Kuiper Airborne Observatory, were used to characterize the dynamics of the neutral gas outflow from the comet. The spatial distribution and the expansion velocity of the H2O molecules were deduced from the positions, widths, and shapes of the H2O lines. The character of line profiles suggested that the H2O distribution was anisotropic in the circular field-of-view of aproximately 10,000 radius centered on the nucleus. Direct observations of the H2O spatial brightness distribution showed that the outflow was concentrated in the sunward direction (although some H2O emission was also detected in the tail). An obvious property of the gas outflow from the Comet P/Halley was its variability. The specific intensities of the H2O lines demonstrated that the gas production from the comet varied significantly on many time scales; there were pre- and postperihelion asymmetries, day-to-day variations, and outbursts.

Larson, H. P.↗
NASA NTRS1987

Infrared investigation of water in Comet P/Halley

The results of the investigation of gaseous water, detected in the IR spectra of the Comet Halley's coma taken by the NASA's Kuiper Airborne Observatory, are summarized. High-resolution spectra revealed, near the wavelength of 2.65 microns, solar-pumped fluorescent emission in the (001-000) band of H2O and three lines of the (011-010) 'hot' band. The absolute production rate of H2O and its spatial distribution, temporal variability, and outflow velocity were measured along with the H2O ortho/para ratio. The measured line intensities indicate that water in the coma is rotationally relaxed. The aperture-averaged excitation temperatures for the low-lying rotational transitions were about 40 K, indicating that the coma kinetic temperature is extremely cold. Significant short-term temporal variations in the H2O line intensities were detected together with a strong pre- to postperihelion asymmetry (a factor of about 7) in the water production rate at a heliocentric distance of about 1 AU.

Weaver, H. A.↗
NASA NTRS1987

Plasma wave measurements in the magnetosphere of Uranus

As Voyager 2 traversed the magnetosphere of Uranus, the plasma-wave instrument detected very significant phenomena related to local wave-particle interactions, radio emissions, and dust impacts. Here the region between the inbound and outbound bow shock traversals (covering a distance of more than 250 Uranus radii) is considered, and it is demonstrated that intense plasma-wave activity developed only in the inner magnetosphere (r less than 12 Uranus radii); this result is similar to that found at Saturn, but it is in marked contrast with the Jupiter case, where very strong wave activity was detected out to distances of 250 Jupiter radii. The Uranus plasma-wave observations in the inner magnetosphere are compared with corresponding results from the Jupiter and Saturn encounters, and it is shown that the Uranus wave measurements are unique in several significant ways. These new aspects include: (1) the detection of a marked inbound-outbound asymmetry and (2) the detection of whistler-mode waves that yield the strongest wave-particle interactions found in outer planet magnetospheres.

Scarf, F. L.↗
NASA NTRS1987

Survey of electrons in the Uranian magnetosphere - Voyager 2 observations

Results of an analysis of the Voyager 2 plasma-science-experiment electron measurements made during the Uranus encounter are presented. The energy coverage is in the range of 10 to 5950 eV. The large day-night asymmetry together with the spin axis alignment with the solar direction and the large tilt of the planetary magnetic dipole suggest that solar-wind-driven time-dependent magnetospheric convection will be an important transport mechanism within the Uranian magnetosphere. The steady state convection time of the plasma is estimated to be between 1 and 3 days.

Sittler, Edward C., Jr.↗
NASA NTRS1988

On nonsteady accretion in stellar wind-fed X-ray sources

The paper reports on the results of hydrodynamical simulations of nonaxisymmetric gas flow past a finite-sized gravitating object. The asymmetry in the flow is assumed to be due to a transverse density gradient. The solutions are found to be highly time-dependent and exhibit phases in which a disk forms. The direction of circulation of the disk alternates between the direct and counter sense on time scales of approximately 1 hr. While the disk is present, the specific angular momentum is high, and the mass accretion rate, low. The angular momentum capture oscillates about a mean which secularly increases. Such temporal behavior may be relevant to those X-ray pulsing systems (e.g., Vela X-1) which exhibit fluctuations in the X-ray luminosity and pulse period on time scales much shorter than the orbital period of the binary system.

Taam, Ronald E.↗
NASA NTRS1988

On the relationship between dynamics of the polar thermosphere and morphology of the aurora - Global-scale observations from Dynamics Explorers 1 and 2

The effect of the neutral-air circulation in the high-latitude F region on the spatial distribution of the aurora is investigated on the basis of simultaneous global-scale auroral images and neutral-wind vectors obtained over the northern polar cap by the DE 1 and DE 2 satellites in December 1981. The results are presented graphically, along with DE 2 data on the composition, ion densities, and cross-track ion drift of the neutral winds and ISEE-3 IMF measurements, and analyzed. It is found that the large-scale features of the neutral circulation follow the substorm-driven expansion and contraction of the auroral oval, and that the drag effect from ions convecting sunward extends to much lower latitudes and has a more complex morphology than that predicted by GCMs. A number of asymmetries in the flow patterns are considered in detail.

Killeen, T. L.↗
NASA NTRS1988

Plasma deposition of amorphous hydrogenated carbon films on III-V semiconductors

Amorphous hydrogenated carbon films were grown on GaAs, InP and fused silica substrates using plasmas generated from hydrocarbon gases. Methane and n-butane sources were utilized. The effects of flow rate and power density on film growth were investigated. Carbon was the major constituent in the films. The degree of asymmetry at the carbon-semiconductor interface was approximately independent of the power density. Different H-C bonding configurations were detected by the technique of secondary-ion mass spectrometry. Band gaps up to 3 eV were obtained from optical absorption studies. Breakdown strengths as high as 600 MV/m were measured.

Pouch, John J.↗
NASA NTRS1988

The generation mechanism of mixed Rossby-gravity waves in the equatorial troposphere

Numerical experiments are performed to clarify the excitation mechanism of mixed Rossby-gravity (Yanai) waves in the tropical troposphere, as well as the selection of zonal wavenumbers four through five and of the five-day period. The results show that the nonlinear wave-CISK mechanism can select the same wavenumber and period of Yanai waves as observed, that in the absence of asymmetries in lateral forcing, symmetric Kelvin modes are dominant over antisymmetric Yanai modes, and that if lateral antisymmetric forcing with arbitrary periods is given, the correct spectral peak of Yanai waves becomes apparent. It is shown how the nonlinear wave-CISK mechanism selects the observed spectral peaks in zonal wavenumber and frequency. The question of why symmetric modes dominate antisymmetric Yanai waves is addressed, as is the reason why the tropical atmosphere selects the periods of Yanai waves independently of those of lateral forcing.

Itoh, Hisanori↗
NASA NTRS1987

Multiple trellis coded modulation (MTCM) performance on a fading mobile satellite channel

The author recently introduced the notion of multiple trellis coding, in which more than one channel symbol per trellis branch is transmitted. He showed that on the ideal additive white Gaussian noise (AWGN) channel, the combination of multiple trellis coding with M-ary modulation yields a performance gain with symmetric signal sets comparable to and in some cases better than that previously achieved only with signal constellation asymmetry. The combination of conventional trellis coding with multiple phase-shift-keyed (MPSK) signaling has recently been shown by the author to be a well-suited modulation/coding scheme for transmission over the fading mobile satellite channel. In particular, a rate 2/3 coded 8-PSK scheme operating at 4800 b/s is currently under development for use in NASA's Mobile Satellite Experiment (MSAT-X). The author applies the multiple trellis-coded modulation technique in the same fading mobile satellite environment, extending the analysis results previously found for its performance over the AWGN channel to the MSAT-X channel.

Simon, Marvin K.↗
NASA NTRS1988

A general model for Io's neutral gas clouds. II - Application to the sodium cloud

A previously developed neutral cloud model is applied to the neutral sodium cloud produced by Io. Past models for the cloud are reviewed and the motivation for developing better models is discussed. The interactions of sodium in the plasma torus are discussed. It is shown how the new model can be used to provide a consistent physical explanation for the basic physical characteristics of the spatially extended Io sodium cloud and for the east-west intensity asymmetry observed very near Io. This leads to definite conclusions regarding the flux distribution and density of sodium in Io's exosphere, the level of the collisional exobase in the atmosphere, the nature of the interaction of the corotating plasma torus and the satellite atmosphere/corona, and the presence of an east-west electric field in the planetary system.

Smyth, W. H.↗
NASA NTRS1987

Chromospheric downflow velocities as a diagnostic in solar flares

The dynamics of chromospheric condensations driven by evaporation during the impulsive phase of solar flares are explored. Specifically, it is found that the maximum chromospheric downflow speed is equal to 0.4(F/rho sub ch) exp 1/3, where F is that part of the flare energy flux driving chromospheric evaporation, and rho sub ch is the mass density in the preflare chromosphere just below the preflare transition region. This implies that chromospheric downflows as measured by H-alpha asymmetries may be a powerful probe of flare energetics.

Fisher, G. H.↗
NASA NTRS1988

The effects of canard-wing flow-field interactions on longitudinal stability, effective dihedral and potential deep-stall trim

The literature available on high aspect ratio canard configurations shows them to have some unique stability characteristics. Using a generic canard-wing model, the effects of canard-wing flow-field interactions on stability were investigated in the NASA Langley Twelve-Foot Low-Speed Wind Tunnel. Results for the attached flow regime indicate linear interactions shift the neutral point of a canard configuration forward, but the effect of a canard on a wing can change significantly when the flow over the surface begins to separate, even several degrees below stall. The asymmetry of the canard downwash in a sideslip condition can result in an increment in effective dihedral roughly proportional to canard lift coefficient. At very high angles of attack the presence of a wing can cause an incremental normal force on a canard, contributing to the possibility of a deep-stall trim point. This effect is greater for a high canard and less for a low one.

Muchmore, C. B., Jr.↗
NASA NTRS1988

Linear simulation of the stationary eddies in a GCM. II - The 'Mountain' model

Linear stationary wave theory is used to account for zonal asymmetries of the winter-averaged tropospheric circulation obtained in a GCM. The eddy zonal velocity field in the upper troposphere indicates that the orographic and thermal plus transient contributions are nearly equal in amplitude, while the eddy meridional velocity field (which is dominated by shorter zonal scales) shows the orographic contribution to be dominant. The two contributions are found to be roughly in phase over the east Asian coast, and they contribute roughly equal amounts to the low level Siberian high. Results indicate that the 300 mb extratropical response to tropical forcing reaches 50 gpm over Alaska, and that the responses to sensible heating and lower tropospheric transients are strongly anticorrelated.

Nigam, Sumant↗
NASA NTRS1988

Hypersonic shock structure with Burnett terms in the viscous stress and heat flux

The continuum Navier-Stokes and Burnett equations are solved for one-dimensional shock structure in various monatomic gases. A new numerical method is employed which utilizes the complete time-dependent continuum equations and obtains the steady-state shock structure by allowing the system to relax from arbitrary initial conditions. Included is discussion of numerical difficulties encountered when solving the Burnett equations. Continuum solutions are compared to those obtained utilizing the Direct Simulation Monte Carlo method. Shock solutions are obtained for a hard sphere gas and for argon from Mach 1.3 to Mach 50. Solutions for a Maxwellian gas are obtained from Mach 1.3 to Mach 3.8. It is shown that the Burnett equations yield shock structure solutions in much closer agreement to both Monte Carlo and experimental results than do the Navier-Stokes equations. Shock density thickness, density asymmetry, and density-temperature separation are all more accurately predicted by the Burnett equations than by the Navier-Stokes equations.

Chapman, Dean R.↗
NASA NTRS1988

Very Large Array observations of Uranus at 2.0 cm

Radio observations of Uranus obtained at 2.0 cm with the B configuration of the VLA during April 1985 are reported. The calibration and data-reduction procedures are described in detail, and the results are presented in tables, maps, and graphs and compared with IRIS 44-micron observations (Hanel et al., 1986). Features discussed include highest brightness centered on the pole rather than on the subearth point, a decrease in brightness temperature (by up to 9 K) at latitudes between -20 and -50 deg (well correlated with the IRIS data), and disk-center position (corrected for the observed radio asymmetry) in good agreement with that found on the basis of the outer contours of the image.

Berge, G. L.↗