Editing Thermosphere (section)

===Solar XUV radiation===
The solar X-ray and extreme ultraviolet radiation (XUV) at wavelengths < 170&nbsp; nm is almost completely absorbed within the thermosphere. This radiation causes the various [[Ionosphere|ionospheric layer]]s as well as a temperature increase at these heights (Figure 1).
While the solar visible light (380 to 780&nbsp; nm) is nearly constant with the variability of not more than about 0.1% of the [[solar constant]],<ref>Willson, R.C., Measurements of the solar total irradiance and its variability, Space Sci. Rev., '''38''', 203, 1984</ref> the solar XUV radiation is highly variable in time and space. For instance, X-ray bursts associated with [[solar flares]] can dramatically increase their intensity over preflare levels by many orders of magnitude over some time of tens of minutes. In the extreme ultraviolet, the Lyman α line at 121.6&nbsp;nm represents an important source of [[ionization]] and [[dissociation (chemistry)|dissociation]] at ionospheric D layer heights.<ref>Brasseur, G., and S. Salomon, "Aeronomy of the Middle Atmosphere", Reidel Pub., Dordrecht, 1984</ref> During quiet periods of [[solar activity]], it alone contains more energy than the rest of the XUV spectrum. Quasi-periodic changes of the order of 100% or greater, with periods of 27 days and 11 years, belong to the prominent variations of solar XUV radiation. However, irregular fluctuations over all time scales are present all the time.<ref>Schmidtke, G., Modelling of the solar radiation for aeronomical applications, in Flügge, S. (ed), Encycl. Phys. ''49/7'', Springer Verlag, Heidelberg, 1</ref> During the low solar activity, about half of the total energy input into the thermosphere is thought to be solar XUV radiation. That solar XUV energy input occurs only during daytime conditions, maximizing at the equator during [[equinox]].