Modeling of light depolarization by cubic and hexagonal particles in noctilucent clouds

Noctilucent clouds (NLCs) play an important indicative role in the physics of the summer polar mesopause. They consist of tiny ice crystals with characteristic dimensions generally smaller than 200 nm. However, the predominant shape of particles is not known. Therefore, biases in the size of crystals obtained from ground and space by light scattering and polarimetric techniques in the assumption of spherical scatterers can be considerable. This is due to the influence of shape effects on the scattering characteristics of particles.We test the assumption of the hexagonal and cubical particles as candidates for the predominant shapes of particles in NLCs using Maxwell electromagnetic theory to calculate the linear depolarization ratio (LDR). We compare results of recent measurements of LDRs with our calculations. Generally, theory and experiments agree very well at the NLC peak.The shape of crystals close to the cloud top cannot be explained by the model of compact particles. Relatively high light depolarization ratios detected from the upper part of the NLC are in agreement with models of elongated needle-like particles or particles having dimensions much larger than those usually attributed to NLC events.