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At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Liquid content of the lower clouds of Venus as determined from Mariner 10 radio occultation

S- and X-band radio occulation data obtained during the Mariner 10 flyby of Venus are analyzed to determine the effects of signal attenuation due to the clouds of Venus. The signal-amplitude data are corrected for errors introduced by spacecraft motion and antenna steering during the occultation, absorption-coefficient profiles are computed, and the liquid content of the Venusian clouds is estimated. Excess attenuation effects are found and ascribed to absorption by small liquid particles, consisting of a water-sulfuric acid mixture containing 75% + or - 25% sulfuric acid, in the clouds. Three absorbing cloud layers at respective heights of 68, 60, and 48 km are suggested by the X-band absorptivity profile, but only the 68- and 48-km layers are clearly indicated by the S-band profile. The relatively slow S-band absorption at 60 km is tentatively attributed to an unknown constituent that is much more absorbing in the X-band than in the S-band. The results are compared with nephelometer measurements performed at 0.9 micron by the Venera 9 and 10 probes.

Kliore, A. J.↗
NASA NTRS1981

Cloud retrieval using infrared sounder data - Error analysis

An error analysis is presented for cloud-top pressure and cloud-amount retrieval using infrared sounder data. Rms and bias errors are determined for instrument noise (typical of the HIRS-2 instrument on Tiros-N) and for uncertainties in the temperature profiles and water vapor profiles used to estimate clear-sky radiances. Errors are determined for a range of test cloud amounts (0.1-1.0) and cloud-top pressures (920-100 mb). Rms errors vary by an order of magnitude depending on the cloud height and cloud amount within the satellite's field of view. Large bias errors are found for low-altitude clouds. These bias errors are shown to result from physical constraints placed on retrieved cloud properties, i.e., cloud amounts between 0.0 and 1.0 and cloud-top pressures between the ground and tropopause levels. Middle-level and high-level clouds (above 3-4 km) are retrieved with low bias and rms errors.

Wielicki, B. A.↗
NASA NTRS1987

Limitations to modeling the thermosphere and exosphere

Correlations were noted between solar 10cm radio flux, the indices of geomagnetic activity, and what happens in the atmosphere. There are also correlations between events in the troposphere and density in the thermosphere. Gravity waves in the thermosphere are not handled in existing models. A reasonable estimate is that they contribute perhaps ten percent to the deviation between model density values and the effective density as it influences satellite orbital motion. Another factor is atmospheric composition which influences density through the different scale heights of components of different molecular weight in this regime.

Slowey, John↗
NASA NTRS1994

Evaluation of the TOPEX/POSEIDON dual-frequency ionosphere correction

The NASA altimeter on board TOPEX/POSEIDON exploits the difference in the delays of the Ku and C band radar pulses to estimate an ionosphere correction to the range measurement. The dependence of the ionosphere correction on ocean and satellite parameters is less than 1 cm. The standard deviation of the 1-s averaged ionosphere correction depends on the height of the ocean waves and ranges from 5 to 14 mm. The accuracy of the ionosphere correction is better than 1 cm at the 1 sigma confidence level. The ionosphere correction should be averaged over 140 km (20 s) along track in order to minimize its noise without sacrificing its accuracy. Ionosphere models must achieve an independent sample spacing of 900 km or less in order to allow a single-frequency altimeter to have an ionosphere correction comparable in accuracy to that of the NASA dual-frequency altimeter.

Imel, David A.↗
NASA NTRS2010

Water in the Oceanic Lithosphere: Salt Lake Crater Xenoliths, Oahu, Hawaii

Water can be present in nominally anhydrous minerals of peridotites in the form of hydrogen bonded to structural oxygen. Such water in the oceanic upper mantle could have a significant effect on its physical and chemical properties. However, the water content of the MORB source has been inferred indirectly from the compositions of basalts. Direct determinations on abyssal peridotites are scarce because they have been heavily hydrothermally altered. Here we present the first water analyses of minerals from spinel peridotite xenoliths of Salt Lake Crater, Oahu, Hawaii, which are exceptionally fresh. These peridotites are thought to represent fragments of the Pacific oceanic lithosphere that was refertilized by alkalic Hawaiian melts. A few have unradiogenic Os and radiogenic Hf isotopes and may be fragments of an ancient (~2 Ga) depleted and recycled lithosphere. Water contents in olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) were determined by FTIR spectrometry. Preliminary H_{2}O contents show ranges of 8-10 ppm for Ol, 151-277 ppm for Opx, and 337-603 ppm for Cpx. Reconstructed bulk rock H_{2}O contents range from 88-131 ppm overlapping estimates for the MORB source. Water contents between Ol minerals of the same xenolith are heterogeneous and individual OH infrared bands vary within a mineral with lower 3230 cm^{-1} and higher 3650-3400 cm^{-1} band heights from core to edge. This observation suggests disturbance of the hydrogen in Ol likely occurring during xenolith entrainment to the surface. Pyroxene water contents are higher than most water contents in pyroxenes from continental peridotite xenoliths and higher than those of abyssal peridotites. Cpx water contents decrease with increasing degree of depletion (e.g. increasing Fo in Ol and Cr# in spinel) consistent with an incompatible behavior of water. However Cpx water contents also show a positive correlation with LREE/HREE ratios and LREE concentrations consistent with refertilization. Opx water contents increase with increasing degree of depletion and decrease with LREE/HREE ratios which is inconsistent with the incompatible behavior of H. Calculated water contents of melts in equilibrium with Cpx or Opx range from 1.4 to 3.8 wt % which is higher than that of all Hawaiian lavas. Calculated melts in equilibrium with Cpx and Opx have variable but mostly high H_{2}O/Ce ratios (194 to 1146) consistent with those of rejuvenated stage lavas from Niihau and the South Arch volcanic field, but unlike the drier shield building stage tholeiites. Whether the high water contents recorded in Salt Lake Crater xenoliths were acquired before and/or during interaction of the oceanic lithosphere with the Hawaiian plume will be discussed.

Peslier, Anne H.↗
NASA NTRS1986

Ozone perturbations by enhanced levels of CFCs, N2O, and CH4 A two-dimensional diabatic circulation study including uncertainty estimates

Observations made over the last few years suggest that the tropospheric concentrations of N2O, CH4, and O3 are increasing. Increases in the concentration of chlorofluorocarbons (CFCs) have been observed for some time. The present study is concerned with combined scenarios of future releases of N2O, CH4, and CFCs, which can affect the height profiles of ozone, while changes in latitudinal gradients of ozone may also be expected. Ozone perturbation calculations performed in the two-dimensional transport-chemistry model described by Stordal et al. (1985) are also presented, and the effects of increased levels of CFCs, N2O, and CH4 are examined. It is found that CH4 may be the most important ozone-perturbing trace species in connection with future tropospheric climatic impacts. A substantial increase in the tropospheric abundancy of CH4 could lead to large future ozone enhancements throughout the troposphere and lower stratosphere at middle and low latitudes.

Isaksen, I. S. A.↗
NASA NTRS2001

Intercomparison of Satellite Derived Gravity Time Series with Inferred Gravity Time Series from TOPEX/POSEIDON Sea Surface Heights and Climatological Model Output

The upcoming GRACE mission promises to open a window on details of the global mass budget that will have remarkable clarity, but it will not directly answer the question of what the state of the Earth's mass budget is over the critical last quarter of the 20th century. To address that problem we must draw upon existing technologies such as SLR, DORIS, and GPS, and climate modeling runs in order to improve our understanding. Analysis of long-period geopotential changes based on SLR and DORIS tracking has shown that addition of post 1996 satellite tracking data has a significant impact on the recovered zonal rates and long-period tides. Interannual effects such as those causing the post 1996 anomalies must be better characterized before refined estimates of the decadal period changes in the geopotential can be derived from the historical database of satellite tracking. A possible cause of this anomaly is variations in ocean mass distribution, perhaps associated with the recent large El Nino/La Nina. In this study, a low-degree spherical harmonic gravity time series derived from satellite tracking is compared with a TOPEX/POSEIDON-derived sea surface height time series. Corrections for atmospheric mass effects, continental hydrology, snowfall accumulation, and ocean steric model predictions will be considered.

Cox, C.↗
NASA NTRS1983

Nonlinear anelastic modal theory for solar convection

Solar envelope models are developed using single-mode anelastic equations as a description of turbulent convection which provide estimates for the variation with depth of the largest convective cellular flows, with horizontal sizes comparable to the total depth of the convection zone. These models can be used to describe compressible motions occurring over many density scale heights. Single-mode anelastic solutions are obtained for a solar envelope whose mean stratification is nearly adiabatic over most of its vertical extent because of the enthalpy flux explicitly carried by the big cell, while a subgrid scale representation of turbulent heat transport is incorporated into the treatment near the surface. It is shown that the single-mode equations allow two solutions for the same horizontal wavelength which are distinguished by the sense of the vertical velocity at the center of the three-dimensional cell. It is found that the upward directed flow experiences large pressure effects which can modify the density fluctuations so that the sense of the buoyancy force is changed, with buoyancy braking actually achieved near the top of the convection zone. It is suggested that such dynamical processes may explain why the amplitudes of flows related to the largest scales of convection are so weak in the solar atmosphere.

Latour, J.↗
NASA NTRS1992

Aerocapture and aeromaneuvering at Mars

This paper presents the results of a guidance analysis study for aerocapture and aeromaneuvering at Mars. A 4-state, near-optimal, hybrid guidance algorithm is derived for applications throughout the paper. It is based on the critical assumption that Loh's function remains constant between successive guidance updates. Besides the guidance algorithm, the paper presents a performance evaluation of guidance derived from estimated or 'navigated' spacecraft state, provided by an onboard navigation subsystem for real-time applications on Mars. For the performance evaluation, the spacecraft state is augmented by such parameters as the reference atmospheric density and scale-height, the banking angle and angle of attack for spacecraft control and the accelerometer measurement model errors. Finally, the paper examines also, the influence of random uncertainties in atmospheric density on aeroassisted maneuvers, by a Monte Carlo type of simulation.

Vijayaraghavan, A.↗
NASA NTRS2002

Cloud Thickness from Offbeam Returns - Thor Lidar

Physical thickness of a cloud layer, and sometimes multiple cloud layers, can be estimated from the time delay of off-beam returns from a pulsed laser source illuminating one side of the cloud layer. In particular, the time delay of light returning from the outer diffuse halo of light surrounding the beam entry point, relative to the time delay at beam center, determines the cloud physical thickness. The delay combined with the pulse stretch gives the optical thickness. The halo method works best for thick cloud layers, typically optical thickness exceeding 2, and thus compliments conventional lidar which cannot penetrate thick clouds. Cloud layer top and base have been measured independently over the ARM/SGP site using conventional laser ranging (lidar) and the top minus base thickness are compared with a cloud top halo estimate obtained from the NASA/Goddard THOR System (THOR = THickness from Offbeam Returns). THOR flies on the NASA P3, and measures the halo timings from several km above cloud top, at the same time providing conventional lidar cloud top height. The ARM/SGP micropulse lidar provides cloud base height for validation.

Cahalan, R.↗
NASA NTRS2011

The Next Generation of Mars-GRAM and Its Role in the Autonomous Aerobraking Development Plan

The Mars Global Reference Atmospheric Model (Mars-GRAM) is an engineering-level atmospheric model widely used for diverse mission applications. Mars-GRAM 2010 is currently being used to develop the onboard atmospheric density estimator that is part of the Autonomous Aerobraking Development Plan. In previous versions, Mars-GRAM was less than realistic when used for sensitivity studies for Thermal Emission Spectrometer (TES) MapYear=0 and large optical depth values, such as tau=3. A comparison analysis has been completed between Mars-GRAM, TES and data from the Planetary Data System (PDS) resulting in updated coefficients for the functions relating density, latitude, and longitude of the sun. The adjustment factors are expressed as a function of height (z), Latitude (Lat) and areocentric solar longitude (Ls). The latest release of Mars-GRAM 2010 includes these adjustment factors that alter the in-put data from MGCM and MTGCM for the Mapping Year 0 (user-controlled dust) case. The greatest adjustment occurs at large optical depths such as tau greater than 1. The addition of the adjustment factors has led to better correspondence to TES Limb data from 0-60 km as well as better agreement with MGS, ODY and MRO data at approximately 90-135 km. Improved simulations utilizing Mars-GRAM 2010 are vital to developing the onboard atmospheric density estimator for the Autonomous Aerobraking Development Plan. Mars-GRAM 2010 was not the only planetary GRAM utilized during phase 1 of this plan; Titan-GRAM and Venus-GRAM were used to generate density data sets for Aerobraking Design Reference Missions. These data sets included altitude profiles (both vertical and along a trajectory), GRAM perturbations (tides, gravity waves, etc.) and provided density and scale height values for analysis by other Autonomous Aero-braking team members.

Justh, Hilary L.↗
NASA NTRS1999

Satellite Altimetry, Ocean Circulation, and Data Assimilation

Ocean circulation is a critical factor in determining the Earth's climate. Satellite altimetry has been proven a powerful technique for measuring the height of the sea surface for the study of global ocean circulation dynamics. A major objective of my research is to investigate the utility of altimeter data for ocean circulation studies. The 6 years' data record of TOPEX/POSEIDON have been analyzed to study the spatial and temporal characteristics of large-scale ocean variability. A major result obtained in 1998 is the discovery of large-scale oscillations in sea level with a period of 25 days in the Argentine Basin of the South Atlantic Ocean (see diagram). They exhibit a dipole pattern with counterclockwise rotational propagation around the Zapiola Rise (centered at 45S and 317E), a small seamount in the abyssal plain of the basin. The peak-to-trough amplitude is about 10 cm over a distance of 500-1000 km. The amplitude of these oscillations has large seasonal-to-interannual variations. The period and rotational characteristics of these oscillations are remarkably similar to the observations made by two current meters deployed near the ocean bottom in the region. What TOPEX/POSEIDON has detected apparently are manifestations of the movement of the entire water column (barotropic motion). The resultant transport variation is estimated to be about 50 x 10(exp 6) cubic M/S, which is about 50% of the total water transport in the region. Preliminary calculations suggest that these oscillations are topographically trapped waves. A numerical model of the South Atlantic is used to investigate the nature of and causes for these waves. A very important property of sea surface height is that it is directly related to the surface geostrophic velocity, which is related to deep ocean circulation through the density field. Therefore altimetry observations are not only useful for determining the surface circulation but also for revealing information about the deep ocean. Another objective of my research is to investigate the extent of information one can infer about the deep ocean from altimetric observations. The approach is data assimilation by ocean models. The objective is to develop a modeling/data assimilation system that will produce estimates of the 3-dimensional state of the ocean for the entire duration of the TOPEX/POSEIDON Mission.

Fu, Lee-Lueng↗
NASA NTRS1978

A new use of high resolution magnetograms

Ground-based solar magnetograms are frequently in error by as much as twenty percent and contribute to the poor correlation between magnetic changes and solar flares. High resolution measurement of the magnetic field component, which is normal to the photosphere and measured at photospheric height, can be used to construct a magnetic flux partition function F. Therefore, dF/dt is an EMF which drives atmospheric currents in reconnecting solar active regions. With a high quality magnetograph, the solar probe can be used to obtain good estimates of F and dF/dt and thereby the energy stored as induced solar atmospheric currents during quiescent interflare periods. Should a flare occur during a favorable observing period, the present method of analysis should show characteristic signatures in F, DF/dt, and especially, in the stored flux computed from dF/dt.

Baum, P. J.↗
NASA NTRS1978

The expanding envelope of Tau Sco /B0 V/

Results are reported for an analysis of the UV spectrum of Tau Sco as observed by Copernicus. It is found that the spectrum exhibits a number of resonance lines of C III, C IV, N III, N V, O VI, Si IV, and P V, all of which have extended violet absorption wings that reach several different velocities. The UV lines of Tau Sco are compared with those of Zeta Pup, the line profiles are studied by a comparison with predicted profiles, and the optical depth of the lines is derived as a function of velocity in the envelope by means of line fitting. The velocity-height relation is examined, the mass-loss rate is derived, and the ionization equilibrium in the envelope is investigated. The data are then evaluated in light of three proposed mass-loss mechanisms. The mass-loss rate is estimated to be about 7 billionths of a solar mass per year, and the observed rapid acceleration of the stellar wind is attributed to radiation pressure in the envelope. It is shown that an imperfect-flow model for the stellar wind gives qualitative agreement with the observations.

Lamers, H. J. G. L. M.↗
NASA NTRS1986

Comments on Lambeck and Coleman - 'The earth's shape and gravity field: A report of progress from 1958 to 1982'

The accuracy and validation of global gravity models based on satellite data are discussed, responding to the statistical analysis of Lambeck and Coleman (1983) (LC). Included are an evaluation of the LC error spectra, a summary of independent-observation calibrations of the error estimates of the Goddard Earth Models (GEM) 9 and L2 (Lerch et al., 1977, 1979, 1982, 1983, and 1985), a comparison of GEM-L2 with GRIM-3B (Reigber et al., 1983), a comparison of recent models with LAGEOS laser ranging, and a summary of resonant-orbit model tests. It is concluded that the accuracy of GEMs 9, 10, and L2 is much higher than claimed by LC, that the GEMs are in good agreement with independent observations and with GRIM-3B, and that the GEM calibrations were adequate. In a reply by LC, a number of specific questions regarding the error estimates are addressed, and it is pointed out that the intermodel discrepancies of the greatest geophysical interest are those in the higher-order coefficients, not discussed in the present comment. It is argued that the differences among the geoid heights of even the most recent models are large enough to call for considerable improvements.

Lerch, F. J.↗
NASA NTRS1988

Structure and growth of the mixing layer over the Amazonian rain forest

The structure and growth of the atmospheric mixed layer over the Amazonian rain forest were examined using measurements obtained during the NASA Amazon Boundary Layer Experiment. Measurements of temperature, moisture, and horizontal wind were carried out in and above the mixed layer by means of a tethered balloon, rawinsonde, and aircraft; fluxes of sensible and latent heat were measured at the top of the canopy. It was found that the mixing layer grows rapidly, at 5-8 cm/sec, soon after sunrise to a mean maximum height of 1200 m by 1300 LT; during undisturbed conditions, mixed layer heights of 1000 are common between 1000 and 1600 LT. No horizontal inhomogeneities in the mixed layer structure or depth were found over large distances. A simple mixed layer model was applied to show how fluxes of species might be estimated using only quantities measured at the surface and prescribing an initial condition and boundary condition for the mixed layer.

Martin, Charles L.↗
NASA NTRS1994

The 630 nm MIG and the vertical neutral wind in the low latitude nighttime thermosphere

It is shown that large negative divergences (gradients) in the horizontal neutral wind in the equatorial thermosphere can support downward neutral winds in excess of 20 m/s. With attention to the meridional and vertical winds only, the pressure tendency equation is used to derive the expression U(sub z0) approximately equals (Partial derivative U(sub y)/Partial derivative y)H for the vertical wind U(sub z0) at the reference altitude for the pressure tendency equation; H is the atmospheric density scale height, and (Partial derivative U(sub y)/Partial derivative y) is the meridional wind gradient. The velocity gradient associated with the Meridional Intensity Gradient (MIG) of the O((sup 1)D) emission (630 nm) at low latitudes is used to estimate the vertical neutral wind in the MIG region. Velocity gradients derived from MIG data are about 0.5 (m/s)/km) or more, indicating that the MIG region may contain downward neutral winds in excess of 20 m/s. Though direct measurements of the vertical wind are scarce, Fabry-Perot interferometer data of the equatorial F-region above Natal, Brazil, showed downward winds of 30 m/s occurring during a strong meridional wind convergence in 1982. In-situ measurements with the WATS instrument on the DE-2 satellite also show large vertical neutral winds in the equatorial region.

Herrero, F. A.↗
NASA NTRS1987

Logistic Risk Model for the Unique Effects of Inherent Aerobic Capacity on (+)G(sub z) Tolerance Before and After Simulated Weightlessness

Small sample size (n less than 1O) and inappropriate analysis of multivariate data have hindered previous attempts to describe which physiologic and demographic variables are most important in determining how long humans can tolerate acceleration. Data from previous centrifuge studies conducted at NASA/Ames Research Center, utilizing a 7-14 d bed rest protocol to simulate weightlessness, were included in the current investigation. After review, data on 25 women and 22 men were available for analysis. Study variables included gender, age, weight, height, percent body fat, resting heart rate, mean arterial pressure, Vo(sub 2)max and plasma volume. Since the dependent variable was time to greyout (failure), two contemporary biostatistical modeling procedures (proportional hazard and logistic discriminant function) were used to estimate risk, given a particular subject's profile. After adjusting for pro-bed-rest tolerance time, none of the profile variables remained in the risk equation for post-bed-rest tolerance greyout. However, prior to bed rest, risk of greyout could be predicted with 91% accuracy. All of the profile variables except weight, MAP, and those related to inherent aerobic capacity (Vo(sub 2)max, percent body fat, resting heart rate) entered the risk equation for pro-bed-rest greyout. A cross-validation using 24 new subjects indicated a very stable model for risk prediction, accurate within 5% of the original equation. The result for the inherent fitness variables is significant in that a consensus as to whether an increased aerobic capacity is beneficial or detrimental has not been satisfactorily established. We conclude that tolerance to +Gz acceleration before and after simulated weightlessness is independent of inherent aerobic fitness.

Ludwig, David A.↗