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At least 829 records · Page 46

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NASA NTRS1986

The vibrational distribution of O2(+) in the dayside ionosphere

The vibrational distributions of O2(+) in the X2Pi(g), A2Pi(u), a4Pi(u), and b4Sigma(-)g states in the dayside terrestrial ionosphere are calculated for both low and high solar activity models. The distributions are found to be significantly different from the O2(+) vibrational distributions found by Fox (1985) for the Venusian ionosphere. The sources and sinks of vibrational excitation and the implications for the chemistry, dayglow, and hot oxygen coronas are discussed. Finally, intensities of the first negative and second negative band systems of O2(+) are presented.

Fox, J. L.↗

NASA NTRS1987

Assignments of autoionization states of O2-asterisk

Attention is given to the uncertainties that remain concerning the autoionization states of O2 leading to the a 4Pi(u) and A 2Pi(u) states of O2(+), as well as some of the assignments of the autoionization states and the determinations of effective quantum numbers and quantum defects. The former problems of vibrational assignments are unambiguously established in view of a study of isotopic oxygen molecules. A systematic examination of the known Rydberg series is conducted, and new assignments and interpretations for several autoionization states leading to the various states of O2(+) are suggested.

Wu, C. Y. Robert↗

NASA NTRS1987

Dust charges, cloud potential, and instabilities in a dust cloud embedded in a plasma

The cloud potential V and dust potential U in dusty clouds are calculated for two limiting theoretical cases between which actual cases are expected to exist. In one case particles are not thermalized within the cloud, and in the other the plasma is thermalized in the cloud interior to form a Maxwellian plasma with densities given by Boltzmann relations as functions of the local cloud potential. The results show that the values of V and U are strongly dependent on the character of the internal plasma and that, most of the time during which the internal plasma retains its non-Maxwellian character, the creation of double layers and the occurrence of transitions with considerable readjustments of the cloud structure are possible. Such processes should not occur if the internal plasma is thermalized.

Havnes, O.↗

NASA NTRS1987

Precise timing of the last interglacial period from mass spectrometric determination of thorium-230 in corals

The development of mass spectrometric techniques for determination of Th-230 abundance has made it possible to reduce analytical errors in (U-238)-(U-234)-(Th-230) dating of corals even with very small samples. Samples of 6 x 10 to the 8th atoms of Th-230 can be measured to an accuracy of + or - 3 percent (2sigma), and 3 x 10 to the 10th atoms of Th-230 can be measured to an accuracy of + or - 0.2 percent. The time range over which useful age data on corals can be obtained now ranges from about 50 to about 500,000 years. For young corals, this approach may be preferable to C-14 dating. The precision with which the age of a coral can now be determined should make it possible to critically test the Milankovitch hypothesis concerning Pleistocene climate fluctuations. Analyses of a number of corals that grew during the last interglacial period yield ages of 122,000 to 130,000 years. The ages coincide with, or slightly post-date, the summer solar insolation high at 65 deg N latitude which occurred 128,000 years ago. This supports the idea that changes in Pleistocene climate can be the result of variations in the distribution of solar insolation caused by changes in the geometry of the earth's orbit and rotation axis.

Chen, J. H.↗

NASA NTRS1987

Robust nonlinear attitude control of flexible spacecraft

This paper presents an approach to large-angle rotational maneuvers of a spacecraft-beam-tip body configuration based on nonlinear invertibility and linear feedback stabilization. A control law u sub d is derived for the decoupled control of attitude angles, lateral elastic deflections, slopes due to bending and angular deflection due to torsion at the tip of the beam using torquers and force actuators. For the stabilization of the elastic modes, a linear feedback control law u sub s is obtained based on a linearized model augmented with a servocompensator. Simulation results are presented to show that large slewing and elastic mode stabilization can be accomplished.

Singh, Sahjendra N.↗

NASA NTRS1987

Liposomes with polyribonucleotides as model of precellular systems

Three types of liposomes were prepared under anoxic conditions: from dipalmitoyl phosphatidyl choline (DPPC), from egg yolk phosphatidyl choline (PC), and from PC with cholesterol (PC:Chol). These were used for encapsulation of poly(U) and poly(C). It was found that 36 to 70 percent of the available liposome lipids and 2 to 5 percent of the polyribonucleotides could be entrapped. An enhanced encapsulation of poly(U) and poly(C) by all three types of liposomes was observed in the presence of 0.001 to 0.01 M Zn(2+), with the effect being greatest with DPPC. The presence of 1.0 M urea inhibited the formation of PC liposomes.

Baeza, Isabel↗

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 NTRS1985

Coating life prediction

The investigation combines both experimental studies and numerical modeling to predict coating life in an oxidizing environment. The experimental work provides both input to and verification of two numerical models. The coatings being examined are an aluminide coating on Udimet 700 (U-700), a low-pressure plasma spray (LPPS) Ni-18Co-17Cr-24Al-0.2Y overlay coating also on U- 700, and bulk deposits of the LPPS NiCoCrAlY coating.

Nesbitt, James A.↗

NASA NTRS1986

Stability of semidiscrete approximations for hyperbolic initial-boundary-value problems: An eigenvalue analysis

A hyperbolic initial-boundary-value problem can be approximated by a system of ordinary differential equations (ODEs) by replacing the spatial derivatives by finite-difference approximations. The resulting system of ODEs is called a semidiscrete approximation. A complication is the fact that more boundary conditions are required for the spatially discrete approximation than are specified for the partial differential equation. Consequently, additional numerical boundary conditions are required and improper treatment of these additional conditions can lead to instability. For a linear initial-boundary-value problem (IBVP) with homogeneous analytical boundary conditions, the semidiscrete approximation results in a system of ODEs of the form du/dt = Au whose solution can be written as u(t) = exp(At)u(O). Lax-Richtmyer stability requires that the matrix norm of exp(At) be uniformly bounded for O less than or = t less than or = T independent of the spatial mesh size. Although the classical Lax-Richtmyer stability definition involves a conventional vector norm, there is no known algebraic test for the uniform boundedness of the matrix norm of exp(At) for hyperbolic IBVPs. An alternative but more complicated stability definition is used in the theory developed by Gustafsson, Kreiss, and Sundstrom (GKS). The two methods are compared.

Warming, Robert F.↗

NASA NTRS1987

Optical alignment of Centaur's inertial guidance system

During Centaur launch operations the launch azimuth of the inertial platform's U-accelerometer input axis must be accurately established and maintained. This is accomplished by using an optically closed loop system with a long-range autotheodolite whose line of sight was established by a first-order survey. A collimated light beam from the autotheodolite intercepts a reflecting Porro prism mounted on the platform azimuth gimbal. Thus, any deviation of the Porro prism from its predetermined heading is optically detected by the autotheodolite. The error signal produced is used to torque the azimuth gimbal back to its required launch azimuth. The heading of the U-accelerometer input axis is therefore maintained automatically. Previously, the autotheodolite system could not distinguish between vehicle sway and rotational motion of the inertial platform unless at least three prisms were used. One prism was mounted on the inertial platform to maintain azimuth alignment, and two prisms were mounted externally on the vehicle to track sway. For example, the automatic azimuth-laying theodolite (AALT-SV-M2) on the Saturn vehilce used three prisms. The results of testing and modifying the AALT-SV-M2 autotheodolite to simultaneously monitor and maintain alignment of the inertial platform and track the sway of the vehicle from a single Porro prism.

Gordan, Andrew L.↗

NASA NTRS1987

On connection between evolution of troposphere fronts and changes of circulation regime in meteor zone

The interactions between the thermobaric fields at heights from 5 to 95 km at the beginning of a winter period are considered, based on experimental radar measurements of meteor drifts obtained in Frunze in November to December 1983. During this period the high atmosphere readily responds to even slight changes in the thermal regime of the stratosphere. The interdiurnal variations of average daily values of wind u, v, U, and azimuth phi are shown.

Karimov, K. A.↗

NASA NTRS1987

What SNC meteorites tell us about Martian magmatism

The SNC meteorites record relatively recent (less than b.y.) basaltic volcanism on a hydrous planet with a core. This much was known about Mars even before the Viking mission. The SNCs tell that the basalts were unusually low in Al2O3 and, despite overall similarities in major elements, apparently sampled two isotopically distinct regions of Mars: one is relatively depleted in incompatible elements (Nahklite source) with a U-Pb model age of 4.3 b.y. and is probably a portion of the upper Martian mantle, whereas the other is relatively enriched in incompatible elements (Shergottite source) with a 4.5 b.y. U-Pb age and is probably Martian crust assimulated into the mantle-derived Shergottite magma. The SNCs tell that the overall abundances of moderately volatile elements, such as alkalies, are higher on Mars than on earth. Given extensive volcanism and a depleted mantle, the present day absence of a thick atmosphere is more likely due to catastrophic removal of a primordial atmosphere than lack of degassing.

Longhi, John↗

NASA NTRS1988

Evidence of the in situ decay of Al-26 in a Semarkona chondrule

A long-standing problem in the study of meteorites, with broad implications for the evolution of the solar system, is the source of heat for melting small planets. Meteorites, such as the eucrites, were obviously the result of a melting process on a small planet, yet the decay of the long-lived radionuclides present on the earth, K-40, U-235, U-238, and Th-232, could not have provided nearly enough heat to initiate melting approximately 4.5 aeons ago, even on bodies 1000 km in diameter. This problem addressed by H.C. Urey in 1955, who recognized that the presence of short-lived (and now extinct) radionuclides such as Al-26 (half-life of approximately 720,000 y) in the early solar system would have provided an abundant heat source for melting small asteroid-size bodies. Since any Al-26 initially present in the solar nebula would have completely decayed after only a few tens of millions of years, the problem became one of finding evidence of Al-26, in the form of excess Mg-26, in meteorites. The search for evidence of the Al-26 is briefly examined.

Hutcheon, I. D.↗

NASA NTRS1987

Quadrupolar Kondo effect in uranium heavy-electron materials?

The possibility of an electric quadrupole Kondo effect for a non-Kramers doublet on a uranium (U) ion is a cubic metallic host is demonstrated by model calculations showing a Kondo upturn in the resistivity, universal quenching of the quadrupolar moment, and a heavy-electron anomaly in the electronic specific heat. With inclusion of excited crystal-field levels, some of the unusual magnetic-response data in the heavy-electron superconductor UBe13 may be understood. Structural phase transitions at unprecedented low temperatures may occur in U-based heavy-electron materials.

Cox, D. L.↗

NASA NTRS1988

The production of O(1D) from dissociative recombination of O2(+)

The results of large scale ab initio calculations of the rates for production of O(1D) by dissociative combination of O2(+) are presented for electron temperatures in the range 100 to 3000 K. A 1-delta-u state is the dominant dissociative route from v = 0 and a 3-sigma-u(-) state is the most important route from v = 1 and v = 2. The calculated total rate for O(1D) production from v = 0 is 2.21(+0.21, -0.24) x 10(-7) x (T sub e/300) exp -.46 near room temperature. The v = 1 and v = 2 rates are about 17 percent and 47 percent smaller respectively, than the v = 0 rate at 300 K.

Guberman, Steven L.↗

NASA NTRS1987

Theoretical determination of the ground state of N2(2+)

The low-lying 1Sigma(g)(+), 3Pi(u), and 3Sigma(g)(-) states of N2(2+) in the immediate region of the minima in their respective potential curves are investigated. Spectroscopic constants for the 1Sigma(u)(+) state are computed for comparison with experiment. Approaches to computing the entire potential curves of many states N2(2+) are calibrated.

Taylor, Peter R.↗

NASA NTRS1988

Recombination line intensities for hydrogenic ions. II - Case B calculations for C VI, N VII and O VIII

The intensities of recombination lines formed in extended, optically thim, photoionized plasmas such as those found in PN, H II regions, and winds of certain hot stars are an important source of information on chemical abundances and can sometimes provide estimates of electron temperature. In this paper, the intensities of the recombination lines of C VI, N VII, and O VIII are calculated, accounting for both electron and heavy particle collisions and assuming case B of Baker and Menzel. The computational procedure is explained. The intensities of lines formed by transitions n(u) - n(l) are tabulated for n(u) of 50 or less, n(l) of 29 or less, at log N(e) = 4(1)13 and 10 values of electron temperature in the interval 10,000 K to 500,000 K.

Storey, P. J.↗

NASA NTRS1987

Stochastic estimation of organized structures in turbulent channel flow

Linear stochastic estimation is used to approximate the conditional vector fields associated with high Reynolds stress producing events in numerically simulated turbulent channel flow. Joint probability density distributions of u-v are presented for y(+) between 1.35 and 180, and at each position the u-v values that make the greatest contributions to the average uv3 are used to define conditional ejection- and sweep-type events. In the y-z plane the conditional fields appear to be pairs of counter-rotating vortices. When the conditional event defined by the weighted probability density function is specified at a point close to the wall, the spacing and size of these eddies are consistent with the accepted wall streak spacing. As this point moves outwards, the eddy size increases significantly. An abrupt change in the flow angle occurs in the buffer layer and may indicate transition from streamwise oriented wall layer structures to hairpin vortices characterizing the outer layer.

Moin, P.↗