Jupiter's largest moon, Ganymede, is also its only moon with a strong magnetic field. Now, using thousands of images from the Hubble Space Telescope, scientists have discovered that the spectacular auroras seen at Jupiter's poles are generated in part by the pull of Ganymede's magnetosphere.

Both Ganymede and the volcanically active moon Io interact with Jupiter's plasma as they orbit around the planet, generating bright spots at the poles called "auroral footprints." Until now, however, no one knew how big Ganymede's footprint was or why the moon caused these beautiful light shows.

By analyzing the Hubble images, researchers measured the exact size of Ganymede's footprint and determined that it's too big to be a projection of the planet itself, but corresponds almost perfectly with the diameter of the moon's protective magnetic field. Scientists also measured the size and shape of Io's footprint, which is caused by charged particles ejected from its many active volcanoes.

"Each of these auroral structures is telling an ongoing story about vast transfers of energy taking place far away from the planet," astrophysicist Denis Grodent of the University of Liege in Belgium said in a press release. "By analyzing the exact locations of these features and how their shape and brightness changes as Io and Gaynmede move in their orbit around Jupiter, we have created the most detailed picture to date of how Jupiter and these moons are electromagnetically interconnected." Grodent presented his research Thursday at the European Planetary Science Congress in Germany.

In addition to linking Ganymede's footprint with its magnetic field, Grodent and his team discovered unexpected periodic variations in the brightness of the moon's aurora, happening on three different timescales. The researchers think each variation reflects a specific interaction between Jupiter's plasma and Ganymede's magnetic field, but they don't yet know what's causing the interactions.

Image 1: Grodent/Hubble Space Telescope Team; HST program GO-10862, May 2007. Image 2: Grodent/Hubble Space Telescope Team; HST Program GO-10140, PI: Denis Grodent (ESA Univeristy of Liege), filter F125LP, exposure time 110 sec, April 2005.

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