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News Space 07 September 2017

Power supply for Jupiter’s aurora puzzles scientists


The intense polar lights of the giant gas planet don’t work the way the ones on Earth do, writes Michael Lucy.


A composite image from the Hubble Space Telescope showing Jupiter in visible light and its aurora in ultraviolet.
A composite image from the Hubble Space Telescope showing Jupiter in visible light and its aurora in ultraviolet.
NASA / ESA / J. Nichols

Evidence from the Juno probe’s close flights past Jupiter indicate that the gas giant’s dazzling polar lightshows are caused by a mysterious mechanism different from the one responsible for intense auroras here on Earth.

On Jupiter, as on Earth, the northern and southern lights are produced by charged particles from the Sun colliding with gas atoms in the atmosphere and releasing energy in flashes of light.

A complete reconstruction of what the northern and southern auroral configurations looked like to the Juno Ultraviolet Spectograph (UVS) as Juno, in its large elliptical orbit, approached Jupiter, passed over the north pole, rapidly traveled to the southern hemisphere to pass over the southern pole, and finally receded from Jupiter. Jupiter itself is represented as a digital model as it might be observed in visible light, and the auroral configurations are static images in ultraviolet light measured by the UVS instrument.
A complete reconstruction of what the northern and southern auroras looked like to the Juno Ultraviolet Spectograph (UVS) as Juno approached Jupiter, passed over the north pole, rapidly traveled to the southern hemisphere to pass over the southern pole, and receded from Jupiter.
Bertrand Bonfond.

There are two different ways this can happen in terrestrial auroras. The more intense auroras are generated when electrons are carried towards the poles by strong electric fields that follow the planet’s magnetic field and accelerated down into the atmosphere near the poles. These are single, serpentine streamers of light, known as discrete auroras.

The less intense auroras are caused by electrons trapped in the magnetic field which are slightly accelerated by magnetic fluctuations. They collide with atoms less energetically and produce a much dimmer glow, called stochastic or broadband auroras.

Jupiter’s aurora is the brightest in the solar system, so planetary scientists assumed it was produced by the discrete process.

However, a paper in Nature analysing data from Juno’s low-altitude passes over Jupiter’s poles shows that, while there are extremely intense electric fields aligned with the magnetic field and signs that electrons are being accelerated downwards, the resulting auroras were much dimmer than those produced by the broadband process.

Why? The authors don’t know, though they speculate that Jupiter’s intense auroras may be started by a discrete process creating a stream of electrons that is then disrupted and diffused by the magnetic field fluctuations that produce the broadband process.

More data will be required. Luckily, Juno will continue its mission around Jupiter until February 2018, when it is scheduled to plunge into the Jovian atmosphere.

Michael Lucy is features editor of Cosmos.
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