ANALYSIS OF PHOTOPOLARIMETRIC DATA OF COMETS AT SMALL PHASE ANGLES BY ROUGH-SURFACE SCATTERING

Photopolarimetric data for comets in the backscattering region, including negative polarization and the opposition effect, are interpreted on the basis of rough scattering by large irregular particles combined with Mie scattering by submicron grains. Both types of grains are assumed to have the same average optical constants, i.e., n = 1.385 and k = 0.035 at wavelength λ = 0.62 μm, where n and k, respectively, denote a refractive index and absorption coefficient of grain material. Scattering from large, rough particles reduces the strong negative polarization in the backward direction contributed by Mie scattering from small grains and is adequate to model observed amplitudes of negative polarization in cometary comae. The introduction of rough scattering also leads to a somewhat steeper increase of intensity with decreasing phase angle, α, in the range 0° ≤ α ≤ 20°, and is more consistent with observations than that predicted from Mie theory alone and Halley's grain-size distribution. © 1990.