Investigation of the shape of noctilucent cloud particles by polarization lidar technique

Abstract

We report on the shape of particles in a noctilucent cloud (NLC) as deduced from measurements by groundbased lidar. The experiment was performed on August 3, 2000 using the ALOMAR Rayleigh/Mie/Raman lidar, located at 69°N, 16°E. Over a period of 74 min, the instrument performed high quality measurements of the polarization state of 532 nm laser light backscattered from NLC particles. From the experiment we derive that the observed depolarization, averaged over the altitude range 84.2 to 85.5 km was δNLC = (1.7 ± 1.0) %. Considering the small ratio of particle size over wavelength this is an unexpectedly large depolarization. The layer of enhanced depolarization was centered 1 km above the maximum of the NLC layer. We compare the observed depolarization with that calculated for cylinder-shaped NLC particles. The observed depolarization can be explained by the presence of elongated particles with a length-over-diameter ratio larger than 2.5.