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At least 307 records · Page 17

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NASA NTRS1989

Organic solids produced from simple C/H/O/N ices by charged particles - Applications to the outer solar system

The effects of charged particle irradiation by cold plasma discharge on surfaces of H2O:CH4 clathrate with a 200:1 ratio and on ices composed of H2O and C2H6 or C2H2 are examined. The molecules studies are found in Comet Halley and are plausible constituents in icy outer solar system objects. The IR transmission spectra of four ice-tholin residues obtained in the laboratory are compared with spectra produced by irradiation of gases and ices containing simple hydrocarbons. The similarities between CH4 clathrate residue and Halley organic grains, and the surface transport or atmospheric replenishment activity on Triton and Pluto are discussed.

Khare, B. N.↗

NASA NTRS1990

Resonant decay of global MHD modes at 'thick' interfaces

A global surface mode can exist on a 'surface' which is not a true discontinuity, but without sufficient dissipation it is not a normal mode and it decays in time via resonance absorption. The decay rate can be calculated analytically when the 'surface' is thin. The goal of this paper is to numerically estimate the decay rate when the surface is not thin. A cold plasma in a uniform magnetic field is considered in which density varies linearly across the 'surface'. In this linearized calculation, the global surface mode is driven in steady state by an antenna located in one of the uniform regions external to the surface. The frequency is a free parameter, and resonance curves are computed numerically without approximations. The widths of the resonance curves are used to estimate the free decay times of undriven surface modes, via the uncertainty principle. When applied to the active solar corona, the decay rates are large enough to account for the coronal heating, but it should be kept in mind that the role of nonlinearity in resonance absorption is still undetermined.

Hollweg, Joseph V.↗

NASA NTRS1990

A statistical characterization of plasmasphere density structure and boundary locations

Plasmaspheric density profiles and their cold plasma boundaries observed by the retarding ion mass spectrometer aboard the DE-1 satellite were classified using the results of a statistical study. Six different categories of the plasmasphere density profiles were identified: 'featureless', featureless with enhancement near left, multiple-plateau, featureless with trough, multiple-plateau with trough, and other complex structure profiles. An explanation for the occurrence of these different profiles is presented.

Horwitz, J. L.↗

NASA NTRS1990

Innovative techniques for the production of energetic radicals for lunar materials processing including photogeneration via concentrated solar energy

The Department of Materials Science and Engineering (MSE) is investigating the use of monatomic chlorine produced in a cold plasma to recover oxygen and metallurgically significant metals from lunar materials. Development of techniques for the production of the chlorine radical (and other energetic radicals for these processes) using local planetary resources is a key step for a successful approach. It was demonstrated terrestrially that the use of UV light to energize the photogeneration of OH radicals from ozone or hydrogen peroxide in aqueous solutions can lead to rapid reaction rates for the breakdown of toxic organic compounds in water. A key question is how to use the expanded solar resource at the lunar surface to generate process-useful radicals. This project is aimed at investigating that question.

Osborn, D. E.↗

NASA NTRS1990

Relaxation of the loss-cone by quasi-linear diffusion of the electron-cyclotron maser instability in the solar corona

A self-consistent numeric two-dimensional code using kinetic-wave particle equations has been developed and applied to the maser dynamics of the solar corona. Time histories generated by the code reveal details of the evolution of the linear phase, the saturation and subsequent relaxation of the loss cone. Quantitative values are obtained for the saturation time, the amount of the converted free energy, the critical wave energy level for onset of maser diffusion, the velocity and pitch angle range of the resonant particles, and the relative importance of the different magnetoionic modes and harmonics. A wide range of initial conditions is covered by varying the loss-cone distribution and the ambient cold plasma parameters.

Aschwanden, Markus J.↗

NASA NTRS1990

Theory of electromagnetic cyclotron wave growth in a time-varying magnetoplasma

The effect of a time-dependent perturbation in the magnetoplasma on the wave and particle populations is investigated using the Kennel-Petchek (1966) approach. Perturbations in the cold plasma density, energetic particle distribution, and resonance condition are calculated on the basis of the ideal MHD assumption given an arbitrary compressional magnetic field perturbation. An equation is derived describing the time-dependent growth rate for parallel propagating electromagnetic cyclotron waves in a time-varying magnetoplasma with perturbations superimposed on an equilibrium configuration.

Gail, William B.↗

NASA NTRS1990

Auroral kilometric radiation - An example of relativistic wave-particle interaction in geoplasma

The earth's auroral kilometric radiation (AKR) is believed to be produced by the electron-cyclotron maser instability. This instability is the result of a wave-particle interaction in which relativistic effects are crucial. An explanation is given as to how these relativistic effects alter the shape of the resonance curve in velocity space and modify the R - X mode wave dispersion near the electron cyclotron frequency compared to the results obtained in the nonrelativistic limit and from cold-plasma theory. The properties of the cyclotron maser instability in a driven system are illustrated using two-dimensional electromagnetic particle simulations which incorporate a continual flow of primary energetic electrons along the magnetic field.

Pritchett, P. L.↗

NASA NTRS1991

ULF waves upstream of the Venus bow shock - Properties of one-hertz waves

Pioneer Venus Orbiter data are used here to study the properties of a class of ULF upstream waves with relatively high observed frequencies. These waves show significant similarity to 'one-Hz' waves identified at earth in the ISEE 1 and 2 observations and the whistler waves identified earlier by IMP 6 observations. The waves appear almost immediately after the spacecraft crosses the magnetic field tangent line to the bow shock surface into the region of connected field lines. The wave amplitude decreases with distance from the shock measured along the magnetic field line. Group velocities calculated using the cold plasma dispersion relation indicate that the waves have sufficient upstream velocities to propagate form the shock into the solar wind. The totality of observations seem best explained by a source of right-handed whistler mode waves at the bow shock.

Orlowski, D. S.↗

NASA NTRS1991

Evolution to nonequilibrium in simple models of prominence filaments

The support of cold plasma against gravity by a line-tied magnetic field is considered. The normal component of the field at the boundaries, the mass/unit flux, and the magnetic shear are all specified. Fixing the first two of these quantities, and varying the third leads to evolutionary sequences. When the magnetic shear is less than a critical value, an infinite number of solutions exist. When the magnetic shear is sufficiently large, there are no solutions with the desired topology. These results support the idea that prominences erupt as a result of evolution to magnetic nonequilibrium.

Zweibel, Ellen G.↗

NASA NTRS1991

Solidification processing superalloys in an electric field

The phase morphology of a given superalloy's microstructure is established during solidification. Solidification-processing in an applied electric field is a technique currently undergoing experimental characterization at NASA-Marshall. The method is predicated on the simplest model of a metallic solid, in which ions are arranged in basic space lattices that are permeated by an 'electron gas' whose valence electrons are free to move in the solid. When in a superheated liquid state, the metal is seen as a dense 'cold plasma' of electrons and ions. The application of an electric field during solidification establishes a steady, continuous current flow; the direction of propagation of the plasma waves coincides with the direction of the electric field. Such an alignment introduces an ordered arrangement of the electric vectors and introduces an additional degree of order in an already highly ordered alloy.

Ahmed, Shaffiq↗

NASA NTRS1992

Electromagnetic ion cyclotron waves observed near the oxygen cyclotron frequency by ISEE 1 and 2

The first results of observations of ion cyclotron waves by the elliptically orbiting ISEE 1 and 2 pair of spacecraft are reported. The most intense waves (8 nT) were observed in the outer plasmasphere where convection drift velocities were largest and the Alfven velocity was a minimum. Wave polarization is predominantly left-handed with propagation almost parallel to the ambient magnetic field, and the spectral slot and polarization reversal predicted by cold plasma propagation theory are identified in the wave data. Computations of the experimental wave spectra during the passage through the plasmapause show that the spectral slots relate to the local plasma parameters, possibly suggesting an ion cyclotron wave growth source near the spacecraft. A regular wave packet structure seen over the first 30 min of the event is attributed to the modulation of this energy source by the Pc 5 waves seen at the same time.

Fraser, B. J.↗

NASA NTRS1992

Wave-particle interaction at the plasmasphere-ring current interface

During the plasmasphere filling process following geomagnetic storms, an outward density decrease of the cold plasma at L = 3-4 is typically observed. When this structure overlaps with the sharp inner edge of the ring current, wave activity is detected at linearly stable phase velocities. The excitation of these waves around the lower hybrid frequency and their effects on the heating of thermal ions is analyzed. It is found that lower hybrid drift waves are most effective at heating lower mass ions, e.g. plasmaspheric H(+), versus He(+), which may be heated more effectively by electromagnetic ion cyclotron waves driven unstable by the ring current loss cone.

Roth, Ilan↗

NASA NTRS1992

The WIDe-angle GEocoronal Telescope (WIDGET)

We describe the WIDe-angle GEocoronal Telescope, or WIDGET. The telescope is designed to image the cold plasma constituent of the magnetosphere, the plasmasphere, by imaging the He II 304 A emission. It utilizes a spherical normal incidence f/2 mirror with a microchannel plate imaging detector positioned near the focal point of the mirror. The instrument has a 30-deg field of view with a .5-1-deg angular resolution. The mirror has a multilayer coating which, combined with an Al/C filter and the detector response, provides a peak sensitivity of approximately 0.5 ct/sec/Rayleigh/bin at 304 A with a bandpass of about 60 A. The instrument will be flown on a sounding rocket in the Fall of 1992 as a test of its capabilities. It can easily be adapted to different wavelengths (584, 834, and 1216 A) using suitable mirror coatings and filters. In this way, a series of WIDGETs could image different aspects of the inner magnetosphere simultaneously, thus providing a more unified picture of this complex region.

Cotton, Daniel↗

NASA NTRS1993

Propagation characteristics of Pc 3 compressional waves generated at the dayside magnetopause

New, 3D ray tracing of Pc 3 compressional waves from the magnetosheath reveals that the magnetosphere can present a major propagation barrier to the penetration of these waves to the plasmasphere. This barrier is the ion-ion cutoff between the He(+) and O(+) gyroresonances. As a result of the frequency-dependent location of this cutoff, the magnetosphere behaves like a filter for Pc 3 compressional waves, and only low-frequency components of Pc 3 compressional waves can penetrate to inner magnetosphere. Results are in agreement with previous satellite observations. This 'filter action' strongly depends on the relative concentration of He(+) and O(+) and is therefore sensitive to solar and magnetic activity. Ray-tracing results are based on a cold plasma dispersion relation, a semiempirical model of plasma density, and the Mead-Fairfield (1975) magnetic field model.

Zhang, X.↗

NASA NTRS1994

Simultaneous measurements of waves and precipitating electrons near the equator in the outer radiation belt

An investigation of wave-particle interactions is made using several simultaneous electron and wave measurements performed at near-equatorial positions from the Combined Release and Radiation Effects Satellite (CRRES) satellite. Bursts of electron precipitation were observed, most frequently at local times near dawn. Examples of bursts are presented in which the fluxes of the precipitating electrons and the wave intensities are correlated with coefficients as high as 0.7. During bursts the frequencies of the enhanced waves spanned a wide range from 311 Hz to 3.11 kHz, and the energies of the enhanced electrons were in the range 1.7 keV to 288 keV. The changes of the precipitating fluxes were generally less pronounced at the lowest energies. On the basis of electron-cyclotron resonant calculations using the cold plasma densities and ambient magnetic fields taken from the CRRES measurements it was found that the wave frequencies and precipitating electron energies were generally consistent with those expected from electron resonance with parallel propagating whistler waves. The electron data of principal concern here were acquired in and about the loss cone with narrow angular resolution spectrometers covering the energy range 340 eV to 5 MeV. The wave data included electric field measurements spanning frequencies from 5 Hz to 400 kHz and magnetic field measurements from 5 Hz to 10 kHz.

Imhof, W. L.↗

NASA NTRS1995

An MHD simulation study of the poloidal mode field line resonance in the Earth's dipole magnetosphere

The poloidal mode field line resonance in the Earth's dipole magnetic field is investigated using cold plasma ideal MHD simulations in dipole geometry. In order to excite the poloidal mode resonance, we use either an initial or a continuous velocity perturbation to drive the system. The perturbation is localized at magnetic shell L = 7 with plasma flow in the radial direction (electric field component in the azimuthal direction). It is found that with the initial perturbation alone, no polodial mode resonance can be obtained and the initially localized perturbation spreads out across all magnetic L shells. With the continuous perturbation, oscillating near the poloidal resonance frequency, a global-scale poloidal cavity mode can be obtained. For the first time, a localized guided poloidal mode resonance is obtained when a radial component of electric field is added to the initial perturbation such that the curl of the electric field is everywhere perpendicular to the background dipole magnetic field. During the localized poloidal resonance, plasma vortices parallel/antiparallel to the background dipole magnetic field B(sub 0). This circular flow, elongated radially, results in twisting of magnetic field flux tubes, which, in turn, leads to the slowdown of the circular plasma flow and reversal of the plasma vortices. The energy associated with the localized poloidal resonance is conserved as it shifts back and forth between the oscillating plasma vortices and the alternately twisted magnetic flux tubes. In the simulations the eigenfunctions associated with the localized poloidal resonance are grid-scale singular functions. This result indicates that ideal MHD is inadequate to describe the underlying problem and nonideal MHD effects are needed for mode broadening.

Ding, D. Q.↗

NASA NTRS1994

The streaming-trapped ion interface in the equatorial inner magnetosphere

Spacecraft measurements of core ions on L=4-7 field-lines typically show trapped ion distributions near the magnetic equator, and frequently indicate field-aligned ion streams at higher latitudes. The nature of the transition between them may indicate both the microphysics of hot-cold plasma interactions and overall consequences for core plasma evolution. We have undertaken a statistical analysis and characterization of this interface and its relation to the equatorial region of the inner magnetosphere. In this analysis, we have characterized such features as the equatorial ion flux anisotropy, the penetration of field-aligned ionospheric streams into the equatorial region, the scale of the transition into trapped ion populations, and the transition latitude. We found that most transition latitudes occur within 13 deg of the equator. The typical values of equatorial ion anisotropies are consistent with bi-Maxwellian temperature ratios of T(sub perpendicular)/T(sub parallel) in the range of 3-5. The latitudinal scales for the edges of the trapped ion populations display a rather strong peak in the 2-3 deg range. We also found that there is a trend for the penetration ratio, the anisotropy half width, and the transition scale length to decrease with a higher equatorial ion anisotropy. We may interpret these features in terms of Liouville mapping of equatorially trapped ions and the reflection of the incoming ionospheric ion streams from the equatorial potential peaks associated with such trapped ions.

Lin, J.↗

NASA NTRS1999

Polar Spacecraft Observations of the Turbulent Outer Cusp/Magnetopause Boundary Layer of Earth

The orbit of the Polar spacecraft has been ideally suited for studying the turbulent region of the cusp that is located near or just outside the magnetopause current sheet at 7-9 R(sub E). The wave data obtained in this region show that electromagnetic turbulence is dominant in the frequency range 1-10 Hz. The waves responsible for this turbulence usually propagate perpendicular to the local magnetic field and have an index of refraction that generally falls between the estimated cold plasma theoretical values of the electromagnetic lower hybrid and whistler modes and may be composed of both modes in concert with kinetic Alfven waves and/or fast magnetosonic waves. Fourier spectra of the higher frequency wave data also show the electromagnetic turbulence at frequencies up to and near the electron cyclotron frequency. This higher frequency electromagnetic turbulence is most likely associated with whistler mode waves. The lower hybrid drift and current gradient instabilities are suggested as possible mechanisms for producing the turbulence. The plasma and field environment of this turbulent region is examined and found to be extremely complex. Some of the wave activity is associated with processes occurring locally, such as changes in the DC magnetic field, while others are associated with solar wind and interplanetary magnetic field changes.

Pickett, J. S.↗