Coherent backscattering

Photometric and polarimetric data obtained from telescopic observations and laboratory measurements at small phase angles have been accnmnlating for many years however, the interpretation of these effects, especially of the polarization branch, was not satisfactory until recently [7-20] when the coherent backscattering (interference) mechanism was developed to explain both effects. Since that time, additional experimental [21-23] and theoretical [18, 24-28] data were obtained, showing that the effects are more complex than initially considered. For example, negative polarization branches with two minima were discovered for some Jupiter satellites and bright asteroids [29,30], and unusual behavior of the opposition spike amphtude with albedo for the Moon, asteroids, and Jupiter s satellite Europa were found [31-33]. 

In general, a decrease of the opposition spike amplitude with increasing albedo contradicts the coherent backscattering mechanism that predicts an increase of the spike amplitude with increasing albedo. One possible explanation of this contradiction is that with the albedo increase, the width of the spike becomes significantly smaller than 0.1° and the phase ratio 7(0.1°)//(3°) is not sensitive to this spike. 

A modehng effort has demonstrated that the polarization opposition effect can be reproduced using the enhanced backscatter mechanism, namely the constractive interference of reciprocal rays. Varions models have been developed that reprodnce this featnre. For instance, a vector theory of coherent backscattering for a semi-infinite medinm of Rayleigh scatterers is able to reproduce the stracture of the polarization featnre when the cyclical (reciprocal) terms are inclnded [8-12]. In addition, various ray-tracing methodologies from different types of scattering systems also are able to produce the features of the polarization opposition effect when the constmctive interference mechanism is inclnded [13-21]. Several review articles address this topic [2, 7, 12, 18]. 

The system of a small particle near a planar surface is of interest not only because the ray-tracing solution converges to the exact solution to very high accuracy, but also because the components that contribute to the coherent backscattering can be separated from the other components. Figure 5 shows the contribution of the polarization state of scattered light by the second-order reciprocal components of ray paths 2 and 3 (see Figure 2). The polarization... 

V.D. Ozrin, Exact solution for the coherent backscattering of polarized light from a random medium of Rayleigh scatterers. Waves Random Media 2, 141-164 (1992). 

M. Mishchenko, V. Tishkovets, and P. Litvinov, Exact results of the vector theory of coherent backscattering from discrete random media an overview, in Optics of Cos mic Dust, ed. by G. Videen and M. Kocifaj (Kluwer Academic Publishers, Dordrecht, 2002) 239-260. 

K. Muinonen, Coherent backscattering by solar system dust particles, in Asteroids, Comets and Meteors, ed. by A. Milani, M. Di Martino, and A. Cellino (Kluwer, Dordrecht, 1974) 271-296. 

Recently Mishchenko et al. [10] predicted that at small phase angles near opposition, asteroids produce a sharp narrow spike of the measurable linear polarization due to coherent backscattering of reciprocal rays (named Opposition Effect). This mechanism operates in planets, asteroids and comets. The polarimetric observations made recently by Rosenbush et al. [11] with 125cm telescope at the Crimean Astrophysical Observatory confirm this prediction. 

Akkermans E, Wolf PE, Maynard R, Maret G. Optical coherent backscattering by random media an experimental study. J Phys (Paris) 1988 49 63-98. 

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