RADIATION PRESSURE CROSS-SECTION FOR FLUFFY AGGREGATES

We apply the discrete dipole approximation (DDA) to estimate the radia tion pressure cross section for fluffy aggregates by computing the asy mmetry parameter and the cross sections for extinction and scattering. The ballistic particle-cluster aggregate and the ballistic cluster-cl uster aggregate consisting of either dielectric or absorbing material are considered to represent naturally existing aggregates. We show tha t the asymmetry parameter perpendicular to the direction of wave propa gation is maximized where the wavelength is comparable to the aggregat e size, which may be characterized by the area-equivalent radius or th e radius of gyration rather than the volume-equivalent radius. The asy mmetry parameter for the aggregate depends on the morphology of the pa rticle, but not on the constituent material. Therefore, the dependence of the radiation pressure cross section on the material composition a rises mainly from that of the extinction and scattering cross sections , in other words, the single-scattering albedo. We find that aggregate s consisting of high-albedo material show a large deviation of radiati on pressure from the direction of incident radiation. When the aggrega tes are illuminated by blackbody radiation, the deviation of the radia tion pressure increases with increasing temperature of the blackbody. Since the parallel component of the radiation pressure cross section f or the aggregates is smaller than that for the volume-equivalent spher es at the size parameter close to unity, the Planck-mean radiation pre ssure cross section for the aggregates having radius comparable to the effective wavelength of radiation shows a lower value, compared with the volume-equivalent sphere. Consequently, the slope of the radiation pressure force per mass of the particle as a function of particle mas s shows a lower maximum for the aggregates than for compact spherical particles. (C) 1998 Elsevier Science Ltd. All rights reserved.