Science: Hubble reveals spectacle of Jupiter's `Northern Lights'

The "Northern Lights" seen on Earth have their equivalents on all the other planets. Jupiter, the biggest, doesn't skimp when it comes to a show, as the latest pictures from the Hubble Space Telescope reveal. Charles Arthur, Science Editor, explains.

The "aurora borealis", the free light show put on by nature at our north and south poles, may be impressive on Earth. But it hardly compares to that available on Jupiter.

These new images, captured by Hubble, show that the largest planet in our solar system can match the best of Earth's northern lights. They extend hundreds of miles upwards into the Jovian atmosphere, and thousands of miles down over the planet's poles.

The same phenomenon has been observed on Earth, and photographed by the Space Shuttle. Usually they are called the Northern Lights (or Southern Lights, depending which hemisphere you're in).

Yet the ones pictured are produced by a very different process from that on the Earth.

Here the flickering auroras are caused by fast-moving electrons, thrown out in the "solar wind" from the Sun, hitting the Earth's upper atmosphere. On Jupiter, the lights are caused by particles thrown out by volcanoes on Io, one of the planet's moons. The particles are then magnetically trapped and begin rotating with the planet, producing ovals of auroral light centred on its magnetic poles (where the magnetic flux is most intense) during both day and night.

The electrons thrown out from Io generate an invisible electric current equivalent to one million amperes, and generate localised aurorae where they enter the planet's magnetic field. Faint traces, looking like white, comet-shaped streaks outside the polar ovals, show where those streams are trapped in Jupiter's magnetosphere. They persist for hours after the moon has passed on its orbit, which takes almost two days.

While the light show on Jupiter may be impressive, it does have one disadvantage. It is invisible to human eyes, because the light created by the electrical energy storms is in the ultraviolet part of the spectrum. That also means that it cannot be observed from the Earth's surface, since our atmosphere absorbs most of the light at these wavelengths. (The ultraviolet lenses used means that sunlight reflected from the planet's surface appears brown.) The Hubble telescope is the only convenient way we presently have to witness such natural phenomena.