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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Periodic solar EUV flux monitored near Venus

A detector sharing the orbital rate of Venus has a unique perspective on solar periodicities. Fourier analysis of the 8.6 year record of solar EUV output gathered by the Langmuir probe on Pioneer Venus Orbiter shows the influences of global oscillation modes located in the convective envelope and in the radiative interior. Seven of the eight lowest angular harmonic r-mode familes are detected by their rotation rates which differ almost unmeasurably from ideal theoretical values. This determines a mean sidereal rotation rate for the envelope of 457.9 + or - 2.0 nHz which corresponds to a period of 25.3 days. Many frequencies are aliased at + or - 106 nHz by modulation from the lowest angular harmonic r-mode in the envelope.

Wolff, Charles L.↗
NASA NTRS1990

Diagnostics and performance of a 1/4-scale MPD thruster

The primary purpose of this study is to evaluate the performance and scaling characteristics of a 1/4-scale magnetoplasmadynamic (MPD) thruster operating with and without applied magnetic nozzle fields. The experiment was carried out with separate pulse forming networks for the thruster and the applied field solenoidal coil. A strong correlation of impact pressure signal with thruster current was noted. Also striking was the larger impact signal when the magnetic nozzle field was applied. Measurements of N(e) and T(e) from Langmuir probes have been made. Compatible interpretation of pressure with N(e), T(e), allow local velocity to be mapped, thus enhancing understanding of the acceleration process.

York, T. M.↗
NASA NTRS1990

A test of convection models for IMF Bz north

The Utah State University Ionospheric Model was run to obtain diurnally reproducible ionospheric densities and temperatures for summer and winter conditions using both distorted two-cell and three-cell convection patterns. Differences due to the different convection patterns manifest themselves in the depth and location of polar holes in the F-region electron density. While the total depth of the model holes is a characteristic of the diurnally reproducible pattern, the features appear and are recognizable within 0.5 h. Langmuir probe data from 41 DE-2 passes, during which the IMF Bz component was northward, have been qualitatively checked against the model predictions. The cross polar cap electron density profiles of a large majority of the passes more closely conform to the distorted two-cell runs for both polarities of the IMF By component. This test can be generalized to rule out proposed convection patterns based on the presence/absence and position of polar electron density holes.

Maynard, N. C.↗
NASA NTRS1990

Measurement realities of current collection in dynamic space plasma environments

Theories which describe currents collected by conducting and non-conducting bodies immersed in plasmas have many of their concepts based upon the fundamentals of sheath-potential distributions and charged-particle behavior in superimposed electric and magnetic fields. Those current-collecting bodies (or electrodes) may be Langmuir probes, electric field detectors, aperture plates on ion mass spectrometers and retarding potential analyzers, or spacecraft and their rigid and tethered appendages. Often the models are incomplete in representing the conditions under which the current-voltage characteristics of the electrode and its system are to be measured. In such cases, the experimenter must carefully take into account magnetic field effects and particle anisotropies, perturbations caused by the current collection process itself and contamination on electrode surfaces, the complexities of non-Maxwellian plasma distributions, and the temporal variability of the local plasma density, temperature, composition and fields. This set of variables is by no means all-inclusive, but it represents a collection of circumstances guaranteed to accompany experiments involving energetic particle beams, plasma discharges, chemical releases, wave injection and various events of controlled and uncontrolled spacecraft charging. Here, an attempt is made to synopsize these diagnostic challenges and frame them within a perspective that focuses on the physics under investigation and the requirements on the parameters to be measured. Examples include laboratory and spaceborne applications, with specific interest in dynamic and unstable plasma environments.

Szuszczewicz, Edward P.↗
NASA NTRS1991

SEPAC data analysis in support of the environmental interaction program

Data analyses of the Space Experiments with Particle Accelerators (SEPAC) data and computer modeling were conducted to investigate spacecraft environmental effects associated with injection of electron beams, plasma clouds, and neutral gas clouds from the Shuttle orbiter. The data analysis indicates that Extremely Low Frequency oscillations from 150 to 200 Hz were seen in the Langmuir probe current when the beam was fired in a continuous mode. The strongest oscillations occurred when the ambient pressure was augmented by neutral gas releases from the SEPAC plasma accelerator magnetoplasma-dynamic (MPD) arcjet. To understand the dependence of spacecraft charging potential on beam density and other plasma parameters, a two-dimensional electrostatic particle code was used to simulate the injection of electron beams from an infinite conductor into a plasma. The simulations show that the conductor charging potential depends critically on the reflection coefficient of the conductor surface, which is defined as the percentage of incident particles reflected by the conductor. The ionization effects on spacecraft charging were examined by including interactions of electrons with neutral gas. The simulations show that the conductor charging potential decreases with increasing neutral background density due to the production of secondary electrons near the conductor surface. The simulations also indicate that the beam radius is generally proportional to the beam electron gyroradius when the conductor is charged to a large potential. It appears that the charge buildup at the beam stagnation point causes the beam radial expansion. A survey of the simulation results suggests that the ratio of the beam radius to the beam electron gyroradius increases with the square root of beam density and decreases inversely with beam injection velocity. These results are useful for explaining the spacecraft charging phenomena observed during SEPAC experiments from Spacelab 1.

Lin, Chin S.↗
NASA NTRS1990

Global models of Ne and Te at solar maximum based on DE-2 measurements

Newly developed global models of the electron density of (Ne) and (Te) in the F-region at solar maximum, based on Langmuir probe measurements from the Dynamics Explorer-2 satellite, are compared with solar minimum models that were developed earlier from Atmosphere Explorer data. Spherical harmonics are used in both models to describe the variations with geomagnetic latitude and local time, but the solar maximum model also includes longitudinal variations. The solar minimum models were for the fixed altitudes of 300 km and 400 km, while the solar maximum model covers all altitudes between 300 and 1000 km. In this paper, the global patterns of Ne and Te at 400 km at solar maximum and minimum are compared with the IRI model for the corresponding parts of the solar cycle. In most respects, the IRI model describes the empirical models quite well at both solar maximum and solar minimum.

Brace, L. H.↗
NASA NTRS1990

Pressure-driven sound turbulence in a high-beta plasma

LF turbulence is investigated experimentally in a 2-m-long 1-m-diameter magnetized electron fluid with beta(e) = about 0.5 and unmagnetized ions, generated in a double-pulsed linear dc discharge under a uniform external magnetic field of 15 G. The results of measurements with Langmuir probes, electric probes, and a directional particle analyzer are presented in graphs and characterized in detail. It is shown that the strong cross-field sound turbulence observed near the lower hybrid frequency is caused by the electron pressure gradient rather than E x B drift, with (1) temperature-gradient wave refraction as the dominant saturation mechanism, (2) wave-enhanced ion mass flow, and (3) only negligible ion-tail formation. The relevance of the present findings for studies of magnetic shock propagation is indicated.

Stenzel, R. L.↗
NASA NTRS1991

Electron attachment line shapes, cross sections, and rate constants at ultralow energies in CF3SO3H, (CF3SO2)2O, and CF3I

Electron attachment cross sections are reported in the energy range 0-160 meV, and at resolutions of 6.0-6.5 meV (FWHM) for the molecules CF3SO3H (triflic acid), (CF3SO2)2O (triflic anhydride), and CF3I (methyl iodide). Use is made of the Kr photoionization method. Attachment line shapes are deconvoluted from the spectral slit (electron energy) function, and are converted to cross sections by normalization to thermal attachment rate constants at 300 K. Rate constants as a function of mean electron energy are calculated from the cross sections using a Maxwellian electron energy distribution function. Present data are compared with flowing-afterglow, Langmuir-probe results in triflic acid and anhydride, and with high-Rydberg ionization results in CF3I.

Alajajian, S. H.↗
NASA NTRS1991

Dependence of polar hole density on magnetic and solar conditions

Electron densities from the Langmuir probes on the Atmospheric Explorer C and Dynamics Explorer 2 are used for analyzing the behavior of the high-altitude night-side F region polar hole as a function of solar and magnetic activity and of universal time (UT). The polar region of invariant latitude from 70 deg to 80 deg and MLT from 22 to 03 hours is examined. The strongest dependencies are observed in F10.7 and UT; a strong hemispherical difference due to the offset of the magnetic poles from the earth's rotation axis is observed in the UT dependence of the ionization hole. A seasonal variation in the dependence of ion density on solar flux is indicated, and an overall asymmetry in the density level between hemispheres is revealed, with the winter-hole density about a factor of 10 greater in the north than in the south.

Hoegy, W. R.↗
NASA NTRS1991

Seasonal variations in the subauroral electron temperature enhancement

A statistical study of the seasonal variations of the subauroral electron temperature enhancement was undertaken using data from the Langmuir probe experiment on the DE 2 satellite throughout most of the mission (1981-1982). In the winter hemisphere the nighttime background electron temperature is the highest and the magnitude of the peak Te responds most weakly to the geomagnetic activity. This behavior can be explained by seasonal trends in the nighttime downward heat flux due to conjugate photoelectrons. Moreover, model results indicate that a factor of about three increase in heat inflow during equinox relative to solstice is required to raise the electron temperature to a given level. This is a consequence of the higher electron densities at the Te peak near equinox. The Te peak occurs on field lines which thread the outer plasmasphere in the vicinity if the plasmapause and thus can be used as a tracer of the plasmapause position.

Fok, M.-C.↗
NASA NTRS1991

An equatorial temperature and wind anomaly (ETWA)

Data obtained from the WATS (Wind and Temperature Spectrometer) and LP (Langmuir Probe) experiments on board DE-2 (Dynamic Explorer) during high solar activity show evidence of anomalous latitudinal variations in the zonal winds and temperature at low latitudes. The zonal winds exhibit a broad maximum centered around the dip equator, flanked by minima on either side around 25 degrees; while the temperature exhibits a pronounced bowl-shaped minimum at the dip equator which is flanked by maxima. The two minima in the zonal winds and the corresponding maxima in the temperature are nearly collocated with the crests of the well known Equatorial Ionization Anomaly (EIA). The maximum in the zonal winds and the minimum in the gas temperature are collocated with the trough of the EIA. The differences between the maxima and minima in temperature and zonal winds, on many occasions, are observed to exceed 100 K and 100 m/s, respectively. The characteristics of this new phenomenon have eluded present day empirical models of thermospheric temperature and winds. The connection among these variables can be understood from the ion-neutral drag effect on the motions of the neutrals that in turn affect their energy balance.

Raghavarao, R.↗
NASA NTRS1991

Venus ionospheric tail rays - Spatial distributions and interplanetary magnetic field control

The overall properties of Venus ionospheric tail rays (such as density, spatial extent, and distribution) and their relationship to the draped magnetic field configuration behind the planet were investigated using measurements obtained by the Pioneer Venus Orbiter Langmuir probe, a magnetometer, and a plasma-wave detector. The results suggest that tail rays are a normal feature of the steady solar wind interaction with Venus and are not generally associated with a central tail plasma sheet. The statistics of the tail rays occurrence point toward the existence of a distributed terminator ionosphere source, consistent with findings of Brace et al. (1990).

Ong, M.↗
NASA NTRS1992

Plasma contactor research - 1991

A report describing the operating principles of hollow-cathode-based plasma contactors emitting or collecting electrons from an ambient plasma is summarized. Preliminary experiments conducted to determine the noise generated by these plasma contactors in the emission-current return line and in the plasma near it are described. These noise data are measured as current fluctuations in the return line and to the Langmuir probe and then analyzed using a fast Fourier transform technique. The spectral compositions of the data are characterized using power spectral density plots which are examined to identify possible noise source(s) and production mechanism(s). The precautions taken in the construction and calibration of the instrumentation to assure adequate frequency response are described. Experimental results show that line-current noise levels are typically 2 percent of the electron current being emitted or collected. However, noise levels increase to as much as 20 percent of the electron current at a few electron-collection operating conditions. The frequencies associated with most of the noise were harmonics of the 60 Hz input to system power supplies. Plasma noise had characteristics similar in magnitude and frequency to those for the return-line noise, but they contained additional features at frequencies considered to be related to ion-acoustic instabilities. Also discussed is a new probe positioning system built to facilitate future plasma-contractor research.

Buchholtz, Brett↗
NASA NTRS1991

Electron and ion temperature data for ionospheric modelling

The major sources ionospheric temperature data are the incoherent scatter radars and in situ instruments (Langmuir probe, retarding potential analyzer) flown on several long-lasting satellite missions. The paper provides an overview over the different data sets and discusses their volume, temporal and spatial resolution, accuracy, and their availability. The state-of-the-art of empirical modeling of ionospheric plasma temperatures is examined, and present shortcomings and future data needs are pointed out. Special emphasis is given to the representation of auroral features, temperature anisotropies, and of solar-cycle variations. Finally, the possibility of using theoretical results to fill data gaps for empirical modeling is considered.

Bilitza, Dieter↗
NASA NTRS1992

A reconsideration of the effects of terminator ionopause height on the nightward ion transport at Venus

The ionopause altitude near the terminator is a crucial parameter for studies dealing with the maintenance of the nightside ionosphere of Venus. It is generally thought that, during high solar wind dynamic pressures (P(SW)) or during solar minimum conditions, the ionopause comes down to very low altitudes so that the dayside ionosphere is not able to supply sufficient plasma to maintain the observed nightside densities. However, there are a number of workable definitions of the ionopause. Near the terminator, the altitude of the ionopause differs considerably depending upon the definition. The ionopause deduced from the radio occultation experiment as well as the pressure ionopause can be significantly lower than the density ionopause deduced from the Langmuir probe at these locations. The latter refers to the altitude where the electron density falls to 100/cu cm. Using in situ data from the Pioneer Venus Orbiter, it is shown that the density ionopause remains fairly high even for high P(SW) conditions. Simple quantitative estimates indicate that significant flow of plasma is still possible under these conditions. Thus, nightward transport of plasma during high P(SW) conditions may be more efficient than has been assumed so far. Since such conditions are more prevalent during solar cycle minimum, it is argued that transport may be relevant in the maintenance of nightside ionosphere at that time also.

Kar, J.↗
NASA NTRS1992

OL-AC Phillips Laboratory MPD thruster research program

The topics are presented in viewgraph form and include the following: facility construction; quadruple langmuir probe measurements; hollow/porous anode magnetoplasmadynamic (MPD) thruster; the measurement of the ionization fraction inside of the MPD thruster; and the experimental investigation of the effects of microturbulence on MPD thruster performance.

Tilley, Dennis L.↗
NASA NTRS1993

CIV experiments on ATLAS-1

A test of the Critical Ionization Velocity (CIV) theory was made with neutral xenon releases from the Space Experiments with Particle Accelerators hollow cathode plasma contactor onboard the Shuttle Orbiter Atlantis during the ATLAS-1 mission. The gas velocity perpendicular to the Earth's magnetic field was essentially the orbital velocity (7.5 km/s), and thus it exceeded the CIV for xenon. The releases were observed with onboard instrumentation. A factor of 60 enhancement was seen in the Langmuir probe current. Calculations confirmed that release conditions generally satisfied criteria for CIV and predicted a maximum factor of 20 increase in plasma density. Thus, CIV effects were likely to have occurred during the ATLAS-I experiments.

Marshall, J. A.↗
NASA NTRS1993

Studies of electromagnetic ion cyclotron waves using AMPTE/CCE and Dynamics Explorer

The principal activity during the past six months has involved the analysis of ion cyclotron waves recorded from DE-2 using the magnetic field experiment and electric field experiment. The results of this study have been published in the Geophysical Research Letters (GRL). The primary finding of this paper is that ion cyclotron waves were found to heat electrons, as observed in the DE-2 Langmuir probe data, through a Landau damping process. A second activity, which was started during the last six months, involves the study of large amplitude approximately one Hz electric and magnetic field oscillations recorded in the nightside auroral zone at substorm onset. Work is under way to determine the properties of these waves and investigate any association these waves may have with the substorm initiation process. A third activity under way involves a comprehensive study of ion cyclotron waves recorded at ionospheric altitudes by DE-2. This study will be an extension of the work reported in the GRL paper and will involve a larger sampling of wave events. This paper will focus on wave properties at ionospheric altitudes. A fourth activity involves a more in-depth analysis of the acceleration mechanisms and the resulting electron distributions based on the observations presented in the GRL paper.

Erlandson, Robert E.↗