Mesosphere

The mesosphere (/ˈmɛsoʊsfɪər/; from Greek mesos, 'middle') is the third major layer of the Earth's atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it begins at the top of the stratosphere (sometimes called the stratopause), and ends at the mesopause, which is the coldest part of Earth's atmosphere with temperatures below −143 °C (−225 °F; 130 K). The exact upper and lower boundaries of the mesosphere vary with latitude and with season (higher in winter and at the tropics, lower in summer and at the poles), but the lower boundary is usually located at altitudes from 50 to 65 kilometres (31 to 40 mi; 164,000 to 213,000 ft) above the Earth's surface and the upper boundary (the mesopause) is usually around 85 to 100 kilometres (53 to 62 mi; 279,000 to 328,000 ft). The stratosphere and the mesosphere are sometimes collectively referred to as the 'middle atmosphere', which spans altitudes between approximately 10 and 100 kilometers above the Earth's surface. The mesopause, at an altitude of 80–90 km (50–56 mi), separates the mesosphere from the thermosphere—the second-outermost layer of the Earth's atmosphere. This is also approximately 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 because the scale heights of different chemical species differ according to their molecular masses. The term near space is also sometimes used to refer to altitudes within the mesosphere. This term does not have a technical definition, but typically refers to the region of the atmosphere up to 100 kilometres (62 mi; 330,000 ft), roughly between the Armstrong limit (above which humans require a pressure suit in order to survive) and the Kármán line (where astrodynamics must take over from aerodynamics in order to achieve flight); or, by another definition, to the range of altitudes below which commercial airliners fly but above which satellites orbit the Earth. Some sources distinguish between the terms 'near space' and 'upper atmosphere', so that only the layers closest to the Kármán line are described as 'near space'. Within the mesosphere, temperature decreases with increasing height, due to decreasing absorption of solar radiation by the rarefied atmosphere and increasing cooling by CO2 radiative emission. The top of the mesosphere, called the mesopause, is the coldest part of Earth's atmosphere. Temperatures in the upper mesosphere fall as low as −101 °C (172 K; −150 °F), varying according to latitude and season. The main dynamic features in this region are strong zonal (East-West) winds, atmospheric tides, internal atmospheric gravity waves (commonly called 'gravity waves'), and planetary waves. Most of these tides and waves start in the troposphere and lower stratosphere, and propagate to the mesosphere. In the mesosphere, gravity-wave amplitudes can become so large that the waves become unstable and dissipate. This dissipation deposits momentum into the mesosphere and largely drives global circulation. This helps the Earth. Noctilucent clouds are located in the mesosphere. The upper mesosphere is also the region of the ionosphere known as the D layer. The D layer is only present during the day when some ionization occurs with nitric oxide being ionized by Lyman series-alpha hydrogen radiation. The ionization is so weak that when night falls, and the source of ionization is removed, the free electron and ion form back into a neutral molecule. The mesosphere has been called the 'ignorosphere' because it is poorly studied relative to the stratosphere (which can be accessed with high-altitude balloons) and the thermosphere (in which satellites can orbit). A 5 km (3.1 mi; 16,000 ft) deep sodium layer is located between 80–105 km (50–65 mi; 262,000–344,000 ft). Made of unbound, non-ionized atoms of sodium, the sodium layer radiates weakly to contribute to the airglow. The sodium has an average concentration of 400,000 atoms per cubic centimetre. This band is regularly replenished by sodium sublimating from incoming meteors. Astronomers have begun utilizing this sodium band to create 'guide stars' as part of the adaptive optical correction process used to produce ultra-sharp ground-based observations. Other metal layers, e.g. iron and potassium, exist in the upper mesosphere/lower thermosphere region as well. Millions of meteors enter the Earth's atmosphere, averaging 40,000 tons per year. The ablated material, called meteoric smoke, is thought to serve as condensation nuclei for noctilucent clouds.