Absorption-Line Formation in a Scattering Planetary Atmosphere: a Test of Van de Hulst's Similarity Relations

Abstract

view Abstract Citations (45) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Absorption-Line Formation in a Scattering Planetary Atmosphere: a Test of Van de Hulst's Similarity Relations Hansen, James E. Abstract Van de Huist's similarity relations, which reduce the problem of anisotropic scattering in a homoge- neous atmosphere to one of isotropic scattering by scaling the optical thickness and the single-scattering albedo, are tested for line formation in clouds and hazes. The relations are shown to give good approxi- mations for a useful range of scattering angles when k (the first characteristic exponent occurring in the solution of the transfer equation in unbounded media) is the basis for the scaling relations. Moreover, except for the center of strong lines, the results are nearly as accurate if the scaling factor were simply (1 - (cos 0)), where (cos 0) is the asymmetry factor of the phase function; this indicates that the mean free path of a photon in a planetary atmosphere is less by the factor (1 - (cos 0)) than the value determined from synthetic spectra under the assumption of isotropic scattering. The above results indicate that the density of cloud particles on Venus is about 6 times greater than the value suggested by the synthetic-spectra calculations of Belton, Hunten, and Goody for isotropic scattering, if it is assumed only that the cloud particles are at least `-~.`1 p in radius. This implies that the density of cloud particles on Venus is comparable with that of cirrus clouds on Earth, a conclusion in agreement with a recent conclusion of Potter. The relation of the computations to Belton's theory for the curve of growth and for the phase effects in a cloudy planetary atmosphere is indicated; for scattering angles at which the similarity relations are valid, anisotropic scattering may be accounted for by simply scaling the single-scattering albedo ob- tained from the curve-of-growth analysis with isotropic scattering. However, the variation of equivalent width with phase angle is increased significantly by anisotropic scattering, and, within observational uncertainties, the use of realistic phase functions provides a probable explanation for a discrepancy be- tween the theoretical and the observed phase effects Publication: The Astrophysical Journal Pub Date: October 1969 DOI: 10.1086/150196 Bibcode: 1969ApJ...158..337H full text sources ADS |