Upper atmosphere
The middle layer of the Uranianatmosphere is the stratosphere, where temperature generally increaseswith altitude from 53 K (−220 °C; −364 °F) in the tropopause tobetween 800 and 850 K (527 and 577 °C; 980 and 1,070 °F) at thebase of the thermosphere. The heating of the stratosphere is causedby absorption of solar UV and IR radiation by methane and otherhydrocarbons, which form in this part of the atmosphere as a resultof methane photolysis. Heat is also conducted from the hotthermosphere. The hydrocarbons occupy a relatively narrow layer ataltitudes of between 100 and 300 km corresponding to a pressure rangeof 1000 to 10 Pa and temperatures of between 75 and 170 K (−198 and−103 °C; −325 and −154 °F). The most abundant hydrocarbonsare methane, acetylene and ethane with mixing ratios of around 10−7relative to hydrogen. The mixing ratio of carbon monoxide is similarat these altitudes. Heavier hydrocarbons and carbon dioxide havemixing ratios three orders of magnitude lower. The abundance ratio ofwater is around 7×10−9. Ethane and acetylene tend to condense inthe colder lower part of stratosphere and tropopause (below 10 mBarlevel) forming haze layers, which may be partly responsible for thebland appearance of Uranus. The concentration of hydrocarbons in theUranian stratosphere above the haze is significantly lower than inthe stratospheres of the other giant planets.
The outermost layer of the Uranianatmosphere is the thermosphere and corona, which has a uniformtemperature around 800 to 850 K. The heat sources necessary tosustain such a high level are not understood, as neither the solar UVnor the auroral activity can provide the necessary energy to maintainthese temperatures. The weak cooling efficiency due to the lack ofhydrocarbons in the stratosphere above 0.1 mBar pressure level maycontribute too. In addition to molecular hydrogen, thethermosphere-corona contains many free hydrogen atoms. Their smallmass and high temperatures explain why the corona extends as far as50,000 km (31,000 mi), or two Uranian radii, from its surface. Thisextended corona is a unique feature of Uranus. Its effects include adrag on small particles orbiting Uranus, causing a general depletionof dust in the Uranian rings. The Uranian thermosphere, together withthe upper part of the stratosphere, corresponds to the ionosphere ofUranus. Observations show that the ionosphere occupies altitudes from2,000 to 10,000 km (1,200 to 6,200 mi). The Uranian ionosphere isdenser than that of either Saturn or Neptune, which may arise fromthe low concentration of hydrocarbons in the stratosphere. Theionosphere is mainly sustained by solar UV radiation and its densitydepends on the solar activity. Auroral activity is insignificant ascompared to Jupiter and Saturn.
Magnetosphere
Before the arrival of Voyager 2, nomeasurements of the Uranian magnetosphere had been taken, so itsnature remained a mystery. Before 1986, scientists had expected themagnetic field of Uranus to be in line with the solar wind, becauseit would then align with Uranus's poles that lie in the ecliptic.
Voyager's observations revealed thatUranus's magnetic field is peculiar, both because it does notoriginate from its geometric centre, and because it is tilted at 59°from the axis of rotation. In fact the magnetic dipole is shiftedfrom Uranus's centre towards the south rotational pole by as much asone third of the planetary radius. This unusual geometry results in ahighly asymmetric magnetosphere, where the magnetic field strength onthe surface in the southern hemisphere can be as low as 0.1 gauss (10µT), whereas in the northern hemisphere it can be as high as 1.1gauss (110 µT). The average field at the surface is 0.23 gauss (23µT). Studies of Voyager 2 data in 2017 suggest that this asymmetrycauses Uranus's magnetosphere to connect with the solar wind once aUranian day, opening the planet to the Sun's particles. Incomparison, the magnetic field of Earth is roughly as strong ateither pole, and its "magnetic equator" is roughlyparallel with its geographical equator. The dipole moment of Uranusis 50 times that of Earth. Neptune has a similarly displaced andtilted magnetic field, suggesting that this may be a common featureof ice giants. One hypothesis is that, unlike the magnetic fields ofthe terrestrial and gas giants, which are generated within theircores, the ice giants' magnetic fields are generated by motion atrelatively shallow depths, for instance, in the water–ammoniaocean. Another possible explanation for the magneto-sphere'salignment is that there are oceans of liquid diamond in Uranus'sinterior that would deter the magnetic field.
