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NASA NTRS1989

Magnetic fields at Neptune

The Voyager 2 magnetic field experiment discovered a complex and powerful magnetic field in Neptune, as well as an associated magnetosphere and magnetic tail. As the spacecraft exited the magnetosphere, the magnetic tail appeared to be monopolar. The auroral zones are probably located far from the rotation poles, and may possess complex geometry. The Neptune rings and all its known moons are imbedded deep within the magnetosphere (except for Nereid, which is outside when it lies sunward of the planet); the radiation belts have a complex structure due to the absorption of energetic particles by the moons and rings of Neptune, as well as losses associated with the significant changes in the diurnally varying magnetosphere configuration.

Ness, Norman F.↗

NASA NTRS1990

Characteristics of wave-particle interactions during sudden commencements. II - Spacecraft observations

Wave data from the DE 1 spacecraft for 50 sudden commencements were analyzed for amplitude and spectral modifications and correlated with data from ground-based observatories. Changes in wave activity were identified in 14 of the events studied. The changes, which were commonly observed at frequencies below the local electron gyrofrequency, exhibited a complex structure both in frequency and in time and varied considerably among events. The typical event lasted 1-10 min with a maximum narrowband amplitude increase or decrease of 10-30 dB. The onset time of the wave growth was correlated with the arrival of the magnetic disturbance at the spacecraft. Changes were most commonly observed at L shells in the range of L values between 3 and 6. Growth was observed on the nightside as well as the dayside, and no clear local time dependence was found. In several cases, growth at frequencies above the local gyrofrequency was noted and identified as auroral kilometric radiation.

Gail, W. B.↗

NASA NTRS1990

Microwave and X-ray observations of a major confined solar flare

Observations of an X4 flare of May 19, 1984 using the VLA and the SMM X-ray Polychromator are discussed. The 6 cm radio source remained stationary throughout the decay phase. Combined with other evidence, this indicates that the flare was magnetically confined and did not disrupt the complex structure of its region. The observed structures may be associated with a neutral sheet connecting the two bipoles.

Schmahl, E. J.↗

NASA NTRS1990

On lightly damped linear systems

A simple engineering approximation using undamped modes is shown to be valid for determining the effect of small viscous or equivalent viscous-type damping forces on the forced vibration response of a lightly damped linear systems. It is shown that intermodal coupling of the undamped modes may be neglected with negligible error, except when the system has degenerate or quasi-degenerate modes. This can occur in complex structural systems such as aircraft or spacecraft. In such cases, one of these two modes has virtually no damping, whereas the other effectively has the damping of both.

Young, M. I.↗

NASA NTRS1990

Spatially resolved optical and near infrared spectroscopy of I Zw 18

For a long time, I Zw 18 has been recognized as the lowest abundance extra-galactic HII region (with the possible recent exception of SBS 0335-052; Izotov 1989). As such, it is important for many studies, including the determination of the primordial helium abundance. Recent imaging studies of I Zw 18 have revealed a more complex structure to I Zw 18 than the simple two-component model previously assumed. This has given rise to concern about the reliability of chemical abundance measurements derived for I Zw 18. Researchers have obtained long-slit spectra covering the wavelength range lambda 3650 to lambda 10,000, which allow us to measure physical parameters and chemical abundances as a function of position. With these new data we can investigate the SE component, which has not been studied previously, and we can address some of the concerns about abundance uncertainties.

Skillman, Evan D.↗

NASA NTRS1990

Multiband radar characterization of forest biomes

The utility of airborne and orbital SAR in classification, assessment, and monitoring of forest biomes is investigated through analysis of orbital synthetic aperature radar (SAR) and multifrequency and multipolarized airborne SAR imagery relying on image tone and texture. Preliminary airborne SAR experiments and truck-mounted scatterometer observations demonstrated that the three dimensional structural complexity of a forest, and the various scales of temporal dynamics in the microwave dielectric properties of both trees and the underlying substrate would severely limit empirical or semi-empirical approaches. As a consequence, it became necessary to develop a more profound understanding of the electromagnetic properties of a forest scene and their temporal dynamics through controlled experimentation coupled with theoretical development and verification. The concatenation of various models into a physically-based composite model treating the entire forest scene became the major objective of the study as this is the key to development of a series of robust retrieval algorithms for forest biophysical properties. In order to verify the performance of the component elements of the composite model, a series of controlled laboratory and field experiments were undertaken to: (1) develop techniques to measure the microwave dielectric properties of vegetation; (2) relate the microwave dielectric properties of vegetation to more readily measured characteristics such as density and moisture content; (3) calculate the radar cross-section of leaves, and cylinders; (4) improve backscatter models for rough surfaces; and (5) relate attenuation and phase delays during propagation through canopies to canopy properties. These modeling efforts, as validated by the measurements, were incorporated within a larger model known as the Michigan Microwave Canopy Scattering (MIMICS) Model.

Dobson, M. Craig↗

NASA NTRS1990

AEOSS runtime manual for system analysis on Advanced Earth-Orbital Spacecraft Systems

Advanced earth orbital spacecraft system (AEOSS) enables users to project the required power, weight, and cost for a generic earth-orbital spacecraft system. These variables are calculated on the component and subsystem levels, and then the system level. The included six subsystems are electric power, thermal control, structure, auxiliary propulsion, attitude control, and communication, command, and data handling. The costs are computed using statistically determined models that were derived from the flown spacecraft in the past and were categorized into classes according to their functions and structural complexity. Selected design and performance analyses for essential components and subsystems are also provided. AEOSS has the feature permitting a user to enter known values of these parameters, totally and partially, at all levels. All information is of vital importance to project managers of subsystems or a spacecraft system. AEOSS is a specially tailored software coded from the relational database program of the Acius' 4th Dimension with a Macintosh version. Because of the licensing agreements, two versions of the AEOSS documents were prepared. This version, AEOSS Runtime Manual, is permitted to be distributed with a finite number of the restrictive 4D Runtime version. It can perform all contained applications without any programming alterations.

Lee, Hwa-Ping↗

NASA NTRS1991

Mechanical response tissue analyzer for estimating bone strength

One of the major concerns for extended space flight is weakness of the long bones of the legs, composed primarily of cortical bone, that functions to provide mechanical support. The strength of cortical bone is due to its complex structure, described simplistically as cylinders of parallel osteons composed of layers of mineralized collagen. The reduced mechanical stresses during space flight or immobilization of bone on Earth reduces the mineral content, and changes the components of its matrix and structure so that its strength is reduced. Currently, the established clinical measures of bone strength are indirect. The measures are based on determinations of mineral density by means of radiography, photon absorptiometry, and quantitative computer tomography. While the mineral content of bone is essential to its strength, there is growing awareness of the limitations of the measurement as the sole predictor of fracture risk in metabolic bone diseases, especially limitations of the measurement as the sole predictor of fracture risk in metabolic bone diseases, especially osteoporosis. Other experimental methods in clinical trials that more directly evaluate the physical properties of bone, and do not require exposure to radiation, include ultrasound, acoustic emission, and low-frequency mechanical vibration. The last method can be considered a direct measure of the functional capacity of a long bone since it quantifies the mechanical response to a stimulus delivered directly to the bone. A low frequency vibration induces a response (impedance) curve with a minimum at the resonant frequency, that a few investigators use for the evaluation of the bone. An alternative approach, the method under consideration, is to use the response curve as the basis for determination of the bone bending stiffness EI (E is the intrinsic material property and I is the cross-sectional moment of inertia) and mass, fundamental mechanical properties of bone.

Arnaud, Sara B.↗

NASA NTRS1989

Independent modal space control with positive position feedback

An independent modal space control (IMSC) algorithm is presented, whose modal control forces are generated from a positive position feedback (PPF) strategy. The proposed algorithm combines the attributes of both the IMSC and the PPF, and maintains the simplicity of the IMSC as it designs the controller of a complex structure at the uncoupled modal level. The effectiveness of the algorithm in damping out the vibration of flexible structures is validated experimentally. A simple cantilevered beam is employed as an example of a flexible structure whose multimodes of vibration are controlled by a single actuator. Performance of the active control system is determined in the frequency and the time domains. The experimental results indicate the potential of the proposed methodology as a viable method for controlling the vibration of large flexible structures.

Baz, A.↗

NASA NTRS1991

Circular polarization in 1612 MHz OH maser emission from OH/IR stars

In order to study the incidence of circular polarization in the 1612 MHz OH emission from envelopes of OH/IR stars, the Arecibo radio telescope was used to obtain high-resolution, high signal-to-noise 1612 MHz spectra in both circular polarizations for several OH/IR stars. The method of Troland and Heiles (1982) was used to estimate the intensities of magnetic fields in the envelopes. The fields appear to have complex structures, and intensities on the order of 1-100 microG. The spectra are weakly polarized, probably as a result of weakly polarized emission from individual emitting elements. Integration over the Stokes parameter V yields nonzero results for three of the stars, suggesting that another polarizing mechanism (in addition to simple Zeeman splittings) is at work.

Zell, Philip↗

NASA NTRS1991

Force-free magnetic fields - Generating functions and footpoint displacements

This paper presents analytic and numerical calculations that explore equilibrium sequences of bipolar force-free magnetic fields in relation to displacments of their magnetic footpoints. It is shown that the appearance of magnetic islands - sometimes interpreted as marking the loss of equilibrium in models of the solar atmosphere - is likely associated only with physically unrealistic footpoint displacements such as infinite separation or 'tearing' of the model photosphere. The work suggests that the loss of equilibrium in bipolar configurations, sometimes proposed as a mechanism for eruptive solar events, probably requires either fully three-dimensional field configurations or nonzero plasma pressure. The results apply only to fields that are strictly bipolar, and do not rule out equilibrium loss in more complex structures such as quadrupolar fields.

Wolfson, Richard↗

NASA NTRS1991

Proton-induced radiation damage in germanium detectors

High-purity germanium (HPGe) detectors will be used in future space missions for gamma-ray measurements and will be subject to interactions with energetic particles. To simulate this process, several large-volume n-type HPGe detectors were incrementally exposed to a particle fluence of up to 10 to the 8th protons/sq cm (proton energy: 1.5 GeV) at different operating temperatures (90 to 120 K) to induce radiation damage. Basic scientific and engineering data on detector performance were collected. During the incremental irradiation, the peak shape produced by the detectors showed a significant change from a Gaussian shape to a broad complex structure. After the irradiation, all detectors were thoroughly characterized by measuring many parameters. To remove the accumulated radiation damage, the detectors were stepwise-annealed at temperatures below 110 C, while kept in their specially designed cryostats. This study shows that n-type HPGe detectors can be used in charged-particle environments as high-energy resolution devices until a certain level of radiation damage is accumulated and that the damage can be removed at moderate annealing temperatures and the detector returned to operating condition.

Brueckner, J.↗

NASA NTRS1991

Efficient numerical method for computation of the thermohydrodynamics of laminar lubricating films

The purpose of this paper is to describe an accurate, yet economical, method for computing temperature effects in laminar lubricating films in two dimensions. Because of the marked dependence of lubricant viscosity on temperature, the effect of viscosity variation both across and along a lubricating film can dwarf other deviations from ideal constant-property lubrication. In practice, a thermohydrodynamics program will involve simultaneous solution of the film lubrication problem, together with heat conduction in a solid, complex structure. In pursuit of computational economy, techniques similar to those for Gaussian quadrature are used; it is shown that, for many purposes, the use of just two properly positioned temperatures (Lobatto points) characterizes the transverse temperature distribution.

Elrod, H. G.↗

NASA NTRS1991

Numerical models of extragalactic radio sources

When supercomputer-implemented numerical simulations analyzing the nonlinear physics inherent in the hydrodynamic and MHD equations are applied to extragalactic radio sources, many of the complex structures observed on telescopic images are reproduced. Attention is presently given to recently obtained results from 2D and 3D numerical simulations of the formation and evolution of extended radio morphologies; these numerical models allow the exploration of such physical phenomena as the role of magnetic fields in the dynamics and emissivity of extended radio galaxies, intermittent outflow from the cores of active galaxies, fluid-jet instabilities, and the bending of collimated outflows by motion through the intergalactic medium.

Burns, Jack O.↗

NASA NTRS1991

On the nature of pulsar radiation

A key question in the interpretation of the emission of pulsars is that of the excitation and propagation of waves in the magnetospheric plasma. The magnetosphere of a pulsar has an extremely complex structure and there are many difficulties in the development of its self-consistent model. At present there exist some sufficiently well-grounded models not exactly agreeing with each other. However, the creation of a dense, relativistic, electron-positron plasma in the polar regions of a rotating neutron star magnetospheres is the point of similarity among these models. The pulsar radiation should be generated in such a plasma. The following subject areas are discussed: the pulse radiation mechanism and applications to the central object in SN1987A remnant.

Kazbegi, A. Z.↗

NASA NTRS1992

Influence of airfoil geometry on delta wing leading-edge vortices and vortex-induced aerodynamics at supersonic speeds

An assessment of the influence of airfoil geometry on delta wing leading edge vortex flow and vortex induced aerodynamics at supersonic speeds is discussed. A series of delta wing wind tunnel models were tested over a Mach number range from 1.7 to 2.0. The model geometric variables included leading edge sweep and airfoil shape. Surface pressure data, vapor screen, and oil flow photograph data were taken to evaluate the complex structure of the vortices and shocks on the family of wings tested. The data show that airfoil shape has a significant impact on the wing upper surface flow structure and pressure distribution, but has a minimal impact on the integrated upper surface pressure increments.

Wood, Richard M.↗

NASA NTRS1992

A conceptual design of a large aperture microwave radiometer geostationary platform

A conceptual design of a Large Aperture Microwave Radiometer (LAMR) Platform has been developed and technology areas essential to the design and on-orbit viability of the platform have been defined. Those technologies that must be developed to the requirement stated here for the LAMR mission to be viable include: advanced radiation resistant solar cells, integrated complex structures, large segmented reflector panels, sub 3 kg/m(exp 2) areal density large antennas, and electric propulsion systems. Technology areas that require further development to enhance the capabilities of the LAMR platform (but are not essential for viability) include: electrical power storage, on-orbit assembly, and on-orbit systems checkout and correction.

Garn, Paul A.↗

NASA NTRS1992

Estimating structural attributes of Douglas-fir/western hemlock forest stands from Landsat and SPOT imagery

Relationships between spectral and texture variables derived from SPOT HRV 10 m panchromatic and Landsat TM 30 m multispectral data and 16 forest stand structural attributes is evaluated to determine the utility of satellite data for analysis of hemlock forests west of the Cascade Mountains crest in Oregon and Washington, USA. Texture of the HRV data was found to be strongly related to many of the stand attributes evaluated, whereas TM texture was weakly related to all attributes. Data analysis based on regression models indicates that both TM and HRV imagery should yield equally accurate estimates of forest age class and stand structure. It is concluded that the satellite data are a valuable source for estimation of the standard deviation of tree sizes, mean size and density of trees in the upper canopy layers, a structural complexity index, and stand age.

Cohen, Warren B.↗