able fraction of naturally occurring dust particles would be divided into a small number of basic classes. Light scattering Numerical computations were performed to study light scattering by clusters of up to 200 interacting spheres to better by these classes could then be calculated and results comunderstand the structure of dust particles in the Solar System. pared with measurements and observations. The size parameter of a cluster is about 13. We, however, vary Lumme et al. (1995) suggested one possibility for such several other parameters to see their impact on the scattering a classification. They divided the dust particles in the folof light. The packing density of a cluster varies between 0.1 lowing categories: polyhedral solids, stochastically rough and 0.2 and the size parameter of a constituent sphere is either particles, and stochastic aggregates. All basic crystal 1.2 or 1.9. We use the refractive indices of 1.29 and 1.57 with shapes, for instance, would form just a subclass of the a small imaginary part. These indices roughly correspond to polyhedral solids. Typical sand particles also seem to fit in ice and silicates. The clusters are formed either by the diffusion this class. Stochastically rough particles do not have any limited aggregation process or by a random thinning of a maxisharp edges. Examples of this class are some cosmic and mally packed cluster. All light scattering computations have atmospheric particles such as flyash and products of cement been done with the discrete-dipole approximation (DDA) which is the most flexible method for complicated cluster geometries. plants. A mathematical model for these particles has been
We calculate all the 16 elements of the scattering matrix tosuggested by Muinonen et al. (1996). Stochastic aggregates gether with efficiency factors for our model particles in random of small particles form a class in which most of the cosmic orientation. The first order scattering approximation, which dust seems to fall. To model the geometry of these particles includes only the electromagnetic phase correction between the we assume two extreme processes. In the first one we constituent particles without any mutual interactions, explains randomly remove constituent particles from a maximally remarkably well the angular dependence (including diffraction) packed cluster. In this process the packing density remains of all the matrix elements below the scattering angle of about almost constant as a function of the radial distance. For 60Ψ. For large scattering angles the signatures of the constituent the other model we assume a diffusion-limited aggregation particles are completely washed out.