Space - Tiny volcanic moon controls Jupiter's auroras

The northern lights are in my eyes – all right, Hubble's (Image: NASA and the Hubble Heritage Team (STScI/AURA))

Sometimes the puppets control the puppeteer. It seems volcanic outbursts on Jupiter's moon Io control brilliant auroras on its parent planet.

Auroras are shimmering curtains of light caused when charged particles slam into a planet's magnetic field. Earth's northern and southern lights are active only when the sun releases a big blob of charged plasma, as it did on 24 January.

Jupiter has a permanent ring of auroral light surrounding each of its poles (see photo). Most of the charged particles responsible for the light have long been thought to originate from tiny, hyperactive Io, which burps out about a tonne of sulphur per second in its persistent, violent volcanic activity. However, the sun was thought to cause any variations in the rings via changes in the pressure of the solar wind.

New observations suggest that Io can control these changes as well. "Variations we thought were connected to the sun we now see are connected to the volcanic activity," says Bertrand Bonfond of the University of Liège in Belgium.

Mega plume

Bonfond and colleagues observed Jupiter and Io with the Hubble Space Telescope once a day between February and June 2007. In those five months, the team captured twice as much data as had been collected in the previous 10 years.

"Never before did we have daily observations of the auroras," Bonfond says. "[Until then] we couldn't disentangle the day-by-day variations and the global trends."

Over that time, Jupiter's constant auroral rings grew significantly, but it was not clear why. The solution came when NASA's New Horizons spacecraft flew by Jupiter in May 2007 on its way to Pluto. The probe snapped spectacular images of a volcanic plume hundreds of kilometres high erupting out of Io's surface.

The influx of plasma from this huge eruption flooded Jupiter's magnetic field, widening the auroral rings. "The auroras are really responding to this increased amount of material coming out of Io," Bonfond says.

Open question

Margaret Kivelson of the University of California, Los Angeles, says the new paper shows that Io can control Jupiter's auroras over long timescales. However, she says the sun is still a player on timescales of a day or so. "This does not rule out contributions from the solar wind," she says.

Team member Denis Grodent of the University of Liège agrees. "The question is certainly still open," he says. "The role of the solar wind is unclear, because of the lack of long-term observations."

There are also other mysteries to be solved, like how the charged particles are accelerated along Jupiter's magnetic field lines. For answers, Bonfond looks to the Juno spacecraft that launched last August and will reach Jupiter in 2016. "For the first time we can analyse [the magnetic interactions] on another planet than the Earth," he says. "It will be a game-changer."

Journal reference: Geophysical Research Letters, DOI: 10.1029/2011gl050253

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