The mystery of the aurora
Are there auroras on Jupiter, too ? Do they occur at the poles like on the earth ?
Yes, they do. In addition to the auroras witnessed at the poles, which resemble headbands, three auroras have been observed at the foot of the magnetic field line passing Jupiter's satellites. We call these "footprint auroras". The auroras occur in different locations because of the origin of the magnetic field line, and because the satellites themselves are separated.
Are the auroras at the foot of the magnetic field line circular ?
No, actually they take the shape of a line in the direction of Jupiter's rotation. The three places where footprints have been observed are at the bottom of the magnetic field line passing the satellites Io, Europa and Ganymede, all Galilean moons. Io's footprints can be explained by the volcanic gas it emits, which appears to interact with the magnetic field line passing by it. This does not explain why footprints were observed at the other two locations, though. Neither of the two satellites has volcanoes, so why would we see auroras there? This is another mystery we hope to unlock. Since Jupiter has so many moons, we may be able to observe other footprints in the future.
Can the auroras of Jupiter be observed from the earth ?
They are difficult to view with visible light from the earth, because auroras on Jupiter glow mainly in ultraviolet light, and such short-wavelength light is absorbed by the earth's atmosphere.
In 2001, when the space craft Cassini passed the dark side of Jupiter on its way to Saturn, an aurora was observed only with visible light. We cannot visually confirm auroras on the side of the planet illuminated by the sun from Earth even if there are actually auroras there. I thought the only way this might become possible was if we were to achieve sufficiently high wavelength resolution to suppress unwanted illumination by the Sun. To my dismay, even using a 188cm-diameter reflecting telescope and a high-resolution spectroscope at the National Astrophysical Observatory in Okayama, I could not confirm the occurrence of auroras. It appears that they don't glow so brightly on the illuminated side of Jupiter. This is yet another perplexity that I am hoping to solve.
Do auroras occur on planets other than Earth and Jupiter ?
Yes, auroras have also been seen on Saturn, Uranus and Neptune. Keys to the occurrence of auroras are a planet's magnetic field and atmosphere. The intensity of the magnetic field depends on the planet's components, size and rotation speed. These characteristics of Mars and Venus are not conducive to the occurrence of auroras.