Satellites criss-cross northern lights

Two satellites have simultaneously flown through a natural particle accelerator just above Earth's atmosphere for the first time to discover how the dramatic light displays of auroras are generated.

The European Space Agency’s (ESA) C3 Cluster satellite first crossed the region at an altitude of 6,400 km, followed five minutes later by C1 at 9,000 km, and the readings have allowed the electrical landscape of the acceleration region to be mapped.

“Cluster has now shown us the very heart of the acceleration process responsible for most bright auroras. It has given us our first look at the electrical structure and stability of such an accelerator,” said Göran Marklund from the Royal Institute of Technology in Stockholm, Sweden.

Mapping the creation of auroras

“This is like geography,” said Marklund, “Only instead of the contours being the height of a landscape, they are the electrical potentials that span the region.” These electrical potentials act in both uphill and downhill directions, accelerating particles towards and away from Earth, according to their charges.

When particles strike the atmosphere, they create the shimmering curtains of light known as the aurora, or more commonly the northern and southern lights. About two-thirds of the bright auroras are estimated to be produced in this way.

Since 2006, the Cluster satellites have been drifting away from their initial orbits because they are being constantly nudged by the gravity of the Moon and the Sun. Fortuitously, the current orbit occasionally passes through the Auroral Acceleration Region, which spans 4000 km to 12000 km above our planet.

Insight into the secrets of space plasma

The satellites do not encounter a natural particle accelerator on every orbit. Those responsible for the bright auroras are temporary alignments of the electrical fields around Earth. They are highly variable in altitude and so not always present.

This first encounter with a natural particle accelerator associated with a large-scale aurora has proved that they may be stable for at least five minutes. A few more encounters are expected in the near future before Cluster’s orbit drifts back out of the region.

Such natural particle accelerators pop up ubiquitously throughout the Solar System, especially in the strong magnetic fields of the gas giants Jupiter and Saturn.