Scattering by aggregates with and without an absorbing mantle: microwave analog experiments

Abstract

We present angular scattering functions for loosely packed aggregates of 250 and 500 identical spheres near the Rayleigh size limit before and after the application of successive layers of an absorbing mantle. All measurements were obtained by using the microwave analog technique. Gross features of the scattering by aggregates without a mantle can be interpreted in terms of coherent scattering from the unit spheres acting independently of each other. The largest deviations from this approximation occur after the first minimum in forward scattering and extend to a scattering angle of 60° or 80° for our models. This intermediate range is also where the largest differences occur in the scattering from one aggregate to another. The angular extent of the range is largest for aggregates with the smallest dimensions. The scattering function is usually flat in the backscattering hemisphere and has little or no backscattering increase. The coherent scattering approximation breaks down when the aggregates are coated, and an equivalent spheres approximation becomes a better representation. The maximum degree of polarization near a scattering angle of 90° first decreases and then increases again as the mantle grows thicker.

Polarization of Light Scattered by Fluffy Particles (PROGRA2 Experiment)

“…The DOPE experiment (Combet et al 2000) studies compact particles of different materials but their nickel samples can be considered as aggregates in terms of their structure. The most comparable experimental work uses microwave analogs (Zerull et al 1993, Gustafson andKolokolova 1999). All these works deal with particles smaller than in the present work; nevertheless, the comparison could help to understand remote observations.…”

“…Their result is similar to those of Xing and Hanner (1997) for a mixture of tetrahedral aggregates of silicate and carbon. Using the microwave analog technique, Zerull et al (1993) studied fluffy aggregates with carbon mantles. When the mantle on the silicate grains is thin, P max decreases, and it increases when the mantle become thicker.…”

“…Laboratory measurements obtained for particles lying on a surface, with multiple scattering between the particles, therefore apply well enough to the case of planets (Lyot 1929, Ebisawa andDollfus 1993) and asteroids (Goidet-Devel et al 1994) but not to cometary and interplanetary dust, for which particles are indeed separated without multiple scattering between them. Microwave analog experiments have also been used to study single scattering for particles of various materials, shapes, and size parameters (Giese et al 1978, Zerull et al 1993, Gustafson and Kolokolova 1999.…”

Microwave measurements of the full amplitude scattering matrix of a complex aggregate: a database for the assessment of light scattering codes

“…Nevertheless, this phase information was never exploited to its full potential and measurements were almost always restricted to co-polarized or total intensities [4,5,6,7,8]. Furthermore, the types of targets studied were predominantly either complex (but not controlled [4,5,7,8]), or controlled (but regular [6,9]). Hence it became desirable to extend those measurements to complex targets with fully known geometrical properties, and at the same time analyze the phase of the scattered fields for all (co-and cross-) polarizations.…”

Optical Properties of Nonspherical Particles of Size Comparable to the Wavelength of Light: Application to Comet Dust

“…West (1991) and Zerull et al (1993) have demonstrated both by computations and laboratory measurements that scattering properties of an aggregate particle are strongly influenced by the scattering properties of its individual constituent particles. This important result means that the optical properties of nonspherical particles comparable in size to the wavelength of light have to be understood before modeling cometary dust as aggregate particles.…”