Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...310..686r&link_type=abstract
Astronomy and Astrophysics, v.310, p.686-698
Astronomy and Astrophysics
Astrophysics
16
Dust, Extinction, Scattering, Polarization
Scientific paper
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 reasonable first approximation to the likely topology of interstellar grains. The orientationally averaged scattering properties of a compact cluster of spheres using this exact method are compared to those of three models approximating the electromagnetic interactions between the spheres in the cluster. Model A includes only dipole interactions. In models B and C, the spheres are assumed to scatter light independently of each other. The only difference is that model B treats the phase differences between the fields scattered by the various spheres in the cluster. None of these approximate models give satisfactory and consistent results 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 material as the cluster. Models include a compact sphere, a sphere whose radius encloses the cluster, and a sphere with the same projected area as 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 radius. The refractive index is determined from the distribution of material within the cluster using effective medium theory. A good agreement is also found. Results for a polydisperse cluster of spheres are also reported. Changes to the scattering properties of such a cluster through either gradual dispersal of the constituent spheres or modification of the complex refractive index are discussed.
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