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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
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NASA NTRS1990

Simultaneous multifrequency observations of the Seyfert 1 galaxy NGC 4051 - Constant optical-infrared emission observed during large-amplitude X-ray variability

Simultaneous infrared, optical, ultraviolet, and X-ray observations of the low-luminosity Seyfert 1 galaxy, NGC 4051 are reported. A new method of reduction was developed to correct the optical flux for atmospheric variations due to seeing and extinction. The X-ray flux varied by factors of up to 2 on time-scales of tens of minutes, while the optical flux remained steady to within 1 percent. The results rule out all models, including Compton scattering and synchrotron models, in which a single electron population is responsible for the formation of both the infrared-to-optical and the X-ray spectra. The optical emission region must be an order of magnitude larger than, or completely separate from, the X-ray source.

Done, C.↗
NASA NTRS1989

The abundances of isotopes in the cosmic radiation

Studies of the isotopic composition of nuclei in the cosmic radiation are reviewed, including abundances of the isotopes of elements from H to Ni, together with their implications for cosmic ray origin, acceleration, and transport in the Galaxy. The review focuses on determinations of the composition of cosmic ray source material, and the extent to which the isotopic composition of this material is different from, or similar to, typical solar system material and other samples of Galactic matter. Theoretical models that have been advanced to explain the observed overabundance of neutron-rich isotopes in cosmic rays are described. Also discussed are studies of various radioactive clocks that record the time-scales associated with the nucleosynthesis, acceleration, and transport of cosmic ray nuclei, and studies of the so-called anomalous cosmic ray component, thought to represent a sample of the neutral interstellar medium.

Mewaldt, R. A.↗
NASA NTRS1990

The relative timing of microwaves and X-rays from solar flares

The delay of microwaves relative to hard X-rays from solar flares is investigated. For short-time scale delays, it was found that for reasonable thick-target model parameters and for injected electron distributions which are separable in time, energy, pitch angle, and position, the observed delay cannot be explained by magnetic trapping of electrons in the corona. It can be accounted for if higher energy microwave producing electrons are accelerated later than lower energy hard X-ray producing electrons. For larger time-scale delays, it is found that the flux during the rising phase of the flares can be explained well in terms of the thick-target model, but during the decay phase this model predicts too little microwave flux. A number of possibilities for this excess microwave flux are explored including spectral hardening, magnetic trapping, and thermal synchrotron and free-free emission.

Lu, Edward T.↗
NASA NTRS1990

The ephemeris of X 1822 - 371

From the analysis of eclipses observed with the HEAO-1, Einstein, Exosat and Ginga satellites, it is shown that the orbital period of X1822 - 371 is increasing on a time-scale of 2.9 million yr. The period change, one of only three determined amongst the LMXBs, is in the opposite sense to the expected evolutionary trend, but in the same sense as the period change of Cyg X-3. The observation of an eclipse simultaneously in the X-ray and optical bands shows that the optical eclipse is asymmetric and has a minimum 3 min later than that of the X-ray eclipse, reflecting a pronounced asymmetry in the optically emitting disk structure.

Hellier, Coel↗
NASA NTRS1990

CDAW 7 revisited - Further evidence for the creation of a near-earth substorm neutral line

Eastman et al. (1988) have interpreted the CDAW 7 substorm of April 24, 1979, previously taken as unambiguously supporting the near-earth neutral line model of magnetospheric substorms, in terms of spatial movements of a preexisting plasma-sheet boundary layer (PSBL) and its associated current sheets across the observing ISEE 1 and 2 spacercraft. It is presently noted that, by contrast, a reinvestigation of ISEE 1 and 2 energetic particle measurements around substorm onset on short time-scales shows the observed flux pattern to require the formation of a particle source eastward of the ISEE spacecraft, well within the plasma sheet, associated with the substorm onset. Strong flows were absent prior to substorm onset, indicating the temporal nature of the event, as opposed to an encounter with a preexisting PSBL containing large flows.

Kettmann, G.↗
NASA NTRS1990

Hot-bench simulation of the active flexible wing wind-tunnel model

Two simulations, one batch and one real-time, of an aeroelastically-scaled wind-tunnel model were developed. The wind-tunnel model was a full-span, free-to-roll model of an advanced fighter concept. The batch simulation was used to generate and verify the real-time simulation and to test candidate control laws prior to implementation. The real-time simulation supported hot-bench testing of a digital controller, which was developed to actively control the elastic deformation of the wind-tunnel model. Time scaling was required for hot-bench testing. The wind-tunnel model, the mathematical models for the simulations, the techniques employed to reduce the hot-bench time-scale factors, and the verification procedures are described.

Buttrill, Carey S.↗
NASA NTRS1990

Ultraviolet remote sensing technology

The Nimbus 7 satellite's Total Ozone Mapping Spectrometer (TOMS) has obtained images of the Antarctic ozone hole, and demonstrated that volcanic eruptions can be unambiguously detected from space in view of the absorption of the plumes' sulfur dioxide. While the TOMS currently has an average spatial resolution of 66 km and a 24-hr temporal resolution, total ozone features at midlatitudes change within time-scales of hours and may involve structural features as small as 15-25 km. It is projected that these observational requirements are achievable with TOMS-like instruments in GEO, using CCD detector arrays in place of mechanical scanning.

Krueger, Arlin J.↗
NASA NTRS1990

Dispersal of gases generated near a lunar outpost

The dispersal of gases evolved by prospective lunar base operations is presently modeled analytically using continuous (mining and habitat-venting) and impulsive (primarily rocket exhaust) injections of gases. In the case of impulsive injection, the neutral atmosphere and associated ionosphere both decay on time-scales of about 20 min; in that of continuous injection, the atmosphere near the base grows and achieves a steady state after about 20 min. Both direct and diffusive transport mechanisms are considered, and it is concluded that for the injection rates and assumptions presently employed the artificial lunar atmospheres produced are not significantly detrimental to astronomical observations and high-vacuum materials processing operations.

Burns, Jack O.↗
NASA NTRS1989

Results of the 1988 NASA/JPL airborne SAR calibration campaign

During the spring of 1988, the NASA/JPL multifrequency, multipolarization SAR (synthetic aperture radar) flew in series of calibration experiments over the Goldstone dry lake bed. An array of calibration devices was deployed, including dihedral and trihedral corner reflectors, polarimetric active radar calibrators, passive receivers, and CW (continuous wave) tone generators. The aim of the campaign was to calibrate both amplitude and phase of the resulting SAR images, over long and short time scales. The results of the analysis of the calibration data collected in the spring of 1988 are presented. Trihedral corner reflector signatures and certain image background measures are used to externally calibrate relative amplitude and phase between polarizations at a given frequency, and to calibrate across frequencies. Assessments are made of the calibration accuracy as a function of image frame position for each frequency, and the stability of the radar calibration over long and short time-scales.

Freeman, A.↗
NASA NTRS1990

Hot-bench simulation of the active flexible wing wind-tunnel model

Two simulations, one batch and one real-time, of an aeroelastically-scaled wind-tunnel model were developed. The wind-tunnel model was a full-span, free-to-roll model of an advanced fighter concept. The batch simulation was used to generate and verify the real-time simulation and to test candidate control laws prior to implementation. The real-time simulation supported hot-bench testing of a digital controller, which was developed to actively control the elastic deformation of the wind tunnel model. Time scaling was required for hot-bench testing. The wind-tunnel model, the math models for the simulations, the techniques employed to reduce the hot-bench time-scale factor, and the verification procedures are described.

Buttrill, Carey S.↗
NASA NTRS1990

Aerospace plane guidance using geometric control theory

A reduced-order method employing decomposition, based on time-scale separation, of the 4-D state space in a 2-D slow manifold and a family of 2-D fast manifolds is shown to provide an excellent approximation to the full-order minimum-fuel ascent trajectory. Near-optimal guidance is obtained by tracking the reduced-order trajectory. The tracking problem is solved as regulation problems on the family of fast manifolds, using the exact linearization methodology from nonlinear geometric control theory. The validity of the overall guidance approach is indicated by simulation.

Van Buren, Mark A.↗
NASA NTRS1991

The development of the CO(+) coma of Comet P/Schwassmann-Wachmann 1

The development of the CO(+) coma of Comet P/Schwassmann-Wachmann 1 is studied in spectroscopic observations indicating that the maximum rise time of the CO(+) gas is 1.2 days. This result implies a maximum creation time-scale for the CO(+) gas of the order of 100,000 sec at 5.8 AU. Evidence is adduced that dust events and gas events observed in this comet are not necessarily connected.

Cochran, Anita L.↗
NASA NTRS1991

Closed-loop transfer recovery with observer-based controllers. II - Design

The paper focuses on the design problem related to the recovery of a target closed-loop transfer function using both full-order and reduced-order observer-based controllers. Design schemes for the reduced-order observer-based controllers are similar to those of full-order controllers; hence, for simplicity, only the case for full-order observer-based controller designs is given. Three types of design schemes are discussed in detail. The first one is an asymptotic time-scale eigenstructure assignment method, and the other two are optimization-based methods where a certain norm of the recovery matrix is minimized. Asymptotic behavior associated with each design method is highlighted. Relative advantages and disadvantages of both the ATEA and optimization-based schemes are discussed. A numerical example is used to illustrate the design possibilities from each of these methods, the available design freedom, and the significance of selecting the limit of the recovery matrix, especially in the case of a nonrecoverable design.

Chen, Ben M.↗
NASA NTRS1991

Cosmic-ray transport and gamma-ray emission in supernova shells

The mixing and transport of cosmic rays accelerated by a pulsar inside an expanding supernova remnant are examined, and the resulting high-energy gamma-ray emission from nuclear interactions of these accelerated particles in the shell is investigated. Rayleigh-Taylor instability at the interface between a pulsar wind cavity and the inner supernova envelope is assumed to be the mixing mechanism. The analysis is applied to the model of Gaisser, Harding, and Stanev (1987), where protons are accelerated at the reverse shock in the pulsar wind. The instability time-scale is estimated from the dynamics of the pulsar wind cavity, and model the injection, diffusion, and interaction of protons in the shell. The resulting gamma-ray flux is lower than previous estimates due to proton adiabatic losses in the expanding pulsar wind. The protons mix and diffuse only into the innermost regions of the envelope before interacting. Energy-dependent diffusion causes the higher energy gamma-ray light curves to decay faster than those at lower energy.

Harding, A. K.↗
NASA NTRS1991

Thermal equilibration of the earth following a giant impact

A thermal evolution model for the cooling of a terrestrial planet, a prescribed fraction of which is melted during a giant impact, is described. Two model geometries are considered, one having a laterally inhomogeneous distribution of melt and solid through a mega-crater geometry and the other having a global magma ocean. It is found that almost the whole planet may melt if the impact originally melted half of the planet and if the first melt temperature was double a typical planet melting temperature. For reasonable choices of parameter values it is determined that thermal equilibration of the earth occurs on a time-scale of 1 to ten million years.

Spohn, Tilman↗
NASA NTRS1991

Incoherent vs. coherent behavior in the normal state of copper oxide superconductors

The self-consistent quantum fluctuations around the mean-field Hartree-Fock state of the Hubbard model provide a very good description of the ground state and low temperature properties of a 2-D itinerant antiferromagnet. Very good agreement with numerical calculations and experimental data is obtained by including the one- and two-loop spin wave corrections to various physical quantities. In particular, the destruction of the long-range order above the Neel temperature can be understood as a spontaneous generation of a length-scale epsilon(T), which should be identified as the spin correlation length. For finite doping, the question of the Hartree-Fock starting point becomes a more complex one since an extra hole tends to self-trap in antiferromagnetic background. Such quantum defects in an underlying antiferromagnetic state can be spin-bags or vortex-like structures and tend to suppress the long-range order. If motion of the holes occurs on a time-scale shorter than the one associated with the motion of these quantum defects of a spin background, one obtains several important empirical features of the normal state of CuO superconductors like linear T-dependence of resistivity, the cusp in the tunneling density of states, etc. As opposed to a familiar Fermi-liquid behavior, the phenomenology of the above system is dominated by a large incoherent piece of a single hole propagator, resulting in many unusual normal state properties.

Tesanovic, Zlatko↗
NASA NTRS1990

A brief introduction to coronal 'loops'

A discussion is presented of elementary aspects of the physics of coronal loops, in order to more securely approach the question of their existence. It is noted that the time-scales for pressure equilibration are far shorter than typical cooling times in loop structures, and that the loop structures are accordingly in pressure balance with their surroundings. Those surroundings, however, are dark, implying a decreased density and gas pressure outside the bright loops. It is deduced that the loop interiors must have a weaker magnetic field than the gas immediately outside them.

Rosner, Robert↗
NASA NTRS1984

Selection of Slow and Fast Variables in the Three-Dimensional Flight Dynamics

A transformation from the altitude and velocity state variables of three-dimensional flight mechanics to a new set of more desirable variables is found. The new variables provide a greater time-scale separation, decrease system coupling, and give better estimates of the fast-variable values along the reduced solution. One of the new variables is the often-used specific energy, whereas the other variable changes along a given trajectory, depending on the nature of the local reduced solution. Numerical examples are included.

Ardema, M.↗