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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Physicochemical and dynamical processes in cometary ionospheres. I - The basic flow profile

The paper deals with the basic aspects of solar wind interaction with the ionosphere of a large comet and with detailed hydrodynamic models developed to describe the flow exterior to the tangential discontinuity surface that separates the outflowing purely cometary ions from the inflowing contaminated solar wind. The equation of the basic flow model and the results of a preliminary analysis of these equations are given, and the shape and structure of the shock layer are analyzed. The shock layer structure associated with fan-shaped comet emanating from the cometary nucleus in the general direction to the sun is identified.

Houpis, H. L. F.↗
NASA NTRS1981

Parasitic current losses due to solar electric propulsion generated plasmas

Solar electric propulsion is a leading candidate for many upcoming space missions. Under many circumstances plasma produced by charge-exchange reactions within the ion beam dominates the ambient environment near the spacecraft. The calculations presented here contain a predictive hydrodynamic model for the charge-exchange plasma expansion, and a fully three-dimensional model for the structure of the plasma sheath around the solar array wing. Results of calculations for several configurations and voltage levels indicate that with kilovolt biases power losses of approximately 10 percent or more are likely, even with only one engine in operation, and that ameliorative measures should focus on the inboard portion of the solar arrays.

Katz, I.↗
NASA NTRS1981

Nucleosynthesis of neutron-rich heavy nuclei during explosive helium burning in massive stars

The production of heavy nuclei during explosive helium burning has been calculated using a hydrodynamical model of a 15-solar mass (Type II) supernova and an n-process nuclear reaction network. It is found that the resulting neutron-rich heavy nuclei are not produced in the relative abundances of solar-system r-process material, especially in the vicinity of Pt, nor are any actinides produced. These deficiencies reflect an inadequate supply of neutrons. However, some neutron-rich isotopes, normally associated with the r-process, are produced which may be significant for the production of isotopic anomalies in meteorites.

Blake, J. B.↗
NASA NTRS1981

Rates of fixation by lightning of carbon and nitrogen in possible primitive atmospheres

A thermochemical-hydrodynamic model of the production of trace species by electrical discharges has been used to estimate the rates of fixation of C and N by lightning in the primitive atmosphere. Calculations for various possible mixtures of CH4, CO2, CO, N2, H2, and H2O reveal that the prime species produced were probably HCN and NO and that the key parameter determining the rates of fixation was the ratio of C atoms to O atoms in the atmosphere. Atmospheres with C more abundant than O have large HCN fixation rates, in excess of 10 to the 17th molecules/J, but small NO yields. However, when O is more abundant than C, the NO fixation rate approaches 10 to the 17th molecules/J while the HCN yield is small. The implications for the evolution of life are discussed.

Chameides, W. L.↗
NASA NTRS1982

Turbulence in transient solar phenomena

If theta dependence is kept in the Navier-Stokes equations for the solar wind, than a density enhancement will grow. This growth is followed in the nonlinear equations until a streamer is formed. Viscosity stops the streamer's growth when there is a large difference in speeds inside and outside of the streamer. Using classical fluid mechanics and a latitude dependent hydrodynamical model, it is shown that unmagnetized perturbed flow evolves into high and low density regions. The growth mechanisms for density enrichments are discussed along with a nonlinear solution for their large amplitude development. It was found that a higher Reynolds number is needed to start turbulence in the presence of a magnetic field because energy is required to bend the field lines attached to the fluid. If cosmological gas was turbulent shortly after the big bang, then galaxies could have been formed by turbulent eddies.

Cross, M.↗
NASA NTRS1983

Analytical calculation of a single jet in cross-flow and comparison with experiment

A series of progressively finer grid systems to calculate the single jet in cross-flow experimentally measured were employed. The experimental measurements provide a collection of velocities, turbulence intensities, and jet concentration profiles with measurements of the inlet field. The use of a series of progressively finer grid systems allows a differentiation between numerical errors and the hydrodynamic modeling assumptions embodied in the 3D combustor code.

Claus, R. W.↗
NASA NTRS1984

A magnetohydrodynamic simulation of the bifurcation of tail lobes during intervals with a northward interplanetary magnetic field

The interaction of the solar wind with the earth's magnetosphere during a northward interplanetary magnetic field was studied by using a three-dimensional magneto-hydrodynamic model. For a northward interplanetary magnetic field on 5 nT, the plasma sheet thickens near the noon-midnight meridian plane. When projected onto the polar cap this appears as a narrow channel extending from midnight towards noon. This plasma pattern is associated with three pairs of convection cells. The high latitude sunward convection and northern B(z) Birkeland current are caused by magnetic merging in the polar region.

Ogino, T.↗
NASA NTRS1984

The multiple modes of interaction of the solar wind with a comet as it approaches the sun

A quasi-steady one-dimensional hydrodynamic model is used to study the various modes of interaction between the solar wind and a medium-bright water-dominated comet approaching the sun. It is found through a series of calculations that at large heliocentric distances (greater than 5 AU) the solar wind penetrates to the surface of the comet. As mass loading reaches a critical value, a standing shock wave is created in the solar wind upstream from the cometary nucleus, and as distance decreases, the cometary atmosphere becomes dense enough to stand off the solar wind ahead of the nuclear surface. It is shown that the present scenario of solar wind/comet interaction is qualitatively valid for comets with different sizes, chemical compositions and optical properties.

Mendis, D. A.↗
NASA NTRS1984

The interplanetary electric field, cleft currents and plasma convection in the polar caps

The relationship between the pattern of plasma convection in the polar cleft and the dynamics of the interplanetary electric field (IEF) is examined theoretically. It is shown that owing to the geometrical properties of the magnetosphere, the East-West component of the IEF will drive field-aligned currents which connect to the ionosphere at points lying on either side of noon, while currents associated with the North-South component of the IEF will connect the two polar caps as sheet currents, also centered at 12 MLT. In order to describe the consequences of the Interplanetary Magnetic Field (IMF) effects upon high-latitude electric fields and convection patterns, a series of numerical simulations was carried out. The simulations were based on a solution to the steady-state equation of current continuity in a height-integrated ionospheric current. The simulations demonstrate that a simple hydrodynamical model can account for the narrow 'throats' of strong dayside antisunward convection observed during periods of southward interplanetary IMF drift, as well as the sunward convection observed during periods of strongly northward IMF drift.

Banks, P. M.↗
NASA NTRS1985

Persistence of shocks to large distances in the solar wind

Present hydrodynamic models of solar wind streams predict that interactions will cause interplanetary shocks to decay and large-scale structures to coalesce and smooth out, with a decay length of 10-15 AU for moderate or small-amplitude shocks. The Pioneer 10 plasma data, extending 1-30 AU, are examined in the light of such predictions. It is found that coalescence of streams into a single stream per solar rotation does occur, in general, but that considerable structure remains by 30 AU. The leading edge of a stream often exhibits a velocity jump of greater than 20 km/s; many of these may be shocks. There is a characteristic velocity-density-temperature signature of these distant streams which differs from the close-in double-shock signature. A unique transient was seen in July 1982, at 28 AU, with a velocity jump of about 235 km/s.

Kayser, S. E.↗
NASA NTRS1985

Molecular dynamics test of the Brownian description of Na(+) motion in water

The present paper provides the results of molecular dynamics calculations on a Na(+) ion in aqueous solution. Attention is given to the sodium-oxygen and sodium-hydrogen radial distribution functions, the velocity autocorrelation function for the Na(+) ion, the autocorrelation function of the force on the stationary ion, and the accuracy of Brownian motion assumptions which are basic to hydrodynamic models of ion dyanmics in solution. It is pointed out that the presented calculations provide accurate data for testing theories of ion dynamics in solution. The conducted tests show that it is feasible to calculate Brownian friction constants for ions in aqueous solutions. It is found that for Na(+) under the considered conditions the Brownian mobility is in error by only 60 percent.

Wilson, M. A.↗
NASA NTRS1986

The X-ray spectrum of Tycho

Baade (1945) has identified Tycho's supernova (SN) of A.D. 1572 as Type I, taking into account an analysis of the light curve recorded by Tycho. However, in spite of the obvious interest of Tycho's spectrum, to date the spectrum remains poorly understood. The present study is concerned with a reevaluation of the X-ray spectrum of Tycho. Attention is given to detailed nonequilibrium calculations and a self-consistent hydrodynamic model for ejecta consisting of pure heavy elements moving into a uniform external medium. The model provides a satisfactory fit to the spectrum, and avoids any requirement for a large mass of material in Tycho. It is demonstrated that the observed iron line emission is compatible with the presence of perhaps 0.7 solar mass of iron, of which 0.3 solar mass is shocked.

Hamilton, A. J. S.↗
NASA NTRS1985

Supersonic meteorology of Io - Sublimation-driven flow of SO2

The horizontal flow of SO2 gas from the day side to the night side of IO is calculated on the basis of a hydrodynamic model. The flow speed is found to be supersonic for all realistic values of the parameters. The surface pressure follows the frost vapor pressure within a factor of 2 in spite of day-night pressure ratios of 10,000 or more. Atmospheric temperature is generally below the surface temperature due to decompression in the expanding flow. The greatest sensitivity of the solution is connected with the frost temperature at the subsolar point. The quantities that involve the mass of the atmosphere (density, pressure, mass transport, and condensation rate) all vary as the vapor pressure of the frost, which is a sensitive function of frost temperature.

Ingersoll, A. P.↗
NASA NTRS1986

The origin of CS in comet IRAS-Araki-Alcock 1983d

It is shown that the IUE observations of CS in the atmosphere of comet IRAS-Araki-Alcock 1983d are consistent with its formation by the photodissociation of CS2 released from the nucleus. The observed scale length can be matched by a calculated scale length, which is based on the photochemical lifetime of CS2 at 1 AU and hydrodynamic models of the velocity field in the coma. A flow velocity of about 0.6 km/s is required to fit the data, which is in excellent agreement with recent calculations of the flow regime in the inner coma.

Jackson, W. M.↗
NASA NTRS1986

Cosmic-ray shock acceleration in oblique MHD shocks

A one-dimensional, steady-state hydrodynamical model of cosmic-ray acceleration at oblique MHD shocks is presented. Upstream of the shock the incoming thermal plasma is subject to the adverse pressure gradient of the accelerated particles, the J x B force, as well as the thermal gas pressure gradient. The efficiency of the acceleration of cosmic-rays at the shock as a function of the upstream magnetic field obliquity and upstream plasma beta is investigated. Astrophysical applications of the results are briefly discussed.

Webb, G. M.↗
NASA NTRS1986

A numerical study of the thermal stability of low-lying coronal loops

The nonlinear evolution of loops that are subjected to a variety of small but finite perturbations was studied. Only the low-lying loops are considered. The analysis was performed numerically using a one-dimensional hydrodynamical model developed at the Naval Research Laboratory. The computer codes solve the time-dependent equations for mass, momentum, and energy transport. The primary interest is the active region filaments, hence a geometry appropriate to those structures was considered. The static solutions were subjected to a moderate sized perturbation and allowed to evolve. The results suggest that both hot and cool loops of the geometry considered are thermally stable against amplitude perturbations of all kinds.

Klimchuk, J. A.↗
NASA NTRS1986

Venus mesosphere and thermosphere. II - Global circulation, temperature, and density variations

The Dickinson and Ridley (1977) symmetric, two-dimensional hydrodynamical model framework is presently used as the basis of a reexamination of the circulation and structure of the Venus thermosphere recently revealed by Pioneer Venus observations. The observed day-to-night variation of composition and temperatures can largely be derived by a wave-drag parameterization yielding a weaker circulation system than that predicted prior to Pioneer Venus. It is also suggested that eddy diffusion is a minor contributor to the maintenance of observed day and nightside densities, and that eddy coefficients are smaller than than those of one-dimensional composition models previously employed.

Bougher, S. W.↗
NASA NTRS1986

Effect of an impact-generated gas cloud on the acceleration of solid ejecta

A hypothesis that vaporization of a large amount of volatiles such as H2O and CO2 in the Martian surface by the impact of a large object could have accelerated solid ejecta to earth is examined. A hydrodynamic model is used to approximate a hemispherical gas cloud expanding into an atmosphere and entraining solid ejecta. Account is taken of the target material, the impactor materials, mass vaporized, impact velocity, drag coefficient, and crater sizes. A Martian crater larger than 30 km diam is found to be a necessary remnant of any impact that could have produced the shergottites, nakhlites and Chassigny meteorites which have been found on earth and possess similarities to analyzed Martian rocks.

Vickery, Ann↗