Download Why study the mesosphere? One can wonder about the importance

Why study the mesosphere?
One can wonder about the importance of the research of physical and chemical
phenomena in the mesosphere. We don't live there and apparently, there is no
process that threatens life on Earth, in contrast to the events in the stratosphere
and the "ozone hole".
As in many scientific domains, the study of the mesosphere is primarily
fundamental research, i.e. studying our environment and trying to understand why
certain phenomena occur. But mesospheric research can be applied research as
well, i.e. in this case, checking the effects of human activity on the mesosphere
and gathering information about the way in which we influence our own
environment.
Fundamental research
The "classical" atmosphere we live in contains 78% nitrogen and 21% oxygen; all
other chemical species form the remaining 1%. Winds are movements of air that
move all molecules, independently of their chemical composition, in a similar way.
They are the main transport mechanism in the troposphere and the stratosphere.
The region above the mesosphere is the thermosphere, where artificial satellites
orbit the Earth. Here, air is extremely rarefied and the composition is rather
variable depending on time and place. The main transport phenomenon there is
"molecular diffusion", which influences the distribution of the species differently
according to their molecular mass. As a consequence of the very small density of
air, the differences in temperature between day and night are considerable. A large
part of the gas molecules are ionised; they carry an electric charge and are
therefore governed by physical laws that differ totally from the laws governing
neutral gasses.
In many ways the mesosphere is a "crossing zone" between these two completely
different areas. The physical and chemical processes are progressively sliding
from one regime into the other, leading to complex interactions between dynamical
phenomena (wind, turbulence, molecular diffusion), photochemistry (ozone,
nitrogen oxides) and heating (absorption of ultraviolet light, radiation of infrared
light). These interactions are about as complex as those in the troposphere, in
which we live, and we're still far away from a complete understanding of these
interactions.
The magnetosphere turns away the
charged particles, emitted by the Sun,
and prevents them from entering the
lower layers of the atmosphere,
except for the polar regions where
collisions between these particles
and the neutral air molecules in the
mesosphere cause the polar aurorae.
These "northerns lights" and "southern lights" are another research topic because
one still badly understands their spread and effects (heating, chemical reactions)
The fundamental research is a goal on itself; motivated by the human desire to get
to know and to understand our environment. Moreover applied research is
impossible without fundamental research.
Applied research
The mesosphere plays a role in two important atmospheric phenomena: the
"ozone hole" in the stratosphere, influenced by particular processes in the
mesosphere, and the global climate change, also called the "greenhouse effect",
for which the mesosphere gives us the first indications.
The size of the "ozone hole" depends largely on the circulation of air around the
poles. In the whole stratosphere winds are driven by dispersion of the gravitation
waves in the mesosphere. These waves are vertical oscillations of air masses,
which arise in the troposphere as a consequence of the winds above mountain
ranges and of storms. These waves then move upward, like water waves moving
in the sea. And like water waves breaking on the beach, gravitation waves end up
in the mesosphere because the air density is too weak to keep on transmitting the
waves. With this breaking, strong winds driving the total air circulation in the
stratosphere arise.
One of the necessary conditions for a complete prediction of the evolution of the
"ozone hole" is a complete understanding of the complex dynamic phenomena in
the mesosphere. This understanding is very difficult because these gravity waves
are very variable in time.
Concerning the global (worldwide) climate change, long-term measurements show
that the mesosphere has cooled during the last fifty years. This decreasing
temperature is probably caused by an increase in the amount of carbon dioxide
(CO2) due to human activities. Although carbon dioxide heats up the bottom layers
of the atmosphere by absorption of infrared radiation, carbon dioxide emits infrared
radiation in the mesosphere, of which half escapes the Earth's atmosphere. This
loss of energy causes a decreasing temperature.
These perturbations in the mesosphere have faster and more severe
consequences than those in the bottom layers of the atmosphere, due to the lower
air density in the mesosphere.
The perturbations can be observed with the naked eye in the form of polar
mesospheric clouds.
These clouds condense at very high
altitudes, around 80 km. Ordinary clouds
only appear in the troposphere. Polar
mesospheric clouds are probably
clouds of ice crystals becoming visible in
the skimming light of the setting sun.
It is only with extreme low temperatures
that the moistness in the rarefied air of
the mesosphere can condense into ice
crystals. In northern countries these clouds are seen more and more frequently.
This is an additional indication for a global change in the thermal condition of our
atmosphere.
The considerable sensitivity of the mesosphere to external perturbations makes it
an outstanding place to test our knowledge of physics and chemistry in the
atmosphere and to warn us for future changes in the lower parts of the
atmosphere. So either we can explain the evolution of the climate in the
mesosphere, and we will have more confidence in the validity of our predictions
concerning the influence of human activity on the global climate, or (more likely)
we will not be able to fully explain the evolution and we will be obliged to do further
research before we can rely on calculated predictions concerning the increase in
the carbon dioxide emissions and the related changes in our climate.