The middle layer of the Uranian atmosphere is the stratosphere, where temperature generally increases with altitude from 53 K in the tropopause to between 800 and 850 K at the base of the thermosphere.[67] The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons,[77] which form in this part of the atmosphere as a result of methane photolysis.[72] Heat is also conducted from the hot thermosphere.[77] The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 300 km corresponding to a pressure range of 10 to 0.1 mbar (1000 to 10 kPa) and temperatures of between 75 and 170 K.[70][73] The most abundant hydrocarbons are methane, acetylene and ethane with mixing ratios of around 10-7 relative to hydrogen. The mixing ratio of carbon monoxide is similar at these altitudes.[70][73][75] Heavier hydrocarbons and carbon dioxide have mixing ratios three orders of magnitude lower.[73] The abundance ratio of water is around 7×10-9.[74] Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause (below 10 mBar level) forming haze layers,[72] which may be partly responsible for the bland appearance of Uranus. The concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets.[70][78]
The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature around 800 to 850 K.[12][78] The heat sources necessary to sustain such a high value are not understood, since neither solar far UV and extreme UV radiation nor auroral activity can provide the necessary energy. The weak cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1 mBar pressure level may contribute too.[67][78] In addition to molecular hydrogen, the thermosphere-corona contains many free hydrogen atoms. Their small mass together with the high temperatures explain why the corona extends as far as 50 000 km or two Uranian radii from the planet.[67][78] This extended corona is a unique feature of Uranus.[78] Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.[67] The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus.[69] Observations show that the ionosphere occupies altitudes from 2 000 to 10 000 km.[69] The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere.[78][79] The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.[80] Auroral activity is insignificant as compared to Jupiter and Saturn
