On the dark side of the Earth, the solar wind stretches out Earth’s magnetic field, forming a region known as the magnetotail. When it is stretched too far, it snaps, releasing energy that creates a pair of counter-spiralling vortices in the solar wind. One vortex sends particles into Earth’s ionosphere; charged particles return up through the other vortex, completing the electrical circuit (Illustration: A Keiling et al/THEMIS/NASA)

On the dark side of the Earth, space is riddled with giant plasma tornadoes that power shimmering auroras, new observations reveal.

Researchers operating the five spacecraft of NASA’s THEMIS mission reported the discovery today at the European Geosciences Union meeting in Vienna, Austria.

They were measuring how the solar wind, a flow of charged particles from the sun, interacts with Earth’s magnetic field.

On the Earth’s dark side, the solar wind stretches out the field, forming a region known as the magnetotail. The magnetotail is like a rubber band; when it is stretched too far, “eventually it snaps and releases the energy”, says team member Andreas Keiling of the University of California, Berkeley.


Counter spirals

The sudden energy release creates a pair of counter-spiralling vortices, which THEMIS detected by measuring the speed and direction of the solar wind in the magnetotail. Each one is 70,000 kilometres long and wide enough to envelop the Earth (see illustration).

One vortex in the pair sends particles spiralling along Earth’s magnetic field lines until they hit molecules of the ionosphere 400 km above the surface. The energy released by the collisions creates the auroral glow, like the gas in a neon light.

Charged particles return up through the other vortex, completing the electrical circuit. The vortices appear every three hours or so, and each one channels a current of 100,000 amps into the ionosphere while it is there.

Little warning

Indeed, magnetometers across North America confirmed that auroral events happened about a minute after THEMIS registered the formation of vortices in the magnetotail. Also, the auroras moved towards the poles in pairs, mirroring the paired vortices in space.

Though the discovery will help researchers understand more about what drives space weather, it won’t help us avoid the disaster scenarios such weather can cause (see Space storm alert: 90 seconds from catastrophe). “It takes about a minute from when the tornado forms for it to propagate down to the ionosphere,” Keiling told New Scientist. “Any warning has to come before that.”

The observations are limited, and it is not yet proven that all auroras are caused by giant space tornadoes, but Keiling thinks this is likely. “A tornado will be involved at some point in the process,” he says.

See a gallery of auroras