Earth's atmosphere as it appears from space, as bands of different colours at the horizon. From the bottom,afterglow illuminates thetroposphere in orange with silhouettes of clouds, and thestratosphere in white and blue. Next themesosphere (pink area) extends to just below theedge of space at one hundred kilometers and the pink line ofairglow of the lowerthermosphere (dark), which hosts green and redaurorae over several hundred kilometers.Diagram showing the five primary layers of the Earth's atmosphere:exosphere,thermosphere, mesosphere,stratosphere, andtroposphere. From Earths surface to the top of the stratosphere (50 km) is just under 1% of Earth's radius.
Themesosphere (/ˈmɛsəsfɪər,ˈmɛz-,ˈmiːsə-,-zə-/;[1] from Ancient Greekμέσος (mésos)'middle' and -sphere) is the third layer of theatmosphere, directly above thestratosphere and directly below thethermosphere. In the mesosphere,temperature decreases asaltitude increases. This characteristic is used to define limits: it begins at the top of the stratosphere (sometimes called thestratopause), and ends at themesopause, which is the coldest part ofEarth'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 47 to 51 km (29 to 32 mi; 154,000 to 167,000 ft) abovesea level, and the upper boundary (the mesopause) is usually from 85 to 100 km (53 to 62 mi; 279,000 to 328,000 ft).[2][3][4][5]
The stratosphere and mesosphere are sometimes collectively referred to as the "middle atmosphere",[6] which spans altitudes approximately between 12 and 80 km (7.5 and 49.7 mi) above Earth's surface. Themesopause, at an altitude of 80–90 km (50–56 mi), separates the mesosphere from thethermosphere—the second-outermost layer of Earth's atmosphere. On Earth, the mesopause nearly co-incides with theturbopause, below which different chemical species are well-mixed due to turbulenteddies. Above this level the atmosphere becomes non-uniform because thescale heights of different chemical species differ according to theirmolecular masses.
The termnear 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 roughly between theArmstrong limit (about 62,000 ft or 19 km, above which humans require apressure suit in order to survive) and theKármán line (whereastrodynamics must take over fromaerodynamics in order to achieve flight); or, by another definition, to the space between the highest altitude commercialairliners fly at (about 40,000 ft (12.2 km)) and the lowest perigee ofsatellites being able to orbit the Earth (about 45 mi (73 km)). 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 increasingheight. This is a result of decreasing absorption of solar radiation by the rarefied atmosphere having adiminishing relative ozone concentration as altitude increases (ozone being the main absorber in the UV wavelengths that survived absorption by the thermosphere).[7] Additionally, this is also a result of increasing cooling byCO2radiative emission. The top of the mesosphere, called themesopause, is the coldest part of Earth's atmosphere.[8] Temperatures in the upper mesosphere fall as low as about −100 °C (173 K; −148 °F),[9] varying according tolatitude andseason.
The main most important features in this region are strong zonal (East-West) winds,atmospheric tides, internal atmospheric gravity waves (commonly called "gravity waves"), andplanetary waves. Most of these tides and waves start in thetroposphere and lowerstratosphere, 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.
Noctilucent clouds are located in the mesosphere. The upper mesosphere is also the region of theionosphere known as theD layer, which is only present during the day when someionization occurs withnitric oxide being ionized byLyman series-alpha hydrogen radiation. The ionization is so weak that when night falls, and the source of ionization is removed, the free electron andion form back into a neutral molecule.
A 5 km (3.1 mi; 16,000 ft) deepsodium 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 theairglow. The sodium has an average concentration of 400,000 atoms per cubic centimetre. This band is regularly replenished by sodiumsublimating from incoming meteors. Astronomers have begun utilizing this sodium band to create "guide stars" as part of theadaptive optical correction process used to produce ultra-sharp ground-based observations.[10] Other metal layers, e.g. iron and potassium, exist in the upper mesosphere/lower thermosphere region as well.
Beginning in October 2018,[11] a distinct type ofaurora has been identified, originating in the mesosphere. Often referred to as 'dunes' due to their resemblance to sandy ripples on a beach, the green undulating lights extend toward the equator. They have been identified as originating about 96 km (60 mi; 315,000 ft) above the surface. Since auroras are caused by ultra-high-speed solar particles interacting with atmospheric molecules, the green color of these dunes has tentatively been explained by the interaction of those solar particles with oxygen molecules. The dunes therefore occur where mesospheric oxygen is more concentrated.[12]
Millions ofmeteors enter the Earth's atmosphere, averaging 40,000 tons per year.[13] The ablated material, called meteoric smoke, is thought to serve as condensation nuclei fornoctilucent clouds.
The mesosphere lies above altitude records foraircraft,[14] while only the lowest few kilometers are accessible toballoons, for which the altitude record is 53.0 kilometres (32.9 mi).[15] Meanwhile, the mesosphere is below the minimum altitude fororbitalspacecraft due to high atmospheric drag.[16][17][18] It has only been accessed through the use ofsounding rockets, which are only capable of taking mesospheric measurements for a few minutes per mission.[19] As a result, it is the least-understood part of the atmosphere, resulting in the humorous monikerignorosphere.[20][21] The presence ofred sprites andblue jets (electrical discharges orlightning within the lower mesosphere),noctilucent clouds, and density shears within this poorly understood layer are of current scientific interest.
An astronaut onboard theInternational Space Station observes lightning at the horizon extending into the mesosphere as redsprite just below the line ofairglow.Noctilucent clouds (not to be mistaken with the slightly higher upairglow), at the upper edge of the mesosphere.
