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NASA NTRS1990

Lightning/rainfall relationships during COHMEX

Initial results of relationships between cloud-to-ground lightning and radar-derived precipitation measurements in air mass thunderstorms (multi- and single cell) occurring over the Southeastern United States are presented. Buoyancy values computed from soundings are used to estimate wind shear, the energy available to storms, and the ability of the environment to support strong updrafts. Precipitation measurements are utilized for sampling the rainfall history of entire storm life cycles. It is shown that the average rain volume per ground discharge decreases as the buoyancy increases; this is attributed to stronger updrafts elevating mass to higher levels in the storm where more frequent ice particle interactions may play a major role in separating charge leading to enhanced lightning activity. Thus, total lightning rates are expected to correlate better with rainfall production.

Goodman, S. J.↗

NASA NTRS1990

High-temperature slow-strain-rate compression studies on CoAl-TiB2 composites

Results are presented of compressive deformation tests performed on particulate-reinforced CoAl-TiB2 composites in the temperature range 1100-1300 K. Hot-pressed and postdeformation microstructures were characterized by TEM and by optical microscopy. It was found that the addition of TiB2 particles improves the deformation resistance of the matrix, due to dislocation-particle interactions.

Mannan, S. K.↗

NASA NTRS1989

Abundances from solar flare line spectroscopy

Updating and improvement of computer codes used in the abundance analyses of solar-flare gamma-ray data are described. These modifications are itemized in detail. The abundance analysis technique was then applied to data obtained by the Gamma-Ray Spectrometer of the Solar Maximum Mission spacecraft for the 27 April 1981 solar flare to set constraints on the interacting-particle angular distribution. The results are presented. Research in several other areas was also performed and they are discussed.

Murphy, Ronald J.↗

NASA NTRS1990

Steric stabilization of nonaqueous silicon slips. I - Control of particle agglomeration and packing. II - Pressure casting of powder compacts

The application of steric stabilization to control particle agglomeration and packing of silicon powder in benzene and trichloroethylene is reported. The results provide useful guidelines for controlling unfavorable particle-particle interactions during nonaqueous processing of silicon-based ceramic materials. The application of steric stabilization to the control and improvement of green processing of nonaqueous silicon slips in pressure consolidation is also demonstrated.

Kerkar, Awdhoot V.↗

NASA NTRS1990

Evidence for the presence of quasi-two-dimensional nearly incompressible fluctuations in the solar wind

Assuming that the slab and isotropic models of solar wind turbulence need modification (largely due to the observed anisotropy of the interplanetary fluctuations and the results of laboratory plasma experiments), this paper proposes a model of the solar wind. The solar wind is seen as a fluid which contains both classical transverse Alfvenic fluctuations and a population of quasi-transverse fluctuations. In quasi-two-dimensional turbulence, the pitch angle scattering by resonant wave-particle interactions is suppressed, and the direction of minimum variance of interplanetary fluctuations is parallel to the mean magnetic field. The assumed incompressibility is consistent with the fact that the density fluctuations are small and anticorrelated, and that the total pressure at small scales is nearly constant.

Matthaeus, William H.↗

NASA NTRS1991

On the precipitation of relativistic electrons from the outer belt

A model in which a partially filled drift loss cone plays a prominent role is presently used to investigate relativistic electron data obtained by the Defense Meteorological Satellite Program spacecraft in the outer belt region. The good agreement obtained between model predictions and observations indicates that electron drift in the drift loss cone, which is known to dominate electron precipitation phenomena at lower latitudes, is also important at outer beta latitudes. In conjunction with the degree of partial filling of the drift loss cone, this effect indicates that slow pitch angle diffusion dominates the morphology of outer belt electrons. It is concluded that half of the electron precipitation from the outer belt is due to wave-particle interactions, and the other half by auroral activity and electron drift in the drift loss cone.

Sheldon, W. R.↗

NASA NTRS1991

Computational astrophysical fluid dynamics

The field of astrophysical fluid dynamics (AFD) is described as an emerging discipline which derives historically from both the theory of stellar evolution and space plasma physics. The fundamental physical assumption behind AFD is that fluid equations of motion accurately describe the evolution of plasmas on scales that are large in comparison with particle interaction length scales. Particular attention is given to purely fluid models of large-scale astrophysical plasmas. The role of computer simulation in AFD research is also highlighted and a suite of general-purpose application codes for AFD research is discussed. The codes are called ZEUS-2D and ZEUS-3D and solve the equations of AFD in two and three dimensions, respectively, in several coordinate geometries for general initial and boundary conditions. The topics of bipolar outflows from protostars, galactic superbubbles and supershells, and extragalactic radio sources are addressed.

Norman, Michael L.↗

NASA NTRS1990

The coherent Cerenkov radiated power from a group of field-aligned test particles in a magnetoplasma

An expression is derived that describes the coherent Cerenkov radiated power from a group of test particles in a plasma medium moving parallel to a magnetic field. In this analysis, each particle has an arbitrary position and velocity along a field line and, as a consequence, both the spatial and temporal coherence of the radiation are considered. As an example, it is demonstrated that a monoenergetic electron beam consisting of small pulses can generate wave powers well above incoherent levels if the pulse spacing is comparable to an integer number of emission wavelengths. It is also shown that, if the beam particles have a velocity spread, Delta-V, the wave powers will decrease in time due to the reduced temporal coherence of the particle radiators, where this coherence scales as 1/Delta-V. This latter effect applies to any charged particle beam propagating in a magnetoplasma, because even an initially monoenergetic beam becomes thermalized by electrostatic wave-particle interactions reducing the radiated power.

Farrell, W. M.↗

NASA NTRS1991

Pitch-angle scattering of cometary ions - Computer simulations

The relationships between the rate of pitch-angle scattering induced by wave-particle interactions and two parameters, alpha, and the cometary ion interjection rate, Lambda-j, are studied with the aid of one-dimensional electromagnetic hybrid computer simulations of homogeneous plasmas. The simulations are done with relatively weak ion injection rates commensurate with those in the distant environment of the Comet Halley. In response to the linear temporal growth of the fluctuating magnetic field energy, the injected ions pitch-angle scatter toward isotropy in both the quasi-parallel and quasi-perpendicular regimes. For the injection of cometary oxygen ions, the simulations show pitch-angle scattering rates that increase as the square root of the injection rate. The oxygen ion pitch-angle scattering rates are roughly twice as large at perpendicular injection, again in qualitative agreement with observations.

Gary, S. P.↗

NASA NTRS1991

Magnetospheric radio and plasma wave research - 1987-1990

This review covers research performed in the area of magnetospheric plasma waves and wave-particle interactions as well as magnetospheric radio emissions. The report focuses on the near-completion of the discovery phase of radio and plasma wave phenomena in the planetary magnetospheres with the successful completion of the Voyager 2 encounters of Neptune and Uranus. Consideration is given to the advances made in detailed studies and theoretical investigations of radio and plasma wave phenomena in the terrestrial magnetosphere or in magnetospheric plasmas in general.

Kurth, W. S.↗

NASA NTRS1992

Microgravity nucleation and particle coagulation experiments support

This project is a part of a program at GSFC to study to formation and growth of cosmic dust grain analogs under terrestrial as well as microgravity conditions. Its primary scientific objective is to study the homogeneous nucleation of refractory metal vapors and a variety of their oxides among others, while the engineering, and perhaps a more immediate objective is to develop a system capable of producing mono-dispersed, homogeneous suspensions of well-characterized refractory particles for various particle interaction experiments aboard the Space Shuttle and Space Station Freedom. Both of these objectives are to be met by a judicious combination of laboratory experiments on the ground and aboard NASA's KC-135 experimental research aircraft. Major effort during the current reporting period was devoted to the evaluation of our very successful first series of microgravity test runs in Feb. 1990. Although the apparatus performed well, it was decided to 'repackage' the equipment for easier installation on the KC-135 and access to various components. It will now consist of three separate racks: one each for the nucleation chamber, the power subsystem, and the electronic packages. The racks were fabricated at the University of Virginia and the assembly of the repackaged units is proceeding well. Preliminary analysis of the video data from the first microgravity flight series was performed and the results appear to display some trends expected from Hale's Scaled Nucleation Theory of 1986. The data acquisition system is currently being refined.

Lilleleht, L. U.↗

NASA NTRS1991

Comets - A laboratory for plasma waves and instabilities

The review discusses the various plasma waves and instabilities detected in the vicinity of comets. Particular emphasis is placed on nonlinear wave evolution, as well as the role of nonlinear waves in wave-particle interactions. Areas that need further research are identified to help persons who wish to contribute to the field.

Tsurutani, Bruce T.↗

NASA NTRS1991

The linear Boltzmann equation in slab geometry - Development and verification of a reliable and efficient solution

The linear Boltzmann equation can be cast in a form mathematically identical to the radiation-transport equation. A multigroup procedure is used to reduce the energy (or velocity) dependence of the transport equation to a series of one-speed problems. Each of these one-speed problems is equivalent to the monochromatic radiative-transfer problem, and existing software is used to solve this problem in slab geometry. The numerical code conserves particles in elastic collisions. Generic examples are provided to illustrate the applicability of this approach. Although this formalism can, in principle, be applied to a variety of test particle or linearized gas dynamics problems, it is particularly well-suited to study the thermalization of suprathermal particles interacting with a background medium when the thermal motion of the background cannot be ignored. Extensions of the formalism to include external forces and spherical geometry are also feasible.

Stamnes, K.↗

NASA NTRS1991

Uranus

The present publication discusses Uranus and its interior, atmosphere, rings, satellites, and magnetosphere. Attention is given to photochemistry and vertical mixing, spectroscopy and chemistry of the Uranian atmosphere, the thermal structure and energy balance of Uranian atmospheric dynamics and circulations, and clouds and aerosols in the Uranian atmosphere. Topics considered include particle properties and processes in Uranus' rings, the origin and properties of the Uranian satellites, the geology and cratering of the Uranian satellites, and the planet's magnetic field and magnetospheric configuration. Also addressed are wave-particle interactions in the Uranian magnetosphere, the magnetic field and magnetospheric configuration of Uranus, the plasma environment of Uranus, and Uranus as a radio source.

Bergstralh, Jay T.↗

NASA NTRS1992

Electrostatic waves due to field-aligned electron beams in the low-latitude boundary layer

Mass-resolved ion, electron, and plasma wave data obtained from several low-latitude boundary layer (LLBL) crossings by the AMPTE CCE satellite are analyzed. The data clearly separate the LLBL from the adjacent magnetosheath and magnetosphere. Attention was focused on wave-particle interactions involving electrons. Electron beams were found to be present in the LLBL during the southward interplanetary magnetic field, along with a simultaneous enhancement of electrostatic waves with parallel polarization. Linear theory analysis shows that for plasma conditions in the LLBL, electron beams are unstable to electrostatic waves that propagate parallel to the local magnetic field, in agreement with observations. A numerical simulation study of the beam-plasma interaction in the LLBL shows that the instability saturates by thermalization of the beam but that a beamlike structure can still remain in the electron distribution for certain initial parameters. It is suggested that peaks in the electron velocity distribution function may be found in the LLBL away from the beam source region.

Peroomian, V.↗

NASA NTRS1990

Uranus - The planet, rings and satellites

The present overview of available data regarding Uranus and its planetary structures encompasses ground- and space-based observations of the planet with specific attention given to the interpretation of Voyager 2 data. A brief examination of historical observations is given which includes its discovery, position determination, and the related discoveries of Uranus' five large satellites and rings. The observational data preceding the Voyager 2 mission are reviewed in terms of the planetary interior, atmosphere, and magnetosphere. The Voyager 2 mission is given detailed treatment with descriptions of NASA's development and deployment of the spacecraft as well as detailed data from the Uranus encounter. Reference is given to structure and composition of Uranus' atmosphere, valid models of the interior, and wave-particle interactions in the magnetosphere. The structures of the rings and satellites are examined with reference to specific observational requirements from future missions.

Miner, Ellis D.↗

NASA NTRS1992

Comparative observations of plasma waves at the outer planets

Observations of the plasma-wave spectra from the four major outer planets are examined with particular attention given to the instabilities related to the precipitation of energetic particles. Wave-particle interactions involving hiss, chorus, and electron-cyclotron harmonic emissions are studied. These interactions contribute about 10 exp 13 W into the Jovian atmosphere, and the electron-cyclotron emissions on Saturn are shown to contribute significantly to the precipitation of energetic electrons. Whistler-mode waves are shown to exist at Uranus with intensities that could put 3-40 keV electrons on strong diffusion, but the amplitudes of the emissions are not sufficient to make them a significant part of precipitation. Weak electron-cyclotron harmonic emissions and whistler modes are detected at Neptune, suggesting that the present dataset is based on regions of low wave intensities.

Kurth, W. S.↗

NASA NTRS1988

Gamma ray astrophysics to the year 2000. Report of the NASA Gamma Ray Program Working Group

Important developments in gamma-ray astrophysics up to energies of 100 GeV during the last decade are reviewed. Also, the report seeks to define the major current scientific goals of the field and proposes a vigorous program to pursue them, extending to the year 2000. The goals of gamma-ray astronomy include the study of gamma rays which provide the most direct means of studying many important problems in high energy astrophysics including explosive nucleosynthesis, accelerated particle interactions and sources, and high-energy processes around compact objects. The current research program in gamma-ray astronomy in the U.S. including the space program, balloon program and foreign programs in gamma-ray astronomy is described. The high priority recommendations for future study include an Explorer-class high resolution gamma-ray spectroscopy mission and a Get Away Special cannister (GAS-can) or Scout class multiwavelength experiment for the study of gamma-ray bursts. Continuing programs include an extended Gamma Ray Observatory mission, continuation of the vigorous program of balloon observations of the nearby Supernova 1987A, augmentation of the balloon program to provide for new instruments and rapid scientific results, and continuation of support for theoretical research. Long term recommendations include new space missions using advanced detectors to better study gamma-ray sources, the development of these detectors, continued study for the assembly of large detectors in space, collaboration with the gamma-ray astronomy missions initiated by other countries, and consideration of the Space Station attached payloads for gamma-ray experiments.

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