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Context. Optical characterization of dust particles in cometary comae is based on a comparison between observations and simulations of solar radiation scattered by cometary dust. Aims. Our recent studies suggest that all the observed features can be reproduced by dust aggregates consisting of optically dark submicrometer-size grains when the overall sizes of the aggregates are much larger than visible wavelengths. Methods. To put constraints on the properties of cometary dust aggregates, we thoroughly investigate the angular and spectral dependencies of intensity and polarization for aggregate particles consisting of identical homogeneous spheres based on a rigorous light-scattering theory. Results. The optimum parameters found through our comprehensive survey are ${a}_{{\rm V}} \gg 0.6~\mu$m for the radius of aggregates, ${a}_{{\rm m}} \approx 0.1~\mu$m for the radius of constituent spheres, $n \approx 1.8$–2.0 for the real part of the refractive index, $k \approx 0.4$–0.6 for the imaginary part and ${\rm d}m/{\rm d}\lambda \ga 0$ for the spectral gradient of the refractive index. In particular, the best results are obtained for the refractive indices derived from a synthetic mixture of carbonaceous material, magnesium-rich silicate, and iron-bearing sulfide with their abundances simulating the composition of dust in the comet 1P/Halley. Conclusions. Despite the simplicity of our model, these parameters are entirely consistent with our best knowledge of the properties of cometary dust. The common angular and spectral dependencies of intensity and polarization observed for cometary dust are explained with similarities in the average size and element composition of the constituent grains forming dust aggregates. Slight differences in the observed optical data may result from variations in the processing of cometary matter.

Light scattering by cometary dust numerically simulated with aggregate particles consisting of identical spheres