AIM: Aeronomy of Ice in the Mesosphere

The Aeronomy of Ice in the Mesosphere, or “AIM”, experiment is a NASA space mission designed to study the highest clouds in the earth’s atmosphere – clouds at the edge of space. These clouds are made of frozen water, or ice crystals, just like some of the clouds that appear in the sky every day. Unlike more common clouds that form up to 5 miles above the surface of the earth, these clouds are 50 miles high in a layer of the atmosphere called the mesosphere. Also unlike normal clouds, these clouds can only be seen near twilight, when the sun is just below the horizon and the sky is dark. For this reason, they are often called “noctilucent” clouds, or NLCs, because the word noctilucent means “night-shining”. Scientists also call these clouds “polar mesospheric clouds”, or PMCs for short, because they usually form only at high latitudes near the north and south poles.

In recent years, several people have reported seeing NLCs at lower latitudes, even as low as 40°N in the continental United States, in Utah and Colorado. Also, NLCs seem to be getting brighter over time. Scientists do not understand why this is happening, and would like to find out. In particular, they wish to determine if these changes are caused by natural variations in the earth’s atmosphere, or if they are influenced by human activities.

Electric blue clouds viewed from the International Space Station.
Don Pettit and NASA TV

The overall goal of the Aeronomy of Ice in the Mesosphere (AIM) experiment is to resolve why PMC's form and why they vary. By measuring PMC's and the thermal, chemical and dynamical environment in which they form, we will quantify the connection between these clouds and the meteorology of the polar mesosphere. In the end, this will provide the basis for study of a long-term variability in the mesospheric climate and its relationship to global change. The results of AIM will be a rigorous validation of predictive models that can reliably use past PMC changes and present trends as indicators of global change. This goal will be achieved by measuring PMC abundances, spatial distribution, particle size distributions, gravity wave activity, dust influx to the atmosphere and precise, vertical profile measurements of temperature H2O, OH, CH4, O3, CO2, NO, and aerosols. These data can only be obtained by a complement of instruments on an orbiting spacecraft.

Over the last 30 years ground based observations from NW Europe of the number of noctilucent clouds (NLC's) show dramatic increases. These clouds, known more recently to satellite observers as PMC's, are believed to respond dramatically to even small changes in their environment. Since cooling of the upper atmosphere (PMC's occur near 85 km) is expected to accompany the possible warming of the lower atmosphere due to an increased greenhouse effect, an increase in mesospheric cloudiness could be one consequence of mesospheric climate change.

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