Survey of Photopolarimetric Properties of Dust/Aerosols Presented as an Ensemble of Rough Spheroids

Abstract:

We use the software package developed by Dubovik et al. (J. Geophys. Res. 111, D11208, 2006) to study the light scattering properties of randomly oriented spheroids. The package allows to compute light scattering by a mixture of spheroids for 25 logarithmically equidistant axis ratios ranging from 0.3 (oblate spheroids) to 3.0 (prolate spheroids) and 41 size bins covering the size parameter (x) from 0.012 to 625. To cover such a large range, two complementary methods were used for computing kernels. For x < 30 - 60, the simulations were performed using the T-matrix method by Mishchenko et al. (JQSRT, 55(5), 535, 1996). For x exceeding the T-matrix convergence limits, the approx. geometric-optics-integral-equation method of Yang & Liou (Appl. Opt. 5.33, 6568, 1996) was used. The roughness is described by a parameter σ that is equal to 0 for smooth surface and > 0 for rough surface. In the package, look-up tables were simulated for quadrature coefficients employed in the numerical integration of spheroid optical properties over size and shape to reduce computation time. The pre-calculated kernels in the software package allow fast, accurate, and flexible modeling of different size and shape distributions.

We compare smooth and rough spheroids that obey log-normal size distribution with standard deviation 0.1 and effective radius 8.71 mm. Size parameter was varied by variation of wavelength within 0.34 – 25 mm. We considered silicate-like refractive index (1.53, 0.008), spheroids of two aspect ratios a/b=2.07 and 0.48, and two roughnesses σ = 0 and 0.2. We study how different photopolariemtric properties of smooth and rough spheroids are depending on the size parameter and scattering angle.

We also make computations for rough spheroids that have composition typical for cometary dust and find characteristics of an ensemble of rough spheroids that fit cometary photopolarimetric observations.