Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.4210t&link_type=abstract
American Astronomical Society, DPS meeting #38, #42.10; Bulletin of the American Astronomical Society, Vol. 38, p.561
Astronomy and Astrophysics
Astronomy
Scientific paper
The far-field radiation pattern of coherent radio waves scattered from an aggregate of dielectric spheres of arbitrary spacing and orientation is accurately described by a multi-particle formulation of Mie theory. Using this approach, Maxwell's equations_subject to the appropriate boundary conditions_are solved by computing field expansions in vector spherical harmonics. Both near- and far-field interaction between the particles is accounted for, providing a rigorous electromagnetic solution to the multiple-scattering problem. However, as the size and number of spheres in the aggregate increases, the amount of time and memory required to compute the Mie solution quickly becomes prohibitively large.
Diffraction screen models for computing near-forward scattering from planetary rings have been proposed in the past (Marouf, BAAS 26, 1150, 1994). We investigate the approach validity by computing the diffraction pattern of amplitude screens, which are constructed by projecting the shadows of an aggregate of spheres onto a plane perpendicular to the wave vector of the incident radio wave. The far-field (Fraunhofer) diffraction patterns are then compared with the Mie theoretic result. We show that for EM scattering from an aggregate of electrically large spheres-both singly-sized and belonging to a size distribution-there is excellent agreement between the exact Mie solution and its diffraction theory approximation when near-forward scattering is the angular range of interest. This excellent agreement holds over a broad range of particle separation and orientation configurations. This predictive tool allows for the simulation of coherent radio wave scattering from many thousands of particles in any arbitrary combination of spacings and orientations relative to the illumination direction. Initial results on the use of this method as a predictive tool for detecting the diffraction signature of microstructure in Saturn's A and B rings are presented.
Marouf Essam A.
Thomson Fraser S.
Tyler Leonard G.
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