Why is the upper atmosphere relevant for life on earth?

Air pressure in the upper atmosphere is so low that you cannot breath, and your blood would boil unless your body was protected by a space suit. So, why should we care? Astronauts are also part of "life on earth", and therefore their opinion also counts: When they approach earth from space, their space ship suffers an atmospheric breaking effect already as high as 120 km (this is what NASA calls "re-entry interface").

And for people on the ground? See below, "mesopause change".

There is also a coupling between the upper and lower atmosphere. This means that the global circulation of the atmosphere cannot be understood by studying only the lower atmosphere. The global circulation is influenced by waves that can break in the upper atmosphere. Those waves are produced in the lower atmosphere, but their upward propagation is favored or inhibited by wind systems at intermediate levels. The old idea that the "stratification" of the stratosphere means that interactions between atmospheric layers are impossible, and meteorology therefore has to deal only with the lower atmosphere, the "troposphere", is a thing of the past. The atmosphere can only be successfully described and understood as a whole. The upper limit of this "whole" is continually shifted upwards by the need to include more and more of the upper atmosphere in order to model the atmosphere reliably.


What is airglow?

Airglow is part of the reason why the night sky appears brighter than from starlight alone (at places far from contamination by artificial illumination). This is light emitted directly by the upper atmosphere. In the mesopause region (at altitudes of 80 to 100 km), the main source of airglow are hydroxyl molecules (OH), from a narrow layer at 87 km (and less than about ten kilometers thick). There is also emission from oxygen atoms (a green spectral line), and of sodium (a pair of yellow lines). OH airglow becomes most intense in the near infrared, where also emissions from molecular oxygen (O2) play a role. The O2 emission comes from an altitude at 95 km, close to where the atomic oxygen green line comes from.

The molecular emissions can be used to determine temperature at the respective emission heights. Doing so with the atomic lines requires very high spectral resolution, but it can also be done.

See also the definition used by the American Meteorological Society.


What is a "network"?

Like an "old boy network", it´s a social system where people with similar background help each other.

But also, nowadays it´s a way of using hi tech communications to bring people from different parts of the world together to make them work more efficiently.

In NDMC, neither are we all "old", nor are we all "boys"; that´s why the second interpretation applies.


What is "mesopause change"?

While "global change" means increasing temperatures in the lower atmosphere due to increased shielding of outgoing infrared radiation by higher concentration of greenhouse gases, above heights where greenhouse gases are effective, temperatures must go down. This cooling effect is expected to be much stronger than the heating at ground level, because of the smaller heat capacity of the rarified air, so that the long-term trend should be easier to detect in the upper atmosphere.

In the mesopause region, where temperature data are readily available from airglow observations for up to the last few decades, we have first indications that such a cooling does indeed exist, more strongly in some places, and weaker in other parts, so that efforts to intensify and extend these studies are worthwhile.