Electrical circuit powering aurora revealed

Eugenie Samuel

The mysterious electrical circuit powering the Northern lights has been revealed for the first time by four satellites flying in formation through the upper reaches of an aurora.

Researchers already knew that the colourful curtains of light are caused by high energy electrons zipping into the Earth's atmosphere in high-latitude night skies.

But they have always been puzzled by calculations that show the electrons' temperature must be several million degrees Celsius. The temperature of the solar wind, which blows electrons from the Sun out into the inner Solar System, is about a thousand times cooler than that.

Now data from the European Space Agency's four Cluster satellites, launched in 2000, shows electrons draining out of the ionosphere - the part of the atmosphere where molecules get ionised by solar radiation - into outer space. The data was analysed by Goran Marklund and colleagues at the Royal Institute of Technology in Stockholm, Sweden.

Patrick Newell of John Hopkins University in Laurel, Maryland says once the ionosphere is drained, huge electric fields can build. These catapult the solar wind electrons that enter the Earth's magnetic sphere of influence down toward Earth, causing a light show in the upper atmosphere.

"This solves the puzzle of how the electric fields are sustained," he says. The acceleration of the electrons by the fields explains why their temperature rises.

Wax and wane

In January 2001, Cluster's satellites passed 20,000 kilometres above a northern aurora as it waxed and waned at an altitude of 200 km. Although previous satellites have measured electric field and current flow during aurora, the fact that they were flying solo made it difficult to reconstruct the evolution over time of electric fields at fixed points in and above the atmosphere.

"When you're driving, you can't tell if the rain stopped or you simply drove out of it," explains Newell.

Marklund presented his work on Tuesday at the American Geophysical Union meeting in San Francisco, California.

Journal reference: Nature (vol 414, p 724)

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