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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
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NASA NTRS1986

Origin of large magnetic fluctuations in the magnetosheath of Venus

The origin of large-amplitude hydromagnetic waves in the Venus magnetosheath downstream of the quasi-parallel bow shock is investigated by means of numerical simulations. It is shown that the most likely source of these waves is the bow shock itself, rather than an instability involving the solar wind and oxygen ions of planetary origin. Pickup of O(+) ions by these waves is also examined and shown to be in agreement with previous test particle calculations. The effect of mass loading on the structure of the shock is also discussed.

Winske, D.↗
NASA NTRS1987

Magnetotails at unmagnetized bodies - Comparison of Comet Giacobini-Zinner and Venus

It is found that the near ionopause environs play a crucial role in the tail formation process at both Venus and G-Z and that draping at the two very different sized bodies occurs on ionopause scale sizes. On the other hand, ion densities, downtail mass fluxes, tailward J x B forces, and lobe betas are factors of about 10,000, 50, 100, and 20 times greater in the G-Z tail than in Venus', while bulk flow speeds and ion temperatures are factors of about 15 and 240 times lower. These large quantitative differences in the properties within the two magnetotails are attributable to the significantly greater upstream mass loading of the solar wind by the extended neutral atmosphere at G-Z (comets in general) compared to the gravitationally bound atmosphere of Venus.

Mccomas, D. J.↗
NASA NTRS1987

Comparative plasma tails of Venus and comets

A review of current models of solar wind flow in the plasma tails of weakly magnetized bodies is presented. Measurements conducted by the Mariner 5 spacecraft, the Veneras 9 and 10, and the PVO orbiters, in the Venus tail, and with the ICE spacecraft in the tail of Comet Giacobini-Zinner, reveal common plasma properties which suggest that similar physical processes are operative. Most notable is the observation of decreased flow velocities and enhanced plasma temperatures in the vicinity of their plasma tails. In Venus, the measured velocity and temperature fields are consistent with the effects of frictional forces between the mass-loaded ionosheath flow and the ionosphere along the (magnetic) polar regions of the ionopause. It is argued that similar conditions exist at a cometary ionopause and that the distribution of magnetic fluxes in a cometary tail is controlled by the entry of plasma fluxes from the (magnetic) polar regions of the comet's ionospheric obstacle. This question is further addressed in connection with the two-step shape of the magnetic profile measured across the tail of the comet. It is suggested that the low intensity outer increases of the magnetic lobes are associated with the draping of the interplanetary magnetic field lines around the comet's ionospheric obstacle, and that the higher intensity increases seen in the inner regions of the magnetic lobes are due to an additional compression of magnetic fluxes produced by the entry of plasma particles into the tail.

Perezdetejada, H.↗
NASA NTRS1987

Solar wind effects on the outer ion coma of Comet Halley

A simple two-dimensional model is developed to examine the composition of the cometary ion coma in the region outside the ionopause which is strongly affected by the solar wind. Two-dimensional ion distributions are obtained assuming a cylindrically symmetric ion coma which accounts for the dynamic effects of the mass-loaded solar wind flow around the cometary ionosphere. The results of this model are discussed in the context of analyzing the GIOTTO ion data.

Flammer, K. R.↗
NASA NTRS1987

Waves in the magnetic field and solar wind flow outside the bow shock at comet P/Halley

The existence of hydromagnetic waves in the mass-loaded solar wind upstream from the bow shock of a comet is well-established both for comet Giacobini-Zinner and for comet Halley. Whereas previous reports have been concerned either with the magnetic field observations or with plasma observations, here observations of the magnetic field with the solar wind proton and alpha particle distributions are combined. This allows the three possible modes of propagation for these waves to be separated. The magnetic component is predominantly transverse to the magnetic field and linearly polarized. The flow vector also has a substantial amount of power parallel to the magnetic field. An examination of the pressure variations shows that slow magnetosonic waves are more common than the fast mode.

Johnstone, A.↗
NASA NTRS1988

A unidimensional model of comet ionosphere structure

The one-dimensional continuity and momentum equations for the plasma formed in the expanding coma of a comet near the sun are studied. An analytic expression for the magnetic field configuration in the presence of outflow, photoionization, dissociative recombination, plasma fluid pressure, and friction between the ions and neutrals is obtained. It is suggested that for a Halley-type comet there will be a region sunward of the nucleus from which the magnetic field is excluded, consistent with Giotto observations. Calculations have been performed for Halley and Giacobini-Zinner type comets, and in the field-free regions it is shown that the dominant terms in the momentum equation balancing the magnetic pressure gradient are the ion neutral friction and the net mass loading momentum gain.

Eviatar, Aharon↗
NASA NTRS1988

A three-dimensional MHD simulation of the interaction of the solar wind with Comet Halley

The interaction between the solar wind and cometary plasmas is simulated using a three-dimensional time-dependent MHD simulation model, and the results are compared with the recent satellite observations of Comet Halley. The model, which includes cometary mass loading, reproduces many of the features observed by the Suisei probe and the Giottot, including the weak bow shock, the enhancement of the magnetic field in front of the contact surface, and the plasma temperature increase across the bow shock (while it decreased near the comet).

Ogino, Tatsuki↗
NASA NTRS1988

Effects of variables upon pyrotechnically induced shock response spectra, part 2

Throughout the aerospace industry, large variations of 50 percent (6 dB) or more in shock response spectra (SRS) derived from pyrotechnic separation events continue to be reported from actual spaceflight data and from laboratory tests. As a result of these variations, NASA funded a research program for 1984 through 1986. The purpose of the 1984 through 1986 project was to analyze variations in pyrotechnically induced SRS and to determine if and to what degree manufacturing and assembly variables and tolerances, distance from the shock source, data acquisition instrumentation, and shock energy propagation affect the SRS. Sixty-four free-free boundary plate tests were performed. NASA funded an additional study for 1987 through 1988. This paper is a summary of the additional study. The purpose was to evaluate shock dissipation through various spacecraft structural joint types, to evaluate shock variation for various manufacturing and assembly variables on clamped boundary test plates, and to verify data correction techniques. Five clamped boundary plate tests investigated manufacturing and assembly variables and mass loading effects. Six free-free boundary plate tests investigated shock dissipation across spacecraft joint structures.

Smith, James Lee↗
NASA NTRS1989

Aircraft propeller induced structure-borne noise

A laboratory-based test apparatus employing components typical of aircraft construction was developed that would allow the study of structure-borne noise transmission due to propeller induced wake/vortex excitation of in-wake structural appendages. The test apparatus was employed to evaluate several aircraft installation effects (power plant placement, engine/nacelle mass loading, and wing/fuselage attachment methods) and several structural response modifications for structure-borne noise control (the use of wing blocking mass/fuel, wing damping treaments, and tuned mechanical dampers). Most important was the development of in-flight structure-borne noise transmission detection techniques using a combination of ground-based frequency response function testing and in-flight structural response measurement. Propeller wake/vortex excitation simulation techniques for improved ground-based testing were also developed to support the in-flight structure-borne noise transmission detection development.

Unruh, James F.↗
NASA NTRS1985

Thermophoretically enhanced mass transport rates to solid and transpiration-cooled walls across turbulent (law-of-the-wall) boundary layers

Convective-diffusion mass transfer rate predictions are made for both solid wall and transpiration-cooled 'law-of-the-wall' nonisothermal turbulent boundary layers (TBLs), including the mechanism of thermophoresis, i.e., small particle mass transport 'down a temperature gradient'. The present calculations are confined to low mass-loading situations but span the entire particle size range from vapor molecules to particles near the onset of inertial ('eddy') impaction. It is shown that, when Sc is much greater than 1, thermophoresis greatly increases particle deposition rates to internally cooled solid walls, but only partially offsets the appreciable reduction in deposition rates associated with dust-free gas-transpiration-cooled surfaces. Thus, efficient particle sampling from hot dusty gases can be carried out using transpiration 'shielded' probe surfaces.

Gokoglu, Suleyman A.↗
NASA NTRS1985

Viscous dissipation effects on thermophoretically augmented aerosol particle transport across laminar boundary layers

The effect of viscous dissipation on mass transport across nonisothermal low-mass-loading laminar boundary layers of dusty gas is investigated theoretically by means of numerical simulations. The derivation of the model is outlined, and numerical results are presented in extensive graphs and characterized in detail. The dissipation effects are found to be significant, increasing total particle-deposition rates; the intensity of the effects depends on the ratio of wall temperature to mainstream static temperature.

Gokoglu, S. A.↗
NASA NTRS1988

Observations of intense ULF pulsation activity near the geomagnetic equator during quiet times

This paper analyzes observations, made by particle and field instruments on the AMPTE CCE satellite, of intense ULF pulsations in the earth's magnetosphere near the geomagnetic equator. These pulsations were observed during magnetically quiet periods in regions characterized by intense fluxes of warm strongly trapped light ions, predominantly H(+), and often with streaming low-energy plasma. The strong latitudinal localization of these pulsations is interpreted to be due to equatorial mass loading or to partial reflection of Alfven wave energy by latitudinal gradients in plasma density. Possible sources of wave energy for these events are discussed.

Engebretson, M. J.↗
NASA NTRS1989

Results of the first statistical study of Pioneer Venus Orbiter plasma observations in the distant Venus tail - Evidence for a hemispheric asymmetry in the pickup of ionospheric ions

Pioneer Venus Orbiter plasma and magnetometer observations from the first nine tail seasons of crossings of the Venus wake are used to study ion pickup in the far wake of an unmagnetized object embedded in the solar wind. This statistical study treats all of the plasma spectra containing pickup ions in the vicinity of the Venus tail. A hemispheric asymmetry is found in the pickup of ionospheric ions, with approximately four times more O(+) events observed in the 'northern' magnetic hemisphere (where the induced electric field points outward) than in the 'southern' magnetic hemisphere. Out of a total of 167 large O(+) events, 125, or 75 percent, occurred in the northern hemisphere when position is calculated in terms of Venus radii, and 129 or 77 percent occurred in the 'northern' hemisphere when position is expressed in gyroradii. This hemispheric asymmetry in ion pickup is consistent with the prediction of the Cloutier et al. (1974) mass loading model for Venusian ions above the ionopause boundary.

Intriligator, Devrie S.↗
NASA NTRS1988

Radial Alfven velocity profiles in the magnetosphere and their relation to ULF wave field-line resonances

The radial Alfven velocity, V(A), in the magnetosphere was investigated using magnetic field and plasma data from the ISEE-1 spacecraft. The harmonic frequency structure calculated from these data was compared with the observed resonant wave structure in the frequency range 5-100 mHz and over the region L = 3-9. It was found that the frequency of these resonances depend on the A(V), which is a function of the magnetic field strength and plasma density. The toroidal mode resonance frequency structure was modeled for the first four harmonics, with excellent agreement between the modeled and observed harmonics for a passs which does not show a plasmapause gradient. In the presence of a steep plasmapause gradient, the model was found to predict a correspondingly steep increase in the resonance frequencies at the plasmapause, by about 5 mHz for the first harmonic and by 20 mHz for the fourth harmonic. Special consideration is given to the effect of heavy ion mass loading on the modeling.

Fraser, B. J.↗
NASA NTRS1989

Cometary environments; Proceedings of Symposium 5, Workshop IV, and Topical Meeting of the 27th COSPAR Plenary Meeting, Espoo, Finland, July 18-29, 1988

Papers on the environment of comets are presented, covering topics such as constraints on the interstellar dust model of comet dust, dust particles and comet nuclei, models of cometary nuclei, subliming gas in the near-nuclear layer of the comet coma, the nucleus and rotation of Comet Halley, the surface albedo of comet nuclei, observational studies on Comet Halley, comet simulations, comet ion composition, and chemical abundances in comets. Additional topics include the O 1D and H2O production rate from comets, collisional coma models, the gas coma of Comet Giacobini-Zinner, IR properties of rough comet grains, studies of Comet Halley by Giotto, the impact of large dust particles on the Vega spacecraft, and carbonaceous materials as components of comet dust. Also, consideration is given to comet plasma boundaries, the comet ionopause, the solar wind-comet interaction, MHD turbulence and particle acceleration in a mass-loaded solar wind, combined first and second order Fermi acceleration at comets, discrete wave packets upstream from the earth and comets, and the visual appearance of comets under varying solar wind conditions.

Gombosi, T. I.↗
NASA NTRS1989

The visual appearance of comets under varying solar wind conditions

Three-dimensional MHD simulations have been performed for four different sets of solar wind conditions and cometary outgassing rates appropriate to the Halley encounters. Even though the simulations are single fluid calculations, it is possible to separate the solar wind and cometary ions using the divergenceless nature of the solar wind ions. The cometary ion density is then integrated along the line-of-sight from the observer through the comet to determine how the comet would look to a distant observer under these different conditions. In general, comet tails appear longer when the interplanetary magnetic field lies in the plane of the sky rather than along the line-of-sight. Also, the tail shrinks as the speed of the solar wind increases and/or the mass loading rate decreases.

Russell, C. T.↗
NASA NTRS1989

The structure of Io's thermal corona and implications for atmospheric escape

A steady-state model of Io's exospheric corona and its interaction with the Io plasma torus is used to study the escape of species from Io's atmosphere. It is found that atmospheric sputtering is the major escape mechanism for models in which the plasma flow reaches the critical level, and that such models produce total mass-loading rates an order of magnitude larger than values inferred from observations. The results suggest the presence of an extended Na coronal component in the thermal exosphere, and are consistent with an O-dominated corona, an exospheric temperature of about 1000 K, an Na critical level mixing ratio of 0.001, and a critical level radius of about 1.5 Io radii.

Summers, M. E.↗
NASA NTRS1990

Investigation of plasma density bulge in front of Venus bow shock

Significant ion and electron flux enhancements immediately upstream of the Venus bow shock were observed by the Electron Temperature Probe on the Pioneer Venus Orbiter. Mass loading of the solar wind by oxygen ions accounts for only about 10 percent of the observed effect.

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