Michael I. Mishchenko

Improved T-matrix computations for large, nonabsorbing and weakly absorbing nonspherical particles and comparison with geometrical-optics approximation.

We show that the use of a matrix inversion scheme based on a special lower triangular-upper triangular factorization rather than on the standard Gaussian elimination significantly improves the numerical stability of T-matrix computations for nonabsorbing and weakly absorbing nonspherical particles. As a result, the maximum convergent size parameter for particles with small or zero absorption can increase by a factor of several and can exceed 100. We describe an improved scheme for evaluating Clebsch-Gordon coefficients with large quantum numbers, which allowed us to extend the analytical orientational averaging method developed by Mishchenko [J. Opt. Soc. Am. A 8, 871 (1991)] to larger size parameters. Comparisons of T-matrix and geometrical optics computations for large, randomly oriented spheroids and finite circular cylinders show that the applicability range of the ray-tracing approximation depends on the imaginary part of the refractive index and is different for different elements of the scattering matrix.

POLARIZATION PROPERTIES OF THE GALILEAN SATELLITES OF JUPITER: OBSERVATIONS AND PRELIMINARY ANALYSIS

We present new, detailed polarimetric measurements of the Galilean satellites of Jupiter with U, B, V , and R -lters at phase angles ranging from 12i to nearly 0i. The polarization phase curves of Io, Europa, and Ganymede in the B, V , and R -lters clearly show the presence of the polarization opposition e†ect in the form of a sharp peak of negative polarization centered at a very small phase angle of and 0i.6E0i.7 superimposed on the regular negative polarization branch. This phase angle is comparable to the width of the spikelike photometric opposition e†ect observed for Europa, thus indicating that both opposition phenomena are likely to be produced by the coherent backscattering mechanism. The U -lter values of for Io and Europa are close to 0.60% and 0.47%, respectively, and exceed the respective BV R o P min o values by a factor of almost 2. The BV R polarization for the trailing hemispheres of Io, Europa, and, especially, Ganymede is systematically stronger than for the respective leading hemispheres. For Callisto, the leading hemisphere polarization is signi-cantly stronger than for the trailing hemisphere. The inver- sion angles for Io, Europa, and Ganymede are nearly wavelength independent and close to 10i.0, 8i.6, and respectively. The inversion angle for the trailing hemisphere of Callisto is also wavelength inde- 8i.8, pendent and is in the range of 12iE13i. Subject headings: minor planets, asteroids E planets and satellites: individual (Jupiter) E polarization E radiative transfer

Diffuse and coherent backscattering by discrete random media—I. Radar reflectivity, polarization ratios, and enhancement factors for a half-space of polydisperse, nonabsorbing and absorbing spherical particles

Abstract It has been demonstrated recently that diffuse, incoherent multiple scattering of electromagnetic waves by media composed of randomly positioned, discrete scattering particles is always accompanied by coherent backscattering and may explain intriguing opposition phenomena observed for some solar system bodies, in particular peculiar characteristics of radar returns from icy satellite surfaces. In this paper, we study theoretically photometric and polarization characteristics of diffuse and coherent backscattering by discrete random media. The cyclical component of the Stokes reflection matrix at exactly the backscattering direction is expressed in terms of the ladder component, and the ladder component is accurately computed by numerically solving the vector radiative transfer equation. We give formulas expressing the radar reflectivity, radar linear and circular polarization ratios, and backscattering enhancement factors in the elements of the Stokes reflection matrix and describe in detail the computational technique used. Assuming that the scattering medium is homogeneous and semi-infinite and that scattering particles are polydisperse spheres, we report the results of a comprehensive theoretical survey of the dependence of the photometric and polarization characteristics of the radar return on the illumination zenith angle and on the particle effective size parameter and real and imaginary parts of the refractive index.

Calculations of the soil hot-spot effect using the coherent backscattering theory

Abstract The shadowing theory has been widely used to account for the hot-spot effect for the canopy and soil. However, it is not valid for soils that are composed of fine particles and do not have well-defined shadows. The coherent backscattering theory is used in this study to calculate both the magnitude and the angular width of the hot-spot peak for clay and silt soils. The soil hot-spot magnitude is calculated by solving the vector radiative transfer equation. The dense nature of the soil medium is taken into account in the calculations of the hot-spot angular width. The results are also linked with the Hapke hot-spot empirical function. The effects of the wavelength dependence and of particle size and shape also are exainined.