Researchers operating the five spacecraft of NASA’s THEMIS mission reported the discovery today at the European Geophysical 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.
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 channel around 100,000 amps of electrical current to and from Earth’s ionosphere every three hours or so.