LIGHT-SCATTERING BY DENSE CLUSTERS OF SPHERES

Numerical computations were performed to study light scattering by clu sters of up to 200 interacting spheres to better understand the struct ure of dust particles in the Solar System, The size parameter of a clu ster is about 13, We, however, vary several other parameters to see th eir impact on the scattering of light, The packing density of a cluste r varies between 0.1 and 0.2 and the size parameter of a constituent s phere is either 1.2 or 1.9, We use the refractive indices of 1.29 and 1.57 with a small imaginary part, These indices roughly correspond to ice and silicates, The clusters are formed either by the diffusion lim ited aggregation process or by a random thinning of a maximally packed cluster, All light scattering computations have been done with the di screte-dipole approximation (DDA) which is the most flexible method fo r complicated cluster geometries. We calculate all the 16 elements of the scattering matrix together with efficiency factors for our model p articles in random orientation. The first order scattering approximati on, which includes only the electromagnetic phase correction between t he constituent particles without any mutual interactions, explains rem arkably well the angular dependence (including diffraction) of all the matrix elements below the scattering angle of about 60 degrees. For l arge scattering angles the signatures of the constituent particles are completely washed out. (C) 1997 Academic Press.