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K. Muinonen (Univ. Helsinki)
Coherent backscattering is an essential physical mechanism contributing to the opposition effects and negative degrees of linear polarization observed for numerous asteroids and other airless solar system bodies. In a particulate medium like the regolith of an asteroid, the multiply scattering electromagnetic waves propagating in opposite directions along the same paths always interfere constructively at the backward direction whereas, in other directions, the interference varies between constructive and destructive, the net intensity effect being a peak close to the backward direction. The interference implies larger (smaller) backscattering peak widths for wave paths typically producing negative (positive) linear polarization, the net polarization effect being negative polarization near the backward direction. An efficient vector Monte Carlo technique is presented for computing coherent backscattering and radiative transfer by particulate media. What is crucial for the treatise of coherent backscattering, instead of collecting reflected rays into bins of finite angular size, the Stokes vector of scattered light is updated at all reflection angles at each individual scattering process. Theoretical modeling is offered for the opposition effects and negative linear polarization degrees of the asteroids (44) Nysa and (64) Angelina.
Research supported by the Academy of Finland.