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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Magnetic reconnection during magnetospheric substorms

The near earth reconnection model of substorms represents an attempt to place a broad range of observations into a consistent framework. The roles and requirements of reconnection are discussed. High speed plasma sheet flows, thin current sheet instability, substorm triggering, plasmoids and flux ropes in the distant tail, and magnetohydrodynamic simulations are discussed. Substorms are global, coherent sequences of processes involving solar wind/magnetosphere/ionosphere interaction. Magnetic reconnection is required to explain different dayside and polar cap phenomena, which required nightside reconnection. The modification and expansion of the standard near earth neutral line (NENL) model can integrate breakup arcs, current disruption, current wedge features, and localized plasma flows into the magnetic reconnection framework.

Baker, Daniel N.↗
NASA NTRS1981

The ionospheric peak on the Venus dayside

The behavior of the ionospheric peak on the dayside of Venus is described and interpreted by combining radio occultation measurements with theoretical calculations. The theoretical models are shown to be able to reproduce the measured electron densities very accurately when careful consideration is given to such parameters as the level of solar activity, the electron temperature, and the neutral density. What is more, the models are able to provide a check on the accuracy of neutral atmospheric models in the vicinity of 140 km. Chemical equilibrium is assumed for the calculation of ion and electron densities. A table giving Pioneer Venus radio occultation measurements of the Venus dayside ionosphere is included.

Cravens, T. E.↗
NASA NTRS1995

Lyman alpha emissions in the outer heliosphere

Since 1993, the Ultraviolet Spectrometers aboard both Voyager 1 and 2, were used to study the Ly-alpha glow pattern backscattered by neutral hydrogen atoms at great distance from the sun. When compared to a radiative transfer calculation for an unperturbed interstellar hydrogen flow into the solar system, the data present an excess of intensity seen in the direction of the incoming interstellar wind. An absolute estimate of this Lyman alpha intensity yields 10 to 15 Rayleighs for each spacecraft. We discuss the possibility that this excess is caused by a gradient of hydrogen at large distance from the Sun due to the filtration of interstellar hydrogen when crossing the heliopause. Comparisons with results from the Baranov-Malama model of neutral-plasma coupling at the heliopause are presented. The data yield a range of possible values for the hydrogen gradient which may be used to infer the location of the heliopause.

Quemerais, E.↗
NASA NTRS1995

Time-dependent MHD modeling of the global structure of the heliosphere

We present results from time-dependent modeling of the global structure of the heliosphere with neutral and magnetic field effects included. The magnetic field is assumed parallel to the interstellar flow in this two-dimensional axisymmetric model; the neutrals are treated as a fluid. The effects of interstellar neutrals and the interplanetary magnetic field on the location of the termination shock are studied using the most recent estimate of the interstellar medium parameters, results will be compared to those of Baranov and Zaitsev. The effect of the solar wind - VLISM interaction on the density and velocity of interstellar neutrals within the heliosphere will also be presented and related to observations. The response of the termination shock to the solar cycle variation in the solar wind will be compared to the response found previously using an axisymmetric hydrodynamic model without neutrals.

Liewer, P. C.↗
NASA NTRS1985

Theoretical spectral distributions and total cross sections for neutral subthreshold pion production in carbon-carbon collisions

A coherent isobar formalism is employed to model subthreshold production of neutral pions in carbon-carbon collisions at energies below 100 MeV/nucleon. No arbitrary scale factors or adjustable free parameters are used in calculation of the Lorentz-invariant cross sections for pion production in the projectile, which produces an excited state that goes to M1 resonance in the target by conservation of spin and isospin. Pion production is also modeled for the projectile, which also reaches M1 resonance. The overall pion spectral distribution in the center of mass system is then integrated over the energy range 35-84 MeV/nucleon. The results expose an energy loss in the incident ions, as observed experimentally, and indicate that an isobar mechanism is responsible for higher energy pion production. Lower energy pions are a result of thermal processes.

Norbury, J. W.↗
NASA NTRS1986

Satellite tori at Saturn

The inner satellites of Saturn are icy bodies imbedded in a plasma environment in which they are continuously bombarded by energetic ions, corotating plasma, and solar radiation. Laboratory sputtering experiments indicate that this should result in the injection of substantial amounts of neutral H, H2, OH, H2O, and O2 into the magnetosphere. The atomic processes affecting these neutrals and the neutrals and ions formed from them are modeled, and the steady state neutral and ion densities expected in the plasma tori of Enceladus, Dione-Tethys, and Rhea are calculated. Comparison with observations shows that recombination can limit the Enceladus and Dione-Tethys tori to the observed densities, but that transport rates of at least 4 x 10 to the -8th Saturn radii squared/s are required to limit torus densities at Rhea to the observed values.

Richardson, J. D.↗
NASA NTRS1988

A design methodology for neutral buoyancy simulation of space operations

Neutral buoyancy has often been used in the past for EVA development activities, but little has been done to provide an analytical understanding of the environment and its correlation with space. This paper covers a set of related research topics at the MIT Space Systems Laboratory, dealing with the modeling of the space and underwater environments, validation of the models through testing in neutral buoyancy, parabolic flight, and space flight experiments, and applications of the models to gain a better design methodology for creating meaningful neutral buoyancy simulations. Examples covered include simulation validation criteria for human body dynamics, and for applied torques in a beam rotation task, which is the pacing crew operation for EVA structural assembly. Extensions of the dynamics models are presented for powered vehicles in the underwater environment, and examples given from the MIT Space Telerobotics Research Program, including the Beam Assembly Teleoperator and the Multimode Proximity Operations Device. Future expansions of the modeling theory are also presented, leading to remote vehicles which behave in neutral buoyancy exactly as the modeled system would in space.

Akin, David L.↗
NASA NTRS1985

Effects of a wavy neutral sheet on cosmic ray anisotropies

The first results of a three-dimensional numerical code calculating cosmic ray anisotropies is presented. The code includes diffusion, convection, adiabatic cooling, and drift in an interplanetary magnetic field model containing a wavy neutral sheet. The 3-D model can reproduce all the principal observations for a reasonable set of parameters.

Kota, J.↗
NASA NTRS1968

Atmospheric neutral points over water.

Positions of skylight neutral points for model of Rayleigh atmosphere and smooth water surface specularly reflecting light according to Fresnel law

Fraser, R. S.↗
NASA NTRS1971

Review of magnetic field observations

Recent observations of magnetic fields in the magnetosphere are reviewed, and critical experiments and data are identified for theoretical analysis and interpretation. Quantitative studies of the solar wind interaction with the earth's magnetic field, regional measurements near the earth's equator at R = 2-8 R sub E, the polar cusp region of the geomagnetosphere, and structural models of the neutral sheet region in the geomagnetic tail are considered.

Ness, N. F.↗
NASA NTRS1976

The inconsistency between proton charge exchange and the observed ring current decay

The equatorial pitch-angle distributions of ring-current ions observed during a storm recovery phase at L values between 3 and 4 are compared with the pitch-angle distributions predicted by proton charge exchange with neutral hydrogen. Large disagreements are found, and three alternative explanations are explored. (1) A strong proton source acts to mask the effects of charge exchange. It is believed that the required strong continual source with a unique pitch-angle and energy dependence is unrealistic at these low L values. (2) Presently accepted neutral hydrogen density models have densities well over an order of magnitude too large for a storm recovery phase. No evidence is known to support the required large errors in the densities. (3) The ring current at particle energies not exceeding 50 keV was dominated by some ion species other than protons during the storm recovery phase. Such ions must have much longer lifetimes for charge exchange with hydrogen than do protons. This alternative is strongly favored, with He(+) being an attractive candidate.

Lyons, L. R.↗
NASA NTRS1980

The latitudinal variation of the charge exchange induced atomic hydrogen escape flux

Using plasma data from the Isis 2 spacecraft and Arecibo radar, diffusive equilibrium models of the ionosphere were constructed for equinox conditions. These plasmaspheric models were combined with models of the neutral atmosphere to calculate the atomic hydrogen escape flux due to charge exchange between thermal protons and to calculate cooler hydrogen and oxygen atoms as a function of dipole latitude and local time. These calculations showed that the daytime escape flux increases as the absolute value of the dipole latitude decreases, reaching its maximum value at the magnetic equator. At 15 hours local time (LT) on March 23, 1972, the calculated escape flux varied from an insignificant amount at 55 deg dipole latitude, to 3 x 10 to the 8th atoms/sq cm sec at the magnetic equators.

Maher, L. J., Jr.↗
NASA NTRS1981

Multiple crossings of a very thin plasma sheet in the Earth's magnetotail

High resolution magnetic field, plasma and energetic particle data from the IMP-8 spacecraft were studied for multiple crossings of the Earth's magnetotail plasma sheet when it becomes thin during magnetospheric substorms. Traversals recur on a time scale of several minutes and they are associated with high velocity plasma flows that are usually directed tailward but are occasionally directed earthward for brief intervals. Observations are explained by rapid oscillations of a plasma sheet that is only a few thousand km thick, a dimension comparable to the gyroradius of energetic protons. Differences in the angular distributions of the two energies indicate that the higher energy protons are preferentially located on field lines deeper in the tail lobe. A neutral line acceleration model is supported tailward streaming energetic electrons which are occasionally present at the lobe plasma sheet interface.

Fairfield, D. H.↗
NASA NTRS1980

Observations of a nonthermal ion layer at the plasma sheet boundary during substorm recovery

Measurements of the energy and angular distributions of energetic protons and alpha particles (not less than 30 keV/charge) in the geomagnetic tail are presented. The measurements were made during the recovery phase of a geomagnetic substorm on Apr. 19, 1978, with the Max-Planck-Institut/University of Maryland sensor system on the Isee 1 satellite. The measurements were also correlated with plasma observations made by the LASL/MPE instrument on Isee 1. The data reveal the presence of a thin nonthermal layer of protons and alpha particles at the plasma sheet boundary. The particles have their maximum flux at 60 keV/charge and are streaming highly collimated in the earthward direction. The alpha particle layer is confined within the proton layer. Many aspects of the observations are in agreement with an acceleration model near the neutral line proposed by Jaeger and Speiser (1974)

Moebius, E.↗
NASA NTRS1981

On the role of the lower hybrid drift instability in substorm dynamics

Recent studies of the lower hybrid drift instability have shed new light on the role of this mode in field-reversed plasmas. For substorm magnetotail conditions it is found that the lower hybrid drift instability can penetrate to the neutral line and can dissipate magnetic energy at a rate of approximately 4 x 10 to the 17th erg/s. Thus this instability is capable of playing a major role in the onset of substorms and providing resistivity for reconnection processes in the context of the neutral line substorm model.

Huba, J. D.↗
NASA NTRS1982

Magnetotail dynamics excited by the streaming tearing mode

Magnetotail reconnection in the presence of plasma streaming parallel to the neutral sheet is modeled. The tearing mode is excited much more violently in the case with parallel plasma flow in the plasma sheet than in the case with no flow. The flow patterns for the nonlinear resistive tearing mode and the streaming tearing mode are much more complex than those for the linear tearing mode. Flow vortices are observed in both cases.

Sato, T.↗
NASA NTRS1985

Viscoplasticity based on overstress with a differential growth law for the equilibrium stress

Two coupled, nonlinear differential equations are proposed for the modeling of the elastic and rate (time) dependent inelastic behavior of structural metals in the absence of recovery and aging. The structure of the model is close to the unified theories but contains essential differences. It is shown that the model reproduces almost elastic regions upon initial loading and in the unloading regions of the hysteresis loop. Under loading, unloading and reloading in strain control the model simulated the experimentally observed sharp transition from nearly elastic to inelastic behavior. When a formulation akin to existing unified theories is adopted the almost elastic regions reduce the points and the transition upon reloading is very gradual. For different formulations the behavior under sudden in(de)creases of the strain rate by two orders of magnitude is simulated by numerical experiments and differences are noted. The model represents cyclically neutral behavior and contains three constants and two positive, decreasing functions. The determination of constants and functions from monotonic loading with strain rate changes and relaxation periods is described.

Krempl, E.↗