Multiple scattering of light in a comet atmosphere.

Abstract

A numerical study of the anisotropic multiple scattering of light in a cometary atmosphere is performed. The atmosphere is assumed to be a spherical shell, illuminated by parallel solar rays. The spatial variation of dust in the coma is symmetric about the sun-comet axis ("r-$\theta$ geometry"). Two model dust profiles were studied: (1) a spherically symmetric profile, and (2) an axisymmetric dust jet at the comet's subsolar point. The specific intensity of multiply-scattered photons is determined throughout the coma by successive approximation to the monochromatic (one-group) transport equation. Direction dependence is treated with discrete ordinates, which are bunched in function of position to avoid ray effects. For each step in the iterative solution we integrate along characteristics in curvilinear geometry. Calculation is made of the flux of visible energy impinging on the nucleus surface, the mean intensity of visible light throughout the coma, and an approximate solution for the flux of thermal radiation at the comet surface due to emission from dust particles in the coma. Coma optical depths are determined at which the flux of scattered photons exceeds the flux of unscattered photons. For all conditions studied, the total radiant flux to the nucleus does not exceed the visible flux on a bare nucleus. Simulated images of a model comet atmosphere as it would be seen by a near-flying satellite, or by general wide-angle imaging techniques, are presented. Nucleus visibility, the effect of forward-scattering, and overall jet appearance is examined for varying phase angle. For both the study of energy transfer and the image simulation, conditions are determined under which single and multiple scattering each become significant.