On the size, shape, and orientation of noctilucent cloud particles

Conclusions about the size of noctilucent cloud particles are often made under the assumption that they are spherical. The small degree of circular polarization observed in the light from noctilucent clouds suggests, however, that the particles may be aligned, and hence nonspherical: unpolarized light is elliptically polarized upon single scattering by aligned particles. Yet no suitable mechanism-aerodynamic, electrical, magnetic-has been proposed which will align small (< 10 μm) mesospheric particles. A more likely explanation for the observed circular polarization is that the light illuminating noctilucent clouds has acquired ellipticity because of multiple scattering in the long atmospheric paths it traverses. Even if noctilucent cloud particles are nonspherical, however, presently accepted upper limits on their size (∼0.1 μm) and volumetric scattering need not be revised: scattering by small ice spheres as well as ice ellipsoids of any shape is consistent with the observed angular dependence of the degree of linear polarization. Moreover, particles much larger than 0.1 μm, regardless of shape, will lower the maximum degree of linear polarization below that which has been observed.