Page last updated at 18:13 GMT, Thursday, 15 October 2009 19:13 UK

Glimpses of Solar System's edge

By Jason Palmer
Science and technology reporter, BBC News

Ibex data (Nasa)
The flow of particles into our solar neighbourhood is uneven

The first results from Nasa's Interstellar Boundary Explorer (Ibex) spacecraft have shown unexpected features at our Solar System's edge.

Ibex was launched nearly one year ago to map the heliosphere, the region of space defined by the extent of our Sun's solar wind.

Ibex's first glimpses show that the heliosphere is not shaped as many astronomers have believed.

A series of papers in the journal Science outlines the results.

Our Solar System is whipping around the centre of the galaxy. Just like a hand held out of a moving car, the Solar System feels a "wind" of particles from the region between our star and its nearest neighbours.

At the same time, the solar wind - a constant stream of fast-moving particles in all directions - blows outwards from the Sun.

The boundary at which the incoming and outgoing particles are at equivalent pressures, known as the heliopause, defines the heliosphere - the "bubble" in space generated by our own Sun's exhalations.

True shape

The true extent and shape of the heliosphere has been a subject of debate for more than half a century. Until now, the best clues came from the two Voyager spacecraft, which are believed to have passed through the heliopause at two different distances.

Completes a large, elliptical orbit around the Earth
Measures uncharged atoms that enter the Solar System from the interstellar medium
Can detect hydrogen, helium, and oxygen moving at a wide range of speeds through the Solar System
Results should explain how interstellar magnetic fields form our heliosphere

Through a process known as "charge exchange" at the heliosphere's edge, fast-moving neutral or uncharged particles are created, and it is these energetic neutral atoms or ENAs that the Ibex spacecraft aims to measure.

It orbits the Earth in a vast ellipse, gathering incoming ENAs flying back from the heliopause at a range of speeds.

What a number of researchers have found is that the flow of the ENAs is uneven, with a significantly higher flow in a "ribbon" across the sky.

"The Ibex results are truly remarkable, with emissions not resembling any of the current theories or models of this never-before-seen region," said lead researcher Dr David McComas of the Southwest Research Institute in New Mexico.

"We expected to see small, gradual spatial variations at the interstellar boundary. However, Ibex is showing us a very narrow ribbon that is two to three times brighter than anything else in the sky."

Near miss

These concentrations of incoming particles were just missed by the Voyager spacecraft, Dr McComas explained.

"The most astounding feature in the Ibex sky maps - the bright narrow ribbon - snakes through the sky between the Voyager spacecraft, where it remained completely undetected until now," he said.

Further measurements were made by the Cassini spacecraft orbiting Saturn. It too has a "camera" that can capture incoming neutral atoms, and also observed a ribbon-shaped region across the sky, but from ENAs moving at slightly different speeds.

What is clear is that the heliosphere is not shaped like a comet, as previously thought, with a head pointed at the oncoming interstellar medium and a tail of matter trailing behind.

The research groups agree that the magnetic field interactions at the heliopause have as-yet undetermined effects on the overall shape. But the exact shape, and the forces that cause it, are still a matter of debate between the teams.

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