Engineering topics

Stewart, A. I. F.

Publications and source records attributed to Stewart, A. I. F..

At least 19 records

NASA NTRS2016

Enhanced O2 Loss at Mars Due to an Ambipolar Electric Field from Electron Heating

Recent results from the MAVEN Langmuir Probe and Waves (LPW) instrument suggest higher than predicted electron temperatures (T sub e) in Mars dayside ionosphere above approx. 180 km in altitude. Correspondingly, measurements from Neutral Gas and Ion Mass Spectrometer (NGIMS) indicate significant abundances of O2+ up to approx. 500 km in altitude, suggesting that O2+ may be a principal ion loss mechanism of oxygen. In this article, we investigate the effects of the higher T(sub e) (which results from electron heating) and ion heating on ion outflow and loss. Numerical solutions show that plasma processes including ion heating and higher T(sub e) may greatly increase O2+ loss at Mars. In particular, enhanced T(sub e) in Mars ionosphere just above the exobase creates a substantial ambipolar electric field with a potential (e) of several k(sub b)T(sub e), which draws ions out of the region allowing for enhanced escape. With active solar wind, electron and ion heating, direct O2+ loss could match or exceed loss via dissociative recombination of O2+. These results suggest that direct loss of O2+ may have played a significant role in the loss of oxygen at Mars over time.

Mars↗

NASA NTRS2006

Enceladus’ water vapor plume

The Cassini spacecraft flew close to Saturn’s small moon Enceladus three times in 2005. Cassini’s UltraViolet Imaging Spectrograph observed stellar occultations on two flybys and confirmed the existence, composition, and regionally confined nature of a water vapor plume in the south polar region of Enceladus. This plume provides an adequate amount of water to resupply losses from Saturn’s E ring and to be the dominant source of the neutral OH and atomic oxygen that fill the Saturnian system.

West, R.↗

NASA NTRS2003

An Ultraviolet Imaging Spectrograph for JIMO

It is vital to include an ultraviolet spectrograph as part of the JIMO payload to Europa, Ganymede and Callisto. Ultraviolet measurements are key for understanding the atmospheres, auroral activity and surfaces of these icy satellites, and a UV imaging spectrograph will also complement a visible camera and near-IR spectrometer, to achieve full wavelength coverage in remote sensing of the icy satellites. The UV instrument proposed for JIMO will be similar to that currently on board the Cassini spacecraft. The design draws on the experience of building UV spectrometers for Mariner, Pioneer, Galileo and Cassini. It will have three spectrographic channels that provide images and spectra of the atmosphere, aurorae and surface: An EUV channel (800-110 nm), an FUV channel (110 to 190 nm) range, and an NUV channel (180 to 350 nm).

Hendrix, A. R.↗

NASA NTRS1997

Pioneer Venus Orbiter Ultraviolet Spectrometer: Operations and Data Analysis

The Ultraviolet Spectrometer investigation on the Pioneer Venus Orbiter mission was extremely successful. The instrument was designed, built and tested at CU/LASP and delivered on time and within budget. The spacecraft and its instruments were required to operate for 243 days in Venus orbit. OUVS operated successfully for a further 13 years with only minor problems. The major scientific results listed above that deal with Venus were all unexpected and significant discoveries. The Comet Halley observations came about because of a favorable alignment of Halley, the Sun, and Venus, and were an important contribution to the international study of this comet. The scientific results of the OUVS investigation are to be found in the 41 papers listed in section 4 below. OUVS data provided material for 6 PhD and one MS dissertations, listed in section 5 below.

Stewart, A. I. F.↗

NASA NTRS1997

Pioneer Venus Orbiter Ultraviolet Spectrometer: Operations and Data Analysis

The Pioneer Venus spacecraft orbited Venus 5,055 times between 4th December 1978 and 6th October 1992, before entering Venus' atmosphere and burning up on the latter date. On 255 of these orbits, science operations were suspended because of superior conjunction (Venus' proximity to the Sun as seen from Earth). Of the remaining 4800 orbits, about 85% yielded good-quality OUVS science data; 15% were lost to various problems, including loss of uplink (commands) to and downlink (data) from the spacecraft, errors in commanding OUVS, and one or other of the two instrument anomalies mentioned below.

Stewart, A. I. F.↗

NASA NTRS1994

Observations of interplanetary Lyman-alpha with the Galileo Ultraviolet Spectrometer: Multiple scattering effects at solar maximum

The Galileo Ultravilet Spectrometer Experiment (UVS) obtained a partial celestial sphere map of interplanetary Lyman-alpha (IP L alpha) on 13-14 December 1990 during the first Earth encounter. The Galileo spacecraft was near the downwind axis of the local interstellar medium flow. These UVS measurements sampled the downwind, anti-sunward hemisphere. The data were modeled using a hot model of the interplanetary hydrogen density distribution with the goal of studying multiple scattering effects in the inner solar system. The derived ratio in the downwind direction of the observed brightness and a single scattering model brightness, both normalized to unity in the upwind direction, is 1.82 +/- 0.2. This brightness ratio requires a multiple scattering correction which is 36% larger than can be accounted for by theoretical calculations. The hot model may require: (1) a temperature perturbation of the interstellar wind velocity distribution or (2) an additional downstream source of interplanetary hydrogen. However, a more likely exlanation which affects the hot model is the latitude dependence of the radiation pressure. This dependence, based on the known solar L alpha flux latitude variation at solar maximum, causes a downwind brightness enhancement by preferential focusing of H-atoms with trajectory planes containing the solar poles. This result implies that radiation pressure near the solar poles is nearly independent of solar cycle and is insufficient to lead to a net repulsion of hydrogen atoms by the sun, as can occur near the ecliptic plane during the solar maximum. In addition, the UVS performed 13 observations of IP L alpha while in cruise between Venus and the Earth in 3 directions fixed in ecliptic coordinates.

Ajello, J. M.↗

NASA NTRS1993

Local time asymmetries in the Venus thermosphere

A comparison is presented of the 130-m images taken in the Venus thermosphere by the Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUVS) to predictions by a model which incorporates current understanding of the global structure of the thermosphere, the mechanisms which excite the 130-nm transition in O, and the radiative transport of the 130-nm triplet in the thermosphere. The features identified in the data/model comparison appear as a local time asymmetry in B(130) and O at altitudes poleward of 30 deg. Oxygen densities at the evening terminator are typically a factor of 2 higher than those at the morning terminator. This asymmetry in O has never before been observed or predicted in the global thermospheric models.

Alexander, M. J.↗

NASA NTRS1992

The Galileo and Pioneer Venus ultraviolet spectrometer experiments - Solar Lyman-alpha latitude variation at solar maximum from interplanetary Lyman-alpha observations

Solar Ly-alpha latitude variation at solar maximum is examined on the basis of interplanetary Ly-alpha observations made during the Galileo and Pioneer Venus UV spectrometer experiments. A comparison is made of the latitude variation of the interplanetary (IP) Ly-alpha signal in 1986 at solar minimum from Pioneer Venus and in 1990 at solar maximum from Galileo. The Galileo EUV spectrometer shows that a large enhancement of the IP Ly-alpha emission occurred over the intervening four years near the solar equator. An IP Ly-alpha model is developed which considers the latitude variation of the solar Ly-alpha flux. The model fit to the data shows a 25-percent decrease of the full disk solar Ly-alpha flux from solar equator to solar pole in 1990. A detailed study of the Galileo IP Ly-alpha observations on day-of-year 190, 193, 197, and 200 in 1990 reveals that large variations occur in response to the 27-d solar variation. Analysis of these data shows that a maximum variation of 20 percent can be expected in the IP Ly-alpha upwind intensity over this 27-d period.

Pryor, W. R.↗

NASA NTRS1992

Galileo Ultraviolet Spectrometer experiment

The Galileo ultraviolet spectrometer experiment uses data obtained by the Ultraviolet Spectrometer (UVS) mounted on the pointed orbiter scan platform and from the Extreme Ultraviolet Spectrometer (EUVS) mounted on the spinning part of the orbiter with the field of view perpendicular to the spin axis. The UVS is a Ebert-Fastie design that covers the range 113-432 nm with a wavelength resolution of 0.7 nm below 190 and 1.3 nm at longer wavelengths. The UVS spatial resolution is 0.4 deg x 0.1 deg for illuminated disk observations and 1 deg x 0.1 deg for limb geometries. The EUVS is a Voyager design objective grating spectrometer, modified to cover the wavelength range from 54 to 128 nm with wavelength resolution 3.5 nm for extended sources and 1.5 nm for point sources and spatial resolution of 0.87 deg x 0.17 deg. The EUVS instrument will follow up on the many Voyager UVS discoveries, particularly the sulfur and oxygen ion emissions in the Io torus and molecular and atomic hydrogen auroral and airglow emissions from Jupiter. The UVS will obtain spectra of emission, absorption, and scattering features in the unexplored, by spacecraft, 170-432 nm wavelength region. The UVS and EUVS instruments will provide a powerful instrument complement to investigate volatile escape and surface composition of the Galilean satellites, the Io plasma torus, micro- and macro-properties of the Jupiter clouds, and the composition structure and evolution of the Jupiter upper atmosphere.

Hord, C. W.↗

NASA NTRS1992

Atomic oxygen in the Martian thermosphere

The Mariner 9 Ultraviolet Spectrometer (UVS) made extensive observations of air-glow emissions from the thermosphere of Mars throughout the nominal mission (November 1971 - February 1972), during late summer in the southern hemisphere. Limb and disc measurements of the 130 nm triplet emission from thermospheric atomic oxygen were modelled by Strickland et al. Recently, the thermospheric general circulation models (TGCMs) developed for the Earth and Venus have been applied to Mars; we refer to it as the MTGCM. Our analysis shows that the oxygen mixing ratio is the fundamental unknown controlling the 130 nm brightness. Our radiative transport calculation shows that the emergent intensity at 130 nm is not very sensitive to variations in thermospheric temperature. The pattern of diurnal variation derived from our analysis is roughly the same as Strickland et al. although with somewhat lower values for the O mixing ratio. The main reasons for this difference are the more important role played by the photoelectron source in our model, and the somewhat larger 130 nm solar flux; thus, we require less oxygen to match the observed brightnesses. Strickland et al. also found that the OI 130 nm emission on Mars is correlated with solar activity. We find that the correlation is virtually non-existent during the early orbits when the planet was covered with a thick global dust storm, but later orbits, during the clearing of the storm, show a persistent correlation.

Stewart, A. I. F.↗

NASA NTRS1992

Atomic oxygen in the Martian thermosphere

Modern models of thermospheric composition and temperature and of excitation and radiative transfer processes are used to simulate the O I 130-nm emission from Mars measured by the Mariner 9 ultraviolet spectrometer. This paper uses the Mars thermospheric general circulation model calculations (MTGCM) of Bougher et al. (1988) and the Monte Carlo partial frequency redistribution multiple scattering code of Meier and Lee (1982). It is found that the decline in atomic oxygen through the daylight hours predicted by the MTGCM cannot be reconciled with the excess afternoon brightness seen in the data. Oxygen concentrations inferred from the data show a positive gradient through the day, in agreement with the original analysis by Strickland et al. (1973). In addition, the data suggest that the oxygen abundance increases toward high southerly latitudes, in contrast with the MTGCM prediction of high values in the Northern Hemisphere. It appears that solar forcing alone cannot account for the observed characteristics of the Martian thermosphere and that wave and tidal effects may profoundly affect the structure, winds, and composition.

Stewart, A. I. F.↗

NASA NTRS1992

Local-time asymmetries in the Venus thermosphere

Our current understanding of the global structure and dynamics of the Venus thermosphere is embodied in models such as the Venus Thermospheric General Circulation Model (VTGCM) and empirical composition models such as VIRA and VTS3. We have completed an analysis of ultraviolet images of Venus at 130 nm acquired by the Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUVS). We have examined 97 images spanning the 10-year period between 1980 and 1990, and have developed a technique for global radiative transfer modeling with which we create synthetic models of each image analyzed. We have developed a hypothesis for understanding the persistent local-time asymmetry observed as a signature of vertically propagating internal gravity waves interacting with the thermospheric SS-AS circulation. This hypothesis is presented.

Alexander, M. J.↗

NASA NTRS1992

Aeronomy of the current Martian atmosphere

The thermal structure of the Martian atmosphere, which varies diurnally, seasonally and episodically, is discussed. The atomic oxygen airglow at 1304 A is used to determine the density of atomic oxygen, and the 1216-A Lyman-alpha line is used to calculate the density of atomic hydrogen and, when coupled with the temperature measurement, the escape flux of atomic hydrogen. The most intense airglow is the IR atmospheric band of O2 at 1.27 micron that results from the photodissociation of ozone. The escape mechanism for atomic hydrogen is thermal, or Jeans, escape, while the atomic oxygen escape is caused by a nonthermal process, namely, the dissociative recombination of O2(+). The ratio of deuterium to hydrogen is enriched by a factor of 6. Three-dimensional models of the Mars thermospheric circulation show that planetary rotation has a significant effect on the wind, composition, and temperature structure.

Barth, C. A.↗

NASA NTRS1991

Galileo ultraviolet spectrometer experiment - Initial Venus and interplanetary cruise results

The Galileo Extreme Ultraviolet Spectrometer obtained a spectrum of Venus atmospheric emissions in the 55.0- to 125.0-nm wavelength region. Emissions of helium (58.4 nm), ionized atomic oxygen (83.4 nm), and atomic hydrogen (121.6 nm), as well as a blended spectral feature of atomic hydrogen (Lyman-beta) and atomic oxygen (102.5 nm), were observed at 3.5-nm resolution. During the Galileo spacecraft cruise from Venus to earth, Lyman-alpha emission from solar system atomic hydrogen (121.6 nm) was measured. The dominant source of the Lyman-alpha emission is atomic hydrogen from the interstellar medium. A model of Galileo observations at solar maximum indicates a decrease in the solar Lyman-alpha flux near the solar poles. A strong day-to-day variation also occurs with the 27-day periodicity of the rotation of the sun.

Hord, C. W.↗

NASA NTRS1990

The Venus nitric oxide night airglow - Model calculations based on the Venus Thermospheric General Circulation Model

The mechanism responsible for the Venus nitric oxide (0,1) delta band nightglow observed in the Pioneer Venus Orbiter UV spectrometer (OUVS) images was investigated using the Venus Thermospheric General Circulation Model (Dickinson et al., 1984), modified to include simple odd nitrogen chemistry. Results obtained for the solar maximum conditions indicate that the recently revised dark-disk average NO intensity at 198.0 nm, based on statistically averaged OUVS measurements, can be reproduced with minor modifications in chemical rate coefficients. The results imply a nightside hemispheric downward N flux of (2.5-3) x 10 to the 9th/sq cm sec, corresponding to the dayside net production of N atoms needed for transport.

Bougher, S. W.↗

NASA NTRS1988

Sulfur dioxide at the Venus cloud tops, 1978-1986

Ultraviolet spectroscopy from the Pioneer Venus Orbiter shows a decline in the cloud top abundance of SO2 from about 100 ppb to about 10 ppb in the period 1978-1986. A consistent decline in polar haze has occurred over the same period, with the correlation coefficient between these two observables of r = 0.8. Star calibrations determine the instrument sensitivity to within 10 percent which rules out the possibility that this is an instrumental effect. Systematic errors could increase the SO2 abundance to twice the inferred values in later orbits. Tracking of SO2 features and power spectral analysis give rotation periods for the longer-lived features of 3.6-5.2 days, consistent with cloud-tracked winds observed at other wavelengths. The behavior of SO2 and polar haze can be plausibly explained by episodic injection of SO2 into the cloud top regions, for example, by active volcanism.

Esposito, Larry W.↗

NASA NTRS1988

Analysis of Pioneer Venus Orbiter ultraviolet spectrometer Lyman alpha data from near the subsolar region

Pioneer Venus Orbiter ultraviolet spectrometer data from 20 orbits which span 3 years have been analyzed to determine the atomic hydrogen number density and vertical flux at the exobase as a function of solar zenith angle, F(10.7) index, and spacecraft latitude. From 1979 through 1981, the exobase number density n(c) and flux phi(c) are remarkably constant at n(c) = 6.0 + or - 1.5 x 10 to the 4th/cu cm and phi(e) = 7.5 + or - 1.5 x 10 to the 7th/sq cm per s in the subsolar region. The integrated vertical column density above 110 km is 3.6 + or - 1 x 10 to the 13th/sq cm. An empirical relationship is determined between the line center solar flux at H Lyman alpha, piF(0), and the F(10.7) index.

Paxton, L. J.↗

NASA NTRS1987

Pioneer Venus measurements of H, O, and C production in comet P/Halley near perihelion

The Pioneer Venus Orbiter Ultraviolet Spectrometer measured the production of H, O, and C in comet Halley from late December 1985 to early March 1986. Water production rates obtained from the H data rose from 3.3 x 10 to the 29th/s at 1 AU inbound to 1.2 x 10 to the 30th/s at perihelion; there was a further increase to 1.6 x 10 to the 30th/s followed by a slow decline to 1.4 x 10 to the 30th/s at the time of the Vega-1 encounter. The H measurements show a periodic behavior that imply variations in the water production rate having structure within a 7.4-day sidereal period, in phase with similar variations seen in radical and dust production in March and April. Average H:O:C ratios of 1.0:0.7:0.07 were measured from 10 to 26 days after perihelion.

Stewart, A. I. F.↗