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At least 1,531 records · Page 85
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NASA NTRS1985

Volatile loss following very large impacts

Large impacts on growing planets can be fundamentally different in outcome than small impacts because they can lead to a planet-enveloping cloud of siliate vapor with a radiative cooling time long compared to dynamic time scales. Under these circumstances, there can be preferrential volatile loss by hydrodynamic outflow immediately above the silicate cloud deck. This loss is in ddition to the prompt, nonpreferential loss immediately following the impact event. During this time, evaporative loss (Jeans loss) can be 0.00001 of the planetary mass, provided the impact has substantial angular momentum and a magma disk forms. The loss is preferentially fromt he extremities of the disk and can be easily s100 bar-equivalents of CO2 or H2O. This implies devolatilization of Moon-forming material in an impact origin and may have important implications for the CO2 reservoirs of Venus, Earth, and Mars.

Stevenson, D. J.↗
NASA NTRS1985

Permafrost features on Earth and Mars: Similarities, differences

Typical permafrost features on Earth are polygonal structures, pingos and soli-/gelifluxion features. In areas around the poles and in mountain ranges the precipitation accumulates to inland ice or ice streams. On Mars the same features were identified: polygonal features cover the larger part of the northern lowlands indicating probably an ice wedge-/sand wedge system or desiccation cracks. These features indicate the extend of large mud accumulations which seem to be related to large outflow events of the chaotic terrains. The shore line of this mud accumulation is indicated by a special set of relief types. In some areas large pingo-like hills were identified. In the vicinity of the largest martian volcano, Olympus Mons, the melting of underlying permafrost and/or ground ice led to the downslope sliding of large parts of the primary shield which formed the aureole around Olympus Mons. Glacier-like features are identified along the escarpment which separates the Southern Uplands from the Northern Lowlands.

Joens, H. P.↗
NASA NTRS1985

White Light Coronograph (WLC) and Ultra-Violet Coronal Spectrometer (UVCS)

The white light coronagraph (WLC) and ultraviolet coronal spectrometer (UVCS) together reveal the corona and the roots of the solar wind from 1.5 to 6 solar radii from Sun center. The WLC measures the plasma density and spatial structure of the corona and coronal mass ejections at a resolution of about 20 arcseconds. The UVCS, in combination with the WLC, measures the temperature and radial outflow speed of the coronal plasma. These instruments will detect mass ejections from active regions and high speed solar wind streams from coronal holes a few days before the source regions rotate onto the face of the Sun, thus giving a week or more of advanced warning for disturbed geomagnetic conditions at Earth.

Moore, R. L.↗
NASA NTRS1984

Kinematic analysis of Hurricane Frederick (1979) using satellite, aircraft and rawinsonde data

Results of kinematic analyses of Hurricane Frederick (1979) performed using satellite wind vectors, aircraft measurements and rawinsonde data are presented. The data were gathered, reduced, preliminarily integrated and used to analyze the structure of the storm in the inflow and outflow layers. The mean structure at large radii is in good agreement with composite analyses of tropical cyclones. Changes in the storm structure from 11 to 12 Sep. 1979 suggests that remote sensing techniques may be useful in monitoring storm intensity.

Frank, W. M.↗
NASA NTRS1984

The structure of Herbig-Haro object 43 and Orion dark cloud extinction

New ultraviolet and optical observations of Herbig-Haro Object No. 43 are reported. Continuum and emission line fluxes in the range 1250 A equal to or less than lambda equal to less than 7350A have been measured. The continuum fluxes are best matched by an enhanced H two photon component added to H free bound emission, assuming theta Ori extinction curve with E(B-V) = 0.2, R = 5. The strucutre and dynamics of three components within the object are discussed. The object has a radiative output of equal to or greater than 0.23 infrared luminosity in ultraviolet and optical radiation combined. The energy requirements are discussed in terms of the production of shock waves by a collimated, supersonic mass outflow from a nearby infrared source.

Schwartz, R. D.↗
NASA NTRS1984

Theoretical model of the solar corona during sunspot minimum. I - Quasi-static approximation

The model of the solar corona (1.5 to 5 solar radius) during sunspot minimum is constructed. It is suggested that pole-equator asymmetry is caused by the interaction between global azimuthal electric currents in the corona and potential magnetic fields originated below. The asymmetry between the two hemispheres, which can be due to the magnetic quadrupole, is considered in detail. It is shown that an additional quadrupole term in the magnetic field representation affects the position of polar coronal hole boundaries differently in the two hemispheres. As a result, the outflow from the two polar regions is found to be different.

Osherovich, V. A.↗
NASA NTRS1984

Detection of submillimeter polarization in the Orion nebula

Linear polarization has been observed in the submillimeter radiation (270 microns) from two regions of Orion: one centered in the Kleinmann-Low nebula and one centered 1.5 arcmin south of the nebula. The observations were performed in September of 1983 and January of 1984 with the NASA Kuiper Airborne Observatory (KAO). The polarizations measured for the two regions were both 1.7 percent, plus or minus 0.4 and 0.5 percent, respectively. The angle of the outflow from both sources was 27 degrees, plus or minus seven degrees. An upper limit for polarization in the submillimeter radiation from the nebula W3(OH) was established at 1.6 percent. The observational data are compared with results from several other recent polarimetric observations of Orion, and some of their implications are discussed.

Hildebrand, R. H.↗
NASA NTRS1984

Plasma diagnostics for the outer solar corona - UV and XUV Fe XII lines

The formation of the UV and XUV spectral lines of Fe XII in the outer corona was investigated. The results indicate that these lines can be an important tool for probing the physical conditions in the coronal regions where the solar wind is generated. The Fe XII lines can be particularly useful for determining outflow velocities of heavy ions in the solar wind and for providing critical empirical constraints on energy transport and dissipation mechanisms in the solar wind acceleration region. Application of the theoretical calculations to existing UV, EUV, and XUV observations of the low corona provide evidence that: (1) the inclusion of the effects of resonances in the collision strengths for the low-lying levels of Fe XII is important, (2) that the contributions of resonances for these levels are even larger than the estimates given in the present paper, and (3) that the collision strengths used here yield populations for these levels which are accurate to a factor of 2. These results have implications concerning the accuracy of abundances and densities derived from observations of the Fe XII lines in the low corona.

Withbroe, G. L.↗
NASA NTRS1984

Circumstellar material around rapidly rotating B stars. II - On the nature of ultraviolet shell lines in the spectra of Be and shell stars

Using high-resolution (FWHM approximately 0.05 A) ultraviolet spectral data obtained with Copernicus, the nature of shell lines in Be and shell stars. Scans were obtained for various excited fine-structure lines of Fe III, Si II, and O I. It is found that Be stars, in general, do not display shell lines in the ultraviolet, contrary to the findings of previous work at lower resolution. The only Be stars that display strong UV shell lines are the 'classical' shell stars, i.e., stars that also display shell lines in the optical region. Furthermore, it is found that the shell lines, when present, have essentially zero velocity shift with respect to the star. The resonance shell lines seen in the shell stars are generally asymmetric on the short-wavelength side, indicating that the lines are formed in a low-velocity outflowing region. The results presented here argue strongly in favor of a flattened disk structure for the shell line forming region, and against some recent scenarios involving spherical symmetry.

Oegerle, W. R.↗
NASA NTRS1984

Brackett-alpha line profiles of young stellar objects

Profiles of the Br-alpha line of H I at a velocity resolution of 45 km/s are presented for the compact imbedded infrared objects BN, S106/IRS 3, GLS 490, GL 961, GL 989, Mon. R2/IRS 2, and for the visible objects LkH-alpha 101, T Tau, and R Mon. A proportionality obtained between Br-alpha luminosity and bolometric luminosity is shown to extend over three orders of magnitude, supporting the idea that the physical conditions and gas motions in the circumstellar envelopes of stellar objects are closely related over a wide range of luminosities. The Br-alpha line strengths are compared to radio continuum flux densities in the context of stellar wind models. Momentum deposition rates deduced from Br-alpha or radio continuum fluxes are consistent with those available in the radiation fields, which appear capable of driving the ionized gas outflows in the vicinity of the core sources. The results of a comparison of the H-alpha and Br-alpha profiles for T Tau are discussed.

Persson, S. E.↗
NASA NTRS1984

The Earth Radiation Budget Experiment (ERBE)

The three-satellite ERBE will be flown in the mid-1980s and will provide upgraded data on changes in the input and outflow of energy which drives climatic phenomena. The spacecraft will carry radiometers, self-calibration equipment, and be programmed with inversion and averaging algorithms. Radiation will be scanned at 0.2-5 and 5-50 microns. The ACRIM detector, similar to that on the SMM spacecraft, will measure the incoming solar radiation at the 600 km sun-synchronous orbit travelled by the three ERBE spacecraft. Data transmitted to the NASA-Langley Center will be analyzed with algorithms which extract radiant flux data from the measured radiances.

Barkstrom, B. R.↗
NASA NTRS1984

Cloud interactions and merging - Numerical simulations

A total of 48 numerical experiments have been performed to study cloud interactions adn merging by means of a two-dimensional multi-cell model. Two soundings of deep convection during GATE and two different magnitudes of large-scale lifting have been used as the initial conditions and as the main forcing on the model. Over two hundred groups of cloud systems with a life history of over sixty minutes have been generated under the influence of different combinations of the stratification and large-scale lifting. The results demonstrate the increase in convective activity and in amount of precipitation with increased intensity of large-scale lifting. The results also show increased occurrence of cloud merger with increased intensity of large-scale lifting. The most unfavorable environmental conditions for cloud merging are (1) less unstable stratification of the atmosphere and (2) weaker large-scale lifting. A total of fourteen cloud systems qualify as mergers. Two selected cases will be described dynamically and thermodynamically in this paper. Although these cloud mergers have been simulated under the influence of different synoptic-scale conditions, the major physical mechanism related to the cloud merging process is the same as that proposed by Simpson. Cumulus downdrafts and associated cold outflows play a dominant role in the merging process in all cases studied.

Tao, W.-K.↗
NASA NTRS1984

The complex X-ray emission of NGC 253

High-resolution X-ray observations of the starburst galaxy NGC 253 with the Einstein Observatory show that there are at least four components to its X-ray emission. Eight bright point-like sources were detected. Diffuse emission is visible in the inner disk of NGC 253. A more intense emission region is associated with the nuclear region and with an extension along the southern portion of the minor axis. This last component, in particular, suggests the presence of shock-heated gas associated with outflow from the starburst nucleus. A siimlar feature was seen in the X-ray emission of M82. A comparison between the nuclear regions of NGC 253, M82, and the Milky Way suggests an excess of X-ray emission in the nucleus of M82.

Fabbiano, G.↗
NASA NTRS1984

Mass loss from red giants - A simple evolutionary model for NGC 7027

NGC 7027 is a young planetary nebula with the remnants of a red giant circumstellar envelope surrounding the central, ionized region. By comparing the outer molecular envelope with the inner ionized material, it is argued that the mass loss rate has decreased by at least a factor of 3, and more probably by about a factor of 10, during the past 1000 years. From this result, it is argued that the luminosity of the central star has also decreased substantially during the same time, consistent with models for the rapid evolution of stars just after they evolve off the asymptotic giant branch. In this picture, the distance to NGC 7027 is less than 1300 pc. NGC 7027 was the last (and best) example of a star where apparently the momentum in the outflowing mass /M(dot)v/ is considerably greater than the momentum in the radiation field (L/c). With the above description of this object, the evidence is now strong that quite often the mass lost from late-type giants is ultimately driven to infinity by radiation pressure on grains. If M(dot)v is as large as L/c for asymptotic branch stars, then it is expected that the total amount of mass lost during this stage of evolution is of the same magnitude as the initial mass of the star, and therefore this mass loss can profoundly affect the star's ultimate fate.

Jura, M.↗
NASA NTRS1985

Superthermal ion signatures of auroral acceleration processes

The occurrence of non-Maxwellian superthermal features in the auroral topside ionosphere distribution functions has been documented by means of the retarding ion mass spectrometer on the Dynamics Explorer 1 spacecraft. Attention is given to a representative sampling of the observed features and their spatial morphology, as observed at altitudes ranging from a few thousand km to a few earth radii. The observations in question reveal a clear distinction between classical polar wind ion outflow and O(+)-enhanced superthermal flows, and confirm the importance of low altitude transverse acceleration in ionospheric plasma transport, as suggested by previous observations.

Moore, T. E.↗
NASA NTRS1984

The solution of non-linear hyperbolic equation systems by the finite element method

A finite-element method for the solution of nonlinear hyperbolic systems of equations, such as those encountered in non-self-adjoint problems of transient phenomena in convection-diffusion or in the mixed representation of wave problems, is developed and demonstrated. The problem is rewritten in moving coordinates and reinterpolated to the original mesh by a Taylor expansion prior to a standard Galerkin spatial discretization, and it is shown that this procedure is equivalent to the time-discretization approach of Donea (1984). Numerical results for sample problems are presented graphically, including such shallow-water problems as the breaking of a dam, the shoaling of a wave, and the outflow of a river; compressible flows such as the isothermal flow in a nozzle and the Riemann shock-tube problem; and the two-dimensional scalar-advection, nonlinear-shallow-water, and Euler equations.

Loehner, R.↗
NASA NTRS1985

Solar wind stagnation near comets

The nature of the solar wind flow near comets is examined analytically in this paper. In particular, typical values for the stagnation pressure and magnetic barrier strength are estimated, taking into account magnetic field line tension and change-exchange cooling of the mass-loaded solar wind. A knowledge of the strength of the magnetic barrier is required in order to determine the location of the ionopause surface which separates the contaminated solar wind plasma from the outflowing plasma of the cometary ionosphere.

Galeev, A. A.↗
NASA NTRS1983

Airborne Doppler-lidar and ground-based Doppler radar observations of a thunderstorm in Oklahoma

The results of airborne Doppler-lidar and ground-based Doppler radar observations of multicellular storms, marked by heavy rainfall, strong surface outflow, and a gust-front tornado, on June 30, 1981 are analyzed. The airborne lidar comprised a CO2-laser operating in the IR region, which was discharged once each second at 20 deg fore and aft, alternatively, of the normal to the aircraft's heading, and a quarter-wave plate for registering the returning frequency-shifted beams. Wind fields are plotted taking into account the advection effects. The lidar data set is noted for its self-consistency, though limited to a range of 5 km by the high moisture levels. Fair agreement was found between the lidar-derived and radar-derived average ground-based radial wind fields, with discrepancies on the order of 1.5 m/s.

Bluestein, H. B.↗