Light Scattering by Aggregates with Sizes Comparable to the Wavelength: An Application to Cometary Dust

Abstract

Cometary dust is likely to have aggregate structure. Therefore the optical properties of aggregates must be understood if one wants to obtain information on cometary dust from optical observations. In this paper the scattering properties of polydisperse clusters with sizes comparable to the visible wavelengths are studied. It is shown that the sizes of the constituent monomers play a vital role in establishing the phase functions of intensity and linear polarization. Irregularly structured aggregates composed of a moderate number of touching spheres (<50) with size parameters ranging from 1.3 to 1.65 display properties typical of cometary dust particles, namely, a weak increase of the backscattering intensity, negative polarization at small phase angles, and a positive wavelength gradient of polarization. More compact particles have a more pronounced negative branch of polarization. With increasing wavelength the negative branch of polarization will become shallower and the inversion angle will approach the backscattering direction (if the refractive index is constant). This is in agreement with observations of Comet Hale–Bopp in the K band. The increase of polarization with wavelength is reduced if the imaginary part of the refractive index decreases with wavelength. The negative wavelength gradient of polarization observed in Comet Giacobini–Zinner can be explained by a particularly strong decrease of absorption with increasing wavelength or by smaller particle sizes. The spectral dependence of extinction efficiency for aggregates is less steep than that of equivalent spheres, and its maximum is shifted to larger size parameters. Therefore, in order to avoid an underestimation of the particle size derived from extinction measurements, one should take into account the shape of the scattering particles.