NLC Climatology from GOMOS Observations

Noctilucent clouds (NLC), also called polar mesospheric clouds when observed from space, are the visible manifestation of water ice particles persistently present in the polar summer mesopause region, which is the coldest place on Earth. Because of their extraordinary height of about 83 km, they can become visible to the naked eye when the sun sinks below the horizon, providing a dazzling display of bluish light. Since these clouds are extremely sensitive to changes in their environment, their observation conveys unique information concerning the various processes taking place in the mesosphere. GOMOS is a stellar occultation instrument combining 4 spectrometers in the spectral range 250 to 950 nm (UV - visible - near IR) and 2 fast photometers (470-520 nm and 650-700 nm). On the day side, in addition to star light, GOMOS measures also the solar light scattered by the atmospheric molecules. In the summer polar days, NLC are clearly detected using the photometers signals. The sun-synchronous orbit of ENVISAT allows observing them in both hemispheres. The main properties of these clouds (occurrence frequency, radiance, altitude) have been retrieved from GOMOS data. A very high accuracy is possible thanks to the stellar occultation technique. Moreover, the observation of these clouds with the spectrometers provides the spectral dependence of the light scattered by the NLC particles, from which it is possible to derive their radii. These clouds at the edge of space have been studied using GOMOS data from 2002 to 2010. After a brief overview of retrieval methods, the climatology obtained for the main NLC characteristics will be presented, focusing on the seasonal and latitudinal coverage