Particle characteristics from the spectra of polar mesospheric clouds

Abstract

Spectrographic imagers on the Midcourse Space Experiment (MSX) satellite have obtained the first spectra of polar mesospheric clouds (PMCs) from the far ultraviolet to the visible (164–585 nm). Several hundred PMC spectra were extracted over a 3 week period during the summer of 1999. To improve statistics, the spectra were normalized and averaged within 32 groups of clouds having common latitudes and scattering angles. The subsequent spectra, from three spectrographic imagers, produced a PMC signal that fell into the noise at wavelengths short of 200 nm and showed Earth albedo effects longward of ∼315 nm. The spectra between these limiting wavelengths were subjected to a least squares analysis to determine characteristics of the scattering particles assuming a lognormal distribution of Mie scatterers made of ice. Averaged over all PMCs, the distribution mode (size) was 65.2 ± 2.2 nm and the distribution dispersion (width) was 1.15 ± 0.04. Over the period of observation, neither the mode nor the dispersion exhibited systematic variations in time or latitude, although less intense clouds at lower latitudes may have been undersampled.