Publication Abstracts

Hansen 1969

Hansen, J.E., 1969: Absorption-line formation in a scattering planetary atmosphere: A test of Van de Hulst's similarity relations. Astrophys. J., 158, 337-349, doi:10.1086/150196.

Van de Hulst's similarity relations, which reduce the problem of anisotropic scattering in a homogeneous 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 approximations 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 θ>), where <cos θ> 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 θ>) 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 μ 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 obtained 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 between the theoretical and the observed phase effects.

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BibTeX Citation

@article{ha07400c,
  author={Hansen, J. E.},
  title={Absorption-line formation in a scattering planetary atmosphere: A test of Van de Hulst's similarity relations},
  year={1969},
  journal={Astrophys. J.},
  volume={158},
  pages={337--349},
  doi={10.1086/150196},
}

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RIS Citation

TY  - JOUR
ID  - ha07400c
AU  - Hansen, J. E.
PY  - 1969
TI  - Absorption-line formation in a scattering planetary atmosphere: A test of Van de Hulst's similarity relations
JA  - Astrophys. J.
VL  - 158
SP  - 337
EP  - 349
DO  - 10.1086/150196
ER  -

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