PMC Particle Distributions from Inversion of MUV Spectra

The spectrographic imagers on the MSX satellite have made the first observations of the middle ultraviolet spectra of polar mesospheric clouds (200-320 nm). After suitable averaging, the PMC spectra from the peak altitudes of the clouds are divided by the solar irradiance to produce a scattering ratio as a function of wavelength. The scattering ratio can be expanded in terms of particle distribution weights and Mie scattering functions. This expansion can be cast in the form of a matrix equation in which one side is the vector of observed scattering ratios in wavelength and the opposite side is the product of a scattering function matrix and the particle distribution in size. Direct inversion of this equation determines the actual particle distribution without recourse to assumptions about its shape or amplitude. This method is applied to MSX data and the results compared to commonly-assumed PMC distributions such as Gaussian and lognormal.