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Callahan, P. S.

Publications and source records attributed to Callahan, P. S..

29 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

On the accretion disk model of QSOs

The accretion-disk model for the optical continuum of QSOs has been investigated using the standard viscous-stress ('alpha-P') formalism. It is found that with a maximum allowable mass of 10 billion solar masses, the standard model with sub-Eddington flow falls a factor of 10 short of the luminosity of the brightest QSOs. Models with the higher mass-flow rates required to reach the maximum luminosities produce most of their energy in an optically thin region which has not yet been self-consistently treated. The disk model with sub-Eddington flow has difficulty in producing a power law of the correct slope over the optical band. It is suggested that nonthermal processes may be needed to explain the optical properties of QSOs.

Callahan, P. S.↗
NASA NTRS1977

A preliminary analysis of Viking S-X Doppler data and comparison to results of Mariner 6, 7, and 9 DRVID measurements of the solar wind turbulence

S-X Doppler data obtained during more than 135 Viking passes from Aug. 3 to Dec. 15, 1976 were analyzed and compared with data from previous investigations using Mariner differenced range versus integrated Doppler to determine changes in the turbulence over the sunspot cycle. A comparison of the data indicates that: (1) electron density fluctuations decline with heliocentric distance; (2) the level of turbulence may be a factor of two lower near sunspot minimum than at maximum; and (3) the spectrum of the fluctuations may be steeper near sunspot minimum. The expected range error for various time scales and geometries is derived from the results.

Callahan, P. S.↗
NASA NTRS1975

Electromagnetic wave energy conversion research

Known electromagnetic wave absorbing structures found in nature were first studied for clues of how one might later design large area man-made radiant-electric converters. This led to the study of the electro-optics of insect dielectric antennae. Insights were achieved into how these antennae probably operate in the infrared 7-14um range. EWEC theoretical models and relevant cases were concisely formulated and justified for metal and dielectric absorber materials. Finding the electromagnetic field solutions to these models is a problem not yet solved. A rough estimate of losses in metal, solid dielectric, and hollow dielectric waveguides indicates future radiant-electric EWEC research should aim toward dielectric materials for maximum conversion efficiency. It was also found that the absorber bandwidth is a theoretical limitation on radiant-electric conversion efficiency. Ideally, the absorbers' wavelength would be centered on the irradiating spectrum and have the same bandwith as the irradiating wave. The EWEC concept appears to have a valid scientific basis, but considerable more research is needed before it is thoroughly understood, especially for the complex randomly polarized, wide band, phase incoherent spectrum of the sun. Specific recommended research areas are identified.

Bailey, R. L.↗
NASA NTRS1975

LIBRA: An inexpensive geodetic network densification system

A description is given of the Libra (Locations Interposed by Ranging Aircraft) system, by which geodesy and earth strain measurements can be performed rapidly and inexpensively to several hundred auxiliary points with respect to a few fundamental control points established by any other technique, such as radio interferometry or satellite ranging. This low-cost means of extending the accuracy of space age geodesy to local surveys provides speed and spatial resolution useful, for example, for earthquake hazards estimation. Libra may be combined with an existing system, Aries (Astronomical Radio Interferometric Earth Surveying) to provide a balanced system adequate to meet the geophysical needs, and applicable to conventional surveying. The basic hardware design was outlined and specifications were defined. Then need for network densification was described. The following activities required to implement the proposed Libra system are also described: hardware development, data reduction, tropospheric calibrations, schedule of development and estimated costs.

Fliegel, H. F.↗
NASA NTRS1975

Columnar content measurements of the solar-wind turbulence near the sun

Turbulence in the solar wind near the sun is investigated using records of changes in the integrated electron columnar content along the line of sight to Mariners 6, 7, and 9. The comoving turbulence spectrum is found to be well represented by a power law with an index of about 3.9. The turbulence is shown to decline with heliocentric distance, and the data suggest a region of enhanced turbulence near the sun. The Mariner-9 spectral amplitudes are found to correspond with Zurich sunspot numbers. Implications of the present observations are discussed for contemporary theories on solar-wind heating.

Callahan, P. S.↗
NASA NTRS1974

Interpretation of columnar content measurements of the solar-wind turbulence

The temporal spectrum of measurements of the integrated electron columnar content between the earth and a distant source are considered. The relation between this spectrum and the three-dimensional wavenumber spectrum of the solar-wind density is derived under the assumption that the correlation scale is the smallest scale of interest. The specific case of round-trip measurements to a spacecraft is investigated for a power-law solar-wind density spectrum. The resulting spectrum has a power-law envelope one power less steep than the solar-wind density spectrum with several deep minima superposed.

Callahan, P. S.↗
NASA NTRS1973

Plasma column changes at small solar elongations

The differenced range versus integrated Doppler (DRVID) technique was used to study charged particle changes in the ray path between earth and Mariner 9. For plasma activity near the sun, DRVID data were obtained from August 10 to October 24, 1972, surrounding the Mariner 9 superior conjunction on September 7. If the records are viewed in terms of range change or range change rate, the day-to-day fluctuations in these quantities mask the changes expected due to the varying solar elongation. Thus, while the steady-state total electron content varies by a factor of 4, the columnar content changes show no systematic variations.

Callahan, P. S.↗
NASA NTRS1973

Plasma column changes at small solar elongations.

By using the difference between phase and group velocities in a plasma, the changes in the columnar content between earth and Mariner 9 have been measured for elongations of 3 to 16 deg. Rates of change were observed up to 2.5 x 10 to the minus 15th electrons per sq m per sec. Structures of 3,000,000 km in size could be inferred.

Callahan, P. S.↗
NASA NTRS1971

Probing the solar plasma with Mariner radio tracking data

The range and Doppler radio tracking close to the sun made it possible to measure solar plasma dynamics. These were measured by means of a method known as differenced range versus integrated Doppler, which exploits the opposite change of group and phase velocity as the plasma density changes along the radio raypath. A simple solar plasma propagation model is proposed.

Macdoran, P. F.↗