Size and shape of ice grains in the mesopause region
The paper is devoted to optical remote-sensing techniques successfully applied to investigations of the mesospheric aerosol layer, starting with a short history of the development of optical sounding experiments. In the following part, a theoretical basis of scattering measurements is discussed. A brief description of the theoretical principles of optical analysis of aerosol properties, ending with a survey of current activity and, as yet, unresolved problems is presented. It can be concluded that there is a consensus regarding the size range of mesospheric aerosol particles. The contribution of the smallest particles cannot be assessed by current optical methods. The combination of the scattering measurement with an electromechanical impact detector is a promising technique. The true size distribution, more correctly a statistical average of many possible distributions, must rely on numerical modelling. Regarding the shape of the particles, the noctilucent-cloud particles are not perfect spheres. New insight into this problem can be expected from the ongoing observational program of the Atmospheric Chemistry Experiment (ACE) and Aeronomy of Ice in the Mesosphere (AIM).