Light scattering by aggregates with sizes comparable to the wavelength: Anapplication to cometary dust

Cometary dust is likely to have aggregate structure. Therefore the optical properties of aggregates must be understood if one wants to obtain informat ion on cometary dust from optical observations. In this paper the scatterin g properties of polydisperse clusters with sizes comparable to the visible wavelengths are studied. It is shown that the sizes of the constituent mono mers play a vital role in establishing the phase functions of intensity and linear polarization. irregularly structured aggregates composed of a moder ate number of touching spheres (<50) with size parameters ranging from 1.3 to 1,65 display properties typical of cometary dust particles, namely, a we ak increase of the backscattering intensity, negative polarization at small phase angles, and a positive wavelength gradient of polarization. More com pact particles have a more pronounced negative branch of polarization. With increasing wavelength the negative branch of polarization will become shal lower and the inversion angle will approach the backscattering direction (i f the refractive index is constant). This is in agreement with observations of Comet Hale-Bopp in the K band. The increase of polarization with wavele ngth is reduced if the imaginary part of the refractive index decreases wit h wavelength. The negative wavelength gradient of polarization observed in Comet Giacobini-Zinner can be explained by a particularly strong decrease o f absorption with increasing wavelength or by smaller particle sizes. The s pectral dependence of extinction efficiency for aggregates is less steep th an that of equivalent spheres, and its maximum is shifted to larger size pa rameters. Therefore, in order to avoid an underestimation of the particle s ize derived from extinction measurements, one should take into account the shape of the scattering particles. (C) 2000 Academic Press.