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At least 721 records · Page 40

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NASA NTRS1978

Vertical temperature and density patterns in the Arctic mesosphere analyzed as gravity waves

Three series of rocket soundings including pitot soundings, grenade soundings, and paired pitot-grenade soundings, were conducted from high latitude sites during winter. Temperature and wind profiles and one density profile were observed independently to obtain the thermodynamic structure, the wind structure, and thus their interdependence in the mesosphere. Temperature profiles from all soundings in each series were averaged, and a smooth curve (or series of smooth curves) drawn through the points. A hydrostatic atmosphere based on the average, measured temperature profile was computed, and deviations from the mean atmosphere were analyzed in terms of gravity wave theory. The vertical wavelengths of the deviations were 10-20 km, and the wave amplitudes slowly increased with height. The experimental data were matched by calculated gravity waves having a period ranging between 15 and 80 minutes and horizontal wavelengths of 60 to 280 km. The interpretation is generally consistent with the results of others who have studied gravity-acoustic waves in the atmosphere. The wind measurements are consistent with the thermodynamic measurements. The results also suggest that gravity waves traveled from east to west with a horizontal phase velocity of approximately 60 m/sec.

Eberstein, I. J.↗

NASA NTRS1979

Bubble behavior during solidification in low-gravity

The trapping and behavior of gas bubbles were studied during low gravity solidification of carbon tetrabromide. The flight experiments were performed during two sounding rocket flights (SPAR 1 and SPAR 3) and involved gradient freeze solidification of gas saturated melts. Gas bubbles were evolved at the solid-liquid interfaces during the low gravity intervals. No large-scale thermal migration of bubbles, bubble pushing by the solid-liquid interface, or bubble detachment from the interface were observed during the low gravity experiments. During the SPAR 3 experiment, a unique bubble motion-fluid flow event occurred in one specimen: a large bubble moved downward and caused some circulation of the melt. The gas bubbles that were trapped by the solid in commercial purity material formed voids that had a cyclindrical shape in SPAR 3, in contrast to the spherical shape that had been observed in SPAR 1. These shapes were not influenced by the gravity level, but were dependent upon the initial temperature gradient. In higher purity material the shape of the voids changed from cylindrical in one-g to spherical in low-g.

Papazian, J. M.↗

NASA NTRS1978

The gravity method and interpretive techniques for detecting vertical crustal movements

Observations of temporal variations of gravity can be used as an inexpensive and rapid means of detecting, monitoring, and studying crustal deformation associated with many active geological processes. Such observations can yield qualitative or semiquantitative information on elevation changes and, when combined with independent elevation data, can yield information about changes of the subsurface density field arising from both subsurface displacements and temporal variations of the density of materials in the subsurface. The effectiveness of gravity techniques in specific cases of crustal deformation depends on the configuration of the local gravity field, the physical processes involved in the deformation, and the accuracy with which temporal gravity variations can be measured.

Jachens, R. C.↗

NASA NTRS1978

A comparison of satellite systems for gravity field measurements

A detailed and accurate earth gravity field model is important both to geophysical progress and to the precise tracking necessary for interpretation of geophysical experiments. Various satellite techniques which may be used to determine the earth's gravity field are compared and their ability to recover the long wavelength and short wavelength features of the field are described. A high-low configuration satellite-to-satellite tracking mission is recommended for the determination of the long wavelength portion of the gravity field. Satellite altimetry and satellite gradiometry experiments are recommended for determination of the short wavelength portion of the gravity field.

Argentiero, P.↗

NASA NTRS1979

Combustion of solid carbon rods in zero and normal gravity

In order to investigate the mechanism of carbon combustion, spectroscopic carbon rods were resistance ignited and burned in an oxygen environment in normal and zero gravity. Direct mass spectrometric sampling was used in the normal gravity tests to obtain concentration profiles of CO2, CO, and O2 as a function of distance from the carbon surface. The experimental concentrations were compared to those predicted by a stagnant film model. Zero gravity droptower tests were conducted in order to assess the effect of convection on the normal gravity combustion process. The ratio of flame diameter to rod diameter as a function of time for oxygen pressures of 5, 10, 15, and 20 psia was obtained for three different diameter rods. It was found that this ratio was inversely proportional to both the oxygen pressure and the rod diameter.

Spuckler, C. M.↗

NASA NTRS1979

Combustion of solid carbon rods in zero and normal gravity

In order to investigate the mechanism of carbon combustion, normal and zero gravity experiments were conducted in which spectroscopic carbon rods were resistance ignited and burned in an oxygen environment. Direct mass spectrometric sampling was used in the normal gravity tests to measure gas phase concentrations. The gas sampling probe was positioned near the circumference of the horizontally mounted carbon rods, either at the top or at angles of 45 or 90 deg from the top, and yielded concentration profiles of CO2, CO, and O2 as a function of distance from the carbon surface. The experimental concentrations were compared to those predicted by a stagnant film model. Zero gravity droptower tests were conducted in order to assess the effect of convection on the normal gravity combustion process. The ratio of flame diameter to rod diameter as a function of time for oxygen pressures of 5, 10, 15, and 20 psia was obtained for three different diameter rods. It was found that this ratio was inversely proportional to both the oxygen pressure and the rod diameter.

Spuckler, C. M.↗

NASA NTRS1980

Gravity fields of the terrestrial planets - Long-wavelength anomalies and tectonics

The paper discusses the gravity and topography data available for four terrestrial planets (earth, moon, Mars, and Venus), with particular emphasis on drawing inferences regarding the relationship of long-wavelength anomalies to tectonics. The discussion covers statistical analyses of global planetary gravity fields, relationship of gravity anomalies to elastic and viscoelastic models, relationship of gravity anomalies to convection models, finite strength, and isostasy (or the state of isostatic compensation). The cases of the earth and the moon are discussed in some detail. A summary of comparative planetology is presented.

Phillips, R. J.↗

NASA NTRS1980

Ultraviolet colors of W Ursae Majoris - Gravity darkening, temperature differences, and the cause of W-type light curves

The paper presents photometry of the prototype W UMa binary system in three ultraviolet bands with the ANS satellite. It was found that W UMa has low-gravity darkening beta of 0.03; that temperature differences between the components not established by gravity darkening are insignificant; and that the bolometric albedo is not very large. It was also found that W UMa is limb-darkened in the ultraviolet region, and that the inner hemisphere of the less massive component is hotter than that predicted by gravity darkening and the reflection effect. It was concluded that about 20% of the surface area of the component responsible for large gravity darkening is covered by dark spots distributed uniformly in the longitudinal direction. An observational value of the convective darkening exponent of 0.054 plus or minus 0.02 is proposed.

Eaton, J. A.↗

NASA NTRS1980

Theoretical regime diagrams for thermally driven flows in a beta-plane channel in the presence of variable gravity

The effect of a power law gravity field on baroclinic instability is examined, with a focus on the case of inverse fifth power gravity, since this is the power law produced when terrestrial gravity is simulated in spherical geometry by a dielectric force. Growth rates are obtained of unstable normal modes as a function of parameters of the problem by solving a second order differential equation numerically. It is concluded that over the range of parameter space explored, there is no significant change in the character of theoretical regime diagrams if the vertically averaged gravity is used as parameter.

Geisler, J. E.↗

NASA NTRS1981

Ionospsheric observation of enhanced convection-initiated gravity waves during tornadic storms

Atmospheric gravity waves associated with tornadoes, with locally severe storms occuring with tornadoes, and with hurricanes were studied through the coupling between the ionosphere and the troposphere. Reverse group ray tracing computations of gravity waves observed by an ionospheric Doppler sounder array were analyzed. The results of ray tracing computations and comparisons between the computed location of the wave sources and with conventional meteorological data indicate that the computed sources of the waves were near the touchdown of the tornadoes, near the eye of the hurricanes, and directly on the squall line of the severe thunderstorms. The signals excited occurred one hour in advance of the tornadoes and three hours in advance of the hurricanes. Satellite photographs show convective overshooting turrets occurring at the same locations and times the gravity waves were being excited. It is suggested that gravity wave observations, conventional meteorological data, and satellite photographs be combined to develop a remote sensing technique for detecting severe storms.

Hung, R. J.↗

NASA NTRS1978

Gravity gradient determination with tethered systems

A detailed investigation of the Earth's gravity field is needed for application to modern solid earth and oceanic investigations. The use of gravity gradiometers presents a technique to measure the intermediate wavelength components of the gravity field. One configuration of a gradiometer involves a tethered pair of masses orbiting the Earth and stabilized by vertical gravity gradient of the earth. A mesurement of the tension in such a system, called the DUMBBELL system is described. It allows the determination of the vertical gradient of the anomalous component of the Earth's gravtiy field. Preliminary analysis of the dynamics, mechanization, expected signal levels and noise environment indicates that the Dumbbell system is feasible.

Kalaghan, P. M.↗

NASA NTRS1980

Density response of the mesospheric sodium layer to gravity wave perturbations

Lidar observations of the mesospheric sodium layer often reveal wavelike features moving through the layer. It is often assumed that these features are a layer density response to gravity waves. Chiu and Ching (1978) described the approximate form of the linear response of atmospheric layers to gravity waves. In this paper, their results are used to predict the response of the sodium layer to gravity waves. These simulations are compared with experimental observations and a good correlation is found between the two. Because of the thickness of the sodium layer and the density gradients found in it, a linear model of the layer response is not always adequate to describe gravity wave-sodium layer interactions. Inclusion of nonlinearities in the layer response is briefly discussed. Experimental data is seen to contain features consistent with the predicted nonlinearities.

Shelton, J. D.↗

NASA NTRS1980

Optical measurements and tests performed in a low-gravity environment

Shadowgraph and schlieren techniques have been used on board the NASA KC-135 low-gravity simulation aircraft to measure gravity-related fluid flow occurring in various experimental configurations. These configurations included cooled and solidified ammonium chloride and water, electrodeposited cobalt in CoSO4 solution, and immiscible and crystal melt solutions. Hardware tests have also been performed for such critical optical system components as lasers and spatial filters to determine their performance and operation under low-gravity conditions. These test results will be used in the design of the various future Shuttle experiments which will utilize advanced optical measurement techniques. The design and configuration of the optical systems used to make these measurements and tests are presented, and the performance and operation of these systems and their components under low-gravity conditions are discussed. Some preliminary results are included.

Owen, R. B.↗

NASA NTRS1981

Gas-laser behavior in a low-gravity environment

In connection with several experiments proposed for flight on the Space Shuttle, which involve the use of gas lasers, the behavior of a He-Ne laser in a low-gravity environment has been studied theoretically and experimentally in a series of flight tests using a low-gravity-simulation aircraft. No fluctuation in laser output above the noise level of the meter (1 part in 1000 for 1 hr) was observed during the low-gravity portion of the flight tests. The laser output gradually increased by 1.4% during a 1.5-hr test; at no time were rapid variations observed in the laser output. A maximum laser instability of 1 part in 100 was observed during forty low-gravity parabolic maneuvers. The beam remained Gaussian throughout the tests and no lobe patterns were observed.

Owen, R. B.↗

NASA NTRS1979

Synthesis of regional crust and upper-mantle structure from seismic and gravity data

Available seismic and ground based gravity data are combined to infer the three dimensional crust and upper mantle structure in selected regions. This synthesis and interpretation proceeds from large-scale average models suitable for early comparison with high-altitude satellite potential field data to more detailed delineation of structural boundaries and other variations that may be significant in natural resource assessment. Seismic and ground based gravity data are the primary focal point, but other relevant information (e.g. magnetic field, heat flow, Landsat imagery, geodetic leveling, and natural resources maps) is used to constrain the structure inferred and to assist in defining structural domains and boundaries. The seismic data consists of regional refraction lines, limited reflection coverage, surface wave dispersion, teleseismic P and S wave delay times, anelastic absorption, and regional seismicity patterns. The gravity data base consists of available point gravity determinations for the areas considered.

Alexander, S. S.↗

NASA NTRS1981

Ionospheric remote sensing of medium-scale gravity waves and tornadic storms

Gravity waves associated with severe storms are investigated on the basis of ionospheric sounding using a ground-based Doppler system. Reverse-group ray tracing computations are used to determine the origin of over 20 gravity waves detected within 800 km of Huntsville, Alabama in association with a group of tornadoes, isolated tornadoes in the presence of a squall line, and isolated tornadoes in the absence of a squall line. Gravity waves associated with tornadoes are found either to be generated by thunderstorms with enhanced convection embedded in a squall line, or in an isolated cloud with enhanced convection. The computed wave sources are observed in all cases to be located near the points where the tornadoes touched down more than an hour after wave excitation. Results show that gravity waves play an important role in troposphere-ionosphere coupling during times of intense convection associated with tornadic storm activity.

Hung, R. J.↗

NASA NTRS1981

Superconducting tensor gravity gradiometer

The employment of superconductivity and other material properties at cryogenic temperatures to fabricate sensitive, low-drift, gravity gradiometer is described. The device yields a reduction of noise of four orders of magnitude over room temperature gradiometers, and direct summation and subtraction of signals from accelerometers in varying orientations are possible with superconducting circuitry. Additional circuits permit determination of the linear and angular acceleration vectors independent of the measurement of the gravity gradient tensor. A dewar flask capable of maintaining helium in a liquid state for a year's duration is under development by NASA, and a superconducting tensor gravity gradiometer for the NASA Geodynamics Program is intended for a LEO polar trajectory to measure the harmonic expansion coefficients of the earth's gravity field up to order 300.

Paik, H. J.↗

NASA NTRS1982

Venus - Comparison of gravity and topography in the vicinity of Beta Regio

The Doppler tracking data obtained from the Pioneer Venus Orbiter when it passed near Beta Regio yielded a peak vertical anomaly of 150 mGal when analyzed by our two stage procedure. A comparison of maps of the gravity and topography at comparable resolution shows a striking correlation. A scatter plot shows that the observed gravity anomaly is approximately 0.4 of that expected from uncompensated topography of half the mean density of Venus. However, the spectral admittance shows that the gravity anomalies can not be explained either by Airy compensation at fixed depth or by a model comprising an elastic plate atop an inviscid fluid. The gravity and topography variations may signify deep compensation or dynamic support for Beta Regio and more shallow compensation for other features.

Reasenberg, R. D.↗