The Northern Lights: powered by an immense electrical circuit.
Satellites have detected the shifting forces that weave the Northern Lights.
A group of four spacecraft has given scientists their first glimpse of the immense electrical circuit above the Earth that creates the shimmering veil of the aurora borealis, or Northern Lights1.
In January 2001 the four satellites of the European Space Agency’s Cluster mission encountered a beam of electrons moving away from the Earth near the North Pole. The beam was on the outward-bound leg of its journey from the Sun, through the Earth’s atmosphere - where it creates the aurora - and back into space.
But the electrons must keep moving. The beam gets bent sideways in the auroral region so that it runs parallel to the Earth’s surface, before turning upward and streaming back into space.
It has long been thought that another, positively charged electric field draws the electrons up from the ionosphere and fires them back into space. According to this idea, the aurora is a consequence of this vast electrical circuit, in which electrons flow from a negative to a positive terminal, like those of a battery.
The aurora owes its ever-changing beauty to the inconstancy of the circuit. Solar winds ’beat’ the magnetic fields, causing them to flicker on and off.
Cluster consists of four satellites launched in 2000 - Rumba, Salsa, Samba and Tango - that orbit between 19,000 and 119,000 kilometres above the Earth, passing in and out of the planet’s magnetic field.
Previous spacecraft have detected the upward flow of electrons. But it hasn’t been possible to follow how the beam changes over time. The Cluster mission can study this because the four satellites, orbiting in formation, pass through the same region of the magnetic field at different times.
The upward beam of electrons creates a kind of ’anti-aurora’ or black aurora, sometimes visible from the ground as black patches or rings in the Northern Lights.
PHILIP BALL | © Nature News Service
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