Optical properties of dust aggregates II. Angular dependence of scattered light

The angular dependence of light scattered by dust aggregates has been investigated by means of the discrete dipole method. Two types of aggregates have been treated; BPCA and BCCA whose fractal dimensions are ~ 3 and ~ 2 in the limit of large size respectively, and whose number of constituent particles range from N = 256 to 4096. The radii of constituent particles are 0.01 μm and are partly 0.03μm. Two kinds of minerals are considered as the constituents; silicate and magnetite. The calculations have been carried out at the wavelength λ = 0.6μm. The angular dependence of the degree of linear polarization and the scattered intensity is sensitive to the structure as well as the chemical composition with increasing size of the aggregates. The degree of linear polarization of BCCA shows the behavior similar to that of the individual constituent particles independent of N and of the chemical composition. In BPCA, the maximum value is depressed and the scattering angle at the maximum shifts towards larger scattering angles with increasing A', which is more remarkable in the magnetite aggregates than in the silicate ones. The difference in the behavior of polarization between BPCA and BCCA can be interpreted as the result of multiple scattering in the aggregates reflecting the difference in the structure, The forward scattering lobe is well-developed with increasing N in BPCA in spite of the smaller size than BCCA with the same number of constituent particles. For a given aggregate, the scattered intensity shows almost the same behavior of angular dependence regardless of, but the strength is sensitive to the chemical composition of the constituent particles. The intensity at the scattering angle θ = 0° is independent of the structure of the aggregates and is proportional to N² in the range of our calculations. The angular dependence of the scattered intensity exhibits a similarity relation when the size parameter of the aggregates A' = kR ≳ 3, where k is the wavenumber and R is the characteristic radius of aggregate. The angular dependence can be divided into at least 4 regimes according to the value of q = 2kR sin(θ/2). The scattered intensity in the region of q ≲ 2 is dominated by the coherently scattered light from the constituent particles and well reflects the size and the mass of aggregates, independent of the structure. The distinction between BCCA and BPCA occurs in the region of 2.4 ≲ q ≲ 5.0 through the effect of multiple scattering in the aggregates.