The
Effect of Polar
Mesospheric Clouds on the Environment at the Edge of Space
Abstract
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.