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 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.
Noctilucent clouds 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 deep
sodium layer is located between . 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. Beginning in October 2018, 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 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. 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. ==Exploration==