Astronomers have detected a bright spot with a trailing tail in Jupiter's aurora, caused by an electromagnetic connection to the planet's moon, Europa.

For more than a decade, the Hubble Space Telescope has returned images of the aurora at Jupiter's poles as the emissions light up in ultraviolet wavelengths. You can watch movies of the aurora here.

Scientists have long known that Io, the innermost Galilean moon, triggers a bright auroral spot, or "footprint", in the light show. That footprint boasts a tail that streaks halfway around the planet.

Now Denis Grodent at the University of Liège, Belgium, and his team say Europa interacts in a similar way, although on a fainter scale. And for the first time, they have detected a dim, short auroral tail associated with Europa's footprint.

Volcanic action

Jupiter's auroras are produced by charged particles zipping down the planet's magnetic field lines to the upper atmosphere where they excite gases, causing them to glow. On Earth, the same process produces the Northern and Southern lights.

But scientists are still unlocking the secrets of the enigmatic lights on Jupiter, which seem to have a wide range of influences.

Several of Jupiter's moons are thought to play a role. In the case of Io, scientists believe the moon's ongoing volcanism produces gases which become charged and carry strong electrical currents along the arcing magnetic lines into Jupiter's atmosphere.

Something in the air?

"A very similar process apparently occurs at Europa," Grodent's team writes, having studied 45 Hubble images of the aurora showing the moon's footprint and tail.

But Europa is not thought to be volcanic, so what could produce the electrical current that zips along and eventually gives rise to Europa's auroral footprint?

"We're looking for something either within the moon's surface or in its atmosphere that's electrically conducting," says John Clarke, a space physicist at Boston University, US. While the surface is a possibility, Clarke told New Scientist, the ionosphere should be the first place to look.

Journal reference: Geophysical Research Letters (DOI: 10.1029/2005GL025487)

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