LIGHT-SCATTERING BY NONSPHERICAL ICE GRAINS - AN APPLICATION TO NOCTILUCENT CLOUD PARTICLES

The light scattered by noctilucent cloud particles is nearly fully polarized at scattering angles in the vicinity of 90-degrees. This was one of the reasons to conclude that the upper limit of their sizes is not larger than about 0.12-mu-m. Nevertheless, this estimate was made on the basis of the Mie scattering theory for spherical particles, whereas many investigators noted usefulness of highly aspherical shapes of noctilucent cloud particles. In this paper, we used rigorous light scattering theory for randomly oriented nonspherical particles to calculate the degree of linear polarization of the scattered light for ice grains of different shape. By comparing these calculations with rocket polarization measurements of noctilucent clouds, we show that, as for spherical particles, the upper limit of particle equal-volume radii for slightly flattened and elongated grains is of about 0.12-mu-m, while for highly aspherical plate-like and needle-like particles this upper limit is substantially larger and is of about 0.18-0.20-mu-m. We also report calculations of the volumetric scattering cross-section for particles of different shape and show that randomly oriented spheroids have (slightly) smaller scattering cross section per unit particle mass than equal-volume spherical grains. Nevertheless, if in noctilucent clouds plate-like and needle-like grains grow to much larger sizes than spherical particles, their scattering efficiency may be much greater.