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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]

In the present work, the light-scattering properties of comet Levy 1990XX are studied through simulations using Ballistic Particle-Cluster Aggregation (BPCA) or Ballistic Cluster–Cluster Aggregation (BCCA) aggregates of up to 128 spherical monomers of different compositions (e.g. silicates, carbonaceous materials etc.) and the best-fitting theoretical polarization curve is generated using theSuperposition T-matrix code. The best-fitting refractive indices coming out from the present analysis show silicate behaviour when monomer radius is and provide excellent results on the maximum and negative degrees of linear polarization at a single wavelength for BCCA aggregates. [ABSTRACT FROM AUTHOR]