Solar Mesosphere Explorer measurements of polar mesospheric clouds (noctilucent clouds)

Abstract

The Ultraviolet Spectrometer experiment on board the Solar Mesosphere Explorer satellite has measured scattering of sunlight from the polar mesospheric cloud layer in the 0.2–0.3 μm spectral range. The layer is manifested in the limb-scanning measurements as large increases in radiance at heights near 80 km, at latitudes 60–90°N and 60–90°S and during the summer season. They are similar to noctilucent clouds with respect to their height, geometrical thickness (less than 3.5 km) and morphology. However, as shown by Donahue et al. (1972), they are much brighter and occupy the entire polar ‘cap’ area, a region largely inaccesible to ground-based observation. The measurements have revealed a forward-scattering asymmetry, which increases with layer brightness. This behaviour shows that the brighter clouds are composed of particles with radii according to Mie theory up to 0.07 μm, at least for the limited set of data studied so far. The variation of layer brightness with asymmetry factor is consistent with the layer being limited by the available water content of the atmosphere. The calculated water content of the particles, assuming them to consist of pure water ice, is about 100 μg m −2, provided the particle distribution is monodisperse. This corresponds to the total amount of atmospheric water vapor residing in a vertical column above 80 km for a water vapor mixing ratio of 1.2 ppmv. This is consistent with the amount of water vapor believed to exist at mesopause heights (a few ppmv). The large amounts of ice reported by Donahue et al. (1972) are too large by a factor ten. Their corrected values are consistent with the present analysis. A brief description is given concerning additional research topics which are being pursued using the extensive SME data base, which now consists of five complete summer seasons from 1981 to the present.