Light scattering by aggregate particles comparable in size to wavelength: Application to cometary dust

Many naturally occurring particles (including, most likely, cometary dust) have an aggregate structure. Wt: study the scattering properties of polydis perse independent aggregate particles (clusters) comparable in size to visi ble wavelengths. The sizes of the monomers constituting a cluster play a si gnificant role in forming the angular dependences of intensity and linear p olarization of the scattered light. Irregularly structured aggregates compo sed of a moderate number of spheres (< 50) with size parameters 1.3-1.65 ex hibit properties typical of cometary dust particles: a slight increase in b ackscattering intensity, a negative polarization at small phase angles, an inversion phase angle close to the observed one, an increase in brightness, and a linear polarization with increasing wavelength. In this case, the im aginary part of the refractive index for particles can increase with decrea sing wavelength in the visible spectral range. which is typical of silicate s with an admixture of iron or organic material. The spectral dependence of extinction efficiency For aggregates is less steep than that for equivalen t spherical particles, and its maximum is shifted to larger size parameters . Therefore, when analyzing extinction measurements, the scatterer shape mu st be taken into account to avoid underestimation of the scattering-particl e sizes.