Question: On which planets have scientists seen auroras?
About the article "Aurora", section "On other planets".

Candidate passages:
Auroras occur on Mars, but they do not occur at the poles as on Earth, because Mars has no planetwide magnetic field.
Rather, they occur near magnetic anomalies in Mars's crust, which are remnants from earlier days when Mars did have a magnetic field.
Martian auroras are a distinct kind not seen elsewhere in the solar system.
They would probably also be invisible to the human eye, being largely ultraviolet phenomena. 
[[Image:Saturns Northern Aurora still.jpg|thumb|An aurora high above the northern part of Saturn; image taken by the Cassini spacecraft,a movie (click on image), shows images from 81 hours of observations of Saturn's aurora]]Both Jupiter and Saturn have magnetic fields much stronger than Earth's (Jupiter's equatorial field strength is 4.3 gauss, compared to 0.3 gauss for Earth), and both have large radiation belts.
Auroras have been observed on both, most clearly with the Hubble Space Telescope.
Uranus and Neptune have also been observed to have auroras.
The auroras on the gas giants seem, like Earth's, to be powered by the solar wind.
In addition, however, Jupiter's moons, especially Io, are powerful sources of auroras on Jupiter.
These arise from electric currents along field lines ("field aligned currents"), generated by a dynamo mechanism due to the relative motion between the rotating planet and the moving moon.
Io, which has active volcanism and an ionosphere, is a particularly strong source, and its currents also generate radio emissions, studied since 1955.
Auroras also have been observed on the surfaces of Io, Europa, and Ganymede, using the Hubble Space Telescope.
These auroras have also been observed on Venus and Mars.
Because Venus has no intrinsic (planetary) magnetic field, Venusian auroras appear as bright and diffuse patches of varying shape and intensity, sometimes distributed across the full planetary disc.
Venusian auroras are produced by the impact of electrons originating from the solar wind and precipitating in the night-side atmosphere.
An aurora was also detected on Mars, on 14 August 2004, by the SPICAM instrument aboard Mars Express.
The aurora was located at Terra Cimmeria, in the region of 177° East, 52° South.
The total size of the emission region was about 30 km across, and possibly about 8 km high.
By analyzing a map of crustal magnetic anomalies compiled with data from Mars Global Surveyor, scientists observed that the region of the emissions corresponded to an area where the strongest magnetic field is localized.
This correlation indicates that the origin of the light emission was a flux of electrons moving along the crust magnetic lines and exciting the upper atmosphere of Mars.
Auroras are classified as diffuse and discrete.
The diffuse aurora is a featureless glow in the sky that may not be visible to the naked eye, even on a dark night.
It defines the extent of the auroral zone.
The discrete auroras are sharply defined features within the diffuse aurora that vary in brightness from just barely visible to the naked eye, to bright enough to read a newspaper by at night.
Discrete auroras are usually seen in only the night sky, because they are not as bright as the sunlit sky.
Auroras occasionally occur poleward of the auroral zone as diffuse patches or arcs, which are generally subvisual.
In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621.
Auroras seen near the magnetic pole may be high overhead, but from farther away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the Sun were rising from an unusual direction.
Discrete auroras often display magnetic field lines or curtain-like structures, and can change within seconds or glow unchanging for hours, most often in fluorescent green.
The aurora borealis most often occurs near the equinoxes.
The northern lights have had a number of names throughout history.
The Cree call this phenomenon the "Dance of the Spirits".
In Medieval Europe, the auroras were commonly believed to be a sign from God.
Its southern counterpart, the aurora australis (or the southern lights), has features that are almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone.
It is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia.
Auroras occur on other planets.
Similar to the Earth's aurora, they are visible close to the planet's magnetic poles.
Modern style guides recommend that the names of meteorological phenomena, such as "aurora borealis", be uncapitalized. 
On August 5, 2011, the combined cloud of three consecutive CMEs produced brilliant auroras, reported as far south as Oklahoma and Alabama.
The geomagnetic storm reached a G4 (severe) level, enough to make power outages.
It was one of the strongest geomagnetic storms in years.
In the southern hemisphere, auroras could have been seen as far north as South Africa, Southern Chile and Southern Australia.
The CMEs were hurled by three M-class flares erupted in active sunspot 1261: M1.4 on August 2, M6.0 on August 3 and M9.3 on August 4. 
In order to assess the likelihood of finding any sort of life on various planets and moons, Dirk Schulze-Makuch and other scientists have developed a Planetary Habitability Index which takes into account factors including characteristics of the surface and atmosphere, availability of energy, solvents and organic compounds.
Using this index, based on data available in late 2011, the scientists found that Titan has the highest current habitability rating of any known world, other than Earth.