On the nature and shape of noctilucent cloud particles

Abstract

The exact theory of the scattering of light from spheres, double-layer spheres, infinite long cylinders and coaxial cylinders is presented here in detail. The theory of scattering from spheres and infinite long cylinders is then applied to the noctilucent Cloud (NCL) problem. The intensity and polarization versus scattering angle, particle size, and wavelength for spherical particle scattering with index of refraction 1.33 (corresponding to ice and stone) were calculated with an IBM 1620 electronic computer and the results are compared with the available experimental data. The experimental data was also compared with the results of Deirmendjian, Clasen, etc., allowing conclusions with regard to the possibility of spherical pure metallic particles. The results indicate that the NLC particles are either stony dust or ice coated stony dust rather than pure metallic in nature. Consideration is given to the possibility of detecting through polarization and spectrographic studies the possible growth of NLC particles resulting from the formation of ice on them. If the NLC become visible only as a result of an increase of the number of particles, then the shape of the polarization versus scattering angle curve will not change, and the intensity versus wavelength curve will not change in shape but only in amplitude. However, if particle growth is responsible for the NLC becoming visible, then the shape of the polarization versus scattering angle curve will change. Careful experimental observations of these quantities should then answer this question about particle growth. A detailed analysis of the NLC particle sampling data obtained in Sweden during 1962 is made. A particle size distribution of the form Nα(diameter)⁻⁴ is required for the sampling data to be consistent with the polarization measurements that have been made.