Vertical structure of NLCs observed by Odin/OSIRIS

The vertical variation of NLC particle sizes is investigated with limb scattering observations in the UV-B spectral range performed with the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite. The particle sizes are determined under the assumptions that NLC particles can be treated as Mie-scatterers with known refractive index, and that their size distribution can be approximated by a log-normal distribution. OSIRIS is particularly suited to resolve vertical NLC structure due to its narrow vertical field of view of about 1 km. A sample data set of several dozen bright NLCs detected on different days during the 2003 NLC season in the northern hemisphere and the 2003/2004 NLC season in the southern hemisphere was selected for this study. In almost all the cases the derived NLC particle size increases with decreasing altitude. This confirms the standard picture of larger particles at the bottom of the cloud due to sedimentation. The derived particle mode radii estimates fall within the range from about 20 to 60 nm assuming a width parameter of σ =1.4 for the log-normal distribution.