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All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

The effect of realistic conductivities on the high-latitude neutral thermospheric circulation

The dynamics of the high latitude thermosphere are dominated by the ion circulation pattern driven by magnetospheric convection. The reaction of the neutral thermosphere is influenced by both the magnitude of the ion convection velocity and by the conductivity of the thermosphere. Using a three-dimensional, time-dependent, thermospheric, neutral model together with different ionospheric models, the effect of changes in conductivity can be assessed. The ion density is described by two models: the first is the empirical model of Chiu (1975) appropriate for very quiet geomagnetic conditions, and the second is a modified version of the theoretical model of Quegan et al. (1982). The differences in the neutral circulation resulting from the use of these two ionospheric models emphasizes the need for realistic high latitude conductivities when attempting to model average or disturbed geomagnetic conditions, and a requirement that models should couple realistically the ionosphere and the neutral thermosphere. An attempt is made to qualitatively interpret some of the features of the neutral circulation produced at high latitudes by magnetospheric processes.

Fuller-Rowell, T. J.↗
NASA NTRS1985

Models of Venus neutral upper atmosphere - Structure and composition

Models of the Venus neutral upper atmosphere, based on both in situ and remote sensing measurements, are provided for the height interval from 100 to 3500 km. The general approach in model formulation was to divide the atmosphere into three regions: 100-150 km, 150-250 km, and 250-3500 km. Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements. A paramount consideration was to keep the models simple enough to be used conveniently. Available observations are reviewed.

Keating, G. M.↗
NASA NTRS1981

Magnetospheric substorm models - Comparison with neutral sheet magnetic field observations

Four models for geomagnetic substorms, a quiet tail model, and models incorporating structural effects of the tail are examined for consistency with magnetic-field data during satellite crossings of the tail neutral sheet/plasma sheet. For this data the tearing mode instability model is always consistent, and inward moving distant neutral line is sometimes consistent, quasi-steady reconnection with slow shock and intermediate wave structure and locally quiet tail rarely consistent, and an outward propagating rarefaction wave is never consistent with the magnetic observations. In several cases structural effects of the tail are consistent with key features of the magnetic signatures.

Speiser, T. W.↗
NASA NTRS1986

Studies of the aurorally-induced ultraviolet emissions on the nightside of Venus

The effect of a monoenergetic flux of electrons on a model atmosphere of the nightside thermosphere of Venus was examined. The neutral model chosen is that of Hedin for high solar activity and l65 degrees solar zenith angle. The model is based on measurements made by the Pioneer Venus Orbiter Neutral Mass Spectrometer. Four species were included in the calculation: CO2, O, CO, and N2. The numerical method that was chosen for energy deposition of the primary electrons is the continuous slowing down approximation. The secondary electron distribution was computed using the empirically determined shape of the differential cross section.

Fox, J. L.↗
NASA NTRS1974

Guiding center model to interpret neutral particle analyzer results

The theoretical model is discussed, which accounts for drift and cyclotron components of ion motion in a partially ionized plasma. Density and velocity distributions are systematically precribed. The flux into the neutral particle analyzer (NPA) from this plasma is determined by summing over all charge exchange neutrals in phase space which are directed into apertures. Especially detailed data, obtained by sweeping the line of sight of the apertures across the plasma of the NASA Lewis HIP-1 burnout device, are presented. Selection of randomized cyclotron velocity distributions about mean azimuthal drift yield energy distributions which compared well with experiment. Use of data obtained with a bending magnet on the NPA showed that separation between energy distribution curves of various mass species correlate well with a drift divided by mean cyclotron energy parameter of the theory. Use of the guiding center model in conjunction with NPA scans across the plasma aid in estimates of ion density and E field variation with plasma radius.

Englert, G. W.↗
NASA NTRS1993

Modeling the structure of H I in the galactic disk

For comparison with a detailed H I survey, we calculated models for the H I intensity distribution, T(B)(l, b, v), for a 18 x 12 deg region in the disk of the Galaxy toward l = 205 deg. In one class of models, neutral hydrogen is in the form of clouds, while in the other class of models it is a continuous medium with holes whose sizes are typical of supernova remnants and superbubbles. The cloud models are a poor representation of the data in that the amplitude fluctuations are dissimilar and the continuity of the H I observations is not reproduced. A continuous H I medium with holes provides a much better match to the data, and the best-fit models suggest that the filling factor for the H I is 20-50 percent.

Bregman, Joel N.↗
NASA NTRS1983

Sensitivity /comparison study between the Jacchia 1970, 1971, and 1977 upper atmospheric density models

The neutral upper atmospheric models for the Earth's thermosphere currently used in NASA-MSFC programs are the Jacchia 1970 (J70), 1971 (J71), and 1977 (J77). The Jacchia 1970 model (modified) is used in all MSFC orbital mechanics analyses. Since total density and its variations are the main environmental parameters of interest in orbital lifetime and attitude control studies, the total neutral density was selected for this analysis. This report presents the results of a parametric study of the total density (at 400 km altitude) as computed with three MSFC/Jacchia models. The sensitivity of each of the density models at the summer solstice to varying solar conditions (flux) and geomagnetic (index) values is discussed.

Johnson, D. L.↗
NASA NTRS1981

SERT II 1980 extended flight thruster experiments

The flight results obtained from mid 1979 through December 1980 are presented. Near continuous solar power in 1979 and 1980 has enabled long periods of thruster endurance testing. Three of four propellant tanks were exhausted with no significant change in thruster system operation before being empty. A new plasma mode thrust was characterized and direct thrust measurements obtained. Other tests, including beam neutralization by various neutralizer sources, give insight to electron conduction across plasmas in space and provide a basis to model neutralization of thruster arrays.

Kerslake, W. R.↗
NASA NTRS1981

SERT II 1980 extended flight thruster experiments

The SERT II spacecraft, launched in 1970, has been maintained in an operational, but intermittent status since 1971. This paper presents the flight results obtained from mid 1979 through December 1980. Near continuous solar power in 1979 and 1980 has enabled long periods of thruster endurance testing. Three of four propellant tanks have been exhausted with no significant change in thruster system operation before being empty. A new plasma mode thrust has been characterized and direct thrust measurements obtained. Other tests, including beam neutralization by various neutralizer sources, give insight to electron conduction across plasmas in space and provide a basis to model neutralization of thruster arrays.

Kerslake, W. R.↗
NASA NTRS2009

NEXT Long-Duration Test Neutralizer Performance and Erosion Characteristics

The NASA's Evolutionary Xenon Thruster (NEXT) program is developing the next-generation ion propulsion system with significant enhancements beyond the state-of-the-art to provide future NASA science missions with enhanced capabilities at a low total development cost. A Long-Duration Test (LDT) was initiated in June 2005, to verify the NEXT propellant throughput capability to a qualification-level of 450 kg, 1.5 times the anticipated throughput requirement of 300 kg per thruster based on mission analyses. As of September 2, 2009, the thruster has accumulated 24,400 hr of operation with extensive durations at the following input powers: 6.9, 4.7, 1.1, and 0.5 kW. The thruster has processed 434 kg of xenon, surpassing the NASA Solar Technology Application Readiness (NSTAR) program thruster propellant throughput demonstrated during the extended life testing of the Deep Space 1 flight spare ion thruster and approaching the NEXT development qualification throughput goal of 450 kg. The NEXT LDT has demonstrated a total impulse of 16.1 10(exp 6zzz0 N s; the highest total impulse ever demonstrated by an ion thruster. A reduction in neutralizer flow margin has been the only appreciable source of thruster performance degradation. The behavior of the neutralizer is not easily predicted due to both erosion and deposition observed in previous wear tests. Spot-to-plume mode transition flow data and in-situ erosion results for the LDT neutralizer are discussed. This loss of flow margin has been addressed through a combination of a design change in the prototype-model neutralizer to increase flow margin at low emission current and to update the NEXT throttle table to ensure adequate flow margin as a function of propellant throughput processed. The new throttle table will be used for future LDT operations. The performance of the NEXT LDT neutralizer is consistent with that observed for long-life hollow cathodes. The neutralizer life-limiting failure modes are progressing as expected and the neutralizer data indicate none of the neutralizer failures are imminent.

Herman, Daniel A.↗
NASA NTRS1973

Incompatibility of solar EUV fluxes and incoherent scatter measurements at Arecibo.

Photoionization rates have been calculated from recent solar EUV flux measurements and are compared with the electron-ion recombination rates estimated from electron density profiles and current reaction rates. These photoionization rates also provided the first step in calculating the photoelectron heating rates of the ambient electrons, which are compared with the cooling rates deduced from ion and electron temperature measurements at Arecibo. The effects of changes in the assumed neutral models, the rate coefficients, and the temperature measurements are discussed. Comparisons of the profiles of energy input and loss with similar profiles of ion production and recombination suggest that the solar EUV fluxes have been underestimated. These comparisons also imply lower molecular neutral densities in winter than in summer.

Swartz, W. E.↗
NASA NTRSDate not supplied

Simulation of Vacuum Chamber Pressure Distribution with Surrogate Modeling and Uncertainty Quantification

A major challenge in understanding differences in electric propulsion performance in ground tests and in space operations concerns the pressure distribution within the test vacuum chamber. The chamber backpressure is much higher than experienced in space, modifying thruster performance and plume dynamics. Numerical simulation is a key element to determining the background conditions in non-ideal vacuum chamber environments. An important parameter for the accurate simulation of chamber backpressure is the probability that an atom will stick to a cryogenic panel used to pump away the plume gases. This quantity can be used to model vacuum pumps in particle-based kinetic numerical methods. In this work, a three-dimensional direct simulation Monte Carlo code is used to model neutral xenon atoms flowing from the anode of the H9 Hall Effect Thruster within the University of Michigan’s Large Vacuum Test Facility. Simulated pressures are compared with ion gauge pressure measurements to infer the effective sticking coefficient of the chamber’s vacuum pumps. A pressure predicting surrogate model is developed for inference of pump sticking coefficients and for uncertainty quantification. This information enables accurate and useful kinetic simulations of electric propulsion thruster plasma plumes in vacuum chambers.

DSMC↗
NASA NTRS1987

Dynamics of the Venus ionosphere revisited

The Whitten et al. (1984) dynamical model of the Venus ionosphere is presently modified in order to obtain a better match between observations and predictions. It is found that the nighttime ion densities are comparatively insensitive to the height of the ionopause above a value of 450 km. The density increases by only about 25 percent when the ionopause is raised to 800 km, but is noted to be very sensitive to the choice of a neutral model atmosphere, and to the ion and electron temperatures that influence both plasma scale height and ion diffusivity.

Mccormick, P. T.↗
NASA NTRS1974

Guiding center model to interpret neutral particle analyzer results

An attempt is made to model the essential features of ion motion in a plasma. A guiding center model is hypothesized for a plasma heating device whose radial electric fields and axial magnetic fields are crossed. The guiding center model is shown to give results which agree with experimental data from four different plasma heating devices and are helpful for the interpretation of neutral particle analyzer results.

Englert, G. W.↗
NASA NTRS1971

Model for rotating and nonuniform planetary exospheres.

The model neutral exosphere with a uniformly rotating exobase is generalized by allowing variations in exobase density and temperature which characterize the thermosphere just below the base. The corresponding velocity distribution function, satisfying the collisionless Boltzmann equation, is constructed and used to form a general expression for the velocity moments. Resulting density profiles of rotating exospheres with nonuniform densities and temperatures on the exobase are compared with corresponding nonrotating exospheres. Density enhancements due to rotation are found to be greatest above regions of exobase density or temperature minima. Equatorial density enhancements of terrestrial hydrogen, resulting from rotation, are estimated to be 15%-17% at altitudes of 10-20 earth radii. Corresponding increases in terrestrial helium are 30%-50% on the equator at altitudes of 0.7 to 1 earth radii even when there is a polar density bulge in the barosphere.

Hartle, R. E.↗
NASA NTRS1982

A new look at the ionosphere of Jupiter in light of the UVS occultation results

Neutral model atmospheres derived from the Voyager UV Spectrometer's solar and stellar occultation data are used to calculate Jupiter ionospheric electron density profiles, demonstrating the inadequacy of such methods. It is also shown that electron densities calculated by means of standard parameters are larger than those measured by both Pioneer and Voyager. McElroy's (1973) suggestion as to the importance of H2 vibrational excitation is investigated, and it is found that no single vibrational temperature is appropriate for the entire thermosphere. The long recombination time constants involved in the plasma's movement, due to either meridional winds or electric fields, through large vertical distances, is shown to account for the observed electron density profiles. It is suggested that the outflow, and possibly the inflow, of plasma may play a role in the definition of electron density profiles.

Mcconnell, J. C.↗
NASA NTRS1984

Dynamics of the Venus ionosphere - A two-dimensional model study

A two-dimensional model of the ionosphere of Venus which simulates ionospheric dynamics by self-consistently solving the plasma equations of motion, including the inertial term, in finite difference form has been constructed. The model, which is applied over the solar zenith angle range extending from 60 to 140 deg and the altitude range 100 to 480 km, simulates the measured horizontal velocity field quite satisfactorily. The ion density field is somewhat overestimated on the dayside, because of the choice model neutral atmosphere, and underestimated on the nightside, because of setting the ionopause height at too low an altitude. It is concluded that solar photoionization on the dayside and ion recombination on the nightside are the processes mainly responsible for accelerating the plasma to the observed velocities. The plasma flow appears to be sufficient to maintain the nightside ionosphere at or near the observed median level of ion densities.

Whitten, R. C.↗