Editing Mesosphere (section)

==Dynamic features==
[[File:ISS-46 Soyuz TMA-17M reentry.jpg|thumb|300px|[[Afterglow]] of the [[troposphere]] (orange), the [[stratosphere]] (blue) and the '''mesosphere''' (dark) at which [[atmospheric entry]] begins, leaving smoke trails, such as in this case of a [[spacecraft]] reentry.]]
The main most important features in this region are strong zonal (East-West) winds, [[atmospheric tide]]s, internal atmospheric gravity waves (commonly called "[[gravity waves]]"), and [[planetary wave]]s. 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.

[[Noctilucent cloud]]s are located in the mesosphere. The upper mesosphere is also the region of the [[ionosphere]] known as the ''D layer'', which 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.

A {{convert|5|km|mi ft|abbr=on}} deep [[sodium layer]] is located between {{convert|80|-|105|km|abbr=on|mi 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 [[Sublimation (phase transition)|sublimating]] from incoming meteors. Astronomers have begun utilizing this sodium band to create "guide stars" as part of the [[adaptive optics|adaptive optical correction process]] used to produce ultra-sharp ground-based observations.<ref>{{cite web| url = http://www.laserfocusworld.com/articles/print/volume-52/issue-07/features/adaptive-optics-eso-s-very-large-telescope-sees-four-times-first-laser-light.html| title = Martin Enderlein et al., ''ESO's Very Large Telescope sees four times first light'', Laser Focus World, July 2016, pp. 22-24| date = 11 July 2016}}</ref> Other metal layers, e.g. iron and potassium, exist in the upper mesosphere/lower thermosphere region as well.

Beginning in October 2018,<ref>The "dunes" were first spotted by photographers in Finland and Sweden.</ref> a distinct type of [[aurora]] 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 {{convert|96|km|abbr=on|mi 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.<ref>{{cite web| url = https://www.smithsonianmag.com/smart-news/new-type-aurora-ripples-across-sky-horizontal-green-dunes-180974083/?spMailingID=41658140&spUserID=NzQwNDUzNzY2MzMS1&spJobID=1682695015&spReportId=MTY4MjY5NTAxNQS2| title = Wu, Katherine J. A New Type of Aurora Ripples Across the Sky in Horizontal Green "Dunes". Smithsonian Magazine (29 January 2020)}}</ref>

Millions of [[meteor]]s enter the Earth's atmosphere, averaging 40,000 tons per year.<ref>Leinert C.; Gruen E. (1990). "Interplanetary Dust". Physics and Chemistry in Space (R. Schwenn and E. Marsch eds.). Springer-Verlag. pp. 204-275</ref> The ablated material, called meteoric smoke, is thought to serve as condensation nuclei for [[noctilucent clouds]].