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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Effect of stratospheric aerosol on ozone profile from BUV measurements

Results of a simulation study on the effects of optically thick stratospheric sulfate aerosol layers on the backscattered ultraviolet radiation (buv) in the range 256-340 nm are presented. In general, the increased Mie scattering produced by the aerosols results in an enhancement of the buv radiation. The increase is approximately linear with optical depth and strongly depends on solar zenith angle and aerosol layer altitude in relation to the ozone maximum. The effect is greatest at those wavelenghts whose contribution functions peak in the vicinity of the densest part of the aerosol layer. The aerosol induced perturbation of the buv field affects the ozone profile retrieval from space measurements by the Solar Backscatter Ultraviolet Experiment (SBUV) experiment. In the tropical stratosphere, the retrieved ozone between 25 and 45 km is underestimated as a result of increased Mie scattering. On the other hand, an algorithm related effect causes the retrieved ozone below 25 km to be overestimated by an amount similar to the stratospheric deficit.

Torres, Omar↗
NASA NTRS1980

Electron temperature and heat flow in the nightside Venus ionosphere

A steady-state two-dimensional heat balance model is used to analyze the night side Venusian ionospheric electron temperatures given by the Pioneer Venus orbiter electron temperature probe. The energy calculation includes the solar EUV heating at the terminator, electron cooling to ions and neutrals, and heat conduction within the ionospheric plasma. An optimum magnetic field is derived by solving for the heat flux directions which force energy conservation while constrained by the observed temperatures within the range of 80-170 deg solar zenith angle and 160-170 km. The heat flux vectors indicate a magnetic field that connects the lower night side ionosphere to the day side ionosphere, and connects the upper ionosphere to the ionosheath. The lower ionosphere is heated through conduction of heat from the dayside, and the upper ionosphere is heated by the solar wind in the ionosheath with heat flowing downward and from the nightside to the day side.

Hoegy, W. R.↗
NASA NTRS1973

Daytime ion chemistry of N2+.

Rocket measurements of the emission of the (0, 0) first negative band of N2+ at 3914 A in the day airglow have been made between 120 and 300 km. The solar zenith angle at the time of the flight was 60 deg, and the zenith intensity above 120 km was 1.6 kR averaged over both legs of the flight. The data are compared with a model in which charge exchange of N2 with metastable O+(2D) ions is included as an additional source of N2 ionization. Above 240 km this mechanism is dominant over the photoionization of N2 by solar extreme ultraviolet radiation. The derived N2+ density is consistent with the results of rocket-borne mass spectrometric observations.

Feldman, P. D.↗
NASA NTRS1975

Signature extension for sun angle, volume 1

The author has identified the following significant results. Within a restricted zenith sun angle range of 35 - 50 degrees, it was empirically observed that canopy reflectance is mainly Lambertian. Reflectance changes with crop stage were simple shifts in scale in the sun angle range. It was noted that sun angle variations depend on canopy characteristics. Effects of the vegetative canopy were most pronounced at the larger solar zenith angles (20 %). The linear sun angle correction coefficients demonstrate a dependency on both crop stage (15-20 %) and crop type (10-20 %). The use of canopy reflectance modeling allowed for the generation of a simulated data set over an extremely broad envelope of sun angles.

Smith, J. A.↗
NASA NTRS2003

MGS Radio Science Electron Density Profiles: Interannual Variability and Implications for the Martian Neutral Atmosphere

Martian electron density profiles provided by the Mars Global Surveyor (MGS) Radio Science (RS) experiment over the 95-200 km altitude range indicate that the height of the electron peak and the longitudinal structure of the peak height are sensitive indicators of the physical state of the Mars lower atmosphere. The present analysis is carried out on five sets of occultation profiles, all at high solar zenith angles (SZA). Variations spanning 2-Martian years are investigated near aphelion conditions at high Northern latitudes (64.7-77.6N). A mean ionospheric peak height of 133.5-135 km was obtained for all aphelion profiles near SZA = 78-82; a corresponding mean peak density of 7.3-8.5 x 10(exp 4)/cu cm was also measured, reflecting solar moderate conditions. Strong wave 2-3 oscillations in peak heights were observed as a function of longitude over both Martian seasons. The Mars Thermospheric General Circulation Model (MTGCM) is exercised for Mars aphelion conditions. The measured interannual variations in the mean and longitude structure of the peak heights are small (consistent with MTGCM simulations), signifying the repeatability of the Mars atmosphere during aphelion conditions. A non-migrating (semi-diurnal period, wave#l eastward propagating) tidal mode is likely responsible for the wave#3 longitude features identified. The height of this photochemically driven peak can be observed to provide an ongoing monitor of the changing state of the Mars lower atmosphere. The magnitudes of these same peaks may reflect more than changing solar EUV fluxes when they are located in the vicinity of Mars crustal magnetic field centers.

Bougher, Stephen W.↗
NASA NTRS1982

Observations of energetic ions near the Venus ionopause

Ions (primarily O/+/) with spacecraft rest frame energies greater than 40 eV have been observed by the Pioneer Venus Neutral Mass Spectrometer. The signature occurs in about 13% of the 700 orbits examined, primarily near the ionopause and at all solar zenith angles. The energetic ions coincide in location with superthermal ions observed by the Ion Mass Spectrometer and more rarely occur in some of the plasma clouds observed by the Electron Temperature Probe. These observations in conjunction with measurements by the Plasma Wave Instrument near the ionopause suggest that the ions are accelerated out of ionospheric plasma by the shocked solar wind through plasma wave-particle interactions.

Kasprzak, W. T.↗
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.↗
NASA NTRS2000

Aerosol Climate Effects: Local Radiative Forcing and Column Closure Experiments

In an effort to reduce uncertainties in climate change predictions, experiments are being planned and conducted to measure anthropogenic aerosol properties and effects, including effects on radiative fields. The global average, direct anthropogenic aerosol effect on upwelling shortwave fluxes is estimated to be about +1/2 W/sq m, whereas errors in flux changes measured with airborne and spaceborne radiometers are 2 to 8 W/sq m or larger. This poses the question of whether flux changes expected in field experiments will be large enough to measure accurately. This paper obtains a new expression for the aerosol-induced change in upwelling flux, compares it to two-stream and adding-doubling (AD) results, and uses all three methods to estimate expected flux changes. The new expression accounts for the solar zenith angle dependences of aerosol transmission and reflection, as well as of surface albedo, all of which can have a strong effect in determining flux changes measured in field experiments. Despite its relative simplicity, the new expression gives results similar to previous two-stream results. Relative to AD results, it agrees within a few watts per square meter for the intermediate solar elevation angles where the flux changes peak (roughly 10 to 30 degrees), but it has negative errors for higher Sun and positive errors for lower Sun. All three techniques yield aerosol-induced changes in upwelling flux of +8 to +50 W/sq m for aerosol midvisible optical depths of 0.1 to 0.5. Because such aerosol optical depths occur frequently off the U.S. and European Atlantic coasts in summer, the flux changes they induce should be measurable by airborne, and possibly by spaceborne, radiometers, provided sufficient care is taken in experiment design (including measurements to separate aerosol radiative effects from those of absorbing gases). The expected flux changes are about 15 to 100 times larger than the global average flux change expected for the global average anthropogenic sulfate optical depth of 0.04. Reasons for the larger flux changes include the larger optical depths considered here (factor 2.5 to 12), plus restricting the measurements to cloud-free, daytime conditions over the ocean (factor 5 to 9).

Russell, P. B.↗
NASA NTRS2004

MGS Radio Science Electron Density Profiles: Interannual Variability and Implications for the Martian Neutral Atmosphere

Martian electron density profiles provided by the Mars Global Surveyor (MGS) Radio Science (RS) experiment over the 95-200 km altitude range indicate what the height of the electron peak and the longitudinal structure of the peak height are sensitive indicators of the physical state of the Mars lower and upper atmospheres. The present analysis is carried out on five sets of occultation profiles, all at high solar zenith angles (SZA). Variations spanning 2 Martian years are investigated near aphelion conditions at high northern latitudes (64.7 - 77.6 N) making use of four of these data sets. A mean ionospheric peak height of 133.5 - 135 km is obtained near SZA = 78 - 82 deg.; a corresponding mean peak density of 7.3 - 8.5 x l0(exp 4)/ qu cm is also measured during solar moderate conditions at Mars. Strong wave number 2 - 3 oscillations in peak heights are consistently observed as a function of longitude over the 2 Martian years. These observed ionospheric features are remarkably similar during aphelion conditions 1 Martian year apart. This year-to-year repeatability in the thermosphere-ionosphere structure is consistent with that observed in multiyear aphelion temperature data of the Mars lower atmosphere. Coupled Mars general circulation model (MGCM) and Mars thermospheric general circulation model (MTGCM) codes are run for Mars aphelion conditions, yielding mean and longitude variable ionospheric peak heights that reasonably match RS observations. A tidal decomposition of MTGCM thermospheric densities shows that observed ionospheric wave number 3 features are linked to a non-migrating tidal mode with semidiurnal period (sigma = 2) and zonal wave number 1 (s = -1) characteristics. The height of this photochemically determined ionospheric peak should be monitored regularly.

Bougher, S. W.↗
NASA NTRS2013

Assessment of Error in Aerosol Optical Depth Measured by AERONET Due to Aerosol Forward Scattering

We present an analysis of the effect of aerosol forward scattering on the accuracy of aerosol optical depth (AOD) measured by CIMEL Sun photometers. The effect is quantified in terms of AOD and solar zenith angle using radiative transfer modeling. The analysis is based on aerosol size distributions derived from multi-year climatologies of AERONET aerosol retrievals. The study shows that the modeled error is lower than AOD calibration uncertainty (0.01) for the vast majority of AERONET level 2 observations, ~99.53%. Only ~0.47% of the AERONET database corresponding mostly to dust aerosol with high AOD and low solar elevations has larger biases. We also show that observations with extreme reductions in direct solar irradiance do not contribute to level 2 AOD due to low Sun photometer digital counts below a quality control cutoff threshold.

AERONET↗
NASA NTRS1994

Thermospheric O I 844.6-nm emission in twilight

The thermospheric O I 844.6-nm column emission rate was measured over Toronto, a midlatitude station, in the autumn of 1991 using an imaging Fabry-Perot spectrometer. Twilight decay curves were measured on four clear evenings when the solar zenith angle was between 95 degs and 104 degs, giving corresponding column emission rates between 874 R and 130 R at 20 degs elevation angle in the azimuth of the Sun. The expected decay curves were calculated from the field line interhemisperic plasma model assuming only photoelectron impact excitation as the production mechanism with a cross section appropriate to an optically thin atmosphere. The agreement was good when the solar and geomagnetic activity levels were low to moderate, but the emission rate was overestimated during high activity periods. The comparison indicates that the photoelectron impact mechanism with a thin-atmosphere cross section is sufficient to explain the twilight decay of the thermospheric O I 844.6-nm emission.

Bahsoun-Hamade, F.↗
NASA NTRS2024

Parameterizing Spectral Surface Reflectance Relationships for the Dark Target Aerosol Algorithm Applied to A Geostationary Imager

Originally developed for the Moderate Resolution Imaging Spectroradiometer (MODIS) in polar, sun-synchronous low-earth orbit (LEO), the Dark Target (DT) aerosol retrieval algorithm relies on the assumption of a Surface Reflectance Parameterization (SRP) over land surfaces. Specifically for vegetated and dark-soiled surfaces, values of surface reflectance in blue and red visible-wavelength bands are assumed to be nearly linearly related to each other and to the value in a shortwave infrared (SWIR) wavelength band. This SRP also includes dependencies on scattering angle and a normalized difference vegetation index computed from two SWIR bands (NDVISWIR). As the DT retrieval algorithm is being ported to new sensors to continue and expand the aerosol data record, we assess whether the MODIS-assumed SRP can be used for these sensors. Here, we specifically assess SRP for the Advanced Baseline Imager (ABI) aboard, the Geostationary Operational Environmental Satellite (GOES)-16/East (ABIE). First, we find that using MODIS-based SRP leads to higher biases and artificial diurnal signatures in aerosol optical depth (AOD)retrievals from ABIE. The primary reason appears to be that geostationary orbit (GEO) encounters an entirely different set of observation geometry than does LEO, primarily with regards to solar angles coupled with fixed view angles. Therefore, we have developed a new SRP for GEO that draws the angular shape of the surface bidirectional reflectance. We also introduce modifications to the parametrization of both red-SWIR and blue-red spectral relationships to include additional information.The revised Red-SWIR SRP includes solar zenith angle, NDVISWIR, and land-type percentage from an ancillary database. The blue-red SRP adds dependencies on the scattering angle and NDVISWIR. The new SRPs improve the AOD retrieval of ABIE in terms of overall less bias and mitigation of the overestimation around local noon. The average bias of DT AOD compared to AERONET AOD shows a reduction from 0.08 to 0.03, while the bias of local solar noon decreases from 0.12 to 0.03.The agreement between DT and AERONET AOD is established through regression slope of 1.06 and y-intercept of 0.01with correlation coefficient is0.74. By using the new SRP, the percentage of data falling within the expected error range (±0.05 + 15%) is notably risen from 54% to 78%.

Mijin Kim↗
NASA NTRS1980

Effect of ice-albedo feedback on global sensitivity in a one-dimensional radiative-convective climate model

The feedback between the ice albedo and temperature is included in a one-dimensional radiative-convective climate model. The effect of this feedback on global sensitivity to changes in solar constant is studied for the current climate conditions. This ice-albedo feedback amplifies global sensitivity by 26 and 39%, respectively, for assumptions of fixed cloud altitude and fixed cloud temperature. The global sensitivity is not affected significantly if the latitudinal variations of mean solar zenith angle and cloud cover are included in the global model. The differences in global sensitivity between one-dimensional radiative-convective models and energy balance models are examined. It is shown that the models are in close agreement when the same feedback mechanisms are included. The one-dimensional radiative-convective model with ice-albedo feedback included is used to compute the equilibrium ice line as a function of solar constant.

Wang, W.-C.↗
NASA NTRS1975

A cloud physics investigation utilizing Skylab data

The author has identified the following significant results. The Lowtran 2 program, S191 spectral response, and solar spectrum were used to compute the expected absorption by 2.0 micron band for a variety of cloud pressure levels and solar zenith angles. Analysis of the three long wavelength data channels continued in which it was found necessary to impose a minimum radiance criterion. It was also found necessary to modify the computer program to permit the computation of mean values and standard deviations for selected subsets of data on a given tape. A technique for computing the integrated absorption in the A band was devised. The technique normalizes the relative maximum at approximately .78 micron to the solar irradiance curve and then adjusts the relative maximum at approximately .74 micron to fit the solar curve.

Alishouse, J.↗
NASA NTRS1985

Inferring hemispherical reflectance of the earth's surface for global energy budgets from remotely sensed nadir or directional radiance values

The relationship between directional reflectances spanning the entire reflecting hemisphere and hemispherical reflectance (albedo) and the effect of solar zenith angle and cover type on these relationships were investigated, using the results obtained from NOAA's 7/8 AVHRR ground-level reflectance measurements. Bands 1 (0.58-0.6B microns) and 2 (0.73-1. 1 microns) were used for reflectance measurements of 11 natural vegetation surfaces ranging from bare soils to dense vegetation canopies. The results show that errors in inferring hemispherical reflectance from nadir reflectance can be between 11 and 45 percent for all cover types and solar angles, depending on the viewing angles. A technique is described in which a choice of two specific view angles reduces this error to less than 6 percent for both bands and for all sun angles and cover types.

Kimes, D. S.↗
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 NTRS2003

Angular Distribution Models for Top-of-Atmosphere Radiative Flux Estimation from the Clouds and the Earth's Radiant Energy System Instrument on the Tropical Rainfall Measuring Mission Satellite: Validation - Part II

Top-of-atmosphere (TOA) radiative fluxes from the Clouds and the Earth s Radiant Energy System (CERES) are estimated from empirical angular distribution models (ADMs) that convert instantaneous radiance measurements to TOA fluxes. This paper evaluates the accuracy of CERES TOA fluxes obtained from a new set of ADMs developed for the CERES instrument onboard the Tropical Rainfall Measuring Mission (TRMM). The uncertainty in regional monthly mean reflected shortwave (SW) and emitted longwave (LW) TOA fluxes is less than 0.5 W/sq m, based on comparisons with TOA fluxes evaluated by direct integration of the measured radiances. When stratified by viewing geometry, TOA fluxes from different angles are consistent to within 2% in the SW and 0.7% (or 2 W/sq m) in the LW. In contrast, TOA fluxes based on ADMs from the Earth Radiation Budget Experiment (ERBE) applied to the same CERES radiance measurements show a 10% relative increase with viewing zenith angle in the SW and a 3.5% (9 W/sq m) decrease with viewing zenith angle in the LW. Based on multiangle CERES radiance measurements, 18 regional instantaneous TOA flux errors from the new CERES ADMs are estimated to be 10 W/sq m in the SW and, 3.5 W/sq m in the LW. The errors show little or no dependence on cloud phase, cloud optical depth, and cloud infrared emissivity. An analysis of cloud radiative forcing (CRF) sensitivity to differences between ERBE and CERES TRMM ADMs, scene identification, and directional models of albedo as a function of solar zenith angle shows that ADM and clear-sky scene identification differences can lead to an 8 W/sq m root-mean-square (rms) difference in 18 daily mean SW CRF and a 4 W/sq m rms difference in LW CRF. In contrast, monthly mean SW and LW CRF differences reach 3 W/sq m. CRF is found to be relatively insensitive to differences between the ERBE and CERES TRMM directional models.

Loeb, N. G.↗
NASA NTRS1975

The Mariner 10 radio occultation measurements of the ionosphere of Venus

Data from the Mariner 10 radio occultation experiment have been utilized to determine the vertical electron density distribution in the ionosphere of Venus. The ingress measurements, which were made at latitude 1.3 deg N on the nightside of the planet, show two distinct layers. The main layer was located at 142 km altitude and had a peak density of 9000 electrons per cu cm. A secondary layer with a peak density of 7000/cu cm was detected at 124 km altitude. During egress, the ionosphere was probed at latitude 56.0 deg S on the dayside of Venus. The solar zenith angle in this region was 67.0 deg. The dayside ionosphere consisted of a main layer with a peak density of 290,000 per cu cm at 142 km altitude and several minor layers. At the top of the dayside ionosphere, the measurements showed an abrupt drop in the density from 2000 per cu cm at 335 km altitude to below the level of detectability, i.e., less than 200 per cu cm, at 360 km altitude. This abrupt density change may be the ionopause where the solar wind plasma interacts with the ionized components of the atmosphere.

Fjeldbo, G.↗