Optical characteristics of mesospheric aerosol distributions in relation to noctilucent clouds

Abstract

It has not hitherto been possible to establish whether the appearance of noctilucent clouds is due to an increased concentration or locally increased size of solid particles at the mesopause, or to a combination of these mechanisms. In this contribution the possibility of a relationship between the optical characteristics of noctilucent clouds and ice sublimation is discussed on the basis of present-day knowledge of the structure and photochemistry of the mesosphere. The first part is devoted t o a discussion of the character of the sublimation process at these altitudes. It is found that such a process is possible in conditions prevailing at the high-latitude summer mesopause. The time scale of sublimation and evaporation is slow compared with horizontal translation velocities observed in this region. Below the mesopause evaporation becomes rapid compared with the settling speed of sub-micron particles. In the second part the scattering properties of monodisperse and polydisperse particle distributions, and the possible changes following a sublimation process are reviewed on the basis of numerical computations of scattering functions of coated spherical particles. It is found that the optical consequences of sublimation depend critically upon the presence and relative concentration of particles obeying the Rayleigh scattering law in the visible spectrum. The characteristic time scale of brightness changes by sublimation is at least 10 to 100 minutes. Different optical methods for further investigation of the nature of mesospheric aerosols are summarized and a method is proposed for the detection of scattering layers in the mesosphere by means of ultraviolet intensity and polarization measurements from rockets and satellites.