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At least 1,225 records · Page 68
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NASA NTRS1984

Uniformly spaced field-aligned ionization ducts

A number of interesting cases of combination mode ducted echoes for mid- and low-latitude regions are presented that show nearly uniformly spaced multiple combination mode traces on the ionograms in the frequency range above 1 MHz. These traces suggest that a parallel system of field-aligned ducts is present. Ray tracing studies are made to determine the structure that would explain the observations using the electron density profile derivable from the vertical trace and assuming field-aligned ducts. Spacing perpendicular to the ducts is found to be as much as 70 km. Some of these parallel duct structures are found to extend to the conjugate hemisphere, possibly to the F peak.

Gross, S. H.↗
NASA NTRS1984

Convective rain rates and their evolution during storms in a semiarid climate

The semiarid climate of the U.S. northern High Plains region has been studied with respect to rain rates and their evolution during summertime convective storms, using radar data from a total of 750 radar echo clusters. Analysis of this data suggests that the average rain rate R among storms is in a first approximation independent of the total rain volume, if the entire storm duration is considered in the averaging process. R primarily depends on the reflectivity threshold considered in calculating the area coverage integrated over the lifetime of the storm. R evolution during storms is analyzed by dividing each storm lifetime into 10 min, 1, 2, and 4 hours, as well as growing and decaying periods. The value of R remained independent of the total rain volume when the growing or decaying periods of storms were considered separately.

Doneaud, A. A.↗
NASA NTRS1984

A radar study of Comet IRAS-Araki-Alcock 1983d

Radar echoes from Comet IRAS-Araki-Alcock at wavelengths 3.54 and 12.9 cm indicate that the comet's nucleus is very rough on a scale larger than the radar wavelengths; however, the low polarization ratio (25 percent at 3.54 cm) indicates that the scattering is not dominated by multiple reflections, internal reflections, or large abundances of sharp edges, cracks, and pits. The shape of the nucleus probably departs greatly from a sphere with average radii near 3-4 km. The nucleus does not appear to look significantly different from a number of Apollo and Amor asteroids except that: (1) there is a suggestion that minor structure moves rapidly across the spectra, and (2) the debris not gravitationally bound to the comet was detected, and contributes 25 percent of the total radar cross section at the 12.9-cm wavelength. Other considerations suggest that the pole was at least 45 deg away from the line of sight on two days of observation, and that the rotation period is approximately 1-2 days.

Goldstein, R. M.↗
NASA NTRS1984

A model for equatorial explosive spread F

Explosive spread F was first described by Woodman and LaHoz (1976). In such events the 50 MHz radar signal rises at Jicamarca to 10-20 dB above the noise level within a few milliseconds but then disappears within 100 ms or less. It has been suggested that electric fields from thunderstorms might drive a plasma instability which could cause these echoes, and recent observations (Woodman and Kudeki, 1984) provide convincing evidence for this triggering. Here, the instability mechanism is discussed and it is shown that transient thunderstorm electric fields comparable to those recently observed by rockets in the midlatitude ionosphere could excite a rapidly growing two-stream plasma instability that is driven by the ExB drift of the F region electrons in the short period (less than the ion gyroperiod) before the ions reach the same velocity.

Kelley, M. C.↗
NASA NTRS1985

Relationship between scattered power and correlation time in VHF radar signals

Equations describing the wave number spectra of wind shear-generated turbulent velocities and refractive index irregularities are discussed, and relations between radar echo power and signal correlation time are derived. If the radar backscatter wavelength is within the inertial subrange of the spectrum, a positive correlation between the scattered power and the signal correlation time is expected. For radar Bragg wavelengths within the dissipative subrange of turbulence, the correlation between scattered power and signal correlation time will be negative as usually expected in turbulence.

Royrvik, O.↗
NASA NTRS1985

The energization of electrons and ions by electron beams injected in the ionosphere

This paper represents a continuation of an investigation of the nature and origin of the particle populations associated with an electron beam-emitting rocket experiment in the ionosphere. In this investigation, the intense flux of suprathermal electrons created around such a rocket has been studied by several groups. Arnoldy and Winckler (1981) have discussed the vehicle floating potential problem. The present paper is concerned with results which have been obtained from the Echo 4 and 5 experiments considered by Swanson (1983). Attention is given to instrumentation, suprathermal electrons, thermal ions, energetic ions, and ion resonances.

Arnoldy, R. L.↗
NASA NTRS1985

Field-aligned electron density irregularities near 500 km Equator to polar cap topside sounder observations

In addition to spread F, evidence for field-aligned electron density irregularities is commonly observed on Alouette 2 topside sounder ionograms recorded near perigee (500 km). This evidence is provided by distinctive signal returns from sounder-generated Z mode waves. At low latitudes these waves become guided in wave ducts caused by field-aligned electron density irregularities and give rise to strong long-duration echoes. At high latitudes, extending well into the polar cap, these Z mode waves (and stimulated electrostatic waves at the plasma frequency) produce a series of vertical bars on the ionogram display as the satellite traverses discrete field-aligned density structures. The radio frequency (RF) noise environment to be expected in the 400 to 500 km altitude region from low to high latitudes was examined by analyzing perigee Alouette 2 topside sounder data. All observed noise bands were scaled on nearly 200 topside sounder ionograms recorded near perigee at low, mid, and high latitude telemetry stations. The minimum and maximum frequencies of each noise band were entered into a data base or computer analysis. The signals of primary interest in the perigee study were found to be sounder-generated.

Benson, R. F.↗
NASA NTRS1985

The rotation period and pole orientation of asteroid 4 Vesta

In 1971 asteroid Vesta was observed in a region of the sky in which it had never been observed before. Its photometric lightcurve had two distinct maxima. Those observations have been the only strong evidence to support a rotation period of about 10 hr 41 min. Lightcurves made in 1982, when Vesta was at the same aspect as 1971, do not show two different maxima. It is concluded that there was a systematic error in the 1971 observations. At this time a definitive statement cannot be made about the true period of Vesta, although the 5 hr 20 min period does appear more plausible. Radar echoes in 1988 and 1992 should resolve the problem. The shorter rotation period was assumed and the photometric astrometry method applied. The sidereal period is 5 hr 20 min 31.68 sec (0.2225889 + or 0.0000002 days), the rotation is prograde, and the coordinates of the north pole are 103 deg longitude and + 43 deg latitude with an uncertainty of about 6 deg.

Taylor, R. C.↗
NASA NTRS1985

The role of the reflection coefficient in precision measurement of ultrasonic attenuation

Ultrasonic attenuation measurements using contact, pulse-echo techniques are sensitive to surface roughness and couplant thickness variations. This can reduce considerable inaccuracies in the measurement of the attenuation coefficient for broadband pulses. Inaccuracies arise from variations in the reflection coefficient at the buffer-couplant-sample interface. The reflection coefficient is examined as a function of the surface roughness and corresponding couplant thickness variations. Interrelations with ultrasonic frequency are illustrated. Reliable attenuation measurements are obtained only when the frequency dependence of the reflection coefficient is incorporated in signal analysis. Data are given for nickel 200 samples and a silicon nitride ceramic bar having surface roughness variations in the 0.3 to 3.0 microns range for signal bandwidths in the 50 to 100 MHz range.

Generazio, E. R.↗
NASA NTRS1985

Observations on the propagation, growth, and predictability of Gulf Stream meanders

During a three-year period, Gulf Stream positions determined by satellite infrared imagery on a grid northeast of Cape Hatteras have been correlated against 'inlet' path parameters (displacement from the mean position, angle, and curvature) monitored by an array of inverted echo sounders. By cross-spectral calculations between these measurements, the downstream coherences, phase speeds, and spatial growth rates were determined. The downstream path predictability from these inlet parameters was tested in a multiple input linear response model. The most energetic meanders, with periods of 33-50 days, remained highly coherent for 300 km downstream, roughly the dominant wavelength, with marginal coherence again at 500-575 km. Phase speeds are frequency dependent and decrease with distance downstream. For the 50- (33) day meanders, phase speeds decreased from 10 (17) km/d at the inlet to 6 (8) km/d at a distance downstream of about 550 km. The average growth rate (spatial e-folding wave number) over the entire region is 1.3 x 10 to the -3rd per km. Within 300 km of the inlet, the multiple coherence accounts for 55-65 percent of the total path-displacement variance, with displacement and angle being the best individual predictors in the first and last halves of this region, respectively.

Vazquez, J.↗
NASA NTRS1985

Performance comparisons between information extraction techniques using variable spatial resolution data

The decreased instantaneous field of view (IFOV) is one of the principal advances noted for the Thematic Mapper (TM) sensor. The 42.5 microradian IFOV of TM and the 710 km nominal orbit altitude result in a 30 m nominal spatial resolution at the earth surface. This is a considerable decrease in the projected pixel area when compared to the 79 m nominal spatial resolution of the Landsat Multispectral Scanner (MSS). An experiment was conducted which allowed a rigorous test of the influence of classifier design, with data spatial resolution of TM (30 m) and approximately that of the Landsat MSS (90 m), on classification performance for a particular TM scene. The experiment involved evaluation of the results for the per-point Gaussian maximum likelihood (GML) classifier and the supervised ECHO (Extraction and Classification of Homogeneous Objects) classifier.

Latty, R. S.↗
NASA NTRS1986

Two-Element Transducer for Ultrasound

Separation of transmitting and receiving units improves probing of deep tissue. Ultrasonic transducer has dual elements to increase depth at which sonic images are made of biological tissue. Transducer uses separate transmitting and receiving elements, and frequency response of receiving element independently designed to accommodate attenuation of higher frequencies by tissue. New transducer intended for pulse-echo ultrasonic systems in which reflected sound pulses reveal features in tissue.

Lecroissette, D. H.↗
NASA NTRS1986

Ultrasonic Inspection Near Small Bores

Portable ultrasonic probe makes it possible to inspect for hidden cracks near insides of narrow tubes. Using pulse-echo technique, instrument detects cracks as small as 0.015-in. (0.38-mm) deep. Used for nondestructive inspection of other hard-to-reach places where conventional large transducers will not fit or where difficult to apply coupling liquid for contact ultrasonic testing. Inspects bore of tubelike fitting. Instrument makes it unnecessary to disassemble fitting to check for cracks. Precise orientation of transducer with respect to part not necessary for detecting cracks.

Parent, R. G.↗
NASA NTRS1985

Aerial laser sensing of ocean upper layer

Applications of laser sensing of the ocean, such as deep bathymetry; determination of the luminescence spectrum of phytoplankton as a sensitive indicator of changes in the external physical parameters of the studied region; monitoring the state of underwater pipelines; conducting search and rescue missions; monitoring pollution; biological observations of the state of algae; searching for schools of fish, etc., are discussed. The Chayka apparatus for laser sensing is discussed. A block diagram is given which is used in describing functioning of this unit. Particular attention is given to the time structure of an echo signal appearing when sensing the upper ocean layer by a short laser pulse propagating through the wave-covered surface.

Vlasov, D. V.↗
NASA NTRS1985

Satellite microwave observations of a storm complex: A comparative analysis

The hypothesis that cold events correspond to a particular stage in a class of thunderstorms was tested. That class is a storms class which updrafts are: (1) strong, broad and moist, and (2) extend well above the freezing level. Condition (1) implies strong mesoscale forcing. Condition (2) implies a tall updraft or a relatively low freezing level. Such storms should have big, intense radar echoes and cold, fast-growing anvils. The thunderstorm events were analyzed by radar, rain gauge and GOES infrared observations. Radar was the starting point for detection and definition of the hypothesized thunderstorms. The radar signature is compared to the signature of the storm in rain gauge observations, satellite infrared images and satellite microwave images.

Martin, D. W.↗
NASA NTRS1985

Examination of nonadiabatic effects during an electron beam experiment in the ionosphere

Prompt and delayed echo fluxes and superacceleration of electrons were observed during the sounding rocket flight Several Coordinated Experiments using an electron accelerator. Magnetospheric electric fields parallel to the magnetic field were investigated. The degree of nonadiabatic behavior of the injected electron beam and the return flux was examined. It is shown that the region of strong beam-plasma interaction is limited to 300 km in length along the magnetic field but that backscatter or energetic electrons could occur from much larger distances. By appraising the difference in transit times of 1.9 and 3.9 keV electrons it is established that a continuous backscatter model operating out to several thousand kilometres is not consistent with the observations.

Wilhelm, K.↗
NASA NTRS1986

Ultrasonic verification of microstructural changes due to heat treatment

Ultrasonic attenuation was measured for polycrystalline samples of nickel and copper with various grain-size distributions produced by heat treatment. Attenuation as a function of frequency was determined for a sample having a known mean grain diameter. Once this function was determined, it could be scaled to determine the mean grain size of other samples of the same material with different mean grain diameters. These results were obtained by using broadband pulse-echo ultrasound in the 25 to 100 MHz frequency range. The results suggest an ultrasonic, nondestructive approach for verifying heat treatment of metals.

Generazio, E. R.↗
NASA NTRS1986

The effect of breaking gravity waves on the dynamics and chemistry of the mesosphere and lower thermosphere (invited review)

The influence of breaking gravity waves on the dynamics and chemical composition of the 60 to 110 km region is investigated with a two dimensional model that includes a parameterization of gravity wave momentum deposition and diffusion. The dynamical model is described by Garcia and Solomon (1983) and Solomon and Garcia (1983) and includes a complete chemical scheme for the mesosphere and lower thermosphere. The parameterization of Lindzen (1981) is used to calculate the momentum deposited and the turbulent diffusion produced by the gravity waves. It is found that wave momentum deposition drives a very vigorous mean meridional circulation, produces a very cold summer mesopause and reverse the zonal wind jets above about 85 km. The seasonal variation of the turbulent diffusion coefficient is consistent with the behavior of mesospheric turbulences inferred from MST radar echoes. The large degree of consistency between model results and various types of dynamical and chemical data supports very strongly the hypothesis that breaking gravity waves play a major role in determining the zonally-averaged dynamical and chemical structure of the 60 to 110 km region of the atmosphere.

Garcia, R. R.↗