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NASA NTRS2016

Lighting Condition Analysis for Mars Moon Phobos

A manned mission to Phobos may be an important precursor and catalyst for the human exploration of Mars, as it will fully demonstrate the technologies for a successful Mars mission. A comprehensive understanding of Phobos' environment such as lighting condition and gravitational acceleration are essential to the mission success. The lighting condition is one of many critical factors for landing zone selection, vehicle power subsystem design, and surface mobility vehicle path planning. Due to the orbital characteristic of Phobos, the lighting condition will change dramatically from one Martian season to another. This study uses high fidelity computer simulation to investigate the lighting conditions, specifically the solar radiation flux over the surface, on Phobos. Ephemeris data from the Jet Propulsion Laboratory (JPL) DE405 model was used to model the state of the Sun, the Earth, and Mars. An occultation model was developed to simulate Phobos' self-shadowing and its solar eclipses by Mars. The propagated Phobos' state was compared with data from JPL's Horizon system to ensure the accuracy of the result. Results for Phobos lighting condition over one Martian year are presented in this paper, which include length of solar eclipse, average solar radiation intensity, surface exposure time, total maximum solar energy, and total surface solar energy (constrained by incident angle). The results show that Phobos' solar eclipse time changes throughout the Martian year with the maximum eclipse time occurring during the Martian spring and fall equinox and no solar eclipse during the Martian summer and winter solstice. Solar radiation intensity is close to minimum at the summer solstice and close to maximum at the winter solstice. Total surface exposure time is longer near the north pole and around the anti- Mars point. Total maximum solar energy is larger around the anti-Mars point. Total surface solar energy is higher around the anti-Mars point near the equator. The results from this study and others like it will be important in determining landing site selection, vehicle system design and mission operations for the human exploration of Phobos and subsequently Mars.

Li, Zu Qun↗
NASA NTRS2016

An Improved Approach for Analyzing the Oxygen Compatibility of Solvents and other Oxygen-Flammable Materials for Use in Oxygen Systems

Solvents used to clean oxygen system components must be assessed for oxygen compatibility, as incompatible residue or fluid inadvertently left behind within an oxygen system can pose a flammability risk. The most recent approach focused on solvent ignition susceptibility to assess the flammability risk associated with these materials. Previous evaluations included Ambient Pressure Liquid Oxygen (LOX) Mechanical Impact Testing (ASTM G86) and Autogenous Ignition Temperature (AIT) Testing (ASTM G72). The goal in this approach was to identify a solvent material that was not flammable in oxygen. As environmental policies restrict the available options of acceptable solvents, it has proven difficult to identify one that is not flammable in oxygen. A more rigorous oxygen compatibility approach is needed in an effort to select a new solvent for NASA applications. NASA White Sands Test Facility proposed an approach that acknowledges oxygen flammability, yet selects solvent materials based on their relative oxygen compatibility ranking, similar to that described in ASTM G63-99. Solvents are selected based on their ranking with respect to minimal ignition susceptibility, damage and propagation potential, as well as their relative ranking when compared with other solvent materials that are successfully used in oxygen systems. Test methods used in this approach included ASTM G86 (Ambient Pressure LOX Mechanical Impact Testing and Pressurized Gaseous Oxygen (GOX) Mechanical Impact Testing), ASTM G72 (AIT Testing), and ASTM D240 (Heat of Combustion (HOC) Testing). Only four solvents were tested through the full battery of tests for evaluation of oxygen compatibility: AK-225G as a baseline comparison, Solstice PF, L-14780, and Vertrel MCA. Baseline solvent AK-225G exhibited the lowest HOC and highest AIT of solvents tested. Nonetheless, Solstice PF, L-14780, and Vertrel MCA HOCs all fell well within the range of properties that are associated with proven oxygen system materials. Tested AITs for these solvents fell only slightly lower than the AIT for the proven AK-225G solvent. Based on these comparisons in which solvents exhibited properties within those ranges seen with proven oxygen system materials, it is believed that Solstice PF, L-14780, and Vertrel MCA would perform well with respect to oxygen compatibility.

Harper, Susan A.↗
NASA NTRS2014

Seasonal Evolution of Saturn's Polar Temperatures and Composition

The seasonal evolution of Saturn's polar atmospheric temperatures and hydrocarbon composition is derived from a decade of Cassini Composite Infrared Spectrometer (CIRS) 7-16 micrometers thermal infrared spectroscopy. We construct a near-continuous record of atmospheric variability poleward of 60 deg from northern winter/southern summer (2004, Ls = 293 deg) through the equinox (2009, Ls= 0 deg) to northern spring/southern autumn (2014, Ls = 56 deg). The hot tropospheric polar cyclones that are entrained by pro-grade jets within 2-3 deg of each pole, and the hexagonal shape of the north polar belt, are both persistent features throughout the decade of observations. The hexagon vertices rotated westward by approx. equal to 30 deg longitude between March 2007 and April 2013, confirming that they are not stationary in the Voyager-defined System III longitude system as previously thought. Tropospheric temperature contrasts between the cool polar zones (near 80-85 deg) and warm polar belts (near 75-80 deg) have varied in both hemispheres, resulting in changes to the vertical wind shear on the zonal jets in the upper troposphere and lower stratosphere. The extended region of south polar stratospheric emission has cooled dramatically poleward of the sharp temperature gradient near 75 deg S (by approximately -5 K/yr), coinciding with a depletion in the abundances of acetylene (0030 +/- 0.005 ppm/yr) and ethane (0.35 +/- 0.1 ppm/yr), and suggestive of stratospheric upwelling with vertical wind speeds of w approx. equal to +0.1 mm/s. The upwelling appears most intense within 5 deg latitude of the south pole. This is mirrored by a general warming of the northern polar stratosphere (+5 K/yr) and an enhancement in acetylene (0.030 +/- 0.003 ppm/yr) and ethane (0.45 +/- 0.1 ppm/yr) abundances that appears to be most intense poleward of 75 deg N, suggesting subsidence at w approx. equal to -0.15 mm/ s. However, the sharp gradient in stratospheric emission expected to form near 75 deg N by northern summer solstice (2017, Ls = 90 deg) has not yet been observed, so we continue to await the development of a northern summer stratospheric vortex. The peak stratospheric warming in the north occurs at lower pressure levels (p less than 1 mbar) than the peak stratospheric cooling in the south (p greater than 1 mbar). Vertical motions are derived from both the temperature field (using the measured rates of temperature change and the deviations from the expectations of radiative equilibrium models) and hydrocarbon distributions (solving the continuity equation). Vertical velocities tend towards zero in the upper troposphere where seasonal temperature contrasts are smaller, except within the tropospheric polar cyclones where w approx. equal to +0.02 mm/s. North polar minima in tropospheric and stratospheric temperatures were detected in 2008-2010 (lagging one season, or 6-8 years, behind winter solstice); south polar maxima appear to have occurred before the start of the Cassini observations (1-2 years after summer solstice), consistent with the expectations of radiative climate models. The influence of dynamics implies that the coldest winter temperatures occur in the 75-80 deg region in the stratosphere, and in the cool polar zones in the troposphere, rather than at the poles themselves. In addition to vertical motions, we propose that the UV-absorbent polar stratospheric aerosols entrained within Saturn's vortices contribute significantly to the radiative budget at the poles, adding to the localized enhancement in the south polar cooling and north polar warming poleward of +/-75 deg.

polar temperatures↗
NASA NTRS2019

Global analysis and forecasts of carbon monoxide on Mars

Spatial and temporal variations in the Martian carbon monoxide (CO) cycle have been investigated through combining Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) retrievals of carbon monoxide mixing ratio and Mars Climate Sounder (MCS) temperature profiles with a Martian global circulation model (GCM) to produce the first global reanalysis of the Martian CO cycle. The reanalysis reduces the root mean square error between the forecast and CRISM CO retrievals by a factor of 2–4, dependent on the time of year. Forecasts initiated from the reanalysis show an improved match to standalone CRISM CO retrievals from northern winter solstice to northern summer solstice, indicating the benefit of data assimilation in simulating the CO abundance. The northern summer solstice CO minimum between 10°S – 50°S in the CRISM CO retrievals is found to be caused by a suppression of CO-enriched air leaking from the Hellas and Argyre basins, and is also strongly influenced by the amount of carbon dioxide condensing at the time. The GCM is the first reported to simulate the local CO minimum however it is weaker in strength, as CO-enriched air is also released northward in the area of Argyre basin in the GCM because of a shifted boundary between the southern polar vortex and mid-latitudes. The reanalysis, as a result of the assimilation of MCS temperature profiles, indicates that the polar vortex boundary is northward of Argyre basin and hence no northerly transport of CO-enriched air should be present over this location. Differences in local CO abundance between the reanalysis and GCM are also evident in spatio-temporal regions where no nearby CRISM CO retrievals are available as a result of alterations in local circulation and the sublimation/condensation of carbon dioxide through the combined assimilation of CRISM CO retrievals alongside MCS temperature profiles, illustrating how constraints can be imposed indirectly on the CO cycle through the powerful technique of data assimilation.

James A. Holmes↗
NASA NTRS2022

Seasonal Variation of Thermospheric Composition Observed by NASA GOLD

We examine characteristics of the seasonal variation of thermospheric composition using column number density ratio ∑O/N(2) observed by the NASA Global Observations of Limb and Disk (GOLD) mission from low-mid to mid-high latitudes. We also use ∑O/N(2) derived from the Global Ultraviolet Imager (GUVI) limb measurements onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and estimated by the NRLMSISE-00 empirical model to aid our investigation. We found that the ∑O/N(2) seasonal variation is hemispherically asymmetric: in the southern hemisphere, it exhibits the well-known annual and semiannual pattern, with highs near the equinoxes, and primary and secondary lows near the solstices. In the northern hemisphere, it is dominated by an annual variation, with a minor semiannual component with the highs shifting toward the wintertime. We also found that the durations of the December and June solstice seasons in terms of ∑O/N(2) are highly variable with longitude. Our hypothesis is that ion-neutral collisional heating in the equatorial ionization anomaly region, ion drag, and auroral Joule heating play substantial roles in this longitudinal dependency. Finally, the rate of change in ∑O/N(2) from one solstice season to the other is dependent on latitude, with more dramatic changes at higher latitudes.

thermosphere↗
NASA NTRS1978

Study of dynamics of minor constituents in the thermosphere, addendum

Numerical studies of a model of the earth's thermosphere are presented. The distribution of thermospheric helium was investigated. Changes in the global transport of helium under solstice conditions caused by a small increase in the latitudes at which the background gas pressure extremes occur lead to much better agreement of the model predictions with data taken by the mass spectrometers on board the ESRO-4 and OGO-6 satellites. The model was applied to a study of the global distributions of atmospheric gases (N2, O2, and O) at both equinox and solstice with emphasis on the winter enhancement of atomic oxygen in the lower thermosphere. Comparison of the results with measurements taken by the ESRO-4 mass spectrometer indicates that the distribution of atomic oxygen is generally a result of global transport by winds.

Straus, J. M.↗
NASA NTRS1980

The behavior of ozone near the stratopause from two years of BUC observations

Two years of Nimbus 4 backscattered ultraviolet data have been recalibrated and reprocessed. A Laplace transform inversion was applied to radiances at 2876 A and below for 560,000 individual scans for the period April 1970-May 1972. The behavior of ozone near 50 km as a function of time, latitude, and longitude is presented. The high-latitude 1-mbar ozone mixing ratio is maximum at the winter solstice, about 10 microgram/g and is minimum at the summer solstice, about 4 microgram/g. Below 30 deg latitude the ozone is fairly constant at 4 microgram/g. Ozone variability is large in winter and spring and small in summer and fall.

Mcpeters, R. D.↗
NASA NTRS1980

Modelling of equatorial phase and amplitude scintillations from OGO-6 and AE irregularity data

The technique of modelling equatorial phase and amplitude scintillations from in-situ measurements of irregularity amplitude, ambient ionization density and irregularity power spectrum is outlined. The in-situ measurements do not directly provide information on irregularity layer thickness for which radar backscatter observations are utilized. The equatorial scintillation models developed from OGO-6 and AE-C in-situ measurements during the two solstices are presented. Considerable differences in the longitude variation of scintillations are predicted during the two solstices. The model estimates are in good agreement with the available ground based phase and amplitude measurements. Problems associated with the use of bottomside spread-F data for transionospheric propagation modelling at VHF/UHF are also discussed.

Basu, S.↗
NASA NTRS1980

A seasonal change in the effect of field-aligned currents at synchronous orbit

The expected signature of substorm field-aligned currents is described for synchronous satellites near 10 deg magnetic latitude. The main effect is a positive D perturbation premidnight and a negative D perturbation postmidnight. This behavior is illustrated for two substorms near the fall equinox. A second pair of substorms taken from winter solstice do not show the expected behavior. The absence of this effect in winter observations is verified statistically by superposed epoch analysis. A simple explanation of this behavior based on the geometry of the plane of magnetic symmetry is presented. During disturbed times at winter solstice a synchronous satellite nominally at 10 deg magnetic latitude is effectively at the magnetic equator. This distortion of the magnetic equator must be considered in future models of the disturbed magnetospheric magnetic field.

Mcpherron, R. L.↗
NASA NTRS1983

Large-scale counterstreaming of H(+) and He(+) along plasmaspheric flux tubes

An interhemispheric plasma transport model is used to study the flow characteristics of H(+), He(+), and O(+) along closed geomagnetic field lines for solstice conditions. The model corresponds to a time-dependent solution of the coupled continuity, momentum, and energy equations for the ions and electrons. The equations are solved along an entire flux tube from 120 km in one hemisphere to 120 km in the other hemisphere. The calculations are carried out for noon conditions for the flux tube passsing through Millstone Hill, at L = 3.2. The main conclusion is that H(+)-He(+) counterstreaming can be expected along a large segment of a plasmaspheric flux tube at solstice. For both symmetric and asymmetric wind patterns, the He(+) flow is from the winter to the summer ionosphere not only in the steady state, but during flux tube refilling due to the winter helium bulge and the depletion of N2.

Richards, P. G.↗
NASA NTRS1984

Effects of atmosphere and view and illumination geometry on visible and near infrared radiance data from the advanced very high resolution radiometer (AVHR)

The use of Dave's models to evaluate satellite off-nadir remote sensing of green vegetation cover types by simulating the visible and near-infrared advanced very high resolution radiometer (AVHRR) NOAA-6 and NOAA-7 radiances for three green-leaf biomass levels and bare soil. Ground measurements of surface reflectances were used. The simulations were done along a scan line at 30 deg latitude during the summer solstice, equinox, and winter solstice. The simulation models are described and the effect of atmosphere over moderately vegetated surfaces is discussed. The results show that sensor response to atmospheric path length can be substantial for the AVHRR visible and near-infrared channels and normalized difference values, but they can be minimized by high sun and clear atmospheric viewing. The results indicate that AVHRR data would be most useful for monitoring low green leaf biomas canopies.

Holben, B. N.↗
NASA NTRS1988

Ultraviolet radiation levels during the Antarctic spring

The decrease in atmospheric ozone over Antarctica during spring implies enhanced levels of ultraviolet (UV) radiation received at the earth's surface. Model calculations show that UV irradiances encountered during the occurrence of an Antarctic 'ozone hole' remain less than those typical of a summer solstice at low to middle latitudes. However, the low ozone amounts observed in October 1987 imply biologically effective irradiances for McMurdo Station, Antarctica, that are comparable to or greater than those for the same location at December solstice. Life indigenous to Antarctica thereby experiences a greatly extended period of summerlike UV radiation levels.

Frederick, John E.↗
NASA NTRS1988

In-flight calibration of solar irradiance measurements by direct comparison with stellar observations

The Solstice instrument aboard the upper atmosphere research satellite (UARS) is designed to correct the deficiencies of both long-term instrument drift and of intercomparison of solar data sets taken at different times by different instruments. The Solstice will make repeated measurements, spaced throughout the mission, of a number of bright early-type stars. In addition, it will determine the ratio of the solar flux to the average flux from an ensemble of stars. This ratio will be monitored throughout the UARS mission and archived.

Rottman, Gary J.↗
NASA NTRS1990

Low-latitude thermospheric neutral winds determined from AE-E measurements of the 6300-A nightglow at solar maximum

Atmosphere Explorer E (AE-E) measurements of the O(1D) 6300-A emission in the nighttime equatorial thermosphere are used to infer the height of the F2 layer peak as a function of latitude and local time. The investigation is conducted both for northern hemisphere winter solstice and for spring equinox, under solar maximum conditions. The layer heights are used to derive magnetic meridional components of the transequatorial neutral wind, in conjunction with the MSIS-86 model and previous Jicamarca incoherent scatter measurements of the zonal electric field. The AE-E wind estimates indicate a predominant summer to winter flow for the winter solstice case. Comparisons are made with the empirical horizontal wind model HWM87 and with winds generated by the thermospheric general circulation model. The model predictions and experimental results are generally in good agreement, confirming the applicability of visible airglow data to studies of the global neutral wind pattern.

Burrage, M. D.↗
NASA NTRS1991

AI techniques for a space application scheduling problem

Scheduling is a very complex optimization problem which can be categorized as an NP-complete problem. NP-complete problems are quite diverse, as are the algorithms used in searching for an optimal solution. In most cases, the best solutions that can be derived for these combinatorial explosive problems are near-optimal solutions. Due to the complexity of the scheduling problem, artificial intelligence (AI) can aid in solving these types of problems. Some of the factors are examined which make space application scheduling problems difficult and presents a fairly new AI-based technique called tabu search as applied to a real scheduling application. the specific problem is concerned with scheduling application. The specific problem is concerned with scheduling solar and stellar observations for the SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) instrument in a constrained environment which produces minimum impact on the other instruments and maximizes target observation times. The SOLSTICE instrument will gly on-board the Upper Atmosphere Research Satellite (UARS) in 1991, and a similar instrument will fly on the earth observing system (Eos).

Thalman, N.↗
NASA NTRS1992

Seawifs Technical Report Series. Volume 2: Analysis of Orbit Selection for Seawifs: Ascending Versus Descending Node

Due to range safety considerations, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color instrument may be required to be launched into a near-noon descending node, as opposed to the ascending node used by the predecessor sensor, the Coastal Zone Color Scanner (CZCS). The relative importance of ascending versus descending near-noon orbits was assessed here to determine if descending node will meet the scientific requirements of SeaWiFS. Analyses focused on ground coverage, local times of coverage, solar and viewing geometries (zenith and azimuth angles), and sun glint. Differences were found in the areas covered by individual orbits, but were not important when taken over a 16 day repeat time. Local time of coverage was also different: for ascending node orbits the Northern Hemisphere was observed in the morning and the Southern Hemisphere in the afternoon, while for descending node orbits the Northern Hemisphere was observed in the afternoon and the Southern in the morning. There were substantial differences in solar azimuth and spacecraft azimuth angles both at equinox and at the Northern Hemisphere summer solstice. Negligible differences in solar and spacecraft zenith angles, relative azimuth angles, and sun glint were obtained at the equinox. However, large differences were found in solar zenith angles, relative azimuths, and sun glint for the solstice. These differences appeared to compensate across the scan, however, an increase in sun glint in descending node over that in ascending node on the western part of the scan was compensated by a decrease on the eastern part of the scan. Thus, no advantage or disadvantage could be conferred upon either ascending node or descending node for noon orbits. Analyses were also performed for ascending and descending node orbits that deviated from a noon equator crossing time. For ascending node, afternoon orbits produced the lowest mean solar zenith angles in the Northern Hemisphere, and morning orbits produced the lowest angles for the Southern Hemisphere. For descending node, morning orbits produced the lowest mean solar zenith angles for the Northern Hemisphere; afternoon orbits produced the lowest angles for the Southern Hemisphere.

Hooker, Stanford B.↗
NASA NTRS1994

Analysis of solar spectral irradiance measurements from the SBUV/2-series and the SSBUV instruments

The measurements of the solar ultraviolet spectral irradiance by the two Upper Atmosphere Research Satellite (UARS) solar instruments are validated to agree within their 2-Sigma calibration uncertainties of about 7 percent, as well as with measurements from the two solar instruments on the Shuttle Atmospheric Laboratory for Applications and Science (ATLAS) missions. Additionally, the precision of the two UARS data sets is better than the original 2 percent goal, especially at wavelengths greater than 160 nm. This excellent agreement can be credited to accurate pre-flight calibrations, comprehensive in-flight calibrations to track instrument degradation, and a coordinated validation program among the UARS and ATLAS solar instrument teams. The solar irradiance results presented here include those derived from UARS SUSIM, UARS SOLSTICE, ATLAS SUSIM and ATLAS SSBUV measurements on 29 March 1992 during the ATLAS-1 mission and on 15 April 1993 during the ATLAS-2 mission. Two ultraviolet spectra from 119 to 410 nm are derived as the weighted average of the UARS SOLSTICE and SUSIM measurements and are recommended as representative solar spectra for the period of the ATLAS-1 and ATLAS-2 missions. The ATLAS-1 mission occurred during the initial phase of the solar cycle 22 decline when solar activity was moderately high. The ATLAS-2 mission occurred later during the declining phase of the solar cycle 22 when solar activity was more moderate.

Cebula, Richard P.↗
NASA NTRS1993

Quasi-decadal variability of the stratosphere: Influence of long-term solar ultraviolet variations

A multiple regression statistical model is applied to investigate the existence of upper-stratospheric ozone, temperature, and zonal wind responses to long-term (solar cycle) changes in solar ultraviolet radiation using 11.5 years of reprocessed Nimbus-7 Solar Backscattered Ultraviolet (SBUV) data and 12.4 years of National Meteorological Center (NMC) data. A positive solar cycle variation of independently measured ozone and temperature occurs with maximum amplitude near the low-latitude stratopause. The seasonal solar regression coefficients near 1 mb for both ozone and temperature occur at low latitudes supporting a role for photochemical and radiative forcing in their origin. Zonal wind perturbations that correlate with long-term solar ultraviolet variations are a strong function of season and pressure level. Above approximately 2 mbar, the largest solar-correlated zonal wind enhancements occur at middle winter latitudes near the time of winter solstice in both hemispheres. The Northern Hemisphere December enhancement at 1 mb was especially large, 23 +/- 9 m/s from solar minimum to maximum during the last solar cycle. The derived ozone, temperature, and zonal wind increases with increasing solar ultraviolet flux near the stratopause are larger than predicted by models that consider primarily photochemical and radiative processes. The higher ozone and temperature response amplitudes at low latitudes may be due to modified ozone transport and adiabatic temperature changes induced by the dynamical response. If the midlatitude winter solstice wind enhancements are solar induced, their high amplitudes require a positive feedback due to wave-mean flow interaction such that the planetary wave drag on the flow is reduced under solar maximum conditions.

Hood, L. L.↗