Fast superposition T-matrix method for computing light scattering by structurally diverse aggregates

Abstract

The superposition T-matrix method is widely used to compute the scattering, absorption, and extinction of light by aggregated particles found in cometary comae, planetary atmospheres, and protoplanetary discs. However, commonly used solvers rapidly lose their efficiency with decreasing fractal dimension or with increasing porosity. We present a new open-source implementation of a fast superposition T-matrix method, FaSTMM2, which provides efficient solutions for a wide range of fractal particle aggregates composed of polydisperse monomers. The solver accelerates the superposition T-matrix method using a broadband multilevel fast multipole algorithm. We have implemented diagonal translation operators at super-wavelength levels based on the plane-wave expansion of spherical waves, while sub-wavelength levels are treated using low-frequency stable spherical-wave translators. In addition, we introduced an adaptive octree data structure that enables the efficient treatment of aggregates composed of polydisperse monomers. Numerical examples demonstrate that FaSTMM2 provides an efficient solution with controllable accuracy for a wide range of aggregates and compositions. In particular, it achieves speed-ups of several orders of magnitude for highly porous aggregates with a fractal dimension of around two.