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Mesosphere
The mesosphere is the region of the atmosphere between the stratosphere and the thermosphere, and is characterized by a decrease of temperature with altitude. The upper limit of the mesosphere, the mesopause, is at the altitude at which the temperature reaches a minimum, which is variable on multiple time scales. Many fascinating features are associated with this region of the Earth atmosphere: for example its temperature under polar sunlight is the coldest temperature found naturally on or near Earth, often reaching lower than 110oK. Furthermore, even though there is very little water vapor the highest clouds are found in the mesosphere: these are ice crystal clouds, termed Noctilucent Clouds. It is also the region where turbulent mixing of the lower and middle atmosphere ends and molecular diffusion becomes the dominant transport process. This leads to composition changes: whereas below the mesosphere composition is constant, above the mesosphere it changes drastically with altitude such that the heavier species are concentrated lower down while the light ones dominate at the higher altitudes. Due to these features the mesosphere has been characterized as a transition region, in which many processes of various spatiotemporal scales are taking place; the key processes and current open issues in mesosphere research are discussed in the relevant chapters.
Thermosphere
In the thermosphere, from the Greek word θερμό (thermό) for heat, and at altitudes > 80 km, molecular diffusion is the dominant transport process, and the composition changes drastically with altitude such that the heavier species are concentrated lower down, while the light ones dominate at the higher altitudes. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. Temperatures can rise to 2,000o C. The gases in the Thermosphere exist mainly in the form of thin plasma, i.e. they are ionized due to bombardment by solar ultraviolet radiation and energetic cosmic rays.
Ground Measurements
There are various experiments and techniques to sample the Upper Atmosphere from the ground, such as Lidars, Ionosondes, Incoherent Scatter radarsIncoherent Scatter radars (ISR), Coherent scatter radars (CSR),Auroral Imaging], Photometers and Fabry-Perot Interferometers. There is a wealth of information that these measurements are providing, and there are significant advances in MLT science that have been accomplished, but there are also limitations that arise from the nature of Remote Sencing Techniques. For example, density and temperature measurements are unfortunately not possible in the 120 km region, as radiances become too weak and non-thermal for density and temperature measurements above that altitude. Some major species composition information is obtained by a combination of UV, IR and FPI measurements, but there is a significant gap in the obtainable profiles at ~100-140 km due to lack of appropriate emissions for observation.
Spacecraft Measurements
The lowest in-situ scientific measurements performed in this region by orbiting vehicles were made by the AE (Atmosphere Explorers) series of satellites in the 1970’s. The initial elliptical phase of these satellites extended as low as 140 km, but the interpretation of mass spectrometer composition data has been difficult at low altitudes. Since then, in-situ measurements of composition and MLT processes have been limited to short crossings by Sounding Rockets. Density measurements as low as 130 km inferred from the decay of low-altitude surveillance satellites have been useful for the understanding of the gross features of the lower thermosphere, but variability and composition of the transition region near 120 km remains obscure. At higher altitudes, a series of spacecraft have provided measurements of density and electric fields (AE (Atmosphere Explorers),DE (Dynamics Explorers), DMSP (Defense Meteorological Satellite Program), CHAMP, DEMETER, GRACE), but these are far from the transition region between the Mesosphere and the Thermosphere, which remains undersamled. One of the early objectives of the TIMED (Thermosphere-Ionosphere-Mesosphere Energetics & Dynamics) mission was to perform exploratory in-situ measurements as low as 120 km by use of a low-perigee elliptical orbit in the early mission phase; however this mission objective was lost to the descope process. Thus information on the MLT region arrives almost exclusively from remote sencing, either from satellites (SME, UARS, CRISTA, SNOE, Odin, TIMED, ENVISAT, AIM) or from various experiments and techniques from Ground measurements
