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We investigate the effect of porosity in the Umov effect for the first time using the aggregate dust model. The Umov effect is an inverse correlation between the reflectivity (or geometric albedo) of an object and the degree of linear polarization of light scattered by it. Three different types of fractal aggregates: ballistic agglomeration (BA), ballistic agglomeration with one migration (BAM1), and ballistic agglomeration with two migrations (BAM2) having porosities 0.87, 0.74, and 0.64, respectively (which have the same characteristic radius ∼1  μ m), are considered in our simulations. Using the multisphere T-matrix (mstm) code, maximum positive polarization (P max) and geometric albedo (A) are calculated for three different fractal aggregated structures considering amorphous silicate composition. Then P max and A are plotted against each other in logarithmic scale that shows a linear inverse correlation and a strong porosity dependence. This study shows that the porosity of the aggregates plays a crucial role in the Umov-law diagram. Further, we explore the effect of aggregate size parameter and the effect of composition in the Umov diagram for particles larger than the wavelength of incident radiation. A systematic study is presented in this paper. [ABSTRACT FROM AUTHOR]

Abstract: A T-matrix code tailored to non-axisymmetric particles with finite symmetries is described. The code exploits geometric symmetries of particles by use of group theoretical methods. Commutation relations of the T-matrix are implemented for reducing CPU-time requirements. Irreducible representations of finite groups are employed for alleviating ill-conditioning problems in numerical computations. Further, an iterative T-matrix method for particles with small-scale surface perturbations is implemented. The code can compute both differential and integrated optical properties of particles in either fixed or random orientation. Methods for testing the convergence and correctness of the computational results are discussed. The package also includes a database of pre-computed group-character tables, as well as an interface to the GAP programming language for computational group theory. The code can be downloaded at http://www.rss.chalmers.se/∼kahnert/Tsym.html. [Copyright &y& Elsevier]

Abstract: The use of group theoretical methods can substantially reduce numerical ill-conditioning problems in T-matrix computations. There are specific problems related to obtaining the irreducible characters of high-order symmetry groups and to the construction of a transformation from the basis of vector spherical wave functions to the irreducible basis of high-order symmetry groups. These problems are addressed, and numerical solutions are discussed and tested. An important application of the method is non-convex particles perturbed with high-order polynomials. Such morphologies can serve as models for particles with small-scale surface roughness, such as mineral aerosols, atmospheric ice particles with rimed surfaces, and various types of cosmic dust particles. The method is tested for high-order 3D-Chebyshev particles, and the performance of the method is gauged by comparing the results to computations based on iteratively solving a Lippmann–Schwinger T-matrix equation. The latter method trades ill-conditioning problems for potential slow-convergence problems, and it is rather specific, as it is tailored to particles with small-scale surface roughness. The group theoretical method is general and not plagued by slow-convergence problems. The comparison of results shows that both methods achieve a comparable numerical stability. This suggests that for particles with high-order symmetries the group-theoretical approach is able to overcome the ill-conditioning problems. Remaining numerical limitations are likely to be associated with loss-of-precision problems in the numerical evaluation of the surface integrals. [Copyright &y& Elsevier]

Abstract: Study of cosmic dust and planetary aerosols indicate that some of them contain a large number of aggregates of the size that significantly exceeds the wavelengths of the visible light. In some cases such large aggregates may dominate in formation of the light scattering characteristics of the dust. In this paper we present the results of computer modeling of light scattering by aggregates that contain more than 1000 monomers of submicron size and study how their light scattering characteristics, specifically polarization, change with phase angle and wavelength. Such a modeling became possible due to development of a new version of Multi Sphere T-Matrix (MSTM) code for parallel computing. The results of the modeling are applied to the results of comet polarimetric observations to check if large aggregates dominate in formation of light scattering by comet dust. We compare aggregates of different structure and porosity. We show that large aggregates of more than 98% porosity (e.g. ballistic cluster–cluster aggregates) have angular dependence of polarization almost identical to the Rayleigh one. Large compact aggregates (less than 80% porosity) demonstrate the curves typical for solid particles. This rules out too porous and too compact aggregates as typical comet dust particles. We show that large aggregates not only can explain phase angle dependence of comet polarization in the near infrared but also may be responsible for the wavelength dependence of polarization, which can be related to their porosity. [Copyright &y& Elsevier]

Abstract: We address the question if and how observations of scattered intensity and polarisation can be employed for retrieving particle shape information beyond a simple classification into spherical and nonspherical particles. To this end, we perform several numerical experiments, in which we attempt to retrieve shape information of complex particles with a simple nonspherical particle model based on homogeneous spheroids. The discrete dipole approximation is used to compute reference phase matrices for a cube, a Gaussian random sphere, and a porous oblate and prolate spheroid as a function of size parameter. Phase matrices for the model particles, homogeneous spheroids, are computed with the T-matrix method. By assuming that the refractive index and the size distribution is known, an optimal shape distribution of model particles is sought that best matches the reference phase matrix. Both the goodness of fit and the optimal shape distribution are analysed. It is found that the phase matrices of cubes and Gaussian random spheres are well reproduced by the spheroidal particle model, while the porous spheroids prove to be challenging. The “retrieved” shape distributions, however, do not correlate well with the shape of the target particle even when the phase matrix is closely reproduced. Rather, they tend to exaggerate the aspect ratio and always include multiple spheroids. A most likely explanation why spheroids succeed in mimicking phase matrices of more irregularly shaped particles, even if their shape distributions display little similarity to those of the target particles, is that by varying the spheroids’ aspect ratio one covers a large range of different phase matrices. This often makes it possible to find a shape distribution of spheroids that matches the phase matrix of more complex particles. [Copyright &y& Elsevier]

Abstract: Optical activity is a typical property of the biological materials where left-handed amino-acids and right-handed carbohydrates dominate (so called homochirality). Observationally, optically active materials reveal themselves through the circular polarization in the light they scatter. Thus, circular polarization produced by the optically active particles can serve as a biomarker. It is known that biological (e.g. colonies of bacteria) and pre-biological (e.g. dust in comets) particles often have a complex structure that can be modeled presenting them as aggregates of small monomers. This motivated the development of the T-matrix code presented in this paper, which enables calculation of the scattering matrix – including circular polarization – of the light scattered by aggregated optically active particles. The code can be used for modeling the light scattering by biological objects (e.g. colonies of bacteria, blood cells) and for interpretation of the circular polarization produced by the cosmic dust that contains (pre)biological organic, e.g. comet dust or planetary aerosols. [Copyright &y& Elsevier]