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At least 811 records · Page 45
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NASA NTRS1991

Field-aligned dynamics of chemically induced perturbations to the ionosphere

Ionospheric modification using photochemically reactive vapors is studied with a one-dimensional, multiion, fluid model of plasma flow along magnetic field lines. The magnitudes of ion and electron density changes are determined by considering both chemical processes (i.e., photoionization, ion-molecule reactions, dissociative recombination, electron attachment) and transport processes (i.e., multispecies diffusion, electric currents, ambipolar electric fields). The numerical treatment in the model is not specific to any type of release or any interaction chemistry. It has been used to simulate releases of Ba, CO2, and CF3Br in the ionosphere, but generalization to other species may be easily accomplished. The results of the calculations are found to be in good agreement with experimental observations. The feasibility of modifying the parallel current paths in the auroral F region is examined.

Bernhardt, P. A.↗
NASA NTRS1991

Role of electromagnetic noise in the interhemispheric plasma exchange along closed field lines

Satellite observations have demonstrated that the equatorially trapped superthermal hydrogen ions are a common occurrence in the outer plasmaspheric flux tubes. Such ions occur in conjunction with broadband waves over a frequency spectrum from the proton-cyclotron frequency to the lower-hybrid frequency. Trapped ions are produced by the transverse heating of the ions by the waves. The effect of such ion heating on the interhemispheric plasma flow is studied here. The consequences of the trapped ion production on the plasmaspheric refilling after geomagnetic storms are examined.

Singh, Nagendra↗
NASA NTRS1991

Studies on the dynamics of Venus ionosphere - Effects of varying the ionopause height and the onset of turbulence

The conservation equations of plasma dynamics in the upper ionosphere of Venus have been solved by using a spectral method in the horizontal and finite differencing in the vertical direction. The effect of varying the ionopause height on the computed nightside ion densities is investigated. These ion densities show a sharp decrease as the ionopause altitude is reduced to 300 km. The effect of viscous forces on the horizontal plasma flow is investigated for a wide range of values of the coefficient of viscosity. The Reynolds numbers characteristics of the flow are calculated and the conditions for the onset of turbulence discussed. It is found that the Reynolds number can be large (greater than 1000) in the subsolar region for a coefficient of viscosity of up to 1.6 x 10 to the -10th g/cm s. The influence of magnetic fields on viscosity is also discussed.

Singhal, R. P.↗
NASA NTRS1991

Magnetic effects of large-scale impacts on airless planetary bodies

The analysis of lunar orbital and sample data combined with laboratory measurements of impact-produced plasmas suggest that large-scale impacts on planetary surfaces may have had significant magnetic effects. These effects may potentially explain part of all lunar crustal magnetization and, by extension, may be responsible for producing paleomagnetism on other airless silicate bodies in the solar system. Theoretical studies are presented of the magnetic field and remanent magnetization effects of basin-scale impacts on the Moon. The specific case of a Moon exposed to the solar wind plasma flow and its embedded magnetic field is investigated. It is shown that maximum compressed field amplitudes occur antipodal to the impact point in agreement with the observed tendency for orbital magnetic anomalies to be concentrated antipodal to young large lunar basins. Generalization of these results to include magnetic effects of impacts on other airless or nearly airless bodies in the solar system is presented.

Hood, L. L.↗
NASA NTRS1991

Generation of field-aligned current in the auroral zone

Generation of a magnetic field-aligned current in the auroral zone connecting the magnetospheric and ionospheric plasmas has been studied by means of a three dimensional particle simulation model. The model is of a magnetostatic variety appropriate for a low beta plasma in which the high frequency transverse displacement current has been eliminated. The simulation model is highly elongated along the magnetic field lines in order to model a highly elongated flux tube in the auroral zone. An enhanced field-aligned current was generated by injection of a magnetospheric plasma across the auroral zone magnetic field at the center of the model. Such a plasma injection may correspond to a plasmoid injection at the geomagnetic tail associated with magnetic reconnection during a substorm or a transverse plasma flow along the low latitude magnetopause boundary layer. The results of the simulations show that the field-aligned current can be enhanced over the thermal current by a factor of 5 - 10 via such injection. Associated with the enhanced current are the electrostatic ion cyclotron waves and shear Alfven waves excited in the auroral zone.

Okuda, Hideo↗
NASA NTRS1991

Two-dimensional model of a slow-mode expansion fan at Io

A 2D model for the standing slow-mode expansion fan that is expected to exist downstream of the Jovian moon Io is developed. The leading edge of the expansion fan makes an angle of 45 deg with the upstream magnetic field direction, and the fan width is about 114 deg. The plasma flow returns to its upstream direction by way of a slow-mode shock behind Io where the MHD parameters return asymptotically to their upstream conditions. The magnetic field perturbation within the fan is much smaller than that associated with the Alfven wing, which lies farther upstream.

Krisko, P. H.↗
NASA NTRS1992

The formation and structure of plasma wakes behind large high-voltage space platforms in ionosphere

Theory and particle simulation results are presented for ionospheric plasma flow over space platforms in the large-dimension and high-voltage range. Both the transient formation of the space-charge wake and its steady state structure are studied. The wake-side ion impact and current collection are obtained. It is found that the wake behind a high-voltage plate is characterized by two ion-rich sheaths embedded in a quasi-neutral background wake. The embedded sheath is formed by the ions passing through the sheath around the plate edge and serves as their trajectory path. Depending on the surface potential, the plate dimension, and the angle of attack, the embedded sheath may either extend downstream or curve back to the plate causing a high, localized ion flux density at the location it strikes.

Wang, J.↗
NASA NTRS1992

Observations of plasma dynamics in the coma of P/Halley by the Giotto ion mass spectrometer

The paper reports observations of plasma dynamics in the coma of P/Halley by the Giotto ion mass spectrometer. Measurements of protons and alpha particles from the far upstream region to the near ionopause region and of ions of mass 12-32 at distances of about 250,000 to 40,000 km from the nucleus are presented. The discontinuity known as the magnetic pileup boundary (MPB) is apparent only in proton, alpha particle, and magnetometer data, indicating that it is a tangential discontinuity of solar wind origin. No significant change is found in the properties of the heavy ions across the MPB. The issue of whether a cometopause was unambiguously observed at Comet Halley is discussed; it is concluded that the observations do not convincingly support the idea of a boundary due to internal cometary processes. A comparison of the observations to MHD models is made. The plasma flow directions at all distances greater than 30,000 km from the nucleus are in agreement with MHD calculations.

Goldstein, B. E.↗
NASA NTRS1992

Anisotropy of the energetic neutral atom flux in the heliosphere

Characteristics of the energetic neutral atoms born at the heliospheric interface are considered for plasma flow structure resulting from a two-shock model of the interaction between the solar wind and the interstellar medium. The energy distributions of heliospheric energetic neutral atoms (HELENAs) are calculated and it is shown that the HELENA flux is highly anisotropic at the earth's orbit. The characteristics of the HELENA flux are highly sensitive to the size of the heliosphere. This supports the conclusion that measurements of HELENAs from the earth's orbit would provide an efficient tool to remotely study the heliosphere.

Gruntman, Michael A.↗
NASA NTRS1992

Motions of particle microstructures in the magnetopause boundary layer

Particle flux data acquired by the ISEE 1 and 2 spacecraft reveal many of magnitude changes within the magnetopause boundary layer that occur during less than 1/4 s of time. These changes define small-scale particle structure, or microstructures within the region, many of which are apparently inclusions of plasma from adjacent regions. Particle structures resolved by the high time resolution data include groups with periods as short as 0.375 s. An estimate is derived for the motion of the microstructures with respect to an oscillatory inner boundary for four afternoon sector boundary layer crossings by measuring interspacecraft time differences as the features propagate past the satellites. On average the microstructures move toward the tail of the magnetosphere in concert with the wavy large-scale inner boundary. Furthermore, the orientations of the edges of the structures maintain an aspect similar to that of the boundary. These characteristics are expected for magnetohydrodynamic plasma flows that generate vorticity due to velocity shear stresses.

Larson, Nels R.↗
NASA NTRS1993

Thermosphere and F-region plasma dynamics in the equatorial region

The dynamics of the equatorial thermosphere and the F-region plasma are reviewed, highlighting some features observed with the San Marco satellite, the AE-E, and the DE-2, as well as with ground-based facilities at Arecibo and Jicamarca. Particular attention is given to the midnight temperature maximum and related phenomena, and to results on zonal neutral and plasma flows at F-region heights.

Herrero, F. A.↗
NASA NTRS1992

Hybrid simulations of intermediate shocks - Coplanar and noncoplanar solutions

A hybrid code is used to investigate the kinetic structure and stability of subfast intermediate shocks (IS) formed dynamically by the interaction between a flowing plasma and a stationary piston. Results of the kinetic simulation of noncoplanar ISs are compared with predictions of the MHD theory. The relevance of the results of the study to observations of the magnetopause is discussed.

Karimabadi, H.↗
NASA NTRS1991

Observations of plasma dynamics in the coma of P/Halley by the Giotto Ion Mass Spectrometer

Observations in the coma of P/Halley by the Giotto Ion Mass Spectrometer (IMS) are reported. The High Energy Range Spectrometer (HERS) of the IMS obtained measurements of protons and alpha particles from the far upstream region to the near ionopause region and of ions from mass 12 to 32 at distances of about 250,000 to 40,000 km from the nucleus. Plasma parameters from the High Intensity Spectrometer (HIS) of the IMS obtained between 150,000 to 5000 km from the nucleus are also discussed. The distribution functions of water group ions (water group will be used to refer to ions of 16 to 18 m/q, where m is in AMU and q is in unit charges) are observed to be spherically symmetric in velocity space, indicating strong pitch angle scattering. The discontinuity known as the magnetic pile-up boundary (MPB) is apparent only in proton, alpha, and magnetometer data, indicating that it is a tangential discontinuity of solar wind origin. HERS observations show no significant change in the properties of the heavy ions across the MPB. A comparison of the observations to an MHD model is made. The plasma flow directions at all distances greater than 30,000 km from the nucleus are in agreement with MHD calculations. However, despite the agreement in flow direction, within 200,000 km of the nucleus the magnitude of the velocity is lower than predicted by the MHD model and the density is much larger (a factor of 4). Within 30,000 km of the nucleus there are large theoretical differences between the MHD model flow calculations for the plane containing the magnetic field and for the plane perpendicular to the magnetic field. The observations agreed much better with the pattern calculated for the plane perpendicular to the magnetic field. The data obtained by the High Energy Range Spectrometer (HERS) of the IMS that are published herein were provided to the International Halley Watch archive.

Goldstein, B. E.↗
NASA NTRS1992

Second Magnetoplasmadynamic Thruster Workshop

The meeting focused on progress made in establishing performance and lifetime expectations of magnetoplasmadynamic (MPD) thrusters as functions of power, propellant, and design; models for the plasma flow and electrode components; viability and transportability of quasi-steady thruster testing; engineering requirements for high power, long life thrusters; and facilities and their requirements for performance and life testing.

Source record↗
NASA NTRS1991

A new approach to plasmasphere refilling: Anomalous plasma effects

During the last 10 months of the grant, both laminar and anomalous plasma processes occurring during the refilling of the outer plasmasphere after magnetic storms are investigated. Theoretical investigations were based on two types of models: (1) two-stream hydrodynamic model in which plasma flows from the conjugate ionospheres are treated as separate fluids and the ion temperature anisotropies are treated self-consistently; and (2) large-scale particle-in-cell code.

Singh, N.↗
NASA NTRS1993

Electron cyclotron thruster new modeling results preparation for initial experiments

The following topics are discussed: a whistler-based electron cyclotron resonance heating (ECRH) thruster; cross-field coupling in the helicon approximation; wave propagation; wave structure; plasma density; wave absorption; the electron distribution function; isothermal and adiabatic plasma flow; ECRH thruster modeling; a PIC code model; electron temperature; electron energy; and initial experimental tests. The discussion is presented in vugraph form.

Hooper, E. Bickford↗
NASA NTRS1992

Quasi-linear velocity space diffusion of heavy cometary pickup ions on bispherical diffusion characteristics

We consider an ion transport equation describing the source, convection, adiabatic acceleration, and quasi-linear velocity diffusion of cometary pickup ions. The equation is solved numerically along solar wind plasma flow lines for the environment of Comet Halley, to obtain distributions at positions on the Giotto spacecraft trajectory which may be compared with observations. We obtain full two-dimensional (pitch angle and velocity) numerical distributions which show pitch angle scattering about the wave scattering centers at +/- V(A), where V(A) is the Alfven wave speed in the solar wind frame. Peak ion phase-space densities approximately follow the bispherical shell geometry. The energy distributions F(v) agree well with Giotto observations at lower v, but in the high-energy tail there is evidence possibly for the first-order Fermi acceleration mechanism taking place in the Comet Halley foreshock region.

Huddleston, D. E.↗
NASA NTRS1992

On the formation and evolution of plasmoids - A survey of ISEE 3 Geotail data

The magnetic and plasma properties of plasmoids, their evolution with distance downtail, and the effect of the direction of the IMF on the plasmoid magnetic configuration were investigated by examining the ISEE 3 magnetometer and electron plasma measurements of the 1983 ISEE 3 Geotail Mission. Both data sets were systematically examined for the presence of bipolar magnetic signatures that occur while ISEE 3 was in the plasma sheet. Results revealed 366 events consistent with this signature while ISEE was in the plasma sheet. It was found that plasmoids are characterized by high-speed plasma flow and that many of them have a well-defined magnetic core field characterized by a field strength maximum at the center of the pass through the structure. Once completely formed, plasmoids are relatively stable. It was found that the size, velocity, magnetic core strength, and Bz field amplitude of plasmoids do not depend on the distance downtail beyond -100 R(E).

Moldwin, Mark B.↗