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K. Muinonen (Observatory, Univ. Helsinki, Finland)
Photometric observations of asteroids indicate an opposition effect, a nonlinear increase of disk-integrated brightness at small solar phase angles. Asteroid polarimetric observations show a negative degree of linear polarization for unpolarized incident light. Multiple scattering of light by an asteroid's surface depends on its small-scale physical characteristics---such as surface roughness and particle size, shape, refractive index, and volume density. Here we describe multiple-scattering modeling that accounts for internal interactions within regolith particles (large compared to the wavelength) as well as external interactions among such particles. We model the single-scattering properties as an incoherent sum of low-degree multipole contributions, starting from the electric dipole (Rayleigh scattering) and magnetic dipole terms. The approach results in realistic ensemble-averaged single-scattering phase matrices and allows an approximate Monte Carlo computation of coherent backscattering effects (e.g., Waves in Random Media 14, 365, 2004). In particular, it is possible to explain, simultaneously, the substantial angular widths of the negative polarization surges and the narrow opposition effects. Using numerical simulations, we can take into account shadowing effects due to the rough surface and the porous regolith. Coherent-backscattering and shadowing modeling of asteroid photometric and polarimetric phase effects is shown for different asteroid classes.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.