We consider the scattering and absorption of light in discrete random media of densely packed spherical particles. In what we term "radiative transfer with reciprocal transactions" (RT), we introduce a volume element of the random medium, derive its scattering and absorption characteristics using the superposition T-Matrix method (STMM), and compute its frequency-domain incoherent volume-element scattering characteristics. Using an order-of-scattering approach, we then compute a numerical Monte Carlo solution for the scattering problem with an exact treatment of the interaction between two volume elements. We compute both the direct and reciprocal contributions along a sequence of volume elements, allowing us to evaluate the coherent backscattering effects. We show that the RT and exact STMM solutions are in mutual agreement for large finite systems of densely packed spherical particles. We conclude that the RT method provides a viable numerical solution for scattering by asymptotically infinite systems of particles.
We show that the scattering phase functions of the coma and the nucleus of the comet 67P/Churyumov-Gerasimenko measured by the Rosetta/OSIRIS instrument can be reproduced by a particle model involving clustered densely packed submicrometer-sized grains composed of organic material and larger micrometer-sized silicate grains. The simulated and measured coma phase functions suggest that near the nucleus scattering is dominated by large particles, and the size distribution of dust particles varies with time and/or local coma environment. Further, we show that the measured nucleus phase function is consistent with the coma phase function by modelling a nucleussized object consisting of the same particles that explain the coma phase functions.
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