Engineering Papers⌕ Search

Engineering topics

Bracewell, R. N.

Publications and source records attributed to Bracewell, R. N..

10 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

The solar cycle - A central-source wave theory

Studies stimulated by the interpretation of the Elatina formation in South Australia as a fossil record of solar activity have led to discoveries of previously unnoticed features of the sunspot cycle record and to a theory of origin of the sunspot cycle that postulates a solar core in torsional motion and a magnetomechanical wave that couples to the photosphere. The considerations supporting the solar interpretation of the Elatina formation are gathered together.

Bracewell, R. N.↗
NASA NTRS1988

Varves and solar physics

The relationship between varves (visibly striated rocks deposited by glacial run-off) and solar radiation is discussed, proposing that the thickness of any given varve represents the mean annual sunspot number for the year in which the varve was laid down. The solar cycle may entail a deep torsional oscillator whose influence reaches the surface by wave propagation. Several observed features suggest a wave link between the source of solar activity and its surface manifestations. It is suggested that the cause of varve modulation is solar UV and X-ray radiation, which exerts solar cycle influence in the ozonosphere and ionosphere. The undulations and cycles of the Elatina varve bed formation in South Australia are related to solar phenomena. The varve mechanism is significant for the build-up of the protective shield of biogenic oxygen in the palaeoatmosphere.

Bracewell, R. N.↗
NASA NTRS1988

Three-halves law in sunspot cycle shape

Evidence is provided of a nonlinear effect consisting of a 3/2 power law in annual mean sunspot numbers since 1700. With correction for this nonlinearity, it has been possible to reproduce theoretically the well-known Waldmeier relation between rise time and maximum sunspot number, in addition to other features of the 11 yr sunspot curve such as a break in the decline of strong 11 yr cycles. Eighteenth century sunspot numbers are found to fit theoretical semicycle shapes well. The present model suggests an upward travelling 22 yr wave launched by a reciprocating magnetic oscillator deep in the sun.

Bracewell, R. N.↗
NASA NTRS1988

Spectral analysis of the Elatina varve series

The Elatina formation in South America, which provides a rich fossil record of presumptive solar activity in the late Precambrian, is of great potential significance for the physics of the sun because it contains luminae grouped in cycles of about 12, an appearance suggestive of the solar cycle. Here, the laminae are treated as varves laid down yearly and modulated in thickness in accordance with the late Precambrian sunspot activity for the year of deposition. The purpose is to present a simple structure, or intrinsic spectrum, that will be uncovered by appropriate data analysis.

Bracewell, R. N.↗
NASA NTRS1988

Spectral analysis of the Elatina series

The Elatina formation in South Australia, which provides a rich fossil record of presumptive solar activity in the late Precambrian, is of great potential significance for the physics of the sun because it contains luminae grouped in cycles of about 12, an appearance suggestive of the solar cycle. Here, the laminae are treated as varves laid down yearly and modulated in thickness in accordance with the late Precambrian sunspot activity for the year of deposition. The purpose is to present a simple structure, or intrinsic spectrum, that will be uncovered by appropriate data analysis.

Bracewell, R. N.↗
NASA NTRS1987

Observing rotation and deformation of sea ice with synthetic aperture radar

The ESA's ERS-1 satellite will carry SARs over the polar regions; an important component in the use of these data is an automated scheme for the extraction of sea ice velocity fields from a sequence of SAR images of the same geographical region. The image pyramid area-correlation hierarchical method is noted to be vulnerable to uncertainties for sea ice rotations greater than 10-15 deg between SAR observations. Rotation-invariant methods can successfully track isolated floes in the marginal ice zone. Hu's (1962) invariant moments are also worth considering as a possible basis for rotation-invariant tracking methods. Feature tracking is inherently robust for tracking rotating sea ice, but is limited when features are floe-lead boundaries. A variety of techniques appears neccessary.

Vesecky, J. F.↗
NASA NTRS1986

Automated remote sensing of sea ice using synthetic aperture radar

Two techniques for automated sea ice tracking: image pyramids (hierarchical correlation) and feature tracking were applied to a pair of SEASAT SAR sea ice images. The results compare well with each other and manually tracked estimates of the ice velocity field. Refinement of the image pyramid technique should include removal of a small number of erroneous velocity vectors using constraints dependent on correlation coefficient, near neighbor velocities, and velocity gradients. Estimates of sea ice velocity are successfully obtained over limited areas using the boundary segment tracking technique. Refinement of the feature tracking technique should include use of pressure ridge features, constrained search, and relaxation techniques.

Vesecky, J. F.↗
NASA NTRS1979

Searching for nonsolar planets

The use of infrared techniques to search for nonsolar planets is examined and compared with other possible methods. Long focus astrometry, spectroscopic radial velocity measurements and spaceborne apodization all use visible light and need further refinement to be practical. Infrared offers an advantage of about 10 to the 5th over visible light as regards the ratio of power received from star and planet. Long baseline infrared interferometry from earth orbit could place an interferometric null on the star to enhance planetary radiation and a spinning interferometer would modulate planetary emission to permit synchronous detection; such an interferometer is illustrated. The limit to sensitivity would be set by thermal radiation to the detector and the infrared component of zodiacal light.

Bracewell, R. N.↗
NASA NTRS1979

An orbiting infrared interferometer to search for nonsolar planets

This paper proposes an orbiting infrared interferometer with its fringe null centered on a nearby star at a distance of say 10 parsecs. A large planet ('Jupiter') would have an angular separation from the star of about 0.5 arcsec. To have a fringe crest on the planet, a fringe period of 1.0 arcsec is needed and at 40 microns the required baseline is 8 m. Spinning the interferometer about the line of sight to the star results, even with pointing errors, in a relatively slowly varying but strongly suppressed stellar output and a more rapidly varying fringe-like planetary signal rich in higher harmonics. For pointing errors up to .050 arcsec the planet's fourth harmonic greatly exceeds that from the star, thereby relaxing interferometer pointing tolerances. Indeed, it appears that the limiting factor is zodiacal infrared background radiation and not the intense localized stellar flux which can effectively be eliminated by the fringe null of the spinning infrared interferometer.

Macphie, R. H.↗