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'Inverse energy cascade' powers Jupiter's storms
Posted: October 06, 2009

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The transfer of energy from local winds to large scale circulations – an inverse energy cascade – could power and maintain Jupiter's intense jet streams, say scientists analysing Cassini observations of the giant planet.

Cassini captured Jupiter's storms in this image at its closest approach to the planet in December 2000, en route to Saturn. Image: NASA/JPL/Space Science Institute.

Intense storms race through Jupiter's atmosphere at the same speeds – some one hundred miles per hour – as much shorter lived hurricanes and tornadoes on Earth, but exactly what mechanism powers these ferocious and long duration storms on the giant planet has long been debated. One suggestion, based on previous observations and modelling, proposed that 'inverse energy cascading' could be the answer, and new analysis, presented by David Choi of the University of Arizona at the American Astronomical Society’s Division for Planetary Sciences meeting in Puerto Rico this week strengthens this idea.

“An inverse energy cascade is the transfer of energy from a local scale – local winds or small vortices, for example – to very large-scale circulations, such as big vortices and jet streams,” says Choi. “Imagine if you took a thin coffee stirrer and stirred only one small corner of your coffee within the cup. If there is an inverse energy cascade present, the small stirring would eventually generate a big swirl that encompassed the entire cup, similar to what you would get if you were stirring with a big spoon.”

Applied to Jupiter, the cascade of energy can grow from 'seeds' such as local thunderstorms, to planet-sized phenomena such as the planet’s trademark Great Red Spot or its dozens of alternating east and westward jet streams that define each hemisphere of the planet. These jet streams can reach top speeds of 300 miles per hour, compared with Earth's 50-150 mile per hour main eastward flowing jet streams.

Choi's analysis is based on images taken by the Cassini spacecraft when it flew by Jupiter in 2000 on its way to Saturn. By implementing automated software he developed for tracking cloud features that move with the wind, Choi was able to create a near-global wind vector map of Jupiter’s atmosphere to demonstrate that inverse energy cascading supplies the energy that forms and sustains Jupiter’s jet streams.

There is little evidence for inverse energy cascading occurring on the Earth, adds Choi, where hurricanes last for just a few days or week, and tornadoes on the order of minutes.