Showing papers in "Astronomy and Astrophysics in 1981"

On the definition of albedo and application to irregular particles

Abstract: The various definitions of albedo used in planetary astronomy are reviewed. In particular, the Bond albedo, which refers only to the reflected and refracted components, is not applicable to small particles or highly irregular particles, where diffraction is not restricted to a well-defined lobe at small scattering angles. Measured scattering functions for irregular particles are presented in a normalized form and are applied to the case of zodiacal light.

The forbidden oxygen lines in comets

Abstract: High spatial resolution observations of comet Bradfield (1979X) with the IUE spectrograph have made it possible to tentatively identify a feature at 2972 A as the 1S-3P forbidden transition of oxygen atoms. It is shown here that the most likely source of this emission is the photodissociation of water molecules, mainly by solar Lyman-alpha photons, but that a significant contribution of CO2 cannot be excluded.

Solar wind motion within 30 R solar masses - Spacecraft radio scintillation observations

Abstract: New remote-sensing observations are reported of the solar wind motion within about 30 earth radii. Use is made of the interplanetary scintillation (IPS) spaced receiver technique with the radio source being a spacecraft signal (rather than a natural radio source as in previous spaced receiver studies). The spacecraft used are Helios A and B and the Viking orbiters. The purposes of the study are (1) to augment the scarce estimates of solar wind bulk flow speed near the sun and in the ecliptic with measurements made using spacecraft signals, and (2) to estimate random velocity components and identify the region where the random velocity is a significant fraction of the mean velocity. In addition, the radial evolution of speed and random velocity is compared with that of the plasma density fluctuation spectrum. Also reported are the first accurately normalized IPS scintillation index measurements using a monochromatic point source.