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In the 1990s, the number of the observed bright cloud features grew considerably partly because new high resolution imaging techniques became available.[17] Most were found in the northern hemisphere as it started to become visible.[17] An early explanation – that bright clouds are easier to identify in the dark part of the planet, whereas in the southern hemisphere the bright collar masks them – was shown to be incorrect: the actual number of features has indeed increased considerably.[102][103] Nevertheless there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.[103] They appear to lie at a higher altitude.[103] The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours, while at least one southern cloud may have persisted since Voyager flyby.[17][98] Recent observation also discovered that cloud features on Uranus have a lot in common with those on Neptune.[17] For example, the dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature dubbed Uranus Dark Spot was imaged.[104] The speculation is that Uranus is becoming more Neptune-like during its equinoctial season.[105]

The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper troposphere of Uranus.[17] At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from -100 to -50 m/s.[17][99] Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located.[17][63] Closer to the poles, the winds shift to a prograde direction, flowing with Uranus's rotation. Windspeeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles.[17] Windspeeds at -40° latitude range from 150 to 200 m/s. Since the collar obscures all clouds below that parallel, speeds between it and the southern pole are impossible to measure.[17] In contrast, in the northern hemisphere maximum speeds as high as 240 m/s are observed near +50 degrees of latitude.

Before the arrival of Voyager 2, no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery. Before 1986, astronomers had expected the magnetic field of Uranus to be in line with the solar wind, because it would then align with Uranus's poles that lie in the ecliptic.[91]

Voyager's observations revealed that Uranus's magnetic field is peculiar, both because it does not originate from its geometric center, and because it is tilted at 59° from the axis of rotation.[91][92] In fact the magnetic dipole is shifted from the center of the planet towards the south rotational pole by as much as one third of the planetary radius.[91] This unusual geometry results in a highly asymmetric magnetosphere, where the magnetic field strength on the surface in the southern hemisphere can be as low as 0.1 gauss (10 µT), whereas in the northern hemisphere it can be as high as 1.1 gauss (110 µT).[91] The average field at the surface is 0.23 gauss (23 µT).[91] In comparison, the magnetic field of Earth is roughly as strong at either pole, and its "magnetic equator" is roughly parallel with its geographical equator.[92] The dipole moment of Uranus is 50 times that of Earth.[91][92] Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants.[92] One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giants, which are generated within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean.[58][93] Another possible explanation for the magnetosphere's alignment is that there are oceans of liquid diamond in Uranus's interior which would deter the magnetic field.[94]

Despite its curious alignment, in other respects the Uranian magnetosphere is like those of other planets: it has a bow shock located at about 23 Uranian radii ahead of it, a magnetopause at 18 Uranian radii, a fully developed magnetotail and radiation belts.[91][92][95] Overall, the structure of Uranus's magnetosphere is different from Jupiter's and more similar to Saturn's.[91][92] Uranus's magnetotail trails behind it into space for millions of kilometers and is twisted by its sideways rotation into a long corkscrew.[91][96]

Uranus's magnetosphere contains charged particles: protons and electrons with small amount of H2+ ions.[92][95] No heavier ions have been detected. Many of these particles probably derive from the hot atmospheric corona.[95] The ion and electron energies can be as high as 4 and 1.2 megaelectronvolts, respectively.[95] The density of low-energy (below 1 kiloelectronvolt) ions in the inner magnetosphere is about 2 cm-3.[97] The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps.[95] The particle flux is high enough to cause darkening or space weathering of the moon’s surfaces on an astronomically rapid timescale of 100,000 years.[95] This may be the cause of the uniformly dark colouration of the moons and rings.[83] Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles.[78] Unlike Jupiter's, Uranus's aurorae seem to be insignificant for the energy balance of the planetary thermosphere

At ultraviolet and visible wavelengths, Uranus's atmosphere is remarkably bland in comparison to the other gas giants, even to Neptune, which it otherwise closely resembles.[17] When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.[16][98] One proposed explanation for this dearth of features is that Uranus's internal heat appears markedly lower than that of the other giant planets. The lowest temperature recorded in Uranus's tropopause is 49 K, making Uranus the coldest planet in the Solar System, colder than Neptune.[12][64]

Banded structure, winds and clouds

In 1986, Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands (see figure on the right).[16] Their boundary is located at about -45 degrees of latitude. A narrow band straddling the latitudinal range from -45 to -50 degrees is the brightest large feature on the visible surface of the planet.[16][99] It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2 bar (see above).[100] Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[16] In all other respects Uranus looked like a dynamically dead planet in 1986. Unfortunately, Voyager 2 arrived during the height of Uranus's southern summer and could not observe the northern hemisphere. At the beginning of the 21st century, when the northern polar region came into view, the Hubble Space Telescope (HST) and Keck telescope initially observed neither a collar nor a polar cap in the northern hemisphere.[99] So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.[99] In 2007, when Uranus passed its equinox, the southern collar almost disappeared, while a faint northern collar emerged near 45 degrees of latitude.