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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

On the interpretation of the observed cometary scintillations

A previous interpretation of strong fluctuations in the intensity of a radio source observed during its occultation by the plasma tail and head of Comet Kohoutek a few days after the latter's perihelion passage is examined to explain the consequence of this interpretation that plasma irregularities moved toward the cometary nucleus with speeds of between 66 and 94 km/sec. It is shown that this reversed velocity may be associated with a nonlinear wave coupled to the turbulent plasma and propagating toward the nucleus with a typical Alfven speed. The reported abrupt cessation of the radio intensity fluctuations at a distance of 80,000 km from the nucleus is shown to be due to a rather abrupt change in the plasma density.

Ip, W.-H.↗
NASA NTRS1976

Strong scintillations in astrophysics. 4. Cross-correlation between different frequencies and finite bandwidth effects

The cross correlation of the intensity fluctuations between different frequencies and finite bandwidth effects on the intensity correlations based on the Markov approximation were calculated. Results may be applied to quite general turbulence spectra for an extended turbulent medium. Calculations of the cross-correlation function and of finite bandwidth effects are explicitly carried out for both Gaussian and Kolmogorov turbulence spectra. The increases of the correlation scale of intensity fluctuations are different for these two spectra and the difference can be used to determine whether the interstellar turbulent medium has a Gaussian or a Kolmogorov spectrum.

Lee, L. C.↗
NASA NTRS1975

Strong scintillations in astrophysics. II - A theory of temporal broadening of pulses

A theory of temporal broadening of pulses propagating in a turbulent medium is developed on the basis of the Markov approximation. The theory may be applied to quite general turbulence spectra and to thin or thick turbulent regions. Since the basis of the theory is the wave equation, no reliance is placed on geometrical optics and no assumptions are made about the scattered angular spectrum. The observed smearing is found to be the combination of three effects: the dispersion effect, the pure refractive effect, and the diffraction effect. The last of these dominates for typical pulsar parameters. Pulse shapes are calculated for both Gaussian and Kolmogorov turbulence spectra and it is shown how these scale with the various turbulence parameters.

Lee, L. C.↗
NASA NTRS1975

Phase and amplitude scintillations at 9.6 GHz on an elevated path

Deep fading, e.g., greater than 40 dB, was observed at 9.6 GHz on a 150 km path with terminals at 3025 and 3300 m elevations and with no significant illumination of the earth's surface. Fades at two antennas separated horizontally by 100 m were largely uncorrelated. Phase variability was also measured and is presented in terms of spectra and coherency between antennas with 10 m and 100 m horizontal spacing.

Thompson, M. C.↗
NASA NTRS1976

Strong scintillations in astrophysics. IV - Cross-correlation between different frequencies and finite bandwidth effects

The cross-correlation of the intensity fluctuations between different frequencies and finite-bandwidth effects on the intensity correlations are calculated based on the Markov approximation. The results may be applied to quite general turbulence spectra for an extended turbulent medium. Calculations of the cross-correlation function and of finite-bandwidth effects are explicitly carried out for both Gaussian and Kolmogorov turbulence spectra. The increases of the correlation scale of intensity fluctuations are different for these two spectra, and the difference can be used to determine whether the interstellar turbulent medium has a Gaussian or a Kolmogorov spectrum.

Lee, L. C.↗
NASA NTRS1978

Gain degradation and amplitude scintillation due to tropospheric turbulence

It is shown that a simple physical model is adequate for the prediction of the long term statistics of both the reduced signal levels and increased peak-to-peak fluctuations. The model is based on conventional atmospheric turbulence theory and incorporates both amplitude and angle of arrival fluctuations. This model predicts the average variance of signals observed under clear air conditions at low elevation angles on earth-space paths at 2, 7.3, 20 and 30 GHz. Design curves based on this model for gain degradation, realizable gain, amplitude fluctuation as a function of antenna aperture size, frequency, and either terrestrial path length or earth-space path elevation angle are presented.

Theobold, D. M.↗
NASA NTRS1979

Measurements of turbulence in the Venus atmosphere deduced from Pioneer Venus multiprobe radio scintillations

The 2.3-GHz log-amplitude fluctuations observed in the radio links of the Pioneer Venus entry probes during Venus encounter have been used to study turbulence in the Venus atmosphere. The deduced estimates of the upper bound of the structure constant of the refractive index fluctuations (less than approximately 4 x 10 to the -8th/cu root cm) are inconsistent with similar entry probe measurements by Veneras 4 to 8 but are consistent with the radio occultation measurements by flyby (Mariners 5 and 10) and orbiting (Venera 9) spacecraft. The Pioneer Venus measurements therefore provide a resolution of the long-standing order of magnitude discrepancy between these earlier measurements of the structure constant.

Woo, R.↗
NASA NTRS1980

Radio scintillations during occultations by turbulent planetary atmospheres

The radio occultation experiment which uses the radio link between the earth and spacecraft passing behind planets has proven to be an important method for remote sensing turbulence in planetary atmospheres. The effects of defocusing and anisotropic irregularities on the turbulence-induced fluctuations of the radio occultation signal are examined. Rytov's method along with geometrical optics is employed to study the frequency spectra and coherences of the log amplitude and phase fluctuations of spherical waves operating at one as well as two frequencies. Comparison with the Mariner 5 2.3-GHz measurements shows good agreement with the theoretical results.

Woo, R.↗
NASA NTRS1979

Thin-film scintillators for extended ultraviolet /UV/ response silicon detectors

The preparation and radiometric properties of silicon detectors coated with fluorescent thin films are described. The films are deposited from solutions of clear plastics, such as acrylic resins, polyvinyl toluene or polystyrene, and of organic laser dyes in a common solvent. They are optically clear, mechanically and chemically stable, yet easily applied and removed. Multiple doped films of a few microns thickness exhibit broad-band absorption from less than 250 nm to about 450 nm and narrow band emissions with peaks ranging from 380 nm to 600 nm. Internal quantum efficiencies are close to 100 percent and fluorescence decay times are in the nanosecond range. When deposited on optically denser media, a large fraction of the fluorescent emission is trapped in the substrate. Silicon photodiodes coated with multiple doped films exhibit high external quantum efficiencies and virtually flat photon response in the near UV.

Viehmann, W.↗
NASA NTRS1982

Observations of tropospheric phase scintillations at 5 GHz on vertical paths

The article presents observations of turbulence-induced tropospheric phase fluctuations measured at 5 GHz on the near-vertical paths relevant to many astronomical and geophysical measurements. The data are summarized as phase power spectra, structure functions, and Allan variances. Comparisons to other microwave observations indicate relatively good agreement in both the level and shape of the power spectrum of these tropospheric phase fluctuations. Implications for precision Doppler tracking of spacecraft and geodesy/radio interferometry are discussed.

Armstrong, J. W.↗
NASA NTRS1983

Calibration of a stack of NaI scintillators at the Berkeley Bevalac

An analysis of the carbon and argon data reveals that essentially all of the charge-changing fragmentation reactions within the stack can be identified and removed by imposing the simple criteria relating the observed energy deposition profiles to the expected Bragg curve depositions. It is noted that these criteria are even capable of identifying approximately one-third of the expected neutron-stripping interactions, which in these cases have anomalous deposition profiles. The contribution of mass error from uncertainty in delta E has an upper limit of 0.25 percent for Mn; this produces an associated mass error for the experiment of about 0.14 amu. It is believed that this uncertainty will change little with changing gamma. Residual errors in the mapping produce even smaller mass errors for lighter isotopes, whereas photoelectron fluctuations and delta-ray effects are approximately the same independent of the charge and energy deposition.

Schindler, S. M.↗