The Mesosphere
transition between atmosphere and space...
The mesosphere (after Greek mesos = middle and sphaira = ball) is in the middle of the five layers of the Earth's atmosphere. It is the layer of the Earth's atmosphere that is located directly above the stratosphere and directly below the thermosphere. The mesosphere extends from about 50 km to 80-90 km altitude above Earth's surface. Due to the low density and the fact that there is hardly any more ozone and the absorption of high-energy UV radiation takes place in the stratosphere, the temperature drops again from around 0 ° C at the stratopause with seasonal fluctuations to an average of around -90 ° C at an altitude of about 80 kilometers. The lapse rate is about 3°C/km and thus lower than that of the troposphere. Carbon dioxide has an important role in the temperature structure due to the radiation of infrared radiation into space. At the top of the mesosphere, the mesopause, temperatures drop below -100°C, varying according to latitude and season.
The role of the mesosphere
As a transition region between the lower atmosphere and outer space, the mesosphere is particularly sensitive to external influences from the sun and changes in the lower atmosphere. In addition to its fundamental importance, the mesophere is also important for the areas of space travel, radio wave propagation, space weather and Aurora. Dynamics, chemistry and thermal structure can change very quickly due to natural and / or anthropogenic influences. In this context, atmospheric waves are particularly important which - starting from their sources in the troposphere and stratosphere - spread upwards and thus transport energy and momentum into the mesosphere. In this way, approximately 1016 joules are transported from the lower to the upper atmosphere every day. That is approximately 115 gigawatts and corresponds approximately to the output of 100 nuclear power plants. The transport occurs through small-scale gravity waves, through large-scale planetary waves and through tides. This roughly corresponds to the daily energy input due to geomagnetic storms (on average every 5 days). Particle incidence, radiation, chemistry and dynamics are also the driving force behind the global circulation systems in the mesosphere, which in turn can feed back into tropospheric and stratospheric processes and thus possibly have an impact on long-term weather forecast and climate. The most important coupling processes are summarized in the figure. In the summer hemisphere the air is transported upwards leading to adiabatic cooling, in the winter hemisphere the air transported downwards leading to adiabatic heating. Thus, on the summer hemisphere the mesosphere is much colder than on the winter hemisphere.
As a transition region between the lower atmosphere and space the mesosphere is extremely sensitive to changes of external influences (solar activity) and changes in the lower parts of atmosphere. Because it is located between the maximum altitude for balloons and the minimum altitude for orbital spacecraft, the mesosphere is the most poorly investigated part of the atmosphere. Our present understanding of the mesosphere and the coupling processes with other parts of the atmosphere is far from being complete. Open questions are:
- What are the spatial and temporal variations in temperature, winds and chemical composition, and what is the reason for these variations?
- How large are atmospheric waves and what are their sources? How do they interact and how do they affect mesospheric circulation?
- How do radiation, chemistry, and dynamics contribute to the balance of energy and momentum?
- Is there any anthropogenic effect on the mesosphere?
These questions have lead to increasing research activities in the last years. Numerical models have been developed covering the atmosphere from the Earth's surface to the thermosphere. Ground-based instruments have been improved. A number of satellites have been launched to measure temperatures, wind, and chemical composition (NIMBUS-7, UARS, CRISTA, TIMED). International scientific programs such as PSMOS (Planetary Scale Mesopause Observing System), CAWSES (Climate and Weather in the Sun-Earth System), and NDMC (Network for the Detection of Mesopause Change) have been initiated to increase our understanding of processes affecting and changing the mesosphere.
Mesopause
The mesopause, at an altitude of 80-90 km, separates the mesosphere from the thermosphere. This is also around the same altitude as the turbopause, below which different chemical species are well mixed due to turbulent eddies. Above this level the atmosphere becomes non-uniform; the scale heights of different chemical species differ by their molecular weights.
Contributions of our Group
Our group is engaged in a number of research activities dealing with processes in the mesosphere. Our ground-based measurements (GRIPS) of the mesopause temperature at night, which is in operation since 1980, contribute to the investigation of global change in the upper atmosphere.
Our satellite instrument CRISTA provided three-dimensional trace gas distributions with high spatial resolution, which helped to understand the dynamical processes in this region of our atmosphere. Our group also participates in the CAWSES program. One objective is the analysis and interpretation of atmospheric waves from data of NASA's TIMED satellite.
