The Effect of Polar Mesospheric Clouds on the Environment at the Edge of Space
Lying in a thin layer at the edge of space (82 km altitude), Polar Mesospheric Clouds (PMCs) are earth’s highest clouds. Formed from water ice nucleated on nanometer size meteoric dust particles, PMCs are a manifestation of the extreme environmental conditions present in the upper mesosphere. Paradoxically, PMCs only form during summer because the summer mesosphere is much colder than the winter. This is ultimately due to the nature of the mesospheric circulation in which strong upwelling and cooling are associated with summertime meteorology. PMCs are also of interest because their sensitivity to temperature and humidity should make them indicators of changing solar activity and climate conditions. Much of the research associated with the NASA Aeronomy of Ice in the Mesosphere (AIM) Explorer has focused on separating out the relative roles of meteorology and solar forcing on PMC occurrence. Most recently, there has been interest in the converse problem, namely, how does PMC formation feed back and affect their environment? Satellite data has shown regions of dehydration which seem to propagate from the PMC formation region. Using general circulation models we are now simulating how PMC formation can influence of the chemical composition of the mesosphere and perhaps the overlying thermosphere. This may lead to new insights on how constituents such as water and hydrogen flow upwards and ultimately escape the Earth’s atmosphere.