Five NASA satellites named after Themis, the Greek goddess of justice, have finally revealed the trigger behind the brilliant auroras that dance across the polar skies. The Themis mission is an attempt to settle a long-standing debate on the origins of the rippling lights. Scientists know that the aurora are caused by electrons streaming from space and delivering a kick to gas molecules in the atmosphere. As the molecules relax, they release the energy as light — blue from nitrogen and green and red from oxygen. But the trigger that unleashes those electrons has remained a mystery. The original energy source for the aurora is the solar wind, a stream of charged plasma that billows from the Sun and deforms the Earth’s magnetic field, producing a long ‘magnetotail’ on the far side of the Earth.
While coronal mass ejections can cause larger plasma storms that last for more than 24 hours, deformation of the magnetotail can create smaller substorms that last just a few hours. But scientists disagree on the precise order of events between the solar wind and the substorm. One camp holds that the key step is a disruption, which occurs about 60,000 kilometres away from the Earth, in the electrical current that travels across the magnetotail. The other camp contends that the first step is actually a realignment of the Earth’s magnetic field some 120,000 kilometres away, roughly one-third the distance to the Moon. According the the Themis results, published today by the journal Science,1 this latter explanation is the correct one.