The Mesosphere and Lower Thermosphere
The Mesosphere and Lower Thermosphere (MLT) region is that part of the Earth’s atmosphere at heights between ~ 50 – 110 km. It includes the mesopause, at heights near 85 km or 100 km, which is the coldest naturally occurring place on Earth. The MLT region is believed to play a key role in the coupling between the lower, middle and upper atmosphere.
The dynamics of the MLT region are dominated by atmospheric waves and tides of large amplitude, mostly originating in the lower atmosphere.
As the waves and tides ascend into the rarified MLT region they grow in amplitude and eventually may become unstable - dissipating their energy and momentum rather like ocean waves breaking on a beach. Thermal tides and planetary waves thus combine with gravity waves to provide up to 70% of the MLT region’s energy and momentum budget. This wave driving (forcing) pushes the temperature and planetary-scale circulation of the MLT region into states very different from those predicted by a simple radiative equilibrium model. Physical processes acting in the MLT then filter the field of ascending waves and tides and so control the dynamic coupling of the middle atmosphere to the upper atmosphere.