ELECTROMAGNETIC SCATTERING BY COMPACT CLUSTERS OF SPHERES

The electromagnetic scattering of compact clusters of touching but non -interpenetrating spheres is solved exactly using a Mie-type solution generalized to multiple spheres. These clusters can provide a reasonab le first approximation to the likely topology of interstellar grains. The orientationally averaged scattering properties of a compact cluste r of spheres using this exact method are compared to those of three mo dels approximating the electromagnetic interactions between the sphere s in the cluster. Model A includes only dipole interactions. In models B and C, the spheres are assumed to scatter light independently of ea ch other. The only difference is that model B treats the phase differe nces between the fields scattered by the various spheres in the cluste r. None of these approximate models give satisfactory and consistent r esults for both cross sections and angular scattering when compared to the exact solution. The multiple Mie sphere results are also compared to those of models involving a homogeneous Mie sphere of appropriate radius with a refractive index computed from effective medium theory. The homogeneous Mie sphere is assumed to have the same volume of mater ial as the cluster. Models include a compact sphere, a sphere whose ra dius encloses the cluster, and a sphere with the same projected area a s the cluster. Only the equal-area sphere model shows a good agreement with the exact results. Finally, the multiple Mie sphere results are compared to those of a Mie sphere whose refractive index varies with r adius. The refractive index is determined from the distribution of mat erial within the cluster using effective medium theory. A good agreeme nt is also found. Results for a polydisperse cluster of spheres are al so reported. Changes to the scattering properties of such a cluster th rough either gradual dispersal of the constituent spheres or modificat ion of the complex refractive index are discussed.