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At least 397 records · Page 22
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NASA NTRS1973

Lunar Surface Gravimeter Experiment

The lunar surface gravimeter which was emplaced on the moon by the Apollo 17 flight is described and a schematic diagram of the sensor is provided. The objective of the lunar surface gravimeter is to use the moon as an instrumented antenna to detect gravitational waves. Another objective is to measure tidal deformation of the moon. Samples of signals received during lunar sunrise activity and during quiet periods are presented in graph form based on power spectrum analysis

Giganti, J. J.↗
NASA NTRS1973

Investigation of the Jet Noise Prediction Theory and Application Utilizing the PAO Formulation

Application of the Phillips theory to engineering calculations of rocket and high speed jet noise radiation is reported. Presented are a detailed derivation of the theory, the composition of the numerical scheme, and discussions of the practical problems arising in the application of the present noise prediction method. The present method still contains some empirical elements, yet it provides a unified approach in the prediction of sound power, spectrum, and directivity.

Source record↗
NASA NTRS1973

Statistical analysis of catalogs of extragalactic objects. II - The Abell catalog of rich clusters

The results of a power-spectrum analysis are presented for the distribution of clusters in the Abell catalog. Clear and direct evidence is found for superclusters with small angular scale, in agreement with the recent study of Bogart and Wagoner (1973). It is also found that the degree and angular scale of the apparent superclustering varies with distance in the manner expected if the clustering is intrinsic to the spatial distribution rather than a consequence of patchy local obscuration.

Hauser, M. G.↗
NASA NTRS1973

The distribution of redshifts of radio galaxies with different optical spectra and forms

The distribution of redshifts of 222 radio galaxies has been subjected to a power-spectrum analysis. No statistically significant spectral peaks are present for galaxies showing normal absorption-line characteristics; on the other hand, significant spectral peaks are present for those exhibiting strong emission features. Of these spectral peaks, the most significant, at a wavelength of 0.031 in redshift, occurs for those strong-emission radio galaxies which are also compact. The result is consistent with the proposal suggested earlier by Burbidge that the nonrandom features, which may suggest intrinsic redshifts, are confined to those objects - QSOs, N systems, etc. - in which radiation from stars does not dominate. Possible explanations of the apparently nonrandom features are briefly discussed. Finally, a table of the most recently published small-redshift QSOs and compact emission-line galaxies is presented. The redshifts appear to conform to the 0.061 (or 0.031) periodicity found earlier.

Burbidge, G. R.↗
NASA NTRS1973

Remote optical measurement of wind speed spectra

The paper presents some new analytical and experimental results on the cross-correlation function and power spectrum for the flow speed by the passive optical crossed-beam correlation technique. Cross-correlation functions of crossed-beam measurements for both intersecting beams and separated beams have been reviewed for the case of varying wind speeds. The corresponding spectral density functions were derived and expressed in terms of the probability density of wind speed variations and the correlation of atmospheric turbulent eddies. The associated spectral density for a special approximate model was derived by assuming a Gaussian distribution of transit times. The theoretical predictions were then compared with actual field test data, and a reasonably good agreement in the valid frequency range was observed.

Su, M.-Y.↗
NASA NTRS1974

Ionospheric irregularities causing scintillation of GHz frequency radio signals

Consideration of the recently observed phenomenon of scintillation of satellite signals at GHz frequency range. Based on the scintillation data and results from in situ measurements, several ionospheric irregularity models with different power spectra are studied. Scintillation index is computed for the various models and compared with observed results. Both magnitude and frequency dependence of the scintillation index are investigated. It is found that a thick irregularity slab of the order of 200 km with an electron density fluctuation of about 20 per cent of its background value and with a nonmonotonic power spectrum may account for the maximum observed values of the scintillation index as well as its frequency dependence. Some future observations and measurements are suggested.

Wernik, A. W.↗
NASA NTRS1974

Hard solar flare X-ray bursts on 8 December 1970

Hard X-ray bursts have been observed from two 1B flares located in the same sunspot region and separated in time by about 70 minutes on Dec. 8, 1970. The bursts are composed by many 'flashes' of 2 to 20 seconds duration. Power spectrum analysis reveals no strong periodicities although a significant peak appears at 7.5 sec. The characteristic times of 2 to 20 sec do not seem likely to be electron transit times since the flare size is so small. These times are evidently connected with the acceleration of the electrons. Fitting Takakura's model of the radio noise region to the observations, this region is found to be small, but the density of electrons above 100 keV is high, on the order of 10 million per cu cm.

Anderson, K. A.↗
NASA NTRS1974

Rigid body mode pointing accuracy and stability criteria for an orbiting spacecraft

The effect of gravity gradient torque disturbances (which is an order of magnitude greater than the other torque disturbances and is sinusoidal with a frequency of two cycles per orbit) on pointing accuracy can be predicted very precisely. However, since the sensor noise such as star tracker and gyro inherent noise, is random, their effect can only be predicted statistically. The most common method used is computer simulation with appropriate sensor model and adjusting spacecraft control laws for minimum pointing error, as presented by Harris (1972). The general investigation outlined theoretically relates spacecraft controls to pointing accuracy in terms of the sensor noise power spectrum. The results are presented in graphical form.

Sandhu, G. S.↗
NASA NTRS1974

Statistical analysis of catalogs of extragalactic objects. III - The Shane-Wirtanen and Zwicky catalogs

Statistical investigation of clustering in the galaxy catalogs compiled by Zwicki et al. (1961-1968) and Shane and Wirtanen (1967). The covariance and power spectrum of the angular distributions are estimated. The space region surveyed in the Zwicki catalog is found to represent a 'fair sample of the universe' in that the statistical nature of the clustering in this catalog is found to be mirrored at smaller angular scales in the deeper survey of Shane and Wirtanen.

Peebles, P. J. E.↗
NASA NTRS1974

Rise time in 20-32 keV impulsive X-radiation

A new property of the X-ray impulsive component observed in solar flares is discussed, giving attention to the relation between the slope of the electron power spectrum and the rise time in the 20-32 keV X-ray spike. This particular energy range was chosen because it offered the greatest number of impulsive events while being sufficiently high to avoid contamination by soft X radiation. It is found for the thin-target model that the electron spectrum tends to be softer when the acceleration rate is smaller.

Vorpahl, J. A.↗
NASA NTRS1974

Bursts of relativistic electrons from Jupiter observed in interplanetary space with the time variation of the planetary rotation period

Bursts of Jovian electrons in the energy range 3 to 30 Mev have been detected to distances of the order of 1 AU from the planet. The duration of each burst was about two to three days, and the maximum intensity increased with increasing distance from the sun. These events could not be correlated with known solar activity. The conclusion that these electrons originated at Jupiter and were accelerated within the Jovian magnetosphere is based on observations of three of these bursts. The power spectrum of the temporal variations of the 6- to 30-MeV electron flux showed a persistent peak at Jupiter's rotation frequency (corresponding to the ten hour rotation period) out to distances greater than 100,000,000 km from the planet. The slope, or spectral index, of the differential energy spectrum measured as a function of time displayed the ten-hour cyclic variation in the value of the index, which was earlier found to be characteristic of the electrons within the Jovian magnetosphere.

Chenette, D. L.↗
NASA NTRS1975

Broadband noise generated by turbulent inflow to rotor or stator blades in an annular duct

The Green's function relating the radiated pressure field to the fluctuating forces on rotor or stator blades is developed in the presence of dissipation due to turbulent velocity fluctuations and sound speed fluctuations. The resonances in the output power spectrum which would occur at the cut-off frequencies in the absence of dissipation should be removed and smeared out by the incorporation of dissipation. Wave number dependence is developed for an effective eddy viscosity due to the aforementioned fluctuations in the background medium. The space-time correlation function for blade-normal velocity fluctuations on a single or on two different blades is developed in terms of the velocity correlation tensor for the inflow under the assumptions of isotropy and (Taylor) frozen behavior. The correlation function is then simplified under certain approximations and the behavior of the blade-force correlation function is inferred.

Lnae, F.↗
NASA NTRS1975

Effort of spanwise variation of turbulence on the normal acceleration of airplanes with small span relative to turbulence scale

A rigid airplane with an unswept wing is analyzed. The results show that the power spectrum, relative to that for a one-dimensional turbulence field, is significantly attenuated at the higher frequencies even for airplanes with arbitrarily small ratios of span to scale of turbulence. This attenuation is described by a simple weighting function of frequency that depends only on aspect ratio. The weighting function, together with the attenuation due to the unsteady flow of gust penetration, allows the determination of the average rate of zero crossings for airplanes having very small spans without recourse to an integral truncation which is often required in calculations based on a one-dimensional turbulence field.

Pratt, K. G.↗
NASA NTRS1975

Modulation of low-energy cosmic rays

The relation between the diffusion coefficient of cosmic rays in the solar wind and the power spectrum of interplanetary magnetic field fluctuations, established in recent theories, is tested directly for low energy protons (below 80 MeV). In addition, an attempt is made to determine whether the particles are scattered by magnetic field discontinuities or by fluctuations between discontinuities. Predictions of a perturbation solution of the Fokker-Planck equation are compared with observations of the cosmic ray radial gradient. It is found that at energies between 40 and 80 MeV, galactic cosmic ray protons respond to changes in the predicted diffusion coefficients (i.e., the relationship under consideration holds at these low energies). The relation between changes in the proton flux and modulation parameters is best when the contribution of discontinuities is subtracted, which means that scattering is caused by fluctuations between discontinuities. There appears to be no distinct relation between changes in the modulation parameters and changes in the intensity of 20 to 40 MeV protons.

Sari, J. W.↗
NASA NTRS1975

Low-frequency fluctuations in the solar wind. I - Theory

Several simple relationships between the power spectra of density and velocity fluctuations and the power spectrum of magnetic field fluctuations are derived within the context of plasma kinetic theory. The theory is restricted to the low-frequency regime (less than the proton cyclotron frequency) where hydromagnetic turbulence is expected to play the most important role. The affects of Alfven and magnetosonic waves upon the plasma fluctuations are discussed separately. The results are then applied to proton fluctuations in the solar wind, demonstrating a connection between plasma and field fluctuations.

Wu, C. S.↗
NASA NTRS1975

H alpha oscillations in sunspot umbrae

Results are reported for direct measurement of intensity fluctuations and photographic subtractions of simultaneous Doppler filtergrams of H-alpha oscillations in sunspot umbrae. The observational techniques are described, and data are presented on the intensity variations, power spectra, and specific characteristics of the oscillations. Periods for the velocity oscillations are obtained by power spectrum analysis. It is found that the oscillations are apparently continuous, and that each umbra has its own characteristic period ranging from 145 to 180 plus or minus 10 sec and a duration of at least 8 cycles. A Doppler shift of 10 km/sec, an oscillating element size of 3 sec or less, and visibility to a radius vector of about 0.9 are also observed.

Phillis, G. L.↗
NASA NTRS1975

Hydromagnetic waves and cosmic-ray diffusion theory

Pitch-angle (and energy) diffusion of cosmic rays in hydromagnetic wave fields is considered. The treatment remains strictly within the quasi-linear approximation. It is shown that the popular assumption of an isotropic power spectrum tensor of magnetic fluctuations requires in this case equal forms and magnitudes of Alfven and magnetosonic wave spectra - a situation which is generally unlikely. The relative contributions to the pitch-angle diffusion coefficient from the cyclotron resonances and Landau resonance due to the different types of waves are evaluated for a typical situation in the solar wind. Since the Landau resonance in this approximation also does not lead to particle reflections, a proper consideration of the nonlinear particle orbits is indeed necessary to overcome the well-known difficulties of quasi-linear scattering theory for cosmic rays near 90 deg pitch angle.

Lee, M. A.↗
NASA NTRS1975

Space power application of the all purpose mini-Brayton rotating unit /mini-BRU/

The design of and results of tests on the mini-Brayton rotating unit (mini-BRU) are presented. Results demonstrate the flat performance trends of the mini-BRU system. The system is 'all-purpose'; it is essentially a closed Brayton cycle engine. A power spectrum comprised between 2.1 and 1.4 kWe and recuperator sizes of 106 or 75 lbs are envisioned for the final in-use configuration.

Gable, R. D.↗