Ultraviolet imaging and spectrographic imaging of polar mesospheric clouds Original Research Article

The Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) instrument on the Midcourse Space Experiment (MSX) satellite obtained unique ultraviolet (200–350 nm) images and spectrographic images of polar mesospheric clouds over both the northern and southern poles during the summers of 1997–1999. Imager observations revealed that PMCs have essentially the same altitudes in the north and south (82.7 ± 1.3 km) and that cloud altitudes remain essentially constant with latitude. Imagery from below the horizon showed PMC structures with scales ranging from 100 to over 500 km. Integrated from 235 nm to 265 mn, PMC limb intensities were essentially the same in both hemispheres, averaging 12 mega-Rayleighs at peak altitude. Intensities increased with latitude toward the poles. Spectrographic observations reveal the clouds have a solar-like spectrum from 200 nm to 600 nm, but that short of 315 nm the spectrum is contaminated with ground albedo effects. Between 200 nm and 315 nm, the scattering ratio (PMC spectral intensity to solar irradiance) is less steep than that expected from pure Rayleigh scattering. When the scattering ratio is least-squares fit to a log-normal distribution of Mie scatterers, the distribution has a mode radius of 65 nm and a dispersion of 1.2 with no dependence on latitude or time from solstice. The slope of the scattering ratio changes as a function of altitude relative to the altitude peak of the PMC intensity, suggesting that particles 3 km above this peak are smaller than particles at the peak and 3 km below.