Pre-conditioned backward Monte Carlo solutions to radiative transport in planetary atmospheres Fundamentals: Sampling of propagation directions in polarising media

• Published in: Astronomy and astrophysics (Berlin) Vol. 573; p. A72
• Main Authors: Munoz, A Garcia, Mills, F P
• Format: Journal Article
• Language: English
• Published: 01.01.2015
• Subjects: Algorithms; Estimating; Media; Monte Carlo methods; Planetary atmospheres; Polarization; Polarized radiation; Sampling
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201424042

The interpretation of polarized radiation emerging from a planetary atmosphere must rely on solutions to the vector radiative transport equation (VRTE). Monte Carlo integration of the VRTE is a valuable approach for its flexible treatment of complex viewing and/or illumination geometries, and it can intuitively incorporate elaborate physics. We present a novel pre-conditioned backward Monte Carlo (PBMC) algorithm for solving the VRTE and apply it to planetary atmospheres irradiated from above. We show that the neglect of polarization in the sampling of photon propagation directions in classical BMC algorithms leads to unstable and biased solutions for conservative, optically-thick, strongly polarizing media such as Rayleigh atmospheres. The PBMC algorithm is robust, and its accuracy is extensively demonstrated via comparisons with examples drawn from the literature for scattering in diverse media. By construction, backward integration provides a better control than forward integration over the planet region contributing to the solution, and this presents a clear advantage when estimating the disk-integrated signal at moderate and large phase angles.