This is also the region where cosmic dust particles entering the atmosphere ablate and inject a range of metals like Fe, Mg and Na (Plane et al., 2015). The dominant cooling process is via emission at 15 µm from CO 2 its degenerate bending vibrational mode is efficiently excited by a collision with O atoms (Castle et al., 2012). A roughly similar amount of molecular kinetic energy is deposited from below by the breaking of gravity waves. This generates atomic O, which participates in highly exothermic reactions (Sect. 4) and converts chemical potential energy into kinetic energy. The dominant process is photodissociation of O 2 through absorption in its Schumann–Runge continuum (130–175 nm) and the Schumann–Runge bands (175–195 nm), with a less important contribution from O 3 photolysis (Mlynczak et al., 2013). The resulting photodissociation, photo-ionisation and high-energy collisions generate radicals and ions, often with internal excitation. The MLT is subject to high-energy inputs from space in the form of solar electromagnetic radiation and energetic particles (mostly electrons and protons of solar origin Sinnhuber et al., 2012). Figure 1 shows schematically the important processes governing its composition and chemistry. The mesosphere and lower thermosphere (MLT) is the region between about 70 and 120 km. The influx and ablation of meteoroids lead to a complex chemistry involving metal species and ultimately the formation of meteoric smoke particles, which affect charge balance and ice cloud nucleation. A prominent feature of MLT photochemistry is the emission of airglow (brown arrows arranged in a star). The upward transport (on average) of H 2O, CH 4, CO 2, N 2O and chlorofluorocarbons (CFC) and downward transport of NO x (NO + NO 2) are shown with blue arrows. Through global-scale transport, this also affects chemistry at lower altitudes. Both drive the dissociation and ionisation of major constituents, thus initiating a wealth of chemistry that includes excited and ionised species. Important energy inputs from above are solar radiation, including energetic radiation in the ultraviolet and X-ray spectrum penetrating to the MLT (brown arrows), and energetic particle precipitation (purple arrows), which is connected to solar wind and geomagnetic activity. A large portion of gravity waves break and deposit their momentum in the MLT region, thus driving the MLT general circulation and controlling thermal structures and transport patterns. Wave activity is central for connecting the MLT region to the dynamics of the lower atmosphere (shown schematically with green arrows). The temperature profile to the left (red line) is representative of the high-latitude summer conditions, thereby depicting the extremely cold mesopause that allows for the existence of noctilucent clouds (NLCs). The vertical temperature structure is the basis for defining the layers of the atmosphere. Figure 1Schematic overview of important processes governing the composition and chemistry of the mesosphere and lower thermosphere (MLT).
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