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  • 1
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    Ion-acoustic waves in a plasma with negative ions (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Propagation and damping of ion-acoustic waves have been investigated in a Q-machine plasma consisting of K(+) positive ions, SF6(-) negative ions, and electrons. The phase velocity of the ion-acoustic 'fast' mode increases with increasing epsilon, the concentration of negative ions. The wave damping decreases with increasing epsilon, and nearly disappears, for the highest wave frequencies investigated, when epsilon is more than about 0.9. Both results are in agreement with predictions from Vlasov theory.
    Keywords: PLASMA PHYSICS
    Type: Physics of Fluids B (ISSN 0899-8221); 3; 284-287
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  • 2
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    Electrostatic ion-cyclotron waves in a two-ion component plasma (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The excitation of electrostatic ion cyclotron (EIC) waves is studied in a single-ended Q machine in a two-ion component plasma (Ca+ and K+). Over a large range of relative concentrations of Cs+ and K+ ions, two modes are excited with frequencies greater than the respective cyclotron frequencies of the ions. The results are discussed in terms of a fluid theory of electrostatic ion cyclotron waves in a two-ion component plasma.
    Keywords: PLASMA PHYSICS
    Type: IEEE Transactions on Plasma Science (ISSN 0093-3813); 16; 396-398
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  • 3
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    Transition from moving to stationary double layers in a single-ended Q machine (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Large-amplitude (less than about 100 percent) relaxation oscillations in the plasma potential are known to be generated when the cold endplate of a single-ended Q machine is biased positively. These oscillations are associated with double layers that form near the hot plate (plasma source) and travel toward the endplate at about the ion-acoustic velocity. At the endplate they dissolve and then form again near the hot plate, the entire process repeating itself in a regular manner. By admitting a sufficient amount of neutral gas into the system, the moving double layers were slowed down and eventually stopped. The production of stationary double layers requires an ion source on the high-potential side of the double layers. These ions are provided by ionization of the neutral gas by electrons that are accelerated through the double layer. The dependence of the critical neutral gas pressure required for stationary double-layer formation on endplate voltage, magnetic field strength, and neutral atom mass has been examined. These results are discussed in terms of a simple model of ion production and loss, including ion losses across the magnetic field.
    Keywords: PLASMA PHYSICS
    Type: Physics of Fluids B (ISSN 0899-8221); 2; 1936-194
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  • 4
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    Ion-beam focusing in a double-plasma device (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The authors studied the propagation of a low-energy charge-neutralized ion beam injected into the target region of a long double-plasma device. A magnetic field of up to about 180 G may be applied along the axis of the device. As a result of charge exchange collisions, the ion beam is attenuated as it propagates into the target region. However, under certain conditions of magnetic field strength and neutral gas pressure, the authors have observed a `reemergence' of the beam on axis far downstream in the target. This reemergence of the ion beam is attributed to a focusing of the ions by a self-consistently produced radial ambipolar electric field. The effect may be expected to occur in other types of plasma devices as well, whenever a sufficiently large radially inward electric field is present.
    Keywords: PLASMA PHYSICS
    Type: IEEE Transactions on Plasma Science (ISSN 0093-3813); 16; 590-596
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  • 5
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    Mediterranean Crustal Motions: Numerical Models Constrained by Geologic and Space Geodetic Data (1996)
    In:  Other Sources
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    Publication Date: 2018-06-08
    Keywords: Geophysics
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  • 6
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    High Upward Fluxes of Formic Acid from a Boreal Forest Canopy (2016)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Eddy covariance fluxes of formic acid, HCOOH, were measured over a boreal forest canopy in spring/summer 2014. The HCOOH fluxes were bidirectional but mostly upward during daytime, in contrast to studies elsewhere that reported mostly downward fluxes. Downward flux episodes were explained well by modeled dry deposition rates. The sum of net observed flux and modeled dry deposition yields an upward gross flux of HCOOH, which could not be quantitatively explained by literature estimates of direct vegetative soil emissions nor by efficient chemical production from other volatile organic compounds, suggesting missing or greatly underestimated HCOOH sources in the boreal ecosystem. We implemented a vegetative HCOOH source into the GEOS-Chem chemical transport model to match our derived gross flux and evaluated the updated model against airborne and spaceborne observations. Model biases in the boundary layer were substantially reduced based on this revised treatment, but biases in the free troposphere remain unexplained.
    Keywords: Geophysics
    Type: GSFC-E-DAA-TN47288 , Geophysical Research Letters (ISSN 0094-8276); 43; 17; 9342-9351
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  • 7
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    Two Years of Chemical Sampling on Meridiani Planum by the Alpha Particle X-Ray Spectrometer Onboard the Mars Exploration Rover Opportunity (2006)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: For over two terrestrial years, the Mars Exploration Rover Opportunity has been exploring the martian surface at Meridiani Planum using the Athena instrument payload [1], including the Alpha Particle X-Ray Spectrometer (APXS). The APXS has a small sensor head that is mounted on the robotic arm of the rover. The chemistry, mineralogy and morphology of selected samples were investigated by the APXS along with the Moessbauer Spectrometer (MB) and the Microscopic Imager (MI). The Rock Abrasion Tool (RAT) provided the possibility to dust and/or abrade rock surfaces down to several millimeters to expose fresh material for analysis. We report here on APXS data gathered along the nearly 6-kilometers long traverse in craters and plains of Meridiani.
    Keywords: Geophysics
    Type: 37th Lunar and Planetary Science Conference; Mar 13, 2006 - Mar 17, 2006; South Shore harbor, League City, Texas; United States
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  • 8
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    Plasma density fluctuations observed during Space Shuttle Orbiter water releases (1989)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Observations by the Langmuir probe on the Plasma Diagnostics Package flown as part of the Spacelab 2 mission in the summer of 1985 show a strong increase in the level of turbulence near the Shuttle Orbiter during operations in which liquid water is released. The spectrum of the plasma density fluctuations peaks at the lowest frequencies measured (a few Hz) and extends up to a few kHz, near the lower hybrid frequency. Two potential mechanisms for generating the plasma turbulence are suggested which are both based on the production of water ions as a result of charge exchange with the ambient oxygen ions in the ionosphere. The first mechanism proposed is the ion-plasma instability which arises from the drift of the contaminant with respect to the ambient oxygen ions. The other mechanism proposed is the Ott-Farley instability, which is a result of the ring distribution formed by the 'pick-up' water ions.
    Keywords: PLASMA PHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 94; 12081-12
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  • 9
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    Plasma density, temperature and turbulence in the wake of the Shuttle Orbiter (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: In situ observations of the plasma density, temperature, and turbulence in the near wake, midwake, and far wake of the Shuttle Orbiter are presented. The results show that the plasma disturbances produced by the passage of the Orbiter extend downstream for a distance of several hundred meters behind the Orbiter. It is found that the electron density profile exhibits nonmonotonic behavior, and that most of the 6-40-Hz turbulence in the wake is confined to a region about 20 m downstream of the Orbiter. The saturation Delta N(e)/N(e) spectra rapidly decline in the 10,000-100,000-Hz decade, both in the ambient ionosphere and in the wake.
    Keywords: PLASMA PHYSICS
    Type: Planetary and Space Science (ISSN 0032-0633); 37; 1001-101
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  • 10
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    Impact Crater in Coastal Patagonia (2000)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: Impact craters are geological structures attributed to the impact of a meteoroid on the Earth's (or other planet's) surface (Koeberl and Sharpton. 1999). The inner planets of the solar system as well as other bodies such as our moon show extensive meteoroid impacts (Gallant 1964, French 1998). Because of its size and gravity, we may assume that the Earth has been heavily bombarded but weathering and erosion have erased or masked most of these features. In the 1920's, a meteor crater (Mark 1987) was identified in Arizona and to this first finding the identification of a large number of impact structures on Earth followed (Hodge 1994). Shock metamorphic effects are associated with meteorite impact craters. Due to extremely high pressures, shatter cones are produced as well as planar features in quartz and feldspar grains, diaplectic glass and high-pressure mineral phases such as stishovite (French 1998).
    Keywords: Geophysics
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