Properties of particles forming Noctilucent clouds (NLC)
Noctilucent clouds consist of ice particles by a size of approx. 50 nm, so only about 1/10 of the wavelength of visible light. The exact size and also the number of particles per volume is object of intensive research. In particular the properties of the particle ensemble like the volume density (amount of water per cloud) as well as the surface density (Reactive surface per cloud) are important for the understanding of the processes in NLC.
The ALOMAR RMR-Lidar is well equipped for the investigation of the particle properties. The size can be determined by observation of the clouds with three wavelengths from ultraviolet up to the infrared spectral range by comparison with modeled particle spectra. Details about the instrument construction and the used wavelengths can be found in the description of the ALOMAR RMR-Lidar. At the IAP different models are used to investigate the optical properties of Nano particles even including the effect of the particle shape, since we could show that the particles are not spherical [Baumgarten et al., 2007]).
During the years 1998-2005 we could determine the particle properties from 51 NLC events. These cloud observations were separated in 645 single measurements to take the temporal evolution in the clouds into account. The measurements of the recent years we could observe that 90% of the particle NLC show a size of 20-80 nm. In particular at most 1% of all particles are bigger than 100 nm. It is astonishing that the yearly averaged volume density of the NLC is almost steady over the years. I.e. if NLC are observed they transport the same amount of water. On the other hand, the annual water content in the ice phase changes by the variations of the frequency of NLC.