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At least 199 records · Page 11

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NASA NTRS1990

A current disruption mechanism in the neutral sheet - A possible trigger for substorm expansions

A linear analysis is performed to investigate the kinetic cross-field streaming instability in the earth's magnetotail neutral sheet region. Numerical solution of the dispersion equation shows that the instability can occur under conditions expected for the neutral sheet just prior to the onset of substorm expansion. The excited waves are obliquely propagating whistlers with a mixed polarization in the lower hybrid frequency range. The ensuing turbulence of this instability can lead to a local reduction of the cross-tail current causing it to continue through the ionosphere to form a substorm current wedge. A substorm expansion onset scenario is proposed based on this instability in which the relative drift between ions and electrons is primarily due to unmagnetized ions undergoing current sheet acceleration in the presence of a cross-tail electric field. The required electric field strength is within the range of electric field values detected in the neutral sheet region during substorm intervals. The skew in local time of substorm onset location and the three conditions under which substorm onset is observed can be understood on the basis of the proposed scenario.

Lui, A. T. Y.↗

NASA NTRS1990

The dependence of modeled OI 1356 and N2 Lyman Birge Hopfield auroral emissions on the neutral atmosphere

The sensitivity of selected auroral emissions to anticipated changes in the neutral atmosphere was investigated from the results of a series of sensitivity studies conducted using an auroral emission code developed by Richards and Torr (1990). In particular, the behavior of OI 1356 and two Lyman Birge Hopfield (LBH) bands and their ratios to each other with changing atmospheric composition was examined. It was found that, for anticipated average uncertainties in the neutral atmosphere (factor 2 at auroral altitudes), the resultant change in the modeled intensities is comparable to or less than the uncertainty in the neutral atmosphere. The variation in the I 1356/I 1838 ratio over the equivalent of a solar cycle is less than 50 percent, and the summer-to-winter changes are approximately a factor of 2.

Germany, G. A.↗

NASA NTRS1990

The position of the magnetotail neutral sheet in the near-earth region

The position of the near-earth neutral sheet throughout the nightside region between R = 5 R(E) and R = 8.8 R(E) is examined statistically using magnetic field data from AMPTE/CCE. As has been demonstrated previously, the latitudinal position of the neutral sheet is primarily controlled by the dipole tilt angle. The position also depends on local time, radial distance, and magnetic activity. These dependencis may be represented by the empirical formula MLAT = - (0.14 Kp + 0.69) /cos(Phi)/exp 1/3 (0.065 R exp 0.8 - 0.16) DTA, where MLAT is the position in degrees of the neutral sheet relative to the magnetic equator, Kp is the 3-hourly magnetic activity index, Phi is the magnetic local time in degrees (midnight = 0 deg), R is the radius in R(E), and DTA is the dipole tilt angle in degrees.

Lopez, Ramon E.↗

NASA NTRS1990

VLA neutral hydrogen imaging of compact groups

Images of the neutral hydrogen (H I) in the direction of the compact groups of galaxies, HCG 31, HCG 44, and HCG 79 are presented. The authors find in HCG 31 and HCG 79, emission contained within a cloud much larger than the galaxies as well as the entire group. The H I emission associated with HCG 44 is located within the individual galaxies but shows definite signs of tidal interactions. The authors imaged the distribution and kinematics of neutral hydrogen at the two extremes of group sizes represented in Hickson's sample. HCG 44 is at the upper limit while HCG 18, HCG 31, and HCG 79 are at the lower end. Although the number of groups that have been imaged is still very small, there may be a pattern emerging which describes the H I morphology of compact groups. The true nature of compact groups has been the subject of considerable debate and controversy. The most recent observational and theoretical evidence strongly suggests that compact groups are physically dense, dynamical systems that are in the process of merging into a single object (Williams and Rood 1987, Hickson and Rood 1988, Barnes 1989). The neutral hydrogen deficiency observed by Williams and Rood (1987) is consistent with a model in which frequent galactic collisions and interactions have heated some of the gas during the short lifetime of the group. The H I disks which are normally more extended than the luminous ones are expected to be more sensitive to collisions and to trace the galaxy's response to recent interactions. Very Large Array observations can provide in most cases the spatial resolution needed to confirm the dynamical interactions in these systems.

Williams, B. A.↗

NASA NTRS1990

A model of multigeneration neutrals in cometary comae

The spatial distribution of neutral species in the outer coma of a comet is described using a modified version of the fountain model. This model accounts for neutrals that are produced in multiple generations after release from the nucleus (or production in the collision zone of the inner coma) by an iterative application of the fountain model. A preliminary description of the model is given; however, results are presented in a future paper. Suitable applications of the model include the following: (1) distributed coma sources of neutral gas phase species from anisotropic dust emission; (2) the description of cometary comae at large heliocentric distances; and (3) day/night asymmetry of gas production from the nucleus.

Boice, Dan C.↗

NASA NTRS1992

Energetic ions as remote probes of X type neutral lines in the geomagnetic tail

The study presents a summary of particle orbit signatures of a neutral line in the geomagnetic tail current sheet (CS) as a function of spatial position along the CS boundary layer, as well as within the CS. Six types of CS particle orbits are reviewed: transient B sub z, transient X line, quasi-trapped, trapped, noncrossing X line, and limiting (singular) cases. It is found that for single particle dynamics in a current sheet with neutral line, the phase space ridge is predicted to be found throughout the current sheet if it is not destroyed by collective behavior. An initially tailward flowing distribution causes asymmetries in distributions earthward vs tailward of the neutral line. It is suggested that the chaotic pitch angle scattering may be a dominant mechanism to produce nightside proton isotropy in the auroral zones.

Speiser, T. W.↗

NASA NTRS1992

A hybrid flow model for charged and neutral particles around spacecraft in Low Earth Orbit

A flow simulation for charged and neutral particles around spacecraft is given that combines the fully coupled effects of neutral particle flow, plasma flow, electromagnetic field effects, and spacecraft charging. The simulation allows for chemically reacting flows (associative ionization, dissociation, and chain exchange) and for thermal accommodation at a spacecraft surface. The solution procedure was applied to several types of flows with orbital altitudes ranging from 250 km to 500 km. Excellent agreement with previously verified computational algorithms was obtained for both the neutral and charged particle cases. Results for the full flow simulation are presented for altitudes of 250 km and 500 km.

Justiz, Charles R.↗

NASA NTRS1992

Plasma-neutral interaction processes in the magnetosphere of Saturn

Models of Saturnian magnetospheric processes are reviewed emphasizing the interaction of charged and neutral particles in the gaseous phase and mentioning the role of solid matter. It is found that interpretations of different Voyager datasets regarding the Saturnian magnetosphere can vary. Specific interactions examined to resolve these discrepancies include charge exchange, ion-atom interchange, isotropizing and thermalizing collisions, and interactions between magnetospheric charged particles and surface layers of the icy satellites. The latter interactions result in sputtering of the surface or atmosphere as well as neutral injections into the magnetosphere. Constraints based on known reaction rates are shown to be useful in analyzing the abundances of the water-group molecules. The composition of the magnetospheric plasma is shown to be related to the differences between the interactions of atomic and molecular plasmas with neutral matter.

Eviatar, Aharon↗

NASA NTRS1992

A particle model for magnetotail neutral sheet equilibria

A particle model of the magnetotail neutral sheet self-consistent structure was developed by assuming that the thickness of the neutral sheet is much less than the thickness of the plasma sheet. This made it possible to approximate the neutral sheet as an x-independent structure. The role of nonlinear particle motions in determining the conductivity and the global current sheet structure is investigated.

Burkhart, G. R.↗

NASA NTRS1992

The neutral circulation in the vicinity of a stable auroral arc

The effects of asymmetric potential distributions, arc-neutral feedback, and cross-arc winds on the neutral circulation near auroral arc were examined using a sophisticated two-dimensional high-resolution model of the neutral atmosphere which was preconditioned by allowing weak diffuse aurora to exist prior to the formation of an arc. Simulations were performed with asymmetric potential structures and with an initial state which was warmed and accelerated due to a preexisting diffuse aurora, as well as with arc-zonal wind feedback and with large-scale cross-arc flow. It is shown that, with a preexisting zonal flow and asymmetrical arc structures, strong zonal winds are generated within the arc. Simulations with the arc-zonal feedback indicate that the effect of feedback is not large because of the limited duration of an arc. Simulations with a strong large-scale cross-arc wind indicate that the zonal wind is not strongly sensitive to the large-scale wind.

Walterscheid, R. L.↗

NASA NTRS1992

Neutral buoyancy simulation of space telerobotics operations

A number of neutral buoyancy versions of space telerobots have been tested in the Space Research Laboratory to understand the potential roles for telerobotic systems in space operations. Particular attention is given to details of the well-modeled dynamic environment, the existence of sizable data base on EVA operations in neural buoyancy with correlation to flight experience, and routine access to a number of high-fidelity mockups of past and planned operational spacecraft. The compromises necessary for the design and construction of neutral buoyancy telerobotic systems are discussed, and data from a number of past simulations, including correlation of neutral buoyancy structural assembly with EVA flight data, and preliminary tests of telerobotic servicing of Hubble Space Telescope are summarized.

Akin, David L.↗

NASA NTRS1993

Linear theory of fast reconnection at an X-type neutral point

A complete analytic theory of dynamic linear reconnection at an X-type neutral point is developed. An eigenmode analysis, using cylindrical coordinates centered on the neutral point, extends the work of Craig & McClymont (1991) to include nonazimuthally symmetric perturbations on the flux function. It is demonstrated that all physically significant disturbances, both reconnective and nonreconnective, decay resistively on a 'fast' time scale. The significance of the linear theory is discussed within the general context of steady state and dynamic reconnection studies. Disturbances can be expected to focus explosively in the vicinity of the neutral point. This suggests the formation of a 'flux pile-up' current layer in which the bulk of the magnetic energy is released as heat rather than kinetic energy of mass motion.

Craig, I. J. D.↗

NASA NTRS1993

The effects of neutral inertia on ionospheric currents in the high-latitude thermosphere following a geomagnetic storm

Neutral flywheel effects are investigated in NCAR-TIGCM simulation of geomagnetic storms that occurred in November 23, 1982 and December 7-8, 1982. Theoretical calculations from the latter storm are compared with measurements of currents form instruments on the Dynamics Explorer 2 satellite. It is concluded that neutral flywheel effects can make a contribution to high latitude electrodynamics for a few hours after the main phase of a geomagnetic storm. The Hall currents that are driven by neutral winds during B(Z) northward conditions are generally in the opposite direction to those that occur during B(Z) southward conditions, when they are driven primarily by ion winds. The morphology of the field-aligned current system calculated by the NCAR-TIGCM during southward B(Z) conditions is in general agreement with observations.

Deng, W.↗

NASA NTRS1992

High-flux source of low-energy neutral beams using reflection of ions from metals

Reflection of low-energy ions from surfaces can be applied as a method of producing high-flux beams of low-energy neutral particles, and is an important effect in several areas of plasma technology, such as in the edge region of fusion devices. We have developed a beam source based on acceleration and reflection of ions from a magnetically confined coaxial RF plasma source. The beam provides a large enough flux to allow the energy distribution of the reflected neutrals to be measured despite the inefficiency of detection, by means of an electrostatic cylindrical mirror analyzer coupled with a quadrupole mass spectrometer. Energy distributions have been measured for oxygen, nitrogen, and inert gas ions incident with from 15 to 70 eV reflected from amorphous metal surfaces of several compositions. For ions of lighter atomic mass than the reflecting metal, reflected beams have peaked energy distributions; beams with the peak at 4-32 eV have been measured. The energy and mass dependences of the energy distributions as well as measurements of absolute flux, and angular distribution and divergence are reported. Applications of the neutral beams produced are described.

Cuthbertson, John W.↗

NASA NTRS1993

A new technique for in situ measurement of the composition of neutral gas in interplanetary space

Neutral atoms in interplanetary space play an important role in many processes relevant to the formation and evolution of the Solar System. An experimental approach is proposed for in situ atom detection based on the conversion of neutral atoms to negative ions at a specially prepared sensitive surface. Negative ions are subsequently analyzed and detected in an essentially noise-free mode. The use of the technique for in situ study of the composition of neutral interstellar atoms is considered. It is shown that interstellar H, D, and O atoms and possibly H2 molecules can be measured by the proposed technique. The experiment can be performed from a high-apogee Earth-orbiting satellite or from a deep space probe. Possible applications of the technique are discussed.

Gruntman, Michael A.↗

NASA NTRS1993

A kinematic analysis of the high-latitude thermospheric neutral circulation pattern

'Synthesized' thermospheric neutral wind fields for solar maximum, December solstice conditions for both quiet and active levels of geometric activity, are provided on the basis of averaged measurements from the Dynamics Explorer 2 (DE 2) satellite combined with theoretical model calculations. High-latitude DE 2 wind data obtained from multiorbit averages are merged with modeled winds from the NCAR thermosphere-ionosphere general circulation model. A 'kinematic analysis' is performed to decompose the merged semiempirical wind fields into their respective divergent and nondivergent components at high latitudes and to calculate the corresponding potential and stream functions. It is concluded that the nondivergent component of the high-latitude thermospheric neutral wind is representative of the convection-driven component of the neutral wind at F region altitudes. The nondivergent wind component comprises a large percentage of the total wind field for both quiet and active geomagnetic conditions.

Thayer, J. P.↗

NASA NTRS1990

Energetic neutral atoms: Imaging the magnetospheric ring current

Magnetospheric imaging is a new discipline whose goal is to make pictures of the energetic particle populations trapped in the magnetic field of Earth (or any other planet). This project demonstrated the technical feasibility and scientific validity of magnetospheric imaging using energetic neutral atoms (ENA) with the publication and quantitative analysis of the first ENA images ever obtained from space. ENA's are produced when singly-charged energetic (approximately 100 keV) trapped ions make an atomic collision with the neutral hydrogen atoms which boil of the top of the Earth's atmosphere. These hydrogen atoms suffuse the entire trapping volume of the magnetosphere. The energetic ion steals the electron from the atmospheric hydrogen, so the energetic ion is transformed into an energetic neutral atom with a velocity of several thousands of kilometers/second. Moreover, the new-born ENA preserves the velocity that the trapped ion had at the time of the collision. Consequently, any population of energetic ions emits ENA's.

Roelof, Edmond C.↗

NASA NTRS1995

Modeling of the coupled magnetospheric and neutral wind dynamos

Work at SRI involved modeling the exchange of electromagnetic energy between the ionosphere and magnetosphere to help interpret the DE-B Poynting flux observations. To describe the electrical properties of the high-latitude ionosphere, we constructed a numerical model, from the framework provided by the Vector Spherical Harmonic (VSH) model, that determines the ionospheric currents, conductivities, and electric fields including both magnetospheric inputs and neutral wind dynamo effects. This model development grew from the earlier question of whether an electrical energy source in the ionosphere was capable of providing an upward Poynting flux. The model solves the steady-state neutral wind dynamo equations and the Poynting flux equation to provide insight into the electrodynamic role of the neutral winds. The modeling effort to determine the high-latitude energy flux has been able to reproduce many of the large-scale features observed in the Poynting flux measurements made by DE-2. Because the Poynting flux measurement is an integrated result of energy flux into or out of the ionosphere, we investigated the ionospheric properties that may contribute to the observed flux of energy measured by the spacecraft. During steady state the electromagnetic energy flux, or DC Poynting flux, is equal to the Joule heating rate and the mechanical energy transfer rate in the high-latitude ionosphere. Although the Joule heating rate acts as an energy sink, transforming electromagnetic energy into thermal or internal energy of the gas, the mechanical energy transfer rate may be either a sink or source of electromagnetic energy. In the steady state, it is only the mechanical energy transfer rate that can generate electromagnetic energy and result in a DC Poynating flux that is directed out of the ionosphere.

Thayer, J. P.↗