Editing Thermosphere (section)

==Neutral gas constituents==
It is convenient  to separate the atmospheric regions  according to the two temperature minima at an altitude of about {{convert|12|km|mi}} (the [[tropopause]]) and at about {{convert|85|km|mi}} (the [[mesopause]]) (Figure 1). The thermosphere (or the upper atmosphere) is the height region above {{convert|85|km|mi}}, while the region between the tropopause and the mesopause is the middle atmosphere ([[stratosphere]] and [[mesosphere]]) where absorption of solar UV radiation generates the temperature maximum near an altitude of {{convert|45|km|mi}} and causes the [[ozone]] layer.
[[File:Nomenclature of Thermosphere.jpg|thumb|upright=2.2|left|Figure 1. Nomenclature of atmospheric regions based on the profiles of electric conductivity (left), temperature (middle), and electron number density in m<sup>−3</sup>(right)]]

The density of the Earth's atmosphere decreases nearly exponentially with altitude. The total mass of the atmosphere  is M = ρ<sub>A</sub> H&nbsp; ≃ 1&nbsp;kg/cm<sup>2</sup> within a column of one square centimeter above the ground (with ρ<sub>A</sub> = 1.29&nbsp;kg/m<sup>3</sup> the atmospheric density on the ground at z = 0 m altitude, and H ≃ 8&nbsp;km the average atmospheric [[scale height]]). Eighty percent of that mass is concentrated within the [[troposphere]]. The mass of the thermosphere above about {{convert|85|km|mi}} is only 0.002% of the total mass. Therefore, no significant energetic feedback from the thermosphere to the lower atmospheric regions can be expected.

Turbulence causes the air within the lower atmospheric regions below the [[turbopause]] at about {{convert|90|km|mi}} to be a mixture of gases that does not change its composition. Its mean molecular weight is 29&nbsp;g/mol with molecular oxygen (O<sub>2</sub>) and nitrogen (N<sub>2</sub>) as the two dominant constituents. Above the turbopause, however, diffusive separation of the various constituents is significant, so that each constituent follows its barometric height structure with a scale height inversely proportional to its molecular weight. The lighter constituents atomic oxygen (O), helium (He), and hydrogen (H) successively dominate above an altitude of about {{convert|200|km|mi|0}} and vary with geographic location, time, and solar activity.  The ratio
N<sub>2</sub>/O which is a measure of the electron density at the ionospheric F region is highly affected by these variations.<ref>Prölss, G.W., and M. K.  Bird, "Physics of the Earth's Space Environment", Springer Verlag, Heidelberg, 2010</ref> These changes follow from the diffusion of the minor constituents through the major gas component during dynamic processes.

The thermosphere contains an appreciable concentration of elemental [[sodium]] located in a {{convert|10|km|mi|adj=on}} thick band that occurs at the edge of the mesosphere, {{convert|80|to|100|km|mi}} above Earth's surface. The sodium has an average concentration of 400,000 atoms per cubic centimeter. This band is regularly replenished by sodium sublimating from incoming meteors. Astronomers have begun using this sodium band to create "[[Laser guide star|guide stars]]" as part of the optical correction process in producing 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>