The Aeronomy of Ice in the Mesosphere (AIM) mission is the first satellite dedicated to the study of noctilucent clouds. Noctilucent clouds, sometimes called Polar Mesospheric Clouds, were first reported in 1885. Forming at altitudes above 50 miles, they are so faint that they can only be seen from the ground in the reflected light of the Sun after it has set below the horizon. Since their discovery, their cause has been a subject of study as a possible indicator of climate change. For those interested in observing noctilucent clouds from the ground, there are images and information at SpaceWeather's Gallery of Noctilucent Clouds.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 22.214.171.124.0