Jupiter's magnetic field sweeps across Io, stripping away particles created by the moon's volcanism.
"Material escapes from Io's volcanoes and creates an ionized ring of plasma called the Io torus," explained Rosaly M. Lopes, a planetary geophysicist for NASA's Jet Propulsion Laboratory in California who was not involved in the study.
Io orbits inside this cloud and, like a rock in a stream, disrupts the flow of particles. This generates powerful plasma waves that create the main auroral spots.
"The paper by Bonford et al. reports the discovery of a new spot in the ultraviolet," seen in an image taken by the Hubble Space Telescope, Lopes said in an email.
Each appearance of this upstream "leading spot" occurs in a distinctive pattern, the study authors say.
When a leading spot precedes the main spot in Jupiter's northern hemisphere, a downstream spots follows the main spot in the southern hemisphere, and vice versa.
The team theorizes that this is because the plasma waves are also creating current loops between Io and Jupiter's poles.
The loops propel beams of electrons along the planet's magnetic field lines into Jupiter's atmosphere, creating the leading spot. As Io changes position within the torus, the leading spot moves upstream or downstream.
"Studying these spots and their variations is an important step in understanding how Io's volcanic activity interacts with Jupiter's magnetosphere," Lopes said.
The findings would also mean that the Jupiter-Io system is the only known test case of a conductive body orbiting a magnetized planet, study leader Bonfond added.
Similar interactions could arise between extrasolar planets and their moons, between planets and their parent stars, or even between magnetized and nonmagnetized stars.
"If we were able to detect auroras on exoplanets, this would be rather good news for potential life, because it would mean that the planet has an atmosphere and a protective magnetic field," he said.
Bonfond said Jupiter's light show could also be used to study auroral storms on Earth, which can damage telecommunication and navigation satellites, perturb radio transmissions, and disrupt electric lines.
(Read "Stronger Solar Storms Predicted; Blackouts May Result" [March 7, 2006].)
"If we want to predict and avoid such problems, we have to understand magnetospheric processes in detail," he said.
And it's easier to study subtle aspects of Earth's physics when they're magnified on a bigger stage, such as Jupiter.
"For example, the fact that electrons can be propelled from one hemisphere toward the other along the field lines does also occur on Earth, but the consequences are barely observable on our planet," he said.
"If our theory is verified, they are striking on Jupiter."
Bonfond and his colleagues hope to make further observations of Io's footprint following planned repairs and improvements to the Hubble Space Telescope later this year.
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES